Entrainment and Suspension of fine sediment in open channels with different aspect ratios.

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Yan Liu, Hongwei Fang, Jing Bai

Monday 29 june 2015

16:45 - 17:00h at Mississippi (level 1)

Themes: (T) Sediment management and morphodynamics, (ST) Sediment transport mechanisms and modelling

Parallel session: 3A. Sediment - Erosion

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This Paper investigates how sidewalls affect the entrainment, suspension and transport of fine sediments by means of large eddy simulations. The Roughness closure method proposed by Stoesser is applied to mimic sediment particles on channel bed. A new sediment entrainment boundary is formulated and adopted. This boundary condition is validated by two physical experiments of fine sediment erosion conducted by Van Rijn and Einstein respectively and a fairly good agreement between measured data and computed results are got. Then three open channel flow cases with different aspect ratios (A=B/H=2,4 and infinite, respectively) are performed to study the influence of sidewalls on sediment erosion. Results show that the channel aspect ratio is the determining factor for the development, strength and distribution of turbulence-driven secondary flow. And the secondary flow greatly influences the cross-section distribution of primary flow, coherent structures and vertical flow, which results in a non-uniform erosion of fine sediment particles over the cross-section. In the streamwise direction, cross-averaged sediment concentration increases faster in the channel with A=2 than the ones with A=4 and infinite, which means a strong sediment erosion in the narrow channel. Sediment entrainment is found to be closely related to the vertical velocity, which can be clearly recognized by plotting the velocity vector on instantaneous sediment concentration. Moreover, vortices are visualized to show details of sediment pick-up by these structures. A spiral sediment concentration distribution is observed on the vortices, i.e. concentration is higher at the upward flow side of vortex and lower at the downward flow side, which reveals the importance of vortices on transporting sediment into the outer flow field.