A Unified RANS-LES Model. Part 1. Computational Model Development

Large eddy simulation (LES) is computationally extremely expensive for the investigation of wall-bounded turbulent flows at high Reynolds numbers. A way to reduce the computationalcost of LES by orders of magnitudeis to combine LES equations with Reynolds-averagedNavier-Stokes (RANS) equations used in the near-wall region. A huge variety of such hybrid RANS-LES methods are currently in use such that there is the question of which hybrid RANS-LES method represents the optimal approach. The properties of an optimal hybrid RANS-LES model are formulated here by taking referenceto fundamentalpropertiesof fluid flow equations. It will be shown that unified RANS-LES models derived from an underlying stochastic turbulence model have the properties of optimal hybrid RANS-LES models. The question of how unified RANS-LES models can be computationally developed is addressed. A-priori analyses of channel flow data are used for a thorough analysis of different RANS-LES coupling methods. The application of a dynamic RANS-LES coupling approach is identified to represent the most convenient computational approach. A-posteriori analyses of channel flow data are used to demonstrate that the computational model obtained does also satisfy all the properties of an optimal hybrid RANS-LES model.