LES-RANS coupling for the simulation of complex flows

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LES has clear superiority over RANS-methods for complex flows, especially when large-scale structures dominate the turbulent transport, in transitional situations and when dynamic forces and noise generation are calculated. However, due to the extremely high resolution and time-averaging requirements, LES still is too costly to be applied directly to the flows of practical interest. On the other hand, RANS can determine the mean flow with engineering accuracy at much lower cost in many cases, especially attached flows, which often prevail in subareas of complex flows. Hence, a hybrid method using RANS in subareas with simpler flow behaviour and LES only in the more critical areas appears to be the ideal approach for complex high Reynolds number flows. The present project aims to develop and test such methods in a so-called zonal approach: RANS and LES will be applied in pre-defined subareas of the calculation domain and linked by suitable coupling conditions. Three coupling situations are considered: the LES-region is upstream of the RANS-region, RANS is applied upstream of LES, and the interface is tangential to the flow. The French and German partner will jointly develop coupling procedures for all three types and test them first for simple channel flow. The methods are then applied for testing and further improvement to increasingly complex flows taken from the common test cases of the group, such as to flow over hills, in a combustor and over a car-shaped body by the German partner and to separating boundary layers and finally the flow over a complete airfoil by the French partner.