Free-surface dynamics and hydrodynamic characteristics in the buoyancy-driven exit of axisymmetric bodies

This study experimentally investigates the buoyancy-driven water exit of axisymmetric bodies to improve understanding free-surface dynamics and hydrodynamic loading relevant to marine and offshore applications. Cylindrical and spherical models were released from rest at different initial submergence depths, with motions captured using high-speed imaging and strain-gauge measurements. Results show that early-stage motion prior to surface interaction is self-similar and largely independent of submergence depth, characterized by constant acceleration and negligible viscous effects. As the bodies approach and breach the free surface, strong coupling occurs between body motion, free-surface deformation, and added-mass variation. The amplitude and width of the disturbed free surface, as well as total hydrodynamic load, increase with submergence depth due to enhanced fluid inertia. Maximum hydrodynamic force occurs immediately before or during surface breach, then decreases rapidly as the entrained water layer detaches. The spherical body exhibits smoother and faster exit with reduced surface disturbance compared to the cylinder. The effect of superhydrophobic coating was also examined; while hydrophobicity did not affect overall exit kinematics or surface evolution, it promoted post-exit breakup of the residual water layer. Overall, submergence depth is identified as the dominant parameter governing free-surface morphology, hydrodynamic loading, and exit velocity.