Interaction of short wind waves and turbulent shear flow as revealed by simultaneous wave slope and surface turbulence visualization

Much work has been undertaken to understand the energy balance of wind waves, but almost no studies about the interaction between waves and turbulence are available up to now. In order to observe waves and surface turbulence simultaneously, measurements were performed in the large wind wave flume of Delft Hydraulics, The Netherlands. A new instrumental setup was used, combining an imaging slope gauge (ISG) with an active infrared technique (CFT). The ISG records time series of the two-dimensional slope within an image sector of 0.4m by 0.4m and a temporal resolution of 16ms. With the CFT, infrared radiators are used to force a constant heat flux through the water surface. The resulting surface temperature within the first 20 fim is observed with a Radiance 1 focal plane array IR camera in the 3-5um wavelenght range. With digital image processing techniques the dynamical behaviour of the water surface can be extracted. The resulting image sequences of temperature patterns are used to compute the 2-D flow field at the water surface. The flow field and wave slope image sequences reveal the significant influence of the turbulence distribution on the shape of the short wind waves. By analyzing properties of the wave field as wave length and phase velocity provided by the ISG the spatial structures of the surface waves can be compared with the scales of the turbulence structures and their dynamics.