
Sudholt, A., Glaude, P.A., BattinLeclerc, F. & Pitsch, H., The oxidation of γvalerolactone in a low pressure ﬂame. In Proceedings of the 7th European Combustion Symposium, March 30th  April 2nd, 2015, Budapest, Hungary. 2015.

Baroncelli, M., Kruse, S. & Pitsch, H., Experimental and numerical investigation of extinction strain rates of coal devolatilization products under oxyfuel conditions. In Proceedings of the European Combustion Meeting 2015. 2015.

Foo, K.K., Jocher, A., Thong, C.X., Dally, B., Nathan, G., Medwell, P.R., Alwahabi, Z.T. & Pitsch, H., Preliminary Experimental and Computational Study of Time varying Laminar NonPremixed Ethylene/Nitrogen Flames. Australian Combustion Symposium, December 7th  9th, Melbourne, Australia. 2015.

Jocher, A., Pitsch, H., Gomez, T., Bonnety, J. & Legros, G., Impact of magnetic fields on the stability of nonpremixed flames. In Proceedings of the 7th European Combustion Meeting, March 30th  April 2nd, Budapest, Hungary. 2015.

Beeckmann, J., Hesse, R., Cai, L., Pitsch, H., Heufer, A. & Yang, Y., 2Butanone Laminar Burning Velocities  Experimental and Kinetic Modelling Study. In SAE Technical Paper Series. p. 10. 2015.

Zweigel, R., Korkmaz, M., Pitsch, H. & Abel, D., Modellbasierte Prädiktive Regelung der dieselmotorischen Niedertemperaturverbrennung unter Berücksichtigung von Emissionen. AUTOREG 2015  Auf dem Weg zum automatisierten Fahren, 7. Fachtagung BadenBaden, 9th10th June 2015, VDIBerichte 2233, pp.317328. 2015.

Jochim, B., Zweigel, R., Korkmaz, M., Abel, D. & Pitsch, H., Reduced Order Combustion Model For A Virtual Control Testbed Of A PCCI Diesel Engine. In Symposium for Combustion Control 2015 (SCC15), 17th18th June 2015, Aachen, Germany. pp. 3540. 2015.

Pitsch, H. & Williams, F.A., Professor Norbert Peters (19422015). Combustion and Flame, 162(10), pp.3447  3448. 2015.

Narayanaswamy, K., Pitsch, H. & Pepiot, P., A chemical mechanism for low to high temperature oxidation of methylcyclohexane as a component of transportation fuel surrogates. Combustion and Flame, 162(4), pp.1193  1213. 2015.
Abstract Surrogate fuels consisting of a mixture of wellstudied hydrocarbons are often used to model real fuels in typical combustion studies. A major challenge, however, is the capability to design compact and reliable kinetic models that capture all the specificities of the simpler, but still multicomponent surrogates. This task is further complicated by the diverse nature of the hydrocarbons commonly considered as potential surrogate components, since they typically result in large detailed reaction schemes. Towards addressing this challenge, the present work proposes a single, compact, and reliable chemical mechanism, that can accurately describe the oxidation of a wide range of fuels, which are important components of surrogate fuels. A wellcharacterized mechanism appropriate for the oxidation of smaller hydrocarbon species (Blanquart et al., 2009), as well as several substituted aromatic species and ndodecane (Narayanaswamy et al., 2010, 2014), well suited as a base to model surrogates, has now been extended to describe the oxidation of methylcyclohexane, a representative of the cyclic alkane class, which is often used in jet fuel surrogates. To ensure compactness of the kinetic scheme, a short mechanism for the low to high temperature oxidation of methylcyclohexane is extracted from the detailed scheme of Pitz et al. (2007) and integrated in a systematic way into the previous model. Rate coefficient changes based on recent recommendations from literature, and an additional concerted elimination pathway important at moderate to low temperatures are introduced to the resulting chemical mechanism, which improve the model predictions. Extensive validation of the revised kinetic model is performed using a wide range of experimental conditions and data sets.

Kerschgens, B., Cai, L., Pitsch, H., Janssen, A., Jakob, M. & Pischinger, S., Surrogate fuels for the simulation of diesel engine combustion of novel biofuels. International Journal of Engine Research, 16(4), pp.531546. 2015.
Recently, several promising biomassderived fuels for diesel engines have been identified, produced, and tested. Diesel engine experiments confirmed very low soot and low nitrogen oxide emissions. With regard to further improvements of the combustion system, it is desirable to complement the diesel engine experiments with numerical simulations. To date, this is hindered by the lack of suitable chemical reaction mechanisms for these novel fuels. Therefore, a surrogate approach is presented here and applied in computational fluid dynamics simulations. Combustion and pollutant formation is simulated using the representative interactive flamelet model. Ignition, combustion, and pollutant formation are described in a consistent manner by inclusion of detailed reaction chemistry. Different mixtures of nheptane, toluene, ethanol, dimethylether, ethane, and phenol are employed to describe the combustion chemistry of the biofuels. The compositions of the surrogate fuels are compiled according to hydrogen/carbon ratio, oxygen content, cetane rating, and molecular properties of the experimental fuels. Spray, injection, and evaporation properties of the experimental fuels, as obtained from spray vessel experiments, are included in the computational fluid dynamics simulations. By systematic comparison of experimental and numerical results, the surrogate methodology is validated and an improved understanding of the limitations of the current surrogates is achieved. Thus, a methodology for the fast adoption of novel fuels for simulations is proposed that can be used regardless of the availability of specific chemical reaction mechanisms.

Bode, M., Deshmukh, A., Kirsch, V., Reddemann, M., Kneer, R. & Pitsch, H., Direct numerical simulations of novel biofuels for predicting spray characteristics. 13th International Conference on Liquid Atomization and Spray Systems, ICLASS 2015, August 23rd  27th 2015, Tainan, Taiwan. 2015.

Kim, S.H. & Pitsch, H., On the lattice Boltzmann method for multiphase flows with large density ratios. Journal of Computational Physics, 303, pp.19  27. 2015.
An analysis of the lattice Boltzmann (LB) method for multiphase flows with large density ratios is presented. It is shown that for incompressible, multiphase LB methods, the divergencefree condition is not satisfied within the formal accuracy of the LB method, when the density ratio between the two phases is large enough. The discrete differentiationbyparts rule is responsible for this error. A new multiphase LB method to resolve this issue is proposed.

Bode, M., Falkenstein, T., Le Chenadec, V., Kang, S., Pitsch, H., Arima, T. & Taniguchi, H., A New Euler/Lagrange Approach for Multiphase Simulations of a MultiHole GDI Injector. SAE Technical Paper Series, 2015010949. 2015.

Göbbert, J.H., Gauding, M., Ansorge, C., Hentschel, B., Kuhlen, T. & Pitsch, H., Direct Numerical Simulation of Fluid Turbulence at Extreme Scale with psOpen, ParCo 2015. 2015.

Deshmukh, A., Bode, M., Pitsch, H., Kirsch, V., Reddemann, M., Palmer, J., Kneer, R., van Overbrüggen, T., Schröder, W., Liebergesell, B., Bardow, A., Ottenwälder, T. & Pischinger, S., Spray breakup and mixture formation of novel biofuels. In 3rd TMFB International Conference, June 23rd  25th 2015, Aachen, Germany. 2015.

Trisjono, P. & Pitsch, H., Systematic Analysis Strategies for the Development of Combustion Models from DNS: A Review. Flow, Turbulence and Combustion, 95(23), pp.231259. 2015.

Bode, M., Göbbert, J.H. & Pitsch, H., Novel multiphase simulations investigating cavitation by use of insitu visualization and Euler/Lagrange coupling. PRACEdays15, May 26th  28th 2015, Dublin, Ireland. 2015.

Gauding, M., Goebbert, J.H., Hasse, C. & Peters, N., Line segments in homogeneous scalar turbulence. Physics of Fluids, 27(9). 2015.

Pitsch, H. & Williams, F.A., Professor Norbert Peters 10 July 1942–4 July 2015 Obituary. Journal of Fluid Mechanics, 781, pp.12. 2015.

Cai, L. & Pitsch, H., Optimized chemical mechanism for combustion of gasoline surrogate fuels. Combustion and Flame, 162(5), pp.1623  1637. 2015.
Abstract Since real petroleum fuels are composed of a huge variety of hydrocarbon components, surrogate mixtures of various hydrocarbon fuels are typically employed in computational research and in engine development to represent transportation fuels. In this study, a reduced combustion mechanism of Primary Reference Fuel (PRF) mixtures (nheptane and isooctane) is integrated into the published kinetic model (Narayanaswamy et al., 2010), allowing for the formulation of multicomponent surrogate fuels (e.g. PRF/toluene) and for the prediction of Polycyclic Aromatic Hydrocarbon (PAH) formation in gasoline engines. In order to optimize the model performance, a recently developed optimization technique based on rate rules (Cai and Pitsch, 2014) is extended in this study. The goal is to calibrate automatically the multicomponent kinetic mechanism, which also leads to a chemically consistent \PRF\ mechanism and a computational advantage for the calibration process. In addition, this work contributes to the development of general rate rules for various hydrocarbon fuels. An ethanol model is also incorporated into the proposed mechanism. This facilitates the prediction of gasoline/ethanol blend combustion. The resulting mechanism retains a compact size and is successfully validated against experimental measurements.

Bode, M., Falkenstein, T., Pitsch, H., Kimijima, T., Taniguchi, H. & Arima, T., Numerical study on the impact of cavitation on the spray development
processes for GDI Injection. ICLASS 2015, 13th Triennial International Conference on Liquid Atomization and Spray Systems, August 23rd  27th 2015, Tainan, Taiwan. 2015.

Boschung, J., Gauding, M., Hennig, F., Peters, N. & Pitsch, H., An alternative definition of order dependent dissipation scales, 15th European Turbulence Conference, 2528 August, 2015, Delft, The Netherlands. 2015.

Ye, J., Medwell, P.R., Varea, E., Kruse, S., Dally, B.B. & Pitsch, H.G., An experimental study on MILD combustion of prevaporised liquid fuels. Applied Energy, 151, pp.93  101. 2015.
Abstract This paper presents an experimental study on moderate or intense low oxygen dilution (MILD) combustion of prevaporised liquid fuels burning in a reverseflow \MILD\ combustor under elevated pressures. The influence of fuel type, equivalence ratio, carrier gas, operating pressure and air jet velocity on the combustion stability and emissions are investigated. Ethanol, acetone and nheptane are vaporised and carried to the combustor using either nitrogen or air. It is found that the combustion stability is highly dependent on fuel type, with nheptane being the most unstable due to its fast ignition under all highpressure conditions studied. Measured \CO\ emissions emitted from all fuels are very low except when the equivalence ratio approaches the lean extinction limit, and this effect is not dependent on the pressure. The joint regime of low \CO\ and \NOx\ emission becomes narrower under elevated pressure as \NOx\ emissions emitted from all fuels increased with pressure. The enhanced \NOx\ formation rate via the nitrous oxide mechanism, the slower mixing, the increased flame temperature and residence time are believed to cause higher \NOx\ emissions as pressure increases. The \NOx\ emissions are reduced by increasing the air jet velocity, which is attributed to a lower peak temperature. The \NOx\ emissions are also reduced when the fuel is carried by nitrogen instead of air. Further research is required to understand this trend which will help in reducing \NOx\ emissions under these conditions, especially at elevated pressures.

Fiorina, B., Mercier, R., Kuenne, G., Ketelheun, A., Avdić, A., Janicka, J., Geyer, D., Dreizler, A., Alenius, E., Duwig, C., Trisjono, P., Kleinheinz, K., Kang, S., Pitsch, H., Proch, F., Marincola, F.C. & Kempf, A., Challenging modeling strategies for LES of nonadiabatic turbulent stratified combustion. Combustion and Flame, 162(11), pp.4264  4282. 2015.
Abstract Five different lowMach large eddy simulations are compared to the turbulent stratified flame experiments conducted at the Technical University of Darmstadt (TUD). The simulations were contributed by TUD, the Institute for Combustion Technology (ITV) at Aachen, Lund University (LUND), the \EM2C\ laboratory at Ecole Centrale Paris, and the University of DuisburgEssen (UDE). Combustion is modeled by a premixed flamelet tabulation with local flame thickening (TUD), a premixed flamelet progress variable approach coupled to a level set method (ITV), a 4steps mechanism combined with implicit ŁES\ (LUND), the FTACLES model that is based on filtered premixed flamelet tabulation (EM2C), and a flame surface density approach (UDE). An extensive comparison of simulation and experimental data is presented for the first two moments of velocity, temperature, mixture fraction, and major species mass fractions. The importance of heatlosses was assessed by comparing simulations for adiabatic and isothermal boundary conditions at the burner walls. The adiabatic computations predict a flame anchored on the burner lip, while the nonadiabatic simulations show a flame liftoff of one half pilot diameter and a better agreement with experimental evidence for temperature and species concentrations. Most simulations agree on the mean flame brush position, but it is evident that subgrid turbulence must be considered to achieve the correct turbulent flame speed. Qualitative comparisons of instantaneous snapshots of the flame show differences in the size of the resolved flame wrinkling patterns. These differences are (a) caused by the influence of the ŁES\ combustion model on the flame dynamics and (b) by the different simulation strategies in terms of grid, inlet condition and numerics. The simulations were conducted with approaches optimized for different objectives, for example low computational cost, or in another case, short turn around.

Kruse, S., Kerschgens, B., Berger, L., Varea, E. & Pitsch, H., Experimental and numerical study of MILD combustion for gas turbine applications. Applied Energy, 148, pp.456  465. 2015.
Abstract In this paper, the pressure influence on the \MILD\ combustion process and on emissions is examined. Experiments were performed in a combustion chamber based on the reverse flow configuration to enhance mixing of fresh and burnt gases. First, investigations under atmospheric pressure were performed to determine the regime of jointly low \NOx\ and \CO\ emissions. Afterwards, the effects of burnt gas recirculation and mixing under ambient pressure were evaluated by decreasing the inlet nozzle diameter without changing the residence time. In the second step, measurements were conducted under higher pressure while keeping the mass flow rates constant. Thereby, the residence time is extended, the \NOx\ formation chemistry is changed. These effects result in a strong rise in \NOx\ emissions and simulations indicate that at higher pressure a flame is established at the nozzle exit for higher equivalence ratios. In order to decrease the Damköhler number and shift the combustion process to the wellstirred reactor regime, the inlet nozzle diameter was decreased without changing the residence time. Thereby, burnt gas recirculation and gas mixing is enhanced, while simultaneously extending the chemical time scales. A significant decrease in \NOx\ emissions was detected.

Beeckmann, J., Cai, L., Schaback, D., Hesse, R. & Pitsch, H., Reduced Chemical Mechanism for the Calculation of Ethanol / Air Flame Speeds. In SAE Technical Paper Series. p. 6. 2015.

Falkenstein, T., Bode, M., Kang, S., Pitsch, H., Arima, T. & Taniguchi, H., LargeEddy Simulation Study on Unsteady Effects in a Statistically Stationary SI Engine Port Flow. SAE Technical Paper Series, 2015010373. 2015.

Mayer, D., Moshammer, K., Cai, L., Pitsch, H. & KohseHöinghaus, K., A numerical study of highlydiluted, burnerstabilised dimethyl ether flames. Combustion Theory and Modelling, 19(2), pp.238259. 2015.

Knudsen, E., Shashank, & Pitsch, H., Modeling partially premixed combustion behavior in multiphase LES. Combustion and Flame, 162(1), pp.159180. 2015.
Abstract A partially premixed flamelet model is extended to account for liquid fuel, and its accuracy and dependence on spray modeling inputs are investigated. The spray and combustion model coupling is considered by simulating single drop evaporation experiments. These simulations confirmed previous findings showing that small changes in spray model assumptions significantly influence evaporation rates. The analysis is then extended to Large Eddy Simulation (LES) using a \NASA\ model aircraft combustor burning JetA fuel. Results indicate the partially premixed flamelet approach can describe the range of combustion physics found in the burner. However, they also reveal that the liquid model sensitivities hold in LES, where a tight coupling exists between combustion model predictions and spray model assumptions. Details of the liquid modeling approach, including thermodynamic specifications and descriptions of liquid spray at the nozzle outlet, influence the burning regime, mean gas temperatures, and emissions levels predicted by LES. The results suggest that while the combustion model is capable, spray model influences must be firmly understood before validation can be accomplished in liquid fueled contexts.

Saghafian, A., Terrapon, V.E. & Pitsch, H., An efficient flameletbased combustion model for compressible flows. Combustion and Flame, 162(3), pp.652  667. 2015.
Abstract A combustion model based on a flamelet/progress variable approach for highspeed flows is introduced. In the proposed formulation, the temperature is computed from the transported total energy and tabulated species mass fractions. Only three additional scalar equations need to be solved for the combustion model. Additionally, a flamelet library is used that is computed in a preprocessing step. This approach is very efficient and allows for the use of complex chemical mechanisms. An approximation is also introduced to eliminate costly iterative steps during the temperature calculation. To better account for compressibility effects, the chemical source term of the progress variable is rescaled with the density and temperature. The compressibility corrections are analyzed in an a priori study. The model is also tested in both Reynoldsaveraged Navier–Stokes (RANS) and largeeddy simulation (LES) computations of a hydrogen jet in a supersonic transverse flow. Comparison with experimental measurements shows good agreement, particularly for the ŁES\ case. It is found that the disagreement between \RANS\ results and experimental data is mostly due to the mixing model deficiencies and the presumed probability density functions used in the \RANS\ formulation. A sensitivity study of the proposed model shows the importance of the compressibility corrections especially for the source term of the progress variable.

Cai, L., Uygun, Y., Togbé, C., Pitsch, H., Olivier, H., Dagaut, P. & Sarathy, S.M., An experimental and modeling study of noctanol combustion. Proceedings of the Combustion Institute, 35(1), pp.419427. 2015.
Abstract This study presents the first investigation on the combustion chemistry of noctanol, a long chain alcohol. Ignition delay times were determined experimentally in a highpressure shock tube, and stable species concentration profiles were obtained in a jet stirred reactor for a range of initial conditions. A detailed kinetic model was developed to describe the oxidation of noctanol at both low and high temperatures, and the model shows good agreement with the present dataset. The fuel’s combustion characteristics are compared to those of nalkanes and to short chain alcohols to illustrate the effects of the hydroxyl moiety and the carbon chain length on important combustion properties. Finally, the results are discussed in detail.

Stoehr, K.D., Peters, N. & Beeckmann, J., Low temperature oscillations of DME combustion in a jetstirred reactor. Proceedings of the Combustion Institute, 35(3), pp.36013607. 2015.
Abstract The low temperature oxidation behavior of \DME\ in a jetstirred reactor is investigated. There have been numerous investigations on the low temperature reaction kinetics of DME. In this work, the focus is set on the instabilities that arise at very low temperatures (in the order of 580 K) in a jetstirred reactor and a numerical analysis of the source of these oscillations is performed. A stability diagram was created where the parameters at which the oscillations were observed are displayed. The temperature oscillations at higher temperatures (around 1000 K) have been shown to be caused by ignition–extinction phenomena, whereas the oscillations observed at low temperatures are believed to be of thermokinetical origin, with the buildup of a semistable species (hydroperoxidemethylformate HPMF), its dissociation at increasing temperatures with the subsequent formation of large quantities of formaldehyde that increase the temperature giving rise to the firststage ignition observed and at the same time act as inhibitor to the reaction. The continuous flow through the reactor brings the system back to its original state, and the process begins anew.

Beeckmann, J., Cai, L., Berens, A., Peters, N. & Pitsch, H., An analytical approximation for low and hightemperature autoignition for dimethyl ether–air mixtures. Proceedings of the Combustion Institute, 35(1), pp.275281. 2015.
Abstract Dimethyl ether has proven to be one of the most attractive alternatives to traditional fossil oil derived fuels for compression ignition engines. In this study, a skeletal mechanism consisting of 32 species and 49 elementary reactions, based on the detailed mechanism proposed by Fischer et al. Int. J. Chem. Kinet. 32 (12) (2000) 713–740, is further reduced to a short 36step mechanism describing first and second stage ignition as well as the rapid transition to final products. A global 4step mechanism is derived by introducing steady state assumptions of intermediate species. An analytical solution for the ignition delay times of the first stage of ignition in the low temperature regime and the beginning of its transition to the intermediate temperature regime is presented. The important competition of the β scission and the reaction with molecular oxygen of the hydroperoxymethoxymethyl radical (C2OCH2O2H) is quantified by the introduction of a parameter β , related to the competition of chainbranching and chainpropagation. The calculated values agree very well with those of the 36step mechanism. Also for the high temperature regime, an analytical solution is presented, which agrees well with the experiments and the values calculated with the 36step mechanism.

Jocher, A., Pitsch, H., Gomez, T. & Legros, G., Modification of sooting tendency by magnetic effects. Proceedings of the Combustion Institute, 35(1), pp.889895. 2015.
Abstract Experimental investigations assess for the first time the influence of the gradient of the square of the magnetic flux density ( ∇ ( B 2 ) ) on the soot production in a laminar axisymmetric nonpremixed flame. The steady nonsmoking ethylene flame is established in a coflowing mixture, composed of oxygen and nitrogen, over a Santoro type burner. This burner is located in an electromagnet. The flame experiences different magnitudes of upward ∇ ( B 2 ) , ranging from 0 to 18.2 T2/m, as well as different oxygen contents of the coflow, ranging from 21% to 50% in volume. Soot volume fraction is mapped in the flame by Laser Extinction Measurement technique. Increasing the magnitude of the ∇ ( B 2 ) allows for the modification of soot production in the flame. This modification is enhanced by increasing oxygen content, as oxygen exhibits a relatively high paramagnetic susceptibility. Furthermore, the aforementioned modification is shown to enable the shift among similar soot concentration profiles in the flame, just as the variation of oxygen content can do. Consequently, modification of the inner flame structure by magnetic effects could contribute to the control of oxyfuel combustion.

Sudholt, A., Cai, L., Heyne, J., Haas, F.M., Pitsch, H. & Dryer, F.L., Ignition characteristics of a bioderived class of saturated and unsaturated furans for engine applications. Proceedings of the Combustion Institute, 35(3), pp.29572965. 2015.
Abstract Ignition characteristics in the form of derived cetane numbers (DCN) of (hydro) furanic species are investigated experimentally in an Ignition Quality Tester. Further, quantum chemistry calculations at CBSQB3 level of theory are applied to determine bond dissociation energies (BDEs) and thereby suggest the initial reactions of the ignition process for all of these fuels. Using the calculated BDEs, it is found that the ignition characteristics are similar among furans and among tetrahydrofurans, but strongly differ between these molecular classes. It is shown that the ignition behavior of aromatic furans is determined by the ring structure, which correlates with a negligible side chain influence. Hence, furan fuel structures can be chosen with respect to feasibility of the production pathways and engine compatibility. On the contrary, the side chain length for tetrahydrofurans defines the potential application paradigm. Tetrahydrofurans with short side chains are candidates for \SI\ application, whereas 2butyltetrahydrofuran may be a candidate for diesel application. The influence of the number and location of double bonds in the ring is illustrated with the additional study of dihydrofurans, and the influence of other functional groups is evaluated for (tetrahydro) furfuryl alcohols. To investigate possible fuel application scenarios, a second part of this study investigates \DCNs\ of furanic fuel blends in nheptane and diesel fuel. A \DCN\ mixing rule is found to be approximately linear and for blends with up to 20 mol% furans, the side chain structure (alkyl, alcohol) has no distinct influence on the blend DCN.

Varea, E., Beeckmann, J., Pitsch, H., Chen, Z. & Renou, B., Determination of burning velocities from spherically expanding H2/air flames. Proceedings of the Combustion Institute, 35(1), pp.711719. 2015.
Abstract Laminar burning velocities of hydrogen/air mixtures can show discrepancies up to 30%, making chemical mechanism validation and improvement difficult. The source of uncertainties may come from different factors influencing at each processing and postprocessing steps the final value. Considering a spherically expanding flame configuration, reflection on the accuracy of the formulations, used to derive the desired quantity, is proposed. Starting from the exact definition of the laminar burning velocity, two formulations – direct and indirect flame speeds formulations – are derived for spherical flames. Each single source of uncertainty involved in the formulations is pointed out. The emphasis is focused on a specific mixture at an equivalence ratio of 0.50, atmospheric pressure, and an initial temperature of 300 K. This point represents the best tradeoff between low ratio of flame velocity and recording sampling rate and the occurrence of cellular flames (Le < 1). An extensive experimental and numerical study (1D spherically expanding flames) of this mixture is carried out. As a result, the experimental laminar burning velocities determined by using the direct flame speed or the indirect flame speed formulae depict different values. However, when numerically determined, both formulae yield the same value. This paves the way to understand and identify the experimental error sources. Stretch and Lewis numbers effects (superadiabatic temperatures) as well as radiation processes (burned gas motion) are studied. Nonetheless, they do not show to be the main source of uncertainty. The extrapolation procedure (linear or nonlinear) according to the limited number of experimental points (rapid apparition of cellular structure) appears as the main factor influencing the discrepancy in laminar burning velocities.

Attili, A., Bisetti, F., Mueller, M.E. & Pitsch, H., Damköhler number effects on soot formation and growth in turbulent nonpremixed flames. Proceedings of the Combustion Institute, 35(2), pp.12151223. 2015.
Abstract The effect of Damköhler number on turbulent nonpremixed sooting flames is investigated via large scale direct numerical simulation in threedimensional nheptane/air jet flames at a jet Reynolds number of 15,000 and at three different Damköhler numbers. A reduced chemical mechanism, which includes the soot precursor naphthalene, and a highorder method of moments are employed. At the highest Damköhler number, local extinction is negligible, while flames holes are observed in the two lowest Damköhler number cases. Compared to temperature and other species controlled by fuel oxidation chemistry, naphthalene is found to be affected more significantly by the Damköhler number. Consequently, the overall soot mass fraction decreases by more than one order of magnitude for a fourfold decrease of the Damköhler number. On the contrary, the overall number density of soot particles is approximately the same, but its distribution in mixture fraction space is different in the three cases. The total soot mass growth rate is found to be proportional to the Damköhler number. In the two lowest Da number cases, soot leakage across the flame is observed. Leveraging Lagrangian statistics, it is concluded that soot leakage is due to patches of soot that cross the stoichiometric surface through flame holes. These results show the leading order effects of turbulent mixing in controlling the dynamics of soot in turbulent flames.

Sakai, Y., Ando, H., Chakravarty, H.K., Pitsch, H. & Fernandes, R.X., A computational study on the kinetics of unimolecular reactions of ethoxyethylperoxy radicals employing CTST and VTST. Proceedings of the Combustion Institute, 35(1), pp.161169. 2015.
Abstract Diethyl ether (DEE) has favorable properties as a diesel fuel component, including its outstanding cetane number. To utilize this promising fuel, more and more knowledge on the chemical kinetics of \DEE\ oxidation will be required. For the present article, the rate constants of unimolecular reactions of ethoxyethylperoxy radicals, which are main intermediates in the oxidation of \DEE\ under the engine relevant conditions, have been computationally investigated and compared with those of alkanes. Geometries, vibrational frequencies, and energies of reactants, products, and transition states with pronounced barrier were calculated according to the procedure of the CBSQB3 method. The highpressure limiting rate constants were calculated in the temperature range of 500−2000 K by using a conventional transition state theory with hindered rotor approximation for low frequency torsional mode. The oxygen dissociation reactions have been investigated by using a variational transition state theory based on the CASPT2(7,5)/augccpvdz single point calculations at UB3LYP/CBSB7 geometries and vibrational frequencies. It was found that the oxidation pathways are equal to those of alkane oxidations, however, the rate constants are significantly different from those of alkanes due to the oxygen vicinity. The rate constants of intramolecular hydrogen shift reactions are from 3 to 8 times larger at 700 K than those of alkylic peroxy radical when the abstracted hydrogen is in the βposition of the ether. The rate constant of βscission reactions for 1,5intramolecular hydrogen shift products of 1ethoxyethylperoxy radial is 163 times larger at 700 K than that of alkylic hydroperoxy radical, and this reaction becomes a main reaction pathway, whereas cyclicether is a main product in alkane oxidation. These characteristic rate constants are given in threeparameter modified Arrhenius form for the refinement of predictive chemical kinetic models being developed.

Schmitt, C. & Pitsch, H., Reactive linearized equations of perturbed compressible variables for lowMach number variabledensity flows. Journal of Computational Physics, 281, pp.127. 2015.
Abstract A hydrodynamic/acoustic splitting approach is proposed to study noise emitted from reactive variabledensity flows. A simulation using the variabledensity lowMach number equations provides a solution of the hydrodynamic motions of the flow (baseflow), and a set of equations for perturbed variables is additionally solved to capture the acoustic motions. A rigorous derivation of these equations for the assumed baseflow is given, which compared to its nonreacting counterpart includes additional terms related to variabledensity flows. Two different test cases are presented. First, the Kirchhoff vortex is simulated to highlight instability issues related to constantdensity flows. Second, an academic test case for variabledensity flows is proposed in the form of a reacting dipole, which is used to underline the stability of the proposed perturbed equations in such a scenario. Various intermediate forms of the derived perturbation equations are juxtaposed and analyzed with respect to their stability for these two test cases, and assumptions made in their derivation are numerically justified.

Narayanaswamy, K., Pepiot, P. & Pitsch, H.G., A chemical mechanism for low to high temperature oxidation of ndodecane as a component of transportation fuel surrogates. Combustion and Flame, 161(4), pp.866–884. 2014.
Using surrogate fuels in lieu of real fuels is an appealing concept for combustion studies. A major limitation however, is the capability to design compact and reliable kinetic models that capture all the specificities of the simpler, but still multicomponent surrogates. This task is further complicated by the fairly large nature of the hydrocarbons commonly considered as potential surrogate components, since they typically result in large detailed reaction schemes. Towards addressing this challenge, the present work proposes a single, compact, and reliable chemical mechanism, that can accurately describe the oxidation of a wide range of fuels, which are important components of surrogate fuels. A wellcharacterized mechanism appropriate for the oxidation of smaller hydrocarbon species G. Blanquart, P. PepiotDesjardins, H. Pitsch, Chemical mechanism for high temperature combustion of engine relevant fuels with emphasis on soot precursors, Combust. Flame 156 (2009) 588–607, and several substituted aromatic species K. Narayanaswamy, G. Blanquart, H. Pitsch, A consistent chemical mechanism for the oxidation of substituted aromatic species, Combust. Flame 157 (10) (2010) 1879–1898, ideally suited as a base to model surrogates, has now been extended to describe the oxidation of ndodecane, a representative of the paraffin class, which is often used in diesel and jet fuel surrogates. To ensure compactness of the kinetic scheme, a short mechanism for the low to high temperature oxidation of ndodecane is extracted from the detailed scheme of Sarathy et al. S. M. Sarathy, C. K.Westbrook, M. Mehl, W. J. Pitz, C. Togbe, P. Dagaut, H. Wang, M. A. Oehlschlaeger, U. Niemann, K. Seshadri, Comprehensive chemical kinetic modeling of the oxidation of 2methylalkanes from C7 to C20, Combust. Flame 158 (12) (2011) 2338–2357 and integrated in a systematic way into the base model. Rate changes based on recent rate recommendations from literature are introduced to the resulting chemical mechanism in a consistent manner, which improve the model predictions. Extensive validation of the revised kinetic model is performed using a wide range of experimental conditions and data sets

Paczko, G., Peters, N., Seshadri, K. & Williams, F.A., The role of coolflame chemistry in quasisteady combustion and extinction of nheptane droplets. Combustion Theory and Modelling, 18(45), pp.515531. 2014.

Cai, L. & Pitsch, H., Mechanism optimization based on reaction rate rules. Combustion and Flame, 161(2), pp.405415. 2014.
Abstract Accurate chemistry models form the backbone of detailed computational fluid dynamics (CFD) tools used for simulating complex combustion devices. Combustion chemistry is often very complex and chemical mechanisms generally involve more than one hundred species and one thousand reactions. In the derivation of these large chemical mechanisms, typically a large number of reactions appears, for which rate data are not available from experiment or theory. Rate data for these reactions are then often assigned using socalled reaction classes. This method categorizes all possible fuelspecific reactions as classes of reactions with prescribed rules for the rate constants. This ensures consistency in the chemical mechanism. In rate parameter optimizations found in the published literature, rate constants of single elementary reactions are usually systematically optimized to achieve good agreement between model performance and experimental measurements. However, it is not kinetically reasonable to modify the rate parameters of single reactions, because this will violate consistency of rate parameters of kinetically similar reactions. In this work, the rate rules, that determine the rates for reaction classes are calibrated instead of the rates of single elementary reactions leading to a chemically more consistent model optimization. This is demonstrated by optimizing an npentane combustion mechanism. The rate rules are studied with respect to reaction classes, abstracting species, broken C–H bonds, and ring strain energy barriers. Furthermore, the uncertainties of the rate rules and model predictions are minimized and the pressure dependence of reaction classes dominating low temperature oxidation is optimized.

Kwak, D., Khetan, A., Noh, S., Pitsch, H. & Han, B., First Principles Study of Morphology, Doping Level, and Water Solvation Effects on the Catalytic Mechanism of NitrogenDoped Graphene in the Oxygen Reduction Reaction. ChemCatChem, 6(9), pp.26622670. 2014.
By using first principles DFT calculations, we reveal oxygen reduction reaction mechanisms in Ndoped graphene (NGr). Considering both the morphology and the concentration of dopant N atoms in bulk and edge NGr forms, we calculate the energies of a large number of NGr model systems to cover a wide range of possible NGr structures and determine the most stable NGr forms. In agreement with experiments, our DFT calculations suggest that doping levels in stable NGr forms are limited to less than approximately 30 at. % N, above which the hexagonal graphene framework is broken. The ground state structures of bulk and edge NGr forms are found to differ depending on the doping level and poisoning of the edge bonds. Oxygen reduction reaction mechanisms are evaluated by using Gibbs freeenergy diagrams with and without water solvation. Our results indicate that N doping significantly alters the catalytic properties of pure graphene and that dilutely doped bulk NGr forms are the most active.

Jochim, B., Zweigel, R., Korkmaz, M., Abel, D. & Pitsch, H., Comparing accuracy and efficiency between a multizone model and artificial neural networks in dynamic simulations of PCCI Diesel engine combustion for engine control. THIESEL 2014  Conference on thermo and fluid dynamic processes in direct injection engines, 9th12th September 2014, Valencia, Spain, 385. 2014.

Khetan, A., Pitsch, H. & Viswanathan, V., Solvent Degradation in Nonaqueous LiO2 Batteries: Oxidative Stability versus HAbstraction. Journal of Physical Chemistry Letters, 5(14), pp.2419–2424. 2014.

Cai, L., Sudholt, A., Lee, D.J., Egolfopoulos, F., Pitsch, H.G., Westbrook, C.K. & Sarathy, S.M., Chemical kinetic study of a novel lignocellulosic biofuel: Dinbutyl ether oxidation in a laminar flow reactor and flames. Combustion and Flame, 161(3), pp.798–809. 2014.
The combustion characteristics of promising alternative fuels have been studied extensively in the recent years. Nevertheless, the pyrolysis and oxidation kinetics for many oxygenated fuels are not well characterized compared to those of hydrocarbons. In the present investigation, the first chemical kinetic study of a longchain linear symmetric ether, dinbutyl ether (DBE), is presented and a detailed reaction model is developed. DBE has been identified recently as a candidate biofuel produced from lignocellulosic biomass. The model includes both high temperature and low temperature reaction pathways with reaction rates generated using appropriate rate rules. In addition, experimental studies on fundamental combustion characteristics, such as ignition delay times and laminar flame speeds have been performed. A laminar flow reactor was used to determine the ignition delay times of lean and stoichiometric DBE/air mixtures. The laminar flame speeds of DBE/air mixtures were measured in the stagnation flame configuration for a wide rage of equivalence ratios at atmospheric pressure and an unburned reactant temperature of 373 K. All experimental data were modeled using the present kinetic model. The agreement between measured and computed results is satisfactory, and the model was used to elucidate the oxidation pathways of DBE. The dissociation of ketohydroperoxides, leading to radical chain branching was found to dominate the ignition of DBE in the low temperature regime. The results of the present numerical and experimental study of the oxidation of dinbutyl ether provide a good basis for further investigation of long chain linear and branched ethers.

