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Numerical Modelling of the Influence of Lower Boundary Roughness on Turbulent Sedimentary Flows

Arfaie, Armin (2015) Numerical Modelling of the Influence of Lower Boundary Roughness on Turbulent Sedimentary Flows. PhD thesis, University of Leeds.

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Abstract

Numerical computations have been performed to evaluate the influence of bedform roughness on turbulent transport of sediments in geophysical flows. Special attention is paid to turbidity currents, which are responsible for the transport of sedimentary rocks far into the deep ocean. It has been suggested that enhanced turbulence mixing in flows over rugose topography contributes to the unexpectedly large runout lengths of naturally occurring turbidity currents. One of the objectives of this study is to provide evidence for against this conjecture. We perform computations over a wide range of periodic arrays of rectangular roughness elements, We find that a strong peak in turbulent mixing occurs when the width-to-height ratio equals a critical value of seven. We also find that a strong peak in resistance to flow occurs at the same critical value. These are competing effects, with the former acting to promote, and the latter acting to diminish runout length. So we are not able to conclude definitively that the enhancement of mixing is responsible for long runout lengths. We continue by considering flows over periodic arrays of shapes which are representative of bedforms that occur in the natural environment. We again find a strong correlation between the optimisation of both turbulence mixing and resistance to the flow. We are unable to distinguish bedform shapes that promote long runout length relative to the flat bed case. However, we are able to distinguish those bedform shapes that have large resistance to flow and large turbulence mixing compared to those that have low resistance and low turbulent mixing, with the latter case occurring for widely spaced asymmetric dunes with a long low angled slope facing the flow. Finally, we develop a model for flow and sediment transport which takes into account erosion and deposition from the bottom boundary. We first apply this model to flow over fixed dune shapes, in order to assess the influence of bedform shape on flow capacity, stratification, and the energy budget. An important result of this study is that flow capacity is optimised for the class of bedform shapes that promote low flow resistance and low turbulent mixing. We conclude by applying the model to the two-way coupled flow of a mobile dune, starting from an initially symmetric inherited dune morphology. We find that, for sufficiently large grain sizes, the dune evolves into a sequence of asymmetric dunes, rather than to a flat bed, and that the long-time evolution tends to be towards those dune shapes that promote large relative flow capacity. However, the model has a discrepancy in that it is unable to prevent the dune shape exceeding the maximum angle of repose. Hence, further work is required before these results can be regarded as reliable.

Item Type: Thesis (PhD)
Keywords: Roughness, CFD, Numerical modelling, turbulent Flows, flow capacity, sediment-laden flow, flow resistance, mixing, Fluid-structure interaction, bedform
Academic Units: The University of Leeds > Faculty of Engineering (Leeds) > School of Chemical and Process Engineering (Leeds)
The University of Leeds > Faculty of Engineering (Leeds) > School of Chemical and Process Engineering (Leeds) > Energy and Resources Research Institute (Leeds)
Identification Number/EthosID: uk.bl.ethos.689238
Depositing User: Mr Armin Arfaie
Date Deposited: 28 Jun 2016 12:52
Last Modified: 06 Oct 2016 14:42
URI: http://etheses.whiterose.ac.uk/id/eprint/13419

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