Assessment of Large-Eddy Simulations of Turbulent Round Jets Using Low-Order Numerical Schemes

The basic idea behind large-eddy simulation (LES) is to accurately resolve the large energy-containing scales and to use subgrid-scale (SGS) models for the smaller scales. The accuracy of LES can be significantly impacted by the numerical discretization schemes and the choice of the SGS model. This work investigates the accuracy of low-order LES codes in the simulation of a turbulent round jet which is representative of fuel jets in engines. The turbulent jet studied is isothermal with a Reynolds number of 6800. It is simulated using Converge, which is second-order accurate in space and first-order in time, and FLEDS, developed at Purdue University, which is sixth-order accurate in space and fourth-order in time. The high-order code requires the resolution of acoustic time-scales and hence is approximately 10 times more expensive than the low-order code. It is shown that the high-order code is able to accurately capture both the potential core length and the radial spreading rate, whereas the low-order code over-predicts the potential core length by 200% and the spreading rate by 15%. A systematic study of the influence of the SGS model and the amplitude of the inflow perturbation is then performed with the low-order code to determine if its results can be altered to more closely match those of the high-order code. Based on this study, the recommendation is to increase the amplitude of the inflow perturbation by 10 times to predict the correct potential core length and to use the dynamic structure (DS) SGS model instead of the Smagorinsky model to more accurately predict the radial spreading rate. It is also shown that by employing the DS model, the low-order code is able to correctly predict the energy cascade from the resolved scales to the subgrid scales. The low-order code is then employed to simulate a non-isothermal turbulent round jet with a Reynolds number of 250,000 which is relevant for gasoline direct injection (GDI) engine applications. It is shown that using the recommended values for the vortex ring amplitude and the choice of the SGS model, the low-order code is able to predict the correct potential core length and spreading rate for the jet.