Phase-locking PIV measurement of vortex–vortex interactions inside dual-slit cavity during high-intensity acoustic modulation

This study experimentally investigates the vortex–vortex interaction mechanism inside a dual-slit cavity structure under high-intensity external acoustic modulation. Particular attention was paid to the slit distance, which corresponded to different vortex behaviors under the combined acoustic forcing effect and wall-confinement effect. A phase-locking particle image velocimetry measurement was performed using the incident acoustic wave as a phase-determination signal. The dynamically interactive response between acoustics and vortices was accurately phase-resolved through a field-programmable gate array with a precast wave decomposition method and a hard-computation approach. Four types of interaction mechanisms (rapid vortex merging, mutually interfered vortex shedding, isolated vortex evolution, and wall-confined vortex jets, with $S^{\ast }=$ 2, 16, 32 and 49, respectively) were classified. Subsequent analysis on their vortex trajectory, convection velocity and kinetic energy consolidate our findings on the acoustic impedance and absorption characteristics, which are significantly reduced by the wall-confinement effect, whereas the vortex-merging effect can have a positive augmentation effect. Finally, multi-scale analysis by proper orthogonal decomposition (POD) and turbulence-production theory was conducted on the interactive vortex structures. POD identified the dominant energy distribution and demonstrated that vortex merging will disrupt the independent periodic wavepacket distribution and make the energy distribution mixed. Meanwhile, the production and dissipation processes of small-scale turbulence structures were identified, accompanying with the variations of turbulence stresses by the interactive vortex structures.