Hydrodynamic characteristics of cavity fluctuation behind a cone-rod assembly entering water

This study explores the phenomenon of cavity fluctuation occurring behind a cone entering water at a constant velocity. The current simulations reveal that cavity fluctuations arise following deep pinch-off, leading to pronounced pressure oscillations in both the water and air regions. Concurrently, ripples form along the cavity surface, extending from the nose to the tail, resulting in a wavy cylindrical cavity. Notably, when the water entry Froude number is below 10, the load on the cone is predominantly due to pressure oscillations induced by cavity fluctuations, which exceed the slamming load experienced during initial water impact. The study also identifies a significant impact of an attached rod on cavity evolution. Specifically, the frequency of cavity rippling increases with the rod's radius; however, when the rod-to-cone radius ratio is less than 20%, the rod's impact on the cavity dynamics becomes negligible. A theoretical analysis, modeling the cavity as a hollow cylindrical structure, is developed to elucidate the relationship between rippling frequency and rod size. The research results demonstrate that the cavity fluctuation frequency is inversely proportional to the difference in the squared radii of the cone and rod. Furthermore, when the scaling length of the cavity at the pinch-off moment exceeds a ratio of Lp/Rc > 6, the water entry cavity can be accurately modeled as a long cylindrical cavity. The numerical results confirm that the proposed theoretical model provides reliable predictions of the impact of a solid rod on the fluctuation characteristics of the cavity.