Characteristics of cavity collapse behind a high-speed projectile entering the water

We investigate cavity collapse regimes behind high-speed projectiles entering the water. Using numerical simulations, we confirmed two different collapse phenomena: deep pinch-off and consecutive collapse. We performed a theoretical analysis to develop the relationship between projectile motion and cavity evolution. We found that projectile acceleration is the most significant factor determining the initial cavity collapse. There is a critical acceleration determining the direction of cavity collapse at the location of deep pinch-off. The pressure field is obviously affected by the collapse. The increase in pressure induced by surface pinch-off accelerates the collapse at the cavity tail. Because of the impact of surface pinch-off, consecutive collapse can be seen if a projectile reaches critical acceleration near a free surface. Otherwise, the cavity will pinch off at a distance from the surface of the water and form a deep pinch-off. Particular attention is paid to the impact of consecutive collapse on the projectile. Numerical calculations show that cavity collapse and a high-speed water jet have an obvious impact on the stability of the projectile. The jet exerts great pressure on the projectile, and the accompanying splash droplets contaminate the cavity wall. Cavity pulsation and the asymmetric geometry of the projectile-cavity system aggravate attitude deflection.