Two-Bubble Cavitation Collapse Near a Rigid Wall: Effects of Size Asymmetry and Temporal Delay

Abstract

This study experimentally investigates the collapse dynamics of two laser-induced cavitation bubbles positioned coaxially near a rigid wall, focusing on bubble-size asymmetry and temporal generation delay. Bubble 1 is located closest to the wall, while Bubble 2 is positioned farther from the boundary. High-speed imaging, Schlieren visualization, and synchronized pressure measurements are used to characterize collapse timing, liquid-jet development, pressure-wave propagation, and wall loading. For the bubble-size study, Bubble 2 is maintained at a maximum diameter of 6.10 mm, while Bubble 1 is reduced to 5.50, 5.03, and 4.50 mm at stand-off distances of 0.52, 1.47, and 1.97. An additional highly asymmetric case with a Bubble 1 diameter of approximately 4.10 mm is also examined. The results show that bubble-size asymmetry modifies collapse order, jet direction, vapor merging, collapse topology, and wall pressure. At a stand-off distance of 0.52, the 5.50 and 5.03 mm cases exhibit torus-torus collapse, while the 4.50 mm case exhibits tip-tip collapse. At stand-off distances of 1.47 and 1.97, Bubble 1 undergoes torus collapse and Bubble 2 undergoes tip collapse. Temporal generation delay further modifies the interaction by changing the relative growth and collapse phases of the cavities. The delayed bubble grows to a larger size and restricts the growth of the bubble generated first. When Bubble 2 is delayed, the maximum jet velocity is approximately 62.55 m/s at 10 microseconds, with jet reversal between 50 and 100 microseconds. When Bubble 1 is delayed, the maximum jet velocity reaches approximately 72.23 m/s at 38 microseconds. Overall, bubble size and generation timing strongly influence collapse dynamics, liquid-jet behavior, and wall loading in interacting cavitation bubbles near a rigid boundary.