The main objective of this paper is to evaluate the capability of SST-SAS and SST-DDES to predict massively separated flow. Flow past a circular cylinder at subcritical Reynolds number Re = 3900 is numerically studied. The hybrid characters of SAS and DDES are discussed in principle. Some typical results such as velocity profiles are calculated. Additionally, the effect of the Lvk limiter on the performance of SST-SAS is discussed. Both SST-SAS and SST-DDES achieve results in favorable agreement with the experiment data. However, a finer mesh generation is needed for SST-SAS, which means that SST-SAS still remains improvements.
Flow around a circular cylinder is usually treated as one of the most classic cases in which the massively separated flow appears behind bluff bodies. It has complex features such as laminar boundary-layer separation and periodic vortex shedding despite of the simplicity of geometry. Hence, it is challenging to accurately predict such an unsteady flow phenomenon with economical computation cost. Reynolds- Averaged Navier-Stokes (RANS) equation is always in favor of its cheap cost. However, it is also blamed for its bad performance in predicting massively separated flow due to its incapability of resolving instantaneous small turbulence scales. While direct numerical simulation (DNS) and wall-resolved large-eddy simulation (LES) are supposed to possess high simulation accuracy, their computation cost is too expensive to be afforded. Hybrid RANS/LES method combines the advantages of RANS and LES by simulating the near wall flow region with RANS and the separated flow region with LES. Hence, hybrid RANS/LES method is widely used to predict massively separated flows in current engineering applications.
Detached-eddy simulation (DES) is one of the mostly used hybrid RANS/LES method due to its simplicity in formulation and adaptation in complex geometry. Recently, several investigations have been carried out to validate the capability of DES to be industrial (Zhao, 2016). The first DES model DES97 proposed by Spalart (1997) substitutes the wall distance with the grid scale when the grid is adequately fine for LES simulation. However, DES97 model suffers several problems including the undefined "grey area" inside the RANS/LES interface. One of the most serious problem faced by DES is the modeled stress depletion (MSD) problem. It occurs when the grid is fine enough for activating LES branch but not fine enough to resolve the turbulence fluctuations internal to boundary layers. As a result, MSD leads to the unphysically grid-induced separation. To address this drawback, delayed-detached eddy simulation (DDES) modifies the character length scale to protect the RANS region from being prematurely switched into LES region. However, this kind of modification is incapable of completely preventing the occurrence of MSD problem, which is just postponed to a finer grid spacing as commented by Menter and Egorov (2005).
