On water-entry modes of the latter sphere in tandem configuration with two spheres

Abstract

In this paper, an experimental method is firstly applied to study the effect of time interval on cavity evolution characteristics of two tandem spheres entering water vertically. The water-entry process of is analyzed for different cases by defining the Froude number Fr, which is determined by velocity and time interval. It is found that there are three typical water-entry modes for the latter sphere, with the disturbance from the primary sphere. When Fr > 33.0, the latter sphere directly falls into the opening cavity of the primary sphere and then enters the water, namely “Steady Mode”. When 25.2 < Fr ≦ 33.0, the latter sphere firstly contacts the splash formed by the cavitation of the primary sphere and enters the water afterwards, namely “Transition Mode”. When 3.4 < Fr ≦ 25.2, the latter sphere enters the water after being disturbed by the impact on the Worthington jet, namely “Perturbation Mode”. Numerical simulations are carried out to detail the three water-entry modes, with flow characteristics and drag coefficient being discussed under typical water-entry conditions. It is found that the latter sphere is attracted by a small pressure difference when moving through the opening cavity in Steady Mode and the latter spheres in all three water-entry modes are able to achieve some drag reductions and get a better sustainability of high speed.