An Experimental Investigation and Modelling of the Deffusion Phenomena Under Wind Waves

Abstract

In the aspect of water-environments, the studies on wind waves are very important regarding transport phenomena in atmosphere, lakes and sea. Dispersion phenomena in water under wind waves have been studied by many researchers, but they have taken notice of only surface layer beneath the free surface. The purpose of this study is to elucidate the diffusion in the surface layer and the lower layer as well. Visualization of dye diffusion and velocity field measurements by means of PIV and 2D-electro magnetic velocity meters were performed. The mechanisms of dye diffusion between the surface and the lower layers are different. In the surface layer, the diffusion can be estimated well by using Taylor's turbulent diffusion theory. In the lower layer, however, we found that the diffusion is like the chaotic mixing. To investigate qualitative feature of the chaotic mixing under wind waves, a mathematical model represented by the superposition of a periodic array of vortices and stationary water waves was proposed. The coexistence of free surface waves and large-scale vortices causes chaotic advection of fluid particles. This phenomenon will induce strong mixing of water in the lower layer even if the turbulent intensity in this layer is small.