Active control of separated shear flow downstream of a backward-facing step by using small perturbations

Kurzfassung

An oscillating flap was implemented in a turbulent backward-facing step flow. The oscillating flap was driven by periodic Ampere force and thereby generating small perturbations over the separation edge at f_p = 55 Hz, which was equal to half of the most amplified frequency of the shedding instability of the turbulent shear layer. The Reynolds number was Re_h = 2.0×10^4, based on the free-stream velocity U_0 = 10 m/s and the step height of h = 30 mm. 2D-2C particle image velocimetry was used to measure the perturbed separated shear layer as well as the reattachment area downstream. The time-averaged velocity fields show that the perturbations can reduce the recirculation region behind the step and reduce the reattachment length by 31.0%. The phase-averaged results indicate a small streamwise ejection with a velocity of 0.7·U_0 which was generated into the shear layer when the oscillating flap moves downward in each period. The roll-up and breakdown processes of the spanwise vortices result in considerable increases of coherent and incoherent Reynolds shear stresses. The coherent structures in the turbulent shear layer are further analyzed by snapshot proper orthogonal decomposition and spatial dynamic mode decomposition in energy and dynamic approaches complementarily. It is concluded that the periodic small perturbations enable to effectively manipulate the large-scale separated shear layer and generated quasi-periodic coherent structures.