The mechanisms governing the formation and destruction of soot in turbulent combustion are intimately coupled to thermal radiation due to the strong dependence of sooting processes and radiative loss on temperature. Detailed computational fluid dynamics (CFD) predictions of the radiative and soot output from turbulent non-premixed flames are normally performed by parabolic algorithms. However, the modelling of combustion systems, such as furnaces and unwanted enclosure fires, often require a fully elliptic description of the flow field and its related physical phenomena. Thus, this thesis investigates the intimate coupling between radiative energy exchange and the mechanisms governing soot formation and destruction within a three-dimensional, general curvilinear CFD code. Thermal radiation is modelled by the discrete transfer radiation model (DTRM). Special emphasis is given to approximate solutions to the radiative transfer equation encompassing various models for the radiative properties of gases and soot. A new algorithm is presented, entitled the differential total absorptivity (DTA) solution, which, unlike alternative solutions, incorporates the source temperature dependence of absorption. Additionally, a weighted sum of gray gases (WSGG) solution is described which includes the treatment of gray boundaries. Whilst the DTA solution is particularly recommended for systems comprising large temperature differences, the WSGG solution is deemed most appropriate for numerical simulation of lower temperature diffusion flames, due to its significant time advantage. The coupling between radiative loss and soot concentration is investigated via a multiple laminar flamelet concept applied within the CFD simulation of confined turbulent diffusion flames burning methane in air at 1 and 3 atm. Flamelet families are employed relating individual sooting mechanisms to the level of radiative loss, which is evaluated by the DTRM formulated for emitting-absorbing mixtures of soot, C02 and H20. Combustion heat release is described by an eddy break-up concept linked to the k-c turbulence model, whilst temperature is evaluated from the solved enthalpy field. Detailed comparisons between prediction and experiment for the critical properties of mixture fraction, temperature and soot volume fraction demonstrate the effectiveness of this novel, coupled strategy within an elliptic flow field calculation.

This thesis presents an investigation into the dynamics of vortex rings impacting a sharply-stratified density interface. This problem has a long history and is important for understanding how individual eddies in stratified turbulence mix the density field. We tackle this problem using a combination of experimental, numerical and modelling techniques to understand the flow instability and subsequent mixing induced by the impinging vortex ring. Our findings demonstrate that there exists a critical Richardson number, corresponding to a mixing transition, beyond which the mixing efficiency is constant. Using a novel Stereo Particle Image Velocimetry (Stereo-PIV) technique, we analyze a series of vortex ring experiments. By amalgamating an ensemble of these experiments, we measure the full, time-resolved, three-dimensional velocity field of the vortex-ring interaction. These measurements capture the instability that is produced on the baroclinically generated vorticity field. This instability is identified as a Crow-like instability. At low Richardson numbers, the timescale of the interface rebound is faster than that of the instability. As a result, there exists a critical Richardson number below which the Crow-like instability will not have sufficient time to grow to large amplitude. By generating a large number of vortex-ring interactions, we measure the incremental change to the stratification. After an initialization period, there is strong evidence to suggest that the mixing due to each vortex ring becomes constant over a moderate range of Richardson numbers. We suggest that the mixing efficiency of the vortex rings does drop at low Richardson numbers (below unity) in agreement with the analysis of the Stereo-PIV measurements. A model of the system accurately predicts the dependence of the mixing rate on the Richardson number. Based upon our study of the vortex-ring system, we construct a one-dimensional turbulence model that includes the energy advection from the vortex rings. This model is validated with both physical experiments and numerical simulations of repeated vortex-ring generations. The constant mixing efficiency regime is recovered in all three methodologies. Through examining the detailed dynamics of the flow, this work suggests that there exists a critical Richardson number corresponding to a transition between mixing regimes, and that this critical Richardson number is a result of the growth of a Crow-like instability. We have highlighted how to improve current mixing-models to capture this physics. New avenues of future research are currently underway to study the mixing produced by a stratified mixing-box experiment in light of these new developments.

