Investigation of Numerical Effects on the Flow and Combustion in LES of ICE

Abstract

This work investigates the influence of numerical dissipation on the modeled combustion in Large Eddy Simulations (LES). It is well known that capturing the dynamics of the in-cylinder flow is crucial for engine simulations, as it strongly affects flame propagation. The flame propagation during the power-stroke highly depends on the turbulence level that is developed throughout compression. This turbulence level will be strongly influenced by the accuracy of the numerical schemes employed. Even a small extent of upwinding, filtering, low-order implicit time-stepping, cell-stretching or mapping between grids may affect the flow field, the turbulence level, and hence the turbulent flame speed and the pressure curve. To provide a reference, the LES in-house code PsiPhi is used, which ensures a minimum of dissipation due to high order explicit time-stepping, homogeneous and isotropic filters and cells. Good stability of the code permits the use of a second-order Central Differencing Scheme (CDS) for the transport of momentum, avoiding numerical dissipation. To analyse the effect of numerical dissipation, simulations of a fired engine are performed using different numerical schemes for the convection of momentum. Physical quantities including the total kinetic energy, the velocity gradient, the turbulent viscosity, the in-cylinder pressure, the flame propagation or the burning rate of different test cases are evaluated and compared to each other to show the numerical effects on combustion. Furthermore, the suitability of common LES quality criteria including an energy criterion and viscosity ratio is discussed based on the comparison of simulations with less and more accurate numerics. It is shown that these LES quality indicators can be highly misleading.