Unified RANS-LES Analysis of Turbulent Jets Covering Several Swirl Number Regimes

Abstract

Many turbulent flow simulations require the use of hybrid methods because large eddy simulation (LES) methods are computationally too expensive and Reynolds-averaged NavierStokes (RANS) methods are not sufficiently accurate. However, a large variety of hybrid RANS-LES methods are currently in use, leading to the question of which method should be preferred. Desired properties of a theoretically optimal hybrid RANS-LES method are formulated here. It is shown that a unified RANS-LES model derived from a stochastic turbulence model has all the properties of a theoretically optimal hybrid RANS-LES method. The unified RANS-LES model is applied to turbulent swirling and nonswirling jet flow simulations to illustrate the difference to other commonly applied hybrid RANSLES methods. The unified RANS-LES results are shown to be in a very good agreement with experimental studies of nonswirling and mildly swirling jet flows. The unified RANSLES model does not suffer from the ’modeled-stress depletion’ problem that is observed in detached eddy simulation (DES) and improved delayed DES (IDDES) simulations of nonswirling jet flows. Regarding the simulation of high swirl number flows it is shown that the unified RANS-LES model performs better than segregated RANS-LES models, which suffer from their inability to correctly simulate the central vortex core, the precessing vortex core, and the mixing efficiency of passive scalars. The results presented contribute to a better understanding of swirl flows by the explanation of conditions for the onset and the mechanism of vortex breakdown.