Doctoral Degrees (Civil Engineering)

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Browsing Doctoral Degrees (Civil Engineering) by Subject "ANSYS (Computer system)"

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    Coupled fully three-dimensional hydro-morphodynamic modelling of bridge pier scour in an alluvial bed
    (Stellenbosch : Stellenbosch University, 2019-12) Vonkeman, Jeanine Karen; Basson, G. R.; Smit, G. J. F.; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.
    ENGLISH ABSTRACT: Local scour at piers has been cited as the main mechanism responsible for the collapse of bridges founded in alluvial beds and yet there is no universally agreed upon design procedure to accurately predict the equilibrium scour depth. The scour process was investigated by a 1:15 scale physical model for a combination of different flows, pier shapes and sediment beds, from which the scour patterns and flow velocities were measured. The experimental data was used to evaluate thirty empirical equations for bridge pier scour, which were found to produce a wide range of unreliable results. No single equation is conclusively superior but the HEC-18 equation is proposed, as well as equations that rely on the pier Reynolds number, a parameter which has been shown to be significant in the horseshoe vortex formation. Subsequently, an improved dimensionless shape factor and armouring factor based on the particle Reynolds number were developed for the HEC-18 equation from field data measurements. Although extensive research has been published on bridge pier scour for more than six decades, comparatively few studies have been presented on the detailed 3D numerical modelling of such processes. The key aim of this study was to develop an improved coupled fully three-dimensional hydro-morphodynamic model with the Immersed Boundary method and Reynolds Stress Model to simulate pier scour. The proposed numerical model computes bed shear stresses from implicit wall functions and adopts an Eulerian multi-fluid model to account for rolling and saltating particles. Numerical instabilities were addressed in the sediment transport submodels which were ascribed to the fine mesh resolution required to resolve the crucial horseshoe vortex and the diffusion resulting from the discretization of the Immersed Boundary method. The Reynolds Stress Model was compared with the 𝑘𝑘-ε turbulence model but it was found that the results from the numerical model are more sensitive to the computational grid than to the choice of turbulence model to resolve the horseshoe vortex and to obtain stability. Despite the perceived limitations of the proposed hydro-morphodynamic model, the model demonstrated that the velocity flow field, the horseshoe vortex and the subsequent maximum bridge pier scour upstream of the pier nose can be modelled successfully to simulate the results from the experimental work. The simplicity of conservative empirical equations may be feasible for the conceptual design of bridges. However, advanced numerical models have the ability to better account for the interaction of several interrelated parameters and the intricate vortex systems responsible for the scour process at bridge piers. It is proposed that the primary subject of future studies for bridge pier scour should be on the comparison of numerical models with one another.