Seminario para optar al título de Ingeniero Comercial, Mención Economía / Autor no autoriza el acceso a texto completo de su documento / Este trabajo analiza los determinantes de eventos extremos en ujos de capitales bru- tos. En particular, se focaliza en el rol del stock de activos internacionales, la apertura nanciera, el r egimen cambiario y la acumulaci on de reservas, en la ocurrencia de una fuerte entrada (surges) o signi cativa contracci on (stops) en in ujos de capitales. Adem as, el trabajo estudia la in uencia de estas variables en eventos compensatorios por parte de los inversionistas residentes, es decir, un fuerte aumento de las salidas de los ujos brutos dado una fuerte entrada de in ujos de capitales ( ight en periodos de surges) o una fuerte reducci on de la salida de ujos brutos (retrenchment) dado a caidas abruptas por el lado de los in ujos de capitales (retrenchment en periodos de stops). Para realizar lo anterior, en las estimaciones se utilizan modelos de logit multinomial. Los resul- tados sugieren que factores globales (factores push), en especial el riesgo global, son signi cativos a la hora de estudiar los in ujos de capitales. Por otro lado, para los eventos compensatorios son m as relevantes los factores internos de cada pa s (factores pull), en los cuales el stocks de activos internacionales cumple un rol fundamental. / This paper analyzes the determinants of extreme events in gross capital ows. In particular, it focuses on the role of stock of international assets, nancial openness, exchange rate and reserves accumulation in the occurrence of extreme capital in ows events (stops and surges) and their in uence in joint compensation events (retrenchment given stops and ight given surges). For this purpose, this work estimates multinomial logit models. The results sug- gest that global factors (push factors), in particular, global risk, are signi cantly associated with extreme capital in ow episodes. On the other hand, country speci c factors (pull factors) are relevant for compensatory events, in particular, for the stock of international assets.

This work investigates the dynamics of magnetic flows on closed orientable Riemannian surfaces. These flows are determined by triples (M, g, σ), where M is the surface, g is the metric and σ is a 2-form on M . Such dynamical systems are described by the Hamiltonian equations of a function E on the tangent bundle TM endowed with a symplectic form ω_σ, where E is the kinetic energy. Our main goal is to prove existence results for a) periodic orbits, and b) Poincare sections for motions on a fixed energy level Σ_m := {E = m^2/2} ⊂ T M . We tackle this problem by studying the contact geometry of the level set Σ_m . This will allow us to a) count periodic orbits using algebraic invariants such as the Symplectic Cohomology SH of the sublevels ({E ≤ m^2/2}, ω_σ ); b) find Poincare sections starting from pseudo-holomorphic foliations, using the techniques developed by Hofer, Wysocki and Zehnder in 1998. In Chapter 3 we give a proof of the invariance of SH under deformation in an abstract setting, suitable for the applications. In Chapter 4 we present some new results on the energy values of contact type. First, we give explicit examples of exact magnetic systems on T^2 which are of contact type at the strict critical value. Then, we analyse the case of non-exact systems on M different from T^2 and prove that, for large m and for small m with symplectic σ, Σ_m is of contact type. Finally, we compute SH in all cases where Σ_m is convex. On the other hand, we are also interested in non-exact examples where the contact property fails. While for surfaces of genus at least two, there is always a level not of contact type for topological reasons, this is not true anymore for S^2 . In Chapter 5, after developing the theory of magnetic flows on surfaces of revolution, we exhibit the first example on S^2 of an energy level not of contact type. We also give a numerical algorithm to check the contact property when the level has positive magnetic curvature. In Chapter 7 we restrict the attention to low energy levels on S^2 with a symplectic σ and we show that these levels are of dynamically convex contact type. Hence, we prove that, in the non-degenerate case, there exists a Poincare section of disc-type and at least an elliptic periodic orbit. In the general case, we show that there are either 2 or infinitely many periodic orbits on Σ_m and that we can divide the periodic orbits in two distinguished classes, short and long, depending on their period. Then, we look at the case of surfaces of revolution, where we give a sufficient condition for the existence of infinitely many periodic orbits. Finally, we discuss a generalisation of dynamical convexity introduced recently by Abreu and Macarini, which applies also to surfaces with genus at least two.

No description available.

