The depth of the sulfate-methane transition (SMT) above gas hydrate systems is a direct proxy to interpret upward methane flux and hydrate saturation. However, two competing reaction pathways can potentially form the SMT. Moreover, the pore water profiles across the SMT in shallow sediment show broad variability leading to different interpretations for how carbon, including CH4, cycles within gas-charged sediment sequences over time. The amount and distribution of marine gas hydrate impacts the chemistry of several other dissolved pore water species such as the dissolved inorganic carbon (DIC). A one-dimensional (1-D) numerical model is developed to account for downhole changes in pore water constituents, and transient and steady-state profiles are generated for three distinct hydrate settings. The model explains how an upward flux of CH4 consumes most SO42- at a shallow SMT implying that anaerobic oxidation of methane (AOM) is the dominant SO42- reduction pathway, and how a large flux of 13C-enriched DIC enters the SMT from depth impacting chemical changes across the SMT. Crucially, neither the concentration nor the d13C of DIC can be used to interpret the chemical reaction causing the SMT. The overall thesis objective is to develop generalized models building on this 1-D framework to understand the primary controls on gas hydrate occurrence. Existing 1-D models can provide first-order insights on hydrate occurrence, but do not capture the complexity and heterogeneity observed in natural gas hydrate systems. In this study, a two-dimensional (2-D) model is developed to simulate multiphase flow through porous media to account for heterogeneous lithologic structures (e.g., fractures, sand layers) and to show how focused fluid flow within these structures governs local hydrate accumulation. These simulations emphasize the importance of local, vertical, fluid flux on local hydrate accumulation and distribution. Through analysis of the fluid fluxes in 2-D systems, it is shown that a local Peclet number characterizes the local hydrate and free gas saturations, just as the Peclet number characterizes hydrate saturations in 1-D, homogeneous systems. Effects of salinity on phase equilibrium and co-existence of hydrate and gas phases can also be investigated using these models. Finally, infinite slope stability analysis assesses the model to identify for potential subsea slope failure and associated risks due to hydrate formation and free gas accumulation. These generalized models can be adapted to specific field examples to evaluate the amount and distribution of hydrate and free gas and to identify conditions favorable for economic gas production.

Shock waves create a unique environment of high pressure, high temperature and high strain-rates. It has been observed that chemical reactions that occur in this regime are exothermic and can lead to the synthesis of new materials that are not possible under other conditions. The exothermic reaction is used in the development of binary energetic materials. These materials are of significant interest to the energetic materials community because of its capability of releasing high heat content during a chemical reaction and the relative insensitivity of these types of energetic materials. Synthesis of these energetic materials, at nano grain sizes with structural reinforcements, provides an opportunity to develop a dual functional material with both strength and energetic characteristics. Shock-induced chemical reactions pose challenges in experiment and instrumentation. This thesis is addressed to the theoretical development of constitutive models of shock-induced chemical reactions in energetic composites, formulated in the framework of non-equilibrium thermodynamics and mixture theories, in a continuum scale. Transition state-based chemical reaction models are introduced and incorporated with the conservation equations that can be used to calculate and simulate the shock-induced reaction process. The energy that should be supplied to reach the transition state has been theoretically modeled by considering both the pore collapse mechanism and the plastic flow with increasing yield stress behind the shock wave. A non-equilibrium thermodynamics framework and the associated evolution equations are introduced to account for time delays that are observed in the experiments of shock-induced or assisted chemical reactions. An appropriate representation of the particle size effects is introduced by modifying the initial energy state of the reactants. Numerical results are presented for shock-induced reactions of mixtures of Al, Fe2O3 and Ni, Al with epoxy as the binder. The theoretical model, in the continuum scale, requires parameters that should be experimentally determined. The experimental characterization has many challenges in measurement and development of nano instrumentation. An alternate approach to determine these parameters is through ab-initio calculations. Thus, this thesis has initiated ab-initio molecular dynamics studies of shock-induced chemical reactions. Specifically, the case of thermal initiation of chemical reactions in aluminum and nickel is considered.

