Experimental and theoretical investigation of the cavity dynamics of underwater launched projectiles

During the underwater launch of a projectile using compressed air, a bubble emerges from the launch tube before the projectile. Upon being pierced by the projectile, this bubble has significant effects on the formation and evolution of the projectile’s shoulder cavity. This paper presents experimental and theoretical analyses of the dynamics of such a projectile’s shoulder cavity launched at low Froude numbers. In the experiments, a dynamic sensor and a camera are used to record the internal pressure and profile of the cavity, respectively. The cavity pressure oscillations explain the cavity ripples observed during the experimental sequence. Accordingly, we establish an independent bubble evolution model to predict the internal cavity pressure based on the assumption of spherical expansion; we also separately construct a shoulder cavity evolution model that treats the cavity as a slender body. The matched asymptotic method is used to solve the shoulder cavity evolution model, and quantitative comparisons between the theoretical and experimental results, including the cavity shape and size, show favourable agreement. Finally, the effects of cavity pressure oscillations and of the nose shape and projectile velocity on the cavity’s behaviour are studied.