|Dissipation regimes for short wind waves |
Auteur(s): Caulliez G.
(Article) Publié: Journal Of Geophysical Research C: Oceans, vol. p.00 (2013)
Ref HAL: hal-00773399_v1
Exporter : BibTex | endNote
The dissipation processes affecting short wind waves of centimeter and decimeter scales are investigated experimentally in the laboratory. The processes include damping due to molecular viscosity, generation of capillary waves, microbreaking and breaking. The observations were made in a large wind wave tank for a wide range of fetches and wind speeds, using a laser sheet and a high-resolution side-looking video camera. The work aims at constructing a comprehensive picture of dissipative processes in the short wind wave field, to find for which scales particular dissipative mechanism may become important. Four distinct dissipation regimes have been identified. For capillary-gravity wave fields, i.e. for dominant waves with scales below roughly 4 cm, viscous damping is found to be the main dissipation mechanism. The gravity-capillary wave fields with dominant wavelength less than 10 cm usually exhibit a train of capillary ripples at the front of wave crest, but no wave breaking. For such waves the main dissipation process is molecular viscosity which then occurs through nonlinear energy cascade towards high-frequency motions. Microscale breaking takes place for waves longer than 10 cm and manifests itself in a very localized surface disruption on the forward face of the crest. Such events generate turbulent motions in water and thus enhance wave dissipation. Plunging breaking, characterized by formation of a crest bulge, a microjet hitting the water surface and a splash-up, occurs for short gravity waves of wavelength exceeding 20 cm. Macroscale spilling breaking is also observed for longer gravity waves at high winds. In both cases the direct momentum transfer from breaking waves to the water flow contributes significantly to wave damping.