Experimental analysis of water entry problem considering hollow cylinders: The impact of hole geometry

The water entry of hollow cylinders is investigated experimentally. Three hollow cylindrical projectiles with the same mass, surface coating, outer diameter, holes' average diameter, and length are considered. They have three different holes in the shape of a cylinder, an upward cone, and a downward cone. Hence, all non-dimensional parameters are identical for the same impact velocity. A Pco-DimaxS high-speed camera records the projectiles' trajectory. Given the released heights (from 0.5 cm to 95 cm), the impact velocity approximately varies from 0.31ms−1 to 4.32ms−1. Different air-entrainment characteristics like cavity closure patterns, cavity dimensions/shapes, pinch-off time/depth, water jets, cavity shedding, etc., are detected and discussed. Also, the projectiles' kinetics (velocity, acceleration, and coefficient of hydrodynamic force) is examined. The results show different closure patterns with the change of impact velocity and hole geometry: (1) the deep-seal, (2) the surface-seal on the through-hole-jet, and on the cylinder wall. The through-hole-jet for the cylinder with the downward conical hole has the highest height of the other two projectiles. In addition, as the cylinder with the downward conical hole has the largest impact cross-section, it draws a larger volume of air, and as a result, it shows more resistance to cavity sealing. •An experimental study concerning the water entry of hollow cylinders is done.•These cylinders have three different holes in the shape of a cylinder, an upward cone and a downward cone.•According to the dimensions of the cylinders, all dimensionless parameters are identical for the same impact velocity.•Different closure patterns with impact velocity and hole geometry change are observed.•The cylinder with a downward conic hole draws a larger air volume, showing more resistance to cavity sealing.