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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
41

The rotating-disk boundary-layer flow studied through numerical simulations

Appelquist, Ellinor January 2017 (has links)
This thesis deals with the instabilities of the incompressible boundary-layer flow thatis induced by a disk rotating in otherwise still fluid. The results presented include bothwork in the linear and nonlinear regime and are derived from direct numerical sim-ulations (DNS). Comparisons are made both to theoretical and experimental resultsproviding new insights into the transition route to turbulence. The simulation codeNek5000 has been chosen for the DNS using a spectral-element method (SEM) witha high-order discretization, and the results were obtained through large-scale paral-lel simulations. The known similarity solution of the Navier–Stokes equations for therotating-disk flow, also called the von K ́arm ́an rotating-disk flow, is reproduced by theDNS. With the addition of modelled small simulated roughnesses on the disk surface,convective instabilities appear and data from the linear region in the DNS are anal-ysed and compared with experimental and theoretical data, all corresponding verywell. A theoretical analysis is also presented using a local linear-stability approach,where two stability solvers have been developed based on earlier work. Furthermore,the impulse response of the rotating-disk boundary layer is investigated using DNS.The local response is known to be absolutely unstable and the global response, onthe contrary, is stable if the edge of the disk is assumed to be at radius infinity. Herecomparisons with a finite domain using various boundary conditions give a globalbehaviour that can be both linearly stable and unstable, however always nonlinearlyunstable. The global frequency of the flow is found to be determined by the Rey-nolds number at the confinement of the domain, either by the edge (linear case) or bythe turbulence appearance (nonlinear case). Moreover, secondary instabilities on topof the convective instabilities induced by roughness elements were investigated andfound to be globally unstable. This behaviour agrees well with the experimental flowand acts at a smaller radial distance than the primary global instability. The sharpline corresponding to transition to turbulence seen in experiments of the rotating diskcan thus be explained by the secondary global instability. Finally, turbulence datawere compared with experiments and investigated thoroughly. / <p>QC 20170203</p>
42

Formation et évolution de tourbillons dans la nébuleuse protoplanétaire / Formation and evolution of vortices in protoplanetary nebula

Richard, Samuel 12 November 2013 (has links)
L'objectif de cette thèse est d'étudier la formation de tourbillons dans la zone morte des disques protoplanétaires. Un code numérique 3D compressible a été mis au point et utilisé pour cette étude. Deux instabilités hydrodynamiques sont envisagées pour former les tourbillons: l'instabilité de Rossby et l'instabilité barocline.La première entraine la fragmentation d'une sur-densité annulaire en une chaîne de tourbillons qui se rattrapent les uns les autres et finissent par fusionner en un seul tourbillon qui reste stable sur de très longues durées lorsque son rapport d’aspect est suffisamment grand, et possède une structure quasi bidimensionnelle. En revanche, les tourbillons tridimensionnels de petits rapport d'aspect sont affectés par l’instabilité elliptique qui les détruits en quelques rotations. Seuls persistent ceux de grand rapport d'aspect.L'instabilité barocline, fondamentalement non linéaire, produit des tourbillons à partir de perturbations d'amplitude finies ; ces tourbillons sont ensuite amplifiés et fusionnent en tourbillons plus gros si le disque est stratifié de façon instable et s’il permet aussi le transfert de chaleur. Deux types de transfert thermique ont été envisagés pour étudier cette instabilité qui conduit alors à des différences significatives dans la structure des tourbillons formés. Le rapport d'aspect étant lié à la vorticité, l'amplification des tourbillons se traduit par une diminution de leur rapport d'aspect, et les rend donc sujet à l'instabilité elliptique. Cependant, ils ne sont pas détruit et gardent une structure tourbillonnaire grâce à l'amplification barocline. / The objective of this thesis is to study the formation of vortices in the dead-zone of protoplanetary disks. A 3D compressible numerical code has been performed and used for this study. Two hydrodynamical instabilities are considered for vortex formation: the Rossby wave instability and the baroclinic instability.The first one leads tp the fragmentation of an annular bump into a chain of vortices that catch one another and merge in a single vortex; this vortex remains stable on very long durations when its aspect ratio is large enough and has a quasi two-dimensional structure. In contrast, tridimensional small aspect ratios vortices are affected by the elliptical instability and are destroyed in a few rotation periods. Only vortices with large aspect ratios can survive.The baroclinic instability, a basically non-linear one, can produce vortices from small amplitude perturbations; these vortices are then amplified and merge in bigger vortices if the disk is unstably stratified and also permits heat transfer. Two types of heat transfer have been considered leading to significant differences in the structures of the resulting vortices. As aspect ratio and vorticity are strongly related, the baroclinic amplification reduces the aspect ratio and, so, make the vortex sensitive to the elliptical instability. However, such vortices are not destroyed and keep a vertical structure thanks to the baroclinic amplification.
43

