Tesi: "Dynamical instabilities"

1

Lin, Min-Kai. "Dynamical instabilities in disc-planet interactions". Thesis, University of Cambridge, 2012. https://www.repository.cam.ac.uk/handle/1810/245135.

Testo completo

Abstract (sommario):

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.

Gli stili APA, Harvard, Vancouver, ISO e altri

2

Tomadin, Andrea. "Dynamical instabilities in quantum many-body systems". Doctoral thesis, Scuola Normale Superiore, 2010. http://hdl.handle.net/11384/85874.

Testo completo

Abstract (sommario):

from the introduction: "[...] This thesis addresses the problem of the nonequilibrium time-evolution of many-body sistems realized with quantum simulators. We investigate theoretically the relation between the time-evolution and the equilibrium phase diagram in the presence of a quantum phase transition. The long-time evolution of the systems is investigated, both in the case of conservative dynamics and under the action of dissipative processes. The thesis is organized as follows. Chaps. 1-3 contain theoretical and experimental facts that are relevant to the present work. In the Chap. 1 we describe the quantum simulator realized with fermionic cold atoms in the bulk of an optical trap. In the Chap. 2 we describe the microresonators where light and matter are strongly coupled, and we focus on the implementation of a quantum simulator with defect-cavities in photonic crystals. In Chap. 3 several theoretical works concerning the nonequilibrium dynamics of many-body systems are summarized. Chaps. 4{6 contain the original contributions of this thesis. In Chap. 4 we investigate the time-evolution of an ensemble of fermionic atoms after an abrupt change of the interaction strength from a vanishing to a weak attractive value. In Chap. 5 we demonstrate the possibility to measure the quantum phase transition between a super fluid and a Mott-insulator state in an array of microresonators, in the presence of the leakage of photons out of the cavities. In Chap. 6 we study a system of cold bosonic atoms coupled to a tailored bath that dissipatively drives the system into a super fluid or into a thermal state.

Gli stili APA, Harvard, Vancouver, ISO e altri

3

Persson, Kristin Aslaug. "Thermodynamical and Dynamical Instabilities from Ab initio Electronic-Structure Calculations". Doctoral thesis, KTH, Physics, 2001. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-3137.

Testo completo

Gli stili APA, Harvard, Vancouver, ISO e altri

4

Persson, Kristin. "Thermodynamical and dynamical instabilities from Ab initio electronic-structure calculations /". Stockholm : Tekniska högsk, 2001. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-3137.

Testo completo

Gli stili APA, Harvard, Vancouver, ISO e altri

5

Madden, Francis. "Dynamical instabilities in a fluid spin-up and in an open flow system". Thesis, University of Oxford, 1991. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.293436.

Testo completo

Gli stili APA, Harvard, Vancouver, ISO e altri

6

Rostami, Masoud. "Dynamical influence of diabatic processes upon developing instabilities of Earth and planetary jets and vortices". Thesis, Paris 6, 2017. http://www.theses.fr/2017PA066186.

Testo completo

Abstract (sommario):

Le but de la thèse est de comprendre l'influence dynamique des effets diabatiques, comme la convection humide, sur les instabilités des vortex atmosphériques terrestres et planétaires. Un modèle verticalement intégré, avec les paramétrisations type relaxation des transitions de phase et de dégagement de la chaleur latente, le modèle de St-Venant avec la convection humide, a été utilisé. La version précédente du modèle a été améliorée pour inclure l'eau précipitable, sa vaporisation et son entrainement. L'approche consiste en 1) analyse détaillée de stabilité des profils idéalisés, ou extraits des données, des vortex, 2) étude de saturation non-linéaire des instabilités à l'aide de schéma numérique de haute résolution aux volumes finis. Les résultats principaux de la thèse sont : 1. Démonstration et quantification d'une forte influence des effets humides sur les instabilités des vortex synoptiques, y compris asymétrie cyclone-anticyclone des vortex de faible intensité aux latitudes moyennes, et de l'intensification des vortex type cyclones tropicaux, avec formation des nuages caractéristiques. 2. Explication de l'origine dynamique de l'hexagone au pôle Nord de Saturne, et de l'absence de structure similaire au pôle Sud, en termes d'instabilité du système vortex polaire - jet circumpolaire, et sa saturation non-linéaire. 3. Explication de la structure observée du vortex polaire hivernal sur Mars en termes d'instabilité et sa saturation en présence de réchauffement /refroidissement radiatif et de déposition de CO2 (transition de phase gaz - solide). Une nouvelle paramétrisation simple a été proposée pour ce processus, incluant l'influence des noyaux de déposition
The thesis is devoted to understanding dynamical influence of diabatic effects, like moist convection, on instabilities of vortices in Earth and planetary atmospheres. A vertically integrated atmospheric model with relaxational parameterisation of phase transitions and related heat release, and with convective fluxes included in mass and momentum equations, the moist-convective rotating shallow water model, was used for this purpose. The previous version of the model was improved to include precipitable water and its vaporisation and entrainment. The approach consists in 1)detailed stability analysis of idealised, or extracted from the data, vortex profiles, 2)study of nonlinear saturation of the instabilities with the help of finite-volume high-resolution numerical code. The main results of the thesis are: 1. Demonstration and quantification of strong influence of moist effects upon instabilities of synoptic vortices, including cyclone-anticyclone asymmetry of mid-latitude vortices of weak intensity, and intensification of tropical-cyclone like vortices with formation of typical cloud patterns. 2. Explanation of the dynamical origin of the Saturn's North Polar hexagon, and of the lack of similar structure at the South Pole, in terms of instability of the coupled polar vortex and circumpolar jet, and their nonlinear saturation.3. Explanation of the observed structure of Mars' winter polar vortex in terms of instability of the latter, and its saturation in the presence of radiative heating/cooling and CO2 deposition (gas-solid phase transition). A new simple parameterisation of the latter process, including the influence of deposition nuclei, was developed in the thesis

Gli stili APA, Harvard, Vancouver, ISO e altri

7

Rostami, Masoud. "Dynamical influence of diabatic processes upon developing instabilities of Earth and planetary jets and vortices". Electronic Thesis or Diss., Paris 6, 2017. http://www.theses.fr/2017PA066186.

