RANS and LES of IC Engine Flows: A Comparative Study

Abstract

A comparative cold flow analysis between Reynolds-Averaged Navier-Stokes (RANS) and Large Eddy Simulation (LES) cycle-averaged velocity and turbulence predictions is carried out for a single cylinder engine with transparent combustion chamber (TCC) under motored conditions using high-speed Particle Image Velocimetry (PIV) measurements as the reference data. Simulations are done using a commercial CFD code CONVERGE with the implementation of standard k-ε and RNG k-ε turbulent models for RANS and a one-equation eddy viscosity model for LES. The following aspects are analyzed in this study: 1. The effects of computational domain geometry (with or without intake and exhaust plenums) on mean flow and turbulence predictions for both LES and RANS simulations 2. Comparison of LES versus RANS simulations in terms of their capability to predict mean flow and turbulence Both RANS and LES full and partial geometry simulations are able to capture the overall mean flow trends qualitatively; but the intake jet structure, velocity magnitudes, turbulence magnitudes and its distribution are more accurately predicted by full geometry simulations. The guideline therefore for CFD engineers is that RANS partial geometry simulations (computationally least expensive) are good enough for capturing overall qualitative flow trends. However, if one is interested in getting reasonably accurate estimates of velocity magnitudes, flow structures, turbulence magnitudes and its distribution, they must resort to LES simulations. Furthermore, to get the most accurate turbulence distributions, one must consider running LES full geometry simulations.