A new testing method was introduced and developed to determine Mode I fracture toughness of disc type rock specimens. The new method was named as Straight Notched Disc Bending and it uses disc specimens under three-point bending. 3D Numerical modeling was carried out with a finite element program ABAQUS to find stress intensity factors for both well-known Semi-circular Bending specimen models and Straight Notched Disc Bending specimen models for varying disc geometries. Both specimen types included notches where a crack front is introduced at the tip of the notch to compute the stress intensity factors. For stress intensity analysis, crack front-upper loading point distance and span length between the two roller supports at the bottom boundary of the specimens were changed. Fracture toughness testing was carried on Ankara G&ouml / lbaSi pink colored andesite for both specimen types / crack front-upper loading point distance and span length between the two roller supports at the bottom boundary of the specimens were changed during the tests. For both specimen geometries, notch lengths changing from 5 mm to 20 mm were used. For each notch length, two different roller supports with span lengths 60 mm and 70 mm were used. For both methods, fracture toughness values determined by using numerically computed stress intensity factors and failure loads obtained from the experiments were very close / the new method was verified by comparing the results. The new method had advantages of lower confining pressure at the crack front and lower stress intensities with a possible smaller crack tip plasticity region.

In this thesis, a Proton Exchange Membrane Fuel Cell is modeled with COMSOL Multiphysics software. A cross-section that is perpendicular to the flow direction is modeled in a 2-D, steady-state, one-phase and isothermal configuration. Anode, cathode and membrane are used as subdomains and serpentine flow channels define the flow field . The flow velocity is defined at the catalyst layers as boundary conditions with respect to the current density that is obtained by using an agglomerate approach at the catalyst layer with the help of fundamental electrochemical equations. Darcy&rsquo / s Law is used for modeling the porous media flow. To investigate the effects of species depletion along the flow channels, a different type of cross-section that is parallel to the flow direction is modeled by adding flow channels as a subdomain to the anode and cathode. Differently, Brinkman Equations are used to define flow in the porous electrodes and the free flow in the channels is modeled with Navier-Stokes equations. By running parallel-to-flow model, mass fractions of species at three different locations (the inlet, the center and the exit of the channel) are predicted for different cell po- tentials. These mass fractions are used as inputs to the perpendicular-to-flow model to obtain performance curves. Finally, by maintaining restricted amount of species by having a very low pressure difference along the channel to represent a single mid-cell of a fuel cell stack, a species depletion problem is detected. If the cell potential is decreased beyond a critical value, this phenomenon causes dead places at which the reaction does not take place. Therefore, at these dead places the current density goes to zero unexpectedly.

Static and dynamic interaction mechanism of the retained soil-compressible geofoam buffer and yielding retaining structures requires further investigation. The present study, initiated on this motive, discusses the results of 1-g physical model tests and numerical analyses of cantilever retaining walls with and without deformable geofoam buffers between the wall and cohesionless granular backfill. 0.7m high walls with various wall thicknesses were utilized in the physical modeling. Dynamic tests were carried out by using a laminar container placed on a uni-axial shaking table. Influence of buffer thickness, geofoam type and wall flexibility as well as base excitation characteristics on the lateral earth pressures and flexural wall deflections were under concern. Outcomes of the analyses performed with FLAC-2D (v6.0) finite difference code were validated against the results of the physical model tests. It was observed that the arching effect induced in the retained soil by the lateral compression of the lower half of the geofoam buffer has a positive effect, as this zone is able to absorb a portion of the total unbalanced lateral force exerted by the backfill thus causing a reduction in the static and seismic lateral wall pressures. Relative thickness and stiffness of the geofoam buffer appear to be the most dominant factors affecting the reduction in earth thrust. Lateral earth pressure coefficients determined from physical model tests were compared with those calculated using methods available in the literature. Good agreement was observed between the predictions. Graphs were provided to estimate the static and dynamic lateral earth pressure coefficients for various combinations of wall stiffness and buffer characteristics. Analysis of a 6m high prototype cantilever wall subjected to an excitation recorded in August 17, 1999 Kocaeli earthquake by finite difference method exhibited the contribution of geofoam buffers on seismic performance of cantilever earth retaining walls. It was observed that the presence of an EPS geofoam inclusion provides a reduction of the permanent flexural wall deflections as well as total seismic thrust likely to be experienced by the wall during an earthquake.