Zweigel, R., Jochim, B., Korkmaz, M., Albin, T., Pitsch, H. & Abel, D., Virtual Test Bed of a Cascaded PCCI Combustion Control Loop with Underlying Air Path Controller and MultiZone Combustion Model. FISITA 2014 World Automotive Congress, 2nd6th June 2014, Maastricht, the Netherlands, F2014CET072, pp.110. 2014.

Attili, A., Bisetti, F., Mueller, M.E. & Pitsch, H.G., Formation, growth, and transport of soot in a threedimensional turbulent nonpremixed jet flame. Combustion and Flame, 161, pp.18491865. 2014.

Mittal, V., Kang, S., Doran, E., Cook, D. & Pitsch, H., LES of Gas Exchange in IC Engines. Oil & Gas Science and Technology, 69(1), pp.2940. 2014.

Bisetti, F., Attili, A. & Pitsch, H., Advancing predictive models for particulate formation in turbulent flames via massively parallel direct numerical simulations. Philos Trans A Math Phys Eng Sci., 372(2022). 2014.
Combustion of fossil fuels is likely to continue for the near future due to the growing trends in energy consumption worldwide. The increase in efficiency and the reduction of pollutant emissions from combustion devices are pivotal to achieving meaningful levels of carbon abatement as part of the ongoing climate change efforts. Computational fluid dynamics featuring adequate combustion models will play an increasingly important role in the design of more efficient and cleaner industrial burners, internal combustion engines, and combustors for stationary power generation and aircraft propulsion. Today, turbulent combustion modelling is hindered severely by the lack of data that are accurate and sufficiently complete to assess and remedy model deficiencies effectively. In particular, the formation of pollutants is a complex, nonlinear and multiscale process characterized by the interaction of molecular and turbulent mixing with a multitude of chemical reactions with disparate time scales. The use of direct numerical simulation (DNS) featuring a state of the art description of the underlying chemistry and physical processes has contributed greatly to combustion model development in recent years. In this paper, the analysis of the intricate evolution of soot formation in turbulent flames demonstrates how DNS databases are used to illuminate relevant physicochemical mechanisms and to identify modelling needs.

Gauding, M., Wick, A., Pitsch, H. & Peters, N., Generalised scalebyscale energybudget equations and largeeddy simulations of anisotropic scalar turbulence at various Schmidt numbers. Journal of Turbulence, 15(12), pp.857882. 2014.
A necessary condition for the accurate prediction of turbulent flows using largeeddy simulation (LES) is the correct representation of energy transfer between the different scales of turbulence in the LES. For scalar turbulence, transfer of energy between turbulent length scales is described by a transport equation for the second moment of the scalar increment. For homogeneous isotropic turbulence, the underlying equation is the wellknown Yaglom equation. In the present work, we study the turbulent mixing of a passive scalar with an imposed mean gradient by homogeneous isotropic turbulence. Both direct numerical simulations (DNS) and LES are performed for this configuration at various Schmidt numbers, ranging from 0.11 to 5.56. As the assumptions made in the derivation of the Yaglom equation are violated for the case considered here, a generalised Yaglom equation accounting for anisotropic effects, induced by the mean gradient, is derived in this work. This equation can be interpreted as a scalebyscale energybudget equation, as it relates at a certain scale r terms representing the production, turbulent transport, diffusive transport and dissipation of scalar energy. The equation is evaluated for the conducted DNS, followed by a discussion of physical effects present at different scales for various Schmidt numbers. For an analysis of the energy transfer in LES, a generalised Yaglom equation for the second moment of the filtered scalar increment is derived. In this equation, new terms appear due to the interaction between resolved and unresolved scales. In an apriori test, this filtered energybudget equation is evaluated by means of explicitly filtered DNS data. In addition, LES calculations of the same configuration are performed, and the energy budget as well as the different terms are thereby analysed in an aposteriori test. It is shown that LES using an eddy viscosity model is able to fulfil the generalised filtered Yaglom equation for the present configuration. Further, the dependence of the terms appearing in the filtered energybudget equation on varying Schmidt numbers is discussed.

Herrmann, F., Jochim, B., Oßwald, P., Cai, L., Pitsch, H. & KohseHöinghaus, K., Experimental and numerical lowtemperature oxidation study of ethanol and dimethyl ether. Combustion and Flame, 161(2), pp.384397. 2014.
Lowtemperature combustion (LTC) receives increasing attention because of its potential to reduce NOx and soot emissions. For the application of this strategy in practical systems such as internal combustion engines and gas turbines, the fundamental chemical reactions involved must be understood in detail. To this end, reliable experimental data are needed including quantitative speciation to assist further development of reaction mechanisms and their reduction for practical applications.<br />
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The present study focuses on the investigation of lowtemperature oxidation of ethanol and dimethyl ether (DME) under identical conditions in an atmosphericpressure laminar flow reactor. The gas composition was analyzed by timeofflight (TOF) mass spectrometry. This technique allows detection of all species simultaneously within the investigated temperature regime. Three different equivalence ratios of ϕ = 0.8, 1.0, and 1.2 were studied in a wide, highlyresolved temperature range from 400 to 1200 K, and quantitative species mole fraction profiles have been determined.<br />
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The experiments were accompanied by numerical simulations. Their results clearly show the expected different lowtemperature oxidation behavior of both fuels, with a distinct negative temperature coefficient (NTC) region only observable for DME. With detailed species information including intermediates, differences of the kinetics for both fuels are discussed. Small modifications of the mechanisms served to identify sensitivities in the model. The experimental results may assist in the improvement of kinetic schemes and their reduction.

Bode, M., Diewald, F., Broll, D.O., Heyse, J.F., Le Chenadec, V. & Pitsch, H., Influence of the Injector Geometry on Primary Breakup in Diesel Injector Systems. SAE Technical Paper Series, 2014011427. 2014.
Diesel injection systems have a significant impact on the performance as well as emission and pollutant formation of modern diesel engines. Even though the geometry of atomizers became more and more complex over the last years, injection systems still have a large potential for improving the overall diesel engine combustion process. Due to the complexity of the atomization process, reliable models are not available, yet these are highly desired for supporting the design process. They have to be developed using detailed numerical simulations.<br />
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In this work, the “Spray A” reference case defined by the Engine Combustion Network is simulated under realistic operation conditions using a recently developed numerical framework for multiphase flows. A LargeEddy Simulation of the nozzle internal flow is coupled with a Direct Numerical Simulation of the interfacial outside flow and the resulting primary breakup is analyzed. Additionally, the impact of the injector geometry on primary breakup is studied. For this purpose, the effect of the taper ratio of the injector nozzle and the turbulent kinetic energy at the injector exit on the primary breakup is discussed and a parametric study is performed.<br />
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It is shown that the taper ratio of the nozzle of modern injection systems has a large impact on the primary breakup because it directly influences the turbulent kinetic energy as well as the radial velocity component at the nozzle exit, which in turn affect the primary breakup.

Beeckmann, J., Cai, L. & Pitsch, H., Experimental investigation of the laminar burning velocities of methanol, ethanol, npropanol, and nbutanol at high pressure. Fuel, 117, Part A, pp.340350. 2014.
Due to their increasing share, the combustion of alternative fuels and in particular oxygenated, bioderived fuel components need to be characterised. The laminar burning velocity is one key parameter for the characterisation of fuels, and it also serves as an important quantity to validate chemical kinetic models. Methanol, ethanol, npropanol, and nbutanol laminar burning velocities experiments were conducted in a spherical combustion vessel at an unburnt temperature of 373 K and a pressure of 10 bar. Measured burning velocities from this study and from the published literature are compared with numerical simulation data from published chemical mechanisms. The models tend to underpredict the experimentally measured values. A sensitivity analysis suggests further investigation of the pressure dependence for the fuel specific reactions with hydrogen and hydroxyl radicals.

Bode, M., Le Chenadec, V. & Pitsch, H., High Fidelity multiphase simulations studying primary breakup. PRACEdays14, 20th  22nd May 2014, Barcelona, Spain. 2014.

Gampert, M., Kleinheinz, K., Peters, N. & Pitsch, H., Experimental and Numerical Study of the Scalar Turbulent/NonTurbulent Interface Layer in a Jet Flow. Flow, Turbulence and Combustion, 92(12), pp.429449. 2014.
Based on two largeeddy simulations (LES) of a nonreacting turbulent round jet with a nozzle based Reynolds number of 8,610 with the same configuration as the one that has recently been investigated experimentally (Gampert et al., 2012; J Fluid Mech, 2012; J Fluid Mech 724:337, 2013), we examine the scalar turbulent/nonturbulent (T/NT) interface layer in the mixture fraction field of the jet flow between ten and thirty nozzle diameters downstream. To this end, the LES—one with a coarse grid and one with a fine grid—are in a first step validated against the experimental data using the axial decay of the mean velocity and the mean mixture fraction as well as based on radial selfsimilar profiles of mean and root mean square values of these two quantities. Then, probability density functions (pdf) of the mixture fraction at various axial and radial positions are compared and the quality of the LES is discussed. In general, the LES results are consistent with the experimental data. However, in the flow region where the imprint of the T/NT interface layer is dominant in the mixture fraction pdf, discrepancies are observed. In a next step, statistics of the T/NT interface layer are studied, where a satisfactory agreement for the pdf of the location of the interface layer from the higher resolved LES with the experimental data is observed, while the one with the coarse grid exhibits considerable deviations. Finally, the mixture fraction profile across the interface is investigated where the same trend as for the pdf of the location is present. In particular, it is found that the sharp interface that is present in experimental studies (Gampert et al., J Fluid Mech, 2013; Westerweel et al., J Fluid Mech 631:199, 2009) is less distinct in the LES results and rather diffused in radial direction outside of the T/NT interface layer.

Trisjono, P., Kleinheinz, K., Pitsch, H. & Kang, S., Large Eddy Simulation of Stratified and Sheared Flames of a Premixed Turbulent Stratified Flame Burner Using a Flamelet Model with Heat Loss. Flow, Turbulence and Combustion, 92(12), pp.201235. 2014.
This paper presents large eddy simulations (LES) of the Darmstadt turbulent stratified flame burner (TSF) at different operating conditions including detailed heat loss modeling. The target cases are a nonreacting and two reacting cases. Both reacting cases are characterized by stratification, while one flame additionally features shear. In the regime diagram for premixed combustion, the studied flames are found at the border separating the thin reaction zones regime and the broken reaction zones regime. A coupled level set/progress variable model is utilized to describe the combustion process. To account for heat loss, an enthalpy defect approach is adopted and reformulated to include differential diffusion effects. A novel powerlaw rescaling methodology is proposed to integrate the enthalpy defect approach into the level set/progress variable model which is extensively validated in two validation scenarios. It is demonstrated that the LES with the newly developed model captures the influence of heat loss well and that the incorporation of heat loss effects improves the predictions of the TSFburner over adiabatic simulations, while reproducing the experimentally observed flame liftoff from the pilot nozzle.

Gampert, M., Boschung, J., Hennig, F., Gauding, M. & Peters, N., The vorticity versus the scalar criterion for the detection of the turbulent/nonturbulent interface. Journal of Fluid Mechanics, 750, pp.578596. 2014.
Based on a direct numerical simulation (DNS) of a temporally evolving mixing layer, we present a detailed study of the turbulent/nonturbulent (T/NT) interface that is defined using the two most common procedures in the literature, namely either a vorticity or a scalar criterion. The different detection approaches are examined qualitatively and quantitatively in terms of the interface position, conditional statistics and orientation of streamlines and vortex lines at the interface. Computing the probability density function (p.d.f.) of the mean location of the T/NT interface from vorticity and scalar allows a detailed comparison of the two methods, where we observe a very good agreement. Furthermore, conditional mean profiles of various quantities are evaluated. In particular, the position p.d.f.s for both criteria coincide and are found to follow a Gaussian distribution. The terms of the governing equations for vorticity and passive scalar are conditioned on the distance to the interface and analysed. At the interface, vortex stretching is negligible and the displacement of the vorticity interface is found to be determined by diffusion, analogous to the scalar interface. In addition, the orientation of vortex lines at the vorticity and the scalar based T/NT interface are analyzed. For both interfaces, vorticity lines are perpendicular to the normal vector of the interface, i.e. parallel to the interface isosurface.

Schaefer, P., Gampert, M., Hennig, F. & Peters, N., Geometrical Features of Streamlines and Streamline Segments in Turbulent Flows. In Dillmann, A., Heller, G., Krämer, E., Kreplin, H. P., Nitsche, W., & Rist, U., eds. New Results in Numerical and Experimental Fluid Mechanics IX. Notes on Numerical Fluid Mechanics and Multidisciplinary Design. pp. 8592. 2014.
Streamlines constitute natural geometries in turbulent flows. In this work streamlines are segmented based on local extrema of the field of the absolute value of the velocity along the streamline coordinate. Streamline segments are parameterized based on their arclength and a theoretical scaling of the mean length with the geometric mean of the Kolmogorov length and the Taylor microscale is derived and found to be in good agreement with direct numerical simulations.

Boschung, J., Schaefer, P., Peters, N. & Meneveau, C., The local topology of streamand vortex lines in turbulent flows. Physics of Fluids (1994present), 26(4), p.045107. 2014.

Khetan, A., Han, B. & Pitsch, H., Structure & Activity of Pt on CeO2 (111) Surface Using First Principles Thermodynamics. Fabrication, Structure and Reactivity of Anchored Nanoparticles: Faraday Discussion 162, 10th  12th April 2013, Berlin, Germany. 2013.

Gampert, M., Narayanaswamy, V., Schaefer, P. & Peters, N., Conditional statistics of the turbulent/nonturbulent interface in a jet flow. Journal of Fluid Mechanics, 731, pp.615638. 2013.
Using twodimensional highspeed measurements of the mixture fraction Z in a turbulent round jet with nozzlebased Reynolds numbers Re0 between 3000 and 18 440, we investigate the scalar turbulent/nonturbulent (T/NT) interface of the flow. The mixture fraction steeply changes from Z=0 to a final value which is typically larger than 0.1. Since combustion occurs in the vicinity of the stoichiometric mixture fraction, which is around Z=0.06 for typical fuel/air mixtures, it is expected to take place largely within the turbulent/nonturbulent interface. Therefore, deep understanding of this part of the flow is essential for an accurate modelling of turbulent nonpremixed combustion. To this end, we use a composite model developed by Effelsberg & Peters (Combust. Flame, vol. 50, 1983, pp. 351–360) for the probability density function (p.d.f.) P(Z) which takes into account the different contributions from the fully turbulent as well as the turbulent/nonturbulent interface part of the flow. A very good agreement between the measurements and the model is observed over a wide range of axial and radial locations as well as at varying intermittency factor γ and shear. Furthermore, we observe a constant mean mixture fraction value in the fully turbulent region. The p.d.f. of this region is thus of nonmarching character, which is attributed physically to the meandering nature of the fully turbulent core of the jet flow. Finally, the location and in particular the scaling of the thickness δ of the scalar turbulent/nonturbulent interface are investigated. We provide the first experimental results for the thickness of the interface over the abovementioned Reynolds number range and observe δ/L∼Re−1λ, where L is an integral length scale and Reλ the local Reynolds number based on the Taylor scale λ, meaning that δ∼λ. This result also supports the assumption often made in modelling of the stoichiometric scalar dissipation rate χst being a Reynoldsnumberindependent quantity.

Wada, T., Sudholt, A., Pitsch, H. & Peters, N., Analysis of first stage ignition delay times of dimethyl ether in a laminar flow reactor. Combustion Theory and Modelling, 17(5), pp.906934. 2013.
The combustion chemistry of the first stage ignition and chemistry/flow interactions are studied for dimethyl ether (DME) with a mathematical analysis of two systems: a plug flow reactor study is used to reduce the reaction chemistry systematically. A skeletal reaction mechanism for the low temperature chemistry of DME until the onset of ignition is derived on the basis of the detailed DME mechanism of the Lawrence Livermore National Laboratory  see Curran, Fischer and Dryer, Int. J. Chem. Kinetics, Vol. 32 (2000). It is shown that reasonably good results for ignition delay times can be reached using a simple system of three ordinary differential equations and that the resulting analytical solution depends only on two reaction rates and the initial fuel concentration. The stepwise reduction of the system based on assumptions yields an understanding on why these reactions are so important. Furthermore, the validation of the assumptions yields insight into the influence of the fuel and the oxygen concentration on the temperature during the induction phase. To investigate the influence of chemistry/flow interactions, a 2D model with a laminar HagenPoiseuille flow and 2Dpolynomial profiles for the radial species concentration is considered. For the 2D model, it is found that only the diffusion coefficients and the reactor radius need to be taken into consideration additionally to describe the system sufficiently. Also, the coupling of flow and chemistry is clarified in the mathematical analysis. The insight obtained from the comparison of the 2D model and the plug flow model is used to establish an average velocity for the conversion of ignition locations to ignition delay times in a laminar flow reactor. Finally, the 2D analytical solution is compared against new experimental data, obtained in such a laminar flow reactor for an undiluted DME/air mixture with an equivalence ratio of phi = 0.835 and a temperature range of 555 to 585 K at atmospheric pressure.

Jocher, A., Pitsch, H., Gomez, T. & Legros, G., Modification of Soot Production inside Laminar Diffusion Flames by Static NonUniform Magnetic Fields. In Proceedings of the European Combustion Meeting 2013, 25th  28th June 2013, Lund, Sweden. 2013.

Schaefer, P., Gampert, M. & Peters, N., A model equation for the joint distribution of the length and velocity difference of streamline segments in turbulent flows. Physics of Fluids, 25(11), p.115107. 2013.
Streamlines recently received attention as natural geometries of turbulent flow fields. Similar to dissipation elements in scalar fields, streamlines are segmented into smaller subunits based on local extreme points of the absolute value of the velocity field u along the streamline coordinate s, i.e., points where the projected gradient in streamline direction u s = 0. Then, streamline segments are parameterized using their arclength l between two neighboring extrema and the velocity difference Δ at the extrema. Both parameters are statistical variables and streamline segments are characterized by the joint probability density function (jpdf) P(l, Δ). Based on a previously formulated model for the marginal pdf of the arclength, P(l), which contains terms that account for slow changes as well as fast changes of streamline segments, a model for the jpdf is formulated. The jpdf's, when normalized with the mean length, l m , and the standard deviation of the velocity difference σ, obtained from two different direct numerical simulations (DNS) cases of homogeneous isotropic decaying and forced turbulence at Taylor based Reynolds number of Re λ = 116 and Re λ = 206, respectively, turn out to be almost Reynolds number independent. The steady model solution is compared with the normalized jpdf's obtained from DNS and it is found to be in good agreement. Special attention is paid to the intrinsic asymmetry of the jpdf with respect to the mean length of positive and negative streamline segments, where due to the kinematic stretching of positive segments and compression of negative ones, the mean length of positive segments turns out to be larger than the mean length of negative ones. This feature is reproduced by the model and the ratio of the two length scales, which turns out to be an almost Reynolds number independent, dimensionless quantity, is well reproduced. Finally, a relation between the kinetic asymmetry of streamline segments and the dynamic asymmetry of the pdf of longitudinal velocity gradients in turbulent flows, which manifests itself in a negative velocity gradient skewness, is established and it is theoretically shown that negative streamline segments are only smaller than positive ones, if the gradient is negatively skewed.

Pitsch, H., High Fidelity Modeling of Pollutant Emissions from Real Combustion Systems. In COMBURA Symposium, 9th  10th October 2013, Maastricht, Netherlands. 2013.

Gampert, M., Narayanaswamy, V. & Peters, N., Scalar gradient trajectory measurements using highfrequency cinematographic planar Rayleigh scattering. Experiments in Fluids, 54(12), pp.115. 2013.
In this work, we perform an experimental investigation into statistics based on scalar gradient trajectories in a turbulent jet flow, which have been suggested as an alternative means to analyze turbulent flow fields by Wang and Peters (J Fluid Mech 554:457–475, 2006, 608:113–138, 2008). Although there are several numerical simulations and theoretical works that investigate the statistics along gradient trajectories, only few experiments in this area have been reported. To this end, highfrequency cinematographic planar Rayleigh scattering imaging is performed at different axial locations of a turbulent propane jet issuing into a CO2 coflow at nozzlebased Reynolds numbers Re 0 = 3,000–8,600. Taylor’s hypothesis is invoked to obtain a threedimensional reconstruction of the scalar field in which then the corresponding scalar gradient trajectories can be computed. These are then used to examine the local structure of the mixture fraction with a focus on the scalar turbulent/nonturbulent interface. The latter is a layer that is located between the fully turbulent part of the jet and the outer flow. Using scalar gradient trajectories, we partition the turbulent scalar field into these three regions according to an approach developed by Mellado et al. (J Fluid Mech 626:333–365, 2009). Based on the latter, we investigate the probability to find the respective regions as a function of the radial distance to the centerline, which turns out to reveal the meandering nature of the scalar T/NT interface layer as well as its impact on the local structure of the turbulent scalar field.

Jochim, B., Vanegas, A. & Pitsch, H., Development of an efficient and accurate multizone model for modelbased PCCI Diesel engine control. 26. Deutscher Flammentag, 2161, pp.377386. 2013.

Kleinheinz, K., Mittal, V., Trisjono, P. & Pitsch, H., LargeEddy Simulation of a Piloted Premixed Jet Burner, 2013.

Stoehr, K.D., Peters, N. & Pitsch, H., Niedertemperaturoszillationen in einem DME betriebenen Jetstirredflow Reaktor. 26. Deutscher Flammentag, VDIBericht. 2013.

Pitsch, H., Why You Need LES for Predicting Soot Emissions in Turbulent Combustion. ERCOFTAC WORKSHOP Direct and LargeEddy Simulation, 3rd  5th April 2013, Dresden, Germany. 2013.

Gampert, M., Schaefer, P. & Peters, N., Experimental investigation of dissipationelement statistics in scalar fields of a jet flow. Journal of Fluid Mechanics, 724, pp.337366. 2013.
We present a detailed experimental investigation of conditional statistics obtained from dissipation elements based on the passive scalar field theta and its instantaneous scalar dissipation rate chi. Using highfrequency planar Rayleigh scattering measurements of propane discharging as a round turbulent jet into coflowing carbon dioxide, we acquire with Taylor's hypothesis a highly resolved threedimensional field of the propane mass fraction theta. The Reynolds number (based on nozzle diameter and jet exit velocity) varies between 3000 and 8600. The experimental results for the joint probability density of the scalar difference Delta theta and the length l of dissipation elements resembles those previously obtained from direct numerical simulations of Wang & Peters (J. Fluid Mech., vol. 554, 2006, pp. 457475). In addition, the normalized marginal probability density function (P) over tilde((l) over tilde) of the length of dissipation elements follows closely the theoretical model derived by Wang & Peters (J. Fluid Mech., vol. 608, 2008, pp. 113138). We also find that the mean linear distance l(m) between two extreme points of an element is of the order of the scalar Taylor microscale lambda(u). Furthermore, the conditional mean <Delta theta vertical bar l > scales with Kolmogorov's 1/3 power law. The investigation of the orientation of long dissipation elements in the jet flow reveals a preferential alignment, perpendicular to the streamwise direction for long elements, while the orientation of short elements is close to isotropic. Following an approach proposed by Kholmyansky & Tsinober (Phys. Lett. A, vol. 373, 2009, pp. 23642367), we finally investigate the probability density function of the scalar increment delta theta in the streamwise direction, when strong dissipative events are either retained in or excluded from the measurement volume. In the present study, however, these events are related to maximum points of the scalar dissipation rate chi together with their local extent. When these regions are excluded from the scalar field, we observe a tendency of the probability density function P(delta theta(r)) towards a Gaussian bellshaped curve.

Kleinheinz, K., Gampert, M., Pitsch, H. & Peters, N., Experimental and numerical study of the turbulent/nonturbulent interface in a turbulent round jet flow. In European Turbulence Conference 14, 1st  4th September 2013, Lyon, France. 2013.
Based on two largeeddy simulations (LES) of a nonreacting turbulent round jet with a nozzle based Reynolds number of 8,610 with the same configuration as the one that has recently been investigated experimentally (Gampert et al., 2012; J Fluid Mech, 2012; J Fluid Mech 724:337, 2013), we examine the scalar turbulent/nonturbulent (T/NT) interface layer in the mixture fraction field of the jet flow between ten and thirty nozzle diameters downstream. To this end, the LES—one with a coarse grid and one with a fine grid—are in a first step validated against the experimental data using the axial decay of the mean velocity and the mean mixture fraction as well as based on radial selfsimilar profiles of mean and root mean square values of these two quantities. Then, probability density functions (pdf) of the mixture fraction at various axial and radial positions are compared and the quality of the LES is discussed. In general, the LES results are consistent with the experimental data. However, in the flow region where the imprint of the T/NT interface layer is dominant in the mixture fraction pdf, discrepancies are observed. In a next step, statistics of the T/NT interface layer are studied, where a satisfactory agreement for the pdf of the location of the interface layer from the higher resolved LES with the experimental data is observed, while the one with the coarse grid exhibits considerable deviations. Finally, the mixture fraction profile across the interface is investigated where the same trend as for the pdf of the location is present. In particular, it is found that the sharp interface that is present in experimental studies (Gampert et al., J Fluid Mech, 2013; Westerweel et al., J Fluid Mech 631:199, 2009) is less distinct in the LES results and rather diffused in radial direction outside of the T/NT interface layer.

Gampert, M., Schaefer, P., Narayanaswamy, V. & Peters, N., Gradient trajectory analysis in a Jet flow for turbulent combustion modelling. Journal of Turbulence, 14(1), pp.147164. 2013.
Based on planar highspeed Rayleigh scattering measurements of the mixture fraction Z of propane discharging from a turbulent round jet into coflowing carbon dioxide at nozzlebased Reynolds numbers Re0 = 30008600, we use scalar gradient trajectories to investigate the local structure of the turbulent scalar field with a focus on the scalar turbulent/nonturbulent interface. The latter is located between the fully turbulent part of the jet and the outer flow. Using scalar gradient trajectories, we partition the turbulent scalar field into these three regions according to an approach developed by Mellado et al. (J.P. Mellado, L. Wang, and N. Peters, Gradient trajectory analysis of a scalar field with external intermittency, J. Fluid Mech. 626 (2009), pp. 333365.). Based on these different regions, we investigate in a next step zonal statistics of the scalar probability density function (pdf) P(Z) as well as the scalar difference along the trajectory Delta Z and its mean scalar value Z(m), where the latter two quantities are used to parameterize the scalar profile along gradient trajectories. We show that the scalar pdf P(Z) can be reconstructed from zonal gradient trajectory statistics of the joint pdf P(Z(m), Delta Z). Furthermore, on the one hand we relate our results for the scalar turbulent/nonturbulent interface to the findings made in other experimental and numerical studies of the turbulent/nonturbulent interface, and on the other hand discuss them in the context of the flamelet approach and the modelling of pdfs in turbulent nonpremixed combustion. Finally, we compare the zonal statistics for P(Z) with the composite model of Effelsberg and Peters (E. Effelsberg and N. Peters, A composite model for the conserved scalar pdf, Combust. Flame 50 (1983), pp. 351360) and observe a very good qualitative and quantitative agreement.

Gauding, M. & Peters, N., Statistical Investigation of Turbulent Mixing by Means of Turbulent Line Segments. 66th Annual Meeting of the APS Division of Fluid Dynamics, 24th  26th November 2013, Pittsburgh, USA. 2013.
We examine the turbulent mixing of a passive scalar with imposed mean gradient. The Taylor microscale based Reynolds number varies between 85 and 530. A straight line through the turbulent field of a passive scalar ϕ is decomposed into piecewise monotonously increasing or decreasing segments. These so called turbulent line segments (TLS) start at a local minimum point and end at a local maximum point or vice versa and are parameterized by the distance ℓ between the extreme points and by the corresponding scalar difference Δϕ. The implication is that TLS, whose mean length is about ten times the Kolmogorov length, characterize the dynamic process of scalarenergy dissipation. Firstly, we examine the joint distribution function of Δϕ and ℓ and define the gradient Δϕ/ℓ of TLS. This helps to understand clifframp structures as we can show at which length scale large gradients arise. Based on a statistical approach we can further relate the mean gradient to the local gradient and can examine the scaling of the kurtosis of the local gradient with the Reynolds number. Secondly, we define a structure function based on TLS, that relates the extreme points and calculate the scaling exponents. The result is compared with the KOCtheory.

Boschung, J., Hennig, F. & Peters, N., A Comparison of the Scalar and Vorticity Criterion defining the T/NT Interface. 66th Annual Meeting of the APS Division of Fluid Dynamics, 24th  26th November 2013, Pittsburgh, USA. 2013.

Cai, L., Beeckmann, J. & Pitsch, H., A Consistent Chemical Mechanism for DME Oxidation. In Proceedings of the European Combustion Meeting 2013, 25th  28th June 2013, Lund, Sweden. 2013.

Boschung, J., Meneveau, C. & Peters, N., Properties of the curvature tensor of streamtubes in turbulent flows. European Turbulence Conference 14, 1st  4th September 2013, Lyon, France. 2013.
Important features of the shape of streamtubes can be described by the first and second invariant of the curvature tensor ∂ti/∂xj where t is the unit vector tangent to the streamlines forming the tube. In threedimensional turbulence, three different generic behaviors can be found, as well as four degenerate cases. For instance, the first invariant of t represents the relative change of the tube’s crosssection. The joint PDF of the first and second invariant is evaluated from DNS, and the results stemming from the presented classification scheme are discussed. An attempt to reproduce the observed PDF from a restrictedEuler like model is described.

Beeckmann, J., Cai, L. & Pitsch, H., Reduced Chemical Mechanism for the Calculation of Ethanol/Air Flame Speeds. Proceedings of the 9th International Colloquium Fuels  Conventional and Future Energy for Automobiles, 15th  17th January 2013, Stuttgart, Germany. 2013.

Schaefer, P., Gampert, M. & Peters, N., Joint statistics and conditional mean strain rates of streamline segments. Physica Scripta, T155(014004). 2013.
Based on four different direct numerical simulations of turbulent flows with Taylorbased Reynolds numbers ranging from Re λ = 50 to 300 among which are two homogeneous isotropic decaying, one forced and one homogeneous shear flow, streamlines are identified and the obtained space curves are parameterized with the pseudotime as well as the arclength. Based on local extrema of the absolute value of the velocity along the streamlines, the latter are partitioned into segments following Wang (2010 J. Fluid Mech. 648 183–203). Streamline segments are then statistically analyzed based on both parameterizations using the joint probability density function of the pseudotime lag τ (arclength l , respectively) between and the velocity difference Δ u at the extrema: P ( τ ,Δ u ), ( P ( l ,Δ u )). We distinguish positive and negative streamline segments depending on the sign of the velocity difference Δ u . Differences as well as similarities in the statistical description for both parameterizations are discussed. In particular, it turns out that the normalized probability distribution functions (pdfs) (of both parameterizations) of the length of positive, negative and all segments assume a universal shape for all Reynolds numbers and flow types and are well described by a model derived in Schaefer P et al (2012 Phys. Fluids 24 045104). Particular attention is given to the conditional mean velocity difference at the ending points of the segments, which can be understood as a firstorder structure function in the context of streamline segment analysis. It determines to a large extent the stretching (compression) of positive (negative) streamline segments and corresponds to the convective velocity in phase space in the transport model equation for the pdf. While based on the random sweeping hypothesis a scaling <vertical bar Delta u parallel to tau > proportional to (u(rms)epsilon tau)(1/3) is found for the parameterization based on the pseudotime, the parameterization with the arclength l yields a much larger than expected l 1/3 scaling. A theoretical indication for this finding is given and a scaling of ⟨Δ u ∥ l ⟩∝ l 2/3 is found from the DNS.

Aryanpour, M., Khetan, A. & Pitsch, H., Activity Descriptor for Catalytic Reactions on Doped Cerium Oxide. ACS Catalysis, 3(6), pp.12531262. 2013.
It is wellknown that ceria enhances chemical activity and catalyst durability in several important catalytic reactions, including CO oxidation and NO reduction. Of great practical value is then having a theoretical model to predict the effect of doping on the ceria activity before the actual. synthesis of its compounds. Such a model is developed in the present work on the basis of experimentally observed data, where we verify our hypothesis that the energy for oxygen vacancy formation is a simple yet powerful activity descriptor for this class of materials. We further benchmark and use our DFT + U computations to estimate this descriptor and to suggest a few transition metals that would increase the activity of ceria toward, redox reactions. This new activity descriptor might be an important factor in similar systems because it does not require any knowledge about the exact chemistry or mechanism of catalysis.

Gampert, M., Schaefer, P., Boschung, J. & Peters, N., Gradient trajectory analysis of the scalar superlayer in a jet flow. Proceedings of the 8th Symposium on Turbulence and Shear Flow Phenomena, 28th  30th August 2013, Poitiers, France. 2013.
Based on planar highspeed Rayleigh scattering measurements of the mass fraction of propane discharging from a turbulent round jet into coflowing carbon dioxide at nozzle based Reynolds numbers Re0=3,0008,600, we investigate the scalar superlayer. The latter is located between the fully turbulent part of the jet and the outer flow and has the so called turbulent/nonturbulent interface embedded within it. It is termed in analogy to the laminar superlayer introduced by Corrsin and Kistler (NACA Report 1244, 1955). Using scalar gradient trajectories, we partition the turbulent scalar field into the afore mentioned three regions according to an approach developed by Mellado et al. (J. Fluid Mech. 626:333365, 2009) based on which we in a next step investigate conditioned zonal statistics of the scalar pdf as well as the scalar difference along the trajectory and its mean scalar value. Finally, we relate our results for the scalar superlayer on the one hand to the findings made in other experimental and numerical studies of the turbulent/nonturbulent interface and discuss them on the other hand in the context of the flamelet approach in turbulent nonpremixed combustion.

Chenadec, V.L. & Pitsch, H., A monotonicity preserving conservative sharp interface flow solver for high density ratio twophase flows. Journal of Computational Physics, 249, pp.185203. 2013.
This paper presents a novel approach for solving the conservative form of the incompressible twophase NavierStokes equations. In order to overcome the numerical instability induced by the potentially large density ratio encountered across the interface, the proposed method includes a VolumeofFluid type integration of the convective momentum transport, a monotonicity preserving momentum rescaling, and a consistent and conservative Ghost Fluid projection that includes surface tension effects. The numerical dissipation inherent in the VolumeofFluid treatment of the convective transport is localized in the interface vicinity, enabling the use of a kinetic energy conserving discretization away from the singularity. Two and threedimensional tests are presented, and the solutions shown to remain accurate at arbitrary density ratios. The proposed method is then successfully used to perform the detailed simulation of a round water jet emerging in quiescent air, therefore suggesting the applicability of the proposed algorithm to the computation of realistic turbulent atomization.

Mueller, M.E., Chan, Q.N., Qamar, N.H., Dally, B.B., Pitsch, H., Alwahabi, Z.T. & Nathan, G.J., Experimental and computational study of soot evolution in a turbulent nonpremixed bluff body ethylene flame. Combustion and Flame, 160(7), pp.12981309. 2013.
A turbulent nonpremixed bluff body ethylene flame is studied both experimentally and computationally. Experimentally, the soot volume fraction is measured using laserinduced incandescence (LII). Three distinct regions are observed in the flame: a lowstrain recirculation zone, a downstream jetlike region, and a highstrain neck region connecting these two regions. The maximum soot volume fraction is found in the recirculation zone, but most of the soot volume is contained in the larger jetlike region further downstream. In the neck region between these two zones, soot cannot form due to large strain rates, and the small amounts of soot in this region indicate that soot rarely escapes the recirculation zone before being oxidized. The recirculation zone is characterized by a low soot intermittency, in contrast to the downstream jetlike region and previously investigated jet flames in which the soot intermittency is high. Large Eddy Simulation (LES) is used to further investigate this distinctly different evolution of soot in the recirculation zone. The LES model is found to predict the soot volume fraction profiles quite accurately, albeit with significant sensitivity to the inflow profiles of the fuel jet and air coflow. Soot is formed near the inner shear layer between the fuel jet and recirculation zone where the mixture fraction is sufficiently large to support Polycyclic Aromatic Hydrocarbon (PAH) formation. A portion of this soot is entrained into the interior of the recirculation zone where the soot growth rates are relatively low, despite the rich mixture fraction in this region. The circulation vortex then transports the soot from the interior of the recirculation zone toward less rich mixture fractions near the flame, which is situated in the outer shear layer between the air coflow and the recirculation zone. Here, the majority of soot growth occurs due to surface growth, that is, mass addition due to surface reactions with acetylene. The dominance of acetylenebased surface growth in the recirculation zone contrasts findings in previous simulations of turbulent jet flames that do not exhibit a recirculation zone, in which nucleation and PAH condensation were found to overwhelm acetylenebased surface growth. (C) 2013 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Beeckmann, J., Pitsch, H., Chaumeix, N., Dagaut, P., Dayma, G., Egolfopoulos, F., Foucher, F., Halter, F., MounaïmRousselle, C., Renou, B., Varea, E., de Goey, P. & Volkov, E., Collaborative Study for Accurate Measurements of Laminar Burning Velocity. Proceedings of the European Combustion Meeting 2013, 25th  28th June 2013, Lund, Sweden. 2013.

