Response of Gravity Water Waves to Wind Excitation

The primary purpose of this work was to study the response of gravity water waves to wind excitation, and in particular the applicability of the Miles inviscid shear-flow theory of gravity wave growth, by conducting experiments in a laboratory wind-wave channel under conditions approximating the assumptions of the mathematical model. The secondary objective was to document features of the coupled wave-response/wind-excitation system for comparison with the assumptions of Miles' theory. Mechanically generated wave profiles were measured with capacitance wire sensors and wave energy was calculated at seven stations spaced at 10-ft intervals along the channel test section. Waves varied in length from about 2.5 to 6.5 ft and maximum wind speeds ranged from 12 to 44 fps. Two sources of evidence indicate that without some modification the Miles inviscid theory may be inapplicable to gravity wave growth on a sufficiently rough water surface. First, the measured growth rates were significantly larger than theory predicts. Second, the superposed ripple standard deviations were larger than the respective critical layer thicknesses. Measured energy growth is not inconsistent with the energy transfer expected from flow separation. (Author).