La descripción del flujo de fluidos involucra la solución de las ecuaciones de Navier-Stokes compresible, un problema muy complejo cuya estructura matemática no es del todo comprendida. Por lo tanto, mediante análisis asintótico, se pueden derivar modelos simplificados bajo ciertas hipótesis sobre el problema hechas en términos de parámetros adimensionales que miden la importancia relativa de los diferentes procesos físicos. Los flujos a baja velocidad se pueden describir por diferentes modelos que incluyen las ecuaciones de Navier Stokes incompresible cuya matemática es mucho mas conocida. Sin embargo, algunos flujos importantes no se pueden considerar incompresibles debido a la presencia de efectos térmicos. En esta clase de problemas se pueden derivar otra clase de ecuaciones simplificadas: las ecuaciones de Boussinesq y las ecuaciones de bajo numero de Mach.La complejidad de estos problemas matemáticos hace que su solución numérica sea muy difícil. En estos problemas el método de los elementos finitos es inestable, lo que en la práctica implica soluciones numéricas que presentan oscilaciones nodo a nodo de naturaleza no física. En las ecuaciones de Navier Stokes incompresible, dos fuentes bien conocidas de inestabilidad son la condición de incompresibilidad y la presencia del término convectivo. Muchas técnicas de estabilización utilizadas hoy en día se basan en la separación de escalas, descomponiendo la incógnita en una parte gruesa inducida por la discretización del domino y una parte fina de subescala. Modelar la subescala y su influencia conduce a un problema modificado para la escala gruesa que resulta estable.Aunque las técnicas de estabilización son ampliamente utilizadas hoy en día, importantes problemas permanecen abiertos. Contribuyendo a su comprensión, en este trabajo se analizan varios aspectos del modelado de las subescalas. Para problemas escalares de segundo orden, se encuentra la dependencia de la subescala con el tamaño de la malla en el caso general de mallas anisótropas. Estas ideas son extendidas a sistemas de ecuaciones para considerar el problema de Oseen. También se analiza el modelado de las subescalas en problemas transitorios, obteniendo un mejor esquema de integración temporal para el problema de escala gruesa. Para considerar flujos a baja velocidad, se presenta la extensión de estas técnicas a problemas no lineales acoplados, lo que esta íntimamente relacionado con el problema del modelado de la turbulencia, que es un tema en si mismo.Los flujos acoplados térmicamente, aparte del interés intrínseco que merecen, son importantes desde un punto de vista ingenieril. Una solución precisa del problema de flujo es necesaria para definir las cargas térmicas sobre las estructuras, que en muchos casos responden fuertemente, haciendo el problema acoplado. Esta clase de problemas, que motivaron este trabajo, incluyen la respuesta estructural en el caso de un incendio. / A general description of a fluid flow involves the solution of the compressible Navier-Stokes equations, a very complex problem whose mathematical structure is not well understood. Therefore, simplified models can be derived by asymptotic analysis under some assumptions on the problem, made in terms of dimensionless parameters that measure the relative importance of different physical processes. Low speed flows can be described by several models including the incompressible Navier Stokes equations whose mathematical structure is much better understood. However many important flows cannot be considered as incompressible, even at low speed, due to the presence of thermal effects. In such kind of problems another class of simplified equations can be derived: the Boussinesq equations and the Low Mach number equations.The complexity of these mathematical problems makes their numerical solution very difficult. For these problems the standard finite element method is unstable, what in practice means that node to node oscillations of non physical nature may appear in the numerical solution. In the incompressible Navier Stokes equations, two well known sources of numerical instabilities are the incompressibility constraint and the presence of the convective terms. Many stabilization techniques used nowadays are based on scale separation, splitting the unknown into a coarse part induced by the discretization of the domain and a fine subgrid part. The modelling of the subgrid scale and its influence leads to a modified coarse scale problem that now can be shown to be stable. Although stabilization techniques are nowadays widely used, important problems remain open. Contributing to their understanding, several aspects of the subgrid scale modelling are analyzed in this work. For second order scalar problems, the dependence of the subgrid scale on the mesh size, in the general anisotropic case, is clarified. These ideas are extended to systems of equations to consider the Oseen problem. The modelling of the subgrid scales in transient problems is also analyzed, leading to an improved time discretization scheme for the coarse scale problem. To consider low speed flow models, the extension of these techniques to nonlinear and coupled problems is presented, something that is intimately related to the problem of turbulence modelling, which a entire subject on its own right. Thermally coupled flow problems, despite the intrinsic interest they deserve, are important from an engineering point of view. An accurate solution of a flow problem is needed to define thermal loads on structures which, in many cases have a strong response, making the problem coupled. This kind of problems, that motivated this work, include the problem of a structural response in the case of fires.

