Horizontal and vertical spatial structures of turbulence beneath short wind waves

A poorly understood aspect of surface-wave physics is wave dissipation. Wave breaking may lead to enhanced turbulence levels and consequently enhanced viscous dissipation at the smallest scales. Recent laboratory, field and modelling studies have provided preliminary evidence that wave breaking may lead to dissipation levels one to two orders of magnitude greater than those in the classical logarithmic layer. However, the experimental techniques to study these phenomena are not well developed, and consequently the turbulence structure near the water surface is poorly understood. A method is described to simultaneously investigate the horizontal and vertical structure of the turbulent flow and wave fields at the water surface and in an intersecting two-dimensional vertical plane. The technique combines an active infrared technique (CFT) in the horizontal plane with digital particle imaging velocimetry (DPIV) in the vertical plane. The DPIV uses a CCD camera in an underwater housing to track submerged particles moving through a pulsed vertical laser light sheet aligned in the wind direction. The CFT uses infrared radiators to heat up the upper 20 micrometres of the water column and observes the resulting heat patterns with an infrared camera. Both instruments use digital image processing techniques to compute the flow variables and images of the turbulent structures. These initial experiments were conducted in the large wind-wave channel at Delft Hydraulics, The Netherlands, in September/October, 1994. The images show a dramatic transition from laminar to turbulent flow with the onset of significant wave activity coincident with vorticity transport from the surface to deeper layers.