Control of the self-sustained shear layer oscillations over rectangular cavities using high-frequency vortex generators

Physics of Fluids, Volume 33, Issue 4, April 2021. The excitation of acoustic resonance by flow over a rectangular cavity can generate acute noise, cause damage to equipment, and interrupt operation. In this work, a passive control technique to suppress the excitation of acoustic resonance by the flow over rectangular cavities is experimentally investigated. A span-wise rod that generates high-frequency vortices is mounted upstream of the cavity leading edge to prevent the flapping of the shear layer. The effect of the rod parameters on the mechanism of acoustic resonance suppression is identified by means of acoustic pressure and particle image velocimetry (PIV) measurements. It is found that the effectiveness of this control technique is significantly dependent on the streamwise location of the rod with respect to the cavity leading edge, the gap between the rod and the wind tunnel wall, and the cavity aspect ratio. In addition, PIV measurements revealed that, in effective rod configurations, the vortices generated in the gap between the control rod and the wall alter the development of the shear layer. Moreover, analysis of the Reynolds stresses showed that fluctuations in the wake of the rod prevent the shear layer from impinging on the cavity downstream edge. Consequently, this interaction interrupts the initiation of the feedback mechanism responsible for the onset of acoustic resonance excitation. Finally, a universal criterion is developed to predict an optimum region to implement the control rod upstream of the cavity leading edge to effectively suppress the acoustic resonance excitation.