Dynamics of the recirculating areas of a forward-facing step

The dynamics of the velocity and surface pressure fields in the recirculating regions upstream of the foot of a forward-facing step and downstream of the step corner are investigated experimentally for a thin turbulent oncoming boundary layer at a Reynolds number, based on the step height, of ${10}^5$. Particle Image Velocimetry (PIV) data are acquired concurrently upstream and downstream of the step, synchronized with time-resolved wall-pressure measurements on the front and top walls of the step. Pressure-based multi-time-delay linear stochastic estimation is employed to reconstruct time-resolved, synchronized velocity fields over the entire step. The low-frequency dynamics of the upstream recirculating bubble are found to cover a different range of states when compared to the case of the submerged step. These low frequency dynamics are imprinted onto the windward portion of the separated flow region downstream of the step corner, but upstream of the location of the mean recirculation node over the top of the step. Downstream of this location, the flow intermediately reattaches as vorticity is shed. The latter process is related to instability of the shear layer over the top of the step resulting in a previously described broadband higher frequency flapping motion and cyclical shedding of vorticity. The present analysis further suggests that a coupling mechanism between the two recirculation regions may exist. Broadband frequency spectral signatures observed for the recirculation bubble at the foot of the step are found to be related to dynamics observed over the top of the step. Graphical abstract:

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