Enhanced diffusion from a continuous point source in shallow free-surface flow with grid turbulence
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Shallow flows are ubiquitous in nature and are prone to instabilities that result in the formation of large-scale, two-dimensional coherent structures that are expected to significantly enhance eddy diffusivities compared to the stable turbulent base flow. We present the results of an experimental study in free-surface flow to determine the mixing coefficient for a passive tracer plume in two-dimensional grid turbulence. The grid consists of a row of cylinders 2.5 times the water depth and spaced to achieve a porosity of 50%. Depth-averaged dye concentrations are measured using a light absorption planar concentration analysis method; turbulence statistics are calculated from measurements of horizontal flow velocity using a two-component laser Doppler velocimeter. A base case without grid turbulence (having only bottom-generated turbulence) and three cases with grid turbulence are presented. For experiments with grid turbulence, injections were made at 40 water depths downstream of the grid, at the cylinder shoulders, and in the middle of the gap between cylinders. For the base case without grid turbulence, spreading rates agreed well with turbulent diffusion coefficients evaluated by others for wide channel flows. Eddy diffusivities for injections with grid turbulence were an order of magnitude greater than the base case; injections at the grid had larger diffusivities than for the injection downstream of the grid. For all experiments, the eddy diffusivity is shown to agree with Taylor's theorem and to correlate with the size and turbulent intensity of the largest-scale coherent structures. 2005 American Institute of Physics.
