Improvement of delayed detached-eddy simulation for LES with wall modelling

Abstract

Adjustments are proposed of the Delayed Detached Eddy Simulation (DDES) approach to turbulence. They preserve the DDES capabilities particularly for natural DES uses, and resolve the mismatch of the logarithmic layers discovered earlier for the basic DES technique when used for Wall-Modelled Large-Eddy Simulation (WMLES) of attached flows. The adjustments are defined both for the Spalart-Allmaras and the Menter SST models. The first one concerns the definition of the LES length scale in general for anisotropic grids near a wall, and makes use of the wall distance along with the grid spacing; it clearly benefits even the Smagorinsky model. The second one manages the blending of RANS and LES behaviour within a WMLES to advantage, greatly increasing the resolved turbulence activity near the wall, and finely adjusting the resolved logarithmic layer. This is seen in channel flow over a wide Reynolds-number range, and through some grid variations. Tests show that the new method, although somewhat more complex, returns the desired behaviour not only in channel-flow LES, but also in channel-flow RANS, in a backward-facing-step case with side-by-side LES and RANS regions, and over an airfoil in deep stall.