Phase-locking particle image velocimetry measurements of acoustic-driven flow interactions between tandem deep cavities

Acoustic-driven flow interactions between tandem deep cavities, which manifest as resonances between the natural acoustic standing-wave mode and the intrinsic shear-layer vortex structures, were experimentally investigated by using a pressure transducer array, the planar particle image velocimetry (PIV) technique, and phase-locking PIV measurements. Specifically, in the phase-locking PIV measurements, a field-programmable gate array-based phase-determination strategy was used to improve the phase-locking accuracy. The pressure measurement results demonstrated that under certain Reynolds numbers, significantly intensified acoustic pressure pulsations were excited once the magnitude of the acoustic resonance occurring inside the tandem deep cavities reached almost three times the magnitude of the dynamic pressure head at the channel inlet. Beyond that, the planar-PIV results illustrated the elevated turbulent flow quantities, such as the expanded velocity gradients, amplified shear-layer momentum thickness, intensified velocity fluctuations, and statistical Reynolds shear stresses. Subsequently, a proper orthogonal decomposition (POD) analysis was conducted to successfully extract the dominant flow modes underlying the acoustic-driven flow interactions, namely, the cavity-to-cavity flow mode and the counterrotating shedding vortex mode. The first POD mode gave rise to essential flow streaks that shuttled synchronously between the tandem deep cavities, while the second POD mode contributed to the streamwise vortex-shedding motions. Finally, the phase-locking PIV results comprehensively revealed the spatiotemporal evolutions of the coherent flow structures (the upper shedding vortices and the recirculation zones beneath) and their centroid trajectories. The findings of this study could be useful for revealing the flow–acoustic coupling mechanisms in related industrial facilities.