Experimental study on vertical water entry of twin spheres side-by-side

The cavity dynamics and trajectories of twin spheres vertically entering water side-by-side are investigated experimentally for different time intervals and several lateral distances at initial velocities from 14.1 m/s to 15.2 m/s, with the diameter (D) based Froude number varying from 37.1 to 40.0. A high-speed photograph system and an image processing method are employed to obtain the features of cavity flow and the position of twin spheres. We firstly investigate the cavity shapes and sphere trajectories with lateral distance being 1.5D between the spheres. Results show that the contact lines of two cavities are obliquely pinned on the twin spheres in synchronous water entry, and a molar-shaped cavity is formed by the overlap of the two cavities. Subsequently, the motion characteristics of the second water-entry sphere (Sphere II) are studied as the increase of time interval between the twin spheres. It is found that the influence on Sphere II increases as the decrease of the time interval at lateral distance of a = 1.5D, except a certain condition, i.e. Delta t = 2.5 ms. As we change the lateral distance in synchronous water entry, the disturbance introduced by adjacent sphere is weakened with the increase of the distance. The results demonstrate that both the cavity dynamics and trajectories of Sphere II are similar to that of single water entry when the lateral distance increases to 5.5D. As for the combined effects of lateral distance and time interval on twin water entry, it is found that Sphere II is attracted horizontally to Sphere I at moderate time intervals with the lateral distance of a = 1.5D, while the interaction acted on Sphere II in cases of a = 2.5D and 3.5D changes from repulsion to attraction gradually as the increase of time interval from 3.5 ms to 8.4 ms and then to 13.1 ms.