Testo completo

Abstract (sommario):

Le but de la thèse est de comprendre l'influence dynamique des effets diabatiques, comme la convection humide, sur les instabilités des vortex atmosphériques terrestres et planétaires. Un modèle verticalement intégré, avec les paramétrisations type relaxation des transitions de phase et de dégagement de la chaleur latente, le modèle de St-Venant avec la convection humide, a été utilisé. La version précédente du modèle a été améliorée pour inclure l'eau précipitable, sa vaporisation et son entrainement. L'approche consiste en 1) analyse détaillée de stabilité des profils idéalisés, ou extraits des données, des vortex, 2) étude de saturation non-linéaire des instabilités à l'aide de schéma numérique de haute résolution aux volumes finis. Les résultats principaux de la thèse sont : 1. Démonstration et quantification d'une forte influence des effets humides sur les instabilités des vortex synoptiques, y compris asymétrie cyclone-anticyclone des vortex de faible intensité aux latitudes moyennes, et de l'intensification des vortex type cyclones tropicaux, avec formation des nuages caractéristiques. 2. Explication de l'origine dynamique de l'hexagone au pôle Nord de Saturne, et de l'absence de structure similaire au pôle Sud, en termes d'instabilité du système vortex polaire - jet circumpolaire, et sa saturation non-linéaire. 3. Explication de la structure observée du vortex polaire hivernal sur Mars en termes d'instabilité et sa saturation en présence de réchauffement /refroidissement radiatif et de déposition de CO2 (transition de phase gaz - solide). Une nouvelle paramétrisation simple a été proposée pour ce processus, incluant l'influence des noyaux de déposition
The thesis is devoted to understanding dynamical influence of diabatic effects, like moist convection, on instabilities of vortices in Earth and planetary atmospheres. A vertically integrated atmospheric model with relaxational parameterisation of phase transitions and related heat release, and with convective fluxes included in mass and momentum equations, the moist-convective rotating shallow water model, was used for this purpose. The previous version of the model was improved to include precipitable water and its vaporisation and entrainment. The approach consists in 1)detailed stability analysis of idealised, or extracted from the data, vortex profiles, 2)study of nonlinear saturation of the instabilities with the help of finite-volume high-resolution numerical code. The main results of the thesis are: 1. Demonstration and quantification of strong influence of moist effects upon instabilities of synoptic vortices, including cyclone-anticyclone asymmetry of mid-latitude vortices of weak intensity, and intensification of tropical-cyclone like vortices with formation of typical cloud patterns. 2. Explanation of the dynamical origin of the Saturn's North Polar hexagon, and of the lack of similar structure at the South Pole, in terms of instability of the coupled polar vortex and circumpolar jet, and their nonlinear saturation.3. Explanation of the observed structure of Mars' winter polar vortex in terms of instability of the latter, and its saturation in the presence of radiative heating/cooling and CO2 deposition (gas-solid phase transition). A new simple parameterisation of the latter process, including the influence of deposition nuclei, was developed in the thesis

Gli stili APA, Harvard, Vancouver, ISO e altri

8

Dufour, Oscar. "Enhanced agent-based models for pedestrian crowds : insights from empirical data at the Festival of Lights and refinements of mechanical interactions, pedestrian shapes, and decisional aspects". Electronic Thesis or Diss., Lyon 1, 2024. http://www.theses.fr/2024LYO10338.

Testo completo

Abstract (sommario):

Avec la multiplication des événements de masse, la dynamique des foules est devenue un sujet d’étude de plus en plus important. Comprendre comment les groupes se déplacent et évoluent dans l'espace, en particulier à des densités moyennes et élevées, est crucial pour l'organisation de tels événements. La première section de cette thèse de doctorat présente l'un des premiers ensembles de données de terrain sur les foules denses. Cet ensemble comprend aussi bien des trajectoires de piétons que des données GPS et de contact collectées lors de la Fête des Lumières 2022 à Lyon. Je développe ensuite un cadre théorique de modélisation de la dynamique des foules qui intègre une composante décisionnelle, où les piétons ajustent régulièrement leur vitesse désirée, et une couche mécanique qui confronte ces décisions à la réalité physique environnante. La plupart des modèles existants échouent à reproduire fidèlement les interactions mécaniques, car ils reposent souvent sur des forces d'interaction idéalisées et des formes circulaires simplifiées. En m'inspirant de la littérature scientifique sur la dynamique granulaire, j'intègre aux équations newtoniennes, des interactions mécaniques plus réalistes, en utilisant des ressorts amortis à la fois tangentiels et normaux aux surfaces de contact. J'utilise de plus des données anthropométriques pour représenter le plus fidèlement possible le contour humain, en deux dimensions, plutôt que de recourir à de simples disques. Cela me permet de créer une foule synthétique qui intègre les hétérogénéités individuelles. En ce qui concerne la composante décisionnelle, les piétons s'efforcent de sélectionner une vitesse souhaitée tout en tenant compte de diverses contraintes métaboliques, physiques et psychologiques, la plupart étayées par des données empiriques. Ces contraintes incluent : une contrainte de destination qui prend en compte l'objectif d'atteindre un lieu spécifique ; des limites biomécaniques liées aux capacités musculaires et articulaires des piétons ; un coût lié à la différence d’alignement entre le corps et la direction de mouvement souhaité ; une volonté de préserver sa bulle sociale, une zone que les individus souhaitent maintenir libre de toute intrusion, qu'il s'agisse d'obstacles ou de piétons voisins ; une intention d’éviter les collisions lors d'un déplacement, basée sur des estimations de temps avant collision. Cet espace de confort est modélisé par un champ scalaire d'inconfort dont les courbes de niveau ne sont pas circulaires. Après validation dans des situations simples impliquant des paires de piétons ou un piéton près d'un mur, je compare avec succès les prédictions du modèle à des expériences impliquant la propagation d’une poussée à travers une rangée de personnes, des évacuations, et des mouvements de faufilement entre murs et piétons. J'étudie enfin les phénomènes collectifs qui se manifestent non seulement dans les foules, mais aussi dans le trafic véhiculaire, en particulier les ondes stop-and-go résultant de la croissance d'instabilités dynamiques. Pour mieux comprendre ces phénomènes, je simule un modèle de suivi automobile en file qui repose sur le maintien d'un temps de freinage constant avec le véhicule suivant. Bien que la version déterministe de ce modèle soit inconditionnellement stable, l'ajout de bruit entraîne de manière inattendue l'apparition d’ondes stop-and-go. J'explique cette observation en utilisant une analogie avec le pendule de Kapitza, qui développe un nouvel état stationnaire sous l'effet de fortes vibrations. Plus précisément, une discontinuité d’un paramètre d'ordre apparaît lorsque le niveau de bruit ou la densité dépasse un certain seuil, faisant écho à une transition liquide-gaz. Mes recherches sur la dynamique des foules soulignent l'importance d'intégrer les processus décisionnels aux interactions mécaniques pour approfondir notre compréhension des comportements collectifs complexes, en particulier dans les environnements bondés
With the surge in mass events, crowd dynamics have become an increasingly important subject of study. Understanding how groups move and evolve in space, particularly at medium and high densities, is crucial for organising such events.The first section of this PhD dissertation presents one of the first field datasets on dense crowds. This dataset includes pedestrian trajectories and meta-information collected during the 2022 Festival of Lights in Lyon as part of the Franco-German MADRAS project. It includes up to 7000 trajectories, GPS data, and contact information. In addition, some rare events have been identified, providing an in-depth description of pedestrian dynamics in complex, real-life scenarios. Subsequently, I develop a theoretical framework for modelling crowd dynamics that integrates a decision-making component, where pedestrians regularly adjust their desired speed, and a mechanical layer that confronts these decisions with the surrounding physical reality. Most existing models fail to faithfully reproduce mechanical interactions, often relying on idealised interaction forces and simplified circular shapes. Drawing inspiration from the scientific literature on grain dynamics, I integrate more realistic mechanical interactions into the Newtonian equations, using damped springs that are tangential and normal to the contact surfaces. I also use anthropometric data to represent the human contour as faithfully as possible, in two dimensions, rather than using simple discs. This allows me to create a synthetic crowd that incorporates individual heterogeneity. Regarding decision-making, pedestrians strive to choose a desired speed while adhering to various metabolic, physical, and psychological constraints, largely supported by empirical data. These constraints include:- A destination constraint which considers the goal of reaching a specific location.- Biomechanical limits related to the muscular and articular capacities of pedestrians.- A cost associated with the misalignment between the body and the desired direction of movement.- A desire to preserve one's social bubble, a zone that individuals wish to keep free of any intrusion, whether from obstacles or neighbouring pedestrians.- An intention to avoid collisions or interpenetration of comfort spaces during movement based on the estimation of time to collision.This comfort space is modelled by a scalar field of discomfort whose contours are not simply circular. The model is implemented in C++ and tested in various scenarios. After validation in simple situations involving pairs of pedestrians or a pedestrian near a wall, I successfully compare the model's predictions with experiments involving the propagation of a push through a row of people, evacuations, and weaving movements between walls and pedestrians.Finally, I investigate collective phenomena that occur not only in crowds but also in vehicular traffic, specifically stop-and-go waves resulting from the growth of dynamical instabilities. To better understand these phenomena, I simulate a car-following model that relies on maintaining a constant time gap with the following vehicle. Although the deterministic version of the model is unconditionally stable, introducing noise intriguingly leads to the emergence of stop-and-go waves. I explain this observation using an analogy with the Kapitza pendulum, which develops a new stationary state under strong vibrations. Specifically, discontinuities in a suitably defined order parameter appear when noise or density exceeds a finite threshold, echoing a liquid-gas transition. This noise may stem from inaccuracies in drivers' and pedestrians' observations, difficulties in brain information processing, or unaccounted interactions. My research on crowd dynamics highlights the importance of integrating decision-making processes with mechanical interactions to deepen our understanding of complex collective behaviours, notably in crowded environments

