Dissertations / Theses on the topic 'Instabilities'
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1
Barnaby, Neil. "Cosmological instabilities." Thesis, McGill University, 2007. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=103365.
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Though historically the word "tachyon" has been used to describe hypothetical particles which propagate faster than the speed of light, in a more modern context the term has been recycled to refer to certain unstable states in field theory. This thesis explores the role of tachyonic instabilities in cosmology considering tachyons which arise in string theory and also more conventional, field theoretic instabilities. Our study of such instabilities is, in part, motivated by attempts to embed inflation into string theory. We will argue that the study of string theory models of inflation is well-motivated and may provide a rare potential observational window into string physics.After reviewing the necessary background material concerning inflation, cosmological perturbation theory and tachyonic instabilities we study in detail the dynamics of the tachyonic instability which marks the end of a particular string theory model of inflation, focusing on the processes of reheating and cosmic string production. We show that the peculiar dynamics of the open string tachyon leads to various novelties in these processes and consider also potential observational consequences.
We consider tachyonic preheating at the end of hybrid inflation in a conventional field theory setting and show that the preheating process can leave an observable imprint on the Cosmic Microwave Background, either through n = 4 contamination of the power spectrum or else through large nongaussian signatures. The possibility of large nongaussianity is particularly interesting since it demonstrates that hybrid inflation provides one of the few well-motivated models which can generate an observable nongaussian signature.
Finally, we study a novel string theoretic model of inflation, p-adic inflation. This model is nonlocal, however, it is free of the usual problems (such as ghosts) which plague nonlocal theories. Furthermore, the nonlocal structure of the theory leads to a variety of unexpected dynamics including the possiblity of a slowly rolling inflaton, despite an extremely steep potential.
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Yu, Rui. "Faraday Instabilities." Digital WPI, 2017. https://digitalcommons.wpi.edu/etd-theses/347.
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The shape of a liquid's surface is determined by both the body force and surface force of the liquid. In this report, the body force is solely from the gravitational force. The surface force is generated from the movement of an elastic interface between the solid and liquid. To obtain the shape of the surface, both asymptotic analysis and numerical approaches are used in this report. The asymptotic analysis is applied on the potential flow. The initial conditions are chosen to be the function of the shape of the interface between the solid and liquid and the free stream velocity far away from the interface. The time dependent contributions from the fluid system are also considered. The initial condition changes according to the function of the calculated velocity potential. The numerical approach includes two parts: calculation the velocity potential and a formalism of the change of the system as time evolves. For the first part, two idealized vertical boundaries are utilized to give a unique solution of the Laplace equation. The boundary conditions are determined as the flow under linear viscosity. For the second part, the flow is first assumed to be a potential flow, and a boundary layer is considered to make the no-slip condition valid and to give a more precise approximation for the shear stress.
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Hasan, Haider. "Nearshore hydrodynamical instabilities." Thesis, University of Nottingham, 2006. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.438557.
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Arshad, S. A. "Control of disruptive instabilities." Thesis, University of Oxford, 1991. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.291069.
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Potter, Mark. "Non-Return Valve Instabilities." Thesis, University of Oxford, 2008. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.504453.
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Barclay, Graeme James. "Instabilities in liquid crystals." Thesis, University of Strathclyde, 1998. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.366797.
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Zahniser, Russell 1982. "Instabilities of rotating jets." Thesis, Massachusetts Institute of Technology, 2004. http://hdl.handle.net/1721.1/32752.
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Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Physics, 2004.Includes bibliographical references (p. 33-34).
When a jet of water is in free fall, it rapidly breaks up into drops, since a cylinder of water is unstable. This and other problems involving the form of a volume of water bound by surface tension have yielded a wealth of theoretical and experimental results, and given insight into such phenomena as the shape of the Earth. Particularly interesting behaviors tend to emerge when the fluid in question is rotating; a drop may, for example, form a toroidal or ellipsoidal shape or even stretch out into some multi-lobed, non-axisymmetric form. In this paper, we investigate the properties of a rotating jet of water, and determine what regime of the parameter space are dominated by the various forms of instability. This is both predicted theoretically and demonstrated to be accurate experimentally. If we watch a jet of water as the rotation rate is gradually increased from zero, the drop size will start shrinking gradually, and then suddenly, rather than a single row of drops, we will see the jet breaking up into two-lobed, bar shaped forms, like the rung of a ladder. The point at which this transition occurs is characterized in terms of the rotational Bond number, B₀ = ... . The critical B₀ may be as low as 6, if there is a strong bias imparted by vibration of the table at an appropriate frequency, but for a perfectly quiescent rotating jet the second mode does not become dominant until a higher B₀. As the rotation rate is increased above this, the instability grows gradually more dramatic, and eventually the two lobes of each drop are breaking apart and flying outward. Then a transition to a third mode will occur, with three lobes in each drop; this is possible from a B₀ of 12, and dominant above a B₀ slightly higher than that. In general, mode m may occur whenever
(cont.) B₀ > m(m + 1).
by Russell Zahniser.
S.B.
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Latter, Henrik. "Instabilities in planetary rings." Thesis, University of Cambridge, 2007. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.612787.
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Sobral, Yuri Dumaresq. "Instabilities in fluidised beds." Thesis, University of Cambridge, 2008. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.612185.
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Santos, German R. "Studies on secondary instabilities." Diss., Virginia Polytechnic Institute and State University, 1987. http://hdl.handle.net/10919/49885.
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Significant advances in understanding early stages of transitional flows have been achieved by studying secondary instabilities in selected prototype flows. These secondary instabilities can be modeled as parametric instabilities of the nearly periodic flow that consists of the prototype velocity profile and a superposed finite-amplitude TS-wave (wavelength λ). The generally three dimensional secondary instabilities are governed by a linearized system of partial differential equations with periodic coefficients which are reduced to an algebraic eigenvalue problem through the application of a spectral collocation method
Following Floquet theory, previous analysis looked for subharmonic (wavelength 2 λ) and fundamental (wavelength λ) types of solutions. We extend the Floquet theory to solutions having arbitrary wavelengths, hence including the previous solutions as special cases. Modes with wavelength in between the subharmonic and fundamental values are called detuned modes. Detuned modes lead to combination resonance which has been observed in controlled transition experiments. Knowledge of the bandwidth of amplified detuned or (combination) modes is very important for clarification of the selectivity of the early stages of transition with respect to initial disturbances.
