Experimental observation on water entry of a sphere in regular wave

Abstract

This paper presents a novel experiment to observe the whole water entry process of a free-falling sphere into a regular wave. A time-accurate synchronizing system modulates the moment elaborately to ensure the sphere impacting onto the water surface at the desirable wave phase. Four high-speed cameras focus locally to measure the high-precision size of the cavity evolution. Meanwhile, the aggregated field view of the camera array covers both the splash above the free surface and the entire cavity in the wave. The detailed methodologies are described and verified for the hardware set-up and the image post-processing. The theoretical maximum deviation is 1.7% on the space scale. The integral morphology of the cavity is captured precisely in the coordinate system during the sphere penetrates through the water at four representative wave phases and the still water. The result shows that the horizontal velocity of the fluid particle in the wave impels the cavity and changes the shape distinctly. Notably, the wave motion causes the cavity to pinch off earlier at the wave trough phase and later at the wave crest phase than in the still water. The wave motion influences the falling process of the sphere slightly in the present parameters.