Experimental investigation on the cavity sealing transformation of elliptical cylinders during water entry

The water-entry cavity sealing of elliptical cylinders is systematically investigated with experimental methods, which features the inconsistent lengths of major and minor axes. By comparing with the cylinder entering water, the effects of aspect ratio and Froude number on the cavity sealing are analyzed to reveal the mechanism of sealing mode transformation among deep, transition, and surface seals. In the deep seal mode, the water-entry cavity of cylinders pinches off at a singularity, but the minor-axis cavity of elliptical cylinders shows an oval hole when the cavity in the major axis pinches off. During the water entry of elliptical cylinders, the water hardly splashes in the major axis but mostly splashes in the minor axis. Thus, the minor-axis cavity is larger than the major-axis cavity in the surface seal. Especially, when the splash crown seals to the tail plane, the transition seal mode occurs with surface and deep seals successively appearing. The shedding cavity in the deep seal of cylinders is penetrated by a central water column, while the shedding cavity of elliptical cylinders merges with the downward jet and splits into two separate halves. As the aspect ratio increases, the minor-axis splash crown obtains more kinetic energy and gradually expands outwards. Consequently, the deep seal happens later with a larger pinch-off depth, but the surface seal occurs faster with a smaller cavity volume. Therefore, the sealing mode transformation sequentially undergoes surface, transition, and deep seals, which is contrary to the mode transformation with the increment of Froude number.