Les écoulements pyroclastiques sont des mélanges à haute température de gaz et de particules volcaniques qui peuvent se propager sur de très grandes distances. Cette forte « mobilité » est souvent attribuée à leur capacité à se fluidiser, c’est-à-dire à générer et conserver une forte pression interstitielle de gaz qui réduit les forces de friction interne. L’objectif principal de cette thèse est de comprendre comment les irrégularités des terrains sur lesquelles se propagent les écoulements pyroclastiques peuvent favoriser leur fluidisation. Une première série d’expériences de laboratoire a consisté à générer des écoulements de particules fines (diamètre de 45-90 μm) sur des substrats de différentes rugosités. Les résultats montrent que la distance de parcours des écoulements augmente avec la rugosité, allant jusqu’à doubler par rapport à la distance de parcours sur fond lisse. Des analyses de vidéos haute vitesse et des mesures de pression interstitielle d’air à la base des écoulements montrent que la tête (partie antérieure) des écoulements qui se propagent sur un substrat rugueux s’auto-fluidisent en conséquence de la sédimentation des particules dans les interstices du substrat, chassant l’air qui remonte et percole dans l’écoulement. Ce mécanisme d’auto-fluidisation est efficace pour toutes les inclinaisons étudiées (0-30°), suggérant qu’il est susceptible de se produire tout au long de la mise en place d’un écoulement pyroclastique. Une seconde étude a consisté à faire chuter des lits de particules dans une colonne statique. Les résultats montrent que même pour une hauteur de relâchement relativement faible (20 cm), le mélange peut entièrement s’auto-fluidiser durant sa chute. Quand les particules sont suffisamment fines (<100 μm) la pression interstitielle dans le dépôt diffuse pendant plusieurs secondes, la durée de cette diffusion augmentant avec l’augmentation de l’épaisseur du lit et la diminution de taille des particules. Les temps de diffusions les plus longs sont observés avec un matériau provenant d’un dépôt d’écoulement pyroclastique (~30 s pour des lits de 28.5 cm d’épaisseur). Ces résultats suggèrent que les écoulements pyroclastiques qui se propagent sur des terrains accidentés peuvent s’auto-fluidiser et conserver une faible friction au cours de leur mise en place. / Pyroclastic flows are hot mixtures of gas and particles that can propagate over large distances. This high “mobility” is often attributed to their ability to be fluidized, that is, to generate and retain high gas pore pressure that reduces internal friction forces. The main objective of this thesis is to understand how irregularities of substrates on which pyroclastic flows propagate can enhance their fluidization. A first set of laboratory experiments consisted of the generation of fine-grained flows (diameter of 45-90 μm) on substrate of various roughness. Results show that the flow runout distance increases with the substrate roughness, and is up to twice the runout on a smooth substrate. High speed video analyses and air pore pressure measurements at the flow base show that the flow head propagating over a rough substrate can auto-fluidize because of particles sedimentation into the substrate interstices, which forces the air to escape upward and percolate through the flow. This auto-fluidization mechanism is efficient at all inclinations investigated (0-30°), suggesting that it could occur during the whole emplacement of a pyroclastic flow. A second study consisted of the vertical release of beds of particles in a static column. Results show that the granular mixture can be fully fluidized, even when collapsing from a relatively low height (20 cm). When particles are fine enough (<100 μm), pore pressure in the deposit diffuses for several seconds, the diffusion duration increasing with increasing bed thickness and decreasing particle size. The longest diffusion durations are observed for pyroclastic flow deposit materials (~30 s for 28.5 cm thick beds). These results suggest that pyroclastic flows propagating on irregular terrains can auto-fluidize and preserve low internal friction during their emplacement.

