Experimental investigation of water entry of dimpled spheres

This study presents the experimental results of the vertical water entry of dimpled spheres. The role of the number and arrangement of the surface dimples under different impact velocities in the air cavity formation/ evolution/collapse is mainly considered. The diameter of all the spheres is fixed, and they all enter into a tank filled with tap water. Hence, in all experiments, the Bond number and Capillary length are the same, but the Reynolds, Weber, Capillary, and Froude numbers will vary concerning the difference in impact velocities. A PcoDimax-S high-speed camera visualizes dimpled spheres' trajectory at a frame rate of 1 to 2 kfps. In addition to the cavity dynamics, the motion kinetics of the spheres (comprising trajectory, velocity, acceleration, and hydrodynamic force coefficient) is also studied. One important outcome is that dimples are crucial in air entrainment cavity formation, even if the spheres impact the water at low velocities. Also, the results show that at a given impact velocity, increasing the number of dimples can change the pinch-off regime from shallow-seal to deep-seal. In addition, surface dimples improve the descent velocity and acceleration and reduce the hydrodynamic force coefficient.