Large eddy simulation of periodically perturbed separated flow over a backward-facing step

Large eddy simulation is used to investigate the effects of a periodic perturbation introduced into a separated shear layer that borders a recirculation bubble behind a backward-facing step in a high-aspect-ratio channel. The perturbation is provoked by the injection of a slot jet, at zero net mass-flow rate, uniformly along the spanwise edge at which separation occurs. Attention is focused on one particular jet-forcing frequency, at the Strouhal number 0.2, for which experimental data show the perturbation to cause a maximum change to the properties of the unperturbed flow – in particular, the largest reduction in the time-mean recirculation length. Results are reported for time-mean and phase-averaged velocity and Reynolds stresses, and these are compared with experimental data. The time evolution of phase-averaged properties, including stream-function, pressure and turbulence energy, are investigated in an effort to identify the mechanisms responsible for the observed substantial changes to the time-mean properties. This is aided by a study of some spectral properties. The simulations are shown to reproduce the experimental observations, and these provide clear indications that the high level of sensitivity to the perturbation at the Strouhal number considered is due to a strong interaction between shear-layer instabilities, which are amplified by the perturbation, and shedding-type instabilities, which are induced by the interaction of large-scale structures developing downstream of the step with the wall, causing the shear-layer to flap.