Computer simulation of irregular surface reflection of an underwater shock wave

Computational studies are given for the behavior of a fluid set in motion by a shallow underwater nuclear explosion. Of particular interest is the interaction of the incident shock wave with the reflected rarefaction wave. Under certain conditions, the rarefaction wave can overtake the shock front beneath the water surface in a manner that is analogous to Mach stem formation in shock reflection from a rigid wall. This phenomenon, referred to as irregular surface rarefaction, has important implications in naval tactics, because it can limit the effective range of shallow underwater explosions. The boundary of the region where irregular rarefaction has occurred is determined by a rarefaction fan generated at the point of surface interaction. This fan resembles the Prandtl-Meyer fan for a supersonic flow through a rapidly expanding jet nozzle. A Lagrangian code and the ALE code were applied to simulate explosions of 1015 joules at depths of burst of 3 m, 21 m, and 6.5 m, and to compute the underwater shock wave until the peak pressure decays to less than 0.1 GPa. The results show that the portion of the wave unperturbed by rarefaction closely follows Snay's theoretical description of a shock wave generated by a point explosion in infinite homogeneous water. The onset of the irregular surface rarefaction and the envelope which separates the irregular rarefaction region from the regular rarefaction one are calculated. The theoretical model that developed predicts quite well the onset point at the surface for all three cases plus the region boundary for weak shocks. However, the theory does not match the region boundary with the computed results at large distances for strong shocks. 14 refs., 26 figs