Unsteady characteristics of cloud cavitating flow near the free surface around an axisymmetric projectile

The effect of free surface on unsteady cloud cavitation is important for high-speed surface vehicles, However, previously published experimental and numerical works regarding this topic are limited. In this paper, a typical launching experiment is performed with the presence of free surface. A numerical approach is established by using large eddy simulation and volume-of-fluid methods. Firstly, unsteady evolutions of the cavity and re-entry jet are obtained in both experimental and numerical results, which agree well with each other. Results indicate that the cavity evolution on the upper side of projectile is remarkably different compared to the lower side under the free-surface effect. For instance, on the upper side, cavity growth is slower, the velocity of the re-entry jet is higher, the cavity sheds faster, and the position of shedding cavity collapse is closer to the main cavity. Secondly, the effect of the free surface is studied by analyzing the constraint variation. Because the flow stream around the upper surface is thin, changing its direction under the effect of pressure difference inside and outside the cavity is easy. Non-axisymmetric collapse features generate a mass of strong vortexes on the cylindrical surface, and the non-uniform distribution of high pressure region is also one of the most important factors to induce lateral and vertical forces on the projectile. Finally, the heights of wave elevation in cases with and without cavitation are compared. The presence of cavitation leads to an increase in wave height, but the increment is about half the thickness of the cavity. This finding indicates that the actual constraint effect is between the effects of the infinite water field and the tully free condition. (C) 2016 Elsevier Ltd. All rights reserved.