We have selected two flows: the Blasius boundary layer flow and the hyperbolic-tangent free-shear flow as prototypes of wall bounded Hows and unbounded Hows, respectively. In the Blasius flow we have concentrated on studying detuned modes. We found the growth rates of modes slightly detuned from the subharmonic wavelength to be almost as large as the growth rate of the subharmonic itself. This result is consistent with both the broadband spectra centered at subharmonic frequency observed in the "biased" experiment of Kachanov & Levchenko, wherein only the TS frequency was introduced, and with the large band-width of resonance in the "controlled" experiments, wherein a TS wave and the detuned modes were introduced simultaneously.
In the free-shear flow, our goals were three-fold. The first was to investigate whether the Floquet analysis based on the shape assumption for TS waves would provide results consistent with results for the stability of Stuart vortices. Second, we aimed at revealing the effect of viscosity on these results. Finally, we wanted to evaluate a group of spectral methods for the numerical treatment of the flow in an unbounded domain. We have made a detailed analysis of subharmonic, fundamental, and detuned modes. Results display the basically inviscid, convective character of the secondary instabilities, and their broadband nature in the streamwise and spanwise directions. In the inviscid limit, and for neutral TS waves, a detailed comparison is made with the closely related study on stability of Stuart vortices by Pierrehumbert & Widnall. Good quantitative agreement is obtained. For a wide range of Reynolds numbers and amplitudes of the 2-D primary wave, results reveal that the most unstable subharmonic modes are two-dimensional (vortex pairing). On the other hand, the most unstable fundamental modes are three-dimensional, with short spanwise wavelengths. Detuned modes have characteristics in between, being most unstable in the two-dimensional or three-dimensional form depending on the detuning value.
Comparisons of our results for a superposed TS wave of constant amplitude with results obtained by numerical simulations suggested that the growth of the TS wave may have a significant effect on the secondary disturbance growth. To check this hypothesis, we have developed a numerical method that accounts for small variations in the TS amplitude. However, the results indicate that the discrepancies are due to other yet concealed effects.Ph. D.
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Moenne-Loccoz, Victor. "Analyse expérimentale des instabilités aérodynamiques dans un compresseur centrifuge de nouvelle génération." Thesis, Lyon, 2019. http://www.theses.fr/2019LYSEC005.
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L’étude effectuée au cours de cette thèse a permis la caractérisation expérimentale des instabilités aérodynamiques se développant dans un compresseur centrifuge et une première évaluation de l’efficacité d’une stratégie de contrôle par aspiration de couche limite. Le compresseur, développé par Safran Helicopter Engines et dénommé Turbocel, est composé d’une roue directrice d’entrée, d’un rouet centrifuge splitté, d’un diffuseur radial aubé et splitté et d’un redresseur axial. Des travaux numériques antérieurs réalisés au Laboratoire de Mécanique des Fluides et d’Acoustique ont montré, aux bas régimes de rotation, un comportement singulier caractérisé par une structure d’écoulement dite « alternée » impliquant deux canaux adjacents du diffuseur radial. L’étude stationnaire réalisée sur l’ensemble des régimes de rotation du compresseur a conduit à une ségrégation des vitesses de rotation suite à l’établissement d’une variable– le taux d’asymétrie - caractérisant l’asymétrie de l’aérodynamique du diffuseur. Ce taux, quasi nul à très basse vitesse de rotation, croît jusqu’à atteindre un maximum à vitesse de rotation intermédiaire, puis s’effondre pour ré-augmenter légèrement. Une analyse fine des données instationnaires acquises à bas régimes a permis la description de deux modes de fonctionnement du compresseur associés à des structures de décollements différentes dans le diffuseur. Le premier mode est caractérisé par l’oscillation à une fréquence de l’ordre de 42 Hz d’un décollement localisé sur la face en dépression des aubes principales du diffuseur. Le second mode, à 12Hz, associé au pompage modéré du compresseur, correspond à la mise en place d’un schéma alterné et à son oscillation sur deux canaux adjacents du diffuseur.Les origines probables de ces différents modes de fonctionnement sont discutées à partir de considérations • aérodynamiques -- la mise en place d’une recirculation en tête de rouet est suspectée d’influer sur le taux d’asymétrie en modifiant l’incidence en entrée de diffuseur,• géométriques -- le nombre et le calage des aubes du diffuseur radial ainsi que la distance inter-roue indiquent une prédisposition du diffuseur à fonctionner en régime alterné sous certaines conditions d’incidence,• aéro-acoustiques -- un accrochage des fréquences aérodynamiques avec les fréquences des ondes acoustiques du banc d’essai semble se produire. Enfin, les résultats sur le contrôle d’écoulement par aspiration de couche limite à régime partiel sont présentés. Une amélioration du rendement est observée à certains points de fonctionnement, mais aucune extension de la plage de fonctionnement du compresseur n’est mesurée. Sans l’atténuer, l’aspiration permet de contrôler sur quels canaux s’établit le régime alternéThis thesis presents an experimental characterization of the evolution of aerodynamic instabilities in a centrifugal compressor, and a first evaluation of the effectiveness of boundary layer suction as a control strategy. The compressor used in this study is Turbocel, a centrifugal compressor developed by Safran Helicopter Engines, featuring inlet guide vanes, a backswept splittered unshrouded impeller, a splittered vaned radial diffuser and axial outlet guide vanes.Previous numerical work, conducted at the Laboratoire de Mécanique des Fluides et d’Acoustique de Lyon, revealed a unusual behaviour of the compressor at low rotational speeds characterized by a distinctive alternate flow structure in the radial diffuser that develops across two adjacent blade channels. The steady analysis, which was conducted over the full operating range of rotational speeds, led to the distinction of different operating zones, following the establishment of a new indicator variable - the asymmetry rate - characterizing the asymmetry of the diffuser aerodynamics. This rate, which is close to zero at very low rotation speed, increases until it reaches a maximum value at intermediate rotational speed, before collapsing and slightly increasing again near the nominal rotational speed.Analysis of the unsteady data acquired at low speeds allowed for the characterization of two compressor operating modes, associated with different flow phenomena in the stalled diffuser. The first mode is characterized by the oscillation of a separation at 42 Hz, on the suction side of the main blades in the diffuser. The second mode, at 12Hz, associated with mild surge of the compressor, corresponds to the emergence of an alternate pattern of unsteady flow separation that occurs across two adjacent channels of the diffuser.The probable causes for these different operating modes are discussed in the context of different considerations:• aerodynamic -- the formation of a recirculation near the tip of the impeller is suspected to affect the asymmetry rate by changing the incidence angle at the diffuser inlet.• geometric -- the number and the stagger angle of the radial diffuser blades as well as the distance between the impeller and the diffuser may result in a predisposition of the diffuser to operate in an alternating mode, under certain conditions of incidence.• aero-acoustic -- as there is evidence of a lock-in of the aerodynamic frequencies with the acoustic modes of the test rig.Finally, boundary layer suction is explored as a means of flow control at partial rotational speed. Improvements in performance were observed for some operating points, however no extension of the compressor operating range was measured. Although boundary layer suction did not allow for the intensity of the oscillating separation pattern in the diffuser to be reduced, it was found to be an effective means of controlling the location of the alternate flow structure in the diffuser
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Marant, Mathieu. "Contrôle en boucle ouverte d'un écoulement tridimensionnel décollé par perturbations optimales." Phd thesis, Toulouse, INPT, 2017. http://oatao.univ-toulouse.fr/19494/1/MARANT_Mathieu.pdf.
