LES/RANS Simulations of High-Speed Mixing Processes

Abstract

A large-eddy simulation / Reynolds-averaged Navier-Stokes (LES/RANS) model is applied to three high-speed mixing layers to generate data suitable for evaluating two current RANS models for turbulent mass diffusivity (Xiao, et al., AIAA Journal, Vol. 44, Brinckman, et al., AIAA Journal, Vol. 46). These models solve transport equations for concentration variance and dissipation rate and differ in the constitutive relation for the turbulent mass diffusivity and the form of the evolution equation for the dissipation rate. The predictive capability of the LES/RANS model is assessed through simulations of an air-air mixing-layer experiment due to Barre, and co-workers (Barre, et al., Journal of Fluid Mechanics, Vol. 259). The LES/RANS model provides good predictions of the mean velocity, turbulence intensity, and rms temperature fluctuation throughout the flow field but slightly over-predicts the spreading rate of the mixing layer. Two other LES/RANS data sets, involving the replacement of the lower air stream in the Barre, et al. configuration by an argon stream and a helium stream, are also generated, and all data sets are ‘mined’ to extract turbulent mass diffusivities, concentration variances, and scalar dissipation rates associated with the resolved eddy motion. The results show that the mass diffusivity model of Xiao, et al. provides better agreement with the LES/RANS data. Both models, however, can be optimized to achieve a good match with the exact scalar-variance production rate. An evaluation of the two models for the scalar dissipation rate indicates that both proposals are in good agreement with LES/RANS dissipation values in the fully developed part of the mixing region, but with significant discrepancies present in the near-field mixing region.