Donde, P., Raman, V., Mueller, M.E. & Pitsch, H., LES/PDF based modeling of soot–turbulence interactions in turbulent flames. Proceedings of the Combustion Institute, 34(1), pp.11831192. 2013.
A large eddy simulation (LES)/probability density function (PDF) approach is used to describe the smallscale soot–turbulence–chemistry interactions. The PDF approach directly evolves the joint statistics of the gasphase scalars and a set of moments of the soot number density function. This LES/PDF approach is then used to simulate a turbulent natural gas jet diffusion flame. Since the PDF equation is high dimensional, a Lagrangian method formulated in cylindrical coordinates is coupled to the Eulerian solution technique for the LES flow equations. The LES/PDF simulations show that soot formation is highly intermittent and is always restricted to the fuelrich region of the flow. The PDF of soot moments has a wide spread leading to a large subfilter variance. Further, the conditional statistics of soot moments conditioned on mixture fraction and reaction progress variable show strong correlation between the gas phase composition and soot moments.

Hennig, F., Boschung, J., Gauding, M. & Peters, N., Direct Numerical Simulation of a Temporal Mixing Layer and Detection of the Turbulent/NonTurbulent Interface. 66th Annual Meeting of the APS Division of Fluid Dynamics, 24th  26th November 2013, Pittsburgh, USA. 2013.
The direct numerical simulation of a temporally evolving mixing layer is presented. Using the DNS data we compare two different approaches of detecting the so called turbulent/nonturbulent interface that is found between the fully turbulent and the irrotational outer flow. Standard and conditional statistics are evaluated and compared with literature results.

Chenadec, V.L. & Pitsch, H., A CONSERVATIVE FRAMEWORK FOR PRIMARY ATOMIZATION COMPUTATION AND APPLICATION TO THE STUDY OF NOZZLE AND DENSITY RATIO EFFECTS. Atomization and Sprays, 23(12), pp.11391165. 2013.
The present work focuses on the application of two recent developments of the volumeoffluid method to the computational study of primary atomization: a secondorder unsplit volumeoffluid algorithm and a conservative monotonicity preserving discretization of the twophase NavierStokes equations. This novel combination is motivated in the context of primary atomization at arbitrary density ratios. A validation of this framework is first carried in a controlled yet complex numerical experiment, a turbulent twophase temporal jet. An analysis of the conservation errors and the convergence of the dropsize distribution under grid refinement is provided, and was found to substantiate the ability of the method to resolve small droplets. Finally, a comparison of the performances of a simplified yet realistic injector with the breakup resulting from a fully turbulent cylindrical pipe in similar conditions is provided for varying density ratios (40 and 800). The framework is shown to be able to yield quantitative trends regarding the atomizer performances.

Knudsen, E., Kolla, H., Hawkes, E.R. & Pitsch, H., LES of a premixed jet flame DNS using a strained flamelet model. Combustion and Flame, 160(12), pp.29112927. 2013.
Turbulent premixed flames in the thin and broken reaction zones regimes are difficult to model with Large Eddy Simulation (LES) because turbulence strongly perturbs subfilter scale flame structures. This study addresses the difficulty by proposing a strained flamelet model for LES of high Karlovitz number flames. The proposed model extends a previously developed premixed flamelet approach to account for turbulence's perturbation of subfilter premixed flame structures. The model describes combustion processes by solving strained premixed fiamelets, tabulating the results in terms of a progress variable and a hydrogen radical, and invoking a presumed PDF framework to account for subfilter physics. The model is validated using two dimensional laminar flame studies, and is then tested by performing an LES of a premixed slotjet direct numerical simulation (DNS). In the premixed regime diagram this slotjet is found at the edge of the broken reaction zones regime. Comparisons of the DNS, the strained flamelet model LES, and an unstrained flamelet model LES confirm that turbulence perturbs flame structure to leading order effect, and that the use of an unstrained flamelet LES model underpredicts flame height. It is shown that the strained flamelet model captures the physics characterizing interactions of mixing and chemistry in highly turbulent regimes. (C) 2013 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Doran, E.M., Pitsch, H. & Cook, D.J., A priori testing of a twodimensional unsteady flamelet model for threefeed combustion systems. Proceedings of the Combustion Institute, 34(1), pp.13171324. 2013.
A number of interesting combustion applications involve the mixing of multiple fuel or air streams, such as splitinjection and dualfuel internal combustion engines, which are the main focus of this work, or secondary air in gas turbines. In this study, a twodimensional flamelet formulation for modeling autoignition and combustion in multistream configurations is presented and validated using twodimensional direct numerical simulations (DNS). The DNS simulations were carried out for a multistream nheptane mixture in constant volume isotropic turbulence with finiterate chemistry according to a reduced chemical mechanism which considers 44 species and 185 reactions. The DNS represents two fuel streams and an oxidizer stream, where the second fuel stream is introduced at a time after the first fuel and oxidizer have started to mix. The resulting data are then used to carry out a priori testing of the twodimensional flamelet model. The flamelet formulation is found to accurately represent the interaction of the multiple stream mixing and chemistry, thus enabling the accurate prediction of the autoignition of each fuel stream.

Bode, M., Bisetti, F., Collier, N. & Pitsch, H., Multidimensional Flamelet Lookup Tables Using BSpline Interpolation. In Proceedings of the European Combustion Meeting 2013, 25th  28th June 2013, Lund, Sweden. 2013.

Regele, J.D., Knudsen, E., Pitsch, H. & Blanquart, G., A twoequation model for nonunity Lewis number differential diffusion in lean premixed laminar flames. Combustion and Flame, 160(2), pp.240  250. 2013.
Premixed flames are prone to develop thermodiffusive instabilities when the diffusivity of the fuel is different from the rest of the mixture. Even when a uniform premixed composition is used, the local equivalence ratio across a flame front will not be constant. Therefore, a single quantity such as the progress variable is incapable of modeling accurately the combustion of nonunity Lewis number flames. In this work, a twoequation model is presented for the simulation of premixed laminar flames with nonunity Lewis number fuels. This model relies on the progress variable approach, which is suited for modeling premixed flames in which the fuel's Lewis number is near unity. An additional transport equation for a mixture fraction is derived for nonunity Lewis numbers. The model is verified to be consistent with simple laminar unstretched premixed flames. Hydrogen and propaneair mixtures are used to demonstrate the model's ability to capture the respectively unstable and stable properties of each lean mixture. One dimensional spherical simulations reproduce the effects of flame stretch due to flow strain rate and flame curvature. Finally in two dimensions, the model captures the creation of cellular structures for negative Markstein length flames and the stable propagation of positive Markstein length flames.

Gampert, M., Schaefer, P., Goebbert, J.H. & Peters, N., Decomposition of the turbulent kinetic energy field into regions of compressive and extensive strain. Physica Scripta, 2013(T155), p.014002. 2013.
Based on direct numerical simulations of homogeneous shear turbulence, homogeneous isotropic decaying turbulence and a turbulent channel flow, the scaling of the twopoint velocity difference along gradient trajectories ⟨Δ u n  s<br />
⟩ as well as between the extreme points of the instantaneous turbulent kinetic energy field k is studied. In the first stt<br />
ep, we examine the linear ⟨Δ u n  s ⟩∝ s · a ∞ scaling, where s denotes the separation arclength along a gradient trajee<br />
ctory and a ∞ is the asymptotic value of the conditional mean strain rate of large dissipation elements. Then, we investigate the scaling of the velocity difference between scalar extreme points ⟨Δ u n  l ⟩ as well as the probability densityy<br />
function of the Euclidean distance P (l) between them, conditioned on compressive and extensive strain regions. We observe that while the overall velocity difference along gradient trajectories and between diffusively connected scalar extreme points exhibits linear scaling behaviour ⟨Δ u n  l ⟩∝ l , the conditional velocity differences of extensive ⟨Δ u n  ll<br />
+ ⟩ and compressive regions ⟨Δ u n  l − ⟩ are in contrast to the K41 theory proportional to l 2/3 . The scaling exponn<br />
ent of the overall comes to one part from the purely extensive (compressive) ⟨Δ u n  l + ⟩ (⟨Δ u n  l − ⟩), while thh) >e second contribution is due to the difference in weighting the different regions, thus involving the conditioned pdfs P ( l + ) and P ( l − ). We find that the latter relation scales with l 1/3 . The decomposition of ⟨Δ u n  l ⟩ into two coo ntributions scaling with l 2/3 and l 1/3 , respectively, hence yields an alternative explanation for the observed linear regime.

Chenadec, V.L. & Pitsch, H., A 3D Unsplit Forward/Backward VolumeofFluid Approach and Coupling to the Level Set Method. Journal of Computational Physics, 233, pp.1033. 2013.
This paper presents a novel methodology for interface capturing in twophase flows by combining a LagrangianEulerian VolumeofFluid approach with a Level Set method. While the VolumeofFluid transport relies on a robust and accurate polyhedral library, any highorder Level Set transport may be used. The method is shown to be less restrictive in terms of CFL conditions than split VolumeofFluid methods. Various geometric integration schemes are proposed and tested. For linear velocity fields, mass error is shown to vanish, and to be third order otherwise. The method is validated on 2D and 3D test cases. Conservation properties are shown to be excellent, while geometrical accuracy remains satisfactory even for complex flows such as the primary breakup of a liquid jet.

Vranckx, S., Beeckmann, J., Kopp, W.A., Lee, C., Cai, L., Chakravarty, H.K., Olivier, H., Leonhard, K., Pitsch, H. & Fernandes, R.X., An experimental and kinetic modelling study of nbutyl formate combustion. Combustion and Flame, 160(12), pp.26802692. 2013.
The oxidation of nbutyl formate, a potential biofuel candidate, is studied using three different experimental approaches. Ignition delay times have been measured for stoichiometric mixtures of fuel and air for pressures of about 20 and 90 bar at temperatures from 846 up to 1205 K in a highpressure shock tube. A rapid compression machine has been used to determine the lowtemperature ignition delay times for stoichiometric mixtures at pressures close to 20 bar over the temperature range from 646 K up to 861 K. Laminar burning velocities have been determined for stoichiometric ratios ranging from 0.8 to 1.2 using the highpressure chamber method combined with an optical Schlieren cinematography setup in order to acquire experimental data at elevated pressures of about 10 bar and a temperature of 373 K. A detailed kinetic model has been constructed including hightemperature and lowtemperature reaction pathways. The enthalpies of formation, entropies, and specific heats at constant pressure for the fuel, its primary radicals, and several combustion intermediates have been computed with the CBSQB3 methods and included in the mechanism. This model was validated successfully against the presented data and used to elucidate the combustion of this interesting ester. The importance of accurate inclusion of the lowtemperature peroxy chemistry has been highlighted through sensitivity and reaction path analysis. This study presents the first combustion study of nbutyl formate and leads to an improved understanding of the chemical kinetics of alkyl ester oxidation.

Kang, S., Pitsch, H. & Hur, N., On a robust ALE method with discrete primary and secondary conservation. Journal of Computational Physics, 254, pp.17. 2013.
In this paper, the objective of the present study is to construct a robust, implicit discretization scheme with the ALE method for deforming grids. In order to minimize the need for an artificial stabilization technique, we aim to enforce the property of discrete secondary conservation as well as primary conservation. The requirements for the spatial and temporal discretization methods were discussed based on consistency between the continuous and discrete differential operators. It was shown that the existing methods combined with the ALE method do not satisfy the secondary conservation property discretely and produce secondorder error terms in time. Therefore, the integrated squared scalar value can be affected spuriously by the advection term. Then, a revised method based on a massweighted interpolation was derived to recover secondary conservation. It was found that the proposed method is an extension of the existing approaches for the secondary conservation law to the cases with deforming grids. When applied to laminar and turbulent flow cases with zero diffusivity or viscosity, the proposed method was found to recover discrete secondary conservation and thus can improve the stability of simulations.

Mueller, M.E., Iaccarino, G. & Pitsch, H., Chemical kinetic uncertainty quantification for Large Eddy Simulation of turbulent nonpremixed combustion. Proceedings of the Combustion Institute, 34(1), pp.12991306. 2013.
While the accuracy of chemical kinetic mechanisms continues to improve, these mechanisms are still models with, sometimes considerable, uncertainty. In order to rigorously validate turbulent combustion simulations against experimental data, this uncertainty must be separated from deficiencies in the turbulent combustion model itself. In this work, a method is developed for quantifying the uncertainty in turbulent flame simulations due to input uncertainty in the chemical mechanism. Here the method is developed for Large Eddy Simulation (LES) combined with a steady flamelet model. Rather than a brute force probabilistic approach in which hundreds or thousands of LES runs are required to compute statistics of outputs of interest, the method takes advantage of the actual algorithm employed with the steady flamelet model. First, the highdimensional uncertainty in the chemical kinetics is propagated through the flamelet equations, and the resulting lowerdimensional joint distribution of the temperature, species mass fractions, and other derived quantities is used as a stochastic equation of state in the LES. Since only a few äctive" quantities are needed to evolve the LES governing equations, efficient nonintrusive stochastic collocation is used to propagate the uncertainty in the density, requiring only a few LES runs. This process captures the uncertainty in the flow field induced by the uncertainty in the chemical kinetic rates. The remaining uncertainty in "passive" quantities, that is, quantities needed only for postprocessing such as the temperature and species mass fractions, is computed with random sampling during the LES runs. The uncertainty quantification algorithm is demonstrated with Sandia flame D, and it is shown that the uncertainty in the simulation results caused by uncertainties in the kinetic rates is sufficiently large to account for the discrepancies with the experimental measurements. The implication is that the turbulent combustion model cannot be fairly assessed with such a large uncertainty.

Mittal, V. & Pitsch, H., A flamelet model for premixed combustion under variable pressure conditions. Proceedings of the Combustion Institute, 34(2), pp.29953003. 2013.
Studying premixed combustion under variable pressure conditions is important for modeling devices such as internal combustion engines. Various models have been developed for isobaric premixed combustion, but few models exist for variable pressure conditions. In this study, a new model is proposed for studying combustion at these conditions. The model is based on the flamelet progress variable approach that has been used extensively to study constant pressure premixed combustion. The quantities required by the combustion model are tabulated only at a reference pressure. At other pressure values, they are obtained using a quadratic logarithmic expansion in pressure. Expensive tabulation due to an additional pressure dimension is avoided, and only the coefficients for the expansion need to be tabulated. Additionally, under constant volume conditions, the burnt gas temperature may become larger than the unstretched premixed postflame temperature due to compression of the gases. This effect is modeled by considering the mixture enthalpy. A polynomial expansion method is developed to obtain the temperature from the mixture enthalpy accurately at minimal computational cost. The model is validated against direct numerical simulation data with multistep finite rate chemistry under isochoric conditions. Different turbulence intensities and length scales are studied to better assess the model performance. The model is shown to capture the effects on temperature due to the variable pressure.

Gampert, M., Schaefer, P. & Peters, N., Dissipation element analysis via highspeed rayleigh scattering in a turbulent jet flow. 15th International Symposium on Flow Visualization, 25th  28th June 2012, Minsk, Belarus. 2012.
Based on the extreme points of turbulent scalar fields, Wang and Peters (2006, 2008) developed the theory of dissipation elements. Starting from every grid point, trajectories along the ascending and descending gradient directions can be calculated, which inevitably end in extreme points. All points that share the same two ending points define a finite volume called dissipation element, which is parameterized by its linear length l between and the scalar difference ∆θ at the extreme points. Based on this theory, space filling and nonarbitrary elements are identified, which allow the reconstruction of statistical properties of the field as a whole in terms of conditional statistics within the elements. In the present study, a turbulent round propane jet discharging from a nozzle with diameter d=12mm into surrounding CO2 has been chosen as the core of the experimental setup. The free shear flow, i.e. the mixture fraction of propane and CO2, is visualized via Rayleigh scattering of a diode pumped double cavity Nd:YLF laser at the molecules. As we need a threedimensional test section, in which dissipation elements can be identified, a laser sheet is used to illuminate a twodimensional plane perpendicular to the jet axis. The resulting signal is recorded with a high speed CMOS camera. In a next step, the recorded time series of the planar test section at a fixed downstream position is transformed into a spatial signal based on Taylor’s hypothesis so that we obtain a frozen threedimensional volume of the mixture fraction field. The latter can be postprocessed using the same algorithms as for the DNS and yields a good agreement of the statistical properties of dissipation elements on the one hand and allow their visualization on the other hand.

Lodato, G., Ham, F. & Pitsch, H., Optimal Inclusion of Transverse Effects in the NonReflecting Outflow Boundary Condition. AIAA Journal, 50(6), pp.12911306. 2012.
The inclusion of transverse effects in designing characteristic boundary conditions for the Euler and NavierStokes equations was discussed by different authors and has proved to give some improvement in the perspective of reducing numerical perturbations generated at open boundaries. Based on the most general characteristic formulation using nonorthogonal and rotated reference frames, an analysis of the different terms involved in such an approach is carried out in the present work. To achieve the best performance for the numerical behavior of the boundary condition, it is then shown that different transverse terms need to be treated differently when included in the definition of the incoming wave amplitude variations. The analysis is supported by a series of numerical tests involving an inviscid vortex convected at an angle or a purely acoustic wave propagating radially. It is concluded that the optimal behavior in terms of acoustic reflection by outgoing vorticity can be achieved by minimizing the contribution of the transverse terms related to the material derivatives along the bicharacteristics, either by properly relaxing them or by selecting a characteristic direction which is not necessarily orthogonal to the boundary.

Han, B., Viswanathan, V. & Pitsch, H., FirstPrinciples Based Analysis of the Electrocatalytic Activity of the Unreconstructed Pt(100) Surface for Oxygen Reduction Reaction. The Journal of Physical Chemistry C, 116(10), pp.61746183. 2012.
We apply a rigorous computational procedure combining ab initio DFT calculations and statistical mechanics based methods to examine the electrocatalytic activity of the unreconstructed Pt(100) surface for oxygen reduction reaction. Using the cluster expansion formalism, we obtain stable interfacial water structures using Monte Carlo simulations carried out using parametrized interactions of water–water and water–metal. We find that both longrange and multibody interactions are important to describe the adsorbate interactions as a consequence of the mismatch between the preferred “hexagonal” water overlayer and the underlying square symmetry of the (100) surface. Our results indicate that the stable interfacial water structure is substantially different from that found on the Pt(111) surface. We compute the potentialdependent equilibrium coverages of oxygencontaining adsorbates, which shows that the surface is poisoned by strongly adsorbed OH. We construct the freeenergy diagram of intermediates for oxygen reduction reaction on the Pt(100) surface and find that the limiting step is the reduction of the strongly adsorbed OH. We also find that, at a given potential, a higher degree of poisoning by OH is the reason unreconstructed (100) surfaces are catalytically less active than (111) surfaces. This study shows the importance of accurately capturing atomistic interactions beyond the nearest neighbor pairs.

Mittal, V., Pitsch, H. & Egolfopoulos, F., Assessment of counterflow to measure laminar burning velocities using direct numerical simulations. Combustion Theory and Modelling, 16(3), pp.419433. 2012.
The laminar burning velocity is a fundamental property that is extensively used in the study and modelling of premixed combustion processes. A counterflow flame configuration is commonly used to measure this quantity for different combustion systems. In this procedure, the burning velocities are typically measured at various low stretch conditions and the unstretched burning velocity is extrapolated from these measurements. This extrapolation is done assuming a theoretically onedimensional system along the centreline. We analyse the validity of this assumption by performing DNS studies with finite rate chemistry of the experimental counterflow configuration. The extrapolation process using onedimensional computations is performed on the DNS data and the extrapolated value is compared to the computed laminar burning velocity for the chemical mechanism used. We show that the assumption works well if the nozzle exit velocity has a nearly tophat profile. For nonuniform velocity profiles, it is shown that the temperature curvature at the centreline becomes important. This effect cannot be captured by the onedimensional formulation. Thus, experimental studies measuring laminar burning velocity need to ensure that the nozzle velocity profile is very close to uniform. The extrapolation to zero stretch using 1D counterflow simulations can be performed in different ways. Based on the results obtained in this paper, a simple and accurate extrapolation method is proposed.

Gampert, M., Schaefer, P. & Peters, N., Scalar superlayer contributions to the mixture fraction pdf in a jet flow. 7th International Symposium on Turbulence, Heat and Mass Transfer, 24th  27th September 2012, Palermo, Italy. 2012.
Based on twodimensional highspeed measurements of the mixture fraction Z in a turbulent round jet with nozzle based Reynolds numbers between 3,600 and 8,000, we investigate the scalar superlayer of the flow. Since combustion occurs in the vicinity of the stoichiometric mixture fraction, which is around Z=0.06 for typical fuel/air mixtures, it is expected to take place largely within that layer. Therefore, profound understanding of this part of the flow is essential for an accurate modeling of turbulent nonpremixed combustion. To this end, we validate a composite model of Effelsberg & Peters 1 for the pdf P(Z) which takes into account the different contributions stemming from the fully turbulent as well as the superlayer part of the flow. We observe a very good agreement between the measurements and the model over a wide range of flow conditions.

Schaefer, P., Gampert, M. & Peters, N., Streamline segment topology in the vicinity of stagnation points in turbulent flows. 15th International Symposium on Flow Visualization, 25th  28th June 2012, Minsk, Belarus. 2012.

Won, H.W., Peters, N., Tait, N. & Kalghatgi, G., Sufficiently premixed compression ignition of a gasolinelike fuel using three different nozzles in a diesel engine. Proceedings of the Institution of Mechanical Engineers, Part D: Journal of Automobile Engineering, 226(5), pp.698708. 2012.
Fuels that are more resistant to autoignition allow more time for mixing before combustion occurs and help to reduce nitrogen oxides (NOx) and smoke in a diesel engine. However, hydrocarbon (HC) and carbon monoxide (CO) emissions are high at low loads because combustion is more likely to take place in lean mixture packets with better mixing caused by longer ignition delays. These problems can be significantly alleviated by managing the mixture strength by changing the injection pressure and the nozzle geometries. A singlecylinder diesel engine is run on a mixture of gasoline and diesel with a research octane number of 91, at different speeds, loads, and exhaust gas recirculation levels using three different nozzles. It is much easier to obtain low NOx and low smoke emissions with this fuel than with a European diesel fuel using the standard nozzle. Larger injector holes and lower injection pressures help to reduce the HC and CO emissions at low loads and also enable the gasolinelike fuel to run at a higher speed of 4000 r/min at a reasonably high load (indicated mean effective pressure) of 10 bar.

Ihme, M. & Pitsch, H., On the generation of direct combustion noise in turbulent nonpremixed flames. International Journal of Aeroacoustics, 11(1), pp.2578. 2012.
Generation of combustion noise in an unconfined turbulent nonpremixed flame is investigated. For this, a model is developed, combining Lighthill's acoustic analogy with a flameletbased combustion model to consistently express all thermochemical quantities by a set of reduced scalars. The model is applied in a largeeddy simulation, and the acoustic pressure in the far field is obtained from an integral solution. Three relevant acoustic source terms with different source characteristics and Mach number scaling are identified. The spatial distribution and spectral characteristics of the acoustic sources are analyzed, and it is shown that the acoustic source due to chemical reaction is the main noise contributor, and is located in the upper part of the flame. Contributions from the acoustic sources due to Reynolds stresses and fluctuating mass flux are found to be virtually insignificant at low frequencies. Discrepancies in the prediction of highfrequency sound pressure level in the jet forward direction were analyzed and are attributed to highfrequency acoustic refraction effects due to variations in sound speed. The directivity exhibits a weak directionality in the 30 degrees forward direction, and some phase cancellation between individual acoustic sources is evident.

Cai, L., Beeckmann, J., Pitsch, H. & Raman, V., Iterative Mechanism Optimization Using Modelbased Experimental Design. 34rd International Symposium on Combustion, 29th July  3rd August 2012, Warsaw, Poland. 2012.

Pecnik, R., Terrapon, V., Ham, F., Iaccarino, G. & Pitsch, H., ReynoldsAveraged NavierStokes Simulations of the HyShot II Scramjet. AIAA Journal, 50(8), pp.17171732. 2012.
The internal flow in the HyShot II scramjet is investigated through numerical simulations. A computational infrastructure to solve the compressible Reynoldsaveraged NavierStokes equations on unstructured meshes is introduced. A combustion model based on tabulated chemistry is considered to incorporate detailed chemical kinetics mechanics while retaining a low computational cost. Both nonreactive and reactive simulations have been performed, and results are compared with ground test measurements obtained at DLR, German Aerospace Center. Different turbulence models were tested, and the dependence on the mesh is assessed through grid refinement. The comparison with experimental data shows good agreement, although the computed heat fluxes at the wall are higher than measurements for the reactive case. A sensitivity analysis on the turbulent Schmidt and Prandtl numbers shows that the choice of these parameters has a strong influence on the results. In particular, variations of the turbulent Prandtl number lead to large changes in the heat flux at the walls. Finally, the inception of thermal choking is investigated by increasing the equivalence ratio, whereby a normal shock is created locally and moves upstream, leading to a large increase in the maximum pressure. Nevertheless, a large portion of the flow is still supersonic.

Mueller, M.E. & Pitsch, H., LES model for sooting turbulent nonpremixed flames. Combustion and Flame, 159(6), pp.21662180. 2012.
In this work, an integrated Large Eddy Simulation (LES) model is developed for sooting turbulent nonpremixed flames and validated in a laboratory scale flame. The integrated approach leverages stateoftheart developments in both soot modeling and turbulent combustion modeling and gives special consideration to the smallscale interactions between turbulence, soot, and chemistry. The oxidation of the fuel and the formation of gasphase soot precursors is described by the Flamelet/Progress Variable model, which has been previously extended to account for radiation losses. However, previous DNS studies have shown that Polycyclic Aromatic Hydrocarbons (PAH), the immediate precursors of soot particles, exhibit significant unsteady effects due to relatively slow chemistry. To model these unsteady effects, a transport equation is solved for a lumped PAH species. In addition, due to the removal of PAH from the gasphase, alternative definitions of the mixture fraction, progress variable, and enthalpy are proposed. The evolution of the soot population is modeled with the Hybrid Method of Moments (HMOM), an efficient statistical model requiring the solution of only a few transport equations describing statistics of the soot population. The filtered source terms in these equations that describe the various formation, growth, and destruction processes are closed with a recently developed presumed subfilter PDF approach that accounts for the high spatial intermittency of soot. The integrated LES model is validated in a piloted natural gas turbulent jet diffusion flame and is shown to predict the magnitude of the maximum soot volume fraction in the flame relatively accurately, although the maximum soot volume fraction is shown to be rather sensitive to the subfilter scalar dissipation rate model.

Schaefer, P., Gampert, M. & Peters, N., On the scaling of the mean length of streamline segments in various turbulent flows. Comptes Rendus Mécanique, 340(11–12), pp.859866. 2012.
The geometrical properties of streamline segments (Wang, 2010 1) and their bounding surface (Schaefer et al., 2012 2) in direct numerical simulations (DNS) of different types of turbulent flows at different Reynolds numbers are reviewed. Particular attention is paid to the geometrical relation of the bounding surface and local and global extrema of the instantaneous turbulent kinetic energy field. Also a previously derived model equation for the normalized probability density of the length of streamline segments is reviewed and compared with the new data. It is highlighted that the model is Reynolds number independent when normalized with the mean length of streamline segments yielding that the mean length l(m) plays a paramount role as the only relevant length scale in the pdf. Based on a local expansion of the field of the absolute value of the velocity u along the streamline coordinate a scaling of the mean size of extrema of u is derived which is then shown to scale with the mean length of streamline segments. It turns out that l(m) scales with the geometrical mean of the Kolmogorov scale eta and the Taylor microscale lambda so that l(m) proportional to (eta lambda)(1/2). The new scaling is confirmed based on the DNS cases over a range of Taylor based Reynolds numbers of Relambda = 50300. (C) 2012 Published by Elsevier Masson SAS on behalf of Academie des sciences.

Mittal, V., Cook, D.J. & Pitsch, H., An extended multiregime flamelet model for IC engines. Combustion and Flame, 159(8), pp.27672776. 2012.
Modeling internal combustion engines is challenging due to the various coupled multiphysics phenomena. With the advent of modern supercomputing and advanced modeling techniques, studying and designing these engines through detailed simulations is becoming tractable. Since the combustion process is the primary controlling feature in these engines, a high fidelity combustion model is essential. This model must be efficient and valid across different combustion regimes, since modern engines might operate in hybrid modes. The Representative Interactive Flamelet (RIF) combustion model is a possible choice. This model has been developed to describe ignition, combustion, and pollutant formation in directinjected diesel engines. However, it has recently been shown that the model has the correct asymptotic behavior for both diesel and Homogeneous Charge Compression Ignition (HCCI) regimes, and the model has been applied successfully for HCCI type combustion. In this study, the model is validated against twodimensional direct numerical simulation data with multistep finite rate chemistry to evaluate model performance in the diesel, the HCCI, and hybrid regimes. A wide range of temperature and mixture fraction stratification cases are simulated to evaluate the model performance across different modes. The model performs well for all cases considered, even when high levels of concurrent thermal and charge stratification are present.

Peters, N., Turbulence statistics along gradient trajectories. ZAMM  Journal of Applied Mathematics and Mechanics / Zeitschrift für Angewandte Mathematik und Mechanik, 92(1), pp.47. 2012.
By calculating gradient trajectories in direction of ascending and descending scalar gradients a local maximum and a local minimum point is reached. Dissipation elements may then be defined as the spatial region from which the same pair of maximum and minimum points in a scalar field is reached. By exploring the twopoint correlation of the scalar gradient along such trajectories it was found that for large elements the mean velocity increment scales linearly with the arclength distance along the trajectory. This is different from the classical Kolmogorov scaling and has consequences for the modeling of the length distribution of dissipation elements.

Viswanathan, V., Wang, F. & Pitsch, H., Monte CarloBased Approach for Simulating Nanostructured Catalytic and Electrocatalytic Systems. Computing in Science & Engineering, 14(2), pp.6068. 2012.
This geometrygeneration method within a Monte Carlobased approach enables the simulation of kinetics on nanostructured catalytic and electrocatalytic systems relevant for industrial applications. The method is applicable to realistic catalyst geometries, and an industrially relevant systemfuel cell is studied as an example.

Won, H.W., Pitsch, H., Tait, N. & Kalghatgi, G., Some effects of gasoline and diesel mixtures on partially premixed combustion and comparison with the practical fuels gasoline and diesel in a compression ignition engine. Proceedings of the Institution of Mechanical Engineers, Part D: Journal of Automobile Engineering, 226(9), pp.12591270. 2012.
If fuels that are more resistant to autoignition are injected near top dead centre in compression ignition engines, they ignite much later than diesel fuel does, and combustion occurs when the fuel and air have had more chance to mix. This helps to reduce nitrogen oxide and smoke emissions. Moreover, this can be achieved at much lower injection pressures than for a diesel fuel. However, it is preferable to have fuels with a lower research octane number than those of commonly available gasolines, because this makes lowload operation easier while retaining the advantages at higher loads. A practical approach to making such fuels is to blend the gasoline and diesel fuel available in the market. Such fuel blends have a wide volatility range since they contain highboilingpoint components from the diesel but have a lower research octane number than that of the gasoline used but have a much longer ignition delay than that of the diesel fuel. This work describes the results of running a singlecylinder diesel engine on three such fuel blends. The engine could be run on such blends with extremely low smoke and low nitrogen oxide emissions at speeds of up to 4000 r/min and loads (indicated mean effective pressures) of up to 10 bar with an injection pressure of only 400 bar. The smoke levels at comparable nitrogen oxide levels were extremely high with diesel fuel in these conditions, even with an injection pressure of 1100 bar. The engine could also be run at nearidle conditions on these blends but with higher hydrocarbon and carbon monoxide emissions but much lower nitrogen oxide emissions and maximum pressure rise rate compared with those of the diesel fuel. The wider volatility range might be of benefit in avoiding overmixing and overleaning, which could lead to poor combustion stability. The paper also considers the tradeoffs between the nitrogen oxide, smoke, hydrocarbon and carbon monoxide emissions and the maximum pressure rise rate and discusses approaches to optimise this type of combustion.

Raessi, M. & Pitsch, H., Consistent mass and momentum transport for simulating incompressible interfacial flows with large density ratios using the level set method. Computers & Fluids, 63, pp.7081. 2012.
There exists a sharp density jump at the fluid interfaces in most of interfacial flows encountered in engineering applications. Numerical treatment of the discontinuous fluid density in such flows is a great challenge. In particular, numerical errors can lead to nonphysical results at large density ratios, hence limit a flow solver to only small and usually unrealistic density ratios. We introduce a consistent mass and momentum transport method which allows for simulations of interfacial flows with large density ratios in the context of the level set method. Fluid momentum is transported by discretizing the conservative form of the convective term in the momentum equation. A new technique is introduced to compute the flux density based on the temporal evolution of the fluid interfaces and by using the level set function at two subsequent time levels. For consistency, we use the same flux density for both mass and momentum transport, and thereby establish a tight coupling between them. We assess the performance of the proposed method by using a set of benchmark test cases, in which the density ratio ranges from 650 to 10(6). The new method is stable and accurate even at extreme density ratios. The numerical results agree very well with the theoretical and experimental results. In contrast, the results of a nonconservative formulation show nonphysical fluid behavior, where the dense fluid is slowed down by the light fluid due to numerical errors.

Knudsen, E. & Pitsch, H., Capabilities and limitations of multiregime flamelet combustion models. Combustion and Flame, 159(1), pp.242264. 2012.
Flamelet combustion models typically assume that burning occurs in either a fully premixed or a fully nonpremixed mode. These assumptions tend to limit the applicability of the models to singleregime combustors. Efforts aimed at reducing this limitation have introduced multiregime approaches that account for different types of mixing and chemistry interactions. In this study a multiregime model is applied to two laminar nheptane flames in an effort to characterize the capabilities and limitations of the approach. Both a 2D laminar triple flame and a 2D laminar counterflow diffusion flame are numerically simulated using the multiregime model. Data for comparison is generated by additionally simulating the flames using finite rate chemistry, a purely premixed flamelet model, and a purely nonpremixed flamelet model. Simulations demonstrate that the multiregime approach functions as desired, and tends to access flamelets from the appropriate regime under both nonpremixed and premixed conditions. Some important differences between the flamelet solutions and finite rate solution are observed, however. These differences are caused by the finite rate solution deviating away from the assumed flamelet manifolds, rather than by inadequate regime predictions. In the analyses of these simulations, an emphasis is placed on understanding the formation of the pollutant species NO. It is shown that even when the local combustion regime is correctly predicted, small deviations from an assumed flamelet manifold can lead to changes in the NO production rate. The simulation results confirm that multiregime flamelet models are applicable to a wide variety of reacting flows, but the results also help to characterize the limitations of these models.

Bisetti, F., Blanquart, G., Mueller, M.E. & Pitsch, H., On the formation and early evolution of soot in turbulent nonpremixed flames. Combustion and Flame, 159(1), pp.317335. 2012.
A Direct Numerical Simulation (DNS) of soot formation in an nheptane/air turbulent nonpremixed flame has been performed to investigate unsteady strain effects on soot growth and transport. For the first time in a DNS of turbulent combustion, Polycyclic Aromatic Hydrocarbons (PAH) are included via a validated, reduced chemical mechanism. A novel statistical representation of soot aggregates based on the Hybrid Method of Moments is used M.E. Mueller, G. Blanquart, H. Pitsch, Combust. Flame 156 (2009) 11431155, which allows for an accurate stateoftheart description of soot number density, volume fraction, and morphology of the aggregates. In agreement with previous experimental studies in laminar flames. Damkohler number effects are found to be significant for PAH. Soot nucleation and growth from PAH are locally inhibited by high scalar dissipation rate, thus providing a possible explanation for the experimentally observed reduction of soot yields at increasing levels of mixing in turbulent sooting flames. Furthermore, our data indicate that soot growth models that rely on smaller hydrocarbon species such as acetylene as a proxy for large PAH molecules ignore or misrepresent the effects of turbulent mixing and hydrodynamic strain on soot formation due to differences in the species Dainkohler number. Upon formation on the rich side of the flame, soot is displaced relative to curved mixture fraction isosurfaces due to differential diffusion effects between soot and the gasphase. Soot traveling towards the flame is oxidized, and aggregates displaced away from the flame grow primarily by condensation of PAH on the particle surface. In contrast to previous DNS studies based on simplified soot and chemistry models, surface reactions are found to contribute barely to the growth of soot, for nucleation and condensation processes occurring in the fuel stream are responsible for the most of soot most generation. Furthermore, the morphology of the soot aggregates is found to depend on the location of soot in mixture fraction space. Aggregates having the largest primary particles populate the region closest to the location of peak soot growth. On the contrary, the aggregates with the largest number of primary particles are located much further into the fuel stream.