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On calcule les amplifications d’énergie optimales de structures quasi longitudinales dans le sillage d’un corps épais axisymétrique à culot droit et dans une couche de mélange parallèle. Les amplifications d'énergie sont seulement modérées dans le sillage du corps axisymétrique tandis qu'elles sont grandes dans la couche de mélange. Les amplifications maximales augmentent avec le nombre de Reynolds et lorsque le nombre d’onde transverse (azimutal) décroît. Les structures amplifiées optimalement sont des stries longitudinales. Lorsqu’elles sont forcées à amplitudes finies, les stries optimales réduisent considérablement l’instationnarité du sillage du corps épais axisymétrique. Pour des nombres de Reynolds modérés, l’instationnarité du sillage peut être complètement supprimée si le forçage optimal est combiné avec un soufflage au culot uniforme. Dans le cas de la couche de mélange 2D, le taux de croissance maximal de l'instabilité de KelvinHelmholtz et le ratio de vitesse critique d'apparition de l'instabilité absolue peuvent être soit réduits soit augmentés en fonction des symétries des stries forcées. Dans ce cas, on montre que la déformation non linéaire moyenne doit être incluse dans l'analyse de sensibilité de l'instabilité et que cela n'influe pas sur la dépendance quadratique par rapport à l'amplitude des stries.
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Vavriv, Dmytro. "Chaotic instabilities and their applications." Göttingen Cuvillier, 2009. http://d-nb.info/998762474/04.
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Hu, Bin. "Characterization of gas-lift instabilities." Doctoral thesis, Norwegian University of Science and Technology, Department of Petroleum Engineering and Applied Geophysics, 2005. http://urn.kb.se/resolve?urn=urn:nbn:no:ntnu:diva-1866.
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This dissertation mainly investigates the occurrence and characteristics of density-wave instability in gas-lift wells. The investigation is based on a simplified gas-lift system, in which water and air are used as producing fluid and lifting gas respectively, and heat transfer effect is neglected.
To carry out the investigation, both linear stability analysis and numerical simulation are performed. The linear stability analysis is based on a homogenous two-phase flow model and the numerical simulation is done by using a commercial available dynamic multiphase flow simulator. In this way, a crosscheck between the two methods can be made in order to gain confidence about the results. Both two methods are validated against casing heading problem before they are applied to density-wave instability study.
The results show that it is possible for density-wave instability to occur in those gas-lift wells producing from depleted reservoirs. The linear stability analysis and numerical simulation give the similar parametric trend in characterizing the instability. Within the normal gas-lift operation parameter range, increasing reservoir pressure and gas injection rate increases stability, but increasing tubing diameter, productivity index and system pressure decreases stability. The instability may occur only when the well loses its capability of natural flowing.
Dynamic simulation also shows that the average production rate could be significantly reduced due to the unstable gas-lift compared with the steadystate prediction. An attempt of using feedback control to stabilize the gas-lift system is also tested by using the simulator. Promising results are obtained from the test in both stabilization and increasing production.
The results of this dissertation add new knowledge to gas-lift instability fundamentals and can help in diagnosing and remedying unstable gas-lift problems.
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Li, Rui-Qing. "Hydrodynamic instabilities of cylindrical interfaces." Thesis, McGill University, 1992. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=39521.
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This thesis consists of two parts. In the first part of the thesis, the Rayleigh-Taylor and the Kelvin-Helmholtz instabilities of a cylindrical interface between two inviscid fluids or two viscous fluids are analyzed from first principles (momentum and continuity equations). Dispersion equations, relating wavenumber, k, to growth rate, G, were derived for various conditions. Application of the dispersion equations to film boiling on a cylindrical heater and to breakup of a liquid film around a cylindrical body led to the development of mathematical models for the prediction of the dominant wavelengths formed during these processes for both inviscid and viscous fluids.Experiments were carried out to measure the dominant unstable wavelength during the breakup of a liquid film around a cylindrical body. It was found that the dominant wavelength decreased with a decrease in the radius of the cylindrical body in agreement with the present theory and in contradiction to previously published work.
In another application of the present theory, the breakup of a cylindrical liquid-in-gas jet and a cylindrical gas-in-liquid jet was analyzed based on the Kelvin-Helmholtz instability. It was predicted that the dominant wavelength decreased rapidly with an increase in the jet velocity.
In the second part of the thesis, gas injection through a very narrow slot into a liquid is examined extensively. A modified bubble formation model is proposed taking into consideration the surface tension force and the inertial force.
When gas was injected into liquid through a very narrow slot (50-250$ mu$m), three different bubbling regimes were found as the flow rate of gas was increased. They were: regular bubble regime at low flow rates, coalescence bubble regime at medium flow rates, and gas globe regime at high flow rates. The gas-dispersion characteristics of each of the regimes were discussed and mathematically analyzed. In the regular bubble regime, the bubble formation was dominated by both surface tension force and inertial force. In the coalescence bubble regime, the formation of bubbles was dominated by inertial forces only. In the gas globe regime, due to the Rayleigh-Taylor instability, multiple bubbles were formed at separate nodes of a continuous gas blanket extending the length of the slot. The critical transition condition between the regular bubble formation regime and the coalescence bubble regime is given.
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Hoyle, Rebecca Bryony. "Instabilities of three-dimensional patterns." Thesis, University of Cambridge, 1993. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.318044.