Gli stili APA, Harvard, Vancouver, ISO e altri

9

Cordeiro, Timothy Joseph. "Dynamic instabilities imparted by CubeSat propulsion". Thesis, Massachusetts Institute of Technology, 2016. http://hdl.handle.net/1721.1/105612.

Testo completo

Abstract (sommario):

Thesis: S.M., Massachusetts Institute of Technology, Department of Aeronautics and Astronautics, 2016.
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 120-123).
As the role of CubeSats evolves to include more challenging and complex missions in addition to technology demonstrations, the demand for agility have increased. As the technology improves and gains flight heritage, CubeSats are being deployed to accomplish more difficult missions including, but not limited to, large constellations and missions beyond Low Earth Orbit (LEO). To perform missions like station keeping for constellations, and to move beyond LEO, CubeSat developers are increasingly integrating propulsion into the design of their CubeSats. In addition, more complex payloads and communication systems require more power generation, which leads to larger deployed solar arrays. Meanwhile, the limiting factor for the CubeSat remains the size and weight constraints of the containerized launch deployers. In order to meet these constraints, the solar array design has to trade stiffness and strength for size. In this work, we investigate whether designs that use a combination of propulsion and solar arrays stress the dynamics of the solar panels and the hinges that hold them in place. Our approach uses SimXpert to perform dynamic simulations on CubeSat models, both 3U and 6U, with deployable solar panels and propulsion forces. By default, SimXpert treats every part as a rigid body and stress is not calculated. By doing a modal analysis of the panels in Nastran and importing the results into SimXpert, stress on the panels can be tracked during propulsive maneuvers. We determine that Margin of Safety (MoS) for the solar panels analyzed is over 100 when combined with three different COTS propulsion units. We also show the movement induced on the panels from propulsion can cause errors in body attitude ranging from 0.04 to 90 degrees. The worst case showed a difference becoming one degree in five seconds before growing exponentially to 90 degrees in 30 seconds.
by Timothy Joseph Cordeiro.
S.M.

Gli stili APA, Harvard, Vancouver, ISO e altri

10

Nguyen, Thi Thu Tra. "Dynamic instabilities of model granular materials". Thesis, Lyon, 2019. http://www.theses.fr/2019LYSET007/document.

Testo completo

Abstract (sommario):

Cette thèse étudie les instabilités dynamiques des milieux granulaires modèles saturés à l’aide d’un appareil triaxial classique. Les instabilités englobent la liquéfaction et les effondrements en compression isotrope drainée, les frottements saccadés en compression triaxiale drainée. Ces instabilités apparaissent spontanément à des contraintes effectives de confinement imprévisibles. Elles sont accompagnées de très rapides et très fortes surpressions interstitielles, malgré un drainage approprié; ce que ne présentent pas les milieux granulaires naturels. En compression isotrope drainée (consolidation), des effondrements locaux naissent instantanément. En compression triaxiale drainée, on observe de larges frottements saccadés quasi-périodiques caractérisés par des déformations volumiques et axiales contractantes. De temps en temps, ces effondrements et frottements saccadés locaux peuvent se développer en liquéfaction menant à une destruction complète de la structure granulaire. Les données à haute résolution temporelle issues de ce travail ont permis la découverte d’une nouvelle famille de liquéfaction dynamique et statique. L’étude des émissions acoustiques passives a permis l’identification de signature spectrale caractéristique. Pour les frottements saccadés, la phase de glissement peut être interprétée comme une consolidation dynamique, limitée par l’unique surface en dessous de la ligne critique de rupture dans le plan des contraintes effectives. La séquence temporelle précise des événements exclut que la pression interstitielle soit la cause principale des instabilités. Cependant, le rôle important de la surpression interstitielle est démontré dans des relations quantitatives entre les incréments de contraintes, et de déformations et l’éphémère surpression interstitielle stabilisée développée pendant la phase de glissement. Cela montre finalement la nature quasi-déterministique de ces instabilités dynamiques. Ces relations empiriques sont basées uniquement sur l’amplitude maximale de l’accélération verticale de très courte durée et sont gouvernées indépendamment par la pression de confinement et par l’indice des vides. La similarité de la surpression interstitielle entre différentes instabilités suggère fortement quelques mécanismes similaires de déclenchement, probablement à partir de ré-arrangements de la micro-structure granulaire
This thesis reports a laboratory study on the dynamic instabilities of model saturated granular material using a triaxial apparatus. The term instability consists of isotropic collapse and liquefaction under isotropic compression and of stick-slip under triaxial compression in drained condition. The instabilities spontaneously occur at unpredictable effective stress with unexpected buildup of excess pore pressure irrespective of fully drained condition, contrasting with the instability-free behaviour of natural granular materials. In isotropic compression, instantaneous local collapse happens and in triaxial compression, very large and quasi-periodic stick-slip occurs with sudden volumetric compaction and axial contraction. Sometimes, these local failures (collapse and stick-slip) can develop into total liquefaction failure, destroying completely the granular structure. High time-resolved data permit the discovery of a new family of dynamic and static liquefaction. Passive acoustic measurements allow the identification of typical spectral signature. For stick-slip phenomenon, the slip phase with constant duration of stress drop can be interpreted as dynamic consolidation at constant deviatoric stress, limited by a unique boundary inside the critical state line in the effective stress plane. The precise temporal sequence of mechanical measurements excludes the generated pore pressure as the main cause of the instabilities. However, the role of pore pressure is emphasised by consistent quantitative relations between the amplitude of incremental stresses, incremental strains and the ephemeral stabilised excess pore pressure developed during the dynamic event, leading to the quasi-deterministic nature of granular instabilities. These empirical relations are based only on the short-lived maximum vertical acceleration and governed separately by the confining pressure and the initial void ratio. The similarity of pore pressure evolution for different kinds of instability strongly suggests some common speculative triggering mechanisms, probably originated from different rearrangements of the granular micro-structure