Les travaux présentés dans ce manuscrit portent sur les courants et le transport de matière solide en suspension, au voisinage de grandes structures immergées en eau peu profonde. Nous avons focalisé notre étude sur le cas de cages d'aquaculture installées dans la rade de Cherbourg. Situées dans une zone de fort courant, elles impactent leur environnement en modifiant l'écoulement et la turbidité locale. En dépit de nombreuses études déjà disponibles sur l'impact hydrodynamique de tels obstacles, de nombreuses questions se posent encore concernant l'écoulement sous la cage, la production de turbulence, l'érosion du sol et la remise en suspension de sédiments, ainsi que le devenir des déchets produits par la cage. Nous avons choisi de développer trois approches complémentaires pour apporter des éléments de réponse à ces questions. La thèse débute par une étude purement théorique de l'hydrodynamique au voisinage d'un obstacle immergé, assimilé à un milieu poreux, et dans une géométrie simplifiée. Ce modèle bi-dimensionnel, dans le plan vertical, donne de premiers ordres de grandeurs concernant la chute de pression motrice le long du courant, l'accélération du fluide sous la cage, la production de turbulence. Cette étude est suivie d'une analyse numérique de ce même modèle, dans laquelle certaines approximations seront levées. En particulier, le modèle numérique permet de considérer des cages plus imperméables que celles analysées par le modèle théorique. Il est cependant limité en nombre de Reynolds, et a été appliqué avec des échelles réduites. Ce modèle numérique a permis d'analyser le lien direct entre le coefficient de traînée effectif de la cage et diverses quantités-clés : le débit à travers la cage, la chute de pression le long du courant, la production de turbulence et l'apparition d'une zone de recirculation à l'arrière de l'obstacle. Ce dernier effet, visible seulement pour des cages très imperméables, a un impact notable sur le transport de sédiments et d'effluents : lorsque l'arrière de la cage devient une zone morte (zone de recirculation), l'essentiel du débit passe au dessous, ce qui crée de fortes survitesses et produit de la turbulence. Celle-ci peut alors entraîner des particules indésirables dans la zone morte, ce qui augmente notablement la complexité de leurs trajectoires et retarde leur évacuation. Enfin, ces études théoriques ont été complétées par des mesures vélocimétriques et turbidimétriques à l'amont et à l'aval d'une grande cage installée dans la rade de Cherbourg. Celles-ci confirment la présence d'une survitesse en provenance du dessous de la cage, comparable à celle prédite par le modèle théorique. Dans le cas d'une cage très peu perméable, la persistence d'une forte turbidité a été mesurée à l'aval de celle-ci, en accord qualitatif avec les simulations numériques. / This manuscript presents investigations of current and sediment transport in the vicinity of large immersed structures in shallow water flows. It focuses on aquaculture cages installed in the roadstead of Cherbourg. Being located in a high stream zone, these obstacles have a non-negligible effect on currents as well as turbidity. In spite of numerous references on this topic, many questions still arise concerning the environmental impact of such objects. Quantitative information are still needed about the flow below the cage, turbulence production, erosion of the sand bed, sediments resuspension, and evacuation of effluents. To deal with these open questions, we have chosen to develop three complementary approaches. The thesis starts with an analytical study of the flow in a simplified two-dimensional model containing a large rectangular porous structure. Only the motion in the vertical mid-plane of the obstacle is considered. This preliminary model provides estimations of various dynamic quantities, like the velocity increase below the cage, the decay of piezometric pressure along the stream, and turbulence production. Then follows a numerical analysis of the very same flow, based on less stringent approximations. Because of the high cost of computations, simulations have been done at a reduced scale. Nevertheless, they confirm most of the findings of the analytical approach for very permeable cages, and provide crucial information also about weakly permeable cages which were out of reach of the analytical model. In particular, the formation of a recirculation cell behind such cages, together with turbulence production, have been studied with the numerical model. This effect has been observed to affect sediment and effluent transport: when the flow at the rear of the cage takes the form of a cell with closed streamlines, the cage can be thought of as closed and most of the flow passes below. This creates large velocity gradients and produces turbulence which significantly increases the dispersion of sediments and effluents. These particles are then likely to be captured for some time in the cell, instead of being evacuated away. In addition to these theoretical approaches, in-situ current and turbidity measurements have been performed upstream and downstream of a large cage immersed in the roadstead of Cherbourg. They confirm the existence of a fast stream emerging from below the cage. Vertical streamwise velocity profiles have been shown to agree with theoretical ones, in the case of a very permeable cage. Also, large turbidity levels have been recorded at the rear of a weakly permeable cage, in qualitative agreement with simulations.

