New Strategies for Mitigating the Gray Area in Delayed-Detached Eddy Simulation Models

Abstract

This paper presents a new approach for mitigating the unphysical delay in the Reynolds-averaged Navier–Stokes (RANS) to large-eddy simulation (LES) transition, often referred to as the gray area, which is a common issue for hybrid RANS–LES turbulence models such as delayed-detached eddy simulation. An existing methodology designed for improving the LES performance in complex flows is adapted and tested. This is based on reducing the numerical diffusion in critical areas for permitting a more accurate development of turbulence. The new formulation comprises both a two-dimensional sensitive velocity gradient model and an alternative definition of the subgrid length scale, which are tested both individually and in tandem, and compared with the other formulations commonly used for addressing the gray area. Four test cases are examined, a flat plate, two variants of the incompressible backward-facing step, and an open jet compressible case, all of which are selected to expose the adverse impact of numerical diffusion that this study seeks to address. Furthermore, the proposed changes are implemented in two different codes for the purpose of cross-validation. Encouraging results are observed, supporting the suitability of the new approach as a candidate for addressing the gray area issue in flows of this kind.