Numerical Investigation of Fixed and Non-fixed Separation with Shear Layer Adapted Delayed Detached Eddy Simulation

Abstract

Delayed detached eddy simulation (DDES) has been proved to be suitable for the numerical simulation of massively separated flow. Whereas, there are still some drawbacks in the treatment of gray area, which is the transition zone between Reynolds-Averaged Navier–Stokes (RANS) and large eddy simulation (LES). In this paper, a modified DDES with shear layer adapted (SLA) subgrid length scale was employed, which takes advantage of the peculiarities of flow and grid topology in the initial shear layer, it can rapidly destabilize the separated shear layer and accelerate RANS to LES transition. To evaluate the performance of modified DDES versus conventional DDES, two typical separated flows are considered, they are the flow over backward-facing step with fixed geometry-induced separation and wall-mounted hump with non-fixed pressure-induced separation. The fifth-order Adaptive Dissipative Compact Scheme (ADCS) is also formulated to reduce numerical dissipation in grey area. The results show that the gray area can be slightly alleviated by ADCS, but it cannot be effectively mitigated with conventional DDES model. The visualizations of instantaneous flow reveal that the modified DDES is capable of unlocking the Kelvin–Helmholtz instability rapidly and accelerating the transition to resolved turbulence in the initial shear layer, which is strongly delayed by conventional DDES. The time-averaged pressure and skin friction coefficients show the mitigation of delayed transition as well. The distributions of mean velocity and Reynolds stress of modified DDES exhibit a rapid development in the initial shear layer; thus, more turbulent structures can be distinguished and the accuracy of results can be enhanced.