The acoustic pressure generated by the cavitation bubble expansion and collapse near a rigid wall

Abstract

We analyze systematically the acoustic transients emitted during the collapse of a laser-induced cavitation bubble for 0.4≤γ≤5.2. γ is the standoff parameter, the ratio of the distance between the bubble's nucleation place and its maximum size. At the bubble collapse, the acoustic signals recorded contain several pressure peaks with rising times as fast as 18 ns. The time delay, Δt, between these peaks is a few hundred nanoseconds apart for bubbles nucleated close to the boundary and decreases with γ. The pressure maxima correspond to shock fronts emitted around the time the bubble reaches its minimum volume and is correlated with the high-speed video recordings of the bubble dynamics. We also show that the amplitude ratio of the first to second acoustic transient is strongly dependent on γ. The experimental results are complemented with numerical simulations based on the Volume-of-Fluid method. The simulated results help clarify the physical mechanisms leading to the generation of acoustic transients and unveil in detail the morphology of the bubble approaching its minimum volume, a feature that cannot be resolved with the high-speed data. Furthermore, the numerical results reveal that the impact of the liquid jet on the rigid wall ensues a local increase in pressure over a significant time period, contrary to the shock wave formed during bubble collapse, which produces a sharp transient pressure peak that propagates radially outwards along the rigid wall. We also discuss the validity and shortcomings of the simulation and how to improve them in the future.