Experimental identification on separated and merged sound-induced vortices along lined slit–cavities under flow-convection effect

This study presents an experimental investigation into the vortex dynamics within lined slit–cavities subjected to coupled grazing flow and acoustic excitation. Central to this investigation is the role of velocity ratio \({U}^{*}\) (defined as the ratio of mainstream velocity to acoustic particle velocity) within the range of 0 to 56.6 in modulating flow-acoustic characteristics. An experimental setup integrating microphones, pressure transducer arrays, and particle image velocimetry (PIV) systems was developed to synchronously capture acoustic responses, pressure fluctuations, and unsteady flow behaviors. Crucially, the PIV system incorporated a field-programmable gate array, leveraging its real-time computation capability to ensure precise synchronization of acoustic-fluidic interactions during phase-locked measurements. Transmission loss analysis reveals a critical threshold at \({U}^{*}=\) 14.9 that bifurcates the acoustic response into two distinct regimes: weak influence regime (0 \(\le {U}^{*}<\) 14.9) and strong influence regime (14.9 \(<{U}^{*}\le\) 56.6). Subsequently, the time-averaged flow fields and spatiotemporal vortex evolution characteristics were comparatively investigated in two distinct regimes. In contrast to the symmetric vortex evolution observed in the absence of grazing flow, two distinct evolution patterns are identified: a separated vortex evolution under weak flow-convection effects and a merged vortex evolution under strong flow-convection effects. The systematic analysis of acoustic-vortex conversion efficiency was conducted sequentially through pressure fluctuations, velocity fluctuations, and dominant modes. The results reveal that high-speed grazing flow significantly suppresses coherent structures within the slit.