Trapped supercritical flows: Numerical simulations with idealized topography

Söderberg, Stefan January 1999 (has links)
Numerical simulations of supercritical coastal flows have been performed. The meso-γ-scale model which has been used in this study is non-linear, hydrostatic and has a higher-order turbulence closure. Previous studies of supercritical coastal flows with this model have given rise to some questions and hypotheses, e. g. is the gradual curvature of the main coastal mountains north of Cape Mendocino sufficient to excite an expansion fan? Is the local terrain of Cape Mendocino responsible for the collapse of the marine atmospheric layer (MABL) in Shelter Cove? This study was designed to answer these questions. The terrain used in the simulations was generated by a simple analytical function and fitted to the real terrain north of Cape Mendocino in a sense that it would reflect it as good as possible, neglecting ”small” changes in the orientation of the coast and height of the terrain. This made it possible to test hypotheses related to the coastline shape one by one. Simulations that were performed are: Piecewise linear coasts with constant terrain height where the change in coastline orientation, the height of the inversion and strength of the background wind speed was varied; Piecewise linear coasts where the height of the terrain was varying along the coast; Curved coastlines with different curvatures; Piecewise linear coasts with simplified capes perpendicular to the coast where the height of the cape was varied. It was found that the angle with which the coast turns away from the flow regulates the amount of acceleration in the following expansion fan. A decreasing height of the terrain along the coast lead to an increased acceleration of the flow, the largest acceleration was found when the slope of the terrain was confined to the change in coastline orientation. It was concluded that this is comparable to an increased change in coastline orientation. The simulations with curved coastlines confirmed the hypothesis that the gradual curvature of the main coastal mountains north of Cape Mendocino is sufficient to excite an expansion fan. In fact, a curved coastline leads to a stronger acceleration of the flow than a piecewise linear coastline. One of the most striking features in this study was that the acceleration of the flow started far upstream of the change in coastline orientation, even though the flows were supercritical. This phenomenon was mainly found in the cases with the highest wind speeds. It is suggested that the upstream acceleration of the flow is due to either high amplitude gravity waves propagating within the MABL or internal gravity waves propagating above the inversion. When a cape was inserted perpendicular to the main coastline, this showed that even with a cape as low as approximately half the depth of the MABL, the flow was significantly blocked. Indications of wave-breaking on the lee side of the cape was also found, which confirmed the hypothesis that the local terrain of Cape Mendocino is responsible for the collapse of the MABL in Shelter Cove.
44

Ressauts dans les écoulements granulaires en pente / Jumps in granular flows down incline

