Hybrid LES-RANS Method Based on an Explicit Algebraic Reynolds Stress Model

Abstract

Although LES is a highly promising simulation technique, it still suffers 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. Whereas RANS suits reasonably well to attached boundary layers, requiring much less CPU-time and memory than LES, the latter is recommended for complex large-scale flow phenomena, which RANS often fails to predict correctly. Both characteristics are combined in hybrid LES-RANS methods to obtain an optimal solution at lower cost. Meanwhile a variety of different hybrid concepts were proposed including DES. In the present study a non-zonal approach based on two different but unique models is preferred. The predefinition of RANS and LES regions is avoided and a gradual transition between both methods takes place which weakens the problem of setting up an appropriate coupling strategy. The new hybrid LES-RANS approach relies on a one-equation model for the turbulent kinetic energy in both modes. At this phase the model is of linear type. In addition to this linear eddy-viscosity model (LEVM), an explicit algebraic Reynolds stress model (EARSM) is applied in the RANS mode in order to account for the Reynolds stress anisotropy. This insertion leads to a non-linear model. The linear version is used for comparison in order to emphasize the advantages of the non-linear formulation. Both model variants have been tested on the basis of the standard plane channel flow and the flow over a periodic arrangement of hills.