On hybrid RANS-LES of transition in a separated boundary layer

The numerical simulation of laminar separation followed by transition and re-attachment of a turbulent boundary layer has been investigated with particular focus on the prediction of the transition process. The turbulence has been modeled using hybrid RANS-LES models, namely the variants DDES and IDDES. For reference, a computation with nearly DNS-like resolution is featured. Using differently fine meshes, the prediction of the shear layer vortex breakup process into turbulence has been observed in terms of turbulence intensities and turbulence length scales. It is shown how these properties develop during the transition process going along with flow re-attachment. Slight differences arise between DDES and IDDES, the latter one operating in a mode of wall-modeled LES. To describe the impact on the aerodynamic properties, the probability of flow separation along the transition process is presented and the correlation between length scales, turbulence intensity and flow separation is discussed. Conclusions are drawn on the applicability and qualification of the turbulence models as well as on requirements concerning mesh resolution and discretization schemes.