Mejean, Ségolène 11 March 2019 (has links)
Le dimensionnement des digues paravalanches s’appuie sur la connaissance des processus physiques liés au ressaut, qui se forme lorsqu’un écoulement fin et rapide rencontre un obstacle suffisamment haut pour ralentir et épaissir l’écoulement incident. La hauteur du ressaut est aujourd’hui calculée à partir d’équations strictement valides pour des écoulements de matériaux non frictionnels et non compressibles tels que l’eau, sur fond plat et lisse. Or, les avalanches de neige dense sont des écoulements compressibles qui ne peuvent avoir lieu qu’en pente, et au sein desquels se produit de la dissipation d’énergie par friction et collisions entre les grains. Il est donc essentiel de mieux connaître le comportement des ressauts dans les écoulements granulaires en pente. Pour cela, la thèse s’appuie sur plusieurs approches. Les ressauts granulaires stationnaires sont d’abord étudiés de manière purement théorique, à l’aide des équations de conservation de la masse et de la quantité de mouvement moyennées dans l’épaisseur, afin de trouver une relation générale pour prédire la hauteur des ressauts quelques soient les conditions d’entrée. Nous simulons ensuite numériquement un grand nombre de ressauts granulaires en faisant varier plusieurs paramètres (la pente du plan incliné, le débit, le diamètre des grains, la friction entre les grains) à l’aide de la méthode aux éléments discrets. Cette méthode permet d’accéder à la structure interne des ressauts, et notamment à la mesure des champs de vitesse, de fraction volumique, ou encore de la dissipation d’énergie. Les simulations sont réalisées en deux dimensions. Enfin, un dispositif de mesure innovant, qui utilise la radiographie à rayons X dynamique, a été adapté à une expérience de laboratoire existante pour créer des ressauts granulaires stationnaires. Cette technique de mesure permet, en particulier, de mesurer la distribution spatiale moyennée dans la largeur de l’écoulement de la fraction volumique avant, à l’intérieur et après le ressaut granulaire. La comparaison du nouveau cadre théorique proposé avec les résultats expérimentaux et numériques nous permet de mettre en évidence une grande diversité des types de ressauts granulaires en fonction des conditions initiales. Pour chaque type de ressaut, les lacunes du cadre théorique classique, qui ne tient pas compte des forces mises en jeu dans le ressaut ni de la compressibilité, sont clairement établies. / The design of avalanche protection dams relies on the understanding and modelling of physical processes related to the formation of jumps that form when a thin and fast flow meets an obstacle high enough to slow down and thicken the incoming flow. The jump height is nowadays calculated through equations that are strictly valid for non-frictional incompressible flows on a horizontal and smooth bottom. However, dense-snow avalanches are compressible granular flows taking place on a slope, and inside which energy is dissipated through enduring frictional contacts and collisions between grains. It is then essential to decipher the behaviour of jumps formed during granular flows down inclines. To this extent, the thesis relies on several approaches. Standing granular jumps are first studied in a purely theoretical way, with the help of depth-averaged mass and momentum conservation equations, in order to find a relation to predict the height of the jumps regardless of the input conditions. A great number of granular jumps are then simulated by varying several parameters (the slope angle of the incline, the discharge, the grain diameter, the grain-grain friction) thanks to the discrete element method. This method allows us to access to the internal structure of the jumps, and in particular to the spatial distributions of velocity, volume fraction and energy dissipation. Those simulations are done in two dimensions. Finally, an innovative measurement technique using dynamic X-ray radiography was adapted to an existing small-scale laboratory device to produce standing granular jumps. This technique allows in particular the measurement of the width-averaged spatial distribution of volume fraction before, inside and after the granular jumps. The comparison between the new theoretical framework proposed and both the experimental and numerical data, allows us to evidence a rich variety of granular jump patterns as a function of the input conditions. For each type of jump pattern, the shortcomings of the classical theoretical framework, which does not account for the forces at stake within the jump volume nor the compressibility, are well established.
45

Transições de fase e criticalidade em modelos estocásticos / Phase transitions and criticality in stochastic models.

