PRESSURE OSCILLATIONS DURING WAVE IMPACT ON VERTICAL WALLS

Abstract

This area of study concerns wave impact pressure on vertical structures. Severe damage can be inflicted on coastal defences during storms and numerous laboratory experiments have been undertaken to gain an understanding of the physical processes. This paper gives a mathematical description of high frequency pressure oscillations which are observed in measurements of water-wave impacts; particularly impact against a vertical wall. Our ultimate aim is to trace the physical origins of the pressure fluctuations which are related to the understanding of the role of fluid compressibility in breaking wave impact pressure. It has been suggested by some authors that the recorded pressure oscillations are due to the vibrations of air-filled gas bubbles, for example Weggel & Maxwell (1970) model some details of acoustic wave propagation. In addition, it is well known that the compressibility of a small volume fraction of air in water dramatically reduces the velocity of sound in the mixture. We investigate possible cases by simplifying the geometry and finding the frequency of free oscillations. We ignore the main flow of water since it has a longer time-scale. Our initial model considers compressible aerated water near the wall and compressible non-aerated water with a much higher sound speed further away. This is compared with a simple example of an air pocket trapped against a wall in incompressible water. Comparisons with three experimental results are encouraging.