Observation of a Critical Time Scale for Supercavitation Development and the Effect of Gas Leakage

A critical time scale that describes the spontaneous growth of small stable cavities into developed supercavitation over blunt free flying cylindrical slug projectiles was observed experimentally. The projectiles were ejected using a modified gas-gun mechanism consisting of a barrel and explosive charge. Upon ignition, high pressure gases translated the projectiles down the launch barrel and into quiescent water where they were digitally imaged for analysis. Results indicate that the initially small cavities appeared downstream of the projectile forebody and grew up the projectile, partially or completely enveloping the projectile. This study introduces a mechanism to explain supercavity development over impulsively translated projectiles. As a projectile is accelerated from rest in the barrel a vortex ring is produced. Small nuclei present in the ambient flow field expand as they encounter the separated flow around the blunt forebody and become entrained in the vortex ring. The subsequent increase in vortex circulation as the projectile continues to accelerate causes the pressure in the vortex ring to drop below the Blake critical pressure, at which point the initially stable cavity rapidly grows. The small cavities grow from aft to fore as the supercavity develops, traveling at speeds up to 4 times the initial projectile velocity.