Title

Investigation of flow characteristics in an open channel with one-line semicircular vegetation patches

Date of Award

2022

Degree Type

Thesis

Degree Name

Master of Engineering Science

School

School of Engineering

First Advisor

Mehdi Khiadani

Abstract

Experiments are conducted in an open channel with semi-circular patches distributed on one sidewall of the channel in which velocity distribution, turbulent intensity, and vortex generation are analyzed. The resistance force offered by a vegetation patch causes the change in water depth which have a broader application in river management, bank stabilization, and flood control.

The measurement is conducted in the laboratory with the help of different measuring devices like Laser doppler velocimeter, particle image velocimeter, measuring scale, and so on. From the experiment, the velocity distribution in a longitudinal direction suggests that the flow is fully developed after a certain distance from a channel inlet. When the flow passes through the vegetation patch, it forms three different regions, i.e., canopy region, interference zone, and canopy free region. The flow velocity is maximum at the interference zone because of canopy patches to the flow. Wake formation is observed behind the canopy patch, which plays a vital role in sediment deposition; further moving away in the flow direction, wake formation decreases. Velocity is unstable and very low inside the canopy region due to the resisting force. It increases while moving away from the canopy patch in the transverse direction and becomes stable in canopy free region until it reaches the sidewall. A dip phenomenon is observed for both wide and narrow channels, but a dip phenomenon for narrow channels always lies lower from the surface than for wider channels. Turbulent intensities vary with different factors like the nature of the channel, vegetation density, and vegetation stem diameter. Turbulent intensity for a wide channel is comparatively higher than the narrow channel in a canopy-free region. Also, turbulent intensity is minimum just behind the midpoint of the canopy patches, whereas it is lower for dense vegetation than sparse vegetation behind the patch. Furthermore, turbulent intensity for smaller stem diameters is lower than for larger stem diameters behind the canopy patch.

Kelvin-Helmholtz instabilities are visualized due to the change in velocity in an interference zone and play a vital role in eradicating sediments. Also, the size of instabilities depends on the dimension of patches which shows the importance of vortex generation. The width of the vortex structure is more significant in a narrow channel than in a wide channel due to the substantial flow divergence by vegetation patches. Investigation of velocity distribution, turbulent intensity, and vortex generation with canopy patches on both sides of the channel is left for future work.

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