Effective Synthetic Jet Control for Separation Control on BFS

Abstract

The flow separation behind a two-dimensional backward facing step is controlled by synthetic jet which is different from traditional ones. The synthetic jet is generated from a 2 mm horizontal slot which is at up corner of the step and facing downstream direction. In order to investigate the effectiveness of the synthetic jet at different actuation frequencies (0, 35, 50 and 100 Hz), 2C-2C particle image velocimetry and fluorescence oil-film was used to indicate the flow pattern on the surface and measure quantitatively the skin friction downstream of the step. The velocity vector field as well as global skin friction clearly indicates reduction of the reattachment length under proper actuation. The comparisons among frequencies indicate that the reattachment length depends on the actuation frequencies. The most effective reduction of reattachment length as much as 43.7% is achieved at the frequency of 100 Hz, corresponding to a Strouhal number Sth ≈ 0.3, based on the free-stream velocity and step height. The Reynolds shear stress is considerably increased and large-scale vortices are analyzed. The skin friction downstream is increased by 20% at the frequency of 35 Hz (Sth ≈ 0.026). In the conclusion, the streaming mechanism of a synthetic jet is briefly discussed.