The underlying correlations between cyclic variability in the velocity field, spray boundary conditions and the spatial distribution of equivalence ratio in a realistic direct injection spark ignition engine have been investigated by means of Proper Orthogonal Decomposition (POD). The method of snapshots has been employed to perform both phase-dependent and phase-independent decomposition of the scalar-velocity correlations. LES based simulation of 30 engine cycles has been used for POD analysis

di Mare, Francesca

Goryntsev, Dmitry

Janicka, Johannes

English

Di Mare, F.

Goryntsev, D.

Janicka, J.

TUbiblio

LES of the flow in a DISI engine: analysis of turbulent scalar - velocity correlations,

Institute of Transport Research:Publications

Les Rencontres Scientifiques de l'IFP – LES for Internal Combustion Engine Flows - 18-19 November 2010   Copyright © 2010, IFP    acted. The POD methodology is further described in [2]. ariable spray boundary conditions is shown in Figure 3. LES of the flow in a DISI engine: analysis of turbulent scalar – velocity correlations  Francesca di Mare1*, Dmitry Goryntsev2, Johannes Janicka2  1 German Aerospace Center, Institute of Propulsion Technology, 51147 Köln, Germany, e-mail: francesca.dimare@dlr.de 2 EKT, Department of Mechanical and Process Engineering, TU-Darmstadt, Germany, e-mail: digor@ekt.tu-darmstadt.de * Corresponding author  Abstract The underlying correlations between cyclic variability in the velocity field, spray boundary conditions and the spatial distribution of equivalence ratio in a realistic direct injection spark ignition engine have been investigated by means of Proper Orthogonal Decomposition (POD). The method of snapshots has been employed to perform both phase-dependent and phase-independent decomposition of the scalar-velocity correlations. LES based simulation of 30 engine cycles has been used for POD analysis.  INTRODUCTION Direct injection spark ignition (DISI) engines have a large potential to reduce emissions and specific fuel consumption. Large Eddy Simulation (LES) based analysis is used to characterize the cyclic variations of the flow field and their impact on the mixture preparation and combustion processes in a realistic IC-engine. The analysis of the effect of the cycle-to-cycle velocity fluctuations on the spray injection and mixing preparation processes has been reported in [1]. Proper Orthogonal decomposition has been applied to investigate cyclic variability in motored optically accessible engines [3-5], more rarely to the analysis of numerical data. Large Eddy Simulations can provide very detailed information on the flow field, with almost unlimited ability to capture the finest details of complex geometries, and be therefore effectively paired to advanced analysis techniques as POD.  In the present work, the method of snapshot proposed by Sirovich [6] has been adopted to investigate the existence of correlations between large-scale motions and the distribution of fuel under variable injection conditions.  1 NUMERICAL METHODS AND INVESTIGATED CONFIGURATION The investigated configuration represents a four stroke DISI engine with variable charge motion system. This is a realistic IC engine with four canted valves, an asymmetric cylinder head and an asymmetric piston bowl [1]. Data in the tumble plane at 315° CA (i.e. where the highest cyclic variability has been observed) have been selected over 30 cycles to perform at this initial stage of the investigation only a phase-dependent POD decomposition of the scalar and velocity modes. In both cases 30 modes were extr2 RESULTS AND DISCUSSION Figures 1-3 present the comparison of intensity of cycle-to-cycle fluctuations of velocity (left) and mass fraction (right) in the cross section of combustion chamber for the cases described below. The investigation has been split into 3 stages in order to highlight the following processes: a) the effect of cyclic velocity variations on the mixture preparation processes, see Figure 1. b) The effect of variable spray boundary conditions on the flow field pattern, see Figure 2. This analysis helps to separate the effects of velocity and spray cyclic fluctuations on the mixing process. c) The joint effect of both velocity cyclic fluctuations and v Figure 1: Root mean square of velocity (left) and mass fraction (right) at Les Rencontres Scientifiques de l'IFP -  LES for Internal Combustion Engine Flows - 18-19 November 2010     CA = 315°. Two-phase flow with constant spray boundary conditions.  Figure 2: Root mean square of velocity (left) and mass fraction (right) at CA = 315°. Two-phase flow with variable spray boundary conditions.  