Depth-Averaged Drag Coefficient for Modeling Flow through Suspended Canopies
Publication: Journal of Hydraulic Engineering
Volume 137, Issue 2
Abstract
Aquatic suspended canopies are porous obstacles that extend down from the free-surface but have a gap between the canopy and bed. Examples of suspended canopies include those formed by aquaculture structures or floating vegetation. The major difference between suspended canopies and the more common submerged canopies, which are located on the bottom boundary, is the influence of the bottom boundary layer beneath the suspended canopy. Data from laboratory experiments are presented which explore aspects of the flow through and beneath suspended canopies constructed from rigid cylinders. The experiments, using both acoustic Doppler and two-dimensional (2D) particle tracking velocimetry, give details of the flow structure that may be divided vertically into a bottom boundary layer, a canopy shear layer, and an internal canopy layer. The experimental data show that the penetration of the shear layer into the canopy is limited by the distance between the canopy and bottom boundary layer. Peaks in velocity spectra indicate an interaction between the bottom boundary and canopy shear layer. An analytical model is also developed that can be used to calculate a drag coefficient that includes the effect of both canopy drag and bed friction. This drag coefficient is suitable for use in 2D (depth-averaged) hydrodynamic modeling. The model also allows the average velocity within and beneath the canopy to be calculated, and is used to investigate the effect of canopy density and thickness on both total drag and bottom friction.
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Acknowledgments
The writer thanks Jen Dumas, Matt Enright, and Glenn Cooper (NIWA) for assistance with the laboratory experiments; Vladimir Nikora (University of Aberdeen) for suggestions on the spatial averaging approach; and Roger Nokes (University of Canterbury) for the use of the FluidStream PTV system. This project was supported by the Foundation for Science, Research, and Technology through Contract No. UNSPECIFIEDC01X0507 Sustainable Aquaculture and C01X0513 Complementary Aquaculture.
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Information
Published In
Journal of Hydraulic Engineering
Volume 137 • Issue 2 • February 2011
Pages: 234 - 247
Copyright
© 2011 ASCE.
History
Received: Jul 16, 2008
Accepted: Jul 27, 2010
Published online: Aug 3, 2010
Published in print: Feb 2011
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