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Edwards, D. A. "Stellar instabilities in cataclysmic variables." Thesis, University of Oxford, 1986. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.370249.
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Morris, A. W. "Large scale instabilities in tokamaks." Thesis, University of Oxford, 1985. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.379925.
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Rashid, Fayyaz Qamar. "Instabilities in the solar tachocline." Thesis, University of Leeds, 2010. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.534836.
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Dasgupta, T. "Instabilities in nonperturbative string theory." Thesis, University of Cambridge, 2001. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.598293.
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In chapter 3 the relation between two different classes of perturbative non-BPS bi-spinor states of heterotic string theory and certain non-perturbative non-BPS D-brane states of the dual type I' theory is exhibited. The domains of stability of these states as well as their decay products in both theories are determined and shown to agree with the duality map. In chapter 4 the effects of the non-BPS D-instanton in type I theory and its M-theory origin is described. The starting point is the tree-level amplitude for the scattering of two gauge particles in the Hořava-Witten formulation of M-theory. At low momenta this exactly reproduces the corresponding tree-level scattering amplitude of the E8 x E8 heterotic string theory. After compactification to nine dimensions this amplitude is used to describe the scattering of two massive SO(16) spinor states. The non-BPS D-instanton component of this amplitude is explicitly determined from this expression. In chapter 5 the renormalization group method is used to study tachyon condensation on bosonic D25-brane. The decay of the D25-brane is controlled by a nearby IR fixed point representing D24-branes. The boundary entropy corresponding to the D24-brane tension is calculated in leading order in perturbation theory and agrees with the expected result to an accuracy of 8%. Multicritical behaviour of the IR theory suggests that the end point of the flow represents a configuration of two D24-branes. An analogy with Kondo physics is discussed. Chapter 6 ongoing developments in the context of little string theory and matrix theory.
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Horseman, Nicola Jane. "Nonlinear instabilities of curved flows." Thesis, University of Exeter, 1991. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.293981.
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McLeod, Andrew Duncan. "Magnetic buoyancy instabilities and magnetoconvection." Thesis, University of Newcastle Upon Tyne, 1996. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.318602.
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Cafolla, Gerard James. "Hydroelastic instabilities of compliant panels." Thesis, University of Warwick, 1997. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.323370.
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Geraldes, Paulo. "Instabilities in transonic cavity flows." Thesis, Cranfield University, 2005. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.431961.
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Huang, X. "Active control of aerodynamic instabilities." Thesis, University of Cambridge, 1988. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.237877.
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Woodcock, Jonathan Peter. "Simulations of space plasma instabilities." Thesis, Queen Mary, University of London, 1997. http://qmro.qmul.ac.uk/xmlui/handle/123456789/131.
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This work describes computer simulations of the behaviour of plasmas similar to those observed in the near Earth environment The work can be split into three main threads Firstly we have developed a set of algorithms to allow the implementation of particle type simulation models on parallel computer architectures ranging from small workstation clusters to massively parallel supercomputers These algorithms allow large simulations with many particles to be performed We address the problems of e cient use of available computational resources and the scaling of algorithms as computers get larger Secondly we use a parallel implementation of a two dimensional hybrid simulation code with periodic boundaries to explore the evolution of ion beam distributions similar to those observed upstream of the Earth s bow shock We follow the evolution of the resonant instabilities of these cool tenuous proton beams both isotropic and anisotropic in temperature into the non linear regime We examine the waves generated their e ects on the ion distribution function the phenomenon of gyrophase bunching and describe the life cycles of two dimensional magnetic features including oblique propagating shocklets We suggest that such two dimensional structures may play a role in the saturation of beam instabilities Coherence lengths of the waves are calculated We see some evidence of anisotropy driven mirror waves late on in these simulations Thirdly we explore the nature of parametric instabilities in two dimensions We examine the role of parametric or wave wave instabilities in the late evolution of beam instability generated waves We nd little evidence of any parametric instability in this case The two dimensional evolution of a wave known to be unstable to one dimensional parametric instability is described We nd that in this case the instability develops in a manner similar to that found in one dimensional simulations although with some angular broadening in wavevector space There is some evidence of anisotropy driven instabilities later in the simulation
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Sitnikovs, Dmitrijs. "Instabilities in quasi-efficient markets." Thesis, Aston University, 2017. http://publications.aston.ac.uk/37532/.
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This thesis studies ways of modelling instabilities in quasi-efficient markets. We consider quasi-efficient markets where arbitrage is possible, but is relatively small and short lived. Under such a assumption we derive optimal arbitrage strategy of one agent and consider possible ways of funding optimal strategy under stop-loss constraint. Optimal strategy is used to build a multi-agent model which defines the arbitrage dynamics, i.e. its mean- reverting behaviour. The influence of agents on the asset prices is modelled by means of permanent price impact function. Multi-agent model is self-consistent as it creates mean-reverting term of the same type under which the optimal strategy for one agent was derived. As we show adding stop-loss constraint creates possibility for market instabilities.
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Dupuy, Benjamin 1981. "Instabilities in particle-laden flows." Thesis, Massachusetts Institute of Technology, 2004. http://hdl.handle.net/1721.1/17765.
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Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Aeronautics and Astronautics, 2004.Includes bibliographical references (p. 85-90).
Particles are present in many industrial processes and in nature. Dry granular flows and suspensions have been well studied and present a broad range of problems in terms of rheology and instabilities. In both cases, new phenomena are continually discovered. Indeed, complex behavior can be exhibited whether the granular media acts like a solid, a liquid or a gas. Moreover, the flow of thin liquid films on solid surfaces is a significant phenomenon in nature and in industrial processes (e.g. manufacture of computer chips, solar power cells, capacitors, etc.), where uniformity and completeness of wetting are paramount in importance. Two instabilities in particle-laden flows down an inclined plate are experimentally investigated and compared to theoretical predictions and scaling analysis. One of the fundamental coating flow geometries is the simple wetting of an inclined plane by a thin and uniform liquid film draining under gravity. It is well-known that, in a clear fluid, this leads to a fingering instability of the contact line. We perform experiments in this geometry using a mixture of silicone oil (1000 cSt) and heavy glass particles (d = 2.6) with a broad range of beads diameters, concentrations and angles. Small beads (106-212 [mu]m) do not present any new behavior since they are just convected by the flow and do not sediment fast enough to perturb it. A suspension with larger particles (250-425 ,[mu]m) exhibits different regimes depending on the angle and the concentration. For very high slopes and very dense suspensions a large ridge is formed at the contact line and does not break in regular fingers as observed previously. Quantitative measurements are performed using a laser sheet coupled
(cont.) to a high-performance digital camera and compared to scaling analysis. A flow of suspension with no contact line may also be gravitationally instable. Shear-induced migration of particles can provoke an inversion of density which is the driving force of a Rayleigh-Taylor instability. Qualitative experimental results are compared to recent theories.
by Benjamin Dupuy.
