Convection within an experimental wave flume

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Abstract
The velocity field beneath a series of progressive gravity waves was investigated using laser Doppler anemometry. The drift velocity was observed to vary with respect to both time (t) and distance (x) along the wave flume. Vorticity profiles, calculated from the velocity data, suggest that this development has its origins within the near shore region. In most cases a stable configuration is eventually achieved. A negative shear current is established in which the back flow is between 3 and 4 times larger than the predicted irrotational motion. Even in those regions which are directly adjacent to the viscous boundary layers the motion appears to depend upon the horizontal convection of vorticity, rather than its vertical diffusion. Indeed, there is a clear transition between the dominant mode of vorticity transportation. The final solution is characterised by negative near bed velocities and positive vorticity within the near surface region. This is completely opposite to that predicted by a diffusion solution, and indicates the importance of convection from the near shore region.

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