Air cushioning in water impact -- its effect on liquid free surface*

A flat plate impacting on stationary water entraps an air layer on its impacting side. This air layer, also known as air cushion, deforms the liquid free surface before the moment of impact. We use high-speed imaging and a new technique to measure the free surface deformation caused by this air cushion close to the moment of impact. This intervening air layer is a low-pressure region, which causes the free surface to be pulled upwards towards the plate due to Bernoulli suction. Our observations are qualitatively well reproduced by two-fluid boundary integral simulations. We attempt to explain the upwards suction of the free surface, and other experimentally observed features using potential flow theory and boundary integral simulations. Our observations have significance for air-pocketing in the water-impact problem which influences the consequently generated loads.