Large eddy simulation and statistical turbulence closure 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 by means of a slot jet injected periodically at zero net mass-flow rate into the flow at the step edge at an angle of 45 degrees relative to the flow direction. Experimental data show the effects to depend non-linearly on the perturbation frequency, 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 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.

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