Simulation of Hypervelocity Water Entry by Spherical Projectiles

Hypervelocity water entry of a projectile through a free surface is an important problem with little available experimental data in the literature and largely unvalidated numerical techniques. In preparation for a planned experimental campaign using spherical projectiles, this study examines a novel numerical approach implemented in the CFD code CHAMPS by comparing it to existing experimental and numerical studies. The shock front depth was well captured when compared to an experimental image, but there was disagreement in the projectile depth. In comparison to previous numerical studies, the evolution of the peak pressure compared well for a supersonic case, but was over-predicted for a subsonic case. More investigation into these discrepancies with previous studies is needed, but may be related to the different equations of state used in the studies. It was also found that the pressure jump across the shock front increases with the initial velocity as a power law with an exponent of 2.9, and both the shock pressure jump and the average initial shock velocity tend to increase with the projectile diameter. The pressure was also found to fall off more steeply in the axial direction than radially as a function of distance from the projectile. In addition, the stiffened gas, IAPWS-95, and NASG equations of state were compared in shock tube simulations where it was found that while all three qualitatively captured the same shock and expansion wave structure, the shock speed was found to increase from the stiffened gas, to the NASG, to the IAPWS-95 solutions, respectively. This increase in shock speed is likely due to the inclusion of covolume and other physical effects in the more complex equations of state.