S.M.
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Shackleton, Henry (Henry J. ). "Diffusional instabilities on curved manifolds." Thesis, Massachusetts Institute of Technology, 2018. http://hdl.handle.net/1721.1/120212.
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Thesis: S.B., Massachusetts Institute of Technology, Department of Physics, 2018.Cataloged from PDF version of thesis.
Includes bibliographical references (pages 61-64).
Diffusionally-driven instabilities provide a versatile mechanism for pattern formation, and have found applications in modeling a variety of biological and chemical systems. Although pattern formation has been observed in systems with a variety of geometries, the theoretical study of diffusional instabilities has primarily been restricted to systems of uniform curvature, such as flat planes, spheres, and cylinders. In this thesis, I study a method of analyzing pattern formation on more generally deformed surfaces, with a focus on perturbatively calculating effects due to small deformations from geometries of uniform curvature. Analytical predictions, ranging from pattern modifications to pinning of pattern development, are obtained on deformed drums, cylinders, and spheres. These predicted effects are compared to numerical studies, and additional cases where our analytical methods break down are studied numerically. Finally, the interplay between advection and non-uniform curvature is studied numerically.
by Henry Shackleton.
S.B.
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Aessopos, Angelica. "Surfactant instabilities on thin films." Thesis, Massachusetts Institute of Technology, 2005. http://hdl.handle.net/1721.1/88902.
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Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2005."June 2005."
Includes bibliographical references (pages 89-92).
The deposition of a surfactant drop over a thin liquid film may be accompanied by a fingering instability. In this work, we present experimental results which identify the critical parameters that govern the shape and extend of the fingering phenomena. It was found that the normalized wavelength, [lambda]/t, scales with Marangoni number, Ma=[Delta][gamma]t/[mu]D, to the -1 exponent for any Marangoni higher than 4.3 · 10⁷. On the other end, for Marangoni < 4.3 · 10⁷ the normalized wavelength scales with Ma to the -0.4 but becomes in addition a funcion of the Prandtl number, Pr=[upsilon]/D, which demonstrates the critical significance of bulk diffusion on the spreding behavior. Finally, we present a numerical implementation of a mathematical model which is capable of reproducing the experimentally observed trends.
by Angelica Aessopos.
S.M.
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Lucca-Negro, Oona. "Modelling of swirling flow instabilities." Thesis, Cardiff University, 1999. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.310677.
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This research concentrates on the swirl motion, and in particular the flow structure which develops under its action, in swirl burner/furnace systems. Although the Reynolds numbers for such systems are usually large and well into the turbulent regime, periodic oscillations and associated instabilities are still prevalent. The predominant coherent structure is the so-called precessing vortex core (PVC) which is a three-dimensional, time-dependent phenomenon. It is helical in shape, twisted against the flow, and precesses around the geometric centre of the system, in the sense of the flow. The aim of this work was to numerically model this instability in a 2MW industrial-size system, under isothermal conditions. A fully three-dimensional, time-dependent model was developed using the CFD (Computational Fluid Dynamics) software FLUENT. This study first presents an overview of publications on vortex breakdown, a similar phenomenon observed initially on delta wings, in order to highlight its significant features. A summary was also made of various recent studies, experimental and theoretical, carried out at Cardiff University, in the same equipment as used in the present work. This review allows a better understanding of the phenomenon and constitutes a basis for further validation of the mathematical model. Numerous flow pattern characteristics have- been predicted, which agree qualitatively with different published studies, such as crescent shaped regions of maximum axial and tangential velocities, off-centred reverse flow zone, and spiralling vortex core. Quantitatively, the agreement is good, in terms of range of velocities and frequency. However, the predicted flow pattern could. not be maintained in time and tended back to axisymmetry, possibly due to numerical diffusion. Grid refinement could not, however, be envisaged due to the practical limits of the available machines. Nevertheless, these results are encouraging and prove that mathematical modelling of these complex flows is a realistic objective.
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Holton, David. "Convective instabilities in binary fluids." Thesis, University of Warwick, 1989. http://wrap.warwick.ac.uk/102295/.
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The subject of two-dimensional convection in a binary fluid is treated by analytical methods and through Galerkin models. The analysis will focus on describing the dynamics of convection at onset of convection. Two independent dynamical parameters are present- one more degree of freedom than single fluid convection. We shall derive normal forms for the tricritical bifurcation - describing the transition between a forward and backward pitchfork bifurcation of a two-dimensional array of rolls in a convecting bulk binary fluid mixture. A multiple time perturbation scheme is constructed to fifth order to describe this motion. The coefficients of the equation are determined as a function of the Lewis number (the ratio of the mass to thermal diffusivity). The degenerate Hopf bifurcation is also investigated using a similar perturbative scheme; with a prediction of the coefficients involved. A model system, using a ’minimal representation’ (Veronis 1968) gives rise to a Galerkin truncated scheme (a set of 14 ordinary differential equations). It is claimed that the dynamical character of both the tricritical and degenerate Hopf bifurcation are included in the zoology of the bifurcation behaviour at onset of convection. These and other dynamical aspects of the equations are investigated. In an attempt to improve upon free slip pervious boundaries a projection of the equations is made onto a more appropriate subspace. A comparison with experimental evidence is given.
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Masad, Jamal A. "Secondary instabilities of boundary layers." Diss., Virginia Tech, 1990. http://hdl.handle.net/10919/37867.
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Several aspects of the subharmonic instability of boundary layers are studied. First, the subharmonic instability of incompressible flows over a flat plate is investigated using the resonant triad model and the Floquet model. The primary wave is taken in the form of a two-dimensional (2-0) Tollmien-Schlichting (T-S) wave. The subharmonic wave is taken in the form of a three-dimensional (3-D) wave. Results from both models are presented and compared with the experimental data and numerical simulation. It is found that the results of the Floquet model are in good agreement with the experimental data and numerical simulation, whereas the results of the resonant triad model agree only qualitatively with the experimental data.