In this study, a two-dimensional depth-averaged beach evolution numerical model is developed to study the medium and long term nearshore sea bottom evolution due to non-cohesive sediment transport under the action of wind waves only over the arbitrary land and sea topographies around existing coastal structures and formations. The developed beach evolution numerical model is composed of four sub-models: a nearshore spectral wave transformation model based on energy balance equation including random wave breaking and diffraction terms to compute the nearshore wave characteristics, a nearshore wave-induced circulation model based on the non-linear shallow water equations to compute the nearshore depth averaged wave-induced current velocities and mean water level changes, a sediment transport model to compute the local total sediment transport rates occurring under the action of wind waves and a bottom evolution model to compute the bed level changes in time due to gradients of sediment transport rates in cross-shore and longshore directions. The governing partial differential equations are solved utilizing finite difference schemes. The developed models are applied successfully to several theoretical and conceptual benchmark cases and an extensive data set of laboratory and field measurements. As an alternative approach to be used in beach evolution problems, a distributed total sediment load formula is proposed based on the assumption that the local total sediment transport rates across the surf zone are proportional to the product of the rate of dissipation of wave energies due to wave breaking and wave-induced current velocities. The proposed distribute load approach is validated with the available laboratory and field measurements.

The Batumi Seep Area, offshore Georgia, Black Sea, has been intensively cored (gravity cores and TV-guided multi-cores) to investigate the methane turnover in the surface sediments. The seep area is characterized by vigorous methane gas bubble emanations. Geochemical analyses show a microbial origin of the methane and a shallow fluid source. Anaerobic methane oxidation rapidly consumes the SO4 2- within the top 5-20 cm, but significant upward fluid advection is not indicated by the porewater profiles. Hence, the Batumi Seep Area must be dominated by methane gas seepage in order to explain the required CH4 flux from below. 1-D transport-reaction modelling constrains the methane flux needed to support the observed SO4 2- flux as well as the rate of near-surface hydrate formation. The model results correlate well with the hydro-acoustic backscatter intensities recorded and mapped bubble release sites using the sonar of a ROV.

The European Union is planning to greatly decrease energy consumption during the coming decades. The ultimate goal is to create sustainable communities that are energy neutral. One way of achieving this challenging goal may be to use efficient hydronic (water-based) heating systems supported by heat pumps. The main objective of the research reported in this work was to improve the thermal performance of wall-mounted hydronic space heaters (radiators). By improving the thermal efficiency of the radiators, their operating temperatures can be lowered without decreasing their thermal outputs. This would significantly improve efficiency of the heat pumps, and thereby most probably also reduce the emissions of greenhouse gases. Thus, by improving the efficiency of radiators, energy sustainability of our society would also increase. The objective was also to investigate how much the temperature of the supply water to the radiators could be lowered without decreasing human thermal comfort. Both numerical and analytical modeling was used to map and improve the thermal efficiency of the analyzed radiator system. Analyses have shown that it is possible to cover space heat losses at low outdoor temperatures with the proposed heating-ventilation systems using low-temperature supplies. The proposed systems were able to give the same heat output as conventional radiator systems but at considerably lower supply water temperature. Accordingly, the heat pump efficiency in the proposed systems was in the same proportion higher than in conventional radiator systems. The human thermal comfort could also be maintained at acceptable level at low-temperature supplies with the proposed systems. In order to avoid possible draught discomfort in spaces served by these systems, it was suggested to direct the pre-heated ventilation air towards cold glazed areas. By doing so the draught discomfort could be efficiently neutralized.     Results presented in this work clearly highlight the advantage of forced convection and high temperature gradients inside and alongside radiators - especially for low-temperature supplies. Thus by a proper combination of incoming air supply and existing radiators a significant decrease in supply water temperature could be achieved without decreasing the thermal output from the system. This was confirmed in several studies in this work. It was also shown that existing radiator systems could successfully be combined with efficient air heaters. This also allowed a considerable reduction in supply water temperature without lowering the heat output of the systems. Thus, by employing the proposed methods, a significant improvement of thermal efficiency of existing radiator systems could be accomplished. A wider use of such combined systems in our society would reduce the distribution heat losses from district heating networks, improve heat pump efficiency and thereby most probably also lower carbon dioxide emissions. / <p>QC 20131029</p>

