High-speed water entry of spherical projectiles

Water entry of projectiles leads to the formation of a cavity following the projectile due to air entrainment and/or cavitation for higher speed entries. While cavity dynamics have been extensively studied for low-speed water entry of projectiles, the present work focuses on characterizing cavity dynamics for high-speed water entry of spherical projectiles, where the entry velocity is higher than the water sound speed (≈1475 m/s under standard conditions). Experiments are conducted using the HOPLITE two-stage light-gas gun, which launches the spherical projectiles horizontally at velocities ranging from 1470 to 2314 m/s into a water tank. Two methods are employed to obstruct the water tank entry port prior to projectile entry: a thin Mylar sheet (of 50 μm or 127 μm thickness) or a pneumatically actuated fast-opening sluice gate. In the Mylar case, the projectile punctures the sheet and enters the water; whereas in the sluice gate case, the gate opens just before the projectile entry to approximate a free surface. The flow field immediately downstream of the entry port is imaged using a high-speed camera to track the cavity evolution. The effects of both the entry port closure method and the entry velocity on cavity formation and collapse are studied. It is observed that cavities are larger when a Mylar sheet is used compared to the gate mechanism, indicating that the initial condition plays a significant role in air entrainment and cavity growth. Additionally, an increase in entry velocity leads to a corresponding increase in cavity size, suggesting dependence on kinetic energy input. In all cases, the cavity collapse occurs through the breakdown of the cavity edge (air-water interface), which leads to the development and bursting of bubbles of varying sizes over time.