Second, the subharmonic instability of incompressible flows over a 2-0 hump is studied using the Floquet model. The mean flow over the hump is calculated by using interacting boundary layers, thereby accounting for viscid/inviscid interactions. The results show that increasing the hump height results in an increase in the amplification factors of the primary and subharmonic waves. When the hump causes separation, the growth rates of both the primary and subharmonic waves are considerably larger than those obtained in the case of no separation.
Third, the subharmonic instability of compressible boundary layers over a flat plate is studied using the Floquet model. Results are presented for adiabatic wall boundary conditions and subsonic, transonic, and supersonic flows. For supersonic flows results are presented for first- and second-mode primary waves. The effect of Mach number, spanwise wavenumber, primary-wave amplitude, Reynolds number, and frequency are studied.
Fourth, results for the effect of heat transfer on the subharmonic instability of a two-dimensional compressible boundary layer over a flat plate are presented for different Mach numbers. For supersonic flows results are presented for first- and second-mode waves. The effect of different levels of heat transfer on changing the features of the subharmonic compressible instability is evaluated.
Fifth, results for the effect of suction on the subharmonic instability of a two-dimensional compressible boundary layer over a flat plate are presented. It is found that when the primary wave is a first-mode merging with a second-mode, the subharmonic wave is strongly destabilized by suction.
Sixth, the effect of a bulge on the subharmonic instability of compressible boundary layers is studied. It is found that the effect of compressibility on reducing the growth rate of the disturbances weakens as the hump height increases.Ph. D.
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Reidy, Kelly E. "Fermi liquids near Pomeranchuk instabilities." Kent State University / OhioLINK, 2014. http://rave.ohiolink.edu/etdc/view?acc_num=kent1406286277.
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Nguyen, Lien K. "Predicting catastrophic BGP routing instabilities." Thesis, Monterey, California. Naval Postgraduate School, 2004. http://hdl.handle.net/10945/1642.
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Approved for public release, distribution is unlimitedInter-domain routing connects individual pieces of Internet topology, creating an integral, global data delivery infrastructure. Currently, this critical function is performed by the Border Gateway Protocol (BGP) version 4 [RFC1771]. Like all routing protocols, BGP is vulnerable to instabilities that reduce its effectiveness. Among the causes of these instabilities are those which are maliciously induced. Although there are other causes, e.g., natural events and network anomalies, this thesis will focus exclusively on maliciously induced instabilities. Most current models that attempt to predict a BGP routing instability confine their focus to either macro- or micro-level metrics, but not to both. The inherent limitations of each of these forms of metric gives rise to an excessive rate of spurious alerts, both false positives and false negatives. It is the original intent of this thesis to develop an improved BGP instability prediction model by statistically combining BGP instability metrics with user level performance metrics. The motivation for such a model is twofold. 1) To provide sufficient prior warning of impending failure to facilitate proactive protection measures. 2) To improve warning reliability beyond existing models, by demonstrably reducing both false positives and false negatives. However, our analysis of actual network trace data shows that a widely used BGP instability metric, the total number of update messages received in a time period, is not a good indicator of future user level performance.
Civilian, Department of Defense
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Soma, Kovács István. "Simplified Simulator for BWR Instabilities." Thesis, KTH, Fysik, 2017. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-210626.
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Shinn-Mendoza, Rachel. "Shocks and instabilities in traffic." Diss., Pasadena, Calif. : California Institute of Technology, 1990. http://resolver.caltech.edu/CaltechETD:etd-11132007-085544.
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Dold, Dominic Nicolas. "Instabilities in asymptotically AdS spacetimes." Thesis, University of Cambridge, 2018. https://www.repository.cam.ac.uk/handle/1810/273930.
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In recent years, more and more efforts have been expended on the study of $n$-dimensional asymptotically anti-de Sitter spacetimes $(\mathcal{M},g)$ as solutions to the Einstein vacuum equations \begin{align*} \mathrm{Ric}(g)=\frac{2}{n-2}\Lambda\, g \end{align*} with negative cosmological constant $\Lambda$. This has been motivated mainly by the conjectured instability of these solutions. The author of this thesis joins these efforts with two contributions, which are themselves independent of each other. In the first part, we are concerned with a superradiant instability for $n=4$. For any cosmological constant $\Lambda=-3/\ell^2$ and any $\alpha < 9/4$, we find a Kerr-AdS spacetime $(\mathcal{M},g_{\mathrm{KAdS}})$, in which the Klein-Gordon equation \begin{align*} \Box_g\psi+\frac{\alpha}{\ell^2}\psi=0 \end{align*} has an exponentially growing mode solution satisfying a Dirichlet boundary condition at infinity. The spacetime violates the Hawking-Reall bound $r_+^2 > |a|\ell$. We obtain an analogous result for Neumann boundary conditions if $5/4 < \alpha < 9/4$. Moreover, in the Dirichlet case, one can prove that, for any Kerr-AdS spacetime violating the Hawking-Reall bound, there exists an open family of masses $\alpha$ such that the corresponding Klein-Gordon equation permits exponentially growing mode solutions. Our result provides the first rigorous construction of a superradiant instability for a negative cosmological constant. In the second part, we study perturbations of five-dimensional Eguchi-Hanson-AdS spacetimes exhibiting biaxial Bianchi IX symmetry. Within this symmetry class, the Einstein vacuum equations are equivalent to a system of non-linear partial differential equations for the radius $r$ of the spheres, the Hawking mass $m$ and $B$, a quantity measuring the squashing of the spheres, which satisfies a non-linear wave equation. First we prove that the system is well-posed as an initial-boundary value problem around infinity $\mathcal{I}$ with $B$ satisfying a Dirichlet boundary condition. Second, we show that initial data in the biaxial Bianchi IX symmetry class around Eguchi-Hanson-AdS spacetimes cannot form horizons in the dynamical evolution.
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Lebranchu, Yannick. "Étude d'ondes non linéaires hydrodynamiques : approches théorique et expérimentale." Thesis, Vandoeuvre-les-Nancy, INPL, 2008. http://www.theses.fr/2008INPL005N/document.