Sabag, Munir Machado de Sousa 20 August 2002 (has links)
Neste trabalho, analisamos três modelos definidos sobre redes e governados por dinÂmicas estocásticas. Nosso principal interesse repousa no estudo das transições de fase e no comportamento crítico desses modelos. O primeiro deles é o autômato celular pobabilístico da Domany-Kinzel, ao qual aplicamos o método de expansões em série. Em seguida, estudamos o comportamento para tempos longos de alguns processos de reação-difusão por meio de simulação numérica. Tais processos podem ser relevantes para o entendimento da compactação em sistemas granulares. Finalmente, também através de dimulações numéricas, analisamos o processo de contato conservativo, que é uma versão do modelo original definida em um ensemble onde o número de partículas é conservado. / In this work, we analyzed three lattice models governed by stochastic dynamics. Our main interest lies on the study of the phase transitions and critical behavior of these models. The first of them is the Domany-Kinzel probabilistic cellular automaton, to which we applied the method of series expansions. Next, we studied the long time behavior of some reaction-diffusion processes by means of numerical simulations. Such processes may be relevant to the understanding of granular compaction. Finally, also by means of numerical simulations, we have analyzed the conserved contact process, which is a version of the original model defined on an ensemble where the number of particles is conserved.
46

Transições de fase e criticalidade em modelos estocásticos / Phase transitions and criticality in stochastic models.

Munir Machado de Sousa Sabag 20 August 2002 (has links)
Neste trabalho, analisamos três modelos definidos sobre redes e governados por dinÂmicas estocásticas. Nosso principal interesse repousa no estudo das transições de fase e no comportamento crítico desses modelos. O primeiro deles é o autômato celular pobabilístico da Domany-Kinzel, ao qual aplicamos o método de expansões em série. Em seguida, estudamos o comportamento para tempos longos de alguns processos de reação-difusão por meio de simulação numérica. Tais processos podem ser relevantes para o entendimento da compactação em sistemas granulares. Finalmente, também através de dimulações numéricas, analisamos o processo de contato conservativo, que é uma versão do modelo original definida em um ensemble onde o número de partículas é conservado. / In this work, we analyzed three lattice models governed by stochastic dynamics. Our main interest lies on the study of the phase transitions and critical behavior of these models. The first of them is the Domany-Kinzel probabilistic cellular automaton, to which we applied the method of series expansions. Next, we studied the long time behavior of some reaction-diffusion processes by means of numerical simulations. Such processes may be relevant to the understanding of granular compaction. Finally, also by means of numerical simulations, we have analyzed the conserved contact process, which is a version of the original model defined on an ensemble where the number of particles is conserved.
47

Interactions et structures dans les suspensions polydisperses de colloïdes chargés sphériques / Interactions and structures in polydisperse suspensions of charged spherical colloids

Bareigts, Guillaume 14 December 2018 (has links)
Les suspensions colloïdales se trouvent un peu partout autour de nous, dans les matériauxde constructions, en cosmétique, dans l'alimentation, en biologie. Elles sont composésde particules nanométriques ou micrométriques dispersés dans un gaz, un liquide ou unsolide.Cette thèse porte sur les suspensions colloïdales dans des solutions ioniques,où les colloïdes portent une charge électriques, par exemple des particules de silicedans une solution aqueuse de chlorure de sodium, à un pH basique. Les colloïdes,ici approximés par des sphères, peuvent varier significativement en taille,ce qui peut avoir un effet important sur le comportement de ces systèmes.Cette étude vise à améliorer la compréhension de ces suspensions colloïdales chargéespar des modèles théoriques résolus par des simulations numériques.Un des défis de ces simulations est le grand nombre de degrés de libertés. Pour chaque(micro-)ion il y a des centaines de molécules de solvant, et pour chaque colloïdedes centaines voire des milliers d'ions. Pour s'en sortir, nous avons calculéles interactions effectives à l'échelle colloïdale. Nous avons repris et développéplusieurs approches, présentant chacune un compromis en terme de temps de calcul etprécision.La variation en taille des colloïdes peut introduire des effets intéressants,observés expérimentalement, notamment le fractionnement des suspensions en plusieursphases cristallines quand on augmente la concentration en colloïdes.Des techniques de simulations Monte-Carlo simples associées aux interactions inter-colloïdescalculées précédemment ont permis d'obtenir des résultats en bon accord avec l'expérience. / Colloidal suspensions are found a bit everywhere around us, in construction materials,in cosmetics, in food, in biology. They are composed of nanometric or micrometric particlesdispersed in a gas, a liquid or sometimes a solid.This thesis is about colloidal suspensions in ionic solutions, where colloids bear anelectric charge, for example silica particles in an aqueous solution of sodium chloride,at a basic pH. The colloids, here approximated by spheres, can vary significantly in size,which can have an important effect on the behavior of these systems.This study aims at improving the understanding of these charged colloidal suspensionsby theoretical models solved by numerical simulations.of these charged colloidal suspensionsby theoretical models solved by numerical simulations.One of the challenge of theses simulations is the huge number of degrees of freedom.For each (micro-)ion there is hundreds of solvent molecules, and for each colloidthere is hundreds if not thousands of ions. To get away with it, we calculated theeffective interactions at the colloidal scale. We took and developed several approaches,each showing a trade-off in terms of computational time and accuracy.The size variation of colloids may introduce interesting effects, experimentallyobserved, notably the fractionation of suspensions in several crystalline phaseswhen the colloidal concentration is increased. Some simple Monte-Carlo simulationtechniques in combination with the inter-colloid interactions computed previouslyallowed us to obtain results in good agreement with experiments.
48

