Onset of wind-driven waves

Abstract

The onset and growth of capillary-gravity waves were studied in a closed wind-water tunnel. The focusing-defocusing of light from a collimated flash passing up through the small waves was used to study photographically the air-water interface. Wavelength and amplitude were determined as well as the onset of waves and the changes in growth patterns. Photographic evidence was obtained on the existence of wave streaks parallel to the mean air flow and preceding the onset of more regular waves. These wave streaks appear to be resonant waves of the type predicted by Phillips. Wave onset and growth were found to depend on air velocity, distance of air-water interaction or fetch, and dimensions of the wind tunnel. At a given air velocity, ‘wave streaks’ were observed at short fetches near the tunnel entrance, with two-dimensional waves, rhombic waves, and gravity waves being formed, respectively, toward the leeward end of the tunnel. A dimensionless number, similar to the boundary-layer transition Reynolds number, was found useful in correlating the conditions for wave onset and growth as well as in relating gross wave properties.