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Cette thèse est dédiée à l'étude d'ondes non linéaires dans des écoulements en rotation. Dans une première partie, je me suis intéressé aux ondes de Rossby apparaissant par instabilités de thermoconvection dans une coquille sphérique en rotation représentant un modèle simplifié de noyau planétaire tellurique, et ce pour deux types de forçage: un chauffage interne correspondant à une activité radioactive du noyau et un chauffage différentiel lié à la différence de température entre les frontières interne et externe. Selon le théorème de Proudman-Taylor, l'écoulement possède une faible dépendance en la coordonnée axiale à cause de la rotation rapide. Cela permet de simplifier les modèles 3D en des modèles quasi géostrophiques 2D reposant sur une intégration axiale. Cette thèse présente la première comparaison systématique entre modèles 2D et 3D (Simitev, U-Glasgow) concernant des ondes de Rossby faiblement non linéaires. En 2D l'équation de Landau régissant l'amplitude de l'onde critique est calculée; l'amplitude de la convection et celle des écoulements zonaux ainsi prédites se comparent assez bien aux résultats 3D. L'existence d'une bifurcation sous-critique est établie à très bas nombre d'Ekman en chauffage interne et en chauffage différentiel, à condition dans ce dernier cas que le nombre de Prandtl soit petit. La seconde partie est une étude expérimentale de l'écoulement d'eau et de ses premières instabilités dans un canal annulaire creusé dans un plateau éventuellement en rotation surmonté d'un couvercle tournant. Trois cas sont étudiés: le cisaillement pur correspondant à la rotation du couvercle seul, la corotation rapide et la contrarotation pure. Le seuil d'instabilité détecté par mesures globales (visualisations par caméra vidéo) et locales par Vélocimétrie Laser Doppler se caractérise par des ondes spiralées. Dans le cas de la contrarotation pure, des structures localisées dans l'espace-temps peuvent coexister avec les ondes. Une comparaison est effectuée avec des calculs numériques (Serre, CNRS-Marseille). Un accord relativement bon est obtenu pour l'écoulement de base (vitesse azimutale) et la première instabilité (nombre de Reynolds, nombre d'onde et fréquence angulaire critiques)A first part is devoted to the study of the Rossby waves that appear in a rotating spherical shell representing the core of a terrestrial planet by thermal instabilities for two heating types. Internal heating is driven by radioactive sources and differential heating is driven by a difference of temperature between the internal and external frontiers. According to the Proudman-Taylor theorem, the flow depends only weakly on the axial coordinate because of the high rotation rate. Thus the 3D models can be simplified into quasi-geostrophic 2D models \textit{via} an axial integration. I present the first systematic comparison between 2D and 3D models (Simitev, U-Glasgow) for weakly nonlinear Rossby waves. In 2D the Landau equation that controls the amplitude of the critical wave is calculated. Predicted convection' amplitude and zonal flows agree rather well with the 3D results. The existence of a subcritical bifurcation is established at very low Ekman numbers with internal and differential heating; in this latter case, the Prandtl number also has to be small for the bifurcation to be subcritical. The second part is an experimental study of water flows and its first instabilities in an annular channel digged in a plate which may rotate, and which is sheared by a rotating lid. Three cases are studied: a pure shear where only the lid turns, a rapid corotation and a pure contrarotation. The onset of instability is studied with global measurements (using a video camera) and local ones (Laser Doppler Anemometry) and is characterized by spiralling waves. In the case of contrarotation, patterns localized in space and time may coexist with the waves. The comparison of these results with numerical ones (Serre, CNRS-Marseille) is done and shows a rather good agreement for the basic azimutal flow and the first instability (critical Reynolds number, wavenumber and angular frequency)
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Lorenzani, Silvia. "Fluid instabilities in precessing ellipsoidal shells." [S.l. : s.n.], 2001. http://deposit.ddb.de/cgi-bin/dokserv?idn=964398702.
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Lin, Min-Kai. "Dynamical instabilities in disc-planet interactions." Thesis, University of Cambridge, 2012. https://www.repository.cam.ac.uk/handle/1810/245135.
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Protoplanetary discs can be dynamically unstable due to structure induced by an embedded giant planet. In this thesis, I discuss the stability of such systems and explore the consequence of instability on planetary migration. I present semi-analytical models to understand the formation of the unstable structure induced by a Saturn mass planet, which leads to vortex formation. I then investigate the effect of such vortices on the migration of a Saturnmass planet using hydrodynamic simulations. I explain the resulting nonmonotonic behaviour in the framework of type III planetary migration. I then examine the role of disc self-gravity on the vortex instabilities. It can be shown that self-gravity has a stabilising effect. Linear numerical calculations confirms this. When applied to disc-planet systems, modes with small azimuthal wavelengths are preferred with increasing disc selfgravity. This is in agreement the observation that more vortices develop in simulations with increasing disc mass. Vortices in more massive discs also resist merging. I show that this is because inclusion of self-gravity sets a minimal vortex separation preventing their coalescence, which would readily occur without self-gravity. I show that in sufficiently massive discs vortex modes are suppressed. Instead, global spiral instabilities develop. They are interpreted as disturbances associated with the planet-induced structure, which interacts with the wider disc leading to instability. I carry out linear calculations to confirm this physical picture. Results from nonlinear hydrodynamic simulations are also in agreement with linear theory. I give examples of the effect of these global modes on planetary migration, which can be outwards, contrasting to standard inwards migration in more typical disc models. I also present the first three-dimensional computer simulations examining planetary gap stability. I confirm that the results discussed above, obtained from two-dimensional disc approximations, persist in three-dimensional discs.
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Miller, Joel C. "Shear flow instabilities in viscoelastic fluids." Thesis, University of Cambridge, 2006. https://www.repository.cam.ac.uk/handle/1810/245318.
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This dissertation is concerned with the theoretical study of the stability of viscoelastic shear flows. It is divided into two parts: part I studies inertialess coextrusion flows at large Weissenberg number where the instabilities are due to discontinuities in the elastic properties, and part II studies the effect of elasticity on the well-known inertial instabilities of inviscid flows with inflection points. We begin part I with a previously known short-wave instability of Upper Convected Maxwell and Oldroyd–B fluids at zero Reynolds number in Couette flow. We show that if the Weissenberg number is large, the instability persists with the same growth rate when the wavelength is longer than the channel width. Intriguingly, surface tension does not modify the growth rate. Previous explanations of elastic interfacial instabilities based on the jump in normal stress at the interface cannot apply to this instability. These results are confirmed both numerically and with asymptotic methods. We then consider Poiseuille flow and show that a new class of instability exists if the interface is close to the center-line. We analyse the scalings and show that it results from a change in the boundary layer structure of the Couette instability. The growth rates can be large, and the wavespeed can be faster than the base flow advection. We are unable to simplify the equations significantly, and asymptotic results are not available, so we use numerics to verify the results. In studying these instabilities we encounter some others which we mention, but do not analyse in detail. In part II we study the effect of elasticity on the inertial instability of flows with inflection points. We show that the elasticity modifies the development of cat’s eyes. The presence of extensional flow complicates the analysis. Consequently we use the FENE–CR equations.
