Date of Award

12-15-2014

Document Type

Open Access Dissertation

Department

Civil and Environmental Engineering

First Advisor

Jasim Imran

Abstract

The objectives of the research presented in this dissertation are to develop and apply numerical models to study free surface flows under different boundary conditions, bedform geometry and channel configuration. A 2D depth-averaged model is developed by solving the Saint Venant or the shallow water equations using a finite volume method. An existing 3D model, developed in-house, that solves the Reynoldsaveraged Navier-Stokes equations and conservation equations for suspended sediment and conservative species is adapted for free surface flows by solving the volume of fluid (VOF) equation using a compressive interface capturing scheme for arbitrary meshes, CICSAM. The depth-averaged or shallow water and the free-surface capturing 3D model are verified against experimental data on dam-break flow over a sloping bed. The numerical models are used to resolve the discrepancy between earlier simulation results obtained with finite difference models of shallow water equations and a benchmark data set in an open channel contraction collected in the early 1950s. To verify possible experimental errors, the experiment is repeated in a laboratory flume made of smooth plexiglass. Measurements are taken for several flow rates. By applying the numerical models to the experimental conditions, it is found that the original data might have measurement errors, and shallow water models are incapable of properly predicting free surface elevation in an open channel contraction, especially in the uniform section downstream of the contraction. The shallow water models predict standing waves that are out of phase and steeper than those observed in the experiments. The 3D model satisfactorily predicts the water level in the contraction and wave peaks in the downstream uniform section measured in this study, but underpredicts the wave trough amplitudes. The limitation of the present 3D model in capturing sharp changes in water depth is likely associated with the diffusive nature of 2-equation isotropic turbulence models. The effects of freshwater discharge, dune amplitude and tidal amplitude on mixing between freshwater flow and an intruding salt wedge are studied using a 3D numerical model. Model results reveal intense mixing over bedforms, generation of large internal waves with salt concentration reaching the water surface and competing effects of freshwater discharge and the tidal amplitude. During the ebb tide, the freshwater discharge is able to push the salt wedge near or outside of the ocean side boundary. Four different types of disturbances over the dune field that are consistent with the field measurements are recognized. The morphology and fluid dynamics of meandering fluvial river channels have been studied extensively by others using analytical, numerical and analog physical models. These studies involve transport of sediment as bed load and often do not consider the transport and deposit of fine suspended sediment. Within point bar deposits, mud deposition as discrete layers has a significant effect on hydrocarbon reservoir performance, especially in unconventional (heavy oil) settings. This scale of stratigraphic heterogeneity is not represented in even relatively sophisticated static models. The present study is conducted using the 3D numerical model to investigate the controls, process and pattern for the fine deposits over preexisting point bars. The study covers the effect of river cross section and planform shape, suspended load grain size, the variation in river flow discharge, and the average streamwise bed slope. The distribution of the fine-grained deposits is found to be controlled by flow divergence from the base state of uniform flow in straight channel. Both cross section and planform shapes are found to be major controls in flow divergence, and thus sediment distribution within the channel. Seasonal variation in river flow affects the deposit volume and distribution over the point bars.; higher flow discharge results in thicker deposits over point bars. Also, increasing the suspended load grain size results in thicker deposits on the upstream part of the point bars, and the bar tail seems to attract the finer grain size more than the coarser sizes. Changing the slope has only nominal effect on the distribution of fine sediment.

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