Wind-Generated Current and Phase Speed of Wind Waves

Abstract

Measurements of drift were made in a wind and wave facility at different elevations below the mean water level. The drift profiles were obtained for reference wind speeds, Ur=3.1, 5.7 and 9.6 m sec?1. The measurement technique involved tracing the movement of small paper disks which were soaked in water to become neutrally buoyant at the elevation of release. A logarithmic drift profile is proposed. The water shear velocity U*w predicts a surface stress, τs=?wU*w2, in agreement with that obtained from the wind shear velocity, τs=?aU*a2, where ?a and ?w refer to air and water densities, respectively. The influence of wind on phase speeds of waves was investigated by solving the first-order perturbation problem of the coupled shear flows in air and water. The air velocity profile was described by a logarithmic distribution and the drift profile was described by the proposed drift profile. Adequate agreement is found between the calculated and measured phase speed using Doppler radar in the wavenumber range 1.9?10 cm?1. In the wavenumber range 0.05?0.5 cm?1, measurements of phase speeds were obtained by using two wave gages. The waves were mechanically generated without wind and the wave gages were spaced to obtain coherent signals. The wind was then allowed to blow over the waves and the distance between wave gages was increased to maintain coherence. The wavelength and frequency were obtained from the distance between the gages and from the generator frequency, respectively. The measured phase speeds were found to increase with wind speed consistent with theoretical computations.