Effects of entry angle on the water-entry cavity and motion hydrodynamics of a hollow cylinder

The oblique water entry of a hollow cylinder at various entry angles is numerically studied. The formation characteristics of the internal and external cavities, the curling splash, and the underwater rotation of the cylinder are revealed and analyzed. Our results show that asymmetric left- and right-attached cavities form near both the inner and outer walls of the cylinder. There are different formation patterns for the internal cavity between the left and right sides. The left internal cavity is mainly formed by the cavity shrinkage after closure, whereas the right internal cavity is formed by internal flow separation at small water-entry angles. An inclined concavity forms on the through-hole jet tip, induced by the rightward-curling splash. The rotation direction of the cylinder varies with entry angles and a critical angle exists. A periodic hydrodynamic force and moment are observed as the cylinder swings from side to side, resulting in a steady tail-slap motion.