Interface conditions for hybrid RANS/LES calculations

Hybrid RANS/LES simulations, in which the attached boundary layers are computed using the Reynolds-averaged Navier–Stokes (RANS) equations, and the non-equilibrium regions of the flow by large-eddy simulation (LES), have received considerable attention over recent years. One issue that may affect (sometimes significantly) the accuracy of the results of hybrid methods is the generation from a smooth RANS field (in which the Reynolds stress is due entirely to the model) of turbulent eddies capable of supporting the Reynolds stresses in the LES region. In this paper, we perform hybrid RANS/LES of turbulent boundary-layer flows in zero, favorable and adverse pressure-gradients. The RANS equations are solved in regions of the flow that are at or near equilibrium, while LES is used elsewhere. Synthetic turbulence and controlled forcing are used to generate turbulent structures in the LES region. When only synthetic turbulence is used at the inflow of the LES region, some distance is required before the proper phase relationships between the modes introduced at the inlet are established; the turbulent statistics are unrealistic in this region, which can extend for 10–20 boundary layer thicknesses. When the forcing is used, this development region is significantly shortened, and realistic statistics are obtained 1–2 boundary-layer thicknesses downstream of the interface. The accuracy of the method, however, was found to depend critically on the accuracy of the RANS solution.