International audienceThis article presents a comparison between experiments and Large-Eddy Simulation (LES) of a spark ignition engine on two operating points: a stable one characterized by low cycle-to-cycle variations (CCV) and an unstable one with high CCV. In order to match the experimental cycle sample, 75 full cycles (with combustion) are computed by LES. LES results are compared with experiments by means of pressure signals in the intake and exhaust ducts, in-cylinder pressure, chemiluminescence and OH Planar Laser Induced Fluorescence (PLIF). Results show that LES is able to: (1) reproduce the flame behavior in both cases (low and high CCV) in terms of position, shape and timing; (2) distinguish a stable point from an unstable one; (3) predict quantitatively the CCV levels of the two fired operating points. For the unstable case, part of the observed CCV is due to incomplete combustion. The results are then used to analyze the incomplete combustion phenomenon which occurs for some cycles of the unstable point and propose modification of the spark location to control CCV

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This paper describes extensions and tests of characteristic methods for outlet boundary conditions in compressible solvers. Three methods based on the specification of ingoing waves using one-and multi-dimensional approximations are extended to unstructured grids. They are first compared for weak to strong vortices propagating on low to high speed mean flows through outlet sections. A major issue is to determine the Mach number to be used in the specification of the transverse terms which must be taken into account in the ingoing wave amplitude specifications. For the vortex computations, results show that the averaged Mach number leads to better results than its local value. The boundary conditions are then tested in a more complex case: the flow around a turbine blade. A reference solution using a long distance between the blade trailing edge and the outlet plane is first computed: for this solution, outlet boundary conditions have almost no effect on the flow around the blade. The distance between 1 PhD student, CFD Team CERFACS. 2 Senior Research Fellow, CFD Team CERFACS.

This paper describes a compressible Large Eddy Simulation (LES) used to investigate cyclic variations for nonreacting flow in an optical single cylinder engine setup. The simulated operating point is part of a large experimental database designed to validate LES for cycle-to-cycle prediction, and constitutes a first step towards the realization of fired operating points. The computational domain covers almost the whole experimental setup (intake and exhaust plenums, intake and exhaust ducts, cylinder) to account for acoustic phenomena. The assessment of the computation is performed in two regions of the domain: the intake and exhaust duct predictions are compared to the results of a Helmholtz solver and the experiment (pressure transducers and Particle Image Velocimetry (PIV)) while the in-cylinder dynamics are compared to PIV measurements. The ability of the B. Enaux · V. Granet (B) · O. Vermorel 154 Flow Turbulence Combust (2011) 86:153-177 developed methodology to capture the correct level of cycle-to-cycle variations is demonstrated considering in-cylinder pressure and velocity fields predictions. Cycleto-cycle variations in velocity are highlighted and localized using a proper orthogonal decomposition analysis.