Two-phase modeling of granular sediment for sheet flows and scour
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Abstract
Even though sediment transport has been studied extensively in the
past decades, not all physical processes involved are
yet well understood and represented. This results in a modeling
deficiency in that few models include complete and detailed
descriptions of the necessary physical processes and in that
models that do usually focus on the specific case of sheet flows.
We seek to address this modeling issue by developing a model that
would describe appropriately the physics and would not only focus
on sheet flows.
To that end, we employ a two-phase approach, for which
concentration-weighted averaged equations of motion are
solved for a sediment and a fluid phase. The two phases are
assumed to only interact through drag forces. The correlations
between fluctuating quantities are modeled using the turbulent
viscosity and the gradient diffusion hypotheses. The fluid
turbulent stresses are calculated using a modified k-epsilon model that accounts for the two-way particle-turbulence
interaction, and the sediment stresses are calculated using a
collisional granular flow theory.
This approach is used to study three different problems: dilute
flow modeling, sheet flows and scouring. In dilute models sediment
stresses are neglected. Near bed processes are instead modeled
through the bottom boundary conditions and we consider and compare
two widely used approaches. We also introduce and validate a
concentration dependent Schmidt number.
The sheet flow model is validated for different flow conditions.
Several well-known results and formulae are confirmed such as
reduced turbulence in the diluted region, the bed load layer
thickness and the dependence of the sediment transport rate on the
Shields parameter. It also provides a counterexample to modeling
the bed shear stress in phase with the free stream velocity.
Finally, it provides information on the vertical sediment flux,
which could be used to model the bottom boundary condition in
dilute models.
The two-phase model is also shown to be able to represent
two-dimensional sediment transport issues. A simple benchmark test
case (scouring downstream of an apron) is performed and results
are found to reproduce reasonably well existing experimental data.
Future work on sediment transport modeling is also discussed.
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2007-12-27T10:32:47Z
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sediment transport; sheet flows; scour; two-phase flow
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bibid: 6476491
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Government Document
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dissertation or thesis