Approximate near-wall treatments based on zonal and hybrid RANS–LES methods for LES at high Reynolds numbers

Two strategies, combining a LES scheme with different near-wall RANS approximations, are investigated by reference to simulations for plane channel flow and two separated flows at moderate and high Reynolds numbers, respectively. One strategy is a hybrid modelling scheme, wherein the subgrid-scale model in the outer LES domain is interfaced with a RANS model in a predefined near-wall layer. The other is a zonal method in which a thin-shear-flow RANS solution in the near-wall layer, embedded within the LES domain which covers the entire flow, is used to provide boundary conditions for the LES computation, the two thus being loosely coupled. Both methods allow the thickness of the near-wall RANS layer to be chosen freely. In the hybrid LES–RANS scheme, the near-wall layer is interfaced to the outer LES region, subject to compatibility conditions for velocity and turbulent viscosity imposed across the interface. These conditions are extracted dynamically as the simulation progresses. In the zonal approach, a mixing-layer model provides the eddy-viscosity field in the near-wall layer, while in the hybrid approach, a two-equations (k–ϵ) model is used.