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The performance of an hybrid LES-RANS strategy, the Detached Eddy Simulation (DES), as a predictive tool for turbulent channel flow with massive separation is scrutinized. This is undertaken in a collaborative effort involving five different flow solvers used by five different groups to cover a broad range of numerical methods and implementations. This paper concentrates on DES results obtained with a computational mesh of approximately one million cells. The results are compared to those obtained by Large Eddy Simulations (LES) using the standard and the dynamic Smagorinsky models and an alternative hybrid LES-RANS -all computed on the same grid. Data of a highly resolved LES (roughly 13 million cells) are used for reference. Furthermore, the impact of resolution and, therefore, the location of the LES-RANS interface is studied.

Large-eddy simulations (LES) still suffer from extremely large resources required for the resolution of the near-wall region, especially for high-Re flows. That is the main motivation for setting up hybrid LES-RANS methods. Meanwhile a variety of different hybrid concepts were proposed mostly relying on linear eddyviscosity models. In the present study a hybrid approach based on an explicit algebraic Reynolds stress model (EARSM) is suggested. The model is applied in the RANS mode with the aim of accounting for the Reynolds stress anisotropy emerging especially in the near-wall region. For the implementation into a CFD code this anisotropy-resolving closure can be formally expressed in terms of a non-linear eddyviscosity model (NLEVM). Its extra computational effort is small, still requiring solely the solution of one additional transport equation for the turbulent kinetic energy. In addition to this EARSM approach, a linear eddy-viscosity model (LEVM) is used in order to verify and emphasize the advantages of the non-linear model. In the present formulation the predefinition of RANS and LES regions is avoided and a gradual transition between both methods is assured. A dynamic interface criterion is suggested which relies on the modeled turbulent kinetic energy and the wall distance and thus automatically accounts for the characteristic properties of the flow. Furthermore, an enhanced version guaranteeing a sharp interface is proposed. The interface behavior is thoroughly investigated and it is shown how the method reacts on dynamic variations of the flow field. Both model variants, i.e. LEVM 416 Flow Turbulence Combust (2008) 81:415-448 and EARSM, have been tested on the basis of the standard plane channel flow and even more detailed on the flow over a periodic arrangement of hills using fine and coarse grids.Keywords Hybrid LES-RANS · EARSM · Interface and interface criteria · Modeled and resolved scales

Abstract. The paper is concerned with a new hybrid LES-RANS approach, which splits up the simulation into a near-wall RANS part and an outer LES part, both modes relying on a one-equation model for the turbulent kinetic energy. This model has been tested on the standard plane channel flow test case as well as on the flow over a periodic arrangement of hills. Encouraging results were achieved. In an additional study based on the detached-eddy simulation (DES) concept attempts have been made to improve the accuracy of the simulation by using adaptive local grid refinement. For this purpose, a criterion based on the residual of the budget of the mean momentum equations has been studied.