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.