Unsteady Behavior of a Backward-facing Step in Forced Flow

The unsteady behaviour of a turbulent flow (Re-h =30450) over a backward-facing step was investigated under sinusoidal actuation. Non-time-resolved Piv and time-resolved pressure measurements were done simultaneously to analyse the dynamical aspects. Initial measurements were made varying the periodic forcing (Strouhal number) based on the step height, Sta, from 0.045 to 0.453. Results suggest that an excitation at Sta=0.226, associated to the shedding frequency, leads to a decrease of the external reattachment length Lr and an increase of the internal separation length Xr. A further study of both, the natural and optimal frequency (among the tested) cases is done using modal decomposition techniques; this method separates the dominant spectral contributions. The spectral decomposition showed an increasing peak related to the shedding phenomena. Comparative results highlighted the dynamical flow mechanism involved in forced flow and underlined that the energetic flow structure interactions is due to a periodic actuation close to the natural shedding frequency. This dynamical behaviour is finally confirmed with a phase averaging of the stochastic flow reconstruction showing convective structures induced by periodic forcing.