An Algebraic Sensor for the RANS-LES Switch in Delayed Detached-Eddy Simulation

A new hybrid RANS-LES method of DDES-type for flows over airfoil sections with small/incipient separation is presented. It alters the RANS-LES switch based on the function f(d) in the DDES formulation. Regions of attached boundary layer flow, the separation point and the subsequent region of separation are detected by sensors. They are based on a combination of algebraic properties of the mean velocity profiles in wall-normal direction and of integral boundary layer quantities, namely the shape factor H-12. While the RANS length scale l(RANS) is used in attached boundary layers, the LES length scale C-DES Delta is used in regions of separated flow. In particular, thin regions of (incipiently) separated flow are treated in LES mode except for a near-wall region where l(RANS) is used. Moreover, in the vicinity of the separation point or in the entire separation region, a stochastic SGS model can be used as a simple method to force the generation of turbulent content.