The Grey-Area Improved $$\sigma $$-DDES Approach: Formulation Review and Application to Complex Test Cases

The paper presents a formulation review and recent results for a non-zonal enhanced DES variant called \(\sigma \)-DDES [9], which seeks to address the issue of delayed transition from RANS to LES in free shear layers (also known as the grey-area problem) frequently experienced for standard delayed DES [13]. The two-part approach is based on modifying the LES behaviour of DES (the new model now behaves similar to the \(\sigma \) LES model of Nicoud et al. [10]) as well as using a more elaborate LES filter width formulation. Combining both means delivers an enhanced behaviour in the crucial early shear layer region. First, the derivation of the approach based on two popular RANS models (i.e. Spalart-Allmaras and Menter SST) is discussed. Subsequently, recent OpenFOAM results from different test cases are presented, such as pressure-induced separation from a 2D hump, a generic car flow and aeroacoustic results for a rudimentary landing gear (RLG). The \(\sigma \)-DDES variant is seen to deliver a more consistent flow prediction with varying grid resolution and topology independent of the applied RANS background model, and could also be established as a viable turbulence modelling approach for aeroacoustics prediction at low Mach numbers.