Implementation and Validation of a Hybrid RANS/LES Model in the Spectral Element Solver Nek5000

Abstract

A dynamic hybrid RANS/LES (DHRL) model has been implemented in the spectral-element solver Nek5000 to reduce computational expense for high Reynolds number applications. The model couples a k-ε URANS model and the dynamic Smagorinsky model for LES. The model is validated for plane channel flow at Reτ = 590 using DNS data, and compared with LES predictions. The model is then applied for the ANL-MAX case, which is a test case relevant to nuclear reactor cooling flow simulations. For the channel flow case, DHRL predictions were similar to LES on finer grids, but on coarser grids, the former predicted velocity profiles closer to DNS than the latter in the log-layer region. The improved prediction by the DHRL model was identified to be due to a 30% additional contribution of RANS stresses. For the ANL-MAX case, the URANS simulation predicts quasi-steady flow, with dominant large-scale turbulent structures, whereas LES predicts small-scale turbulent structures comparable with results in rapid mixing of cool and warm flow jets. DHRL simulations predict LES mode in the inlet jet region, and URANS mode elsewhere, as expected.