Bubbles are commonly found in the world around us, from industrial products to carbonated beverages. This thesis will discuss three processes involving of bubbles, from applications to fundamental phenomena. In the first portion of this thesis, I describe the use of bubbles to prevent the formation of marine biofilms and other colonizing organisms onto built structures, collectively referred to as biofouling. Biofouling detrimentally affects the structures upon which they grow, increasing drag and fuel consumption of moving vessels, reducing performance of acoustic sensors, and enhancing degradation of static structures. With recent international bans placed on common biocidal coatings, there is a demand for environmentally friendly antifouling technologies with strong performance. Bubbles rising along a submerged surface have been shown to inhibit biofouling growth, but little work has been done to determine the primary mechanisms responsible for their antifouling behavior. In this thesis I discuss a combination of field and laboratory experiments as well as a theoretical approach used to gain insight into the dominant mechanisms at play, thus laying a foundation for optimization of this antifouling technique. We find that biofouling is inhibited by shear stresses generated throughout the flow, and the degree of biofouling prevention relates to the distribution of bubbles which locally alters the shear stress. Inspired by the potential for direct interactions between bubbles and biofouling, the second topic of this thesis considers the process by which a bubble dewets, or "sticks to", a solid surface. As a bubble approaches a solid surface, the liquid between the gas and solid begins to drain until it resembles a thin film. Upon rupture of this thin film, the air dewets the surface as a contact line is formed and expands. Previous work regarding this contact line motion assumes viscous effects dominate the spreading dynamics while inertial effects are neglected. Studying the early-time dynamics of dewetting bubbles, we find viscosity to be negligible while inertia and capillarity govern the motion of a newly established contact line, suggesting early stages of dewetting are more rapid than anticipated. In the final portion of this thesis, I discuss the fundamental stability of bubbles in volatile liquids. When a bubble arrives at a free surface, we typically expect the film of the bubble cap to thin over some period of time until it ruptures. Traditionally, the drainage of this film has been considered inevitable with evaporation only hastening the film rupture. Here I show air bubbles at the free surface of liquids which appear to defy traditional drainage rules and can avoid rupture, persisting for hours until dissolution. Using pure, volatile liquids free of any surfactants, we highlight and model a thermocapillary phenomenon in which liquid surrounding the bubble is continuously drawn into the bubble cap, effectively overpowering the drainage effects. / 2020-09-28T00:00:00Z

We find a set of conditions on a roof function to ensure topological mixing for suspension flows over a topological mixing base. In the measure theoretic case, such conditions have already been established for certain flows. Specifically, certain suspensions are topologically mixing if and only if the roof function is not cohomologous to a constant. We show that an analogous statement holds to establish topological mixing with the presence of dense periodic points. Much of the work required is to find properties specific to the equivalence class of functions cohomologous to a constant. In addition to these conditions, we show that the set of roof functions that induce a topologically mixing suspension is open and dense in the space of continuous roof functions.

Please read the summary in the section 00front of this document. / Thesis (PhD)--University of Pretoria, 2010. / Economics / unrestricted

The Diamond Fork River, and it’s tributary Sixth Water Creek, has been highly altered in terms of shape, function, and ecologicaly due to large, trans-basin flows additions to the system for irrigation starting in the early 1900s. Flows were exceptionally large for 80 years, after which they were reduced in 2004. Larger than natural flows during the low flow season were then added to the river in an effort to improve ecosystem health and recreational fishing opportunities. Since the prescription additional flow during low flow seasons, the river channel has undergone further change, most noticeably in the form of narrowing. With the channel change that has occurred over the past decade, it has been suggested that the additional flows, which augment baseflows, are too large and that key habitat elements, particularly pools, are lacking throughout the river. We evaluated trout habitat throughout the altered reaches of the Diamond Fork River and Sixth Water Creek which are still subject to flow additions. We used a model to estimate the number of fish a reach can support at a given flow in order to evaluate which flow produces the highest quality trout habitat. We found that the current summer baseflows of 80 cfs for the lower Diamond Fork and 32 cfs for Sixth Water Creek are less than desirable and that flows less than 40 cfs for the lower Diamond Fork and flows between 20 cfs and 30 cfs for Sixth Water Creek would increase the quality of trout habitat. We also evaluated the size and number of pools throughout the system and identified that pools are generally lacking both in size and number relative to standard requirements for trout success. We documented that as the channel narrowed, the number of pools/deep water disappeared. The lack of sediment and the limited number of flows capable of moving sediment were identified as key causes of the channel simplification and loss of pool habitat. Crucial components of future pool formation and maintenance are i) active sediment availability and ii) frequent sediment mobilizing flows. If the baseflow regime were to be lowered, we predict that the channel would narrow, which would increase the effectiveness of floods, increase mobility of sediment, and overall increase pool habitat. This research, as part of a larger, interdisciplinary project, lays the foundation for proposing new flows for the Diamond Fork River that targets ecological goals identified by stakeholders as well as providing information for future habitat restoration projects.