Prediction of flow and sediment transport is an important and challenging problem for stormwater management and river engineering applications. This thesis concerns primarily the computation of flow, sediment deposition and transport processes in stormwater ponds and alluvial channels based on a multiphase flow approach in modeling sediment transport. Starting from an existing hydrodynamic Reynolds Averaged Navier-Stokes flow solver, numerical models are developed to predict flow, sediment deposition and transport under the FLUENT software package. Two types of sediment transport models are formulated to consider quantities of present sediment phase volume fractions: a Discrete Phase Model in a Lagrangian frame where the sediment phase occupies a low volume fraction and particle-particle interactions are neglected; a Eulerian two-phase model where each phase is considered as an interpenetrating continuum and particle-particle interactions are not neglegible. The model is capable to model sediment transport with high volume fractions.
The solution methodologies are implemented numerically for different case studies. The Discrete Phase Model approach, together with a standard k – ϵ turbulence model, is applied to stormwater pond modeling studies. The use of computational fluid dynamics to simulate flow fields and sediment depositions in stormwater tanks is beneficial because one may compare different factors that affect sedimentation efficiency. In particular, two case studies with different inlet and outlet pipes arrangements are investigated under different steady inflow conditions and bed boundary conditions. A method is employed and hooked to FLUENT for accurate simulations of particle settling behavior in stormwater ponds. The method considers critical bed shear stress as a threshold and evaluates local bed shear stress with this value to determine the particle deposition behavior. It is demonstrated that this model is an efficient 3D hydrodynamic flow and sediment transport numeric model for low sediment-laden flows, thus providing engineers and scientists with a useful tool for studying sediment deposition with a variety of sediment sizes, inflow conditions, and geometry arrangements.
In order to gain more insight into the fundamental flow and sediment interaction mechanics of sediment transport, an Eulerian two-phase model embeded in FLUENT is implemented in an open channel with loose bed based on two-phase mass and momentum equations. These equations are used in conjunction with the constitutive relations that are obtained by applying kinetic theory. Different from traditional sediment transport models, this model uses the two-phase theory, and thus, has no need to invoke any empirical sediment transport formulas. In this application, predictions for turbulent fluctuations for the fluid phase are obtained using a modified k – ϵ turbulence model with a supplement of extra terms which take into account the interphase turbulent momentum transfer. Predictions for turbulent quantities for the solid phase are obtained using Tchen-theory correlations for the discrete particles under homogeneous and steady turbulent flows. Besides simulation of sediment transport, the model also provides some ideas for simulating scour and bed deformation. The results presented in this study demonstrate that the model is efficient and quite accurate in dealing with sediment transport and scour simulation with loose bed.
|Adviser||Laura J. Harrell|
|School||OLD DOMINION UNIVERSITY|
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