The AdS/CFT correspondence and its extensions to more general gauge/gravity dualities have provided a powerful framework for the study of strongly coupled gauge theories. This thesis explores properties of a large class of thermal strongly coupled gauge theories using the gravity dual. In order to bring the holographic framework closer to Quantum Chromodynamics (QCD), we study theories with matter in the fundamental representation. In particular, we focus on the holographic dual of SU(Nc) supersymmetric Yang-Mills theory coupled to Nf<<Nc flavours of fundamental matter at finite temperature, which is realised as Nf Dq-brane probes in the near horizon (black hole) geometry of Nc black Dp-branes. We explore many aspects of these Dp/Dq brane systems, often focussing on the D3/D7 brane system which is dual to a four dimensional gauge theory. We study the thermodynamics of the Dq-brane probes in the black hole geometry. At low temperature, the branes sit outside the black hole and the meson spectrum is discrete and possesses a mass gap. As the temperature increases, the branes approach a critical solution. Eventually, they fall into the horizon and a phase transition occurs. At large Nc and large 't Hooft coupling, we show that this phase transition is always first order. We calculate the free energy, entropy and energy densities, as well as the speed of sound in these systems. We compute the meson spectrum for brane embeddings outside the horizon and find that tachyonic modes appear where this phase is expected to be unstable from thermodynamic considerations. We study the system at non-zero baryon density nb and find that there is a line of phase transitions for small nb, terminating at a critical point with finite nb. We demonstrate that, to leading order in Nf/Nc, the viscosity to entropy density ratio in these theories saturates the conjectured universal bound. Finally, we compute spectral functions and diffusion constants for fundamental matter in the high temperature phase of the D3/D7 theory.

Mechanistic studies of electrochemical proton coupled electron transfer (PCET) have attracted attention for many decades due to their importance in many fields ranging from electrocatalysis to biology. However, mechanistic research is confined to only a few groups, and challenges in this field can be found in both theory and experiment. The contributions to mechanistic studies of electrochemical PCET reaction in this thesis can be categorized under the following two headings: 1) mechanistic studies of an aminobenzoquinone modified monolayer system with multiple electron/proton transfer reaction; 2) studies that attempt to develop the relationship between thermochemical data and electrochemical PCET mechanism. An aminobenzoquinone modified monolayer showing nearly ideal electrochemical behavior and high stability was successfully prepared and used as a model system for the mechanistic study of electrochemical multiple electron/proton transfer. This model system has been proposed to undergo a 2e3H transfer at low pH electrolyte and a 2e2H transfer at high pH electrolyte. Two non-destructive electrochemical techniques (cyclic voltammetry and chronocoulmetry) have been applied for the measurement of apparent standard rate constant as a function of pH. Both pH dependent apparent formal potential and pH dependent apparent standard rate constant have been used to determine the charge transfer mechanism of this monolayer system. Under the assumption of an operative PCET mechanism (i.e. electron transfer step is the rate determining step), a theoretical description of this system has been developed based on the refinement and extension of previous models. By combining this extended theoretical model with pH dependent apparent formal potential and apparent standard rate constant, charge transfer pathways have been determined and shown to be consistent with the observed pH dependent electrochemical response, in addition, the determined pathways in this aminobenzoquinone modified monolayer are similar to previous reported pathways for benzoquinone freely dissolved in aqueous buffered electrolyte. A series of analytical expressions built in this thesis demonstrate that the parameters that differentiate stepwise mechanisms from concerted mechanisms can be classified into two aspects: thermodynamic parameters, namely acid dissociation constants, standard formal potentials; and kinetic parameters, namely standard rate constants, standard transfer coefficients. Although attempts to understand the relation between controlling parameters and electrochemical PCET mechanism (stepwise versus concerted) has been reported previously by some groups, there are still lots of unresolved aspects requiring further investigation. In this thesis, an important conclusion has been drawn which is that for the stepwise mechanism, an apparent experimentally observable kinetic isotope effect (KIE) can be induced by solvent isotope induced variation of acid dissociation constants, which contradicts previous understanding. Additionally, for the first time, values of apparent KIE, which were measured for the aminobenzoquinone modified monolayer system with stepwise PCET mechanism, were successfully explained by variation in acid dissociation constants, not by variation in standard rate constants. Based on theoretical prediction, a nitroxyl radical modified bilayer showing one electron one proton transfer reaction has been prepared in an effort to afford experimental verification. After applying similar analytical procedures as those for the aminobenzoquinone modified monolayer system, this bilayer system has been shown to follow the concerted 1e1H transfer pathway in high pH electrolytes. These latter contributions provide evidence that further development in this field will eventually lead to a comprehensive theory that can use known thermochemical variables to fully predict PCET mechanism.

