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Effect of Submerged Vegetation Patch on Hydrodynamics

Author(s): Zhengbing Chen; Chunbo Jiang; Heidi Nepf

Linked Author(s): Heidi M. Nepf

Keywords: Submerged vegetation; Turbulence; Circular patch; Three-dimensional flow

Abstract: This paper uses velocity measurements made with acoustic Doppler velocimetry (ADV) to describe the impact of patch aspect ratio on the flow adjustment near a region of submerged model vegetation. The submerged patches were constructed with rigid cylinders of height h=10. 8cm. One patch was circular, with a diameter of D=22cm, and was placed in the middle of the flume. The flow adjustment for the submerged circular patch was three-dimensional. The other patch spanned the width of flume, but had the same stream-wise length L=22cm. For this rectangular patch the flow adjustment was effectively two-dimensional. The cylinders within the patches were arranged in a staggered pattern at two different densities, described by the frontal area per volume, a; a=5. 1m-1 and 19. 4m-1. The water depth was H = 22cm, that is, H/h=2. Velocity measurements began upstream of the patch and extended 1 to 1. 5m behind the patch. The flow through circular patch was smaller than through rectangular patch. The velocity recovered more quickly behind the circular patch than behind the rectangular patch. For both patches, the peak in Reynolds stress (RS) occurred at the height of the patch and at some distance behind the patch. The RS peak occurred closer to the circular patch and had a lower magnitude, than the peak RS observed behind the rectangular patch. For the rectangular patch, there was no horizontal shear development, and the wake evolution was controlled by the vertical shear layer. For the dense circular patch horizontal shear layer developmentwas important, which made the wake recovering much quicker than behind the rectangular patch. For both aspect ratios, there was a region directly behind the patch in which both the velocity and turbulence near the bed were reduced, and these conditions could promote the deposition of organic particles behind the patch.


Year: 2013

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