Water impact of a surface-patterned disk

This study experimentally investigates the effect of a surface pattern applied to a flat disk on the impact force during water entry. A macroscale mesh-like pattern with square holes is applied to the bottom surface of a flat disk, and the shape of the pattern and the falling speed of the disk are varied to find a universal parameter that characterizes the impact force. When a surface-patterned disk impacts a free surface, air is trapped inside the holes, and the subsequent compression and depressurization of the trapped air is accompanied by a rise and fall in the impact force. Under the cushioning effect of the trapped air, the peak of the impact force decreases and its period extends as the total volume of holes in the surface pattern increases. These changes are independent of the specific shape of the pattern. In contrast, the impulse exerted on the disk remains similar, regardless of the total volume of holes and the pattern. We conduct a simple theoretical analysis based on the added mass of the disk to estimate the impact force, and confirm the trends observed in our experiments.