Experimental study of vortical structures in a periodically perturbed flow over a backward-facing step

The flow field over a backward-facing step for Reh=6940 was studied experimentally under forced flow conditions employing 2D PIV. Pulsating jets of non-zero net mass flux issuing at the step edge area with Strouhal numbers ranging in the interval 0.026-0.23 (based on the step height) caused a 20% maximum reduction of the recirculation length. Ensemble averaging revealed details of the flow evolution at 14 phases of each cycle. Applying POD analysis and the λ2 vortex detection criterion it was shown that optimum periodic blowing generates vortical structures which are characterized by an elevated circulation and smaller population compared to non optimum and unforced cases, being coherent till the next blowing cycle. As a consequence, newly born vortices combined with those of the previous cycle effectively reduce the separation bubble size.