In a globalizing world, trade has become essential to supporting the needs and wants of billions of people. Virtually everyone now consumes resource commodities and manufactured products traded all over the world; the ecological footprints of nations are now scattered across the globe. The spatial separation of material production (resource exploitation) from consumption eliminates negative feedbacks from supporting eco-systems. Most consumers remain unaware of the impacts that their trade dependence imposes on distant ecosystems (out of sight out of mind). I take the first steps in developing a conceptual and practical framework for an ‘interregional ecology’ approach to exploring and analyzing sustainability in an increasingly interconnected world. Such an approach accounts for some of the ‘externalities’ of globalization and international trade. It underscores the increasing dependence and impact of almost any country on resources originating from others and recognizes that the sustainability of any specified region may be increasingly linked to the ecological sustainability of distant supporting regions. I empirically describe and quantify some of the interregional material linkages between selected countries. I document the flows of renewable resources into the U.S. and quantify the U.S. external material footprint (EF) on specific countries. I then document the physical inputs involved in production of most agricultural export products from Costa Rica and Canada. Finally, I focus on major export products such as bananas, coffee and beef in Costa Rica and agricultural activities in the Canadian Prairies and document some of the ecological consequences (loss of habitat, soil degradation, water contamination and biodiversity loss) of that production. My research findings show increasing U.S. imports, increasing reliance on external sources and growing external ecological footprints. They also show how production activities mostly for overseas consumption led to changes in ecological structure and function in the studied export countries. This dissertation adds a missing trans-national dimension to the sustainability debate effectively integrating the policy and planning domain for sustainability in one region with that in others. While my research focuses mainly on documenting the nature and magnitude of interregional connections I also consider some of the implications of the interregional approach for sustainability planning. / Applied Science, Faculty of / Community and Regional Planning (SCARP), School of / Graduate

Large-scale unsteady flow structures play an influential role in the dynamics of many practical flows, such as those found in gas turbine combustion chambers. This thesis is concerned primarily with large-scale unsteady structures that arise due to self-sustained hydrodynamic oscillations, also known as global hydrodynamic instability. Direct numerical simulation (DNS) of the Navier--Stokes equations in the low Mach number limit is used to obtain a steady base flow, and the most unstable direct and adjoint global modes. These are combined, using a structural sensitivity framework, to identify the region of the flow and the feedback mechanisms that are responsible for causing the global instability. Using a Lagrangian framework, the direct and adjoint global modes are also used to identify the regions of the flow where steady and unsteady control, such as a drag force or heat input, can suppress or promote the global instability. These tools are used to study a variety of reacting and non-reacting flows to build an understanding of the physical mechanisms that are responsible for global hydrodynamic instability in swirling diffusion flames. In a non-swirling lifted jet diffusion flame, two modes of global instability are found. The first mode is a high-frequency mode caused by the instability of the low-density jet shear layer in the premixing zone. The second mode is a low-frequency mode caused by an instability of the outer shear layer of the flame. Two types of swirling diffusion flames with vortex breakdown bubbles are considered. They show qualitatively similar behaviour to the lifted jet diffusion flames. The first type of flame is unstable to a low-frequency mode, with wavemaker located at the flame base. The second type of flame is unstable to a high-frequency mode, with wavemaker located at the upstream edge of the vortex breakdown bubble. Feedback from density perturbations is found to have a strong influence on the unstable modes in the reacting flows. The wavemaker of the high-frequency mode in the reacting flows is very similar to its non-reacting counterpart. The low-frequency mode, however, is only observed in the reacting flows. The presence of reaction increases the influence of changes in the base flow mixture fraction profiles on the eigenmode. This increased influence acts through the heat release term. These results emphasize the possibility that non-reacting simulations and experiments may not always capture the important instability mechanisms of reacting flows, and highlight the importance of including heat release terms in stability analyses of reacting flows.

This thesis is concerned with the new formulation of a finite-volume lattice Boltzmann equation method and its implementation on unstructured meshes. The finite-volume discretization with a cell-centered tessellation is employed. The new formulation effectively adopts a total variation diminishing concept. The formulation is analyzed for the modified partial differential equation and the apparent viscosity of the model. Further, the high-order extension of the present formulation is laid out. Parallel simulations of a variety of two-dimensional benchmark flows are carried out to validate the formulation. In Chapter 1, the important notions of the kinetic theory and the most celebrated equation in the kinetic theory, ‘the Boltzmann equation’ are given. The historical developments and the theory of a discrete form of Boltzmann equation are briefly discussed. Various off-lattice schemes are introduced. Various methodologies adopted in the past for the solution of the lattice Boltzmann equation on finite-volume discretization are reviewed. The basic objectives of this thesis are stated. In Chapter2,the basic formulations of lattice Boltzmann equation method with a rational behind different boundary condition implementations are discussed. The benchmark flows are studied for various flow phenomenon with the parallel code developed in-house. In particular, the new benchmark solution is given for the flow induced inside a rectangular, deep cavity. In Chapter 3, the need for off-lattice schemes and a general introduction to the finite-volume approach and unstructured mesh technology are given. A new mathematical formulation of the off-lattice finite-volume lattice Boltzmann equation procedure on a cell-centered, arbitrary triangular tessellation is laid out. This formulation employs the total variation diminishing procedure to treat the advection terms. The implementation of the boundary condition is given with an outline of the numerical implementation. The Chapman-Enskog (CE) expansion is performed to derive the conservation equations and an expression for the apparent viscosity from the finite-volume lattice Boltzmann equation formulation in Chapter 4. Further, the numerical investigations are performed to analyze the apparent viscosity variation with respect to the grid resolution. In Chapter 5, an extensive validation of the newly formulated finite-volume scheme is presented. The benchmark flows considered are of increasing complexity and are namely (1) Posieuille flow, (2) unsteady Couette flow, (3) lid-driven cavity flow, (4) flow past a backward step and (5) steady flow past a circular cylinder. Further, a sensitivity study to the various limiter functions has also been carried out. The main objective of Chapter6is to enhance the order of accuracy of spatio-temporal calculations in the newly presented finite-volume lattice Boltzmann equation formulation. Further, efficient implementation of the formulation for parallel processing is carried out. An appropriate decomposition of the computational domain is performed using a graph partitioning tool. The order-of-accuracy has been verified by simulating a flow past a curved surface. The extended formulation is employed to study more complex unsteady flows past circular cylinders. In Chapter 7, the main conclusions of this thesis are summarized. Possible issues to be examined for further improvements in the formulation are identified. Further, the potential applications of the present formulation are discussed.