Viswanathan, V., Hansen, H.A., Rossmeisl, J., Jaramillo, T.F., Pitsch, H. & Nørskov, J.K., Simulating Linear Sweep Voltammetry from FirstPrinciples: Application to Electrochemical Oxidation of Water on Pt(111) and Pt3Ni(111). The Journal of Physical Chemistry C, 116(7), pp.46984704. 2012.
Cyclic voltaminetry is a fundamental experimental method for characterizing adsorbates on electrochemical surfaces. We present a model for the electrochemical solid liquid interface, and we simulate the linear sweep voltammogram of the electrochemical oxidation of H2O on Pt(111) and Pt3Ni(111), based on kinetic and thermodynamic parameters computed by Density Functional Theory (DFT) and the dynamics of the system solved through Monte Carlobased methods. The model predicts onset of OH and O formation in good agreement with voltammetric and ex situ XPS experiments.

Knudsen, E., Richardson, E.S., Doran, E.M., Pitsch, H. & Chen, J.H., Modeling scalar dissipation and scalar variance in large eddy simulation: Algebraic and transport equation closures. Physics of Fluids, 24(5). 2012.
Scalar dissipation rates and subfilter scalar variances are important modeling parameters in large eddy simulations(LES) of reacting flows. Currently available models capture the general behavior of these parameters, but these models do not always perform with the degree of accuracy that is needed for predictive LES. Here, two direct numerical simulations (DNS) are used to analyze LES dissipation rate and variance models, and to propose a new model for the dissipation rate that is based on a transport equation. The first DNS that is considered is a nonpremixed autoigniting C2H4 jet flame simulation originally performed by Yoo et al. Proc. Combust. Inst.33, 1619–1627 (2011)10.1016/j.proci.2010.06.147. A LES of this case is run using algebraic models for the dissipation rate and subfilter variance. It is shown that the algebraic models fail to adequately reproduce the DNS results. This motivates the introduction of a transport equationmodel for the LES dissipation rate. Closure of the equation is addressed by formulating a new adapted dynamic approach. This approach borrows dynamically computed information from LES quantities that, unlike the dissipation rate, do not reside on the smallest flow length scales. The adapted dynamic approach is analyzed by considering a second DNS of scalar mixing in homogeneous isotropic turbulence. Data from this second DNS are used to confirm that the adapted dynamic approach successfully closes the dissipation rate equation over a wide range of LES filter widths. The first reacting jet case is then returned to and used to test the LES transport equationmodels. The transport equationmodel for the dissipation rate is shown to be more accurate than its algebraic counterpoint, and the dissipation rate is eliminated as a source of error in the transported variance model.

Soliman, A.M., Mansour, M.S., Peters, N. & Morsy, M.H., Dissipation element analysis of scalar field in turbulent jet flow. Experimental Thermal and Fluid Science, 37, pp.5764. 2012.
For better understanding of turbulence, the geometry of turbulent structures in turbulent jet flow should be analyzed. The aim of the present work was to experimentally verify the dissipation element theory on highly resolved twodimensional measurements turbulent jets using Rayleigh scattering technique. The statistical analysis of the characteristic parameters of dissipation elements; namely the linear length connecting the extremal points and the absolute value of the scalar difference at these points, respectively was also investigated. Rayleigh scattering was used to topographically produce 2D images of turbulent mixing to obtain the concentration distribution of two gases in a turbulent shear flow. The scalar field obtained was subdivided into numerous finite size regions. In each of these regions local extremal points of the fluctuating scalar are determined via gradient trajectory method. Gradient trajectories starting from any point in the scalar field phi(x, y) in the directions of ascending and descending scalar gradients will always reach a minimum and a maximum point where del phi = 0. The dissipation element has two extremal points (one maximal and one minimal) and two saddle points at the boundaries.

Schaefer, P., Gampert, M. & Peters, N., The length distribution of streamline segments in homogeneous isotropic decaying turbulence. Physics of Fluids, 24(4). 2012.
Based on the profile of the absolute value u of the velocity field u(i) along streamlines, the latter are partitioned into segments at their extreme points as proposed by Wang J. Fluid Mech. 648, 183203 (2010). It is found that the boundaries of all streamline segments, i.e., points where the gradient projected in streamline direction. partial derivative(u)/partial derivative(s) vanishes, define a surface in space. This surface also contains all local extreme points of the scalar ufield, i.e., points where the gradient in all directions of the field of the absolute value of the velocity and thereby those of the turbulent kinetic energy (k = u(2)/2, where k is the instantaneous turbulent kinetic energy) vanishes. Such points also include stagnation points of the flow field, which are absolute minimum points of the turbulent kinetic energy. As local extreme points are the ending points of dissipation elements, an approach for spacefilling geometries in turbulent scalar fields, such elements in the turbulent kinetic energy field also end and begin on the surface and the temporal evolution of dissipation elements and streamline segments are intimately related. Streamline segments by construction evolve both morphologically and topologically. A morphological evolution of a streamline segment corresponds to a continuous deformation when it is subject to stretching or compression and thus also implies a continuous evolution of the arclength l with time. Such an evolution does not change the number of the overall streamline segments and hence does not involve counting. On the other hand a topological evolution corresponds to either a cutting of a large segment into smaller ones or a connection of two smaller ones to form a larger segment. Such a process changes the number of segments and thus involves counting. This change of integer variables (i.e., counting) yields discrete jumps in the length of the streamline segment which are discontinuous in time. Following the terminology by Schaefer et al. "Fast and slow changes of the length of gradient trajectories in homogenous shear turbulence," in Advances in Turbulence XII, edited by B. Eckhardt (SpringerVerlag, Berlin, 2009), pp. 565572 we will refer to the morphological part of the evolution of streamline segments as slow changes while the topological part of the evolution is referred to as fast changes. This separation yields a transport equation for the probability density function (pdf) P(l) of the arclength l of streamline segments in which the slow changes translate into a convection and a diffusion term when terms up to second order are included and the fast changes yield integral terms. The overall temporal evolution (morphological and topological) of the arclength l of streamline segments is analyzed and associated with the motion of the above isosurface. This motion is diffusion controlled for small segments, while large segments are mainly subject to strain and pressure fluctuations. The convection velocity corresponds to the first order jump moment, while the diffusion term includes the second order jump moment. It is concluded, both theoretically and from direct numerical simulations (DNS) data of homogeneous isotropic decaying turbulence at two different Reynolds numbers, that the normalized first order jump moment is quasiuniversal, while the second order one is proportional to the inverse of the square root of the Taylor based Reynolds number Relambda(1/2). Its inclusion thus represents a small correction in the limit of large Reynolds numbers. Numerical solutions of the pdf equation yield a good agreement with the pdf obtained from the DNS data. The interplay of viscous drift acting on small segments and linear strain acting on large segments yield, as it has already been concluded for dissipation elements, that the mean length of streamline segments should scale with Taylor microscale.

Gampert, M., Schaefer, P. & Peters, N., Experimental investigation of small scale geometries in a turbulent round jet. Journal of Physics: Conference Series, 318(3). 2011.
In the present work, we present a method to gather highly accurate threedimensional measurements of a scalar field in order to experimentally validate the theory of dissipation elements as developped by Wang & Peters (2006, 2008). Combining a twodimensional highspeed Rayleigh scattering technique with Taylor's hypothesis allows to resolve the concentration field of gaseous propane discharging into ambient air from a turbulent round jet at a Reynolds number (based on nozzle diameter and exit velocity) of 2,800 down to the Kolmogorov scale in every spatial direction. Based on the acquired data, the normalized probability density function of the length of dissipation elements (P) over tilde((l) over tilde) is investigated at various downstream positions x/d = 15  40 and an excellent agreement with the theoretically derived model equation is obtained.

Schaefer, P., Gampert, M., Goebbert, J.H., Gauding, M. & Peters, N., Asymptotic analysis of homogeneous isotropic decaying turbulence with unknown initial conditions. Journal of Turbulence, 12(30), pp.120. 2011.
In decaying grid turbulence there is a transition from the initial state immediately behind the grid to the state of fully developed turbulence downstream, which is believed to be selfsimilar. This state is characterized by a power law decay of the turbulent kinetic energy with a timeindependent decay exponent n. The value of this exponent, however, depends on the initial distribution of the velocity, about which we have only general information at the very best. For homogeneous isotropic decaying turbulence, the evolution of the twopoint velocity correlation is described by the von KarmanHowarth equation. In the nondimensionalized form of this equation a decay exponent dependent term occurs, whose coefficient will be called delta. We exploit the fact that delta vanishes for n > 2, which is shown to correspond to the limit d > infinity, where d denotes the dimensionality of space to formulate a singular perturbation problem. It is shown that a distinguished limit exists for d > infinity and delta > 0. We obtain in the limit of infinitely large Reynolds numbers an outer layer of limited, but a priori unknown extension, as well as an inner layer of the thickness of the order O(delta(3/2)), where the Kolmogorov scaling is valid. To leading order, we obtain an algebraic balance in the outer layer between the twopoint correlation and the thirdorder structure function. In the inner layer the analysis yields the emergence of higher order terms to the classical K41 scaling. All leading order solutions are shown to be subject to a band of uncertainty of the order O(delta), which is argued to be due to intrinsically unknown initial conditions.

Kerschgens, B., Vanegas, A. & Pitsch, H., Numerical Assessment of Emission Sources for a Modified Diesel Engine Running in PCCI Mode on a Mixture of Gasoline and Diesel. SAE Technical Paper Series, 2011240014. 2011.
Premixed charge compression ignition (PCCI) is an interesting alternative to conventional diesel combustion, as it allows very low emission levels for part load operation. The difficult control of the onset of combustion is an obstacle to the implementation of PCCI. In a recent study, different mixtures of gasoline and diesel fuel have been used in a modified diesel engine to delay the ignition and thus to allow for a substantial premixing time. For these cases, very low levels of particulate emissions have been reported. However, the emissions of CO and NOx were considerably high. In this study, combustion and pollutant formation in the abovementioned modified diesel engine are simulated using the representative interactive flamelet (RIF) approach. A detailed chemical reaction mechanism for a mixture of nheptane, isooctane, toluene, and ethanol, serving as surrogate fuel for the dieselgasoline blend, is used for the simulations. By systematic comparison of experimental and numerical results, an improved understanding of PCCI combustion is achieved and the origins of the CO and NOx emissions are identified. Finally, measures to reduce these emissions while keeping the low PM levels are suggested.

Mueller, M.E., Blanquart, G. & Pitsch, H., Modeling the oxidationinduced fragmentation of soot aggregates in laminar flames. Proceedings of the Combustion Institute, 33(1), pp.667674. 2011.
While the formation and growth of soot particles has received much attention, the subsequent destruction of the particles is less well understood. Soot particles are destroyed though two parallel processes: oxidation and fragmentation. Oxidation is the removal of mass from particles due to chemical reactions with molecular oxygen and hydroxyl radicals. Fragmentation is the breakup of large aggregates into smaller aggregates. Here, a new model for fragmentation inspired by previous experimental investigations is proposed and formulated within the Hybrid Method of Moments (HMOM). With the formulation, the rate of particle loss due to oxidation is closed, resolving a longstanding problem with the Method of Moments. Less important, secondary unclosed terms are introduced, and models for these terms are proposed. The oxidation and fragmentation models are validated using a set of laminar premixed methane flames and then applied to a series of laminar counterflow diffusion acetylene flames. The role of fragmentation is distinctly different in the two flame types. In the premixed flames, fragmentation only occurs in lean flames with a high oxygen concentration. In the diffusion flames, fragmentation is virtually absent, for soot passes through an OH radical layer and is completely oxidized before reaching high oxygen concentrations.

Heufer, K.A., Fernandes, R.X., Olivier, H., Beeckmann, J., Roehl, O. & Peters, N., Shock tube investigations of ignition delays of nbutanol at elevated pressures between 770 and 1250 K. Proceedings of the Combustion Institute, 33(1), pp.359366. 2011.
The ignition delays of nbutanol, a potential biofuel candidate, have been determined in a highpressure shock tube. Conditions behind the reflected shock are approximately between 1042 bar and 7701250 K. To our knowledge, the ignition delay measurements of butanol at these high pressures are the first of their kind. CH emission and pressure time histories have been probed to determine ignition delay times for all experiments. For stoichiometric fuelairmixtures the influence of the temperature and pressure has been characterized. Interestingly the experimental data deviate from the Arrhenius behavior for temperatures lower than 1000 K. This is in contrast to simulation results which have been obtained by employing the simulation tool CANTERA with different reaction mechanisms applying the typical assumption of isochoric conditions. It has been found out that a positive pressure and temperature gradient behind the reflected shock has a significant influence on the ignition delay below 1000 K causing a pronounced decrease in the ignition delay times. This change of the conditions behind the reflected shock is attributed to the shock attenuation and probably from preignition. Including the measured pressure gradients and assuming an isentropic compression behind the reflected shock, the simulation data and the experimental results show a same trend in the temperature dependence of the ignition delay. Nevertheless, striking differences between experiment and simulation persist especially for higher pressures. By performing sensitivity analysis at different temperatures some critical reactions could be identified and their role under our experimental conditions is discussed. In summary it can be stated that the employed reaction mechanisms may not be fully applicable to highpressure conditions and it seems plausible that the lack of more detailed low temperature fuel specific reactions could be the probable cause for the discrepancies which calls for detailed investigations at elevated pressures.

Dahms, R.N., Drake, M.C., Fansler, T.D., Kuo, T.W. & Peters, N., Understanding ignition processes in sprayguided gasoline engines using highspeed imaging and the extended sparkignition model SparkCIMM: Part B: Importance of molecular fuel properties in early flame front propagation. Combustion and Flame, 158(11), pp.2245  2260. 2011.
Recent highspeed imaging of ignition processes in sprayguided gasoline engines has motivated the development of the physicallybased spark channel ignition monitoring model SparkCIMM, which bridges the gap between a detailed spray and vaporization model and a model for fully developed turbulent combustion. Previously, both SparkCIMM and highspeed optical imaging data have shown that, in sprayguided engines, large variations in turbulence intensity, equivalence ratio, and enthalpy along the stretched and wrinkled spark plasma channel favor localized ignition spot formations in richmixture regions. In combination with strong local flow velocity, multiple successful ignition events along the restriking spark lead to early nonspherical turbulent flame fronts. In this paper, SparkCIMM is enhanced by: (1) criteria to capture localized flame extinction phenomena, (2) a formulation of early flame kernel propagation based on the Gequation theory that includes effects of nonunity Lewis numbers, and (3) an extended equation to compute turbulent burning velocities of stretched flames in stratified mixtures. Localized rich ignition along the spark leads to early flames, whose propagation is, due to initially small turbulent Damkohler numbers, significantly influenced by molecular fuel properties. The analysis reveals that nonunity Lewis number curvature effects, intensified by heavy dilution by exhaust gas recirculation, strongly affect the early flamekernel development in sprayguided gasoline engines. In particular, these effects significantly bias the flammability limit of flame kernels towards richmixtures while inhibiting their propagation in lean regions. Favorable initial conditions for combustion are found in richmixture regions, albeit in the presence of substantial equivalence ratio fluctuations and scalar dissipation rates. This paper demonstrates that the full complexity of the model equations developed here is required to reproduce the characteristic experimental features (spark channel stretching, multiple restrikes, localized flame kernel formation, and early turbulent flame front corrugation) of sprayguided ignition phenomena.

Jochim, B., Felsch, C., Drews, P., Vanegas, A., Hoffmann, K., Abel, D., Peters, N. & Pitsch, H., A multizone combustion model with detailed chemistry including cycletocycle dynamics for diesel engine control design. Proceedings of the Institution of Mechanical Engineers, Part D: Journal of Automobile Engineering, 225(9), pp.12351252. 2011.
This paper reviews the research activities within the subproject B1 Model Reduction for LowTemperature Combustion Processes through CFDSimulations and MultiZone Models of the Collaborative Research Centre SFB 686  ModelBased Control of Homogenized LowTemperature Combustion. The SFB 686 is carried out at RWTH Aachen University, Germany and Bielefeld University, Germany, and is funded by the German Research Foundation (DFG). This paper thereby summarizes the outcome of various publications by the authors, with the appropriate references given in the individual sections. Additionally, some new results are introduced. The particular subject of this work is a dynamic simulation strategy for premixed charge compression ignition (PCCI) combustion that can be used in closedloop control development. A detailed multizone chemistry model for the highpressure part of the engine cycle is extended by a mean value gas exchange model accounting for the lowpressure part. Thus, an efficient model capable of describing PCCI combustion is sufficiently well established. In order to capture cycletocycle dynamics, identified system dynamics influencing the input parameters are incorporated. For this, a Wiener model is set up that uses the combustion model as a nonlinear system representation. In this way, a dynamic nonlinear model for the representation of the controlled plant Diesel engine is created. The model is validated against transient experimental engine data.

Dahms, R.N., Drake, M.C., Fansler, T.D., Kuo, T.W. & Peters, N., Understanding ignition processes in sprayguided gasoline engines using highspeed imaging and the extended sparkignition model SparkCIMM. Part A: Spark channel processes and the turbulent flame front propagation. Combustion and Flame, 158(11), pp.22292244. 2011.
Recent highspeed imaging of ignition processes in sprayguided gasoline engines has motivated the development of the physicallybased spark channel ignition monitoring model SparkCIMM, which bridges the gap between a detailed spray/vaporization model and a model for fully developed turbulent flame front propagation. Previously, both SparkCIMM and highspeed optical imaging data have shown that, in sprayguided engines, the spark plasma channel is stretched and wrinkled by the local turbulence, excessive stretching results in spark restrikes, large variations occur in turbulence intensity and local equivalence ratio along the spark channel, and ignition occurs in localized regions along the spark channel (based upon a Karlovitznumber criteria). In this paper, SparkCIMM is enhanced by: (1) an extended flamelet model to predict localized ignition spots along the spark plasma channel, (2) a detailed chemical mechanism for gasoline surrogate oxidation, and (3) a formulation of early flame kernel propagation based on the Gequation theory that includes detailed chemistry and a local enthalpy flamelet model to consider turbulent enthalpy fluctuations. In agreement with new experimental data from broadband spark and hot soot luminosity imaging, the model establishes that ignition prefers to occur in fuelrich regions along the spark channel. In this highlyturbulent highlystratified environment, these ignition spots burn as quasilaminar flame kernels. In this paper, the laminar burning velocities and flame thicknesses of these kernels are calculated along the mean turbulent flame front, using tabulated detailed chemistry flamelets over a wide range of stoichiometry and exhaust gas dilution. The criteria for flame propagation include chemical (crossover temperature based) and turbulence (Karlovitznumber based) effects. Numerical simulations using ignition models of different physical complexity demonstrate the significance of turbulent mixture fraction and enthalpy fluctuations in the prediction of early flame front propagation. A third paper on SparkCIMM (companion paper to this one) focuses on the importance of molecular fuel properties and flame curvature on early flame propagation and compares computed flame propagation with high speed combustion imaging and computed heat release rates with cylinder pressure analysis.<br />
The goals of SparkCIMM development are to (a) enhance our fundamental understanding of ignition and combustion processes in highlyturbulent highlystratified engine conditions, (b) incorporate that understanding into a physicallybased submodel for RANS engine calculations that can be reliably used without modification for a wide range of conditions (i.e., homogeneous or stratified, low or high turbulence, low or high dilution), and (c) provide a submodel that can be incorporated into a future LES model for physicallybased modeling of cycletocycle variability in engines.

Gampert, M., Goebbert, J.H., Schaefer, P., Gauding, M., Peters, N., Aldudak, F. & Oberlack, M., Compressive and extensive strain along gradient trajectories. Journal of Physics: Conference Series, 318(5), p.052029. 2011.
Based on direct numerical simulations of forced turbulence, shear turbulence, decaying turbulence, a turbulent channel flow as well as a Kolmogorov flow with Taylor based Reynolds numbers Reλ between 69 and 295, the normalized probability density function of the length distribution P̃(l̃) of dissipation elements, the conditional mean scalar difference at the extreme points as well as the scaling of the twopoint velocity difference along gradient trajectories are studied. Using the field of the instantanous turbulent kinetic energy k as a scalar, we find a good agreement between the model equation for P̃(l̃) as proposed by Wang and Peters (2008) and the results obtained in the different DNS cases. This confirms the independance of the model solution from both, the Reynolds number and the type of turbulent flow, so that it can be considered universally valid. In addition, we show a 2/3 scaling for the mean conditional scalar difference. In the second part of the paper, we examine the scaling of the conditional twopoint velocity difference along gradient trajectories. In particular, we compare the linear s/τ scaling, where τ denotes an integral time scale and s the separation arclength along a gradient trajectory in the inertial range as derived by Wang (2009) with the s · a∞ scaling, where a∞ denotes the asymtotic value of the conditional mean strain rate of large dissipation elements.

Luo, K., Pitsch, H., Pai, M.G. & Desjardins, O., Direct numerical simulations and analysis of threedimensional nheptane spray flames in a model swirl combustor. Proceedings of the Combustion Institute, 33(2), pp.21432152. 2011.
Threedimensional nheptane spray flames in a swirl combustor are investigated by means of direct numerical simulation (DNS) to provide insight into realistic spray evaporation and combustion as well as relevant modeling issues. The variabledensity, lowMach number NavierStokes equations are solved using a fully conservative and kinetic energy conserving finite difference scheme in cylindrical coordinates. Dispersed droplets are tracked in a Lagrangian framework. Droplet evaporation is described by an equilibrium model. Gas combustion is represented using an adaptive onestep irreversible reaction. Two different cases are studied: a lean case that resembles a lean direct injection combustion, and a rich case that represents the primary combustion region of a richburn/quickquench/leanburn combustor. The results suggest that premixed combustion contribute more than 70% to the total heat release rate, although diffusion flame have volumetrically a higher contribution. The conditional mean scalar dissipation rate is shown to be strongly influenced, especially in the rich case. The conditional mean evaporation rate increases almost linearly with mixture fraction in the lean case, but shows a more complex behavior in the rich case. The probability density functions (PDF) of mixture fraction in spray combustion are shown to be quite complex. To model this behavior, the formulation of the PDF in a transformed mixture fraction space is proposed and demonstrated to predict the DNS data reasonably well.

Dahms, R., Felsch, C., Roehl, O. & Peters, N., Detailed chemistry flamelet modeling of mixedmode combustion in sparkassisted HCCI engines. Proceedings of the Combustion Institute, 33(2), pp.30233030. 2011.
A detailed chemistry mixedmode flamelet model for the prediction of combustion in sparkassisted homogeneous charge compression ignition (HCCI) engines is presented in this paper. The complex phenomena of sparkchannel processes (turbulent corrugation, multiple restrikes) and of early flame kernel propagation (localized flame kernel formation, nonspherical early flame shapes), both induced by sprayguided sparkignition combustion initiation, are captured by the recently introduced SparkCIMM ignition model. In this paper, laminar burning velocities and flame thicknesses are calculated along the mean turbulent flame front, using tabulated detailed chemistry flamelet calculations to appropriately consider locally rich, highly diluted, and autoigniting stratified mixtures. Flame extinction criterions are formulated and incorporated into an extended Gequation flame front tracking scheme. Autoignition processes in the unburnt mixture, controlled by the flameinduced pressure and temperature increase, are captured by a recently developed multizone flamelet model that accounts for detailed chemistry and effects of scalar mixing on turbulent combustion. The laminar burning velocity is shown to increase significantly as the flame propagates into the chemically reacting mixture within the first stage of autoignition. After the initiation of the thermal runaway, however, flame extinction occurs rapidly. These interactions of mixedmode combustion processes are captured by the presented flamelet model. It is developed based on a time and length scale analysis, revealing a scale separation between the ignition delay of diffusion combustion and the flame time of flame propagation. The analysis of the mixture preparation process, along with the simulation of turbulent flame front propagation and its extinction, demonstrates that the prediction of combustion in sparkassisted HCCI engines requires the on hand comprehensive mixedmode combustion model. A comparison of simulation results from this new model with data from experiments and combustion models of reduced physical complexity proofs its qualification.

Luckhchoura, V., Peters, N. & Diwakar, R., Computational analysis of injectionrate shapes in a smallbore directinjection diesel engine. International Journal of Engine Research, 12(2), pp.145168. 2011.
The purpose of this study is to analyse the differences in the combustion process, and pollutants formation (especially soot) due to tophat and boot injectionrate shapes at one specific highload point of a singlecylinder smallbore diesel engine using multidimensional engine simulations. The simulations are performed using a response interactive flamelet model with detailed chemical kinetics. A detailed chemistrybased soot model is used for the prediction of soot emissions. The heights of the first and second stages of the boot shape are varied to observe the effect of the injected fuel mass distribution. In addition, results of boot shapes are compared with a trapezoidal (tophat) shape. A detailed analysis of soot formation and oxidation is also presented for some selected rate shapes. Through computational analysis it is shown that the boot shapes have the potential to decrease combustiongenerated noise and to lower emissions at the investigated load point compared to the tophat shape. Variations in the temporal distribution of the injected fuel mass show that a lower height of the first stage and a higher height of the second stage of the boot shape result in a relatively slower rise of heat release (lower combustiongenerated noise) in the early part of combustion and enhanced soot oxidation as a result of higher spray momentum near the end of injection.

Hemchandra, S., Peters, N. & Lieuwen, T., Heat release response of acoustically forced turbulent premixed flamesrole of kinematic restoration. Proceedings of the Combustion Institute, 33(1), pp.16091617. 2011.
Predicting the ensemble averaged heat release response of a turbulent premixed flame to acoustic forcing is a fundamental problem associated with understanding combustion instabilities. This paper describes an analysis of this problem, by modeling the response of a flame that is simultaneously perturbed by broadband, turbulent fluctuations and narrowband, acoustic fluctuations of amplitude epsilon(a) and epsilon(T), respectively. It is shown that the response of the flame surface to coherent forcing and turbulent fluctuations are coupled even at linear order in coherent forcing amplitude, epsilon(a), due to flame propagation (kinematic restoration). This coupling effectively causes the local consumption and displacement speeds of the flame to vary in time over a forcing cycle. Turbulent fluctuations also provide a mechanism for destruction of coherent flame surface wrinkling at first order in epsilon(a), an effect which only occurs at O(epsilon(2)(a)) for laminar flames.

Gampert, M., Schaefer, P. & Peters, N., Experimental investigation of small scale geometries in a turbulent round jet. 7th Symposium on Turbulence and Shear Flow Phenomena, 28th  31st July 2011, Ottawa, Canada. 2011.
In the present work, we present a method to gather highly accurate threedimensional measurements of a scalar field in order to experimentally validate the theory of dissipation elements as developped by Wang & Peters (2006, 2008). Combining a twodimensional highspeed Rayleigh scattering technique with Taylor's hypothesis allows to resolve the concentration field of gaseous propane discharging into ambient air from a turbulent round jet at a Reynolds number (based on nozzle diameter and exit velocity) of 2,800 down to the Kolmogorov scale in every spatial direction. Based on the acquired data, the normalized probability density function of the length of dissipation elements () is investigated at various downstream positions x/d = 15 − 40 and an excellent agreement with the theoretically derived model equation is obtained.

ElAsrag, H.A., Pitsch, H., Kim, W., Do, H. & Mungal, M.G., Damköhler Number Similarity for Static Flame Stability in GaseousFueled Augmentor Flows. Combustion Science and Technology, 183(7), pp.718737. 2011.
Afterburners (or augmentors) are used to increase thrust in aircraft engines. Static flame stability, or the robustness to flame blowoff, is an important metric in the performance assessment of combustion in aircraft engine afterburners, where bluffbodytype flame holders are typically used to stabilize the flame. The design of such flame holders is complicated by the operating conditions, which involve flows at high speed, high temperature, and low pressure. In this paper, experimental and computational studies of Damkohler (Da) number similarity are presented with relevance to augmentor flame stability. The Da number describes the ratio of flow and chemical time scales. Hence, as long as a reference Da number is kept constant, similar characteristics of static stability should be expected of the bluffbody stabilized flame at low and high speeds. Flame stability in highspeed vitiated flows could then be studied at low speed if the chemical time scale is reduced, for instance by lowering the oxidizer flow temperature. However, each chemical reaction has its own Da number and not all can be kept constant at the same time. Since different stabilization mechanisms are governed by different chemical reactions, it is not necessarily clear what the relevant Da number is. Here, a consistent method for defining the Da number is provided based on the analysis of the modeled governing equations. Numerical simulations are performed for three different velocities with the inflow temperature adjusted to keep the Da number constant. Results are validated by comparison with experimental PIV data and the reported flame liftoff height. For the same characteristic Da number and constant momentum ratio between the fuel jet and the vitiated cross flow, the three flames show similar mean features for the recirculation zone and the flame shape. The flow field is found to exhibit von Karman vortex shedding with the same Strouhal number for all cases. The average nondimensional flame liftoff height is also found to be the same for all cases. These results suggest a method to properly define the Da number to test augmentor stability features in lowspeed test facilities under the conditions of similarity.

Peters, N., Hoffmann, K., Felsch, C. & Abel, D., A Dynamic Simulation Strategy for PCCI Combustion Control Design. Oil Gas Sci. Technol. – Rev. IFP Energies nouvelles, 66(4), pp.549562. 2011.
A Dynamic Simulation Strategy for PCCI Combustion Control Design  Subject of this work is a dynamic simulation strategy for PCCI combustion that can be used in closedloop control development. A detailed multizone chemistry model for the highpressure part of the engine cycle is extended by a mean value model accounting for the gas exchange losses. The resulting stationary model is capable of describing PCCI combustion sufficiently well. It is at the same time very economic with respect to computational costs. The model is further extended by identified system dynamics influencing the stationary inputs. For this, a Wiener model is set up that uses the stationary model as a nonlinear system representation. In this way, a dynamic nonlinear model for the representation of the controlled plant Diesel engine is created. This paper summarizes an important outcome of the the Collaborative Research Centre "SFB 686  Modellbasierte Regelung der homgenisierten NiedertemperaturVerbrennung" at RWTH Aachen University and Bielefeld University, Germany.

Schaefer, P., Gampert, M. & Peters, N., The local dynamics of streamlines in turbulence. Journal of Physics: Conference Series, 318(5). 2011.
Based on a coordinate transformation, similiar to the one used in the flamelet approach in nonpremixed combustion, we introduce a coordinate locally tangent to streamlines and transform the NavierStokes equations to an equation for the variation of the absolute value of the velocity u i . Different from previous approaches, the unsteady term splits into two terms, one of which contains all information about the time and spacedependence of the streamline coordinate. Based on the DNS of homogeneous isotropic decaying turbulence at a Taylor based Reynolds number of Re λ = 71 we first discuss the temporal variation of the streamline coordinate field, reflecting nonlocal fast and slow changes of the latter. Then, based on the transformed equation we discuss the balance of the different terms along streamlines and discern terms of different orders of magnitude.

Drews, P., Albin, T., Heßeler, F.J., Peters, N. & Abel, D., FuelEfficient ModelBased Optimal MIMO Control for PCCI Engines. Proceedings of the 18th IFAC World Congress, 18th August  2nd September 2011, Milano, Italy, pp.1299813003. 2011.
Recent research in modern combustion technologies, like partial homogeneous charge compression ignition (PCCI), demonstrates the capability of reducing pollutant emissions, e.g. soot and NOx. In addition to this advantage, a possibility to reduce fuel consumption and noise production by modelbased optimal control is presented in this paper. In order to understand the basic properties of the PCCI mode, process measurements were conducted using a slightly modified series diesel engine. Control variables are engine combustion parameters: the indicated mean effective pressure, the combustion average and the maximum gradient of the cylinderpressure. Control inputs are the parameters: quantity of injected fuel, start of injection and the intake manifold fraction of recirculated exhaust gas. The process has very fast, almost proportional behaviour over the engine`s working cycles. Focusing on the static behaviour of the process, a nonlinear neural network model is used for identification. Successive linearization of the nonlinear network is used to build an affine internal controller model for the actual operating point. The presented controller structure is able to consider constraints by individual formulation of the cost function. With this configuration the closedloop process is able to track the combustion setpoints with high control quality with minimal possible fuel consumption and combustion noise.

Goebbert, J.H. & Peters, N., Analysis of Turbulent Premixed Flames using Gradient Trajectories. Proceedings of the 5th European Combustion Meeting, 27th June  1st July, 2011, Cardiff, Wales. 2011.

Aye, M.M., Beeckmann, J.L., Vanegas, A., Peters, N. & Pitsch, H., Experimental Investigation of Diesel and Surrogate Fuels: Spray and Ignition Behavior. SAE Technical Paper Series, 2011011921. 2011.
In this work, surrogate fuels composed of ndecane and alphamethylnaphthalene (AMNL) with different compositions according to the reference cetane numbers 53, 45, 38, and 23 are investigated. In addition to the twocomponent mixtures, we examine a threecomponent mixture composed of ndecane, AMNL, and dinbutyl ether (DNBE) corresponding to a reference cetane number of 53. Spray characteristics of liquid and fuel vapor phase and the relationship between ignition quality and liftoff length are investigated. The experimental results show, first of all, that for these mixtures, the cetane number is a good indicator for the ignition delay. Diesel and surrogate fuels have different liquid penetration lengths, which depend on the evaporation rate, and hence vapor pressure and boiling point of the fuels. Spray tip penetration of all investigated fuels except diesel are similar for the vaporphase, because the differences in fuel properties are too small to considerably affect the momentum flux. Furthermore, we discuss the overlap number (OL) of the fuels to estimate the soot formation tendency under compression ignition (CI) engine like conditions. It is found out that the OL generally increases with decreasing cetane number, although there are substantial variations for the studied fuels with the same cetane number. Most interestingly, the low cetane number fuel, which has a substantially higher content of a tworing aromatic, is shown to have diminished soot formation, which is consistent with its higher OL number.

Gampert, M., Schaefer, P. & Peters, N., Threedimensional measurements of a turbulent scalar field in a round jet. 41st AIAA Fluid Dynamics Conference and Exhibit, 27th  30th June 2011, Honolulu, Hawaii. 2011.

Sudholt, A., Wada, T., Pitsch, H. & Peters, N., First Stage Ignition of Dibutyl Ether. Proceedings of the 5th European Combustion Meeting, 28th June  1st July 2011, Cardiff, Wales. 2011.

Beeckmann, J. & Pitsch, H., Investigation of the Laminar Burning Velocities of C1C4 alcohols. Proceedings of the 5th European Combustion Meeting, 28th June  1st July 2011, Cardiff, Wales. 2011.

Schaefer, P., Gampert, M., Gauding, M., Peters, N. & Treviño, C., The secondary splitting of zerogradient points in a scalar field. Journal of Engineering Mathematics, 71(1), pp.8195. 2011.
The mechanisms related to the secondary splitting of zerogradient points of scalar fields are analyzed using the twodimensional case of a scalar extreme point lying in a region of local strain. The velocity field is assumed to resemble a stagnationpoint flow, cf. Gibson (Phys Fluids 11:2305–2315, 1968), which is approximated using a Taylor expansion up to third order. The temporal evolution of the scalar field in the vicinity of the stagnation point is derived using a series expansion, and it is found that the splitting can only be explained when the thirdorder terms of the Taylor expansion of the flow field are included. The nondimensional splitting time turns out to depend on three parameters, namely the local Péclet number Pe δ based on the initial size of the extreme point δ and two parameters which are measures of the rate of change of the local strain. For the limiting casePe δ → 0, the splitting time is found to be finite but Pécletnumber independent, while for the case of Pe δ → ∞ it increases logarithmically with the Péclet number. The physical implications of the twodimensional mathematical solution are discussed and compared with the splitting times obtained numerically from a Taylor–Green vortex.

