Date of Award

10-17-2019

Publication Type

Doctoral Thesis

Degree Name

Ph.D.

Department

Civil and Environmental Engineering

Keywords

flow measurements, flow turbulence, local scour, scour and erosion, scour countermeasures, sediment transport

Supervisor

Ram Balachandar

Rights

info:eu-repo/semantics/openAccess

Abstract

Local scour modelling has been established as an imperative tool in the understanding of local scour mechanisms and development of effective design methodologies for use in practice. However, there are limitations in physical scale modelling which must be fully understood in order to acquire useful experimental results to this end. In hydraulic modelling, facility constraints often result in dimensionless geometric parameters which are considerably altered from prototype conditions. Channel width b in a typical laboratory flume is limited, resulting in small values of channel aspect ratio AR (b/h, where h is flow depth). To further complicate matters, cylinder diameter D in local scour studies must be maximized to obtain a measurable scour pattern and maintain acceptably high relative coarseness D/d50 (where d50 is the median diameter of bed material). This results in cylinder sizes which pose a significant blockage to flow. While blockage effects have been explored for a cylinder mounted on a fixed bed, the effect of blockage ratio (D/b) for a cylinder in an erodible bed has only been explored through comparison of bed formations. In order to fully understand the effect of sidewall proximity on the flow field surrounding a cylinder under equilibrium of local scour, detailed velocity measurements are required. Furthermore, the effect of D/b on local scour must be isolated by holding all other scour-governing parameters constant. In order to achieve this, channel width b has been altered in the present investigation by movable flume sidewalls. In altering b, the channel aspect ratio AR is also affected. Particle Image Velocimetry (PIV) measurements have been undertaken in order to explore the effects of channel aspect ratio on open-channel flow over a porous bed. The effects of increasing vertical confinement(decreasing h) and horizontal confinement (decreasing b) are explored by comparison of vi mean and turbulence properties as well as third-order turbulent moments and quadrant analysis. The findings are then applied to an investigation of the role of D/b on the flow field mechanisms surrounding an emergent circular cylinder under equilibrium of clear-water scour. Changes in the distribution of time-averaged flow velocity, Reynolds shear stress and spanwise vorticity are presented and related to the bed formation. Increasing sidewall proximity is observed to confine the wake region and therefore influence the geometry of the dune downstream of the cylinder. The dune geometry subsequently affects the features in the surrounding flow field, and the streamwise velocity upstream of the cylinder is reduced as D/b increases. A primary objective of scour modelling is development of countermeasures for mitigation of the mechanisms which drive local scour. Lastly, an investigation of two types of scour countermeasures was carried out for flow around a submerged circular cylinder at equilibrium of clear-water scour. Based on fluid dynamic considerations, a vertical splitter plate and a horizontal base plate were chosen as potential flow-altering devices and the efficacy of these devices for scour mitigation are explored. PIV measurements also facilitate understanding of the methods by which each countermeasure affects the flow field mechanisms surrounding the cylinder, and the distribution of flow velocity and Reynolds shear stress are presented. The vertical splitter plate is found to affect the interaction between shear layers in the wake region, reducing the depth of scour downstream of the cylinder. The horizontal base plate is found to eliminate scour upstream of the cylinder altogether by protecting the bed around the sides of the cylinder where shear stress is maximized. Local scour modelling has been established as an imperative tool in the understanding of local scour mechanisms and development of effective design methodologies for use in practice. However, there are limitations in physical scale modelling which must be fully understood in order to acquire useful experimental results to this end. In hydraulic modelling, facility constraints often result in dimensionless geometric parameters which are considerably altered from prototype conditions. Channel width b in a typical laboratory flume is limited, resulting in small values of channel aspect ratio AR (b/h, where h is flow depth). To further complicate matters, cylinder diameter D in local scour studies must be maximized to obtain a measurable scour pattern and maintain acceptably high relative coarseness D/d50 (where d50 is the median diameter of bed material). This results in cylinder sizes which pose a significant blockage to flow. While blockage effects have been explored for a cylinder mounted on a fixed bed, the effect of blockage ratio (D/b) for a cylinder in an erodible bed has only been explored through comparison of bed formations. In order to fully understand the effect of sidewall proximity on the flow field surrounding a cylinder under equilibrium of local scour, detailed velocity measurements are required. Furthermore, the effect of D/b on local scour must be isolated by holding all other scour-governing parameters constant. In order to achieve this, channel width b has been altered in the present investigation by movable flume sidewalls. In altering b, the channel aspect ratio AR is also affected. Particle Image Velocimetry (PIV) measurements have been undertaken in order to explore the effects of channel aspect ratio on open-channel flow over a porous bed. The effects of increasing vertical confinement(decreasing h) and horizontal confinement (decreasing b) are explored by comparison of vi mean and turbulence properties as well as third-order turbulent moments and quadrant analysis. The findings are then applied to an investigation of the role of D/b on the flow field mechanisms surrounding an emergent circular cylinder under equilibrium of clear-water scour. Changes in the distribution of time-averaged flow velocity, Reynolds shear stress and spanwise vorticity are presented and related to the bed formation. Increasing sidewall proximity is observed to confine the wake region and therefore influence the geometry of the dune downstream of the cylinder. The dune geometry subsequently affects the features in the surrounding flow field, and the streamwise velocity upstream of the cylinder is reduced as D/b increases. A primary objective of scour modelling is development of countermeasures for mitigation of the mechanisms which drive local scour. Lastly, an investigation of two types of scour countermeasures was carried out for flow around a submerged circular cylinder at equilibrium of clear-water scour. Based on fluid dynamic considerations, a vertical splitter plate and a horizontal base plate were chosen as potential flow-altering devices and the efficacy of these devices for scour mitigation are explored. PIV measurements also facilitate understanding of the methods by which each countermeasure affects the flow field mechanisms surrounding the cylinder, and the distribution of flow velocity and Reynolds shear stress are presented. The vertical splitter plate is found to affect the interaction between shear layers in the wake region, reducing the depth of scour downstream of the cylinder. The horizontal base plate is found to eliminate scour upstream of the cylinder altogether by protecting the bed around the sides of the cylinder where shear stress is maximized.

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