Turbulent burning, flame acceleration, explosion triggering

Akkerman, V'yacheslav January 2007 (has links)
The present thesis considers several important problems of combustion theory, which are closely related to each other: turbulent burning, flame interaction with walls in different geometries, flame acceleration and detonation triggering. The theory of turbulent burning is developed within the renormalization approach. The theory takes into account realistic thermal expansion of burning matter. Unlike previous renormalization models of turbulent burning, the theory includes flame interaction with vortices aligned both perpendicular and parallel to average direction of flame propagation. The perpendicular vortices distort a flame front due to kinematical drift; the parallel vortices modify the flame shape because of the centrifugal force. A corrugated flame front consumes more fuel mixture per unit of time and propagates much faster. The Darrieus-Landau instability is also included in the theory. The instability becomes especially important when the characteristic length scale of the flow is large. Flame interaction with non-slip walls is another large-scale effect, which influences the flame shape and the turbulent burning rate. This interaction is investigated in the thesis in different geometries of tubes with open / closed ends. When the tube ends are open, then flame interaction with non-slip walls leads to an oscillating regime of burning. Flame oscillations are investigated for different flame parameters and tube widths. The average increase in the burning rate in the oscillations is found. Then, propagating from a closed tube end, a flame accelerates according to the Shelkin mechanism. In the theses, an analytical theory of laminar flame acceleration is developed. The theory predicts the acceleration rate, the flame shape and the velocity profile in the flow pushed by the flame. The theory is validated by extensive numerical simulations. An alternative mechanism of flame acceleration is also considered, which is possible at the initial stages of burning in tubes. The mechanism is investigated using the analytical theory and direct numerical simulations. The analytical and numerical results are in very good agreement with previous experiments on “tulip” flames. The analytical theory of explosion triggering by an accelerating flame is developed. The theory describes heating of the fuel mixture by a compression wave pushed by an accelerating flame. As a result, the fuel mixture may explode ahead of the flame front. The explosion time is calculated. The theory shows good agreement with previous numerical simulations on deflagration-to-detonation transition in laminar flows. Flame interaction with sound waves is studied in the geometry of a flame propagating to a closed tube end. It is demonstrated numerically that intrinsic flame oscillations coming into resonance with acoustic waves may lead to violent folding of the flame front with a drastic increase in the burning rate. The flame folding is related to the Rayleigh-Taylor instability developing at the flame front in the oscillating acceleration field of the acoustic wave.
49