Schaefer, P., Gampert, M. & Peters, N., On The Dissipation Rate Coefficient In Homogeneous Isotropic Decaying And Forced Turbulence. Seventh International Symposium on Turbulence and Shear Flow Phenomena, 28th  31st July 2011, Ottawa, Canada. 2011.
The normalized nondimensional von KármánHowarth equation for isotropic homogeneous decaying and forced steady turbulence is integrated to obtain expressions for the dissipation rate coefficient Cϵ=(Lϵ)/<u2>3/2, where L denotes the longitudinal integral length scale, ϵ the mean dissipation rate and <u2> the mean variance of the longitudinal velocity fluctuations. For decaying turbulence the final exact expressions for Cϵ for the low and high Reynolds number limit depend on the decay exponent n, which is known to depend on the initial velocity structure at the turbulence production. The dependence on n leads to a nonuniversal coefficient. The expressions for the steady forced case depend on the forcing mechanism and thus are not universal either. Nonetheless, a lower value and considerably less scatter as compared to the decaying turbulence case should be expected when similiar forcing algorithms are employed.

Gampert, M., Goebbert, J.H., Schaefer, P., Gauding, M., Peters, N., Aldudak, F. & Oberlack, M., Extensive strain along gradient trajectories in the turbulent kinetic energy field. New Journal of Physics, 13(4). 2011.
Based on direct numerical simulations of forced turbulence, shear turbulence, decaying turbulence, a turbulent channel flow as well as a Kolmogorov flow with Taylorbased Reynolds numbers Relambda between 69 and 295, the normalized probability density function of the length distribution (P) over tilde((l) over tilde) of dissipation elements, the conditional mean scalar difference <Delta kl > at the extreme points as well as the scaling of the twopoint velocity difference along gradient trajectories <Delta u(n)> are studied. Using the field of the instantaneous turbulent kinetic energy k as a scalar, we find good agreement between the model equation for (P) over tilde ((l) over tilde) as proposed by Wang and Peters (2008 J. Fluid Mech. 608 11338) and the results obtained in the different direct numerical simulation cases. This confirms the independence of the model solution from both the Reynolds number and the type of turbulent flow, so that it can be considered universally valid. In addition, we show a 2/3 scaling for the mean conditional scalar difference. In the second part of the paper, we examine the scaling of the conditional twopoint velocity difference along gradient trajectories. In particular, we compare the linear s/tau scaling, where tau denotes an integral time scale and s the separation arclength along a gradient trajectory in the inertial range as derived by Wang (2009 Phys. Rev. E 79 046325) with the s . a(infinity) scaling, where a(infinity) denotes the asymptotic value of the conditional mean strain rate of large dissipation elements.

Mueller, M.E. & Pitsch, H., Large eddy simulation subfilter modeling of sootturbulence interactions. Physics of Fluids, 23(11), p.115104. 2011.
The smallscale interactions between turbulence, chemistry, and soot have a profound effect on the soot formation, growth, and destruction processes in turbulent reacting flows. In this work, the smallscale subfilter interactions between turbulence and soot are modeled using a presumed subfilter PDF for the statistical moments of the soot number density function. Due to a very large (infinite) Schmidt number, soot is confined to very thin structures. In addition, soot is formed initially from Polycyclic Aromatic Hydrocarbons, which exhibit a strong sensitivity to the local scalar dissipation rate in the flow field. These interactions of soot with the gasphase chemistry, molecular transport, and turbulence result in very high spatial and temporal intermittency. Therefore, the soot subfilter PDF is presumed to be a pair of delta distributions with a sooting mode and a nonsooting mode. In addition to the mean values of the scalars, one additional parameter is needed to specify this distribution. The presumed soot subfilter PDF approach is evaluated a priori using a recent twodimensional direct numerical simulation (DNS) database of soot evolution in a nonpremixed flame. The analysis shows that predictions of the soot intermittency as well as the soot source terms are improved with this presumed soot subfilter PDF approach compared to simply using the mean values of the soot scalars. Several choices for specifying the additional parameter of the PDF are also evaluated with the database. The results show that the parameter is best specified using the second moment of the soot number density. In addition, the transport equation for the second moment of the number density is briefly discussed. A model for the lone unclosed term is proposed and evaluated with the DNS data.

Wang, L., Pitsch, H., Yamamoto, K. & Orii, A., An efficient approach of unsteady flamelet modeling of a crossflowjet combustion system using LES. Combustion Theory and Modelling, 15(6), pp.849862. 2011.
Steady flamelet models have been widely used in turbulent combustion simulations because of their simplicity, efficiency, yet physicsbased nature. They are, however, unable to handle slow chemical and physical processes such as pollutant formation. Unsteady flamelet models have been shown to be able to provide accurate predictions especially for pollutants, but their implementations are usually not as straightforward as for the steady models, and additional assumptions are involved. One relatively straightforward approach of implementing the unsteady flamelet model is to tabulate the time history of unsteady flamelet solutions. This often leads to flamelet libraries of large sizes because of increased dimensions for the new physics. The purpose of this paper is to introduce a new and efficient approach of tabulating unsteady flamelet solutions in the LES of complex systems, here demonstrated in simulations of a crossflowjet combustion system. This approach employs Taylor series expansions to represent the time history of unsteady flamelet solutions. Compared with other approaches, the new approach retains the efficiency and simplicity benefits of steady flamelet models but possesses the accuracy of unsteady flamelet models. Various issues associated with the formulation and implementation of this approach are discussed, which include the selection of the base solution, the order of accuracy of the expansion, and the treatment of simultaneous wall heat losses and heat transfer through thermal radiation. This approach is validated in large eddy simulations of a crossflowjet combustion system. Good agreement with experiments is obtained for both temperature and NO concentration, as well as for major species.

Dhanda, A., Pitsch, H. & O’Hayre, R., Diffusion Impedance Element Model for the Triple Phase Boundary. Journal of The Electrochemical Society, 158(8), pp.B877B884. 2011.
Triple Phase Boundary (TPB) properties at Pt/Nafion interfaces in a Polymer Electrolyte Membrane (PEM) fuel cell depend on the relative rates of reaction and diffusion processes. In this paper, electrochemical impedance analysis is performed to model the coupling between the reaction and diffusion phenomena in the TPB zone. The transfer function for the PEM fuel cell system is obtained for the oxygen reduction reaction (ORR) mechanism with two adsorbed species. An analytical expression is derived for the TPB diffusion impedance element based on the transient concentration profile and the ORR kinetics derived for Pt microelectrodes placed on Nafion electrolyte. For this arrangement, the coupled kineticdiffusion impedance is shown to have a distinct gain and phase relationships, which is closely related to the Gerischer element. The model is used to study the transient response of the coupled impedance element in terms of TPB parameters and operating voltages. The TPB impedance element, in conjunction with existing models of mass transport impedance in GDL and porous electrodes, provides a more comprehensive analysis of combined Faradaic and diffusion processes occurring at the Pt/Nafion interfaces in PEM fuel cells.

Goebbert, J.H., Gauding, M., Gampert, M., Schaefer, P., Peters, N. & Wang, L., A new View on Geometry and conditional Statistics in Turbulence. Inside: Innovatives Supercomputing in Deutschland, 9(1), pp.3037. 2011.

Gauding, M., Goebbert, J.H. & Peters, N., Scalebyscale Statistics of Passive Scalar Mixing with Uniform Mean Gradient in Turbulent Flows. Proceedings of the 6th AIAA Theoretical Fluid Mechanics Conference, 27th  30th June 2011, Honolulu, Hawaii, USA, 20114024. 2011.
The mixing of passive scalars with imposed mean gradient in stationary homogeneous turbulence is studied by means of direct numerical simulation. The scalar field is characterized by strong intermittency and anisotropy due to the mean shear gradient. Statistics in direction of the mean shear gradient are different from statistics normal to the mean shear. A generalized Yaglom equation is derived to take the anisotropy and the mean scalar shear gradient into account. This equation incorporates the balance between production, transport and dissipation of turbulent scalar energy. It is insensitive to smallscale intermittency and only describes the turbulent scalar energy transfer through scales. Various Schmidt numbers between 0.11 and 14.4 are investigated, while the Taylormicroscale Reynolds number is fixed between 119 and 148. A comparison with Yaglom’s 2/3law is performed and limiting cases for large and small scales are discussed.

Knudsen, E.W., Kim, S.H. & Pitsch, H., An analysis of premixed flamelet models for large eddy simulation of turbulent combustion. Physics of Fluids, 22(11), p.115109. 2010.
When premixed flamelet models are applied in the context of large eddy simulation, a number of assumptions are implicity made. The validity of these assumptions depends on, for example, the simulated flame’s location within the premixed regime diagram, the accuracy of the presumed subfilter flamelet coordinate distributions, and the extent to which the asymptotic flamelets capture the turbulenceperturbed chemistry. Here, the errors that arise due to these assumptions are considered, analyzed, and compared using a direct numerical simulation of a premixed turbulentflame propagating in the thin reaction zones regime. Flamelet representations of the progress variable source term are formed in an a priori fashion. Level set flamelet methods in particular are considered because, although they offer a number of advantages, they make some of the most stringent flame structure assumptions. Errors due to the level set model are evaluated relative to other flamelet error sources, such as the shape of the presumed probability density function and the influence of the variance model. The results provide guidance on the importance of the individual modeling assumptions, and are used to propose a new modeling strategy in an effort to improve the level set framework.

Won, H.W., Moon, S.E., Luckhchoura, V., Lee, S.W. & Peters, N., An Experimental Investigation into the Effects of Injection Pressures and Swirl Ratio using Cluster Nozzle Configurations in a DI Diesel Engine. In FISITA 2010 World Automotive Congress: Automobiles and sustainable mobility. Budapest. 2010.
One of the most important parameter that affects diffusion controlled combustion process in diesel engine is air fuel mixing which is mainly controlled by injection. Based on the diesel concept, it is practically impossible to avoid fuel rich and stoichiometric pockets, but the formation of soot and NOx are also time dependent. If the mixing time is sufficiently small, both pollutant could be reduced simultaneously without getting into the well known sootNOx trade off. In order to develop a low emission engine, research is necessary to come up with a new combustion strategy for diesel engine which includes the use of cluster nozzle. The concept of cluster nozzle is that it is possible to improve the condition of air fuel mixture by changing nozzle configuration. The cluster nozzle, grouping multi smaller holes together, has the potential to improve air fuel mixing in the center of spray. The cluster nozzle is characterized by high number of degree of freedom relate to spray figure, number of spray holes and configuration of holes in a group. Three cluster nozzles were tested with a CRI 3.3 piezo injector in a singlecylinder Duramax engine and compared with a conventional nozzle. The experiment was conducted under four test points and various rail pressures were applied to find an optimal rail pressure at each test point. Compared to conventional nozzle, ignition delay of cluster nozzle is shorter and the initial rate of heat release and the maximum rate of heat release of diffusion combustion are higher. Also cluster nozzles combined with high injection pressure shows better fuel consumption rate and emission level. The cluster nozzles achieved improved smokeNOx trade off while maintaining same NOx level with higher EGR. And three different mean swirl ratios were tested because the swirl is an important factor of spray formation and combustion process in combustion chamber. Swirl ratio variation was conducted to find an optimal swirl ratio for cluster nozzle at each test point. Among three different swirl ratio, 0.5, 1.5 and 3.0, better smoke emission was achieved with 1.5 and 3.0 swirl ratio compared with 0.5 swirl ratio. Between 1.5 and 3.0 swirl ratio, even though both resulted similar in smoke emission, 1.5 swirl ratios has lower BSFC and HC level than 3.0 swirl ratio. The findings form those experiments could be helpful for conceptualizing cluster nozzle for application in the whole operation range of diesel engine.

Beeckmann, J., Cai, L., Roehl, O., Pitsch, H. & Peters, N., A Reduced Kinetic Reaction Mechanism for the Autoignition of Dimethyl Ether. SAE Technical Paper Series, 2010012108. 2010.
A reduced kinetic reaction mechanism for the autoignition of dimethyl ether is presented in this paper. Dimethyl ether has proven to be one of the most attractive alternatives to traditional fossil fuels for compression ignition engines. It can either be produced from biomass or from fossil oil. For dimethyl ether, Fischer et al. (Int. J.Chem. Kinet. 32 ( 12 ) (2000) 713740) proposed a detailed reaction mechanism consisting of 79 species and 351 elementary reactions. In the present work, this detailed mechanism is systematically reduced to 31 species and 49 reactions. The reduced mechanism is discussed in detail with special emphasis on the high temperature thermal decomposition of dimethyl ether and on the fuel specific depleting reactions, which produce the methoxymethyl radical. In addition, a reaction pathway analysis for low temperature combustion is applied, where hydroperoxymethylformate is found to be the dominating parameter for the low temperature regime. A validation of the reduced mechanism is performed by calculating the ignition delay times in a pressure range from 1 bar to 40 bar, as well as at different equivalence ratios varying from 0.5 to 2.0 and temperatures from 625 K to 1650 K. These results are compared with experimental findings and values obtained using the detailed mechanism. Overall, very good agreement is achieved.

Gauding, M., Goebert, J.H. & Peters, N., The effect of filtering on gradient trajectories of scalar flow fields. In Proceedings of the 8th International ERCOFTAC Symposium on Engineering Turbulence Modelling and Measurements, 9th  11th June 2010, Marseille, France. pp. 588592. 2010.

Vanegas, A., Kerschgens, B. & Peters, N., The PCCI Combustion Process for Diesel Engine. A Key Element for upcoming Euro 6. In FISITA 2010 World Automotive Congress: Automobiles and sustainable mobility, 30th May  4th June 2010, Budapest, Hungary. 2010.
The Premixed Charge Compression Ignition, a lowtemperature combustion strategy, is one of the most important ways to achieve very low pollutant emissions without sacrificing power and fuel consumption. By using PCCI it is possible to drastically reduce NOx and soot emissions simultaneously. Before this combustion process can be fully implemented into the serial diesel engine, it is necessary to solve some difficulties like the higher unburned HC and CO emissions, as well as the early combustion phasing. In the present study, some strategies to solve this problem are introduced. The first strategy is to find the optimum spray cone angle experimentally and with numerical methods; the second strategy is to find the optimum piston bowl geometry; and the third strategy is to find a good injection strategy. The combination of these strategies shows that it is possible to drastically reduce the pollutant emissions through the PCCI combustion concept, which is used only for lowload conditions. The highload conditions are covered by the conventional diffusive combustion process. The transition from low to highload conditions should be done through the injection strategy by using a combustion closedloop control, which was developed by the Institute of Automatic Control at RWTH Aachen University. This control, however, will not be discussed in this paper.

Schaefer, P., Gampert, M. & Peters, N., Secondary splitting of zero gradient points in turbulent scalar fields. In Progress in Turbulence IV. pp. 35  39. 2010.
The mechanisms related to the secondary splitting of zero gradient points of scalar fields are analyzed using the two dimensional case of a scalar extreme point lying in a region of local strain. The velocity field is assumed to resemble a stagnation point flow, cf. Gibson 1, which is approximated using a Taylor expansion up to third order. It is found that the splitting can only be explained when the third order terms of the Taylor expansion of the flow field are included. The nondimensional splitting time turns out to depend on three parameters, namely the local Péeclet number Peδ based on the initial size of the extreme point δ and two parameters which are measures of the rate of change of the local strain. For the limiting case Peδ → 0, the splitting time is found to be finite but Péclet number independant, while for the case of Peδ → ∞ it increases logarithmically with the Péclet number.

Desjardins, O. & Pitsch, H., Detailed Numerical Investigation of Turbulent Atomization of
Liquid Jets. Atomization and Sprays, 20(4), pp.311336. 2010.

Beeckmann, J., Cai, L., Roehl, O., Pitsch, H. & Peters, N., A Reduced Kinetic Reaction Mechanism for the Autoignition of Dimethyl Ether. In 33rd International Symposium of Combustion, 2nd August 2010, Bejing, China. 2010.

Wada, T., Jarmolowitz, F., Abel, D. & Peters, N., An Instability of Diluted Lean Methane/Air Combustion: Modeling and Control. Combustion Science and Technology, 183(1), pp.119. 2010.
In order to study possibilities of Model Predictive Control (MPC) to low temperature combustion, the authors performed a numerical study of combustion with a lean highly diluted methane/air mixture in a perfectly stirred reactor using a detailed chemical kinetic model. Chosen conditions are the following: equivalence ratio 0.6, residence time of mixture in reactor 0.5 s; molar fraction of Nitrogen 0.9, temperature of incoming unburned gases and reactor 1100 K, and heat loss coefficient 2 × 10−3 cal/(cm2Ks). At these conditions strong oscillations are predicted in agreement with previous experimental findings of M. de Joannon et al. (200411.<br />
de Joannon , M. , Sabia , P. , A. Tregrossi , A. , and Cavaliere , A. 2004 . Dynamic behavior of methane oxidation in premixed flow reactor . Combust. Sci. Tech. , 176 , 769 . Taylor & Francis Online, Web of Science ®View all references). It is found that, at low temperatures (<1300 K), reactions forming CH3O and CH3O2 from CH3 had a strong influence on the oscillations. Once these reactions were subtracted from the kinetic model, no oscillations were observed. A virtual MPC suppressed the oscillations at low temperatures successfully and demonstrated extremely low NO and N2O emissions (<0.1 ppm).

Mellado, J.P., Wang, L. & Peters, N., Investigation of the Conditional Scalar Dissipation Rate Across a Shear Layer Using Gradient Trajectories. In Progress in turbulence III: ITi Conference in Turbulence 2008. Springer Proceedings in Physics. pp. 2124. 2010.
The dependence on the lateral distance y to the centerplane and on time t of the average of the scalar dissipation rate chi = 2D vertical bar del Z vertical bar(2) conditioned on the scalar Z has been investigated in a temporallyevolving shear layer using a direct numerical simulation. As the inviscid scaling of the unconditional mean dissipation rate is approached, the conditional mean (chi) over barz follows also an approximate selfsimilar behavior within the time interval considered in the simulations and (chi) over barz shows two strong peaks at a distance of about one vorticity thickness from the centerplane, with dissipation values about 3 times the one in the central region. It is showed that this spatial variation is introduced by the nonturbulent/turbulent transition regions, which are identified by means of gradient trajectories of the scalar field.

Knudsen, E.W. & Pitsch, H., LargeEddy Simulation for Combustion Systems: Modeling Approaches for partially premixed flows. The Open Thermodynamics Journal, 4(2), pp.7685. 2010.

Zeng, P., Binninger, B., Peters, N., Pitsch, H. & Herrmann, M., Modeling and Simulation of Primary Atomization with Phase Transition. Bulletin of the American Physical Society, 55(16), p.1 S. 2010.

Struckmeier, U., Lucassen, A., Hansen, N., Wada, T., Peters, N. & KohseHöinghaus, K., Demonstration of a burner for the investigation of partially premixed lowtemperature flames. Combustion and Flame, 157(10), pp.19661975. 2010.

Pitsch, H., Shedding New Light on a Burning Question. Journal of Fluid Mechanics, 658, pp.14. 2010.
Only recently, threedimensional direct numerical simulations (DNS) with detailed chemistry have become possible. These types of simulations will have an important impact on combustion science because of the data they provide. Obtaining similar data sets through experimental characterization is very challenging and requires joint measurements of a multitude of chemical species concentrations in addition to temperature and velocity fields. Gruber et al. (J. Fluid Mech., vol. 658, 2010, pp. 5–32) have performed a detailed chemistry DNS to understand the physics of flame/wall interactions. They found that flames interact with streamwise vortical structures close to the wall, and that these structures can push the flame towards or away from the wall. The flames get extinguish when they are too close to the wall. Interestingly, the extinction process actually causes an orderofmagnitude increase in heat release, and therefore, strongly amplifies wall heat transfer.

Narayanaswamy, K., Blanquart, G. & Pitsch, H., A consistent chemical mechanism for oxidation of substituted aromatic species. Combustion and Flame, 157(10), pp.18791898. 2010.
Computational studies of combustion in engines are typically performed by modeling the real fuel as a surrogate mixture of various hydrocarbons. Aromatic species are crucial components in these surrogate mixtures. In this work, a consistent chemical mechanism to predict the high temperature combustion characteristics of toluene, styrene, ethylbenzene, 1,3dimethylbenzene (mxylene), and 1methylnaphthalene is presented. The present work builds on a detailed chemical mechanism for high temperature oxidation of smaller hydrocarbons developed by Blanquart et al. Combust. Flame 156 (2009) 588–607. The base mechanism has been validated extensively in the previous work and is now extended to include reactions of various substituted aromatic compounds. The reactions representing oxidation of the aromatic species are taken from the literature or are derived from those of the lower aromatics or the corresponding alkane species. The chemical mechanism is validated against plug flow reactor data, ignition delay times, species profiles measured in shock tube experiments, and laminar burning velocities. The combustion characteristics predicted by the chemical model compare well with those available from experiments for the different aromatic species under consideration.

Gampert, M., Schaefer, P. & Peters, N., Testing of a new model equation for the mean dissipation via Kolmogorov flows. In Proceedings of the 8th International ERCOFTAC Symposium on Engineering Turbulence Modelling and Measurements, 9th  11th June 2010, Marseille, France. pp. 7378. 2010.

Luckhchoura, V., Kupiek, C., Jochim, B., Won, H.W. & Peters, N., Effects of the ClusterNozzles on the Engine Performance and Soot Formation in a Direct Injection Diesel Engine: Experiments and Numerical Simulations. In FISITA 2010 World Automotive Congress: Automobiles and sustainable mobility, 30th May  4th June 2010, Budapest, Hungary. 2010.

Drews, P., Albin, T., Hoffmann, K., Vanegas, A., Felsch, C., Peters, N. & Abel, D., ModelBased Optimal Control for PCCI Combustion Engines. In AAC 2010: 6th IFAC Symposium Advances in Automotive Control, 12th  14th July, 2010, Holiday Inn Munich – Schwabing, Germany. pp. 288  293. 2010.
New combustion methods for engines have been recently researched very intensively. In diesel engines, the homogenisation of the airfuel mixture by early fuel injection has significant effects on emission reduction. The paper presents a modelbased optimal control strategy for premixed charge compression ignition (PCCI) low temperature combustion in diesel engines. In order to understand the basic properties of the PCCI mode, static and dynamic measurements were conducted using a real conventional diesel engine. The main inputs of the combustion process are the exhaust gas recirculation rate and injection parameters. Outputs are the indicated mean effective pressure and the fuel mass conversion balance point. The process has very fast, almost proportional dynamics over the engine`s working cycles. Focusing on the static behaviour of the process, a nonlinear neural network model is used for identification. Successive linearisation of the nonlinear network is used as predictive controller model. The presented controller structure is able to consider constraints and can be computed very fast. Finally, the controller is validated under real time conditions by experimental tests at the engine test bench. Although the controller structure contains a model and a convex optimisation step with regards to constraints, its implementation is very simple, as no observer is used, and the linearised model consists of static gains only.

Gampert, M., Schaefer, P. & Peters, N., Testing of a new model equations for the mean dissipation via Kolmogorov flows. In Proceedings of the 8th International Symposium on Engineering Turbulence Modelling and Measurements, 9th  11th June 2010, Marseille, France. pp. 7378. 2010.

Zeng, P., Binninger, B., Peters, N. & Herrmann, M., Modeling primary atomization and interface evaporation in turbulent twophase flows by means of levelset method and secondorder boundary layer theory. In 5th workshop 'MicroMacro Modelling and Simulation of LiquidVapour Flows', 14th 16th April 2010, IRMA, Strassbourg. 2010.

Schäfer, P., Gampert, M., Göbbert, J.H., Wang, L. & Peters, N., Testing of Model Equations for the Mean Dissipation using Kolmogorov Flows. Flow, Turbulence and Combustion, 85(2), pp.225243. 2010.

Gampert, M., Schaefer, P. & Peters, N., Testing of a new model equations for the mean dissipation via Kolmogorov flows. In Proceedings of the 8th International ERCOFTAC Symposium on Engineering Turbulence Modelling and Measurements, 9th  11th June 2010, Marseille, France. pp. 7378. 2010.

Mellado, J.P., Stevens, B., Schmidt, H. & Peters, N., Probability density functions in the cloudtop mixing layer. New Journal of Physics, 12(8), p.085010. 2010.
The cloudtop mixing layer is an idealized configuration often employed in the literature to study local aspects (over length scales of the order of 10 m) of the top of stratocumulustopped mixed layers. Latent heat effects are further investigated here by means of direct numerical simulations, discussing the probability density functions of the horizontal and vertical velocities, as well as the mixture fraction (equal to a normalized enthalpy and total waterspecific humidity). The focus is on the turbulent convection layer that develops from the buoyancy reversal instability as a consequence of the evaporative cooling at the upper cloud boundary. An approximately selfsimilar behavior is found, where the convection scales based on the molecular buoyancy flux at the cloud top characterize the distributions at different times, at least to leading order and within the statistical convergence achieved in the simulations. However, a very strong vertical variation in the density functions across the turbulent convection layer is found, which is of relevance to possible models. NonGaussian behavior is often observed, even in the horizontal component of the velocity vector. In particular, large values of skewness and flatness are measured at the lower end of the turbulent zone, where external intermittency is very strong.

Schaefer, P., Gampert, M., Goebbert, J.H. & Peters, N., Dissipation element analysis of inhomogenous turbulence. In Computational Fluid Dynamics. pp. 711716. 2010.

Luckhchoura, V., Pardeshi, R., Sharma, A., Vogel, S., Peters, N., Cardénas, M. & Kneer, R., Investigation of the Opening Angle between Two Sprays in Diverging Nozzles. In ILASS Europe 2010: 23rd Annual Conference on Liquid Atomization and Spray Systems, 6th  8th September 2010, Brno, Czech Republic. p. 129. 2010.

Luckhchoura, V., Pardeshi, R., Sharma, A., Vogel, S., Peters, N., Cardénas, M. & Kneer, R., Influence of the opening angle between two sprays in diverging nozzles. ILASS – Europe 2010: 23rd Annual Conference on Liquid Atomization and Spray Systems, September 2010, Brno, Czech Republic. 2010.
In the cluster nozzle concept, each orifice of a conventional nozzle is replaced by two smaller orifices yielding the same flow rate. The opening angle between two sprays was positive, thus the nozzles were called diverging. In this study, numerical simulations of three cluster nozzles were performed in a chamber at 800 K ambient temperature and 50 bar ambient pressure conditions. First, the computed liquid and vapor penetration lengths were compared with the measured penetrations to validate the model. Then, the computed results were further analyzed to examine the influence of sprayspray interaction due to varying opening angle between the sprays on droplet breakup, spray shape and evaporation leading to mixture preparation.

Schäfer, P., Gampert, M., Göbbert, J.H., Gottschalk, D. & Peters, N., Geometrical Dissipation element analysis if inhomogenous turbulence. In NIC Symposium 2010. Schriften des Forschungszentrums Jülich: IAS Series Vol. 3. Jülich. 2010.

Mellado, J.P., Stevens, B., Schmidt, H. & Peters, N., Twofluid formulation of the cloudtop mixing layer for direct numerical simulation. Theoretical and Computational Fluid Dynamics, 24(6), pp.511536. 2010.
A mixture fraction formulation to perform direct numerical simulations of a disperse and dilute twophase system consisting of water liquid and vapor in air in local thermodynamic equilibrium using a twofluid model is derived and discussed. The goal is to understand the assumptions intrinsic to this simplified but commonly employed approach for the study of twolayer buoyancy reversing systems like the cloudtop mixing layer. Emphasis is placed on molecular transport phenomena. In particular, a formulation is proposed that recovers the actual nondiffusive liquidphase continuum as a limiting case of differential diffusion. Highorder numerical schemes suitable for direct numerical simulations in the compressible and Boussinesq limits are described, and simulations are presented to validate the incompressible approach. As expected, the Boussinesq approximation provides an accurate and efficient description of the flow on the scales (of the order of meters) that are considered.

Won, H.W. & Peters, N., Modified bowl geometry for cluster nozzles in directinjection diesel engines. Proceedings of the Institution of Mechanical Engineers / Part D, Journal of automobile engineering, 224(7), pp.953968. 2010.

Peters, N., Wang, L., Mellado, J.P., Goebbert, J.H., Gauding, M., Schaefer, P. & Gampert, M., Geometrical Properties Of Small Scale Turbulence. In NIC Symposium 2010, 24th  25th February 2010, Jülich, Germany. IAS series. pp. 365373. 2010.
In order to extract smallscale statistical information on turblulent flows from passive scalar fields obtained by direct numerical simulation (DNS), a new method of analysis is introduced. It consists of determining local minimum and maximum points of the fluctuating scalar field via gradient trajectories starting from every grid point in the directions of ascending and descending scalar gradients. The ensemble of grid cells from which the same pair of extremal points is reached determines a spatial region which is called dissipation element. Dissipation elements are highly convoluted and have on average an elongated shape. Their diameter is nearly constant and of the order of a few Kolmogorov scales while the length is highly variable and has a mean of the Taylor length. Since dissipation elements are spacefilling, it is possible to reproduce the overall statistics of the turbulent fields based on the comparably simple statistics of the elements.

Luckhchoura, V., Chivite, A., Vogel, S., Peters, N., Rottmann, M. & Pischinger, S., Modeling the InjectionRate Shapes in Diesel Engines. In Thermo and fluid dynamic processes in diesel engines: THIESEL 2010, 14th  17th September 2010, Valencia, Spain. 2010.

Wang, L. & Peters, N., Mean velocity increment conditioned on gradient trajectories of various scalar variables in turbulence. Physica Scripta, 2010(T142), p.014004. 2010.
This paper focuses on the generality of the velocity structure along the gradients of several field variables in scalar form, such as the passive scalar, the Cartesian projections of the velocity vector, kinetic energy and energy dissipation. These scalar variables behave differently, which can be explained mathematically by the fact that the source terms in their governing equations are different. Consequently, it is reasonable to expect some generalities among different scalars if the respective source terms are instantaneously small. The validity of this expectation has been discussed for individual cases, especially at high Reynolds numbers. It is found that the pointwise mean strain rates along gradients of various scalars are always negative. By analyzing the scalar gradient orientations, it is seen that statistically the gradient vectors with large magnitudes align with each other, whereas those with small magnitudes tend to be randomly organized. In terms of the twopoint velocity increment, for the passive scalar it has recently been derived that there exists a nearly linear scaling of the velocity difference with respect to the arc length of gradient trajectories (Wang 2009 Phys. Rev. E 79 046325). Theoretically, this scaling can also be extended to other scalars with satisfactory agreement with numerical results.

Kerschgens, B., Gauding, M., Felsch, C., Peters, N. & Hasse, C., A consistent flamelet model to describe the interaction of combustion chemistry and mixing in the controlled auto ignition regime. SAE Technical Paper Series, 2010010181. 2010.
In internal combustion engines operating in Controlled Auto Ignition (CAI) mode, combustion phasing and heatrelease rate is controlled by stratification of fuel, fresh air, and hot internally recirculated exhaust gases. Based on the Representative Interactive Flamelet (RIF) model, a twodimensional flamelet approach is developed. As independent parameters, firstly the fuel mixture fraction and secondly the mixture fraction of internally recirculated exhaust gases are considered. The flamelet equations are derived from the transport equations for species mass fraction and total enthalpy, employing an asymptotic analysis. A subsequent coordinate transformation leads to the phase space formulation of the twodimensional flamelet equations. By the use of detailed chemical reaction mechanisms, the effects of dilution, temperature, and chemical species composition due to the internally recirculated exhaust gases are represented. The new approach is tested in phase space, using different variations of operating conditions (i.e., variations of temperature, inhomogeneity, degree of turbulence) typical of CAI engines. Effects of temperature and the composition of the recirculated exhaust gases on combustion, combustion timing, and the location of auto ignition in phase space are individually studied and discussed. The effect of the chemical composition of recirculated exhaust gases is investigated by employing different amounts of oxygen and other chemical species in the internal EGR.

Gauding, M., Goebbert, J.H. & Peters, N., The effect of filtering on gradient trajectories of scalar flow fields. Proceedings of the 8th International ERCOFTAC Symposium on Engineering Turbulence Modelling and Measurements, pp.588592. 2010.

Beeckmann, J., Aye, M.M., Gehmlich, R. & Peters, N., Experimental Investigation of the Spray Characteristics of DinButyl Ether (DNBE) as an Oxygenated Compound in Diesel Fuel. SAE Technical Paper Series, 2010011502. 2010.
Increasing concern for the environment and the impending scarcity of fossil fuels requires continued development in hydrocarbon combustion science. For compressionignition engines, adding oxygenated compounds to the fuel can reduce noise, soot formation, and unburned hydrocarbons while simultaneously increasing thermal efficiency.<br />
<br />
In order to reliably model and design compressionignition engines to use new fuel blends, accurate spray characteristic data is required. In this study, the spray characteristics of various blends of the oxygenated compound dinbutyl ether (DNBE) with standard EN590 Diesel fuel are presented, including spray cone angle and spray penetration length for both liquid and gas phases. The experiments were conducted in a spray chamber at ambient conditions of 50 bar and 800 K, simulating TDC conditions in a Diesel engine. Injection pressures were varied from 7001600 bar. Miescatter and shadowgraphy techniques were used to measure the liquid phase and gas phase spray characteristics, respectively. The liftoff length and ignition delay is investigated using OHmeasurement technique. The spray structure of the DNBE alone and its blends with Diesel fuel have shown a larger spray angle and shorter penetration length than with the pure Diesel fuel, providing improvements to atomization behaviour, and thus expected increases in performance.

Zeng, P., Binninger, B., Peters, N. & Herrmann, M., Direct Numerical Simulation of Spray Primary Breakup with Phase Transition. In Spray 2010: 9. Workshop über Sprays, Techniken der Fluidzerstäubung und Untersuchungen von Sprühvorgängen, 3.5. Mai 2010, Heidelberg. pp. 309  316. 2010.
We perform direct numerical simulation on large distributed memory parallel computers in order to investigate the primary breakup process of diesel spray direct injection. Local refinement algorithm– Refined level–set method has been used to reduce the memory requirement, and we analyze the performance by experiments on a 1024– processor parallel computer.

Schaefer, P., Gampert, M., Göbbert, J.H., Gottschalk, D. & Peters, N., Geometrical Dissipation element analysis of inhomogenous turbulence. In NIC Symposium 2010, 24th  25th February 2010, Jülich, Germany. 2010.

Zeng, P., Binninger, B., Peters, N. & Herrmann, M., Numerical investigation of spray primary breakup with phase transition. In 8th International Symposium on Engineering Turbulence Modelling and Measurements, 9th  11th June 2010, Marseille, France. pp. 782787. 2010.

Beeckmann, J., Kruse, S. & Peters, N., Effect of Ethanol and nButanol on Standard Gasoline Regarding Laminar Burning Velocities. SAE Technical Paper Series, 2010011452. 2010.
Ethanol is frequently used as a blending component in standard gasoline, with blend rates up to 10%vol liq . nButanol has received recent interest as an alternative fuel instead of ethanol for use in spark ignition engines. Similar to ethanol, nbutanol can be produced via the fermentation of sugars, starches, and lignocelluloses obtained from agricultural feedstock.<br />
<br />
It is of great interest to modern engine development to understand the effect of ethanol and nbutanol as blending components on the laminar burning velocity of standard gasoline. The laminar burning velocity is one key parameter for the numerical simulation of gasoline engine combustion processes.<br />
<br />
Tested fuel components are ethanol, nbutanol, and standard marked gasoline without any oxygen content. Fuel blends consist of standardmarked gasoline containing ethanol and butanol. The maximum blend rate of oxygenates is 10%vol liq . Experiments were done at different equivalence ratios between 0.7 and 1.3. Test conditions in this work are pressures of 10 bar and temperatures of 373 K.<br />
<br />
The experimental setup consists of a spherical closed pressurized combustion vessel with optical access. Experimental results are discussed and compared with the numerical simulations, as far as models are available, and references from the literature.

