The NASA 2D wall-mounted hump simulated using DDES-SA with the G3D::Flow solver

Paper in proceeding, 2019

With the interest in simulating complex geometries using the Delayed Detached Eddy Simulation (DDES) model, an initial step should be taken in verifying the methodologies needed. The DDES model is used with a newly proposed modification to the sub-grid length-scale, the shear-layer-adaptive length-scale, to improve the transition from RANS to LES. The well-known 2D NASA wall-mounted hump test case is simulated. RANS simulations are performed to verify a correct implementation of the turbulence model developed by Spalart and Allmaras (SA). The SA model is important as it will serve as the underlying sub-grid-scale model for the DDES. Furthermore, RANS was used in an initial grid study. Two simulations are performed using the DDES model, where the difference lies in the number of cells and the grid topology. The results show an extended steady shear-layer in the separated region, delaying the transition from RANS to LES, where the cause is suggested to be insufficient grid resolution in the focus region. This influences the prediction of the re-attachment location and the velocity profiles downstream of the hump. However, one of the transient simulations improves the predictions of the re-attachment location and downstream velocity profiles. The other transient simulation is, however, not capable of improving the RANS results due to the delayed breakdown of two-dimensional coherent structures generated at the separation location. The results from the two DDES simulations indicate that the grid-resolution near the separation point needs refinement for a faster transition from RANS to LES. Using an explicit CFD solver for transient simulations of wall-bounded flow configurations, special treatment is needed to make the time-step requirements restricted by flow physics, rather than the numerical stability-limit. To achieve this, the dual-time stepping method has been implemented in to the in-house CFD solver, G3D::Flow. When using the dual-time stepping method, in combination with residual smoothing and low-speed reconditioning, a speed-up of approximately 50 is achieved.