Body coupled communication (BCC) is a hot topic in personal networking domain. Many works arepublished suggesting different architectures for BCC since its inception in 1995 by Zimmerman. The number ofelectronic gadgets used by a single person increases as time pass by. Its a tedious job to transfer data betweenthen from a user point of view. Many of these gadgets can share their resources and save power and money.The existing wired or wireless networks does not meet the requirements for this network like scalable data rate,security etc. So here comes the novel idea of using human body as communication medium. The aim of thisthesis is to realize a hardware for BCC based on wide band signaling as part of a big project.The human body consists of 70% of water. This property makes the human body a fairly good conductor.By exploiting this basic property makes the BCC possible. A capacitance is formed if we place a metal platenear to the human body with the skin as a dielectric. This capacitance forms the interface between the humanbody and the analog front-end of the BCC transceiver. Any other metal structures near to the human body canattenuate the signal.A first-order communication link is established in software by the human body model and the transceiver inthe loop along with noise and interference. This communication link is used to verify the human body modeland the base band model done as part of the same big project. Based on the results a hardware prototype isimplemented. Measurements are taken in different scenarios using the hardware setup. The trade-off betweendesign parameters are discussed based on the results. At the end, it suggests a road map to take the projectfurther.

A written report of a paper titled Holographic dual of collimated radiation by Veronika E. Hubeny where a new and easier method is proposed to estimate the “radiation due to an accelerated quark in a strongly coupled medium”. The method is able to reproduce the results from an earlier paper without the need of solving the linearized Einstein equations but by way of calculating geodesics in AdS using the AdS/CFT correspondence and the gravitational dual of the quark being a string. A quick introduction to synchrotron radiation and general relativity is given after which the AdS/CFT correspondence is introduced along with the results and method of V. Hubeny. / A bachelor thesis in theoretical physics.

A novel method by which to calculate the near edge X-rayabsorption fine structure region of the X-ray absorption spectrum has been derived and implemented. By means of damped coupled cluster theory at coupled cluster levels CCS, CC2, CCSD and CCSDR(3), the spectra of neon and methane have been investigated. Using methods incorprating double excitations, the important relaxation effects maybe taken into account by simultaneous excitation of the core electron and relaxation of other electrons. An asymmetric Lanczos-chain driven approach has been utilized as a means to partially resolve the excitation space given by the coupled cluster Jacobian. The K-edge of the systems have been considered, and relativistic effects are estimated with use of the Douglas--Kroll scalar relativistic Hamiltonian. Comparisons have been made to results obtained with the four-component static-exchange approach and ionization potentials obtained by the {Delta}SCF-method. The appropriate basis sets by which to describe the core and excited states have been been determined.  The addition of core-polarizing functions and diffuse or Rydberg functions is important for this description. Scalar relativistic effects accounts for an increase in excitation energies due to the contraction of the 1s-orbital, and this increase is seen to be 0.88 eV for neon. The coupled cluster hierachy shows a trend of convergence towards the experimental spectrum, with an 1s -&gt; 3p excitation energy for neon of an accuracy of 0.40 eV at a relativistic CCSDR(3) level of theory. Results obtained at the damped coupled cluster and STEX levels of theory, respectively, are seen to be in agreement, with a mere relative energy shift.

The AdS/CFT correspondence and its extensions to more general gauge/gravity dualities have provided a powerful framework for the study of strongly coupled gauge theories. This thesis explores properties of a large class of thermal strongly coupled gauge theories using the gravity dual. In order to bring the holographic framework closer to Quantum Chromodynamics (QCD), we study theories with matter in the fundamental representation. In particular, we focus on the holographic dual of SU(Nc) supersymmetric Yang-Mills theory coupled to Nf<<Nc flavours of fundamental matter at finite temperature, which is realised as Nf Dq-brane probes in the near horizon (black hole) geometry of Nc black Dp-branes. We explore many aspects of these Dp/Dq brane systems, often focussing on the D3/D7 brane system which is dual to a four dimensional gauge theory. We study the thermodynamics of the Dq-brane probes in the black hole geometry. At low temperature, the branes sit outside the black hole and the meson spectrum is discrete and possesses a mass gap. As the temperature increases, the branes approach a critical solution. Eventually, they fall into the horizon and a phase transition occurs. At large Nc and large 't Hooft coupling, we show that this phase transition is always first order. We calculate the free energy, entropy and energy densities, as well as the speed of sound in these systems. We compute the meson spectrum for brane embeddings outside the horizon and find that tachyonic modes appear where this phase is expected to be unstable from thermodynamic considerations. We study the system at non-zero baryon density nb and find that there is a line of phase transitions for small nb, terminating at a critical point with finite nb. We demonstrate that, to leading order in Nf/Nc, the viscosity to entropy density ratio in these theories saturates the conjectured universal bound. Finally, we compute spectral functions and diffusion constants for fundamental matter in the high temperature phase of the D3/D7 theory.