Won, H.W. & Peters, N., Optimizing the Injection Pressure for Cluster Nozzle Concepts in DI Diesel Engines. International Journal of Engine Research, 11(2), pp.163174. 2010.
A combination of highpressure injection and small orifices will be one of the strategies to achieve lean combustion. However, equispaced small orifices tend to increase soot under highload condition because the spray tip penetration becomes exceedingly inadequate. For this reason, the cluster concept was chosen as a means to realize lean combustion. Six clusters were investigated with different injection pressures under partload condition and highload condition in a singlecylinder diesel engine, and the results were compared with a reference nozzle. The clusters tend to produce higher smoke than the reference nozzle for low injection pressures under conventional injection timing because the spray from the clusters with a shorter spray tip penetration loses momentum near the piston bowl. However, the clusters show improved smoke emissions with higher injection pressures. The combination of highpressure injection and cluster concepts can be seen as one of the solutions to achieve lean combustion for diesel engines. Clusters with highpressure injection have improved emissions, as better fuel atomization and evaporation are achieved while holding momentum near the piston bowl and maintaining the penetration of the spray.

Luckhchoura, V., Robert, F.X., Peters, N., Rottmann, M. & Pischinger, S., Experimental and Numerical Investigation of Injection Rate Shaping in a SmallBore DirectInjection Diesel Engine. In Towards Clean Diesel Engines: 7th International Symposium, 4th  5th June, AGIT Technology Center, Aachen, Germany. CEUR Workshop Proceedings. 2009.

Jerzembeck, S., Peters, N., PepiotDesjardins, P. & Pitsch, H., Laminar burning velocities at high pressure for primary reference fuels and gasoline: Experimental and numerical investigation. Combustion and Flame, 156(2), pp.292301. 2009.
Spherical flames of n heptane, isooctane, PRF 87 and gasoline/air mixtures are experimentally investigated to determine laminar burning velocities and Markstein lengths under enginerelevant conditions by using the constant volume bomb method. Data are obtained for an initial temperature of 373 K, equivalence ratios varying from ϕ=0.7ϕ=0.7 to ϕ=1.2ϕ=1.2, and initial pressures from 10 to 25 bar. To track the flame front in the vessel a dark field He–Ne laser Schlieren measurement technique and digital image processing were used. The propagating speed with respect to the burned gases and the stretch rate are determined from the rate of change of the flame radius. The laminar burning velocities are obtained through a linear extrapolation to zero stretch. The experimentally determined Markstein numbers are compared to theoretical predictions. A reduced chemical kinetic mechanism for nheptane and isooctane was derived from the Lawrence Livermore comprehensive mechanisms. This mechanism was validated for ignition delay times and flame propagation at low and high pressures. In summary an overall good agreement with the various experimental data sets used in the validation was obtained.

Honnet, S., Seshadri, K., Niemann, U. & Peters, N., A surrogate fuel for kerosene. Proceedings of the Combustion Institute, 32(1), pp.485492. 2009.
Experimental and numerical studies are carried out to develop a surrogate that can reproduce selected aspects of combustion of kerosene. Jet fuels, in particular JetA1, JetA, and JP8 are kerosene type fuels. Surrogate fuels are defined as mixtures of few hydrocarbon compounds with combustion characteristics similar to those of commercial fuels. A mixture of ndecane 80% and 1,2,4trimethylbenzene 20% by weight, called the Aachen surrogate, is selected for consideration as a possible surrogate of kerosene. Experiments are carried out employing the counterflow configuration. The fuels tested are kerosene and the Aachen surrogate. Critical conditions of extinction, autoignition, and volume fraction of soot measured in laminar non premixed flows burning the Aachen surrogate are found to be similar to those in flames burning kerosene.<br />
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A chemicalkinetic mechanism is developed to describe the combustion of the Aachen surrogate. This mechanism is assembled using previously developed chemicalkinetic mechanisms for the components: ndecane and 1,2,4trimethylbenzene. Improvements are made to the previously developed chemicalkinetic mechanism for ndecane. The combined mechanisms are validated using experimental data obtained from shock tubes, rapid compression machines, jet stirred reactor, burner stabilized premixed flames, and a freely propagating premixed flame. Numerical calculations are performed using the chemicalkinetic mechanism for the Aachen surrogate. The calculated values of the critical conditions of autoignition and soot volume fraction agree well with experimental data. The present study shows that the chemicalkinetic mechanism for the Aachen surrogate can be employed to predict non premixed combustion of kerosene.

Wada, T. & Peters, N., Investigation of Chemical Kinetics in ChemicalInduced Oscillations with Diluted Lean CH4/Air Mixture at Low Temperatures. In 7th AsiaPacific Conference on Combustion: National Taiwan Univ. Hospital, Intern. Convention Center, 24th  27th May, 2009, Taipei, Taiwan. 2009.
Numerical simulations of oscillation phenomena are performed using a perfectly stirred reactor model. Reactions were carried out at low temperatures under at mospheric pressure, and diluted lean CH4/air mixtures were used. To specify a predominant reaction path, a skeletal kinetic model was created from a detailed model and evaluated using reaction flow analyses. The results show that reactions from CH3 to CH3O and CH3O2 are the most important reactions with respect to oscillations and indicate the critical temperature of low and hightemperature chemistries. On the basis of the obtained results, we describe the mechanism of all the oscillations in the reactor and extend our fundamental understanding of hightemperature chemistry to chemical phenomena observed below the critical temperature.

Dahms, R., Fansler, T.D., Drake, M.C., Kuo, T.W., Lippert, A.M. & Peters, N., Modeling ignition phenomena in sprayguided sparkignited engines. Proceedings of the Combustion Institute, 32(2), pp.27432750. 2009.
An ignition model for sprayguided (SG) directinjection gasoline engines called SparkCIMMSpark Channel Ignition Monitoring Modelis presented in this paper. The model concept is motivated by highspeed imaging data showing complex processes for spark (formation, turbulent stretching and wrinkling, and multiple restrikes) and ignition (localized flame kernel formation and growth). Turbulent fluctuations occurring on the scale of turbulent sparkchannel wrinkling (similar to 0.050.1 mm) and local ignition (similar to 0.10.5 mm) are analyzed using expressions for the subgrid turbulent kinetic energy, eddy turnover velocity and mixturefraction variance. Computational particles are introduced along the spark channel to monitor its motion and ignitability. Ignition is determined by a local Karlovitznumber criterion that incorporates effects of turbulence, detailed chemical kinetics, and continuous energy deposition from the spark. Wherever suitable ignition conditions are found along the elongated spark channel, a small flame kernel is launched and tracked as it grows and merges with other flame kernels to form the turbulent flame surface. When the flame surface is sufficiently large, a Gequation flamelet combustion model tracks the turbulent flame front. Both the early flamekernelgrowth and Gequation combustion models include effects of local mixturefraction variance. Results from the SparkCIMM ignition model and Gequation combustion model are compared with experiments in an SG directinjection engine. The model reproduces the general experimental features of sparkchannel stretching, corrugation, and localized ignition; and flame front location probabilities are in good agreement. (c) 2009 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Kerschgens, B., Felsch, C., Vanegas, A. & Peters, N., Applying an Interactively Coupled CFDMultiZone Approach to Study the Effects of Piston Bowl Geometry Variations on PCCI Combustion. SAE International Journal of Engines, 2(1), pp.17931810. 2009.
Recently, a consistent mixing model for the twoway coupling of a CFD code and a zerodimensional multizone code was developed. This work allowed for building an interactively coupled CFDmultizone approach that can be used to model HCCI combustion.<br />
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In this study, the interactively coupled CFDmultizone approach is applied to PCCI combustion in a 1.9l FIAT GM Diesel engine. The physical domain in the CFD code is subdivided into multiple zones based on one phase variable (fuel mixture fraction). The fuel mixture fraction is the dominant quantity for the description of nonpremixed combustion. Each zone in the CFD code is represented by a corresponding zone in the zerodimensional multizone code. The zerodimensional multizone code solves the chemistry for each zone, and the heat release is fed back into the CFD code. The thermodynamic state of each zone, and thereby the phase variable, changes in time due to mixing and source terms (e.g., vaporization of fuel, wall heat transfer). An elaborated mixing model and an appropriate treatment of the source terms keep the thermodynamic state of the zones in the CFD code and the zerodimensional multizone code identical.<br />
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The results obtained with the interactively coupled CFDmultizone approach are compared to experimental results for five selected operating conditions, showing very good agreement. In a following step, the engine geometry used in the numerical simulations is modified, yielding two different pistonbowl geometries which vary from the original shape in regard to bowl diameter and bowl depth. The volume of the bowl and thus the compression ratio are kept at the original value. Simulations of PCCI combustion using these modified geometries show the influence of the pistonbowl shape on mixture formation and pollutant emissions, leading the way to numerically optimise the pistonbowl design for PCCI combustion.

Peters, N., Multiscale combustion and turbulence. Proceedings of the Combustion Institute, 32(1), pp.125. 2009.
Multiscale physics is the interaction of different physical processes occurring at largely separated scales. In combustion, many elementary reactions combine to only a few, but still have separated time scales. In flames, owing to the presence of diffusion, time scales manifest themselves as length scales, i.e. thicknesses of reaction layers embedded within each other. For premixed flames there results a single velocity scale, the laminar burning velocity, which in turn defines a flame thickness and a flame time as global length and time scales, respectively. The laminar burning velocity represents the simplest microscale model to be used at a premixed combustion interface.<br />
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While combustion is a multiscale process, this is not so evident for turbulence. Based on the picture of a cascade process traditional turbulent closure approximations treat turbulence as a singlescale problem. Attempts to model turbulent combustion in the same way by using methods developed for nonreacting turbulent flows therefore must fail, because they ignore the multiscale nature of combustion.<br />
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There is, however, a long tradition and much progress in multiscale modeling of combustion, both on the macroscale as well as on the microscale level. Unfortunately much of that work is conceived only in its particular context, not as part of a multiscale approach. For instance, papers in the TURBULENT FLAMES Colloquium and the FIRE RESEARCH Colloquium at this and at previous Combustion Symposia often take the viewpoint of macroscale modeling only, while REACTION KINETICS and LAMINAR FLAMES concentrate on microscale aspects. What seems to be needed is a more explicit reference to the needs of models developed in the other parts of the community. Furthermore, research is needed to develop suitable definitions of the interface between macroscale and microscale models.

Mellado, J.P., Schmidt, H., Stevens, B. & Peters, N., DNS of the turbulent cloudtop mixing layer. In Advances in turbulence XII: proceedings of the 12th EUROMECH European Turbulence Conference, 7th  10th September 2009, Marburg, Germany. Springer Proceedings in Physics. pp. 401404. 2009.
The turbulent cloudtop mixing layer is studied using direct numerical simulation (DNS). This configuration models the top of stratocumulus clouds and is employed to investigate the role of latent heat effects. A partial description of the turbulent flow that develops when the cloud and the cloudfree air mix under buoyancy reversal conditions is presented in this paper.

Beeckmann, J., Keppner, J., Glatz, T. & Peters, N., Laminar Burning Velocities of Dimethyl Ether, nHeptane and isoOctane at High Pressure. SAE Technical Paper Series, 2009012656. 2009.
Oxygenates, such as methanol or ethanol, are frequently used as blending components in standard gasoline. One oxygenate, dimethyl ether (DME), is also used as a fuel component in some regions of the world, for example in Asia. In addition, patent reviews show the potential of DME as a blending component in liquefied petroleum gas (LPG) or mixed with propane. The laminar burning velocity is one key parameter for the numerical simulation of gasoline engine combustion processes. Therefore, it is of great interest for modern engine development to understand the effect of oxygenates on the laminar burning velocity.<br />
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The experimental results have been conducted under enginelike conditions with elevated initial pressures of up to 20 bar and initial temperatures of 373 K. Experiments were done at equivalence ratios between 0.8 and 1.3. The experimental setup consists of a spherical closed pressurized combustion vessel with optical access. The filling procedure and filling setup have been significantly improved compared to previous work done in the same apparatus. Schlieren measurements coupled with a highspeed camera are used for image acquisition to track the expanding flame front. A post processing tool is used to extrapolate the measurements to zero stretch. Tested fuel components were DME, nheptane and isooctane. Nheptane and isooctane results are new data with minimised standard deviations achieved by an optimised filling procedure and therefore presented in this study. Additionally, the experimental results obtained are compared to numerical simulations. Hightemperature chemical kinetic models have been used for calculating laminar burning velocities of the considered fuel components. Finally, the experimental and numerical results are discussed in detail in conjunction with references found in literature.

Beeckmann, J., Röhl, O. & Peters, N., Numerical and Experimental Investigation of Laminar Burning Velocities of isoOctane, Ethanol and nButanol. SAE Technical Paper Series, 2009012784. 2009.
Fuels containing oxygenates have become more and more important for spark ignition engines in recent years. Oxygenates are either used as an octane booster or as a biofuel component for fulfilling legislative regulations. Ethanol has been well established for blend rates up to 10%vol liq . On the other hand butanol has been introduced as an alternative biofuel component.<br />
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The effect of the laminar burning velocity of different fuel components on modern engine development is investigated by conducting experiments under high initial pressure and temperature. Initial conditions in this work are a pressure of p = 10 bar and a temperature of T = 373 K. Experiments were done at different fuel  air ratios between 0.8 and 1.3. Test fuels were the pure fuel components isooctane, ethanol and nbutanol. Different chemical kinetic mechanisms for isooctane, ethanol and nbutanol from literature are used to calculate laminar burning velocities. The experimental setup consists of a spherical closed pressurized combustion vessel with optical access. Schlieren measurements coupled with a high speed camera are used for image acquisition to track the expanding flame front. Finally, a post processing tool is used to extrapolate the measurements to zero stretch. Numerical results are discussed in detail and compared with the experimental results as well as references from the literature.

Jerzembeck, S., Glawe, C. & Peters, N., Development and Experimental Evaluation of a High Temperature Mechanism for blended NHeptaneIsooctaneEthanolAirMixtures and GasolineEthanolAirMixtures. In Recent Advances in Energy and Environment: 4th IASME/WSEAS International Conference on Energy and Environment, 24th  26th March, 2009, Cambridge, England. Energy and Environmental Engineering Series. pp. 7883. 2009.
Laminar burning velocity measurements using the closed vessel bomb method have been done for fuel blendairmixtures at 373 K initial temperature and up to 20 bar initial pressure. The two experimentally investigated fuel blends consist, on the one hand, of 78.3 % vol. isooctane, 11.7 % vol. nheptane, and 10 % vol. ethanol, and on the other hand of 90 % vol. standard gasoline and 10 % vol. ethanol. A detailed hightemperature chemical kinetic model has been developed with the focus on calculating laminar burning velocities of these mixtures. The reduced high temperature mechanism developed by Jerzembeck et al. 1 for PRFs (Primary Reference Fuels consisting of isooctane and nheptane) is the basis for the present work. The dominant reactions of ethanol were added to this mechanism. These reactions were taken from the mechanism of Marinov et al. 2. Laminar burning velocities calculated with the newly developed mechanism are in good agreement with the experimental results for low and high pressure mixtures, as well as for single fuel and blended fuel air mixtures.

Won, H.W. & Peters, N., Investigation of Clusternozzle Concepts for Direct Injection Diesel Engines. Atomization and Sprays, 19(10), pp.983996. 2009.

Mellado, J.P., Wang, L. & Peters, N., Gradient trajectory analysis of a scalar field with external intermittency. Journal of Fluid Mechanics, 626(1), pp.333365. 2009.
The passive scalar field of a temporally evolving shear layer is investigated using gradient trajectories as a means to analyse the scalar probability density function and the conditional scalar dissipation rate in the presence of external intermittency. These results are of significance for turbulent combustion, where improved predictions of the statistics of the conditional dissipation rate are needed in several models. First, the variation of the conventional first and second moments of the conditional dissipation rate across the layer is quantitatively documented in detail. A strong dependence of the conditional dissipation rate on the lateral position and on the conditioning value of the scalar is observed. The dependence on the transverse distance to the centreplane partially explains the doublehump profile usually reported when this dependence is ignored. The variation with the scalar observed in the ratio between the second and first moments would invalidate certain assumptions commonly done in turbulent combustion. It is also seen that conditioning on the scalar does not reduce the fluctuation of the dissipation rate with respect to unconditional values. Next, the role of external intermittency in these results is investigated. For that purpose, the flow is partitioned into different zones based on different types of gradient trajectories passing through each point, thereby introducing nonlocal information in comparison with the standard turbulent/nonturbulent separation based on the conventional intermittency function. In addition to the homogeneous outer regions, three zones are identified: a turbulent zone, a turbulence interface and quasilaminar diffusion layers. The relative contribution from each of these zones to the conventional intermittency factor is reported. The statistics are then conditioned on each of these zones, and the spatial variation of the scalar distribution and of the conditional scalar dissipation rate is explained in terms of the observed zonal statistics. For the Reynolds numbers of the present simulation, between 1500 and 3000 based on the vorticity thickness and the velocity difference, and a Schmidt number equal to 1, it results that the major contribution to both statistics is due to the turbulence interfaces. At the same time, the turbulent zone shows a distinct behaviour, being approximately homogeneous but anisotropic.

Rottmann, M., Menne, C., Pischinger, S., Luckhchoura, V. & Peters, N., Injection rate shaping investigations on a smallbore DI diesel engine. SAE Technical Paper Series, 2009010850. 2009.
So far, the effect of injection rate shaping on the diesel combustion in smallbore DI diesel engines has not been extensively investigated, especially at high partload conditions with high EGR rates. The benefit of injection rate shaping is already verified for heavyduty engines at highload conditions with and without EGR. For this investigation, singlecylinder engine investigations were conducted at the VKA/RWTH Aachen University.<br />
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In order to meet the future NOX legislation limits like US Tier2Bin5 it is crucial to reduce NOX especially at the highload points of the certification cycles, as FTP75 or US06. For the singlecylinder investigations two partload points were chosen, which have relevance for the mentioned certification cycles. The experimental work focuses on different rate shapes as rectangular (CommonRail type), ramp and boot shape at high EGR rates.<br />
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 Ramp and boot shapes were optimized in terms of angle (ramp shape) and length and height (boot shape) in order to find the best emission, efficiency and combustion noise result.<br />
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 The resulting rate shapes were compared in variations of injection timing for constant EGR rates.<br />
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 The measurement data was analyzed in order to find explanations for the observed engine behavior. This was done by means of zerodimensional heat release calculations and threedimensional CFD calculations with the opensource program KIVA 3V.<br />
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 Additionally, CFD calculations were conducted by the Institute of Technical Combustion, using a Representative Interactive Flamelet (RIF) model. This paper contains the model calibration to the experimental results and first emission calculations.<br />
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The comparison of rectangular, ramp and boot injection shows, as expected, a retardation of the combustion for the ramp shape and especially the boot shape in comparison to the rectangular shape. For constant start of injection and constant EGR rate, this leads to reduced NOX emission and improved combustion noise especially for the boot injection shape, without drawbacks in the other emissions or engine efficiency.

Jarmolowitz, F., Abel, D., Wada, T. & Peters, N., Control of a homogeneous stirred reactor: Trajectory piecewiselinear model for NMPC. In Proceedings of the European Control Conference 2009, 23rd  26th August 2009, Budapest, Hungary. pp. 23012306. 2009.
As a consequence of increasing computational power and improved algorithms, model predictive control (MPC) nowadays is becoming applicable to highdynamic processes. If using nonlinear MPC strategies (NMPC), e.g. sequential quadratic programmingsolvers (SQP), it is reasonable, if not mandatory to reduce the prediction model in order to cope with realtime. In this work the trajectory piecewiselinear approach (TPWL) as a model order reduction (MOR) scheme is investigated in simulation on its applicability on controlling combustion instabilities in an open homogeneous reactor using a SQP strategy. The corresponding nonlinear model exhibits, like the experiment, temperature oscillations during combustion. The temperature oscillations are due to chemical kinetics and typical for homogeneous lowtemperature combustion. Since chemical kinetics are highly nonlinear, NMPC is a promising approach to damp these oscillations and to control the system in different operating points.

Felsch, C., Kerschgens, B., Vanegas, A., Glawe, C., Peters, N., Hoffmann, K., Drews, P., Abel, D. & Barths, H., Systematische Reduktion eines interaktiv gekoppelten CFDMehrzonenmodells zu einem rechenzeitoptimierten eigenständigen Mehrzonenmodell für die PCCI Verbrennung. In Motorische Verbrennung  aktuelle Probleme und moderne Lösungsansätze: 19./20. März 2009, Haus der Technik, München. Berichte zur Energie und Verfahrenstechnik. pp. 381393. 2009.

Spiekermann, P., Jerzembeck, S., Felsch, C., Vogel, S., Gauding, M. & Peters, N., Experimental data and numerical simulation of commonrail ethanol sprays at diesel enginelike conditions. Atomization and Sprays, 19(4), pp.357386. 2009.
CommonRail ethanol sprays are experimentally investigated in a highpressure vessel for pressures up to 50 bar and temperatures up to 800 K. Mie, Shadow, and Raman optical measurement techniques are used for the spray investigations. The experimental setup and the measurement techniques are described in detail. Detailed experimental results are presented for various vessel conditions. This includes liquid and gaseous penetration, spray angle of liquid and gaseous phase, liquid and vapor fuel mass fraction, liquid and vapor temperature, as well as Sauter mean radius. The experimental results provide a considerable dataset for numerical validation purposes. In addition to the experimental investigations, numerical simulations are carried out applying the commonly used discrete droplet model (DDM). A short review of the DDM including its inherent submodels is presented along with the simulation setup. Selected experimental results (liquid and gaseous penetration, liquid and vapor fuel mass fraction, liquid and vapor temperature, and Sauter mean radius) are compared to corresponding results obtained from the spray simulations to evaluate the performance of the DDM approach.

Vanegas, A. & Peters, N., Experimental analysis of the effect of very early pilot injection on pollutant formation for a PCCI Diesel Engine. In Towards Clean Diesel Engines: 7th International Symposium, 4th  5th June, 2009, AGIT Technology Center, Aachen, Germany. CEUR Workshop Proceedings. 2009.

Felsch, C., Gauding, M., Hasse, C., Vogel, S. & Peters, N., An extended flamelet model for multiple injections in DI Diesel engines. Proceedings of the Combustion Institute, 32(2), pp.27752783. 2009.
Combustion modeling using the Representative Interactive Flamelet (RIF) model has proven successful in predicting Diesel engine combustion. The RIF model was previously used for Diesel engine combustion processes with not more than two consecutive injections into the combustion chamber. In this study, the RIF model is extended allowing for any number of injection events.<br />
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First, the twodimensional laminar flamelet equations, which can describe the transfer of heat and mass between twointeracting mixture fields, are introduced. This is followed by a description of the various mixture fraction and mixture fraction variance equations that are required for the model extension accounting for multiple injection events. Finally, the modeling strategy for multiple injection events is derived: Different phases of combustion and interaction between the mixture fields resulting from different injections are identified. Based on this, the extension of the RIF model to describe any number of injections is put forward.<br />
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Simulation results using the extended RIF model are compared against experimental data for a CommonRail DI Diesel engine that was operated with three injection pulses. For the pilot injection and the main or post injection, respectively, different ignition phenomena are pointed out and the influence of the scalar dissipation rate on these ignition phenomena is investigated in detail.

Mellado, J.P., Stevens, B., Schmidt, H. & Peters, N., Buoyancy reversal in cloudtop mixing layers. Quarterly Journal of the Royal Meteorological Society, 135(641), pp.963978. 2009.
A theoretical and numerical smallscale study of the evaporative cooling phenomenon that might appear in the stratocumulustopped boundary layers is presented. An ideal configuration of a cloudtop mixing layer is considered as defined by two nonturbulent horizontal layers, stably stratified and with buoyancy reversal within a certain range of mixture fractions due to the evaporative cooling. Linear stability analysis of the shearfree configuration is employed to provide a new interpretation of the buoyancy reversal parameter, namely in terms of a timescale ratio between the stable and the unstable modes of the system. An incompressible highorder numerical algorithm to perform direct numerical simulation of the configuration is described and twodimensional simulations of singlemode perturbations are presented. These simulations confirm the role of the different parameters identified in the linear stability analysis and show that convoluted flow patterns can be generated by the evaporative cooling even for the low levels of buoyancy reversal found in stratocumulus clouds. They also show that there is no enhancement of turbulent entrainment of upperlayer fluid in the shearfree configuration, and turbulent mixing enhancement by the evaporative cooling is restricted to the lower layer.

Zeng, P., Binninger, B., Peters, N. & Herrmann, M., Simulation of Primary Breakup for Diesel spray with phase transition. In Towards Clean Diesel Engines: 7th International Symposium, 4th  5th June 2009, AGIT Technology Center, Aachen, Germany. p. 1. 2009.

Felsch, C., Dahms, R., Glodde, B., Vogel, S., Peters, N., Barths, H., Sloane, T., Wermuth, N. & Lippert, A., An Interactively Coupled CFDMultiZone Approach to Model HCCI Combustion. Flow, Turbulence and Combustion, 82(4), pp.621641. 2009.
The objective of this work is to present an innovative interactively coupled CFDmultizone approach. In a consistent manner, the approach combines detailed flow field information obtained from CFD with detailed chemical kinetics solved in a multizone model. Combustion and pollutant formation in an HCCI engine with recompressing VVA strategy are numerically investigated using the interactively coupled CFDmultizone approach. A surrogate fuel for gasoline is used in the simulation that consists of nheptane (18% liquid volume fraction) and isooctane (82% liquid volume fraction). The underlying complete reaction mechanism comprises 482 elementary reactions and 115 chemical species. The interactively coupled CFDmultizone approach shows to be accurate enough to describe HCCI chemistry, and is at the same time economical enough to allow application in an industrial environment. For the test case investigated, the simulation results are compared to experimental data that has been obtained using real gasoline. The overall agreement between simulation and experiment is found to be very good.

Wada, T., Mizomoto, M., Yokomori, T. & Peters, N., Extinction of methane/air counterflow partially premixed flames. Proceedings of the Combustion Institute, 32(1), pp.10751082. 2009.
Flame interactions of partially premixed flames (PPFs) were experimentally and numerically studied using counterflow configurations with CH4/air mixtures under atmospheric condition. The main aims of this study were to describe the flame interactions in terms of variations in global strain rate (K) and to understand flame extinction. Two premixedfuel streams were characterized by equivalence ratios (ϕlean and ϕrich). Experiments were conducted under the following conditions: ϕlean + ϕrich = 2, 0.5 < ϕlean < 0.7 (i.e., 1.3 < ϕrich < 1.5), and several K values. To describe the interactions, twin flames (TFs), also examined under corresponding ϕ, were used as a criterion, because the ϕ values were within the flammability limits. Experimental results of PPFs showed a different behavior than those of TFs: first, three different luminous layers were observed; second, thin singlelayered flames, with a nearly constant flame width, were observed near extinction for several combinations of ϕ; third, PPFs could exist for significantly higher K than TFs. Numerical results supported the experimental results and showed that two reaction paths, CH4 + OH → CH3 + H2O and CH3 + O → CO + H2 + H, were remarkably intensified by flame interaction of PPF near extinction; the significance of the reaction paths was determined. In addition, it was found that excess CO production plays an important role in extinction of PPFs.

Felsch, C., Hoffmann, K., Vanegas, A., Drews, P., Albin, T., Abel, D. & Peters, N., A dynamic PCCI combustion model for Diesel engine control design. In 7th International Symposium Towards Clean Diesel Engines: 4th  5th June 2009, AGIT Technology Center, Aachen, Germany. p. 3. 2009.
The subject of this work is the derivation of a simulation model for premixed charge compression ignition (PCCI) combustion that can be used in closedloop control development. For the highpressure part of the engine cycle, a detailed threedimensional computational fluid dynamics model is reduced to a standalone multizone chemistry model. This multizone chemistry model is extended by a mean value model accounting for the gas exchange losses. The resulting model is capable of describing PCCI combustion with stationary exactness, and is at the same time very economic with respect to computational costs. The model is further extended by the identified system dynamics that influence the stationary inputs. For this purporse, a Wiener model is set up that uses the stationary model as a nonlinear system representation. In this way, a dynamic nonlinear model for the representation of the controlled plant diesel engine is created.

Bui, T.P., Ihme, M., Schroeder, W. & Pitsch, H., Analysis of different sound source formulations to simulate combustion generated noise using a hybrid LES/APERF method. International Journal of Aeroacoustics, 8(12), pp.95123. 2009.
Combustion noise and sound source mechanisms of the DLRA flame are investigated. A hybridlargeeddy simulation /computational aeracoustics (LES/CAA) approach is employed. In the first step of the hybrid analysis the flamelet/progress variable (FPV) model is employed as combustion model followed by the acoustic simulation in the second step using the acoustic perturbation equations for reacting flows (APERF). The flamelet/progress variable database has been extended in terms of acoustic source terms. The analysis of the acoustic field of low MACH number reacting flows induced by the thermoacoustic sources such as the unsteady heat release leads to a very stiff problem formulation, since the related sources require highly resolved regions in the source area, which restricts the possible time step. To simulate combustion generated noise using such a hybrid approach, a suitable source description has to be used, which preferably satisfies two requirements, i.e, to efficiently and accurately predict the generated sound field, while the source term can be easily evaluated from the LES. Using the source term, which is expressed via the scaled partial time derivative of the density, the acoustic field can be reproduced best up to a maximum STROUHAL number of StD = 2. However, this source formulation requires a rigorous constraint at the interface of the hybrid approach to avoid spurious noise due to artificial acceleration caused by interpolation. To be more precise, the convection speed of density inhomogeneities has to be preserved during interpolation. A compromise between efficiency and accuracy can be achieved using the source formulation expressed via the scaled<br />
material derivative of the density, since by definition this formulation does not describe the convection of density inhomogeneities.

Gauding, M., Felsch, C., Kerschgens, B., Vanegas, A., Won, H.W., Peters, N. & Hasse, C., Applying an Extended Flamelet Model for a Multiple Injection Operating Strategy in a CommonRail DI Diesel Engine. SAE International Journal of Engines, 2(1), pp.727741. 2009.

Felsch, C., Hoffmann, K., Vanegas, A., Drews, P., Barths, H., Abel, D. & Peters, N., Combustion model reduction for diesel engine control design. International Journal of Engine Research, 10(6), pp.359387. 2009.
The subject of this work is the derivation of a simulation model for premixed charge compression ignition (PCCI) combustion that can be used in closedloop control development. For the highpressure part of the engine cycle, a detailed threedimensional computational fluid dynamics model is reduced to a standalone multizone chemistry model. This multizone chemistry model is extended by a mean value model accounting for the gas exchange losses. The resulting model is capable of describing PCCI combustion with stationary exactness, and is at the same time very economic with respect to computational costs. The model is further extended by the identified system dynamics that influence the stationary inputs. For this purporse, a Wiener model is set up that uses the stationary model as a nonlinear system representation. In this way, a dynamic nonlinear model for the representation of the controlled plant diesel engine is created.

Moureau, V., Fiorina, B. & Pitsch, H., A Level Set Formulation for Premixed Combustion LES Considering the Turbulent Flame Structure. Combustion and Flame, 156(4), pp.801812. 2009.
In this paper, a consistent and rigorous formulation is developed for the coupling of the Gequation model to an LES flow solver that describes the interactions of the scales of the flame, the turbulence, and the filtering procedure from the resolved turbulence regime to the broadened preheat regions regime. A progress variable equation is introduced to describe the filtered flame structure. The models provided for the subfilter diffusivity and the filtered reaction term appearing in this equation are consistent with the solution of the Gequation model. The solution of the progress variable equation ensures that the resolved part of the turbulent mixing in the preheat region can be described. However, the Cfield is underresolved if the subfilter Damköhler number is not much smaller than unity, and hence the solution of the Cequation cannot be expected to produce the correct flame propagation speed. The coupling with the Gequation ensures that the flame front described by the filtered reaction progress variable moves with the correct propagation velocity, independent of numerical diffusion caused by an underresolution of the flame. Formulations both for lowMach number flow solvers and for fully compressible solvers are presented. To validate the formulation, the model is applied in compressible LES of two turbulent flames anchored by a triangular flameholder. For the statistically stationary case, the mean and RMS progress variable are in very good agreement with experimental data, demonstrating that the model correctly reproduces the flame anchoring and the flame–turbulence interactions in the recirculation zone. For the acoustically pulsed case, the LES fields show the same large scale fluctuations that are present in the experimental data.

Knudsen, E. & Pitsch, H., A general flamelet transformation useful for distinguishing between premixed and nonpremixed modes of combustion. Combustion and Flame, 156(3), pp.678696. 2009.
The flame index was originally proposed by Yamashita et al. as a method of locally distinguishing between premixed and nonpremixed combustion. Although this index has been applied both passively in the analysis of direct numerical simulation data, and actively using single step combustion models, certain limitations restrict its use in more detailed combustion models. In this work a general flamelet transformation that holds in the limits of both premixed and nonpremixed combustion is developed. This transformation makes use of two statistically independent variables: a mixture fraction and a reaction progress parameter. The transformation is used to produce a model for distinguishing between premixed and nonpremixed combustion regimes. The new model locally examines the term budget of the general flamelet transformation. The magnitudes of each of the terms in the budget are calculated and compared to the chemical source term. Determining whether a flame burns in a premixed or a nonpremixed regime then amounts to determining which sets of these terms most significantly contribute to balancing the source term. The model is tested in a numerical simulation of a laminar triple flame, and is compared to a recent manifestation of the flame index approach. Additionally, the model is applied in a presumed probability density function (PDF) large eddy simulation (LES) of a lean premixed swirl burner. The model is used to locally select whether tabulated premixed or tabulated nonpremixed chemistry should be referenced in the LES. Results from the LES are compared to experiments.

Kim, S.H., Pitsch, H. & Boyd, I.D., Lattice Boltzmann modeling of multicomponent diffusion in narrow channels. Physical Review E, 79(1), p.016702. 2009.
We investigate lattice Boltzmann (LB) modeling of multicomponent diffusion for finite Knudsen numbers. Analytic solutions for binary diffusion in narrow channels, where both molecular and Knudsen diffusion are of importance, are obtained for the standard and higherorder LB methods and validated against the results from the direct simulation Monte Carlo (DSMC) method. The LB methods are shown to reproduce the diffusion slip phenomena. In the DSMC method, while fluid particles are diffusely reflected on a wall, significant component slip and a kinetic boundary layer are observed. It is shown that a higherorder LB method accurately captures the characteristics observed in the DSMC method.

Ihme, M., Pitsch, H. & Bodony, D., Radiation of Noise in Turbulent nonpremixed Flames. Proceedings of the Combustion Institute, 32(1), pp.15451553. 2009.
A model for the prediction of combustiongenerated noise in nonpremixed flames has been developed. This model is based on Lighthill’s acoustic analogy and employs the flamelet/progress variable model to express the excess density as function of mixture fraction and reaction progress variable. In this model, three major sources of sound have been identified, and their individual contribution to the acoustic spectra and overall sound pressure level are analyzed for a nitrogendiluted methane–hydrogen/air flame. The hybrid approach, combining a largeeddy simulation and a computational aeroacoustic method, introduces spurious noise which can pollute the acoustic results. All relevant sources of spurious noise are analyzed, and a physicsbased lowpass filter is proposed which eliminates spurious noise due to the convection of acoustic sources. The numerical predictions for both statistical flow field quantities and acoustic results have been validated with experimental data. The good agreement between experiments and simulation highlights the potential of the method for applications to more complex flow configurations and to provide further understanding of combustion noise mechanisms.

Desjardins, O. & Pitsch, H., A spectrally refined interface approach for simulating multiphase flows. Journal of Computational Physics, 228(5), pp.16581677. 2009.
This paper presents a novel approach to phaseinterface transport based on pseudospectral subgrid refinement of a level set function. In each flow solver grid cell, a set of quadrature points is introduced on which the value of the level set function is known. This methodology allows to define a polynomial reconstruction of the level set function in each cell. The transport is performed using a semiLagrangian technique, removing all constraints on the time step size. Such an approach provides subcell resolution of the phaseinterface and leads to excellent accuracy in the transport, while a reasonable cost is obtained by precomputing some of the metrics associated with the polynomials. To couple this approach with a flow solver, an converging curvature computation is introduced. First, a second order explicit distance to the subgrid interface is reconstructed on the flow solver mesh. Then, a least squares approach is employed to extract the curvature from this distance function. This technique is found to combine the high accuracy and good conservation found in the particle level set method with the converging curvature usually obtained with classical high order PDE transport of the level set function. Tests are presented for both transport as well as twophase flows, that suggest that this technique is capable of retaining the thin liquid structures that are expected in turbulent atomization of liquids.

