LES/RANS Simulations of Turbulent Mixing in Gas Plumes

Abstract

A hybrid Large Eddy Simulation/Reynolds-averaged Navier-Stokes (LES/RANS) model is used to simulate a set of high-speed experiments of sonic gas injection into a Mach 2.0 air stream (Lin, et al, AIAA Paper 2009-1423) in order to gather data for the assessment of two RANS-level models for the turbulent mass diffusivity (Xiao, et al., AIAA Journal, Vol. 44, Brinckman, et al., AIAA Journal, Vol. 46). The LES/RANS closure is validated by comparing the injected ethylene mole fraction with Raman scattering and NO-PLIF data from the experiment. The simulation is in general good agreement with the experiment but slightly under-predicts the level of jet entrainment. Two further simulations are conducted with the ethylene injectant replaced with helium and argon. The LES/RANS data is mined to extract resolved turbulence statistics which are then used to assess the performance of both mass diffusivity models. It is found that both models perform poorly in the context of a classical gradient-diffusion formulation for the scalar flux; a new formulation is proposed which improves the performance of both models. The LES/RANS data is then used to optimize the model constants of both models. Further evaluation of the optimized models shows that they are in good agreement with LES/RANS values of scalar turbulence production for both the sonic injection flow and a planar mixing layer (Barre, et al., Journal of Fluid Mechanics, Vol. 259).