Gli stili APA, Harvard, Vancouver, ISO e altri

11

Ozen, Ozgur. "Effect of vapor dynamics on interfacial instabilities". [Gainesville, Fla.] : University of Florida, 2004. http://purl.fcla.edu/fcla/etd/UFE0006000.

Testo completo

Gli stili APA, Harvard, Vancouver, ISO e altri

12

Gillispie, Brian Douglas Stewart David. "Instabilities of elastic bodies in motion". Thesis supplements, 2009. http://ir.uiowa.edu/etd/238/.

Testo completo

Gli stili APA, Harvard, Vancouver, ISO e altri

13

Kaiser, Thomas. "Impact of Flow Rotation on Flame Dynamics and Hydrodynamic Stability". Thesis, Toulouse, INPT, 2019. http://oatao.univ-toulouse.fr/24115/1/Kaiser_Thomas.pdf.

Testo completo

Abstract (sommario):

This thesis investigates large scale flow rotation in two configurations. In the first, the effect of flow rotation on a laminar flame is investigated. The flame is anchored in the wake of a cylindrical bluff body. The flow rotation is introduced by turning the cylinder along its axis. It is shown by Direct Numerical Simulation (DNS), that the cylinder rotation breaks the symmetry of both flame branches. Flame Transfer Function (FTF) measurements performed by the Wiener-Hopf Inversion suggest, that low rotation rates lead to deep gaps in the gain and the flame becomes almost insensitive to acoustic perturbation at a specific frequency. It furthermore is demonstrated that this decrease in gain of the FTF is due to destructive interference of the heat release signals caused by the two flame branches. The frequency at which the gain becomes almost zero can be adjusted by tuning the cylinder rotation rate. The study suggests that controlling the symmetry of the flame could be a tool of open-loop control of thermoacoustic instabilities.

Gli stili APA, Harvard, Vancouver, ISO e altri

14

Nikitas, Nikolaos. "Wind-induced dynamic instabilities of flexible bridges". Thesis, University of Bristol, 2011. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.546213.

Testo completo

Gli stili APA, Harvard, Vancouver, ISO e altri

15

Sinclair, Stephen W. "Dynamic instabilities in a nonlinear ring cavity". Thesis, University of Strathclyde, 1989. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.293235.

Testo completo

Gli stili APA, Harvard, Vancouver, ISO e altri

16

Citro, Vincenzo. "Unsteady and three-dimensional fluid dynamic instabilities". Doctoral thesis, Universita degli studi di Salerno, 2016. http://hdl.handle.net/10556/2220.

Testo completo

Gli stili APA, Harvard, Vancouver, ISO e altri

17

Harper, C. M. "Turbulence and combustion instabilities in engines". Thesis, University of Leeds, 1989. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.343890.

Testo completo

Gli stili APA, Harvard, Vancouver, ISO e altri

18

Stamper, Megan Andrena. "The evolution and breakdown of submesoscale instabilities". Thesis, University of Cambridge, 2018. https://www.repository.cam.ac.uk/handle/1810/277822.

Testo completo

Abstract (sommario):

Ocean submesoscales are the subject of increasing focus in the oceanographic literature; with instrumentation now more capable of observing them in situ and numerical models now able to reach the resolution required to more fully capture them. Submesoscales are typified by horizontal spatial scales of O(1 − 10) km, vertical scales O(100) m and time-scales of O(1) day and are known to be associated with regions of high vertical velocity and vorticity. Occurring most commonly at density fronts at the ocean surface they can control mixed layer restratification and provide an important control on fluxes between the atmosphere and the deep ocean. This thesis sets out to better understand the fundamental physical processes underpinning submesoscale instabilities using a number of idealised process models. Linear stability analysis complemented by non-linear, high-resolution simulations will be used initially to explore the ways in which submesoscale instabilities in the mixed layer may compete and interact with one another. In particular, we will investigate the way in which symmetric and ageostrophic baroclinic instabilities interact when simultaneously present in a flow, with focus on the growth rates and energetic pathways of previously unexplored dynamic instabilities that arise in this paradigm; three-dimensional, mixed symmetric-baroclinic instabilities. Further, these non-linear simulations will allow us to investigate the transition to dissipative scales that can occur in the classical Eady model via a multitude of small-scale secondary instabilities that result from primary submesoscale instabilities. Finally, observational data, taken aboard the SMILES project cruise to the Southern Ocean, helps to motivate the consideration of a new dynamical paradigm; the Eady model with superimposed high amplitude barotropic jet. Non-linear simulations investigate the extent to which the addition of such a jet is capable of damping submesoscale growth. The causes of this damping are then investigated using linear analysis. With this approach eventually demonstrated as being unable to fully explain growth rate reductions, we introduce a new framework combining potential vorticity mixing by submesoscale instabilities with geostrophic adjustment, which relaxes the flow back to a geostrophic balanced state. This framework will help to explain, conceptually, how non-linear eddies control the linear stability of the flow.

Gli stili APA, Harvard, Vancouver, ISO e altri

19

Kay, S. R. "Instabilities and dynamic behaviour in branched chain reactions". Thesis, University of Leeds, 1987. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.378413.

Testo completo

Gli stili APA, Harvard, Vancouver, ISO e altri

20

Peters, Eric David. "Dynamic instabilities imparted by CubeSat deployable solar panels". Thesis, Massachusetts Institute of Technology, 2014. http://hdl.handle.net/1721.1/93800.

Testo completo

Abstract (sommario):

Thesis: S.M., Massachusetts Institute of Technology, Department of Aeronautics and Astronautics, 2014.
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 85-87).
In this work, multibody dynamics simulation was used to investigate the effects of solar panel deployment on CubeSat attitude dynamics. Nominal and partial/asymmetric deployments were simulated for four different solar panel assemblies. Trend lines were obtained for the evolution of the angular velocities and accelerations of the CubeSat about its center of mass for the duration of the deployment. The partial deployment simulations shed insight into the motions that an attitude control system may need to mitigate in the event of a deployment anomaly.
by Eric David Peters.
S.M.

