Hybrid LES-RANS: Computation of the Flow Around a Three-Dimensional Hill

Book chapter, 2005

This chapter discusses computation of the flow around a three-dimensional hill. The main bottle neck for using large eddy simulations at high Reynolds number is the requirement of very fine meshes near walls. Hybrid large eddy simulation-Reynolds averaged Navier-Stokes (LES-RANS) is invented to get rid of this limitation. The object of hybrid LES-RANS is to get rid of the requirement of high near-wall resolution in wall-parallel planes. In the near-wall region (the URANS region), a low-Re number RANS turbulence model (usually an eddy-viscosity model) is used. In the outer region (the LES region), the usual LES is used. In this method, unsteady RANS (URANS) is used near walls and at a certain distance from the wall, where the LES resolution is sufficiently good, a switch to LES is made. A one-equation RANS turbulence model is used in the URANS region and a one-equation SGS model is employed in the LES region. The matching between URANS and LES takes place in the inner log-region. An improved LES-RANS method is evaluated for computing the flow over a three-dimensional axi-symmetric hill. The improvement consists of adding instantaneous turbulent fluctuations (forcing conditions) at the matching plane between the LES and URANS regions in order to provide the equations in the LES region with relevant turbulent structures. © 2005 Elsevier B.V. All rights reserved.