Aspects of Hybrid RANS/LES Simulations of Subsonic Asymmetric Diffuser Flow

In this paper, the effect of different aspects on modeling of a separated flow in a subsonic asymmetric diffuser with an opening angle of 10° using hybrid RANS/LES methods is studied. The results of using two different types of hybrid methods is studied, namely SST-IDDES that is based on the SST eddy viscosity model, and HSM (hybrid stress method) that is based on non-Boussinesq turbulent stress models. Theoretically, the anisotropy of subgrid scales and their interaction with resolved scales is better described by the second method than the first because in the former, the subgrid stresses are not algebraically related to the resolved strain rate tensor. In a series of simulations, mean velocity and Reynolds stress profiles are compared, as well as pressure and friction distributions along the diffuser walls. Additionally, the instantaneous energy flux from the resolved to the subgrid scales, or backscatter, is studied. It is shown that the HSM method provides a similar level of accuracy to the SST-IDDES method and at the same time allows to simulate the backscatter effect. For each method, a grid convergence study of the statistical characteristics of the flow is carried out. The dependence of the solution on the computational domain width is studied.