Gli stili APA, Harvard, Vancouver, ISO e altri

21

Sundkvist, David. "Space Plasma Dynamics : Instabilities, Coherent Vortices and Covariant Parametrization". Doctoral thesis, Uppsala : Acta Universitatis Upsaliensis : Univ.-bibl. [distributör], 2005. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-6051.

Testo completo

Gli stili APA, Harvard, Vancouver, ISO e altri

22

Hourlier-Fargette, Aurélie. "Soft interfaces : from elastocapillary snap-through to droplet dynamics on elastomers". Thesis, Paris 6, 2017. http://www.theses.fr/2017PA066089/document.

Testo completo

Abstract (sommario):

Dans cette thèse à l’interface entre élasticité et capillarité, nous présentons tout d’abord une instabilité élastique, le claquage, revisitée dans un contexte élastocapillaire. En déposant une goutte d’eau sous une lamelle flambée en position basse, nous parvenons à déclencher une instabilité de claquage à contresens de la gravité. Cette démonstration de la prédominance des effets capillaires à petite échelle s’accompagne d’une étude des positions d’équilibre et de la stabilité de systèmes goutte-lamelle. Nous démontrons l’influence importante de la taille et de la position de la goutte le long de la lamelle, puis étendons notre étude au cas de bulles ou de gouttes condensées à partir de vapeur d’eau. Enfin, nous nous intéressons à l’aspect dynamique de l’instabilité, qui est dictée principalement par l’élasticité, y compris dans le cas élastocapillaire.Nous mettons ensuite en évidence un phénomène surprenant : la dynamique de descente d’une goutte d’eau sur un élastomère silicone présente deux régimes successifs, caractérisés par deux vitesses différentes. Nous montrons que les chaînes libres non réticulées présentes dans l’élastomère sont à l’origine de cette dynamique inattendue. La goutte est progressivement recouverte par des chaînes de silicone, et sa vitesse change brutalement lorsqu’une concentration surfacique critique est atteinte, ce qui se traduit par une transition brutale de tension de surface. Nous nous intéressons aux vitesses de gouttes dans les deux régimes ainsi qu’aux échelles de temps mises en jeu lors de l’extraction de chaînes non réticulées, et montrons que l’extraction de ces chaînes se produit au niveau de la ligne triple
This thesis focuses on interactions between liquids and elastic solids. We first revisit the snap-through instability from an elastocapillary point of view, showing that capillary forces are able to counterbalance gravity by inducing snap-through with a droplet deposited below a downward buckled elastic strip clamped at both ends. Equilibrium, stability, and dynamics of drop-strip systems are investigated, demonstrating the influence of droplet size and droplet position along the buckled strip, and showing that capillarity is driving the system toward instability but elasticity is ruling the subsequent dynamics. Spin-off versions of the experiment are also designed, including a humidity-controlled mechanical switch and upscaled experiments using soap bubbles.We then focus on interactions between silicone elastomers and aqueous droplets to understand the mechanisms underlying an unexpected two-regime droplet dynamics observed on vertical silicone elastomer plates. After demonstrating that this two-regime dynamics is due to the presence of uncrosslinked oligomers in the elastomer, we show that the speed transition coincides with a surface tension transition. A quantitative study of the droplets speeds in the two regimes is performed, and the timescale needed for uncrosslinked oligomers to cover the water-air interface is investigated both for sessile and moving droplets. We eventually show that uncrosslinked chains are extracted from the elastomer at the water - air - silicone elastomer triple line, and demonstrate that extraction occurs in various setups such as partially immersed silicone elastomer plates or air bubbles sliding up PDMS planes immersed in a water bath

Gli stili APA, Harvard, Vancouver, ISO e altri

23

NICCOLINI, MARMONT DU HAUT CHAMP CARLO ALBERTO. ""Analysis of dynamic responses and instabilities in rotating machinery”". Doctoral thesis, Università degli studi di Genova, 2022. http://hdl.handle.net/11567/1080576.

Testo completo

Abstract (sommario):

The first task of the present research is to characterize both experimentally and numerically journal bearings with low radial clearances for rotors in small-scale applications (e.g., micro Gas Turbines); their diameter is in the order of ten millimetres, leading to very small dimensional clearances when the typical relative ones (order of 1/1000) are employed; investigating this particular class of journal bearings under static and dynamic loading conditions represents something unexplored. To this goal, a suitable test rig was designed, and the performance of its bearings were investigated under steady load. For the sake of comparison, numerical simulations of the lubrication were also performed by means of a simplified model. The original test rig adopted is a commercial Rotor Kit (RK), but substantial modifications were carried out in order to allow significant measurements. Indeed, the relative radial clearance of RK4 RK bearings is about 2/100, while it is around 1/1000 in industrial bearings. Therefore, the same original RK bearings are employed in this new test rig, but a new shaft was designed to reduce their original clearance. The new custom shaft allows to study bearing behaviour for different clearances, since it is equipped with interchangeable journals. Experimental data obtained by this test rig are then compared with further results of more sophisticated simulations. They were carried out by means of an in-house developed finite element (FEM) code, suitable for ThermoElasto-HydroDynamic (TEHD) analysis of journal bearings both in static and dynamic conditions. In this work, bearing static performances are studied to assess the reliability of the experimental journal location predictions by comparing them with the ones coming from already validated numerical codes. Such comparisons are presented both for large and small clearance bearings of original and modified RK, respectively. Good agreement is found only for the modified RK equipped with small clearance bearings (relative radial clearance 8/1000), as expected. In comparison with two-dimensional lubrication analysis, three-dimensional simulation improves prediction of journal location and correlation with experimental results. The second main task of the present work is the development and the implementation of a suitable analytical model to correctly capture rolling bearing radial stiffness, particularly nearby the critical speeds of the investigated rotor-bearings system. In this work, such bearing non-linear stiffness lumped parameter model is firstly validated on the commercial RK and then it is applied to both air bladeless turbines (or Tesla turbines) and to an innovative microturbine, in order to assess their global rotodynamic behavior when they are mounted on ball bearings. In order to properly investigate all the issues related to critical speeds and stiffness, an adequate number of experimental tests was performed by exploiting an experimental air Tesla turbine prototype located at TPG experimental facility of the University of Genoa. The correlation between measured flexural critical speeds and their numerical predictions is markedly conditioned by the correct identification of ball bearings dynamic characteristics; in particular, bearings stiffness effect may play a significant role in terms of rotor-bearings system natural frequencies and therefore it must be properly assessed. Indeed, Tesla turbine rotor FE model previously employed for numerical modal analysis relies on rigid bearings assumption and therefore it does not account for bearings stiffness overall contribution, which may become crucial in case of “hard mounting” of rotor-bearings systems. Subsequently, high-speed air Tesla rotor is investigated by means of an enhanced FE model for numerical modal analysis within Ansys® environment, where ball bearings are modelled as non-linear springs whose stiffness is expressed according to the analytic model implemented in Matlab®. Two different numerical FE models are devised for microturbine rotor modelling which respectively rely on beam elements and on three-dimensional solid elements for mechanical system spatial discretization. The obtained results in terms of rotor-bearings system modal analysis exhibit an improvement in experimental-numerical results correlation by relying on such ball bearing stiffness model; moreover, beam-based FE model critical speeds predictions are coherent with experimental evidence and with respect to solid elements model it is characterized by lower computational time and it is more easily interpretable. Thus, such experimentally validated numerical model represents a reliable and easily adaptable tool for highspeed rotating machinery critical speeds prediction in practical industrial application cases. Finally In this work, several signal processing techniques performed on vibro-acoustic signals acquired from a T100 Turbec microturbine (which is furnished with a centrifugal compressor) are illustrated. Research activity goal focuses on the investigation different kinds of system response starting from non-intrusive probes signals like accelerometers and microphones; this is made by means of techniques such as HOSA and Wavelet Transform, developed in Matlab® environment, for early detection of the onset of unstable phenomena in centrifugal compressors. These new and different methods have been applied to the same set of data to get sufficiently independent information useful to synergistically improve knowledge in the diagnostic system. Data were acquired by means of an experimental facility based on a T100 turbine developed by the Thermochemical Power Group (TPG) at the University of Genoa. Sampling rate and sensor placement were carefully taken into account, basing both on the physical phenomena to be observed and on the sensor dynamic characteristics. In this context, it is meant to study microphones and accelerometers signals not from an isolated centrifugal turbomachine installed in a dedicated line, but from a whole compressor placed in a mGT system for energy generation. Indeed, the investigated machine is not operating in standalone mode, but its working point and angular velocity depend on the coupling with several elements. In particular, compressor working point and then its vibro-acoustic signals are expected to convey vibration and sound contributions coming from all the plant components; thus, they are more representative of machine realistic behavior in the energy system.

