Mass Transport in Vegetated Shear Flows |
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Authors: | Marco?Ghisalberti Email author" target="_blank">Heidi?NepfEmail author |
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Institution: | (1) Centre for Water Research, University of Western Australia, Crawley, Western Australia, Australia;(2) Ralph M. Parsons Laboratory, Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA |
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Abstract: | Submerged aquatic vegetation has the potential to greatly improve water quality through the removal of nutrients, particulates
and trace metals. The efficiency of this removal depends heavily upon the rate of vertical mixing, which dictates the timescale
over which these constituents remain in the canopy. Continuous dye injection experiments were conducted in a flume with model
vegetation to characterize vertical mass transport in vegetated shear flows. Through the absorbance–concentration relationship
of the Beer–Lambert Law, digital imaging was used to provide high-resolution concentration profiles of the dye plumes. Vertical
mass transport is dominated by the coherent vortices of the vegetated shear layers. This is highlighted by the strong periodicity
of the transport and its simple characterization based on properties of the shear layer. For example, the vertical turbulent
diffusivity is directly proportional to the shear and thickness of the layer. The turbulent diffusivity depends upon the size
of the plume, such that the rate of plume growth is lower near the source. In the far-field, mass is mixed more than twice
as rapidly as momentum. Finally, plume size is dictated predominantly by X, a dimensionless distance that scales upon the
number of vortex rotations experienced by the plume. |
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Keywords: | mass transport shear layer turbulence turbulent Schmidt number vegetated flow vortex |
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