Does an air-layer cushion hydrodynamic slamming loads?*

Hydrodynamic slamming events are found in the crashing of waves against coastal/breakwater structures, against the walls of shipping containers due to sloshing of liquid cargo, when a diver lands onto water, or during the splash-down landing of sea planes and spacecrafts. In all such events, the ambient air is trapped between the liquid and the impactor on its own accord, but its role in affecting the hydrodynamic loads is not clear. The highest loads are produced when the impact is perfectly parallel - this appears contradictory to the presence of a trapped air `cushioning' layer, which is also the thickest under the same conditions. We shed light on this feature of solid-liquid slamming loads by performing careful experiments with a flat plate that is made to slam perfectly parallel on a bath of water. In the absense of a cushioning air layer, a parallel impact would produce loads with a time singularity, but spatially uniform across the impactor. Whereas with the air cushioning layer, different regions of the plate contact the liquid at different times, such that the loading has a non-uniform spatial distribution. We show how the intially entrapped air-layer's shape, and its time evolution effects the impact pressures at different locations on the plate. Using high-fidelity sensors, we show that at the location where the air layer is at its thickest, there is a very clear reduction of the time-integrated loads (or pressure impulses).