Organized vortex motion in periodically perturbed turbulent separated flow over a backward-facing step

This study considers the relationship between the time-averaged and phase-averaged flow fields in turbulent backward-facing step flow under the influence of periodic perturbation. Attempts are made to clarify the interaction between organized vortex motion and turbulence statistics such as Reynolds stress. The velocity fields are measured using a particle imaging velocimeter (PIV) for three selected perturbation frequencies, one corresponding to the most effective frequency in terms of the reduction of reattachment length, one below it and another above it. The evolution of organized vortex motion due to the imposed perturbation is found remarkable except for the case of perturbation at the highest frequency, at which the organized motion dissipates so quickly behind the step that the flow is not altered. At the most effective perturbation frequency, the regions of large Reynolds stress appear as a result of strong stretching between successive vortices caused by the perturbation. It is concluded that the change in the mean velocity field due to the organized fluid motion alters the production rate of Reynolds stress, which is a key effect of the perturbation on turbulent separated flow.