Shallow water entry of supercavitating darts

The dynamics of the vapor cavity created by shallow water entry of a supercavitating dart are studied experimentally. Flat-tipped darts with cavitator radii of 3 and 6 mm and a mass of 55 g were fired vertically using a gas gun into a 1 m deep, 1.2 m square water tank at velocities between 197 and 438 m/s. Pressure was recorded along the axis of penetration and near the point of impact using piezoelectric transducers. High-speed video is used to extract cavity dynamics. Previous work by Bergmann et al. (J Fluid Mech 633:381–409, 2009) is extended to high-Froude number conditions to model the cavity dynamics assuming purely radial flow. It is found that this model predicts cavity behavior well in the middle of the fluid volume where axial flow is negligible. It is also found that the parameter controlling the timing of cavity dynamics, $\beta$, can be related to the size of the experimental tank. Using the results of the model, the general collapse behavior of the cavity can be predicted.