Towards a generalised dual-mesh hybrid LES/RANS framework with improved consistency

Abstract

This paper presents a combined RANS and LES methodology, which automates the dual-mesh hybrid LES/RANS approach and circumvents the need for wall-functions. Unlike RANS, wall-resolved LES is still unaffordable for studying high Reynolds number complex flows in industry and requires much user expertise when turbulence features are unknown a priori. The present approach avoids these issues by concurrently solving the LES and unsteady RANS equations on separate meshes, which are adapted to each model and are overlapping over the entire domain.The RANS solution guides the LES where its mesh is too coarse, and vice-versa where the LES is well-resolved. The driver- and driven-simulation locally swap roles automatically, depending on which one is deemed more reliable by a blending function. Consistency between the RANS solution and a temporal average of the LES solution is enforced by drift terms, whose strength depend on relaxation times-scales that are provided by the RANS model.Predictions for fully-developed channel flows and the flow through periodic hills are shown to be in excellent agreement with reference data. The LES grids are deliberately too coarse for wall-resolved LES, while the independent RANS mesh uses high aspect-ratio cells to economically resolve the near-wall layer.