Vertical water entry of a hydrophobic sphere into waves: Numerical computations and experiments

Abstract

The water entry cavity evolution and its flow structures for a sphere interacting with periodic waves are investigated numerically and experimentally. The large eddy simulation is applied in the simulation to accurately capture the turbulent flow near the surface and within the cavity of the sphere. An overset mesh-based numerical wave tank is developed, integrating an overset mesh with a method for generating regular waves, to ensure high resolution simulation of velocity fields around the water entry cavity in waves. To validate the numerical model, a physical experiment system is developed, featuring a free-falling setup and an asynchronous pulse trigger system. This experimental setup allows for precise control of the vertical water entry of a sphere at a predetermined phase of a periodic wave. The computed cavity shape and the sphere motion are in good agreement with the experimental results. Notably, the hydrodynamic forces exerted on the sphere exhibit two distinct peaks at the moment of impact and the pinch-off of the cavity, respectively. The gas-phase force acting on the dry surface of the sphere, as the cavity forms and evolves, experiences significant fluctuations along the direction of the sphere's descent. These fluctuations are caused by the accelerating gas flow prior to the pinch-off of the cavity. The changes of the hydrodynamic forces on the sphere for the cases of different water entry phase locations of waves and Froude numbers are discussed.