This study investigated the mobility and trapping of charges and the recombination of x-ray induced charges in a-Se. X-ray induced changes in these parameters were also examined. Prior to exposure to x-rays, the mobilities and deep trapping lifetimes of both holes and electrons were constant. Exposure to x-rays caused no change in the mobility of these charges. Immediately following exposure, the hole deep trapping lifetime would fall ~30% while the electron deep trapping lifetime would only change ~10%. The deep trapping lifetimes continued to change unpredictably several hours after the initial exposure. Following an extended rest period, the charge lifetimes within the a-Se film would return to a stable state, but not necessarily equal to the initial lifetime prior to the x-ray exposure. These changes were proposed to occur because of a relaxation or reordering of the atoms in a-Se, similar to accepted changes thought to occur resulting from exposure to visible light. Analysis of the experimental evidence suggests that intimate valence alternation pair (IVAP) charged defects are created by x-ray irradiation. These defects are relatively unstable, disappearing within two hours after irradiation. Since the hole and electron lifetimes continued to change for at least 12 hours, it was concluded that the traditional view of deep charge trapping into IVAP defects cannot be dominant. A new charge trapping theory, consistent with published optically induced effects, is proposed in this work to explain these observations. The energy required to create a free electron-hole pair in a-Se by exposure to x-rays was measured. This energy was found to exhibit a strong field and temperature dependence, with little dependence on the mean x-ray beam energy. These findings are consistent with the geminate recombination theory, generally agreed to be the dominant charge loss mechanism with optical photons in a-Se. The persistent x-ray photocurrent was found to be thermally activated below ~250 K with an activation energy of 0.16 eV. This does not correspond to the energy level of any known traps in a-Se, and lends further support to the charge trapping theory developed earlier.

Urea is an important nitrogen fertilizer for plant nutrition, but is very susceptible to moisture sorption and caking even at low moisture contents, e.g. 0.25% w/w. When urea particles adsorb moisture followed by drying, crystal bridges form between urea particles. For particles in a bed, this process is called caking. Cakes in stored urea cause a degradation of its quality and value. Investigations of the moisture absorption in beds of manufactured urea particles and adsorption on the external and internal surfaces of urea particles are a necessary step if engineers are to recommend procedures to reduce caking and control inventories. Research on moisture adsorption and cake strength of urea fertilizer has not been sufficiently explored. Only recently have researchers started to devise tests to investigate the crystal bonding between two urea particles. Prior to this research, investigations of the moisture interactions in beds of urea were nearly non-existent. This thesis presents experimental, theoretical and numerical methods to investigate the coupled heat and moisture transfer processes in a bed of urea particles while the bed is exposed to ambient air with changing temperature and humidity.<p> Urea particles are nearly spherical with uniform particle size distribution. The particle size, its internal pore structure and rough crystalline external surface depend on the manufacturing process. In this thesis, two types of urea products are investigated, i.e. prill Georgia urea and granular Terico urea. The rough external surface and internal pore structure of each particle makes the total surface area exposed to water much larger than similar smooth and solid spherical particles. Although Georgia urea has higher external surface area than Terico urea, the latter type has larger total surface area and internal pore volume. For both Terico urea and Georgia, the internal surface area dominates the water sorption process but the external moisture sorption of Georgia urea is more important than that of Terico urea.<p> All the water vapor interaction experiments were carried out with air flow through a test bed because it shortens the duration of each experiment to a few hours in most cases. A series of experiments with step changes in inlet air temperature and humidity for air flow through a urea bed indicated that the measured outlet air temperature and humidity responses, each at a specific air flow rate, reveals a typical exponential or transient time change that can be characterized by a time constant. After formulating the theoretical problem for step changes in the inlet properties, the analytical solutions showed that the time constants of outlet response to whether a temperature step change or a humidity step change are functions of the convection coefficient and air velocity. The predicted outlet air temperature is determined by only one time constant for a temperature step change while it is determined by these two time constants for a humidity step change.<p> A new test cell with sampling test ports was developed to measure the transient moisture uptake of a urea particle bed and its distribution at any time without any interruption of the experiment. A novel particle sampling device, modified from a syringe and pistons, was designed to minimize the particle exposure to ambient air during the moisture content determination using a Karl Fischer titrator. Data from two continuous cyclic step changes in the inlet flow with relative humidities between 4% and 70% at room temperature showed a hysteresis in the isothermal moisture content for only the first cycle. After the second sorption- desorption cycle, the hysteresis disappeared. This implies that the internal pore and particle surface geometry changes are very slow after the first cycle.<p> A new theoretical porous media model was developed for a coupled heat and moisture transport process when humid air flowed uniformly through a large test bed in two coupled computational domains: internal domain (i.e., the particle phase) and the external domain (i.e., the interstitial air space). The moisture migration in two computational domains included: water vapor diffusion inside each particle, and water vapor convection and diffusion in the interstitial air space in the urea particle bed. For energy transport, the temperature was assumed to be uniform inside each particle, but heat convection and conduction between the urea particles and the interstitial air outside particles occurred throughout the bed. Both heat transfer and mass transfer in internal domain and external domain were coupled by the heat and mass convection at the gas-particle interface. The numerical simulation was compared with the data of moisture uptake and showed good agreement implying that the internal moisture diffusion that dominates the moisture uptake process is a very slow process.<p> These above experimental, theoretical and numerical research studies provide a set of information on how urea particles adsorb or desorb moisture from or to ambient air on the external and internal pore surface, which offers a useful suggestion for urea caking prevention and is also a first and necessary step to the study of further caking formation and strength.

