An Enhanced Version of DES with Rapid Transition from RANS to LES in Separated Flows

The paper addresses the issue of the significant delay of transition from RANS to LES in shear layers, which is known to affect the original version of Detached-Eddy Simulation (DES) on typical anisotropic grids. A common remedy has been to disable the subgrid scale model, leading to Implicit LES (ILES). Here, enhanced versions of DES are proposed based on new definitions of the subgrid length-scale. Unlike the original definition attached to DES (i.e., simply the maximum local grid spacing) the new ones include solution-dependent kinematic measures which serve as indicators of the nearly 2D grid-aligned flow regions which are typical of the initial region of free and separated shear layers. This brings about a significant reduction of the subgrid viscosity in such regions. This, in turn, unlocks the Kelvin-Helmholtz instability and drastically speeds-up transition to 2D and then 3D flow structures in shear layers. At the same time, the proposed length-scale is not influenced by the smallest grid dimension, unlike the cube root of the cell volume and other recently proposed definitions, and we view this as a physically plausible, safe and therefore preferable length-scale definition. The advantages of these enhanced versions are demonstrated on a set of numerical examples which include isotropic turbulence, a mixing layer, a jet, a boundary layer, and a backward-facing step. The new definitions are as successful as ILES in liberating the early instabilities, while being non-zonal and compatible with later interactions of the turbulent region with solid bodies. The turbulence statistics of the flows and the radiated noise of the jet are also considerably improved, especially with relatively coarse lateral grid spacings. The new definitions will also improve LES, particularly with the Smagorinsky model.