Experimental study on the collapse of tail cavity induced by underwater ventilation

Abstract

Tail cavities are common in gas-propelled underwater cold emission and jet-propelled underwater vehicles. They can also provide a stable working environment for solid rocket motors. In this study, a 2-mm-diameter ventilation hole was provided at the vehicle's rear for a ventilation-induced cavity. Then, the effect of different initial ventilation flows (Q) and Froude numbers (Fr) on cavity collapse after the air supply was stopped was studied. Three different tail cavity closure types are observed: the intact cavity (IC), partially broken cavity (PBC), and pulsating foam cavity (PFC). The IC changes from a twin vortex tube closure to a reentrant jet closure, eventually collapsing entirely. The cavity collapse time decreases with increasing Fr and increases with increasing Q. The dimensionless cavity length (L/D) has an exponential relationship with time when Fr is small and becomes linear with time when Fr is large. The cavity collapse velocity increases with increasing Fr, while Q has little effect. For PBC collapses, the cavity first transforms into an IC and then collapses as an IC. L/D first increases to a local maximum and then decreases. The effect of the reflux gas on the cavity length is critical. During PFC collapses, the cavity first transforms into a PBC, then into an IC, and finally collapses as an IC. L/D first increases to a local maximum and then decreases exponentially.