A pore-scale understanding of fluid flow underpins the constitutive laws of continuum-scale porous media flow. Porous media flow laws are founded on simplified pore structure such as the classical capillary tube model or the pore-network model, both of which do not include diverging-converging pore geometry in the direction of flow. Therefore, modifications in the fluid flow field due to different pore geometries are not well understood. Thus this may translate to uncertainties on how flow in porous media is predicted in practical applications such as geological sequestration of carbon dioxide, petroleum recovery, and contaminant’s fate in aquifers. To fill this gap, we have investigated the role of a spectrum of diverging-converging pore geometries likely formed due to different grain shapes which may be due to a variety of processes such as weathering, sediment transport, and diagenesis. Our findings describe the physical mechanisms for the failure of Darcy’s Law and the characteristics of Forchheimer Law at increasing Reynolds Number flows. Through fundamental fluid physics, we determined the forces which are most responsible for the continuum-scale porous media hydraulic conductivity (K) or permeability. We show that the pore geometry and the eddies associated therein significantly modify the flow field and the boundary stresses. This has important implications on mineral precipitation-dissolution and microbial growth. We present a new non-dimensional geometric factor β, a metric for diverging-converging pore geometry, which can be used to predict K. This model for K based on β generalizes the original and now widely-used Kozeny (1927) model which was based on straight capillary tubes. Further, in order to better quantify the feasibility of geological CO2 sequestration, we have conducted laboratory fluid flow experiments at reservoir conditions to investigate the controls of media wettability and grain shapes on pore-scale capillary trapping. We present experimental evidence for the snap-off or formation of trapped CO2 ganglion. The total trapping potential is found to be 15% of porosity for a water-wet media. We show that at the pore-scale media wettability and viscous-fingering play a critical role in transport and trapping of CO2. Our investigations clearly show that that in single-phase flow pore geometry significantly modifies pore-scale stresses and impacts continuum-scale flow laws. In two-phase flows, while the media wettability plays a vital role, the mobility ratio of CO2 - brine system significantly controls the CO2 capillary trapping potential- a result which should be taken into consideration while managing CO2 sequestration projects. / text

published_or_final_version / abstract / toc / Mechanical Engineering / Master / Master of Philosophy

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We present here a theory for the generation of mesoscale eddies, in the context of describing the generation of dipoles seen near the Queen Charlotte Islands in British Columbia. The Regional Ocean Modeling System (ROMS) is used to show dipoles forming from the coalescence of small headland eddies at Cape St. James. These headland eddies are formed by frictional generation of potential vorticity (PV) when the tide oscillates across the cape. Only 20% of the PV generated at the cape ends up in the headland eddies, with the remainder lost due to mixing of waters with PV of opposite signs. Coalescence of the headland eddies is achieved with a much higher efficiency - the PV contained in the final eddy is near 80% of the sum of that contained in the small eddies. Not all headland eddies coalesce. Coalescence of a positive PV eddy occurs only when the eddy is formed on a strong tidal flood followed by a weak ebb. Thus, a diurnal inequality in the tides is a requirement for coalescence. The eddies in the final dipole contain roughly equal amounts of PV; each has a radius of approximately 15 km and extends to nearly 100-m depth.

The flow characteristics in compound channels with and without vegetation on the floodplain were investigated experimentally and numerically in this thesis. Detailed measurements of velocity and boundary shear stress, using a Pitot tube and an acoustic Doppler velocimeter together with a Preston tube, were undertaken to understand the flow characteristics in compound channels. Eight no-rod cases, two emergent-rod cases and two submerged-rod cases were tested. Unsteady large eddies that occur in the shear layer were explored numerically with Large Eddy Simulation (LES) to identify its generation and its effects on the flow behaviors. Mean flow parameters were predicted using the quasi-2D model by considering the shear effect. Usirgg the data of depth-averaged velocity and boundary shear stress, the contributions of shear-generated turbulence and bed-generated turbulence to the Reynolds shear stress were identified, the apparent shear stress was calculated using the modified method of Shiono and Knight (1991) and the depth-averaged secondary current force was then obtained. Large eddies were important to the lateral momentum exchange in shallow non-vegetated compound channels and even in deep vegetated compound channels. In the compound channel with one-line rods at the floodplain edge, the secondary current forces were of opposite signs in the main channel and on the floodplain and the bed shear stress was smaller than the standard two-dimensional value of yHSo due to the vegetation effect, where y,H,So are the specific weight of water, water depth and bed slope respectively. In vegetated compound channels, the velocity patterns were different to those and the discharges were smaller than those in non-vegetated compound channels under similar relative water depth conditions. The anisotropy of turbulence was the main contribution to the generation of secondary currents in non-vegetated and vegetated compound channels, but the Reynolds stress term was more important in the vegetated compound channels. Results of cross spectra showed the mechanisms of the turbulent shear generation near the main channel-floodplain junction are due to large eddies in the non-vegetated compound channel and owing to wakes in the vegetated compound channel. LES results indicated that large eddies caused significant spatial and temporal fluctuations of velocity and water level in the compound channel and the instantaneousv alues of these flow parameters were significantly higher than the mean values. In vegetated compound channels, the flow moved from the main channel to the floodplain and from the floodplain to the main channel alternately. The characteristic frequencies of the large eddy were less than 1Hz which was consistent with the experimental data. The capability of the quasi-2D model to predict the 2D mean flow parameters in compound channels were assessed under different flow conditions and also improved by using the mean wall velocity as the boundary condition and appropriate values of the lateral gradient of the secondary current force. In the vegetated compound channels, new approaches were proposed to treat the drag force in the cases of oneline emergent rods at the floodplain edge and submerged rods on the floodplain.