Figure 3: Root mean square of velocity (left) and mass fraction (right) at CA = 315°. Two-phase flow with the joint effect of both velocity cyclic ding reconstruction ons of the reconstruction coefficients are presented. fluctuations and variable spray boundary conditions. It is obviously possible to perform a joint decomposition of the scalar-velocity correlations. Although the physical significance of such quantities is clear (turbulent scalar fluxes), the interpretation of the POD mode and relative reconstruction coefficients is not intuitive. For this reason a different procedure has been adopted, where the velocity and scalar field have been independently decomposed and the corresponcoefficient subsequently analysed. In Figure 4 the eigenvalues and energy convergence for the scalar and velocity fields are shown, while in Figure 5 the temporal (i.e. cyclic) variati ; b) Eigenvalues k .  Figure 5: Time (cycle) dependence of the reconstruction coefficients for velocity (solid line) and scalar (dashed line) field. a)  b)  Figure 6: First scalar (a) and velocity (b) mode. Inspection of the contours of the modes extracted from the scalar and velocity correlations (Figures 6 and 7) indicate the presence of a strong coupling, as expected, between the large structures in the velocity field and the distribution of the scalar variance, however provides little insight in the relationship, if any, between the modal components of the scalar and velocity. If the reconstruction coefficients are represented as scatter plot (see Figure 8), it is evident that a strong correlation exists between component 1 of both scalar and velocity but also between component 3 of the velocity field and 2 of the scalar field. On the contrary, several modes have been found to be in opposition of phase as expected. Although the analysis presented in this work focussed only on one particular crank angle, the results obtained are encouraging and show that the technique adopted could be used to investigate how the correlation identified in this particular case could evolve along the entire cycle and over a number of cycles. Figure 4: a) Energy convergence:  kkLes Rencontres Scientifiques de l'IFP - Advances in Hybrid Powertrains  - 25-26 November 2008 - Proceedings    a)  b)  Figure 7: Second scalar (a) and velocity (b) mode. Figure 8: Correlation between reconstruction coefficients of the first 5 modes of the velocity and scalar fields. Solid line: self correlation of the velocity coefficients for visual aid.  CONCLUSIONS AND FUTURE WORK The results of a highly resolved Large Eddy Simulation of IC engine with variable injection parameters have been analysed by means of phase-dependent POD. Inspection of the scalar and velocity reconstruction coefficient reveal interesting correlation patterns which could be used to determine the relative sensitivity of the corresponding modes to variation in the configuration parameters. A phase-independent investigation is needed to assess further the correlations between the scalar and velocity fields and also the possibility of reconstructing entire dataset from existing information. ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support by the FVV-project No. 896 and SFB 568. F. di Mare wishes to thank Prof. R. Mönig and Dr. E. Kügeler at DLR-Köln for their continued support during this work. REFERENCES 1. Goryntsev D., Sadiki A., Klein M. and Janicka J. (2009) Large Eddy Simulation based analysis of the effects of cycle-to-cycle variations on air-fuel mixing in realistic DISI IC-engines. Proc. of the Combustion Institute, 32-2: 759-2766. 2. Böhm, B., Di Mare, F. and Dreizler, A. Characterisation of cyclic variability in an optically accessible IC engine by means of POD, in proceedings of LES4ICE 2010. 3. Berkooz G., Holmes P., Lumley J. L. (1993) The Proper Orthogonal Decomposition in the analysis of turbulent flows. Annual Review of Fluid Mechanics, vol. 25, pp. 539-575. 4. Bizon K., Continillo G., Mancaruso E., Merola S.S. and Vaglieco B.M. (2010) POD-based analysis of combustion images in optically accessible engines. Combustion and Flame 157, 632-640. 5. Fogleman M., Lumley J., Rempfer, D and Haworth D., (2004) Application of the proper orthogonal decomposition to datasets of internal combustion engine flows. Journal of Turbulence, 5: 23. 6. Sirovich L. (1987) Turbulence and the dynamics of coherent structures, parts i-iii. Quart. J. Appl. Math., 45(3): 561-590.  

Application of the proper orthogonal decomposition to datasets of internal combustion engine flows.

Characterisation of cyclic variability in an optically accessible IC engine by means of POD,

Large Eddy Simulation based analysis of the effects of cycle-to-cycle variations on air-fuel mixing in realistic DISI IC-engines.

POD-based analysis of combustion images in optically accessible engines.

The Proper Orthogonal Decomposition in the analysis of turbulent flows.

Turbulence and the dynamics of coherent structures, parts i-iii.

LES of the flow in a DISI engine: analysis of turbulent scalar-velocity correlations

LES of the flow in a DISI engine: analysis of turbulent scalar-velocity correlations

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