A modification of DDES based on a detection of attached boundary layers and flow separation

Kurzfassung

We present a new hybrid RANS-LES method of DDES-type for flows with small/incipient separation. It alters the RANS-LES switch based on the function f_d in the DDES formulation. Instead we present a method for detecting regions of attached boundary layer flow, the separation point and the subsequent region of separation, developed for flows over airfoil sections. This method is based on a combination of algebraic properties of the mean velocity profiles in wall-normal direction inspired by Stock&Haase (1987) and of integral boundary layer quantities, namely the shape factor H. Then 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 is used as a simple method to force the generation of turbulent content.