In the western tropical Atlantic, the North Brazil Current retroflection periodically sheds large anticyclonic rings, which then propagate northwestward. Between 1998 and 2000, the North Brazil Current Rings Experiment sampled a large number of these rings by shipboard and moored acoustic Doppler current profiler. Ten of the sampled rings are analyzed in this study, focusing on their sea surface dynamic properties. The rings exhibit a radial structure consisting of two regimes, an ``inner'' core region in near solid body rotation and an ``outer'' ring regime with an approximately exponentially decaying structure. The observations show a sharp change in vorticity at the regime transition and the presence of a strong opposite vorticity shield bounding the inner solid body core. We show that Gaussian models, commonly used to represent the surface expression of these and other rings, are adequate for determining the sea surface height anomaly but tend to poorly estimate other properties such as the maximum swirl velocity. Therefore, we propose a new two--part model as a better approximation of the rings' radial structure. According to the cyclogeostrophic balance approximation, the sea surface height distribution across the inner ring has a parabolic shape, while the outer ring has an exponential structure similar to the velocity field. Interestingly, many of the observed rings have an intensity very close to the theoretical limit for anticyclones at these latitudes, which is believed to be due to inertial instability. A climatology of the NBCR is developed from 17 years of satellite altimetry. Usually 5 to 7 NBCR are observed per year, leading to an average of 6.1 rings per year, higher than the previously accepted 5.5 rings per year. A new methodology, more robust and consistent, is developed to track the rings, showing an impressive skill. The methodology can be applied to any velocity field, including irregular data grids. In contrast to what was previously believed, the NBCR do have a seasonal cycle. While so clear in the number of generated rings the seasonal cycle is explicit in the rhythm of formation. The rings are usually formed every 30--70 days, being more frequent during the Spring, when they are generated in a regular pace of near 40 days. In the Fall, the rings are less frequent, with a longer and variable time interval between them. The generation of North Brazil Current Rings (NBCR) has been proposed from numerical simulations to result from westward propagation Rossby waves originating from the instability of the North Equatorial Countercurrent (NECC). Other mechanisms, such as instability of he North Brazil Current where it crosses the equator, are also possible, and the precise mechanisms controlling NBCR formation are still undetermined. Here the ``NECC wave mechanism'' for generation of the near surface NBCR is evaluated for the first time from observations -- 18 years of satellite altimetry. Using a Complex Principal Component analysis on maps of absolute dynamic topography, it is shown that the NECC is the origin of the coherent propagating rings along the NBCR corridor. In agreement with the results proposed from previous simulations, the modal solution has a longer wavelength before the retroflection and shorter wavelength along the ring corridor. The seasonal signal of the wave energy in the NECC is also found to be coherent with the seasonal production cycle of the rings, after taking into account phase lags due to wave propagation. It is therefore confirmed that the of NBCR shedding is defined by wave processes arising in the NECC. Further, in contrast with prior understanding, the NBCR are shown to have a seasonal signal that follows the seasonal cycle of the NECC intensity.

A Large-Eddy Simulation (LES) methodology adapted to the resolution of high Reynolds number turbulent flows in supersonic conditions was proposed and developed. A novel numerical scheme was designed, that switches from a low-dissipation central scheme for turbulence resolution to a flux difference splitting scheme in regions of discontinuities. Furthermore, a state-of-the-art closure model was extended in order to take compressibility effects and the action of shock / turbulence interaction into account. The proposed method was validated against fundamental studies of high speed flows and shock / turbulence interaction studies. This new LES approach was employed for the study of shock / turbulent shear layer interaction as a mixing-augmentation technique, and highlighted the efficiency in mixing improvement after the interaction, but also the limited spatial extent of this turbulent enhancement. A second practical study was conducted by simulating the injection of a sonic jet normally to a supersonic crossflow. The validity of the simulation was assessed by comparison with experimental data, and the dynamics of the interaction was examined. The sources of vortical structures were identified, with a particular emphasis on the impact of the flow speed onto the vortical evolution.

An electrochemical sensor array can provide information about the spatial and temporal distribution of chemicals in liquid turbulent plumes. Planar laser induced fluorescence (PLIF) and amperometric sensor arrays were used to record signals from modulated chemical plumes released into a recirculating aquatic flume. Coherence analysis was applied to extract the frequency components contained in the sensor response. Effects due to release distance, modulation frequency, and array orientation were investigated. This study has demonstrated that frequency encoded information can be extracted from a turbulent chemical plume using an array of amperometric sensors with optimized three-dimensional geometry and tuning.

Thesis (M.App.Sc.) - Carleton University, 2005. / Includes bibliographical references (p. 224-244). Also available in electronic format on the Internet.