Free surface and near-wall effects on the cloud cavitating flow over an axisymmetric projectile

Abstract

For high-speed underwater vehicles, cavitation can be one of the most important speed barriers. The cavitating flow can be even complex when an underwater vehicle is sailing close to the free surface/wall. In this study, the mutual effects of the free surface and near-wall on the cloud cavitating flow that surrounds an axisymmetric projectile are investigated by water tank experiment and the computational fluids dynamics (CFD). The Split-Hopkinson pressure bar (SHPB) technology is applied in the experiment to accelerate the projectile in a short time. In our numerical approach, the volume of fluid (VOF) method, the large eddy simulation (LES) turbulence model, and the Zwart-Gerber-Belamri (ZGB) cavitation model are used. The cloud cavitating flow under the free surface/wall effect is investigated by changing the distance between the projectile and free surface (df) or wall (dw). Both the experiment and the simulation show that the unstable cavity evolution includes four stages induced by the re-entrant jet: cavity growth, re-entrant jet, cavity shedding and collapsing. The results further show that as df decreases, the cavity length is shorter and the cavity becomes more stable; as dw decreases, the cavity length is longer and the cavity becomes less stable. By understanding the free surface/wall effect on the stability of the cloud cavitating flow, the parameters space spanned by df and dw can be divided into several distinct flow regimes, in which the entire shedding cavity (symmetry/asymmetry), steady and non-shedding cavity, partial cavity shedding, and ventilated cavity are identified.