Gli stili APA, Harvard, Vancouver, ISO e altri

24

Gibbons, Brian J. Jr. "Electromigration induced step instabilities on silicon surfaces". The Ohio State University, 2006. http://rave.ohiolink.edu/etdc/view?acc_num=osu1143235175.

Testo completo

Gli stili APA, Harvard, Vancouver, ISO e altri

25

Arnould, Maëlis. "Some surface expressions of mantle convective instabilities". Thesis, The University of Sydney, 2018. http://hdl.handle.net/2123/19901.

Testo completo

Abstract (sommario):

The multiple interactions between Earth’s mantle and lithosphere, which is the upper boundary layer of mantle convection, generate lateral (plate tectonics) and vertical (dynamic topography) motions of Earth's surface. Understanding the influence of the dynamics of mantle convective instabilities on the surface is fundamental to improve our interpretations of a large range of surface observations, such as the formation of sedimentary basins, continental motions, the location of hotspots, the presence of gravity anomalies or sea-level variations. This thesis aims at using numerical models of whole-mantle convection self-generating plate-like tectonics to study the impacts of the development and the dynamics of mantle convective instabilities (such as slabs or mantle plumes) on the continuous reshaping of the surface. First, I focus on the effect of the coupling between mantle convective motions and plate tectonics on the development of dynamic topography at different spatio-temporal scales. The results suggest that Earth's surface can deform over large spatio-temporal scales (> 104 km and several hundreds of millions of years) induced by whole-mantle convection to small-scales (< 500 km and five million years) arising from small-scale upper-mantle convection. I show that subduction initiation and slab break-off events control the existence of intermediate scales of dynamic topography (between 500 and 104 km). A second aim of this thesis is to understand the dynamics of mantle plumes and their interactions with surface. I first characterize in detail the behavior of mantle plumes arising in models of whole-mantle convection self-generating plate-like tectonics, in light of surface observations. Then, I quantify the lateral motions of mantle plumes and unravel the sources of their drift. Finally, I use observations of the mantle thermal signature of plume/ridge interactions to reconstruct the relative motions between the Azores mantle plume and the Mid-Atlantic Ridge.

Gli stili APA, Harvard, Vancouver, ISO e altri

26

THOMAS, DJEISSON HOFFMANN. "CONTROL SYSTEM TO SUPPRESS GAIN DYNAMIC INSTABILITIES OF AN EDFA". PONTIFÍCIA UNIVERSIDADE CATÓLICA DO RIO DE JANEIRO, 2003. http://www.maxwell.vrac.puc-rio.br/Busca_etds.php?strSecao=resultado&nrSeq=3954@1.

Testo completo

Abstract (sommario):

CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO
ERICSSON DO BRASIL
Objetivando suprimir as instabilidades dinâmicas de ganho em um amplificador à fibra dopada com Érbio (EDFA), uma nova configuração de laser em anel é apresentada e demonstrada. Neste trabalho, analizamos os efeitos da variação do nível de atenuação no laço de re-alimentação sobre a resposta transitória do EDFA. Particularmente, observamos as excursões de ganho experimentadas pelo canal sobrevivente quando sete dentre oito canais da rede são adicionados ou removidos, à exemplo do que ocorre em sistemas WDM reais. Sob esta análise, avaliamos o desempenho do sistema em suprimir as instabilidades dinâmicas de ganho do EDFA.
A new ring laser configuration to eliminate the gain dynamic instabilities of an erbium doped fiber amplifier (EDFA) is proposed and demonstrated. We examine the effect of the attenuation level in the optical feedback path over thetransient response of the EDFA. In particular, we look at the transient gain excursions experienced by surviving channel when seven of eight channels are added or dropped, like in real WDM systems. Using this analysis as a guide, we highlight the robustness of the approach and evaluate its performance to EDFA gain stabilization.

Gli stili APA, Harvard, Vancouver, ISO e altri

27

Gorchon, Jon. "Current and field induced magnetization reversal in Pt/Co/Pt and (Ga, Mn)(As, P) ferromagnetic films". Thesis, Paris 11, 2014. http://www.theses.fr/2014PA112143.

Testo completo

Abstract (sommario):

