On “Steady” RANS Modeling for improved Prediction of Wall-bounded Separation

It is well-known that the separation process is inherently a highly unsteady phenomenon. To capture it correctly LES-relevant models – conventional LES and hybrid LES/RANS models (DES schemes, PITM, PANS) have to be applied. Because of their high spatial and temporal requirements the application of these methods is not straightforwardly affordable for the flow configurations of industrial relevance. On the other hand, apart of the backward-facing step flow geometry characterized by the sharpe-edge separation of a flat plate boundary layer which can be reasonably well solved by an advanced steady RANS model, the flows involving separation are in general beyond the reach of the conventional RANS method independent of the modeling level. Typical outcome is a low level of turbulence activity in the separated shear layer and a correspondingly long recirculation zone. The latter issues motivated the present work demonstrating the possibility to appropriately improve the computational results pertinent to the flow configurations featured by wall-bounded separation in the “Steady RANS” framework. An appropriately designed term modeled in terms of the von Karman length scale (adopted from the SAS modeling strategy for ”unsteady” flow computations, Menter and Egorov, 2010) was introduced into the scale-supplying equation governing the homogeneous part of the inverse time scale ( = ⁄ ). This term (denoted by ) being active only in the narrow area of the separation region acts towards an appropriate enhancement of the (fully-modeled) turbulence in the separated shear layer resulting in a correct mean velocity development and proper size of the recirculation zone. Predictive performances of the proposed model equation solved in conjunction with the Jakirlic and Hanjalic’s Reynolds stress model equation (2002) were illustrated by computing several configurations featured by boundary layer separation including the flow over a periodical arrangement of smoothly contoured 2D hills in a range of Reynolds numbers, flow over a wall-mounted fence and in a 3D diffuser.