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Wind waves generation


A. Paquier, F. Moisy, M. Rabaud


Understanding the generation of surface waves under the action of wind is a problem of primary interest for wave forecasting and for evaluation of air-sea exchanges of heat, mass, and momentum on Earth or on natural satellites. It is also important in engineering applications involving liquid and gas transport in pipes.

We investigate experimentally the early stage of the generation of waves by a turbulent wind at the surface of a viscous liquid. The spatio-temporal structure of the surface deformation is analyzed by the optical method Free Surface Synthetic Schlieren, which allows for time-resolved measurements with a micrometric accuracy.


Fig. 1: Liquid tank and wind tunnel. The surface deformations are measured by Free-Surface Synthetic Schlieren, by imaging from above a pattern of random dots located below the liquid tank.



Because of the high viscosity of the liquid, the flow induced by the turbulent wind in the liquid remains laminar, with weak surface drift velocity. Two regimes of deformation of the liquid-air interface are identified. In the first regime, at low wind speed, the surface is dominated by rapidly propagating disorganized wrinkles, elongated in the streamwise direction, which correspond to the surface response to the pressure fluctuations advected by the turbulent airflow. The amplitude of these deformations increases approximately linearly with wind velocity and are essentially independent of the fetch (distance along the channel). Above a threshold in wind speed, we observe the growth of well defined gravity-capillary waves with crests nearly perpendicular to the wind direction. In this second regime, the wave amplitude increases with wind speed but far more quickly than in the first regime


Fig. 2: Instantaneous surface height measured by FS-SS at increasing wind velocities. (a) small-amplitude disorganized wrinkles elongated in the streamwise direction; (b) combination of streamwise wrinkles and spanwise waves; (c) well-defined spanwise waves; (d) large-amplitude waves with increasing disorder.



Movie: 3D reconstruction of the surface height measured by FS-SS at increasing wind velocities.



Publications

  • Viscosity effects in wind wave generation
    A. Paquier, F. Moisy and M. Rabaud, Phys. Rev. Fluids 1, 083901 (2016)
    [Abstract | PDF]
  • Surface deformations and wave generation by wind blowing over a viscous liquid
    A. Paquier, F. Moisy, and M. Rabaud, Phys. Fluids 27, 122103 (2015)
    [Abstract | PDF]

Last modification: December 02 2016, 09:57:48.