L'insuffisance cardiaque est une maladie à grande incidence dont le traitement définitif est difficile. Les pompes d'assistance ventriculaire ont été proposées comme thérapie alternative à long terme, mais la technologie est relativement jeune et selon son design, axial ou centrifuge, le dispositif favorise soit l'hémolyse, soit la stagnation de l'écoulement sanguin. Les pompes à écoulement mixte, combinant certaines propriétés des deux types, ont été proposées comme solution intermédiaire. Pour évaluer leurs performances, nous avons effectué des comparaisons numériques entre huit pompes, deux axiales, deux centrifuges, et quatre mixtes, en employant un modèle Windkessel du système cardiovasculaire avec paramètres optimisés pour l'insuffisance cardiaque résolu avec une méthode Radau IIA3, une méthode de résolution de système d'équations différentielles ordinaires L-stable appartenant à la famille des méthodes Runge-Kutta implicites. Nos résultats semblent suggérer que les pompes d'assistance mixtes ne démontrent qu'un léger avantage comparativement aux autres types en terme de performance optimale dans le cas de l'insuffisance cardiaque, mais il faudrait effectuer plus d'essais numériques avec un modèle plus complet, entre autres avec contrôles nerveux implémentés. / Heart failure is a disease with a high incidence rate that is difficult to treat definitively. Ventricular assist devices have been proposed as alternative long-term therapeutic options, but the technology is fairly recent and depending on the design, axial or centrifugal, the device will favor hemolysis or blood flow stagnation. Mixed flow devices, which combine properties of the two previous types, have therefore been proposed as intermediate solutions. To evaluate their performance, we did a numerical comparison between eight pumps (two axial, two centrifugal, and four mixed) using a Windkessel model of the cardiovascular system with parameters optimised for heart failure and solved using a Radau IIA3 method, an L-stable method used to numerically solve systems of ordinary differential equations that belongs to the family of implicit Runge-Kutta methods. Our results suggest that in terms of optimal performance, mixed flow pumps are modest improvements over the other two types, and more tests would have to be performed using a more complete model with, among other modifications, neural control systems implemented.

In this study, The Beach Dewatering System, a relatively recent technology to combat beach erosion, which is proposed as a practical alternative to more traditional shoreline stabilization methods, is investigated and an informative overview on the genesis, development and recent use of this technique is provided. On the basis of the link existing between the elevation of beach groundwater and erosional or accretionary trends at the beach face, a numerical model that simulates groundwater flow in a coastal aquifer under beach drainage is presented. In this model, the seaward boundary of the domain is considered to be tidally fluctuating in a large scale to represent the occurrence of seepage face significantly. The unsteady groundwater flow equation is solved numerically using the method of finite differences. The results clearly showed that the water table being lowered caused the reduction of the seepage face which is the main aim of Beach Dewatering projects. The positional design parameters, i.e. horizontal and vertical location of the drain, are also investigated by utilizing an efficiency index. It is observed that the system efficiency decreased as the drain is shifted landward. The results also indicated that, the efficiency slightly increased with the vertical drain elevation.