Mechanism of the sharp rise and fall of the impact force during water impact of a flat plate: compression-expansion behavior of trapped air layer

During the water impact of a flat plate, an air cushion is generally entrapped underneath and the impact force history is characterized by pronounced oscillations, typically comprising three distinct stages: a sharp rise to the peak, a sharp drop to the trough, and less intense pulsating forces thereafter. The physics of post-impact pulsating forces is full understood, i.e., due to bubble oscillations; whereas, a clear understanding of the initial sharp rise and fall of the impact force is still lacking, which is numerically investigated in this paper. The results show that the drastic compression-expansion behavior of the trapped air layer is the physical cause. By defining an air control volume confined by the moving plate bottom and its underneath deforming water surface, for the first time, we demonstrate that the compression-expansion mechanism of air layer is governed by two key physical processes: the contraction and expansion of the control volume, and the inflow and outflow of air through the control surface.