In the case of cold accretion disk, coupling between charge neutral gas and magnetic field is too weak such that the magneto-rotational instability will be less effective or even stop working. In such a situation it is of prime interest to investigate the pure hydrodynamic turbulence and transport phenomenon. As the Reynolds number increases, the relative importance of the non-linear term in the hydrodynamic equation increases and in the case of accretion disk where molecular viscosity is too small the Reynolds number is large enough for the non-linear term to bring new effects. We investigate a scenario, the ‘weakly non-linear’ evolution of amplitude of linear mode when the flow is bounded by two parallel walls. The unperturbed flow is similar to plane Couette flow but with Coriolis force included in the hydrodynamic equation. Although there is no exponentially growing eigenmode, due to self-interaction the least stable eigenmode will grow in an intermediate phase. Later on this will lead to higher order non-linearity and plausible turbulence. Although the non-linear term in the hydrodynamic equation is energy conserving, within the weakly non-linear analysis it is possible to define a lower bound of the energy needed for flow to transform to turbulent phase. Such an unstable phase is possible only if the Reynolds number ≥ 103−4. In Chapter-2 we set up equation of amplitude for the hydrodynamic perturbation and study the effect of weak non-linear evolution of linear mode for general angular momentum distribution, where Keplerian disk is obtained as a special case. As we know that to explain observed hard X-rays the choice of Keplerian angular momentum profile is not adequate, we consider the sub-Keplerian regime of the disk. In Chapter-3 we assume that the cooling mechanism is dominated by bremsstrahlung process (without any strict knowledge of the magnetic field structure).We show that in a range of Shakura-Sunyaev viscosity 0.2 ≥ α ≥ 0.0005, flow behavior varies widely, particularly by means of the size of disk, efficiency of cooling and corresponding temperatures of ions and electrons. We also show that the disk around a rotating black hole is hotter compared to that around a Schwarzschild black hole, rendering a larger difference between ion and electron temperatures in the former case. We finally reproduce the observed luminosities(L) of two extreme cases—the under-fed AGNs and quasars and ultra-luminous X-ray sources at different combinations of mass accretion rate, ratio of specific heats, Shakura-Sunyaev viscosity parameter and Kerr parameter. In Chapter-4 we investigate the viscous two temperature accretion disk flows around rotating blackholes. We describe the global solution of accretion flows, unlike that in Chapter-3, with a sub-Keplerian angular momentum profile, by solving the underlying conservation equations including explicit cooling processes self-consistently. Bremsstrahlung, synchrotron and inverse comptonization of soft photons are considered as possible cooling mechanisms. We focus on the set of solutions for sub-Eddington, Eddington and super-Eddington mass accretion rates around Schwarzschild and Kerr black holes with a Kerr parameter 0.998. We analyse various phases of advection–general advective paradigm to radiatively inefficient paradigm. The solution may potentially explain the hard X-rays and γ-rays emitted from AGNs and X-ray binaries. We also compare the solutions for two different regimes of viscosity. We finally reproduce the observed luminosities of the under-fed AGNs and quasars, ultra-luminous X-ray sources at different combinations of input parameters such as mass accretion rate and ratio of specific heats.

