Dynamics of cavitation bubbles near granular boundaries

We study the dynamics of a cavitation bubble near beds sand different grain sizes. use high-speed imaging to observe motion and for values stand-off parameter $\gamma$ (dimensionless bubble-boundary distance) between 0.3 5.3. Compared with rigid boundary, we find that granular boundary leads bubbles shorter lifetimes reduced centroid displacements. Above $\gamma \approx 1.3$ , behaviour is almost independent granularity sand. When lies $0.6$ $1.3$ mound develops beneath bubble, which can force latter assume conical shape as it collapses. For \lesssim 0.6$ bell-shaped form, leading formation thin surprisingly fast micro-jets ( $v_{jet} > 1000\,{\rm m}\,{\rm s}^{-1}$ ). Moreover, 0.3$ jets erupt from surface following collapse. additionally develop simple numerical model, based on integral method, predict bed sand, replace by an equivalent liquid. The simulations are remarkably consistent experimental observations down . also show how anisotropy $\zeta$ dimensionless version Kelvin impulse, be adapted case nearby displacement $\zeta 0.08$