A Prediction of Water-Entry Cavity Shape

Abstract

A method is described for predicting the shape of the cavity formed when a projectile enters the water vertically from air, as a function of time. The method, developed by Birkhoff and based on a hydraulic flow model, has been shown to be applicable for entries of bluff, axisymmetric projectiles. A knowledge of the projectile weight, entry velocity, and cavity-running drag coefficient is required. Predicted and observed cavity shapes are compared for the entry of a sphere at very reduced pressure and the entry of a conical-nosed cylinder from normal air. Excellent agreement is observed over almost the entire cavity length.