Experimental study on cavity dynamics of projectile water entry with different physical parameters

In this paper, we investigate the influences of nose shape, impact velocity (8-14 m/s), and impact angle (60°-90°) on cavity dynamics when a projectile enters water. The Froude number, which characters the kinetic energy against gravitational potential, ranges from 280 to 850. It is found that the cavity diameter changes for different nose shapes, and an elongated cavity is achieved as the impact speed increases. The cavity pinch-off phenomenon is characterized. Experimental data reveal that the nose shape, impact velocity, and impact angle change the pinch-off depth and pinch-off time slightly by changing the occurrence time of the surface seal. For blunt nose shapes, greater impact velocity speeds up the surface seal and then quickens the pinch-off, thus reducing both the pinch-off depth and pinch-off time. Generally, the pinch-off depth follows the Fr^1/3 law in our experiments. Cavity ripples were observed after pinch-off, and the wavelength, amplitude, and rippling frequency were measured. The wavelength of a ripple remains constant throughout, and all ripples are fixed with the experimental frame. The rippling frequencies are approximately identical to the Minnaert frequency. The impact velocity significantly changes the rippling frequency by affecting the radius of the air cavity.