Experimental investigation on high-speed vertical water entry of spheres with different densities and velocities

In this investigation, a series of experiments were carried out by using aluminum, steel and tungsten spherical projectiles to vertically impact a water-filled tank at a speed of 448.5-1018.2 m/s. The effects of impact velocity and sphere density on cavity, shock wave and motion characteristics of spheres were studied based on the shadowgraph visualization technology and digital image processing. Experimental results showed that for the same density of the sphere, as impact velocity increases, the time of surface closure gets longer and the cavity size becomes larger. In particular, the depth where the maximum cavity diameter occurs and the average velocity of cavity wall section are not significantly affected by the impact velocity. The higher the impact velocity is, the larger the peak pressure of the initial shock wave gets, the longer the penetration distance is, and the greater the velocity decay becomes. Additionally, when the impact velocity of the sphere is fixed, with the increase of sphere density, the time of surface closure gets longer and the cavity size becomes larger. It is worth noting that the deeper the maximum cavity diameter appears, the slower the average cavity wall velocity changes. The sphere of greater density produces relatively larger peak pressure of the initial shock wave, and undergoes longer entry distance and less velocity decay for the same entry time. Furthermore, an analytical model of cavity evolution was developed and good agreement was found between the experimental results and model predictions.