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Turbidity Current Flows Over Rough Substrates


Although many turbidity currents propagate over lower boundaries characterized by form roughness, there have been few experimental and numerical investigations of turbidity current flow over dynamically rough substrates. This research aims to investigate the influence of roughness height and spacing on sediment entrainment potential, erosion and rate of sedimentation (deposition). Specific questions include: (i) do lower boundary interactions in the hydraulically smooth vs. rough cases cause differential turbidity current propagation? (ii) is any coupling between the flow and substrate one-way or two way? (iii) in the latter case, what are the implications for linked flow and substrate evolution? Accordingly, a numerical study of flow over rough substrates has been undertaken for a wide range of roughness spacings and Reynolds number. LES and DNS numerical modelling of such wide range of roughness spacings is problematic due to the computational resources, the number of cases and the Reynolds number range. Therefore, a RANS (Reynolds Averaged Navier-Stokes) based turbulence modelling approach is adopted using a commercial CFD code, ANSYS-CFX 14.5. The modeled scenario is a channel with lower boundary roughness comprising square bars on the bottom wall, oriented transverse to the flow direction. Initial results suggest turbulence maxima occur when the ratio of object spacing to object width (the pitch ratio) is ∼ 7. This value is only weakly dependent on Reynolds number. The decay rate of turbulence enhancement beyond this optimum spacing is rather low. Further work has examined the transition from the hydraulically smooth to fully rough regimes by varying object height, across a range of pitch ratios and Reynolds number conditions. The implications of likely lower boundary roughness effects for prediction of flow depletion and run-out distance are investigated.