LNS - An approach towards embedded LES

Abstract

This paper considers the present and possible future state of hybrid turbulence models, which blend statistical Reynolds-Averaged Navier-Stokes (RANS) closures with Large-Eddy Simulation (LES) methods. Experience gathered from existing hybrid formulations is beginning to confirm their ability to bridge the near-wall region in unsteady simulations of high-Reynolds number, separated flow. However, the issue of kinetic energy transfer between regions of differing spatial resolution still limits the range of applicability of the current generation of hybrid methods. A potential for greater generality exists, whereby local grid refinement would automatically lead to a detailed treatment of selected flow regions at a cost lower than that of either conventional LES, or LES with near-wall RANS. Constructing a general hybrid method that interfaces seamlessly between RANS and LES (and vice versa) requires additional considerations relating to spatial/temporal resolution and the transfer of modeled turbulence kinetic energy into resolvable structures. This paper reports some progress in this area. NOMENCLATURE Turbulence kinetic energy, m/s Length scale, m Turbulence production, m/s Reynolds number Strain tensor, Sy Strain magnitude = (2SijSij)Velocity scale, m/s LNS latency parameter Turbulence dissipation rate, m/s Dynamic viscosity coeff., kg/(m.s) Kinematic viscosity coeff., (m/s) Turbulence eddy viscosity , kg/(m.s) Vorticity tensor, Qj . Vorticity magnitude = (2£1^ Qq)' Density, (kg/m) Favre-average of fy Time-average of <j) INTRODUCTION Due to the continued high cost of direct numerical simulation (DNS) and traditional large eddy simulation (LES), engineering predictions of turbulent flow continue to be dominated by simple (single-point, isotropic) ReynoldsAveraged Navier-Stokes (RANS) models. Despite acknowledged uncertainties over the practice of unsteady RANS, these models are frequently entrusted with the task of predicting k L Pk Re S S V