A Hybrid One-Equation Large Eddy Simulation Technique with URANS Computation of the Wall Region. Application to Backward-Step Flow

摘要

In this paper we propose a zonal hybrid between large-eddy simulation (LES) and Reynolds-averaged Navier-Stokes equations (RANS) techniques, the latter taking over in near-wall regions. Both the LES and RANS use a one-equation model; the main difference between the two lies in the specification of the length scale. To avoid a sudden jump in this length scale, a transition region between LES and RANS is specified, in which a smoothly-varying weighted average length scale between LES and RANS is used. This approach was applied to a backward-facing step flow with an expansion ratio of 1.5, at a Reynolds number of 5540 based on inlet bulk velocity and the height of the step. These conditions match the experimental conditions of Kasagi et al. (1995). The backward step flow is a standard test case of massively separated flow. Turbulent structures at the inlet are calculated by channel DNS. The reattachment length is 6.97 step heights, an error of 7.1% compared with Kasagi et al.'s experiment data. The peaks of Reynolds stresses are somewhat underpredicted near the step, but farther downstream they agree well. In contrast, mean stream velocity is well predicted near the step, but far from the step, the mean flow seems to be flexion down. The mean wall-normal velocity components was also agree well with experimental data.