Influence of a Weak Surface Film on the Formation and Propagation of Wind Waves in a Channel

Processes of the formation of wind waves remain poorly understood, despite numerous studies. One of the main reasons, in our opinion, is that simplified theoretical analysis does not take into account the weak film of natural contaminants. In the present work, wave generation in two channels is experimentally studied and compared for ethanol, water, and water with the addition of a soluble surfactant (sodium dodecyl sulphate (SDS)) in various concentrations. These concentrations hardly affect the surface tension coefficient, but they lead to a significant modification of the subsurface flow structure. In ethanol, the surface film is not formed, so it can be considered a reference case. In water and water with the surfactant, the film is broken and the surface becomes pure at certain critical wind speed, which grows for increasing surfactant concentration. For the surface to remain pure, the contaminant adsorption to the surface must be compensated by its removal by the tangential stress. Three experimental techniques are used to study the influence of cool skin on the formation of the wind waves. The surface relief is measured with modified color schlieren technique and the liquid velocity fields are determined with particle image velocimetry (PIV). The surface temperature fields, which allow the identification of the regions of the rupture of cool skin, are obtained with IR thermography. IR thermography is also used to study the surface velocity field (IR PIV). The film is shown to have significant influence on both the wave amplitude and the structure of subsurface flow.