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Rosin, Mark. "Instabilities and transport in magnetized plasmas." Thesis, University of Cambridge, 2011. https://www.repository.cam.ac.uk/handle/1810/237241.
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In a magnetized plasma, naturally occurring pressure anisotropies facilitate instabilities that are expected to modify the transport properties of the system. In this thesis we examine two such instabilities and, where appropriate, their effects on transport. First we consider the collisional (fluid) magnetized magnetorotational instability (MRI) in the presence of the Braginskii viscosity. We conduct a global linear analysis of the instability in a galactic rotation profile for three magnetic field configurations: purely azimuthal, purely vertical and slightly pitched. Our analysis, numerical and asymptotic, shows that the first two represent singular configurations where the Braginskii viscosity's primary role is dissipative and the maximum growth rate is proportional to the Reynolds number when this is small. For a weak pitched field, the Braginskii viscosity is destabilising and when its effects dominate over the Lorentz force, the growth rate of the MRI can be up to 2√2 times faster than the inviscid limit. If the field is strong, an over-stability develops and both the real and imaginary parts of the frequency increase with the coefficient of the viscosity. Second, in the context of the ICM of galaxy clusters, we consider the pressure-anisotropy-driven firehose instability. The linear instability is fast (~ ion cyclotron period) and small-scale (ion Larmor radius ρi) and so fluid theory is inapplicable. We determine its nonlinear evolution in an ab initio kinetic calculation (for parallel gradients only). We use a particular physical asymptotic ordering to derive a closed nonlinear equation for the firehose turbulence, which we solve. We find secular (α t) growth of magnetic fluctuations and a k-||3 spectrum, starting at scales >~ ρi. When a parallel ion heat flux is present, the parallel firehose instability mutates into the new gyrothermal instability. Its nonlinear evolution also involves secular magnetic energy growth, but its spectrum is eventually dominated by modes with a maximal scale ~ρilT/λmfp,(lT is the parallel temperature gradient scale). Throughout we discuss implications for modelling, transport and other areas of magnetized plasma physics.
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Pickering, Paul Frederick. "Instabilities in low pressure boiling systems." Thesis, Imperial College London, 1994. http://hdl.handle.net/10044/1/7784.
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Souza, Max Oliveira de. "Instabilities of rotating and unsteady flows." Thesis, University of Cambridge, 1998. https://www.repository.cam.ac.uk/handle/1810/251641.
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This dissertation is divided into two parts. The first part discusses the stability and breakdown of swirling flows. The second one deals with the stability of time periodic flows. Chapter 1 gives the background and reviews previous work in vortex breakdown. Chapter 2 deals with various aspects of axisymmetric breakdown. These include the study of basic states that do not support waves, and the role played by the downstream boundary conditions in steady solutions to the Euler equations. A description of the bifurcation diagram for pipes is presented, and we also show how the process of wave-steepening can lead to the formation of a highly oscillatory shock. In chapter 3, we study the weakly non-linear stability of trailing vortices. Following numerical calculations by Yang (1992), who found a viscous instability for arbitrary large values of the swirl at sufficiently large Reynolds number, we present an analysis for the steady states and their stability. We obtain fast-swirling, steady states, and study their linear stability to viscous centre-modes. For nearly neutral modes, we investigate their weakly nonlinear stability, accounting for non-parallel effects. Previous work on stability of time-periodic flows is reviewed in chapter 4. In chapter 5, we extend the critical-layer analysis by Lin (1955) to unsteady flows, and use it to investigate the stability of several oscillatory flows. For slightly oscillatory Plane-Poiseuille flow, the method is able to recover periodic modes (asymptotic Floquet modes for large Reynolds numbers). Nevertheless, the method fails for the Stokes layer; the reasons for such failure are discussed. Finally, we present in chapter 6 some results on the completeness of Floquet modes in channels.
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Puhl, Jacqueline L. "Chemical instabilities in isotropic turbulent flows." Doctoral thesis, Universite Libre de Bruxelles, 1988. http://hdl.handle.net/2013/ULB-DIPOT:oai:dipot.ulb.ac.be:2013/213327.
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Müeller, Judith. "Study of stress-induced morphological instabilities." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1998. http://www.collectionscanada.ca/obj/s4/f2/dsk1/tape11/PQDD_0025/NQ50225.pdf.
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Ali, Shahid. "Waves and instabilities in quantum plasmas." Doctoral thesis, Umeå : Department of Physics, Umeå Univ, 2008. http://urn.kb.se/resolve?urn=urn:nbn:se:umu:diva-1520.
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Müller, Judith. "Study of stress-induced morphological instabilities." Thesis, McGill University, 1998. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=35472.
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We propose a model based on a Ginzburg-Landau approach to study a strain relief mechanism at a free interface of a non-hydrostatically stressed solid, commonly observed in thin-film growth. The evolving instability, known as the Grinfeld instability, is of high technological importance. It can be associated with the dislocation-free island-on-layer growth mode in epitaxy which is an essential process used in the semiconductor industry.In our model, the elastic field is coupled to a scalar order parameter in such a way that the solid supports shear whereas the liquid phase does not. Thus, the order parameter has a transparent meaning in the context of liquid-solid phase transitions.
We show that our model reduces in the appropriate limits to the sharp-interface equation, which is the traditional formulation of the problem. Inherent in our description is the proper treatment of non-linearities which avoids the numerical deficiencies of previous approaches and allows numerical studies in two and three dimensions.
To test our model, we perform a numerical linear stability analysis and obtain a dispersion relation which agrees with analytical results. We study the non-linear regime by measuring the Fourier transform of the height-height correlation function. We observe that, as strain is relieved, interfacial structures, corresponding to different wave numbers, coarsen. Furthermore, we find that the structure factor shows scale invariance. We expect that our result on transient coarsening phenomena can be measured through microscopy or x-ray diffraction.
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Li, Yong. "Active control of boundary layer instabilities." Thesis, Queen Mary, University of London, 2005. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.413644.
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