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The physical and numerical representation of turbulent flow over a porous riverbed

Stubbs, Alex 2021. The physical and numerical representation of turbulent flow over a porous riverbed. PhD Thesis, Cardiff University.
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Abstract

There are numerous scenarios in which turbulent flow interactions are of great importance, particularly in nature, where the dispersion and exchange of oxygen and nutrients is directly related to the turbulence over riverbeds. However, due to the highly complex nature of both the flow field and the bed geometry, the study of turbulent flow phenomena in open channel flows above and within porous media is highly challenging. A fundamental challenge is how to represent a natural gravel riverbed in terms of surface roughness characteristics, porosity, particle packing, grain size, and grain shape variation. In both experimental and computational studies alike, impermeable roughness surfaces have been successfully employed and have shed much light on flow interactions with roughness elements. Some progress has also been made investigating turbulent flow behaviours with permeable bed surfaces. However, such experimental studies have limitations in terms of gathering data between roughness elements. Equally, such numerical studies are limited by the numerical representation of the geometry of the bed itself. Therefore, this study aims to provide methodology for the manufacture of a physical representation as well as the generation of a numerical representation of a natural gravel riverbed. Through validation of the porosity, surface roughness distribution and surface roughness spectra against the literature this study shows that both a physical and a numerical riverbed can be successfully generated based on the methodology presented here. The artificial riverbed has a porosity of 31.5% and the numerical riverbed has a porosity of 32.5%. The Hurst exponent, a key indicator of roughness, was found to be 0.97 for the artificial riverbed, 1.4330 for a coarse resolution version of the numerical riverbed, and 1.4305 for a fine resolution version of the numerical riverbed. To further understand the distribution of Turbulent Kinetic Energy (TKE) in the near-bed region of a porous roughness surface, two LES cases at disparate resolutions were also undertaken as part of this study. Streamwise TKE budget components contribute significantly to the overall TKE compared to the spanwise components. This study also shows that with increased elevation away from the bed surface, the magnitude and thus, contribution to TKE of the flow field declines. The shear production, wake production, vertical diffusion transport, pressure transport TKE budget terms were found to be significant both in the near-bed region as well as within the uppermost 2 layers of the riverbed geometry.

Item Type: Thesis (PhD)
Date Type: Completion
Status: Unpublished
Schools: Engineering
Uncontrolled Keywords: Porous Riverbed, Large Eddy Simulation, Turbulent Flow, Interstitial Flow, Near-bed Flow, Turbulent Kinetic Energy
Date of First Compliant Deposit: 24 September 2021
Last Modified: 24 Sep 2021 08:49
URI: https://orca.cardiff.ac.uk/id/eprint/144412

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