A Flow Visualization Study of Flow-Acoustic Coupling at the Mouth of a Resonant Side-Branch

Abstract The coupling of a shear layer with the sound field of a resonant side-branch has been investigated by means of an extensive flow visualization study under controlled excitation level and frequency. The interaction mechanism is visualized at several Strouhal numbers corresponding to: (a) before the onset resonance; (b) within the lock-in range; (c) in the hysteresis region and (d) after the subsidence of resonance. It is shown that the instantaneous position and phase speed of the shear-layer vortices are strongly dependent on the Strouhal number and the amplitude of the acoustic particle velocity. Moreover, the phase speed has been found to be not constant over the oscillation cycle. These findings have important implications when modelling flow-acoustic interaction at large amplitude acoustic pulsations. The study has shed more light also on the phenomenon of hysteresis as well as on the mechanism of "vortex damping" which limits the amplitude of pulsation when the system losses become negligibly small. The flow visualization results give support to the argument made by other researchers that the mechanism of acoustic resonance is different from that which causes the non-resonant oscillations of impinging shear flows.