Effect of Wettability on Dynamics of a Rotating Cylinder When Plunging into Water

Abstract

To investigate the dynamics of the object plunging into water, the sequential images of water entry of rotating hydrophilic and hydrophobic cylinders were captured with high speed cameras, and the underwater trajectories of the cylinders were measured at various impact angles, velocities, and rotational rates. The captured images reveal that the hydrophobic cylinder splashes sheet-shaped water and turns to the left opposite to pre-impact moving direction while the hydrophilic cylinder continues moving in the right direction without the splash. The hydrophobic cylinder commonly produces a fully developed air cavity behind, although the hydrophilic one generates it only at the impact velocity higher than a critical value. The hydrophilic cylinder with the high impact velocity turns more rightward, and the fast rotation of the cylinder changes the trajectory leftward regardless of wettability. From the captured images and the estimated force by trajectory analysis, it can be deduced that the changes of the trajectory are due to the reaction force of the splash, additional hydrostatic force induced by the cavity, and lift force by Magnus effect.