The Dynamics of Wind in the Vicinity of Progressive Water Waves

This paper presents a fundamental study of the dynamics of wind in the vicinity of progressive water waves. The normal pressure distribution and the structure of the velocity profile immediately shows progressive water waves are investigated. The theory of Miles (1957) on the generation of surface waves by shear flows predicts a phase shift between the pressure distribution along a wavy surface and the wave itself. This shift becomes responsible for the transfer of energy from air to water due to normal pressure, thereby causing the waves to grow. An experimental verification of this theory is sought. The aerodynamic perturbation-pressure at the air-water interface was measured under two conditions: (1) the pressure sensor following the water surface and (2) the pressure sensor fixed in space above the crest. The results indicate that a pressure shift does exist as predicted by Miles within the assumptions of the theory. Furthermore, the results demonstrate clearly the importance of using a pressure sensor which follows the water surface in obtaining meaningful pressures at the air-water interface. The boundary layer above the water surface was also investigated and the results correlated with the measured pressures. Mean velocity profiles with and without mechanically-generated waves as well as instantaneous velocity profiles above the crest and trough of a mechanically-generated wave were obtained. Contrary to what is normally assumed, the results indicate that there exists a difference in structure between the boundary layer in the vicinity of water waves and that along a flat plate.