Date of Award

1-1-2011

Document Type

Campus Access Dissertation

Department

Civil and Environmental Engineering

First Advisor

Jasim Imran

Abstract

The hydrodynamics of density currents are difficult to study in the natural envi- ronment, whereas laboratory experiments are limited to small-scale flows, are time- consuming and are not necessarily easier when it comes to measuring flow properties or establishing relationships between the turbulent flow structures and the transport and deposition of sediment. Numerical modeling of turbidity currents can provide valuable insights into current dynamics including turbulence structures and their ef- fects on deposit thickness, grain size, and distribution pattern. Proper modeling of turbulence in these stratified flows is crucial to accurately predict the turbulence en- ergy budget, which consequently affects the velocity, concentration, and bed shear stress.

The present thesis is divided into three closely related research subjects compiled in a paper format. In chapter 2, the k − ω turbulence closure model is extended for a stratified flow by incorporating additional buoyancy-related production terms. I particularly focus on the sensitivity of the predicted profiles of velocity and concen- tration to a parameter Cω3 associated with buoyancy-related turbulence generation. A Comparison of the k − ω model to the buoyancy-extended k − ε model equations provides a basis on which different values of the coefficient are tested. A series of 2-D simulations of a number of saline and turbidity currents are performed. The simu- lated profiles are compared with experimental data to determine a range of values for Cω3 that provides the best match with the data. The results obtained using the proposed range of values also show a good match with those obtained using the k − ε model.

In chapter 3, I use a three-dimensional unsteady numerical model to simulate turbidity currents in a large-scale submarine environment. The model solves the Reynolds-averaged Navier-Stokes equations, along with a two-equation turbulence closure model, the sediment conservation equations for multiple grain-size classes and the Exner equation of bed sediment conservation. The model is applied to a modern seafloor environment in the continental slope of the Niger Delta. Bathymetric data and seven piston cores were collected from the study site. Detailed analyses of the core data provided the grain size distribution for different beds within each core. Because the flow parameters and conditions of recent events that formed this modern seafloor deposit are unknown, I performed simulations with different inflow conditions in order to match the data. The model realistically predicts the physics of the current and its evolution with time over the complex topography and shows the spatial distribution of different grain size classes. The predicted flow field does not display any secondary current at bend apices; instead, a strong lateral flow from the inner to the outer bank is observed. Using the deposition rate of each grain size class, I obtain the fraction of the class at the core locations. The discrete values predicted from the model fall within the range of D50 and D90 observed in different beds in the cores.

In chapter 4, I apply a three-dimensional numerical model to simulate turbidity currents in two segments of a deep submarine canyon located on the continental slope of the Niger Delta. A total of 22 piston cores were collected in the study area at different locations along the canyon’s thalweg providing grain-size and bed thickness at different elevations above the thalweg. Different inflow conditions with four grain-size classes are considered in order to match the observed grain-size and bed thickness. The simulated flow field shows complex evolution of the current due to steep changes in local topography. The deposition rate for each grain-size class at the core locations is used to calculate the fraction of each class in the deposit. The total deposition rate is multiplied by an appropriate duration in order to compare

simulation results with the observed mean bed thickness. The mean diameter and bed thickness decrease with elevation above the channel thalweg and a comparison of the simulation results to the data show a reasonable agreement. The findings of this research provide a better understanding of the distribution pattern, thickness, and grain size of turbidity currents and their deposits on a complex steep topography. The results demonstrate that physics-based models may help constrain the variables of reservoir architecture and, therefore reduce uncertainty in estimating hydrocarbon reservoir quality and connectivity.

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