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Flow behaviour over rough riverbeds

Nassrullah, Shehla 2022. Flow behaviour over rough riverbeds. PhD Thesis, Cardiff University.
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Over the past several decades, many studies have investigated turbulent flows over natural, hydraulically rough-beds to identify behaviour similar to flows over smooth beds. Such studies have made a substantial contribution to our understanding of flows above impermeable and permeable rough-beds, but the effect of permeable beds on specific turbulent flow characteristics, such as bursting and sweep events, remains elusive. Therefore, this study investigates the flow over permeable natural gravel-bed and artificial beds in terms of the velocity profile, turbulence intensity, turbulent kinetic energy, and Reynolds stresses, which are investigated in part utilising quadrant analysis. Various beds are explored in this study and are comprised of natural gravel with median diameters of 20 mm and 35 mm, as well as an artificial bed with particles nominally 28 mm in diameter. Measurements of instantaneous velocities in the three-component directions for six experiments were collected under uniform flow conditions in intermediate and high relative submergence scenarios using an Acoustic Doppler Velocimeter positioned above the beds within a narrow flume. Endoscopes were also used to track dye flow paths within the various beds themselves. The first- and second-order velocity statistics were investigated by applying a time-space averaging methodology to the Navier-Stokes equations. The experimental results were compared against the logarithmic equation and show that a logarithmic velocity profile is valid for rough-bed flows where von Karman’s constant is within the range of 0.38 to 0.41. Additionally, the Nikuradse equivalent sand roughness was found to range from 5 mm to 61 mm for the different beds. The results showed that κ increases with increasing porosity, and decreases with increasing Rek for flow above natural gravel-beds. Turbulence intensity in the vertical direction was found to notably differ in this study compared to the literature, likely due to the high porosity of the beds used here. Additionally, it was found that relative submergence affects the streamwise turbulence intensity more than in the vertical direction. The distribution of double averaged Reynolds shear stress, Reynolds normal stress, and turbulent kinetic energy were found to decrease linearly from the bed towards the water surface. While the influence of K+ on second-order statistics was found to be insignificant, relative submergence and porosity were found to have a significant effect. Quadrant analysis show ejection and sweep events dominate more than inward and outward interaction events. Interstitial flow results show that numerous turbulent upwelling and downwelling events were observed within all the experiments that were conducted.

Item Type: Thesis (PhD)
Date Type: Completion
Status: Unpublished
Schools: Engineering
Uncontrolled Keywords: gravel-bed and artificial-bed, velocity profile ,Turbulent stresses , Turbulence intensity , Quadrant analysis and Interstitial Flows.
Date of First Compliant Deposit: 26 August 2022
Last Modified: 06 May 2023 01:05

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