Shock wave generation by collapse of an explosive bubble in water

Abstract

Shock wave generation by collapse of a single explosive bubble exposed to an underwater shock wave in water has been studied experimentally and numerically. The explosive bubble was made of a stoichiometric ethylene-oxygen mixture and its initial volume-equivalent radius was varied from 1.0 to 2.9 mm. The underwater shock wave, whose strength ranged from 7 to 19 MPa, was driven by gasous detonation in a vertical detonation tube. It was found that after passage of the underwater shock wave, the bubble starts to shrink, resulting in explosion just before reaching the minimum radius. In the bubble rebound phase a shock wave is generated whose strength is about twice as compared to that for the air bubble. The maximum pressure of the shock wave nondimensionalized by the ambient pressure behind the underwater shock wave was found to be inversely proportional to the non-dimensional distance. The conversion rate of the bubble energy to the shock energy was estimated to be 0.10, regardless of chemical heat release. Simple numerical analysis of bubble dynamics including compressibility of water was performed. The calculated bubble motion and the shock propagation agree with the experimental results, although the calculated shock strength is overestimated mainly because of non-sphericity of the initial shape of the bubble floating up in water. There is also a good agreement between the experimental and the calculated results in regard to the ratio of the shock pressure for the explosive bubble to that for the air bubble.