Vortex shedding and evolution at a slit with sound and bias flow

The acoustical response of a slit with a mean bias flow is investigated. The vorticity generation at the rim of the slit, and the subsequent evolution pattern of the vorticity are explored by means of a discrete vortex model. For a low amplitude sound, the bias flow predominates, and the vorticity is convected away as a pair of destabilized vortex sheets. As the sound amplitude increases to a higher level, but not high enough to produce the reversal, the vortex sheets periodically roll up into compact vortex pairs due to the acoustical pulses, showing less sensitivity to the Kelvin-Helmholtz instability. The coherent structure keeps growing in both circulation and extension for one period time, but starts being disturbed after its formation, and eventually becomes an incoherent vortices Cluster. For a high-intensity sound, the vortex shedding, rolling-up, and convection take place alternately on both sides of the plate. The acoustic impedance of the slit responds to the sound amplitude Variation as well, indicating that the slit goes through the linear, transition, and nonlinear regimes.