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

Abstract

Processes of the wind waves formation remain poorly understood, despite the 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 waves generation in two channels is experimentally studied and compared for ethanol, water and water with addition of soluble surfactant SDS (sodium dodecyl sulphate) in various concentrations. The employed concentrations hardly affect the surface tension coefficient, but lead to significant modification of the subsurface flow structure. In ethanol the surface film is not formed, so it can be considered as reference case. In water and water with added surfactant the film gets broken and the surface becomes clean at certain critical wind speed, which grows for increasing surfactant concentration. For the surface to remain clean, 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 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 identification of the regions of the cool skin rupture, 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 waves amplitude and the structure of subsurface flow.