La manipulation de l’état magnétique d’un système ferromagnétique présente un grand intérêt en raison de possibles applications technologiques. Comprendre les mécanismes fondamentaux qui contrôlent l’aimantation est donc particulièrement important. La compréhension de certains mécanismes peut également avoir un impact dans d’autres domaines de la physique. C’est le cas par exemple de la dynamique de déplacement de parois de domaines sous champ magnétique dans le régime de reptation (creep) qui peut être assimilé à celui d’une interface élastique et qui présente un caractère universel. Cette thèse présente tout d’abord, à travers un travail expérimental sur des couches ultra-minces de Pt/Co/Pt, une description complète de la dynamique de déplacement de parois de domaines sous champ magnétique. Une analyse auto-cohérente permet d’extraire tous les paramètres de contrôle, des exposants universels sont confirmés et un nouveau régime dynamique (le TAFF) est identifié. Une deuxième étude porte sur le déplacement de parois sous courant électrique en géométrie étendue dans un film de (Ga,Mn)(As,P). Cette étude met en évidence des instabilités de forme des parois de domaines soumises à un gradient de courant électrique. Les limites de stabilités sont analytiquement prédites et présentent un bon accord avec les expériences. Un troisième travail porte sur le renversement de l’aimantation à l’interface entre un film de (Ga,Mn)(As,P) et une électrode non ferromagnétique. Un renversement stochastique de l’aimantation sous courant continu est mis en évidence. Son origine réside dans l’accumulation de spin à l’interface qui diminue fortement l’aimantation locale. Un modèle simplifié permet de décrire la probabilité de retournement de l’aimantation et d’extraire les temps caractéristiques associés
Effectively manipulating the magnetic state of a ferromagnet has a great interest for possible technological applications. Understanding the underlying fundamental mechanisms is thus particularly important. In some cases, the understanding of some mechanisms may even importantly impact other areas of physics. This is the case for example with field induced magnetic domain walls motion in the creep regime, where the wall can be assimilated to an elastic interface and follows an universal behavior. This thesis presents through an experimental work on Pt/Co/Pt ultra-thin samples, a complete description of the temperature and field dependent domain wall dynamics. A self-consistent analysis allows the extraction of all control parameters, identifying the new Thermally Activated Flux Flow regime, and confirming universal thermal scaling exponents. A second study focuses on current induced domain wall motion in an extended geometry of a (Ga,Mn)(As,P) ferromagnetic film. This study unveils domain wall shape instabilities under a gradient of current. The instability limits are analytically predicted in agreement with the experimental observations. A third work concerns the magnetization reversal mechanism evidenced at the interface between a (Ga,Mn)(As,P) film and a non-ferromagnetic electrode under a current flow. The reversal is shown to be stochastic and mainly governed by the spin accumulation at the interface, which reduces importantly the local magnetization. A simplified model allows the description of the reversal probability and the time scales involved in the mechanism of reversal are accessed and discussed

Gli stili APA, Harvard, Vancouver, ISO e altri

28

Hillier, Andrew Stephen. "Numerical Study of Plasma Instabilities to Investigate Fine-Scale Prominence Dynamics". 京都大学 (Kyoto University), 2011. http://hdl.handle.net/2433/152029.

Testo completo

Gli stili APA, Harvard, Vancouver, ISO e altri

29

Kalmoni, N. M. E. "The role of magnetospheric plasma instabilities in auroral and substorm dynamics". Thesis, University College London (University of London), 2017. http://discovery.ucl.ac.uk/1546163/.

Testo completo

Abstract (sommario):

The auroral substorm is the manifestation of explosive energy release from the rapid and global reconfiguration of the magnetotail. The auroral substorm is marked by a sudden brightening and poleward expansion of the most equatorward auroral arc in the midnight sector of the ionosphere. The temporal sequence of magnetospheric processes which lead to the dynamic auroral substorm display remain disputed to this day. This thesis contains original research on the development and exploitation of novel data analysis techniques in order to analyse ground-based all sky imager data of the aurora, enabling the study of substorm processes in remarkable detail. Fourier analysis techniques are used to find the spatial scales of wave-like signatures (otherwise known as auroral beads/rays), which form along substorm onset arcs. Growth rates of ∼0.05 s−1 are found from the exponential growth of the power spectral density of individual spatial scales. By analysing the dataset in this way, comparisons are made between observations and theoretical predictions of plasma instabilities at the near-Earth edge of the plasma-sheet which have been proposed to play a critical part in the substorm onset process. Auroral arc tracking techniques are developed to automate and increase the size of the database of events analysed. The vast majority of independently identified substorm onsets are preceded by azimuthal structuring along the onset arc with median wavelengths of ∼80 km. These beads grow and develop into a magnetospheric instability around 2 minutes prior to auroral substorm onset. Showing that beads are a common feature along the substorm onset arc provides unprecedented quantitative evidence that a near-Earth instability is a fundamental component of the substorm onset process. Finally, analysis techniques are extended to state-of-the-art high resolution multi-spectral auroral data to investigate the processes driving auroral beads. Beads can be resolved in the green-, blue- and red-line aurora with spatial scales as small as 30 km, which later develop into larger structures of ∼80 km. These observations are consistent with Alfvén wave accelerated auroral particle precipitation and therefore imply that the substorm onset arc and auroral beads are driven unstable by waves.

Gli stili APA, Harvard, Vancouver, ISO e altri

30

Mirat, Clément. "Analyse des instabilités de combustion dans des foyers de centrale thermique fonctionnant au fioul lourd". Thesis, Châtenay-Malabry, Ecole centrale de Paris, 2015. http://www.theses.fr/2015ECAP0037/document.

Testo completo

Abstract (sommario):

