Evaluation of the SST-SAS Model

Abstract

The SAS model (Scale Adapted Simulation) was invented by Menter and co-workers. The idea behind the SST-SAS k-omega model is to add an additional production term -- the SAS term -- in the omega equation, which is sensitive to resolved (i.e. unsteady) fluctuations. When the flow equations resolve turbulence, the length scale based on velocity gradients is much smaller than that based on time-averaged velocity gradients. Hence the von Karman length scale, L_vK, is an appropriate quantity to use as a sensor for detecting unsteadiness. In regions where the flow is on the limit of going unsteady, the objective of the SAS term is to increase omega. The result is that k and nu_t are reduced so that the dissipating (damping) effect of the turbulent viscosity on the resolved fluctuations is reduced, thereby promoting the momentum equations to switch from steady to unsteady mode. The SST-SAS model and the standard SST-URANS are evaluated for three flows: developing channel flow, the flow in an asymmetric, plane diffuser and the flow around a three-dimensional axi-symmetric hill. Unsteady inlet boundary conditions are prescribed in all cases by superimposing turbulent fluctuations on a steady inlet boundary velocity profile.