Morphology of entrapped air bubbles during water impact of a flat plate

When a flat plate impacts upon the water surface, bubbles of different shapes can be entrapped depending on impact velocity. In this paper, the morphology of trapped air bubbles and the underlying physics are numerically investigated. Among the wide range of impact velocities, two typical air bubble patterns have been discovered: small-bubble pattern and large-bubble pattern. Specifically, as the plate descends, an air layer is initially entrapped underneath, and is then enclosed into a large air bubble upon initial contact of plate edge with water surface. Subsequently, this large bubble breaks into lots of small bubbles at low impact velocities, but remains intact at higher impact velocities, exhibiting two distinct patterns. The morphological evolution of air bubbles is found to be dictated by two physical factors: the compression-expansion intensity of trapped air during initial impact stages (before the contact of plate edge with water surface) and wind-driven waves on the water surface below plate. Particularly, the former determines the thickness and shape of the initially enclosed large bubble. At low impact velocities, due to the weak compression-expansion behavior of trapped air, the initially formed large bubble is thin and flat, in which the wind-driven waves can make direct contact with plate bottom easily, thereby splitting the large bubble into many small bubbles. At higher impact velocities, the compression-expansion intensity of trapped air becomes increasingly stronger, causing the initially formed large bubble to be considerably thicker and rounder, in which the wind-driven waves fail to wet the plate bottom; thus, the large bubble is preserved.