Assessment of Delayed DES and Improved Delayed DES Combined with a Shear-Layer-Adapted Subgrid Length-Scale in Separated Flows

A comparative study is conducted between the original versions of Delayed Detached-Eddy Simulation (DDES) and Improved DDES (IDDES) and these approaches combined with a new (shear layer adapted) definition of the subgrid length-scale recently proposed in Shur et al. (Flow Turbul. Combust. 95(4), 709-737, 2015). This definition is aimed at accelerating the transition to resolved turbulence in separated shear-layers, which significant delay is typical of the non-zonal hybrid RANS-LES models, in general, and DES-like approaches, in particular. An objective of the study is widening the validation database of the new solutions-dependent definition of the length-scale compared to that employed in the original work of Shur et al. In order to reach this, three different complex separated flows with well-understood flow physics were considered, which all are widely used for the validation of different CFD approaches. These flows are: a flow with non-fixed pressure-induced separation and reattachment (wall-mounted hump), a massively separated flow (NACA 0021 airfoil beyond stall), and a supersonic separated flow (wake behind a cylindrical forebody). The results of simulations suggest that the DDES and IDDES models combined with the shear-layer adapted subgrid length-scale perform according to their design (no unforeseen interactions of the shear-layer adapted length-scale with the empirical functions involved in the DDES and IDDES formulations are observed) and considerably mitigate the delay of transition from fully modeled to partially resolved turbulence in the separated shear layers compared to the standard DES definition of the length-scale (maximum local grid-spacing).