Stability of plane Couette flow and pipe Poiseuille flow

Åsén, Per-Olov January 2007 (has links)
This thesis concerns the stability of plane Couette flow and pipe Poiseuille flow in three space dimensions. The mathematical model for both flows is the incompressible Navier--Stokes equations. Both analytical and numerical techniques are used. We present new results for the resolvent corresponding to both flows. For plane Couette flow, analytical bounds on the resolvent have previously been derived in parts of the unstable half-plane. In the remaining part, only bounds based on numerical computations in an infinite parameter domain are available. Due to the need for truncation of this infinite parameter domain, these results are mathematically insufficient. We obtain a new analytical bound on the resolvent at s=0 in all but a compact subset of the parameter domain. This is done by deriving approximate solutions of the Orr--Sommerfeld equation and bounding the errors made by the approximations. In the remaining compact set, we use standard numerical techniques to obtain a bound. Hence, this part of the proof is not rigorous in the mathematical sense. In the thesis, we present a way of making also the numerical part of the proof rigorous. By using analytical techniques, we reduce the remaining compact subset of the parameter domain to a finite set of parameter values. In this set, we need to compute bounds on the solution of a boundary value problem. By using a validated numerical method, such bounds can be obtained. In the thesis, we investigate a validated numerical method for enclosing the solutions of boundary value problems. For pipe Poiseuille flow, only numerical bounds on the resolvent have previously been derived. We present analytical bounds in parts of the unstable half-plane. Also, we derive a bound on the resolvent for certain perturbations. Especially, the bound is valid for the perturbation which numerical computations indicate to be the perturbation which exhibits largest transient growth. The bound is valid in the entire unstable half-plane. We also investigate the stability of pipe Poiseuille flow by direct numerical simulations. Especially, we consider a disturbance which experiments have shown is efficient in triggering turbulence. The disturbance is in the form of blowing and suction in two small holes. Our results show the formation of hairpin vortices shortly after the disturbance. Initially, the hairpins form a localized packet of hairpins as they are advected downstream. After approximately $10$ pipe diameters from the disturbance origin, the flow becomes severely disordered. Our results show good agreement with the experimental results. In order to perform direct numerical simulations of disturbances which are highly localized in space, parallel computers must be used. Also, direct numerical simulations require the use of numerical methods of high order of accuracy. Many such methods have a global data dependency, making parallelization difficult. In this thesis, we also present the process of parallelizing a code for direct numerical simulations of pipe Poiseuille flow for a distributed memory computer. / QC 20100825
50

Modelling Effects Of Insufficient Lap Splices On A Deficient Reinforced Concrete Frame

Lin, Wesley Wei-chih 01 February 2013 (has links) (PDF)
assessed and strengthened. Performance evaluation of deficient buildings has become a major concern due to devastating earthquakes in the past. In order to justify new provisions in design and assessment codes, experiments and analyses are inherently necessary. In this thesis study, investigations into the behaviour of two deficient reinforced concrete frames built at Middle East Technical University&rsquo / s Structural and Earthquake Laboratory and tested via pseudo-dynamic tests were made. These frames were modelled on the OpenSees platform by following methods of analyses outlined in the Turkish Earthquake Code of 2007 (TEC 2007) and ASCE/SEI-41-06. Both deficient frames were essentially the same, with the only difference being the presence of insufficient lap splices, which was the focus of the study. Time history performance assessments were conducted in accordance to TEC 2007&rsquo / s damage state limits and ASCE/SEI 41-06&rsquo / s performance limits. The damages observed matched the performance levels estimated through the procedure outlined in TEC 2007 rather well. Specific to the specimen with lap splice deficiencies, ASCE/SEI 41-06 was overly conservative in its assessments. TEC 2007&rsquo / s requirements for lap splice lengths were found to be conservative in the laboratory and are able to tolerate deficiencies up to 25% of the required length. With respect to mathematical models, accounting for materials in deficient systems by using nominal but reduced strength properties is not very efficient and unless joint deformations are explicitly accounted for, local deformations cannot be captured.

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