The collapse of a bubble against infinite and finite rigid boundaries for underwater laser propulsion

Abstract

In order to investigate the influence of a bubble on underwater laser propulsion, the analytical formula of the bubble collapse time near rigid boundary is deduced from Rayleigh collapse time and Rattray prolongation factor. Experiments and numerical simulations are employed to validate the collapse time formula. The collapsing features of a bubble, including the maximum bubble radius Rmax, the collapse time of the bubble TCR, the shock wave and liquid jet emitted during the bubble collapse, are obtained near infinite and finite rigid boundaries. The theoretical, numerical and experimental results for the dimensionless distance γ > 1 all illuminate that TCR increases with Rmax near the rigid boundary. Rmax and TCR increase with the laser energy first, then begin to level out as the laser energy continues to increase, thus it is impossible to increase the propelling force just through increasing the laser pulse energy continuously. In addition, TCR is smaller, and the shock wave pressure and the liquid jet velocity are larger near the finite rigid boundary than that near the infinite rigid boundary, which means that the bubble collapses fiercer in the former case, in other words the finite rigid boundary gets more propelling force.