Comparative LES and unsteady RANS computations for a periodically-perturbed separated flow over a backward-facing step

Large eddy simulation and statistical turbulence closures are used to investigate and contrast the ability of both strategies to represent the effects arising from the unsteady perturbation of a separated backward-facing-step flow caused by a slot jet injected periodically at zero net mass-flow rate into the flow at the step edge, at an angle of 45 deg relative to the flow direction. Experimental data show the effects to depend nonlinearly on the perturbation ftequency, the strongest response arising at a Strouhal number of 0.2, which is the condition investigated herein. The principal response is a shortening of the separation bubble by almost 30%, a result that is highly pertinent to active flow control. As the injection frequency lies within the low-frequency range of the large scales of the turbulence spectrum, an issue of particular interest that is addressed herein is the ability of the statistical models, operating within a phase-averaged URANS framework, to reproduce the experimental observations and the response derived from the simulation.