The role of 'splashing' in the collapse of a laser-generated cavity near arigid boundary

Vapour cavities in liquid flows have long been associated with cavitation d amage to nearby solid surfaces and it is thought that the final stage of co llapse, when a highspeed liquid jet threads the cavity, plays a vital role in this process. The present study investigates this aspect of the motion o f laser-generated cavities in a quiescent liquid when the distance (or stan d-off) of the point of inception from a rigid boundary is between 0.8 and 1 .2 times the maximum radius of the cavity. Numerical simulations using a bo undary integral method with an incompressible liquid impact model provide a framework for the interpretation of the experimental results. It is observ ed that, within the given interval of the stand-off parameter, the peak pre ssures measured on the boundary at the first collapse of a cavity attain a local minimum, while at the same time there is an increase in the duration of the pressure pulse. This contrasts with a monotonic increase in the peak pressures as the stand-off is reduced, when the cavity inception point is outside the stated interval. This phenomenon is shown to be due to a splash effect which follows the impact of the liquid jet. Three cases are chosen to typify the splash interaction with the free surface of the collapsing ca vity: (i) surface reconnection around the liquid jet; (ii) splash impact at the base of the liquid jet; (iii) thin film splash. Hydrodynamic pressures generated following splash impact are found to be much greater than those produced by the jet impact. The combination of splash impact and the emissi on of shock waves, together with the subsequent re-expansion, drives the fl ow around the toroidal cavity producing a distinctive double pressure peak.