Blanquart, G., PepiotDesjardins, P. & Pitsch, H., Chemical Mechanism for High Temperature Combustion of Engine Relevant Fuels with Emphasis on Soot Precursors. Combustion and Flame, 156(3), pp.588607. 2009.
This article presents a chemical mechanism for the high temperature combustion of a wide range of hydrocarbon fuels ranging from methane to isooctane. The emphasis is placed on developing an accurate model for the formation of soot precursors for realistic fuel surrogates for premixed and diffusion flames. Species like acetylene (C2H2C2H2), propyne (C3H4C3H4), propene (C3H6C3H6), and butadiene (C4H6C4H6) play a major role in the formation of soot as their decomposition leads to the production of radicals involved in the formation of Polycyclic Aromatic Hydrocarbons (PAH) and the further growth of soot particles. A chemical kinetic mechanism is developed to represent the combustion of these molecules and is validated against a series of experimental data sets including laminar burning velocities and ignition delay times. To correctly predict the formation of soot precursors from the combustion of engine relevant fuels, additional species should be considered. One normal alkane (nheptane), one ramified alkane (isooctane), and two aromatics (benzene and toluene) were chosen as chemical species representative of the components typically found in these fuels. A submechanism for the combustion of these four species has been added, and the full mechanism has been further validated. Finally, the mechanism is supplemented with a submechanism for the formation of larger PAH molecules up to cyclocdpyrene. Laminar premixed and counterflow diffusion flames are simulated to assess the ability of the mechanism to predict the formation of soot precursors in flames. The final mechanism contains 149 species and 1651 reactions (forward and backward reactions counted separately). The mechanism is available with thermodynamic and transport properties as supplemental material.

Vogel, S. & Peters, N., Simulation of Lifted Diesel Sprays using a newly developed Combined LevelSet Flamelet Model. In Towards Clean Diesel Engines: 7th International Symposium, 4th  5th June 2009, AGIT Technology Center, Aachen, Germany. CEUR Workshop Proceedings. p. 3. 2009.

Won, H.W., Sharma, A., Moon, S.E., Vanegas, A. & Peters, N., An Experimental Study of Cluster Nozzles for DI Diesel Engine. 9th International Conference on Engines and Vehicles, 13th  18th September 2009, Capri, Naples, Italy. 2009.
In a conventional Diesel engine, air is gradually drawn into the fuel spray from the surrounding area. The ignition delay period is short, so combustion starts before the fuel has thoroughly mixed with the air. Consequently, the center of the spray is overly rich, resulting in smoke, while a stoichiometric mixture is formed in the surrounding area, resulting in a high NOx concentration. Based on the Diesel concept it is practically impossible to totally avoid fuel rich and stoichiometric pockets, but the formation of soot and NOx are also time dependent. If the mixing time is sufficiently small both pollutants could be reduced simultaneously without getting into the well known sootNOx tradeoff. In order to develop a low emission engine, research is necessary to come up with a new combustion strategy for Diesel engines, which includes the use of cluster nozzles. Conditions for low raw particulate emission are: suitable start of injection, lean air/fuel mixture, sufficiently high temperatures in the combustion chamber and a sufficient retention time with these conditions. This could be achieved by use of cluster nozzles and a careful tuning of the air/fuel mixing parameters of the engine piston geometry. Decreasing the holesize improves mixing in the center of the spray and therefore the soot production goes down tremendously. It has been observed, that an optimum number of holes exists (6 to 8) and that a certain swirl level is necessary. Based on this experience the cluster concept was developed. The cluster nozzles can also be used for improved homogenization of incylinder charge so as to enable a partly homogenous mode of Diesel combustion. In this study, three cluster designs are investigated through engine measurements. They were tested in a singlecylinder engine with CRI 3.3 piezo injectors under partload conditions for a partly homogenous mode of Diesel combustion, and also under high load conditions for conventional Diesel combustion. The experimental studies give some detailed information on how cluster nozzles could help in lowering soot formation and thereby provide more freedom to reduce other pollutants.

Wada, T., Peters, N. & Mellado, J.P., Linear Stability Analysis of OneStep Model with Diluted Lean CH4/Air Oscillations at Low Temperatures. In Clear Air 2009  10th International Conference on Energy for a Clean Environment, 7th  10th July 2009, Lisbon, Portugal. 2009.
Chemically induced oscillations in a diluted lean CH4 /air mixture were studied using an extended onestep model for lowtemperature conditions (<1300 K) in a perfectly stirred reactor. This extended onestep model was used to find analytical solutions. In this study, linear stability analysis was used to evaluate the system. The results showed the following: First, there was a good agreement between the simulation results done with detailed chemical kinetic models and experimental observations. In addition, these results prove that the onestep model is valid both at highand lowtemperature conditions. Second, the oscillations occur only when the steadystate points of the system are linearly unstable. This proves that linear stability analysis is an effective method for studying not only irreversible phenomena such as ignition or extinction but also the dynamic and reversible behavior of a nonlinear system. Third, the oscillatory condition was determined from the analytical solution; it depended strongly on the heat loss. This is supported by other experimental and numerical observations and provides a theoretical underpinning for these observations.

Röhl, O., Jerzembeck, S., Beeckmann, J. & Peters, N., Numerical Investigation of Laminar Burning Velocities of High Octane Fuel Blends Containing Ethanol. SAE Technical Paper Series, 2009010935. 2009.
Recently, fuels containing ethanol have become more and more important for spark ignition engines. Fuels with up to 10 vol.% ethanol can be used in most spark ignition engines without technical modification. These fuels have been introduced in many countries already. Alternatively, for fuels with higher amounts of ethanol so called flex fuel vehicles (FFV) exist.<br />
<br />
One of the most important quantities characterizing a fuel is the laminar burning velocity. To account for the new fuels with respect to engine design, reliable data need to be existent. Especially for engine simulations, various combustion models have been introduced which rely on the laminar burning velocity as the physical quantity describing the progress of chemical reactions, diffusion, and heat conduction. However, there is very few data available in the literature for fuels containing ethanol, especially at high pressures.<br />
<br />
A detailed chemical kinetic mechanism for isooctane, nheptane, and ethanol is used to calculate laminar burning velocities under engine relevant conditions. The results are validated against data from literature and new experimental measurements using the closedvessel bomb method. Finally, based on the results obtained, the effect of ethanol blended to standard fuels is analyzed.

Ihme, M., Schmitt, C. & Pitsch, H., Optimal Artificial Neural Networks and Tabulation Methods for Chemistry Representation in LES of a Bluffbody Swirlstabilized Flame. Proceedings of the Combustion Institute, 32(1), pp.15271535. 2009.
Largeeddy simulations (LES) of the Sydney bluffbody swirlstabilized methane–hydrogen flame are performed, employing two chemistry representation methods, namely a conventional structured tabulation technique and artificial neural networks (ANNs). A generalized method for the generation of optimal artificial networks (OANNs) has been proposed by Ihme et al. M. Ihme, A.L. Marsden, H. Pitsch, Neural Comput. 20 (2) (2008) 573–601. This method is, for the first time, applied in LES of turbulent reactive flows, guaranteeing an optimal chemistry representation with error control, which was previously not possible. The network performance with respect to accuracy, data retrieval time, and storage requirements is compared with the structured tabulation of increasing resolution, and effects of longtime error accumulation on the statistical results during a numerical simulation are discussed. Using the optimization algorithm, it is demonstrated that ANN accuracies can be achieved which are comparable with structured tables of moderate to fine resolution. Furthermore, it is shown that for a comparable number of synaptic weights, the network fitness increases with increasing number of hidden layers. Compared to the tabulation technique, data retrieval from the network is computationally more expensive; however, the additional overhead associated with the ANN evaluation remains acceptable in LES applications. Results for flow field statistics and scalar quantities which are obtained from LES are in good agreement with experimental data, and possible reasons for the differences between computed and measured temperature profiles near the bluffbody are discussed. The difference in the velocity statistics between simulations employing structured table and network representation are small, and deviations in the CO2CO2 profiles on the fuelrich side of the flame are mainly attributed to the sensitivity of CO2CO2 with respect to changes in progress variable.

Won, H.W., Sharma, A., Hottenbach, P., Gauding, M., Roberts, F.X., Peters, N. & Gruenefeld, G., Investigation of Particulate Emissions for ClusterNozzle Concepts in DI Diesel Engines. In ICLASS 2009: 11th International Annual Conference on Liquid Atomization and Spray Systems, 26th  30th July, Vail, Colorado USA. 2009.
In a conventional Diesel engine, air is gradually drawn into the fuel spray from the surrounding area. The ignition delay period is short, so combustion starts before the fuel has thoroughly mixed with the air. Consequently, the center of the spray is overly rich, resulting in smoke, while stoichiometric mixture is formed in the surrounding area, resulting in a high NOx concentration. Based on the Diesel concept, it is practically impossible to totally avoid fuelrich and stoichiometric pockets, but the formation of soot and NOx are also timedependent. If the mixing time is sufficiently small, both pollutants could be reduced simultaneously without getting into the well known sootNOx tradeoff. In order to develop a lowemission engine, research is necessary to come up with a new combustion strategy for Diesel engines which includes the use of cluster nozzles. Decreasing the holesize improves mixing in the center of the spray and therefore the soot production is lowered tremendously. Based on this experience, three cluster designs were developed for the present work. The basic strategy of the cluster nozzles is to provide a better primary breakup and therefore a better mixture formation caused by the smaller nozzle holes, while keeping a comparable penetration length of the vapor phase due to merging of the sprays. In this study, two different clusternozzle designs were investigated in a combustion vessel and compared to a conventional nozzle with the same flow rate. Two different measurement techniques are employed to investigate the combustion process. The local soot concentration during combustion is measured semiquantitatively using Laser Induced Incandescence (LII). The natural soot luminosity is recorded simultaneously using a double frame camera. The results indicate that soot formation can be reduced by using cluster nozzles, at least in the early combustion phase under the investigated conditions. Three cluster designs similar to the ones used in the combustion vessel, were investigated through engine measurements. The nozzles used in this study were designed for improved homogenization of incylinder charge. They were tested<br />
in a singlecylinder engine with CRI 3.3 piezoinjectors under partload conditions for a partly homogenous mode of Diesel combustion, and also under high load conditions for conventional Diesel combustion. Numerical simulations were also carried out to explain the observations of the engine experiments. Certain test cases were simulated to get some detailed information on performance of cluster nozzles, giving more insight into soot formation. Another nozzle was designed based on the results from the three clusters. Engine experiments with the nozzle show<br />
improvements in soot emissions under highload conditions.

Senoner, J.M., Sanjose, M., Lederlin, T., Jaegle, F., Garcia, M., Riber, E., Cuenot, B., Gicquel, L., Pitsch, H. & Poinsot, T., Eulerian and Lagrangian LargeEddy Simulations of an evaporating twophase flow. Comptes Rendus Mecanique, 337(67), pp.458468. 2009.
LargeEddy Simulations (LES) of an evaporating twophase flow in an experimental burner are performed using two different solvers, CDP from CTRStanford and AVBP from CERFACS, on the same grid and for the same operating conditions. Results are evaluated by comparison with experimental data. The CDP code uses a Lagrangian particle tracking method (EL) while the code AVBP can be coupled either with a mesoscopic Eulerian approach (EE) or with a Lagrangian method (EL). After a validation of the purely gaseous flow in the burner, liquidphase dynamics, droplet dispersion and fuel evaporation are qualitatively and quantitatively evaluated for three twophase flow simulations. They are respectively referred as: CDPEL, AVBPEE and AVBPEL. The results of the three simulations show reasonable agreement with experiments for the twophase flow case.

Mueller, M.E., Blanquart, G. & Pitsch, H., A joint VolumeSurface model of soot aggregation with the method of moments. Proceedings of the Combustion Institute, 32(1), pp.785792. 2009.
In this work, a bivariate model of soot aggregation is formulated within the framework of the Method of Moments with Interpolative Closure (MOMIC). In the bivariate model, soot particles are represented by two independent variables: their volume and surface area. This joint formulation also allows for the blending of aggregation and coalescence with the two as limits. The new formulation is compared to the old formulation with the univariate model as well as both the Direct Quadrature Method of Moments (DQMOM) and Direct Simulation Monte Carlo (DSMC) for a laminar premixed ethylene flame. With the bivariate model, MOMIC is shown to predict volume fraction and number density very accurately and gives some insight into the properties of the aggregates.

Jarmolowitz, F., Abel, D., Wada, T. & Peters, N., Control of a homogeneous stirred reactor: Trajectory PiecewiseLinear Model for NMPC. In 10th European Control Conference, 23rd  26th August 2009, Budapest. 2009.
As a consequence of increasing computational power and improved algorithms, model predictive control (MPC) nowadays is becoming applicable to highdynamic processes. If using nonlinear MPC strategies (NMPC), e.g. sequential quadratic programmingsolvers (SQP), it is reasonable, if not mandatory to reduce the prediction model in order to cope with realtime. In this work the trajectory piecewiselinear approach (TPWL) as a model order reduction (MOR) scheme is investigated in simulation on its applicability on controlling combustion instabilities in an open homogeneous reactor using a SQP strategy. The corresponding nonlinear model exhibits, like the experiment, temperature oscillations during combustion. The temperature oscillations are due to chemical kinetics and typical for homogeneous lowtemperature combustion. Since chemical kinetics are highly nonlinear, NMPC is a promising approach to damp these oscillations and to control the system in different operating points.

Mueller, M.E., Blanquart, G. & Pitsch, H., Hybrid Method of Moments for Modeling Soot Formation and Growth. Combustion and Flame, 156(6), pp.11431155. 2009.
In this work, a new statistical model for soot formation and growth is developed and presented. The Hybrid Method of Moments (HMOM) seeks to combine the advantages of two moment methods, the Method of Moments with Interpolative Closure (MOMIC) and the Direct Quadrature Method of Moments (DQMOM), in an accurate and consistent formulation. MOMIC is numerically simple and easy to implement but is unable to account for bimodal soot Number Density Functions (NDF). DQMOM is accurate but is numerically illposed and difficult to implement. HMOM combines the best of both two methods to capture bimodal NDF while retaining ease of implementation and numerical robustness. The new hybrid method is shown to predict mean quantities nearly as accurately as DQMOM and highfidelity Monte Carlo simulations. In addition, a model for combining particle coalescence with particle aggregation is presented and shown to accurately reproduce experimental measurements in a variety of sooting flames.

Kim, S.H. & Pitsch, H., Reconstruction and Effective Diffusivity of the Catalyst Layer in PEM Fuel Cells. Journal of The Electrochemical Society, 156(6), pp.B673681. 2009.

Schäfer, P., Gampert, M., Wang, L. & Peters, N., Fast and slow changes of the length of gradient trajectories in homogenous shear turbulence. In Advances in Turbulence XII: proceedings of the 12th EUROMECH European Turbulence Conference, 7th  10th September, 2009, Marburg, Germany. Springer Proceedings in Physics. pp. 565572. 2009.
Gradient trajectories in scalar fields have recently received attention in the context of dissipation elements 1, 2 which in turn are of interest for the flamelet concept in nonpremixed combustion 3. Dissipation elements are space filling regions in a scalar field defined such that gradient trajectories starting from any point within the element in ascending and descending directions reach the same minimum and maximum points. Gradient trajectories advance preferentially through regions of the scalar field that have been smoothed by the combined action of diffusion and extensive strain. Since the extensive strain in these regions is of the order of the inverse of the integral time scale T, dimensional analysis predicts the mean length l m of dissipation elements to be of the order of the Taylor length 4.

Barths, H., Felsch, C. & Peters, N., Mixing models for the twoway coupling of CFD codes and zerodimensional multizone codes to model HCCI combustion. Combustion and Flame, 156(1), pp.130139. 2009.
The objective of this work is the development of a consistent mixing model for the twowaycoupling of a CFD code and a multizone code based on multiple zerodimensional reactors. The twowaycoupling allows for a computationally efficient modeling of HCCI combustion. The physical domain in the CFD code is subdivided into multiple zones based on three phase variables (fuel mixture fraction, dilution, and total enthalpy). Those phase variables are sufficient for the description of the thermodynamic state of each zone, assuming that each zone is at the same pressure. Each zone in the CFD code is represented by a corresponding zone in the zerodimensional code. The zerodimensional code solves the chemistry for each zone, and the heat release is fed back into the CFD code. The difficulty in facing this kind of methodology is to keep the thermodynamic state of each zone consistent between the CFD code and the zerodimensional code after the initialization of the zones in the multizone code has taken place. The thermodynamic state of each zone (and thereby the phase variables) will change in time due to mixing and source terms (e.g., vaporization of fuel, wall heat transfer). The focus of this work lies on a consistent description of the mixing between the zones in phase space in the zerodimensional code, based on the solution of the CFD code. Two mixing models with different degrees of accuracy, complexity, and numerical effort are described. The most elaborate mixing model (and an appropriate treatment of the source terms) keeps the thermodynamic state of the zones in the CFD code and the zerodimensional code identical. The models are applied to a test case of HCCI combustion in an engine.

Gauding, M., Brands, T., Felsch, C., Hottenbach, P., Hasse, C., Pauls, C., Grünefeld, G. & Peters, N., Experimental and Numerical Investigation of Ignition Mechanisms for Multiple Injection Strategies at Diesel EngineLike Conditions. In 4th European Combustion Meeting: 14th  17th April 2009, Vienna, Austria. 2009.
Split injections in DI Diesel engines and combustion vessels exhibit different ignition phenomena for the first and for the second injection pulse. In this work, the ignition phenomena of a split injection into a combustion vessel are investigated experimentally and numerically. The first pulse of the split injection is injected into quiescent air; combustion is an autoignition process. The ignition mechanism of the second pulse is fundamentally different. As soon as the mixture fields of the first and the second pulse get into contact, a strained premixed flame between the mixture fields is established. The premixed flame propagates along the region of stoichiometric mixture towards the mixture field of the second pulse. This leads to a triggered ignition of the second pulse.

Zeng, P., Binninger, B., Peters, N. & Herrman, M., Simulation of Primary Breakup for Diesel Spray with Phase Transition. In 11th ICLASS International Conference on Liquid Atomization and Spray Systems, 26th  30th July 2009, Vail, Colorado. 2009.

Drews, P., Hoffmann, K., Beck, R., Gasper, R., Vanegas, A., Felsch, C., Peters, N. & Abel, D., Fast model predictive control for the air path of a turbocharged diesel engine. In Proceedings of the European Control Conference 2009, 23rd  26th August 2009, Budapest, Hungary. pp. 33773382. 2009.

Kaul, C.M., Raman, V., Balarac, G. & Pitsch, H., Numerical errors in the computation of subfilter scalar variance in large eddy simulations. Physics of Fluids, 21(5), p.055102. 2009.
Subfilter scalar variance is a key quantity for scalar mixing at the small scales of a turbulent flow and thus plays a crucial role in large eddy simulation of combustion. While prior studies have mainly focused on the physical aspects of modeling subfilter variance, the current work discusses variance models in conjunction with the numerical errors due to their implementation using finitedifference methods. A priori tests on data from direct numerical simulation of homogeneous turbulence are performed to evaluate the numerical implications of specific model forms. Like other subfilter quantities, such as kinetic energy, subfilter variance can be modeled according to one of two general methodologies. In the first of these, an algebraic equation relating the variance to gradients of the filtered scalar field is coupled with a dynamic procedure for coefficient estimation. Although finitedifference methods substantially underpredict the gradient of the filtered scalar field, the dynamic method is shown to mitigate this error through overestimation of the model coefficient. The second group of models utilizes a transport equation for the subfilter variance itself or for the second moment of the scalar. Here, it is shown that the model formulation based on the variance transport equation is consistently biased toward underprediction of the subfilter variance. The numerical issues in the variance transport equation stem from discrete approximations to chainrule manipulations used to derive convection, diffusion, and production terms associated with the square of the filtered scalar. These approximations can be avoided by solving the equation for the second moment of the scalar, suggesting that model’s numerical superiority.

Hoffmann, K., Drews, P., Abel, D., Felsch, C., Vanegas, A. & Peters, N., A cyclebased multizone simulation approach including cycletocycle dynamics for the development of a controller for PCCI combustion. SAE Technical Paper Series, 2009010671. 2009.
Subject of this work is a simulation model for PCCI combustion that can be used in closedloop control development. A detailed multizone chemistry model for the highpressure part of the engine cycle is extended by a mean value model accounting for the gas exchange losses. The resulting model is capable of describing PCCI combustion with stationary excactness. It is at the same time very economic with respect to computational costs. The model is further extended by identified system dynamics influencing the stationary inputs. For this, a Wiener model is set up that uses the stationary model as a nonlinear system representation. In this way, a dynamic nonlinear model for the representation of the controlled plant Diesel engine is created.

Zeng, P., Sarholz, S., Iwainsky, C., Binninger, B., Peters, N. & Herrman, M., Simulation of Primary Breakup for Diesel Spray with Phase Transition. In Recent advances in parallel virtual machine and message passing interface: 16th European PVM/MPI Users' Group Meeting, 7th  10th September 2009, Espoo, Finland. Lecture Notes in Computer Science. pp. 313320. 2009.
We perform direct numerical simulation on large distributed memory parallel computers in order to investigate the primary breakup process of diesel spray direct injection. Local refinement algorithmRefined levelset method has been used to reduce the memory requirement, and we analyze the performance by experiments on a 1024processor parallel computer.

Gauding, M., Pawlowski, A., Felsch, C., Sonntag, B., Kneer, R. & Peters, N., Fundamental Investigation of Diesel SpraySpray Interaction for ClusterHole Nozzles. In 11th ICLASS International Conference on Liquid Atomization and Spray Systems, 26th  30th July 2009, Vail, Colorado. ILASS International. 2009.

Wada, T., Paczko, G. & Peters, N., Numerical Investigation of Highly Diluted Lean CH4/Air Oscillations at Low Temperatures. In 4th European Combustion Meeting, 14th  17th April, Vienna, Austria. 2009.

Dahms, R. & Peters, N., On Nitrogen Oxide Formation in SprayGuided SparkIgnited Gasoline Engines. In Sixth Mediterranean Combustion Symposium, Corsica, France. 2009.

Jerzembeck, S., Matalon, M. & Peters, N., Experimental investigation of very rich laminar spherical flames under microgravity conditions. Proceedings of the Combustion Institute, 32(1), pp.11251132. 2009.
Very rich premixed outward propagating spherical flames of nheptane (ϕ = 3.5) and isooctane (ϕ = 3.9) spherical flames were experimentally investigated under microgravity conditions at 420 K and initial pressures of up to 30 bar. Inert gases with significantly different molecular weights were used as diluents leading to mixtures with varying effective Lewis numbers. The experimental setup consisted of a spherical closed pressurized vessel that enabled optical access. The experiments were performed in a drop tower under microgravity conditions to prevent the influence of buoyancy on the expanding flames. A Schlieren measurement technique combined with a highspeed CCD camera was used to track the expanding flames. During the entire experiment, the flames were laminar and smooth with no wrinkles due to combustion instabilities observed. This allowed for accurate measurements of the propagation speed and its dependence on stretch, which was then compared to theoretical predictions based a hydrodynamic model for relatively thin flames. The experimental and theoretical results show good agreement with each other. Investigation of such rich mixtures that are close to the upper flammability limit has not been reported before.

Blanquart, G. & Pitsch, H., Analyzing the Effects of Temperature on Soot Formation using a Joint VolumeSurfaceHydrogen Model. Combustion and Flame, 156(8), pp.16141626. 2009.
The intent of the current work is to present and further validate a new trivariate model for the formation of soot particles, to apply this model in analyzing the effects of temperature on the formation and growth of soot, and to compare the findings with the present understanding derived from numerous experimental studies. In this novel model, a particle is represented as a fractal shaped aggregate and is described by three independent quantities: the volume, the surface area, and the number of hydrogenated sites (or active sites) on the surface. The introduction of this third variable allows for a better description of the surface reactivity at high temperatures. This approach is extended by a model for the total carbontohydrogen (C/H) ratio of the particle. The model is validated first in high temperature premixed ethylene flames, premixed benzene flames, an acetylene counterflow diffusion flame, and toluene pyrolysis in shocktubes. Then, the soot volume fraction is computed for a series of atmospheric laminar ethylene premixed flames with varying flame temperatures. The soot model is shown to reproduce the well known bellshaped temperature dependence of soot volume fraction, which was found in many experiments. It is observed that nucleation is the largest contributor to soot volume fraction at low temperatures while growth by surface reactions is more important at higher temperatures. The surface reactivity and the volumetric carbontohydrogen ratio (C/H) are also computed as a function of temperature. The surface reactivity is found to depend not only on the temperature but also on the particle size and the residence time in the flame. Finally, as observed experimentally, the C/H ratio is found to be essentially constant and close to unity for low temperature flames and increases with residence time in high temperature flames.

Gauding, M., Felsch, C., Kerschgens, B., Peters, N. & Hasse, C., Ein verallgemeinertes Flamelet Modell für Mehrfacheinspritzungen in modernen Dieselmotoren. In Motorische Verbrennung: aktuelle Probleme und moderne Lösungsansätze (Tagung), Haus der Technik, 19./20. März, München. 2009.

Peters, N. & Won, H.W., A cluster nozzle concept with high injection pressures for DI Diesel engine. In Towards Clean Diesel Engines: 7th International Symposium, 4th  5th June 2009, AGIT Technology Center, Aachen, Germany. CEUR Workshop Proceedings. p. 3. 2009.

Röhl, O. & Peters, N., A Reduced Mechanism for Ethanol Oxidation. In 4th European Combustion Meeting: Vienna University of Technology, 14th  17th April, Vienna, Austria. 2009.
To account for the increasing share of alternative fuels, reliable chemical kinetic mechanisms are needed. For ethanol, as one of the most important alternative fuels, Marinov 1 proposed a hightemperature mechanism consisting of 57 chemical species and 383 elementary reactions. In the present work, this mechanism is systematically reduced, nally leading to 38 species and 228 reactions. For validation, numerical results using the reduced mechanism are compared to experimental ignition delay times and laminar burning velocities, as well as results obtained with the original mechanism.

Vogel, S. & Peters, N., Simulation of Lifted Diesel Sprays using a newly developed Combined LevelSet Flamelet Model. Towards Clean Diesel Engines: 7th International Symposium, 4th  5th June 2009, AGIT Technology Center, Aachen, Germany, 452. 2009.

Felsch, C., Dahms, R., Glodde, B., Vogel, S., Peters, N., Barths, H., Sloane, T., Wermuth, N. & Lippert, A., Applying an interactively coupled CFDMultiZoneApproach to model HCCI combustion. Flow, Turbulence and Combustion, 82(4), pp.621641. 2009.
The objective of this work is to present an innovative interactively coupled CFDmultizone approach. In a consistent manner,<br />
the approach combines detailed flow field information obtained from CFD with detailed chemical kinetics solved in a multizone<br />
model. Combustion and pollutant formation in an HCCI engine with recompressing VVA strategy are numerically investigated using<br />
the interactively coupled CFDmultizone approach. A surrogate fuel for gasoline is used in the simulation that consists of<br />
nheptane (18% liquid volume fraction) and isooctane (82% liquid volume fraction). The underlying complete reaction mechanism<br />
comprises 482 elementary reactions and 115 chemical species. The interactively coupled CFDmultizone approach shows to be<br />
accurate enough to describe HCCI chemistry, and is at the same time economical enough to allow application in an industrial<br />
environment. For the test case investigated, the simulation results are compared to experimental data that has been obtained<br />
using real gasoline. The overall agreement between simulation and experiment is found to be very good.

Peters, N., Hoffmann, K., Felsch, C. & Abel, D., A Dynamic Simulation Strategy for PCCI Combustion Control Design. In ECOSM'09: IFAC Workshop on Engine and Powertrain Control, Simulation and Modeling, 30th November  2nd December, 2009, RueilMalmaison, France. 2009.
A Dynamic Simulation Strategy for PCCI Combustion Control Design  Subject of this work is a dynamic simulation strategy for PCCI combustion that can be used in closedloop control development. A detailed multizone chemistry model for the highpressure part of the engine cycle is extended by a mean value model accounting for the gas exchange losses. The resulting stationary model is capable of describing PCCI combustion sufficiently well. It is at the same time very economic with respect to computational costs. The model is further extended by identified system dynamics influencing the stationary inputs. For this, a Wiener model is set up that uses the stationary model as a nonlinear system representation. In this way, a dynamic nonlinear model for the representation of the controlled plant Diesel engine is created. This paper summarizes an important outcome of the the Collaborative Research Centre "SFB 686  Modellbasierte Regelung der homgenisierten NiedertemperaturVerbrennung" at RWTH Aachen University and Bielefeld University, Germany.

Beeckmann, J., Röhl, O. & Peters, N., Experimental and numerical investigation of isooctane, methanol and ethanol regarding laminar burning velocity at elevated pressure and temperature. SAE Technical Paper Series, 2009011774. 2009.
The laminar burning velocity is one key parameter for the numerical simulation of gasoline engine combustion processes. In order to understand the effect of the laminar burning velocity of different fuel components on modern engine development it is of great interest to conduct experiments under high initial pressure and temperature. Initial conditions in this publication are a pressure of p = 10bar and a temperature of T = 373K.<br />
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Special focus has been laid on the common C1 and C2 alcohols, methanol and ethanol, which are frequently used for blending components in standard gasoline.<br />
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The experimental setup consists of a spherical closed pressurized combustion vessel with optical access. Schlieren measurements coupled with a high speed camera are used for image acquisition to track the expanding flame front. Finally, a post processing tool is used to extrapolate the measurements to zero stretch.<br />
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Experiments were done at different fuelair ratios between Φ = 0.8 and up to Φ = 1.2. Test fuels were the pure component isooctane, methanol and ethanol or mixtures isooctane / methanol and isooctane / ethanol with a maximum blend rate of 10% vol liq regarding the alcohol.<br />
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Thereafter, numerical simulations using high temperature chemical models were undertaken to estimate the laminar burning velocity for a distinct amount of experimental test points.<br />
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Experimental results are discussed in detail and compared with the numerical simulations as well as references from literature.

Wang, L. & Peters, N., Lengthscale distribution functions and conditional means for various fields in turbulence. Journal of Fluid Mechanics, 608(1), pp.113138. 2008.
Dissipation elements are identified for various direct numerical simulation (DNS) fields of homogeneous shear turbulence. The fields are those of the fluctuations of a passive scalar, of the three components of velocity and vorticity, of the second invariant of the velocity gradient tensor, turbulent kinetic energy and viscous dissipation. In each of these fields trajectories starting from every grid point are calculated in the direction of ascending and descending gradients, reaching a local maximum and minimum point, respectively. Dissipation elements are defined as spatial regions containing all the grid points from which the same pair of minimum and maximum points is reached. They are parameterized by the linear length between these points and the difference of the field variable at these points.<br />
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In analysing the changes that occur during one time step in the linear length as well as in the number of grid points contained in the elements, it is found that rapid splitting and attachment processes occur between elements. These processes are much more frequent than the previously identified processes of cutting and reconnection. The model for the lengthscale distribution function that had previously been proposed is modified to include these additional processes. Comparisons of the lengthscale distribution function for the various fields with the proposed model show satisfactory agreement.<br />
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The conditional mean difference of the field variable at the minimum and maximum points of dissipation elements is calculated for the passive scalar field and the three components of velocity. While the conditional mean difference follows the 1/3 inertialrange Kolmogorov scaling for the passive scalar field, the scaling exponent differs from the 1/3 law for each of the three components of velocity. This is thought to be due to the relatively high shear rate of the DNS calculations.<br />
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The conditional mean viscous dissipation shows, differently from all other field variables analysed, a pronounced dependence on the linear length of elements. This is explained by intermittency. This finding is used to evaluate the production and the dissipation term of the empirically derived xs03F5equation that is often used in engineering calculations.

Jerzembeck, S., Spiekermann, P., Röhl, O., Glawe, C. & Peters, N., Development and experimental evaluation of a hightemperature mechanism for blended nheptaneisooctane–ethanolair mixtures and gasolineethanolairmixtures. In 4th IASME/WSEAS international conference on energy, environment, ecosystems and sustainable development, Wisconsin, USA. pp. 7883. 2008.
Laminar burning velocity measurements using the closed vessel bomb method have been done for fuelblendairmixtures at 373 K initial temperature and up to 20 bar initial pressure. The two experimentally investigated fuel blends consist, on the one hand, of 78.3 % vol. isooctane, 11.7 % vol. nheptane, and 10 % vol. ethanol, and on the other hand of 90 % vol. standard gasoline and 10 % vol. ethanol.<br />
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A detailed hightemperature chemical kinetic model has been developed with the focus on calculating laminar burning velocities of these mixtures. The reduced high temperature mechanism developed by Jerzembeck et al. 1 for PRFs (Primary Reference Fuels consisting of isooctane and nheptane) is the basis for the present work. The dominant reactions of ethanol were added to this mechanism. These reactions were taken from the mechanism of Marinov et al. 2. Laminar burning velocities calculated with the newly developed mechanism are in good agreement with the experimental results for low and high pressure mixtures, as well as for single fuel and blended fuel air mixtures.

Stoehr, K.D., Wada, T. & Peters, N., Development of a ModelBased Controller for Combustion Instabilities in a JSFR. Poster Presentation, 32nd International Symposium on Combustion, Montreal, Canada. 2008.

Jerzembeck, S., Glawe, C., Keppner, J. & Peters, N., Laminar burning velocities from Schlieren  and pressure history measurements. 5th WSEAS International Conference on Fluid Mechanics: Theoretical and experimental aspects of fluid mechanics, 25th  27th January 2008, Acapulco, Mexico, pp.106113. 2008.
In the present work, laminar burning velocity results of nheptane and isooctaneairmixtures determined with two measurement techniques are compared. Both measurement techniques are based on the closedvesselbombmethod. In the first approach, the spherical flame fronts were tracked using the captured pressure history of the fresh mixture during combustion. A thermodynamic model, developed by Lewis and Elbe 1, was used to predict the onedimensional outwardly propagating flame front based on the compression of the fresh mixture. In the second approach, the flame is. tracked by an optical SchlierenMeasurementTechnique using a HeliumNeon Laser as a light source and a highspeed camera to capture the flame photographs. The spherical flame propagation is captured with a highspeed camera as well. The recorded photographs were analyzed using an imageprocessing technique. Both techniques take into account the effect of stretch to determine the laminar burning velocities. Numerical onedimensional calculations of freely propagating flames were done to evaluate the accuracy of the measured results. Nheptane and isooctaneairmixtures were investigated for equivalence ratios from Phi= 0.7 to Phi=1.2, initial pressures of 10 bar and 20 bar, and an initial temperature for all mixtures of 373 K.

Spiekermann, P., Jerzembeck, S. & Peters, N., The Influence of Fuel Boiling Temperature on CommonRail Spray Penetration and Mixture Formation for Ethanol and PropyleneGlycol. SAE Technical Paper, 2008010934. 2008.

Felsch, C., Gauding, M., Vanegas, A., Won, H., Luckhchoura, V., Hasse, C., Ewald, J. & Peters, N., Evaluation of Modeling Approaches for NOx Formation in a CommonRail DI Diesel Engine within the Framework of Representative Interactive Flamelets (RIF. SAE Technical Paper Series, 2008010971. 2008.
Representative Interactive Flamelets (RIF) have proven successful in predicting diesel engine combustion. The RIF concept is based on the assumption that chemistry is fast compared to the smallest turbulent time scales, associated with the turnover time of a Kolmogorov eddy.<br />
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The assumption of fast chemistry may become questionable with respect to the prediction of pollutant formation; the formation of NOx, for example, is a rather slow process. For this reason, three different approaches to account for NOx emissions within the flamelet approach are presented and discussed in this study. This includes taking the pollutant mass fractions directly from the flamelet equations, a technique based on a threedimensional transport equation as well as the extended Zeldovich mechanism.<br />
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Combustion and pollutant emissions in a CommonRail DI diesel engine are numerically investigated using the RIF concept. Special emphasis is put on NOx emissions. A surrogate fuel for diesel consisting of a mixture of ndecane (70% liquid volume fraction) and αmethylnaphthalene (30% liquid volume fraction) is applied in the simulations. One engine operating point is considered with a variation of start of injection. The simulation results are discussed and compared to experimental data.

