Abstract: The problem of water entry of a solid sphere has challenged researchers for centuries and remains of interest to the researchers today, but how the surface condition affect the cavitation during a water entry of sphere has not been studied well. The problem of water entry of a solid sphere is investigated numerically simulated based on the Navier-Stokes equations and volume of fluid method. The numerical results show good agreement with the experimental data. Numerical results with different surface wettabilities and impact speeds are presented. The results show that the condition to create an air cavity is that the impact speed must be strictly above a critical velocity, and the critical velocity is discovered to be dependent on the wetting contact angle of sphere. That means the air entrainment is best inhibited by hydrophilic surfaces, hydrophobic spheres like making a big cavity. Four distinct cavitations are observed at different water entry velocities and contact angles: non-cavitation, deep-seal cavitation, surface-seal cavitation and surface-like cavitation. The simulation results are analyzed, and an empirical theory about the relationship between critical velocity and contact angle is presented.

Key words: fluid mechanics, surface wettability, cavity formation, surface-seal, water entry of sphere