Shockwaves from air bubbles within pits induced by nearby cavitation bubbles

The cavitation intensity in near-boundary regions was significantly affected by the smoothness of boundaries. Micro pits on these boundaries may harbor smaller air bubbles or gas nuclei, and cavitation bubbles within cavitation clouds or cavitation bubble clusters inevitably interact with air bubbles in these pits. In this study, the experimental setup employed underwater Corona Discharge to generate controlled cavitation bubbles, and experimental observations were made with high-speed photography system. The experimental results revealed that for a given pit spatial size (ξ), the presence of air bubbles within pits reduces the evolution period of cavitation bubble (defined as the ratio of the time from cavitation bubble inception to its first collapse to the Rayleigh time) as the dimensionless bubble-boundary distance (γ) increases. Additionally, compared to scenarios without air bubbles, the evolution period of cavitation bubbles decreases, while the velocity of microjet increases. The cavitation bubble shockwave pressure follows a distinct pattern as γ increases: it initially decreases, followed by an increase, and eventually stabilizing. Within the γ range of 0.9 to 1.7, the air bubbles in pits significantly attenuate the shockwave pressure generated during cavitation bubble collapse (air bubble can reduce cavitation bubble collapse pressure by up to 80 %). Through the assistance of Schlieren techniques, a novel 'cavitation' behavior of air bubble within the pits was discovered. The phenomenon is characterized by the generation of an 'implosion shockwave' during the air bubble collapse (the propagation speed of this 'shockwave', as observable in high-speed images, is approximately 1534 ± 39 m/s, which is on the order of the speed of sound in the liquid, around 1500 m/s). Further analysis revealed the critical conditions for the 'implosion shockwave' from the small air bubbles within pits induced by nearby cavitation bubbles. Specifically, the critical dimensionless standoff distance(γ*) exhibits an exponential decay with increasing pit spatial size (ξ), and the coefficient is likely related to the ratio of maximum bubble radii (R_air/R_max) between the air bubble and cavitation bubble. These innovative findings offer valuable references for controlling and evaluating cavitation intensity in defective water flow boundaries.