No description available.

Many problems in fluid mechanics and material sciences deal with liquid-vapour flows. In these flows, the ideal gas assumption is not accurate and the van der Waals equation of state is usually used. This equation of state is non-convex and causes the solution domain to have two hyperbolic regions separated by an elliptic region. Therefore, the governing equations of these flows have a mixed elliptic-hyperbolic nature. Numerical oscillations usually appear with standard finite-difference space discretization schemes, and they persist when the order of accuracy of the semi-discrete scheme is increased. In this study, we propose to use a Chebyshev pseudospectral method for solving the governing equations. A comparison of the results of this method with very high-order (up to tenth-order accurate) finite difference schemes is presented, which shows that the proposed method leads to a lower level of numerical oscillations than other high-order finite difference schemes, and also does not exhibit fast-traveling packages of short waves which are usually observed in high-order finite difference methods. The proposed method can thus successfully capture various complex regimes of waves and phase transitions in both elliptic and hyperbolic regimes

Shallow flows are those whose width is significantly larger than their depth. In these types of flows, two dimensional coherent structures can be generated and can influence the flow greatly by the lateral transfer of mass and momentum. The development of coherent structures as a result of flow instabilities has been a topic of interest for environmental fluid mechanics for decades. Studies on the use of linear modal stability analysis is commonly found in literature. However, the relatively recent development in the field of hydrodynamic stability suggests that the traditional linear modal stability analysis does not describe the behaviour of the perturbations in finite time. The discrepancy between asymptotic behaviour and finite time behaviour is particularly large in shear driven flows and it is most likely to be the case for shallow flows. This study aims to provide a better understanding of finite time growth of perturbation energy in shallow flows. The three cases of shallow flows evaluated are the mixing layer, jet and wake. The critical cases are obtained through the linear modal analysis and nonmodal analysis was conducted to show the transient behaviour in finite time for what is so-called marginally stable. Finally, the thesis concludes by generalizing the finite time energy growth in the S-k space.

Les dysfonctionnements observés dans les enceintes de conversion thermochimique tels que les zones mortes et les courts-Circuits conduisent en général à une mauvaise valorisation de la ressource énergétique et à une pollution atmosphérique. Ils trouvent leur origine dans les conditions aérauliques au sein de ces enceintes. Ils pourront alors être évités par une meilleure maîtrise de ces écoulements. Nous proposons dans ce travail d’une part, le développement d’un nouvel outil pour la détermination de la distribution des temps de séjour de la phase solide, basé sur la luminescence de particules préalablement enrobées de pigments phosphorescents. Cette méthode systémique, optique, non intrusive et souple d’emploi a été mise en place à l’échelle laboratoire sur un banc d’essais aéraulique. D’autre part, nous avons développé un modèle numérique permettant de déterminer la distribution des temps de séjour afin de maîtriser les écoulements à la sortie des enceintes en vue de les optimiser et les extrapoler à l’échelle industrielle. Cette approche analytique est basée sur une modélisation par couplage MFN de type volumes finis du comportement d’un fluide à l’aide du Code Saturne et DEM de type éléments discrets du comportement du solide à l’aide du code SIGRAME. Enfin une confrontation de la DTS du modèle numérique avec laDTS expérimentale a été réalisée. / Dysfunctions observed in thermochemical conversion reactors like dead zones and short circuits generally lead to inaccurate pricing of energy resources and air pollution. They originate in the air flow conditions in these aeraulic reactor. They can then be avoided by a better control of these flows. We propose in this work to develop a new tool for determining the distribution of residence time of the solid phase, based on the luminescence of particles previously coated with phosphorescent pigments. This optical method, non-Intrusive and flexible, has been implemented at a laboratory scale, on an aeraulic test bench.On the other hand, we have developed a numerical model allowing to determine the distribution of the residence time to master the flows at the exit of surrounding walls to optimize them and extrapolate them at the industrial scale. This analytical approach is based on a modelling by coupling MFN by finite volume types via the Code Saturn and DEM by discrete elements of the solid behavior by means of the code SIGRAME. Finally a confrontation of the DTS of the digital model with the experimental DTS has been done.

Business valuation of micro trading company LIPOVICA trade s.r.o; main business activity of the company is selling and distribution of aluminum radiators on the markets in the Czech and Slovak republic. The valuation is based on discounted cash flows method, economic value added method, capitalization of profits method and simple liquidation method.