Local scour around bridge piers can cause serious damages and structural failure to the bridge. Correct prediction of the scour is an important criterion for the engineering design. Though the subject has been investigated for many decades, the theoretical developments have been very limited due to the complicated interaction of three-dimensional flow and the sediment transport. This thesis concerns the flow around a vertical bottom mounted cylinder exposed to currents and is considered as the initial phase of a study towards modeling local scour around vertical bottom-mounted structures. The aim of the present study is to obtain a better understanding of the complex three-dimensional flow and the mechanisms related to the scouring. The study started with the development of a three-dimensional numerical model to simulate flow around cylindrical structures. After validation of the model, the model was applied to investigate flow around an isolated rectangular cylinder. Unsteady flows around cylinders of a square cross section (A/D = 1) and a rectangular cross section (A/D = 2) were simulated to understand the flow properties around a cylinder other than a circular cross section. Three-dimensional flow patterns, pressure distribution, forces on the cylinder and vortex-shedding frequencies were discussed. It was found that the present numerical results generally agree well with the experimental data. Flow around a vertical cylinder mounted on a rigid bed was then investigated by the numerical model. A circular and a square cross sections were considered respectively. Flow structures of horseshoe vortex and the wake vortex which are the major mechanisms leading to the scouring around the base of the vertical cylinder were explored. The bed shear stress distributions that directly affect the scour processes were discussed. Finally the numerical model was applied to study the flow around a submerged square cylinder mounted on a bed, which has significant engineering relevance to the local scour around structures, such as bridge pier foundation itself, or a caisson placed underwater around the pier bottom for scour protection. The numerical calculations were carried out at different cylinder heights to investigate the effect of the cylinder height on the flow properties. It was found that for submerged cylinders with the height of less than one and a half of the cylinder side width the maximum bed shear stress amplification is about 60% of the value of an infinite long cylinder. The quantification of the shear stress reduction is important for scour protection design.
|Qualification||Doctor of Philosophy|
|Publication status||Unpublished - 2007|