Modifying the cavitation bubble collapse in the erosive regime with a surface bar structure

The collapse of cavitation bubbles near surfaces can cause severe erosion, posing significant risks to fast-rotating turbines. Previous studies suggest that the energy concentration of shock waves during non-spherical collapse is a potential mechanism behind this erosion. An effective strategy to mitigate shock wave focusing and subsequent erosion could involve modifying the boundary structure. In an initial effort to influence shock wave focusing, we introduce a symmetry-breaking boundary structure, specifically a slender bar, to quantitatively investigate how asymmetry affects cavitation bubble dynamics during the final collapse stage and the self-focusing process of shock waves. Using two high-speed cameras to capture the behavior of a single laser-induced cavitation bubble near this structure, we identify two distinct regimes based on the characteristic morphologies of the bubble during the final collapse: the Island-Bridge regime and the Asymmetric Torus regime. We analyze how the dynamic and geometric characteristics of the toroidal bubble evolve with increasing distance between the bubble and the structure, revealing distinct trends in each regime. Furthermore, we examine the first collapse location and collapse propagation velocity of the toroidal bubble, which are likely to affect the shock wave energy focusing. The findings of this study provide insights into the role of structures attached to the boundary in cavitation bubble dynamics and may offer a potential methodology for designing surface microstructures to mitigate cavitation erosion or to concentrate cavitation bubble energy for engineering applications.