Experimental Investigation of Hypervelocity Water Entry by Spherical Projectiles

The results of an experimental investigation of the entry of steel spheres into liquid water through either a free surface or a thin Mylar film at a range of velocities from 1740–2314 m/s are presented. Particular attention is given to characterizing the flow features. The entry velocity is observed to affect the sphere deceleration, the shock wave, and cavity formation. With increasing entry velocity, the post-impact deceleration increases due to the effect of compressible drag, and the cavity grows more quickly. The entry material is observed to only weakly influence the sphere deceleration and has a negligible effect on the shockwave and cavity formation. This is possibly due to the thin Mylar film reducing the kinetic energy of the sphere upon impact or causing a flattening of the front of the sphere, leading to increased drag especially at higher entry speeds. The splash occurs in four distinct phases: a fast-moving cloud of atomized spray, a pause, a slower bulk liquid ejection, and finally a faster liquid flow. With increasing impact velocity, the pause phase is shorter in duration, while the slow ejection phase is longer. The effect of adding a thin plastic film over the entry region lengthened the duration of the bulk ejection phase by 28% for an entry velocity of 2000 m/s, but had no effect on the spray phase duration. For all velocity and entry material conditions, the spray phase duration remained nearly constant.