Dahms, R., Peters, N., Stanton, D.W., Tan, Z. & Ewald, J., Pollutant formation modelling in natural gas SI engines using a level set based flamelet model. International Journal of Engine Research, 9(1), pp.114. 2008.
The level set flamelet model for turbulent premixed combustion was recently extended to improve the modelling of the spark ignition process, to incorporate the influence of unsteady flame kernel development on turbulent flame propagation, to embrace flameâ€”wall interaction, and to comprise pollutant prediction models. The formation of nitrogen oxides (NOx) is predicted using the extended Zeldovich mechanism. The concentrations of the involved intermediate species are determined using a chemical equilibrium assumption. The formation of engineout unburnt hydrocarbons (UHCs) and carbon monoxides (COs) is directly linked to incomplete combustion due to flame quenching at the time the flame hits the wall. In this paper, the new model is used for the analysis of combustion in a homogeneous charged natural gas SI engine operating on lean conditions. The investigated engine operating (PAM) points differ in the applied equivalence ratio, ignition timing, and rate of exhaust gas recirculation. The influence of the load on the peak pressures, peak pressure locations, and pollutant formations is discussed in detail. A good agreement with experimental data was obtained among all engine operating points.

Peters, N., MultiScale Mixing and Combustion. In 32nd International Symposium on Combustion, Montreal, Kanada. 2008.

Jerzembeck, S., Röhl, O., Glawe, C. & Peters, N., Laminar Burning Velocities of PrimaryReference FuelEthanolAir Mixtures. 32nd International Symposium on Combustion, Montreal, Canada. 2008.

Jerzembeck, S., Peters, N. & Spiekermann, P., The influence of fuel boiling temperature on common rail spray penetration and mixture formation of ethanol and propyleneglycol. SAE Technical Paper Series, 2008010934. 2008.
An intricate experimental investigation of CommonRail Sprays was done using a HighPressure Chamber, a CommonRailInjection System as well as three optical measurement techniques. Ethanol and PropyleneGlycol (of purity for spectroscopic applications ≻99.9%) were used as fuels. The experimental boundary conditions of the high pressure chamber were up to 5 MPa and 800K. In detail an optical shadowgraph imaging and Miescattering technique were used. Liquid and gas phase spray penetration are investigated for fuels with low and high volatility respectively boiling temperature (propylene glycol, ethanol) for a variation of ambient gas phase temperature and density. Spatial information of the mixing process of both fuels is obtained by the 1D spontaneous Raman scattering (1DRS) technique. That technique provides quantitative mass fraction respectively fuelair ratio data over a wide temperature and pressure range without the need of a tracer as for many techniques that are based on fluorescence (LIF). A brief discussion as well as a comparison of these results of the two investigated fuels will be done.

Wang, L. & Peters, N., A CompensationDefect Model for the Joint PDF of the Scalar Difference and the Length Scale of Dissipation Elements. Physics of Fluids, 20(6). 2008.
Dissipation element analysis is a new approach to study turbulent scalar fields. Gradient trajectories starting from each material point in a fluctuating scalar field ϕ′(x⃗ ,t) in ascending and descending directions will inevitably reach a maximal and a minimal point. The ensemble of material points sharing the same pair ending points is named a dissipation element. Dissipation elements can be parametrized by the length scale l and the scalar difference Δϕ′, which are defined as the straight line connecting the two extremal points and the scalar difference at these points, respectively. The decomposition of a turbulent field into dissipation elements is space filling. This allows us to reconstruct certain statistical quantities of fine scale turbulence which cannot be obtained otherwise. The marginal probability density function (PDF) of the length scale distribution had been modeled in the previous work based on a Poisson random cuttingreconnection process and had been compared to data from direct numerical simulation (DNS). The joint PDF of l and Δϕ′ contains the important information that is needed for the modeling of scalar mixing in turbulence, such as the marginal PDF of the length of elements and conditional moments, as well as their scaling exponents. In order to be able to predict these quantities, there is a need to model the joint PDF. A compensationdefect model is put forward in this work and the agreement between the model prediction and DNS results is satisfactory.

Won, H.W., Vanegas, A. & Peters, N., Experimental study of HC emissions using narrow spray cone angles and different surrogate fuels in low temperature diesel combustion systems. In FISITA World Automotive Congress: 14th  19th September 2008, Munich, Germany. ATZ/ATZautotechnology. 2008.
esel engine exhaust contains emissions other than CO2, reductions in such emissions is a high priority issue. PM and NOx have been major constituents of the Diesel emissions and the regulations are getting stringent. But these Diesel emissions are difficult to reduce due to the broad range of fuelair mixtures present during the combustion process. In a conventional Diesel engine, air is gradually drawn into the fuel spray from the surrounding area. The ignition delay period is short, so combustion starts before the fuel has thoroughly mixed with the air. Consequently, the centre of the spray is overly rich, resulting in smoke, while a stoichiometric mixture is formed in the surrounding area, resulting in a high NOx concentration. In order to develop a low emission engine, research is necessary to come up with a new combustion strategy for Diesel engine. The new combustion strategies such as LTC (low temperature combustion) for Diesel engine have been widely studied as a combustion technology to avoid NOx and smoke formation regions simultaneously. In PCCI (premixed charged compression ignition) approach the combustion temperature is lowered by forming a lean premixture which simultaneously and substantially reduces NOx and smoke.

Luckhchoura, V., Won, H.W., Sharma, A., Paczko, G. & Peters, N., Investigation of combustion noise development with variation in start of injection using 3dimensional simulations by applying representative interactive flamelet (RIF) model. SAE Technical Paper Series, 2008010950. 2008.
Engine noise pollution is as harmful as other forms of pollution to human health. Apart from the health effects, noise also has an adverse effect on the engine structure, thus requiring a sturdier construction to maintain long engine life. In a conventional direct injection diesel engine the fuel ignites spontaneously shortly after the beginning of injection. The combustion process causes fluctuations in heat release and therefore, fluctuations in combustion chamber pressure. Combustiongenerated noise can be lowered by lowering the fluctuations in heat release or pressure, which can be achieved by separating the fuel evaporation and fuelair mixing from start of ignition in space and in time. The noise is mainly affected by the early part of the combustion process due to higher rates of heat release. Combustion noise generation in the early stage of combustion is not yet entirely understood. Threedimensional numerical simulations can be helpful to address this problem and to identify the parameters involved in the generation of combustion noise.<br />
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In the present work, experiments and simulations were performed for different start of injection (SOI) to investigate the effect of mixture formation on combustiongenerated noise. The experiments were done on a 1.9Liter, fourcylinder direct injection General Motors CorporationFIAT diesel engine using European diesel fuel. Representative Interactive flamelet model (RIF) was applied for the numerical simulations. RIF model uses a CFD code coupled interactively with the flamelet model. The diesel fuel for simulations is substituted by a twocomponent surrogate fuel which is a mixture of 70% ndecane and 30% αmethylnaphthalene (by liquid volume fraction) and is known as IDEA fuel. The chemical reaction mechanism consists of 506 elementary reactions and 118 chemical species. The simulated results using RIF are in good agreements with the experimental results. Ringing intensity correlation was used as a quantitative indicator of combustion noise for the experiments and simulations. Additionally, an approach based on unsteady flamelet temperature solutions in mixture fraction space is proposed to investigate combustion noise. This method provided a deeper understanding of noise generation.

Mansour, M., Peters, N. & Schrader, L.U., Experimental study of turbulent flame kernel propagation. Experimental Thermal and Fluid Science, 32(7), pp.13961404. 2008.
Flame kernels in spark ignited combustion systems dominate the flame propagation and combustion stability and performance. They are likely controlled by the spark energy, flow field and mixing field. The aim of the present work is to experimentally investigate the structure and propagation of the flame kernel in turbulent premixed methane flow using advanced laserbased techniques. The spark is generated using pulsed Nd:YAG laser with 20 mJ pulse energy in order to avoid the effect of the electrodes on the flame kernel structure and the variation of spark energy from shottoshot. Four flames have been investigated at equivalence ratios, φj, of 0.8 and 1.0 and jet velocities, Uj, of 6 and 12 m/s. A combined twodimensional Rayleigh and LIPFOH technique has been applied. The flame kernel structure has been collected at several time intervals from the laser ignition between 10 μs and 2 ms. The data show that the flame kernel structure starts with spherical shape and changes gradually to peanutlike, then to mushroomlike and finally disturbed by the turbulence. The mushroomlike structure lasts longer in the stoichiometric and slower jet velocity. The growth rate of the average flame kernel radius is divided into two linear relations; the first one during the first 100 μs is almost three times faster than that at the later stage between 100 and 2000 μs. The flame propagation is slightly faster in leaner flames. The trends of the flame propagation, flame radius, flame crosssectional area and mean flame temperature are related to the jet velocity and equivalence ratio. The relations obtained in the present work allow the prediction of any of these parameters at different conditions.

PepiotDesjardins, P. & Pitsch, H., An Efficient Error Propagation Based Reduction Method for Large Chemical Kinetic Mechanisms. Combustion and Flame, 154(12), pp.6781. 2008.
Production rates obtained from a detailed chemical mechanism are analyzed in order to quantify the coupling between the various species and reactions involved. These interactions can be represented by a directed relation graph. A geometric error propagation strategy applied to this graph accurately identifies the dependencies of specified targets and creates a set of increasingly simplified kinetic schemes containing only the chemical paths deemed the most important for the targets. An integrity check is performed concurrently with the reduction process to avoid truncated chemical paths and mass accumulation in intermediate species. The quality of a given skeletal model is assessed through the magnitude of the errors introduced in the target predictions. The applied error evaluation is variabledependent and unambiguous for unsteady problems. The technique yields overall monotonically increasing errors, and the smallest skeletal mechanism that satisfies a userdefined error tolerance over a selected domain of applicability is readily obtained. An additional module based on lifetime analysis identifies a set of species that can be modeled accurately by quasisteady state relations. An application of the reduction procedure is presented for autoignition using a large isooctane mechanism. The whole process is automatic, is fast, has moderate CPU and memory requirements, and compares favorably to other existing techniques.

Blanquart, G. & Pitsch, H., A joint volumesurfacehydrogen multivariate model for soot formation, in combustion generated fine carbonaceous particles. In Combustion Generated Fine Carbonaceous Particles. pp. 439466. 2008.

Jerzembeck, S., Sharma, A. & Peters, N., Laminar burning velocities of nitrogen diluted standard gasolineair mixture. SAE Technical Paper Series, 2008011075. 2008.
To understand how laminar burning velocities of standard unleaded gasolineair mixtures change by varying the concentration of oxygen in the combustible mixture, experimentally and numerical investigations are conducted in this work. Experiments were performed using a heatable pressure vessel which enables optical access. A monochromatic highspeed Schlieren cinematography measurement system combined with a highspeed CCD camera were used to track the propagating spherical flame fronts in the vessel. Numerically, freely propagating onedimensional laminar steady flame calculations were conducted for PrimaryReferenceFuel Air Mixtures (PRF87 or RON87), corresponding for standard gasoline combustible mixtures. Two combustible mixtures were investigated: (1) with air as oxidizer; (2) oxidizer consisting of 15% O₂ and 85% N₂ by mole fractions. The initial temperature for all investigated mixtures was 373 K. Initial pressures were varied from 10 to 25 bar and equivalence ratios were varied from 0.8 to 1.2. Experimental and numerical laminar burning velocities are compared with each other and show an overall good agreement.

Jerzembeck, S. & Peters, N., Laminar spherical flame kernel investigation of very rich premixed hydrocarbonairmixtures in a closed vessel under microgravity conditions. SAE Technical Paper Series, 2008010471. 2008.
In this work very rich premixed laminar spherical flame kernels of hydrocarbonair combustible mixtures were experimentally and numerically investigated under microgravity conditions. These microgravity combustion experiments were carried out in the Drop Tower of Bremen University. The ClosedVesselBomb Method (CVBM) was applied for all experimental investigations combined with a monochromatic HeliumNeonSchlieren Measurement Technique. Images of the propagating spherical flames were tracked with a HighSpeed Camera. The pressure vessel enables optical access and contains a volume of approx. half a liter. Combustible Mixtures were investigated at initial pressures up to 30 bar and initial temperatures were 420 K for all experiments, whereas the equivalence ratio for investigated NPentaneAir Mixtures was Φ=3.0, NHexaneAir Mixtures was Φ=3.3, NHeptaneAir Mixtures was Φ=3.5 and the equivalence ratio for investigated IsooctaneAir Mixtures was Φ=3.9 for all experiments. All investigated flames appeared to be smooth. Mainly no wrinkles due to flame instabilities could be observed on the flame surface. The propagating spherical radii over time of the spherical flames were tracked with an imageprocessing code. Furthermore, linear extrapolation of flame propagating velocity with respect to the burned mixture to the zero stretch value was done. Interestingly, at these 'very' rich equivalence ratios the laminar flame velocities increase with increase in initial pressure. This diametric effect, due to, e.g., stoichiometric or lean flames, could be shown in the numerical results as well. Onedimensional laminar flame calculations were done with a modified skeletal mechanism for NHeptane and IsooctaneAir Mixtures.

PepiotDesjardins, P., Malhotra, R., Kirby, S.R., Boehman, A.L. & Pitsch, H., Structural Group Analysis for Soot Reduction Tendency of Oxygenated Fuels. Combustion and Flame, 154(12), pp.191205. 2008.
Oxygenated additives are known to reduce soot formation in diesel engines. Numerous studies, both experimental and numerical, have reported that the reduction of particulate emissions depends on the molecular structure of the additives. In this paper, a structural group contribution approach is proposed to interpret experimental observations on the effect of oxygenated additives on the sooting propensities of hydrocarbon fuels. The statistically based method makes it possible to distinguish between chemical effects caused by the presence of oxygenated groups in the fuel mixture and mere dilution of the original fuel by the additive. The analysis was carried out on several experimental databases encompassing both premixed and nonpremixed configurations that include a new extensive set of smoke point measurements for mixtures of a given fuel with several oxygenated molecules. The current approach unifies the conclusions on the relative efficiency of the various oxygenated functionalities such as alcohols, esters, ethers, and carbonyl groups and provides a potential explanation for the seemingly contradictory trends exhibited by some raw experimental data.

Kim, S.H. & Pitsch, H., Analytic Solution for a HigherOrder Lattice Boltzmann Method: Slip Velocity and Knudsen Layer. Physical Review E, 78(1), p.016702. 2008.
We present the analysis of a higherorder lattice Boltzmann (LB) method based on the fourthorder GaussHermite quadrature, with emphasis on the slip velocity and the Knudsen layer. The exact solution of the slip velocity for the higherorder LB equation is obtained for Poiseuille flows with finite Knudsen numbers. Due to increased accuracy in velocity space discretization, the higherorder scheme gives much improved slip coefficients as compared with the standard LB method based on the thirdorder GaussHermite quadrature. A multiple relaxation time model is investigated to show the effects of the relaxation times for higherorder moments on the slip phenomena.

Balarac, G., Pitsch, H. & Raman, V., Development of a dynamic model for the subfilter scalar variance using the concept of optimal Estimators. Physics of Fluids, 20(3), p.035114. 2008.
The concept of optimal estimators, recently introduced by Moreau et al. Phys. Fluids18, 1 (2006) is used as an a priori tool to discuss the accuracy of subfilter models. Placed in the framework of largeeddy simulation of combustion problems, this work focuses on the subfilter models used to evaluate the subfilter variance of a conserved scalar, the mixture fraction. The a priori tests are performed using 512^3 direct numerical simulation data of forced homogeneous isotropic turbulence. First, the performance of the most commonly used models for the subfilter variance is studied. Using optimal estimators, the Smagorinskytype model Pierce and Moin, Phys. Fluids10, 3041 (1998) is shown to have the best set of parameters. However, the conventional dynamic formulation of the model leads to large errors in the variance prediction. It was found that assumptions used in the model formulation are not verified. A new dynamic procedure based on a Taylor series expansion is then proposed to improve the predictive accuracy. The a priori tests show that the new model substantially improves predictive accuracy.

Walch, S., Dhanda, A., Aryanpour, M. & Pitsch, H., Mechanism of Molecular Oxygen Reduction at the Cathode of a PEM Fuel Cell: NonElectrochemical Reactions on Catalytic Pt Particles. The Journal of Physical Chemistry C, 112(22), pp.84648475. 2008.
Various catalytic reactions in proton exchange membrane (PEM) fuel cells are discussed, and the effects of different steps, parallel pathways, and side reactions are analyzed for the oxygen reduction reaction (ORR) mechanism. A suitable mechanism table is formulated for the prominent pathway of ORR. The kinetics of the proposed nonelectrochemical reactions on platinum surfaces are then studied using the B3LYP density functional theory (DFT) with the Wadt and Hay relativistic ECPs and basis sets augmented with a 4f function on Pt. The reactions considered are O2(ads) ↔ O(ads) + O(ads) (1), O2H(ads) ↔ O(ads) + OH(ads) (2), H2O(ads) ↔ H(ads) + OH(ads) (3), OH(ads) + OH(ads) ↔ O(ads) + H2O(ads) (4), and OH(ads) + O(ads) ↔ O(ads) + OH(ads) (5). Calibration calculations are carried out for reaction 1 on a single Pt atom using the CASSCF/MRCI method with a ccpVDZ/relativistic ECP basis set for Pt and the augccpVDZ basis set for O. Comparison with B3LYP DFT calculations shows that the latter method overestimates binding energies by more than a factor of 2, but the barrier heights with respect to reactants are accurate. This result is consistent with calculations for the outer minimum for molecular oxygen on Pt(111), where the computed binding energy is about a factor of 2 larger than what is found from experiments. We find that, while a Pt2 cluster gives qualitatively correct results, the results are strongly influenced by cluster size effects, and inclusion of nearest neighbor atoms in at least the top and second layers is necessary for accurate energetics. The rates computed within the conventional transition state theory plus a Wigner estimate for tunneling using barriers obtained from the largest clusters show that the second, fourth, and fifth reactions are most important on the surface of catalytic Pt particles. Solvation effects were investigated using a model consisting of cyclic (H2O)n structures and were found to be small.

Knudsen, E. & Pitsch, H., A Dynamic Model for the Turbulent Burning Velocity for LES of Premixed Combustion. Combustion and Flame, 154(4), pp.740760. 2008.
Turbulent premixed combustion is particularly difficult to describe using large eddy simulation (LES). In LES, premixed flame structures typically exist on subfilter length scales. Consequently, premixed LES models must be capable of describing how completely unresolved flame structures propagate under the influence of completely unresolved eddies. This description is usually accomplished through the implementation of a model for the turbulent burning velocity. Here, a dynamic model for describing the turbulent burning velocity in the context of LES is presented. This model uses a new surface filtering procedure that is consistent with standard LES filtering. Additionally, it only uses information that comes directly from the flame front. This latter attribute is important for two reasons. First, it guarantees that the model can be consistently applied when level set methods, where arbitrary constraints can be imposed on field variables away from fronts, are used to track the flame. Second, it forces the model to recognize that the physics governing flame front propagation are only valid locally at the front. Results showing model validation in the context of direct numerical simulation (DNS), and model application in the context of LES, are presented.

Balarac, G., Pitsch, H. & Raman, V., Modeling of the SubFilter Scalar Dissipation Rate Using the Concept of Optimal Estimators. Physics of Fluids, 20(9), p.091701. 2008.
In this work, modeling of the subfilter scalar dissipation rate is addressed. First, the best set of quantities to write a model is determined using the concept of optimal estimators. This study shows that the best approach is to assume a proportionality between the turbulent time scale and turbulent scalar mixing time scale. It is shown that the turbulent time scale should be defined by the subfilter kinetic energy. To define the coefficient appearing in this model, a dynamic determination based on a global subfilter equilibrium assumption between the dissipation and the production terms leads to the best results.

Aryanpour, M., Dhanda, A. & Pitsch, H., An algorithm for mass matrix calculation of constrained molecular geometries. Journal of Chemical Physics, 128, p.044113. 2008.
Dynamic models for molecular systems require the determination of corresponding mass matrix. For constrained geometries, these computations are often not trivial but need special considerations. Here, assembling the mass matrix of internally constrained molecular structures is formulated as an optimization problem. Analytical expressions are derived for the solution of the different possible cases depending on the rank of the constraint matrix. Geometrical interpretations are further used to enhance the solution concept. As an application, we evaluate the mass matrix for a constrained molecule undergoing an electrontransfer reaction. The preexponential factor for this reaction is computed based on the harmonic model.

Desjardins, O., Moureau, V. & Pitsch, H., An Accurate Conservative Level Set/Ghost Fluid Method for Simulating Turbulent Atomization. Journal of Computational Physics, 227(18), pp.82098488. 2008.
This paper presents a novel methodology for simulating incompressible twophase flows by combining an improved version of the conservative level set technique introduced in E. Olsson, G. Kreiss, A conservative level set method for two phase flow, J. Comput. Phys. 210 (2005) 225–246 with a ghost fluid approach. By employing a hyperbolic tangent level set function that is transported and reinitialized using fully conservative numerical schemes, mass conservation issues that are known to affect level set methods are greatly reduced. In order to improve the accuracy of the conservative level set method, high order numerical schemes are used. The overall robustness of the numerical approach is increased by computing the interface normals from a signed distance function reconstructed from the hyperbolic tangent level set by a fast marching method. The convergence of the curvature calculation is ensured by using a least squares reconstruction. The ghost fluid technique provides a way of handling the interfacial forces and large density jumps associated with twophase flows with good accuracy, while avoiding artificial spreading of the interface. Since the proposed approach relies on partial differential equations, its implementation is straightforward in all coordinate systems, and it benefits from high parallel efficiency. The robustness and efficiency of the approach is further improved by using implicit schemes for the interface transport and reinitialization equations, as well as for the momentum solver. The performance of the method is assessed through both classical level set transport tests and simple twophase flow examples including topology changes. It is then applied to simulate turbulent atomization of a liquid Diesel jet at Re=3000Re=3000. The conservation errors associated with the accurate conservative level set technique are shown to remain small even for this complex case.

Kim, S.H., Pitsch, H. & Boyd, I.D., Slip Velocity and Knudsen Layer in the Lattice Boltzmann method for microscale flows. Physical Review E, 77(2), p.026704. 2008.
We present mesoscopic fluidwall interaction models for lattice Boltzmann (LB) model simulations of microscale flows. The exact solution of the slip velocity for the LB equation with the BhatnagarGrossKrook collision operator is obtained for Poiseuille flow at finite Knudsen numbers. With a consistent definition of the Knudsen number, the slip coefficients of the LB equation with the standard D2Q9 scheme are found to be slightly larger than those of the Boltzmann equation with the same boundary condition, which makes the standard LB method remain quantitatively accurate only for small Knudsen numbers. By modifying the nonequilibrium energy flux or introducing the effective relaxation time, the LB method is analytically shown to reproduce the slip phenomena up to second order in the Knudsen number. For the standard LB method, the Knudsen layer is captured only with modification of the relaxation dynamics such as in the effective relaxation time model.

Ihme, M. & Pitsch, H., Prediction of Extinction and Reignition in NonPremixed Turbulent Flames Using a Flamelet/Progress Variable Model. Part 1: A Priori Study and Presumed PDF Closure. Combustion and Flame, 155(12), pp.7089. 2008.
Previously conducted studies of the flamelet/progress variable model for the prediction of nonpremixed turbulent combustion processes identified two areas for model improvements: the modeling of the presumed probability density function (PDF) for the reaction progress parameter and the consideration of unsteady effects Ihme et al., Proc. Combust. Inst. 30 (2005) 793. These effects are of particular importance during local flame extinction and subsequent reignition. Here, the models for the presumed PDFs for conserved and reactive scalars are reexamined and a statistically most likely distribution (SMLD) is employed and tested in a priori studies using direct numerical simulation (DNS) data and experimental results from the Sandia flame series. In the first part of the paper, the SMLD model is employed for a reactive scalar distribution. Modeling aspects of the a priori PDF, accounting for the bias in composition space, are discussed. The convergence of the SMLD with increasing number of enforced moments is demonstrated. It is concluded that information about more than two moments is beneficial to accurately represent the reactive scalar distribution in turbulent flames with strong extinction and reignition. In addition to the reactive scalar analysis, the potential of the SMLD for the representation of conserved scalar distributions is also analyzed. In the a priori study using DNS data it is found that the conventionally employed beta distribution provides a better representation for the scalar distribution. This is attributed to the fact that the betaPDF implicitly enforces higher moment information that is in excellent agreement with the DNS data. However, the SMLD outperforms the beta distribution in free shear flow applications, which are typically characterized by strongly skewed scalar distributions, in the case where higher moment information can be enforced.

Jerzembeck, S., Dahms, R., Röhl, O. & Peters, N., Thermo diffusive instability phenomena of stoichiometric up to very rich spherical flames of nheptaneairmixtures. In 5th European ThermalSciences Conference: 18th  22nd May 2008, Eindhoven, Netherlands. 2008.

PepiotDesjardins, P. & Pitsch, H., An automatic chemical lumping method for the reduction of large chemical kinetic mechanisms. Combustion Theory and Modelling, 12(6), pp.10891108. 2008.
A novel approach to the lumping of species in large chemical kinetic mechanisms is presented. Species with similar composition and functionalities are lumped into one single representative species. Simulations using the detailed scheme are used to gather statistical information on the distribution of the isomers within each lump group. These distributions are functions of space and time. Closure is performed in state space by approximating these distribution functions as the conditional averages depending on the independent state space variables of the lumped scheme. This approach is simplified further, so that the resulting chemical mechanisms can be used directly in standard chemistry packages. For this purpose, only the dependence of the isomer distributions on the temperature is retained, and optimal correcting factors are incorporated into the Arrhenius form of the rate coefficients of lumped reactions. Validation is performed using two comprehensive mechanisms for nheptane and isooctane oxidation. In all cases, a very good agreement is observed between the predictions obtained using the detailed and the lumped mechanisms. Effects of the lumping procedure on sensitivities of the kinetic scheme and on isomer concentrations are studied. Also, integration of this lumping approach into a multistage reduction strategy is discussed and illustrated.

Ihme, M. & Pitsch, H., Prediction of Extinction and Reignition in NonPremixed Turbulent Flames Using a Flamelet/Progress Variable Model. Part 2: Application in LES of Sandia Flames D and E. Combustion and Flame, 155(12), pp.90107. 2008.

Rai, V., Aryanpour, M. & Pitsch, H., A FirstPrinciples Analysis of OContaining Adsorbates Formed from the Electrochemical Discharge of Water on Pt(111). The Journal of Physical Chemistry C, 112(26), pp.97609768. 2008.
A combination of density functional theory (DFT) calculations and dynamic Monte Carlo (DMC) simulations was employed to study the process of Ocontaining adsorbates formed from the electrochemical discharge of water on Pt(111) in acidic environment. Potentialdependent activation energy and rate coefficients, as input for DMC simulations, were obtained for the electrochemical reactions and for use in DFT calculations. From DMC simulations, we find that OHads is the dominant adsorbate between 0.5−0.8 V, but above 0.8 V OHads and Oads coexist. Ordered structures are found for OHads at 0.8 V and for Oads at 0.9 V. These results agree well with cyclic voltammetry and electrochemical−Xray photoelectron spectroscopy measurements.

Vanegas, A., Won, H.W., Felsch, C., Gauding, M. & Peters, N., Experimental investigation of the effect of multiple injections on combustion efficiency and pollutant formation in a commonrail DI diesel engine. In SAE 2008 International World Congress: Compression ignition combustion processes, 14th  17th April 2008, Detroit, Michigan, USA. 2008.

Wang, L. & Peters, N., Dissipation element analysis of turbulent scalar fields. Physica Scripta, 2008(T132), p.014006. 2008.
The field of the fluctuating scalar obtained from Direct Numerical Simulation (DNS) in homogeneous shear flow is subdivided into finite size regions within which it varies monotonously. These regions are called dissipation elements and are identified by calculating trajectories normal to isoscalar surfaces starting from every grid point until a minimum and a maximum point is reached. Two parameters describe the statistical properties of dissipation elements sufficiently well: the linear distance between the minimum and maximum points and the absolute value of the scalar difference at these points. The joint probability density function of these parameters decomposes into a conditional pdf of the scalar difference and the marginal pdf of the distance between the minimum and maximum points, the latter being the object of this study.For the length scale distribution function a stochastic evolution equation was derived in a companion paper. It implies the cutting and reconnection of linear elements and the effect of molecular diffusion. The equation is an integral equation and must be solved numerically. In this paper we show how onedimensional simulations of randomly generated scalar profiles would illustrate the cutting and reconnection processes as well as the drift and disappearance of small elements by molecular diffusion. The resulting distribution function from the simulation shows good agreement with the predicted distribution function. It is concluded that the mean distance between extremal points is of the order of the scalar Taylor length

Kim, S.H., Pitsch, H. & Boyd, I.D., Accuracy of higherorder lattice Boltzmann methods for microscale flows with finite Knudsen numbers. Journal of Computational Physics, 227(19), pp.86558671. 2008.
Accuracy of the lattice Boltzmann (LB) method for microscale flows with finite Knudsen numbers is investigated. We employ up to the eleventhorder Gauss–Hermite quadrature for the lattice velocities and diffusescattering boundary condition for fluid–wall interactions. Detailed comparisons with the direct simulation Monte Carlo (DSMC) method and the linearized Boltzmann equation are made for planar Couette and Poiseuille flows. All higherorder LB methods considered here give improved results as compared with the standard LB method. The accuracy of the LB hierarchy, however, does not monotonically increase with the order of the Gauss–Hermite quadrature at moderate and large Knudsen numbers. The results also show the sensitivity to a quadrature chosen, even when the Gauss–Hermite quadratures have the same order of formal accuracy. Among the schemes investigated here, D2Q16 is the most efficient method and offers a quantitative prediction in the slip and transition regimes. The higherorder LB methods predict the Knudsen layer up to Kn=O(0.1)Kn=O(0.1). The Knudsen layer, however, rapidly disappears when the Knudsen number approaches unity due to a finite number of the lattice velocities, while it is still present for Kn=O(1)Kn=O(1) in the Boltzmann equation. It is also found that the higherorder LB methods adopted here do not capture the asymptotic behavior of the Boltzmann equation at large Knudsen numbers.

Barths, H., Felsch, C. & Peters, N., Mixing models for the twowaycoupling of CFD codes and zerodimensional multizone codes to model HCCI combustion. Combustion and Flame, 155(3), p.440. 2008.
The objective of this work is the development of a consistent mixing model for the twowaycoupling of a CFD code and a multizone code based on multiple zerodimensional reactors. The twowaycoupling allows for a computationally efficient modeling of HCCI combustion. The physical domain in the CFD code is subdivided into multiple zones based on three phase variables (fuel mixture fraction, dilution, and total enthalpy). Those phase variables are sufficient for the description of the thermodynamic state of each zone, assuming that each zone is at the same pressure. Each zone in the CFD code is represented by a corresponding zone in the zerodimensional code. The zerodimensional code solves the chemistry for each zone, and the heat release is fed back into the CFD code. The difficulty in facing this kind of methodology is to keep the thermodynamic state of each zone consistent between the CFD code and the zerodimensional code after the initialization of the zones in the multizone code has taken place. The thermodynamic state of each zone (and thereby the phase variables) will change in time due to mixing and source terms (e.g., vaporization of fuel, wall heat transfer). The focus of this work lies on a consistent description of the mixing between the zones in phase space in the zerodimensional code, based on the solution of the CFD code. Two mixing models with different degrees of accuracy, complexity, and numerical effort are described. The most elaborate mixing model (and an appropriate treatment of the source terms) keeps the thermodynamic state of the zones in the CFD code and the zerodimensional code identical. The models are applied to a test case of HCCI combustion in an engine.

Felsch, C., Vanegas, A., Kerschgens, B., Peters, N., Hoffmann, K., Drews, P., Abel, D. & Barths, H., Systematic reduction of an interactively coupled CFDmultizoneapproach towards a standalone multizone model for PCCI combustion. In Conference on Thermo and Fluid Dynamic Processes in Diesel Engines: 9th  12th September 2008, Valencia, Spain. 2008.

Pitsch, H., Desjardins, O., Balarac, G. & Ihme, M., LargeEddy Simulation of Turbulent Reacting Flows. Progress in Aerospace Sciences, 44(6), pp.466478. 2008.
In this paper, a few advances and challenges towards predictive largeeddy simulations of turbulent reacting flow are discussed in the context of numerical simulations of aircraft engines, which require an adequate description of liquid fuel injection, liquid fuel atomization, drop breakup, drop dynamics, and evaporation, largescale turbulent fuel air mixing, small scale molecular fuel air mixing, chemical reactions, and turbulence/chemistry interactions. We have identified three of the most important and most challenging modeling problems in this process, namely primary atomization, subfilter scalar mixing, and pollutant formation. Some recent progress on all three topics is presented.

Ihme, M. & Pitsch, H., Modeling of Radiation and NO Formation in Turbulent NonPremixed Flames using a Flamelet/Progress Variable Formulation. Physics of Fluids, 20(5), p.055110. 2008.
A model for the prediction of the nitric oxide (NO) formation in turbulent nonpremixed flames is proposed. Since the NO formation has a strong temperature sensitivity, the accurate prediction of the flame temperature under the consideration of radiative heat losses is required. The first part of the paper addresses the extension of a flameletbased combustionmodel to account for radiative heat loss effects by introducing enthalpy as an additional parameter. A transport equation for enthalpy is solved, and the radiative sink term in this equation is obtained from unsteady flamelet solutions. The model is applied to a largeeddy simulation(LES) of Sandia flame D, and the importance of the interaction between turbulence and radiation on temperature and mixture fraction is investigated. Based on the radiative flamelet formulation, a consistent model for the prediction of NO formation is developed in the second part of the paper. In this model, an additional transport equation for the NO mass fraction is solved, and the chemical source term is obtained from a flamelet library. Since the consumption rate is dependent on the NO mass fraction, this term requires modeling, which is discussed in this paper. By employing a scale similarity argument, a closure model for application in LES is presented. After the analysis of the proposed model for the thermal, nitrous oxide, and prompt pathways for NO formation, the NO model is integrated into the extended flamelet/progress variable model and applied in LES of Sandia flame D and a Pratt & Whitney aircraft engine combustor configuration.

Ihme, M., Marsden, A.L. & Pitsch, H., Generation of Optimal Artificial Neural Networks Using a Pattern Search Algorithm: Application to Approximation of Chemical Systems. Neural Computation, 20(2), pp.573601. 2008.
A pattern search optimization method is applied to the generation of optimal artificial neural networks (ANNs). Optimization is performed using a mixed variable extension to the generalized pattern search method. This method offers the advantage that categorical variables, such as neural transfer functions and nodal connectivities, can be used as parameters in optimization. When used together with a surrogate, the resulting algorithm is highly efficient for expensive objective functions. Results demonstrate the effectiveness of this method in optimizing an ANN for the number of neurons, the type of transfer function, and the connectivity among neurons. The optimization method is applied to a chemistry approximation of practical relevance. In this application, temperature and a chemical source term are approximated as functions of two independent parameters using optimal ANNs. Comparison of the performance of optimal ANNs with conventional tabulation methods demonstrates equivalent accuracy by considerable savings in memory storage. The architecture of the optimal ANN for the approximation of the chemical source term consists of a fully connected feedforward network having four nonlinear hidden layers and 117 synaptic weights. An equivalent representation of the chemical source term using tabulation techniques would require a 500 x 500 grid point discretization of the parameter space.

Desjardins, O., Blanquart, G., Balarac, G. & Pitsch, H., High Order Conservative Finite Difference Scheme for Variable Density Low Mach Number Turbulent Flows. Journal of Computational Physics, 227(15), pp.71257159. 2008.
The high order conservative finite difference scheme of Morinishi et al. Y. Morinishi, O.V. Vasilyev, T. Ogi, Fully conservative finite difference scheme in cylindrical coordinates for incompressible flow simulations, J. Comput. Phys. 197 (2004) 686 is extended to simulate variable density flows in complex geometries with cylindrical or cartesian nonuniform meshes. The formulation discretely conserves mass, momentum, and kinetic energy in a periodic domain. In the presence of walls, boundary conditions that ensure primary conservation have been derived, while secondary conservation is shown to remain satisfactory. In the case of cylindrical coordinates, it is desirable to increase the order of accuracy of the convective term in the radial direction, where most gradients are often found. A straightforward centerline treatment is employed, leading to good accuracy as well as satisfactory robustness. A similar strategy is introduced to increase the order of accuracy of the viscous terms. The overall numerical scheme obtained is highly suitable for the simulation of reactive turbulent flows in realistic geometries, for it combines arbitrarily high order of accuracy, discrete conservation of mass, momentum, and energy with consistent boundary conditions. This numerical methodology is used to simulate a series of canonical turbulent flows ranging from isotropic turbulence to a variable density round jet. Both direct numerical simulation (DNS) and large eddy simulation (LES) results are presented. It is observed that higher order spatial accuracy can improve significantly the quality of the results. The error to cost ratio is analyzed in details for a few cases. The results suggest that high order schemes can be more computationally efficient than low order schemes.