The Impact of a Deep-Water Plunging Breaker on a Partially Submerged Cube

Abstract

The impact of a plunging breaker on a partially submerged cube is explored experimentally in a wave tank equipped with a programable wave maker. The experiments are conducted with the cube (dimension $L=30.48$ cm) positioned at one streamwise location relative to the wave maker and at three heights relative to the undisturbed water level. A single, repeatable wave maker motion producing a breaker with a nominal wavelength of 1.18 m is used. The water surface profile at the stream wise vertical center plane of the cube is measured with a cinematic Laser-Induced Fluorescence technique and the impact pressures on the front face of the cube are measured with piezoelectric dynamic pressure transducers. The surface profile measurements and the impact pressure measurements are synchronized. When the cube is positioned vertically so that its bottom face is at either $0.5L$ or $0.25L$ below the undisturbed water surface, the water surface profile behaviors are basically similar with a nearly circular arc forming between the water contact point on the front face of the cube and the wave crest. As the impact proceeds, this arc shrinks to zero size and creates a fast-moving vertical jet in a manner similar to that found in previous studies of wave impact on bottom-mounted vertical walls. In the case where the cube is one quarter submerged, a small jet also forms at the crest and impacts the front face of the cube just before the circular arc reaches zero size. When the bottom of the cube is located at the undisturbed water level the wave impact is dramatically different. In this case, it appears that a packet of air is entrapped during the impact and the surface pressure subsequently oscillates with a frequency of about 2,000 Hz.