Flow-dependent DDES/γ−Re‾ _θt coupling model for the simulation of separated transitional flow

A new DDES and γ−Re‾ _θt coupling model for separated transitional flow is proposed. The studies in this paper indicate that the activation of the LES branch in the free stream may lead to an inappropriate decrease in the inflow turbulence intensity and a delayed transition. To address this problem, a new flow-dependent RANS/LES switch function based on identification functions for the boundary layer and wake regions is designed. In this new model, the adoption of the RANS or LES branch is decided by both the local grid density and local flow properties, and the LES branch is only allowed to be active in the wake region. Therefore, the inflow turbulence intensity uncertainties caused by grid dependence of the LES branch in the free stream are largely eliminated. Separated transitional cases including flows over an A-Airfoil (small separation), a DBLN-526 airfoil (moderate separation), and a circular cylinder in subcritical and critical regimes (massive separation) are studied. The performance of two commonly employed boundary layer shielding functions is examined. Good agreements are achieved between the numerical and experimental results, and the accuracy and reliability of the new model are demonstrated.