<p>Scientists expect temperatures on Earth to get substantially warmer over the course of the 21st century, causing storm systems to intensify and sea-level rise to accelerate--these changes will likely have dramatic impacts on how the coastlines of tomorrow will evolve. Humans are also playing an increasingly important role in shaping Earth's coastal systems. Coastal scientists have only a general understanding of how these three factors--humans, storms, and sea-level rise--will alter the evolution of coastlines over the coming century, however. I conduct numerical modeling experiments to shed light on the relative importance of these factors on the evolution of coastline geomorphology.</p><p>In a series of experiments using a numerical model of large-scale (1 to 100's km) and long-term (years to centuries) coastline evolution that results from gradients in alongshore sediment transport, I explore how the patterns and rates of shoreline erosion and accretion are affected by shifts in 'wave climate' (the mix of influences on alongshore sediment transport of waves approaching from different directions) induced by intensified storm systems and the direct manipulation of the shoreline system by humans through beach nourishment (periodically placing sand on an eroding beach). I use a cuspate-cape coastline, similar to the Outer Banks, North and South Carolina, USA, as an important case study in my experiments. I observe that moderate shifts in the wave climate can alter the patterns of shoreline erosion and accretion, potentially increasing migration rates by several times that which we see today, and nearly an order-of-magnitude larger than sea-level rise-related erosion alone. I also find that under possible wave climate futures, beach nourishment may also induce shoreline change on the same order of magnitude as does sea-level rise.</p><p>The decision humans make whether or not to nourish their beach often depends upon a favorable economic outcome in the endeavor. In further experiments, I couple a cost-benefit economic model of human decision making to the numerical model of coastline evolution and test a hypothetical scenario where two communities (one 'rich' and one 'poor') nourish their beaches in tandem, under different sets of economic and wave climate parameters. I observe that two adjacent communities can benefit substantially from each other's nourishment activity, and these effects persist even if the two communities are separated by several tens of kilometers.</p><p>In a separate effort, I employ techniques from dynamic capital theory coupled to a physically-realistic model of coastline evolution to find the optimum time a community should wait between beach nourishment episodes ('rotation length') to maximize the utility to beach-front property owners. In a series of experiments, I explore the sensitivity of the rotation length to economic parameters, including the discount rate, the fixed and variable costs of beach nourishment, and the benefits from beach nourishment, and physical parameters including the background erosion rate and the exponential rate at which both the cross-shore profile and the plan-view coastline shape re-adjusts following a beach nourishment episode ('decay rate' of nourishment sand). Some results I obtained were expected: if property values, the hedonic value of beach width, the baseline retreat rate, the fixed cost of beach nourishment, and the discount rate increase, then the rotation length of nourishment decreases. Some results I obtained, however, were unexpected: the rotation length of nourishment can either increase or decrease when the decay rate of nourishment sand varies versus the discount rate and when the variable costs of beach nourishment increase.</p> / Dissertation