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The aim of this work was to examine the behaviour of sediment, from the perspective of the grain scale mechanics, through computer simulation of grain interactions. A discrete particle model was developed, capable of manipulating the simultaneous motion of large numbers of grains through numerical integration of the equation of motion on an individual basis. Simulations of the critical entrainment shear stress of grains in the surface of single-sized sediment beds demonstrated a distributed nature of threshold values which was dependent upon the arrangement of the randomly deposited bed surface. Geometrical arguments were developed that indicated the existence of general critical entrainment shear stress distributions of both single and mixed-sized beds of a restricted grain size range. Effects of flow sheltering were examined, finding them to hold a significant influence over the effective critical entrainment shear stress distributions. Simulations of saltation trajectories revealed a transition in saltation behaviour associated with a densimetric Froude number, Fr ˜<I> </I>1.2, above which saltation was maintained indefinitely. Trajectory lengths were also investigated over a range of bed grain to saltating grain size ratios and were found to vary linearly with particle Froude number, to a first approximation, for d/D =1. The critical entrainment shear stress and trajectory length results were then incorporated into a probabilistic model which was used to predict fractional bed-load composition, providing valuable insight into the significance of grain scale effects upon large scale phenomena. The results reproduced some of the aspects of the partial transport regime identified by Wilcock (1997), showing evidence of a strong dependency on saltation trajectory length.

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This study uses the PIC (Particle-in-cell) method to simulate unsteady three-dimensional dynamics of particles in argon plasma under low pressure, high density, and weak ionization between two planar electrodes subject to a sudden biased voltage. Plasma has been widely used in materials processing, film manufacturing, nuclear fusion, lamps, etc. Properties of plasmas are also becoming important area for research. This work includes elastic collisions between electrons and neutrals, ions and neutrals, and inelastic collisions resulting in ionization from impacting neutrals by electrons, and charge exchange between ions and neutrals, and Coulomb collisions between electrons and ions. The model ignores magnetic field, secondary electron emission, recombination between ions and electrons, and assumes uniform distribution of the neutrals having velocity of Maxwellian distribution. The computed results show the effects of elastic and inelastic collisions on the characteristics of plasma and sheath (space charge region) in front of the workpiece surface. Unsteady mass, momentum and energy transport from the bulk plasma through sheath to the workpiece is confirmatively and exploratorily studied after successful comparison between PIC prediction and experimental data has been made.

>Magister Scientiae - MSc / Extreme weather events in combination with geographical changes in groundwater utilization, groundwater availability, aquifer recharge, and ultimately changes in the quality of water resources, are expected in the future. As a consequence of changing weather patterns and urbanization the demand for groundwater is likely to increase in certain areas. We know that most waterborne pathogenic health epidemics are associated with contamination of farm water and wastewater. There is however limited understanding of the nature and extent of chemical, physical and biological processes that control the fate and transport of the microorganisms in primary and secondary aquifers. In this thesis, transport results are reported, where E. coli and PDR1 were selected as the biological tracers transported through a primary aquifer at the University of the Western Cape. In conjunction with the microbes salt and Rhodamine (chemical tracers) were injected to compare their fate and transport mechanism in the primary aquifer medium. A series of controlled Darcy experiments under laboratory and field conditions were conducted. Each provided a different data and information. The results from laboratory studies were used to improve design of the field studies. In both cases, the data collected provided information on fate and transport of microbes in groundwater. The field design phase of the experiment was an up-scaling of the laboratory phase of this project. The amount of chemical tracers injected into the aquifer was increased in proportion to the size of the research site. Tracer tests using chemical and microbial tracers were conducted simultaneously. Results of laboratory tests demonstrate a 5 times slower transport of microbes, compared to tests with salts during the laboratory phase. The salts at field scale show a breakthrough occurring after 2 days whereas the microbes –did not break through during the 28 days of the observation period. A new borehole was drilled closer to the pumping borehole to eliminate distance or travel time, but this had no effect on field results for the microbes. / National Research Foundation

Gradient-driven mass diffusion, or species separation, is a transport process which can occur in plasma shocks. Experimental observations of this phenomena are difficult to make, but are of interest to ongoing Inertial Confinement Fusion efforts. This body of work describes the results of two major experimental campaigns conducted at Virginia Tech's Experimental Plasma and Propulsion Laboratory to identify species separation in multi-component plasma shocks. A linear plasma-armature railgun forms and accelerates low temperature, high density, supersonic plasma jets, with the collision between two of these jets shown to induce a collisional plasma shock in the first campaign. The second campaign leverages this experimental setup while employing spatially resolved emission spectroscopy alongside collisional radiative modeling to identify species separation within multi-ion-species plasma shocks consisting of argon, aluminum, and nitrogen. These results are some of the first to be performed in a plasma shock with more than two ion species, and can be used for verification and validation of physics models of fusion plasmas. This body of work was supported by the National Science Foundation under Grant No. PHY-1903442. / Doctor of Philosophy / Plasmas represent the fourth state of matter, where enough energy has been imparted onto a gas for ionization to occur, resulting in a quasi-neutral collection of charged and neutral particles that are subject to both hydrodynamic and electrodynamic effects. Shocks can occur in plasmas, which presents as a transition layer where plasma parameters drastically change over a small region of space. Plasmas hold the key to nuclear fusion, with the topic of gradient-driven mass diffusion, or species separation, in plasma shocks being of great interest to large-scale fusion experiments. This body of work performs experimental measurements using a railgun-based plasma source to create plasma shocks with multiple ion species in the laboratory, and ultimately observe this effect of species separation through the use of spatially resolved spectroscopy. To the author's best knowledge, these measurements represent some of the first to be done in a plasma shock with more than two ion species, and can be used to benchmark physics models of plasmas in fusion experiments.

Reservoir heterogeneities exhibit a wide range of length scales, and their interaction with various transport mechanisms control the overall performance of subsurface flow and transport processes. Modeling these processes at large-scales requires proper scale-up of both heterogeneity and the underlying transport mechanisms. This research demonstrates a new reservoir modeling procedure to systematically quantify the scaling characteristics of transport processes by accounting for sub-scale heterogeneities and their interaction with various transport mechanisms based on the volume averaging approach. Although treatments of transport problems with the volume averaging technique have been published in the past, application to real geological systems exhibiting complex heterogeneity is lacking. A novel procedure, where results from a fine-scale numerical flow simulation reflecting the full physics of the transport process albeit over a small sub-volume of the reservoir, can be integrated with the volume averaging technique to provide effective description of transport at the coarse scale. In a volume averaging procedure, scaled up equations describing solute transport in single-phase flow are developed. Scaling characteristics of effective transport coefficient corresponding to different reservoir heterogeneity correlation lengths as well as different transport mechanisms including convection, dispersion, and diffusion are studied. The method is subsequently extended to describe transport in multiphase systems to study scaling characteristics of processes involving adsorption and inter-phase transport. Key conclusions drawn from this dissertation show that 1) variance of reservoir properties and flow responses generally decrease with scale; 2) scaling of recovery processes can be described by the scaling of effective mass transfer coefficient (Keff); in particular, mean and variance of Keff decrease with length scale, similar in the fashion of recovery statistics (e.g., variances in tracer breakthrough time and recovery); 3) the scaling of Keff depends on the underlying heterogeneity and is influenced by the dominant transport mechanisms. To show the versatility of the approach for studying scale-up of other transport mechanisms, it is also applied to derive scaled up formulations of non-Newtonian polymer flow to investigate the scaling characteristics of the apparent viscosity and effective shear rate in porous media. / text