Fluctuational characteristics of cavities behind an axisymmetric vehicle during water-entry

Cavity fluctuations induced by deep pinch-off are commonly observed in water-entry phenomena. This study investigates the fluctuational characteristics of cavities formed by an axisymmetric vehicle during water-entry. Simulations capture the evolution of the cavity after deep pinch-off, focusing on the behavior of interface waves and significant pressure oscillations within the fluid. The study classifies three cavity types based on the relative length between the cavity and the water-entry vehicle: a cylinder-like cavity, a tube-like cavity, and a cylinder-like cavity with an internal body. These cavity types exhibit different fluctuational behaviors, which result in differing ripple propagation characteristics. A theoretical analysis models the cavity as a tube-like cylindrical structure, elucidating the relationship between rippling frequency and the afterbody radius of the water-entry object. For the cylinder-like cavity with an internal body, an analogical approach is introduced to describe the interface rippling mechanism by transforming the cavity into an equivalent cylinder-like structure. The results reveal that, for tube-like cavities, the rippling frequency increases with the afterbody radius, whereas for cylinder-like cavities with an internal body, the rippling frequency depends on the internal body volume but remains independent of body shape. Furthermore, when the Froude number (Fr) exceeds 24, the water-entry cavity can be effectively modeled as a long cylindrical cavity. The simulations confirm that the proposed theoretical models provide accurate predictions of the impact of the object's afterbody on the fluctuation characteristics of the entrained cavity.