Des crises vibratoires ont été constatées dans plusieurs centrales thermiques d’EDF opérant avec du fioul lourd, certaines ayant entraîné l’arrêt du foyer. Ce travail traite des instabilités de combustion pouvant se déclencher dans ce type de système où le combustible liquide est injecté avec de la vapeur d’eau et où l’écoulement d’air est mis en rotation. Ces phénomènes vibratoires résultent d’un couplage résonant entre la dynamique de la combustion et l’acoustique du foyer. La réponse acoustique des flammes diphasiques non-prémélangées swirlées reste largement méconnue et est difficilement analysable sur le foyer réel. L’objectif de ce travail est donc d’étudier la stabilité des chaudières EDF à partir de l’analyse de la réponse d’une flamme diphasique non-prémélangée swirlée issue d’un injecteur générique et soumise à des perturbations de la vitesse acoustique. Cette réponse est déterminée sur un dispositif original (DIFAV) équipé d’un swirler et d’un injecteur bi-fluides fonctionnant à la vapeur d’eau et au dodécane. Ce système est constitué des principaux éléments des brûleurs utilisés sur les centrales thermiques EDF à une échelle 1/7000. Le dispositif est conçu pour facilement modifier la géométrie de la tête d’injection, les conditions d’injection de combustible et de vapeur et ainsi contrôler le spray généré. Des visualisations à la sortie d’une buse d’injection montrent l’influence de la topologie de l’écoulement diphasique dans l’injecteur sur la taille des gouttes mesurées dans le spray. Des mesures de taille et de vitesse des gouttes lorsque le rapport des débits de vapeur et de combustible (GLR) est modifié sont réalisées. Ces données comparées à des modèles ont permis d’estimer l’évolution de la taille des gouttes générées par l’injecteur qui équipe les centrales thermiques lorsque le GLR varie. Une analyse modale du foyer DIFAV et d’un modèle simplifié de la chaudière réelle est ensuite menée. Les fréquences propres et les taux d’amortissement du foyer DIFAV sont déterminés expérimentalement en soumettant le système à une modulation acoustique externe. Un modèle acoustique simplifié composé de trois cavités couplées représentatif du brûleur DIFAV est également développé. Des simulations acoustiques réalisées avec COMSOL Multiphysics sur une coupe transverse d’une chaudière générique représentative de la chaudière industrielle permettent d’identifier trois modes à basses fréquences établis entre les plenums et la chambre de combustion qui sont susceptibles d’être instables. La sensibilité de ces modes à la géométrie du foyer et aux conditions limites est étudiée. La réponse de la flamme générique lorsqu’elle est soumise à des modulations acoustiques de l’écoulement d’air en amont du brûleur est ensuite mesurée sur le banc DIFAV pour différents niveaux d’excitation et deux topologies de flamme lorsque les conditions d’injection sont modifiées. Les mécanismes qui pilotent l’évolution du gain de l’une des fonctions de transfert généralisées (FDF) de la flamme sont étudiés à l’aide de visualisations en moyenne de phase de l’écoulement et de mesures des vitesses axiale et azimutale de l’écoulement d’air au cours d’un cycle de modulation. Une forte sensibilité de la phase de la FDF à l’amplitude des perturbations acoustiques est observée. Un adimensionnement par le nombre de Strouhal basé sur la vitesse débitante et la longueur efficace de la flamme est proposé pour transposer ces FDFs sur le brûleur réel. Une analyse de stabilité du foyer DIFAV est réalisée en intégrant les FDF au modèle acoustique afin de déterminer les cycles limites des oscillations lorsque la longueur de la chambre de combustion varie. Ces calculs sont comparés aux fréquences des instabilités auto-entretenues mesurées aux cycles limites dans le foyer DIFAV. [...]
Vibratory crises have been observed in EDF thermal power plants operating with heavy fuel oil. Such instabilities may lead to shutdown and damage the boiler. This work deals with combustion instabilities that can take place in boilers equipped with steam-assisted atomizers and where the airflow is swirled. These vibratory phenomena result from a resonant coupling between the combustion dynamics and the boiler acoustics. Analyses of combustion dynamics of non-premixed swirling spray flames remain rare and are difficult to realize on the real system. The objective of this work is to analyze the stability of EDF boilers using the response of generic non-premixed swirling spray flames submitted to acoustic velocity disturbances. This response is determined on an original device (DIFAV) equipped with a swirling vane and a twin-fluid atomizer operated with steam and dodecane. This burner is equipped with the main elements of those used in the thermal power plant, but has a reduced scale of 1/7000. The influence of the injector geometry and of the operating conditions on the spray generated by the injector can be studied. Spray visualizations at the outlet of the injector reveal the relationship between the topology of the two-phase flow in the injector and the measured droplet size. Measurements of the droplet diameter and velocity as a function of the gas-to-liquid ratio (GLR) have been performed at the outlet of the injector. These data have been compared to models and were used to estimate the evolution of the droplets diameter as a function of the GLR generated by the industrial injector. A modal analysis of the DIFAV combustor is then carried out and a simplified acoustic model made of three coupled cavities is developed. The natural frequencies and damping rates of the DIFAV combustor are determined experimentally when it is submitted to acoustic modulation. Acoustic simulations are performed with COMSOL Multiphysics on a simplified geometrical model of the industrial boiler. Three low frequency modes established between the plenums and the combustion chamber have been identified and may be unstable. Their sensitivity to modifications of the boiler geometry and boundary conditions are studied. Flame responses subjected to acoustic modulations of the airflow rate are then measured on the DIFAV combustor for several amplitudes and two flames topologies obtained at globally lean condition. Phase-conditioned flame visualizations and measurements of swirl number fluctuations during an acoustic forcing cycle are conducted to explain the mechanisms that control the evolution of gain of the Flame Describing Function (FDF). A high sensitivity of the phase of the FDF to the amplitude of the acoustic disturbance is observed. The Strouhal number based on the airflow velocity and the effective length of the flame is used to transpose these FDF on the industrial burner. FDF are integrated in the acoustic model of the DIFAV setup to carry out a stability analysis and predict the limit cycle oscillations as a function of the combustion chamber length. These calculations are compared to frequencies of self-sustained instability measured at the limit cycles in the DIFAV combustor. A reasonable agreement is obtained showing the validity of the stability analysis for the non-premixed two-phase flames investigated based on the knowledge of their FDF. Finally, a stability analysis of the EDF boiler is conducted with the COMSOL Multiphysics model by including the acoustic flame response of the industrial burner in the simulation. This FDF is deducted from the dimensionless FDF measured on the generic burner. The Rayleigh criterion is used to analyze the stability of the combustor as a function of the flame length for different boundary conditions. Indications are given to improve the stability of the EDF boiler

Gli stili APA, Harvard, Vancouver, ISO e altri

31

Krichene, Assaad. "Active identification and control of aerodynamic instabilities in axial and centrifugal compressors". Diss., Georgia Institute of Technology, 2001. http://hdl.handle.net/1853/12062.

Testo completo

Gli stili APA, Harvard, Vancouver, ISO e altri

32

Miranda, Juliana Albertoni de. "Dynamics of Brazil Current dipoles: barotropic instabilities and flow-western boundary interactions". Universidade de São Paulo, 2013. http://www.teses.usp.br/teses/disponiveis/21/21135/tde-09022015-095946/.

Testo completo

Abstract (sommario):

This dissertation examines the nature of jet-boundary interactions and the role of barotropic instabilities in the Brazil Current system while still attached to the western boundary. The motivation was due to the frequent observation of bipolar features associated to the Brazil Current flow south of Cape Frio (RJ) and off Santos Bight (22º-28ºS). Such observations were mainly obtained from sea surface temperature images, and also from few \"in situ\" hydrographic and direct velocity measurements data. Therefore, our main focus is on the formation of bipolar features associated with the flow, and the main hypothesis is that barotropic instability is responsible for the generation of such dipoles along the western boundary current jet, while it has to deal with topographic variations along its path poleward. We address the system dynamics from a semi-theoretical perspective, and also through the application of numerical modeling on a process study approach. So it includes semi-theoretical studies of jet-lateral boundary interactions in idealized configurations relevant to the Brazil Current system off Cape Frio, considering the quasi-geostrophic theory as an appropriate approximation of the system we want to investigate. Additionally, numerical modeling is used through the construction of idealized scenarios where we simplify the physics in order to isolate the dynamical process of interest. All the dynamical analyzes were initialized from synoptic hydrographic data set which comprised the Brazil Current system off Cape Frio region. We validated the quasi-geostrophic theory we assumed and obtained the kinematics characteristics of the jet. In the dynamical analyses, we started the investigation from the simplest framework applied here, where we evaluate a piecewise constant potential vorticity field in a quasi-geostrophic contour dynamics model. It dealt with a meridionally-oriented jet flowing southward along a straight western boundary. Next, in a second model, we added more complexity in the system, idealizing western boundary coastline scenarios considering a quasi-continuous potential vorticity field in a quasi-geostrophic numerical model. Finally, in a third model, we constructed a more complex scenario for the Brazil Current jet that incorporated real topography and stratification of the water column in a primitive equation numerical model. Among our main finds, we verified that purely barotropic instability is able to generate vortex dipoles. Moreover, variations in the western boundary can indeed trigger perturbations in the jet and dipoles form. Therefore, sites with abrupt change in bathymetry and coastline orientation are preferred to the formation of the BC vortical dipoles. Vortex streets and instability trains can also develop downstream of such locations as consequence of perturbing a potentially batropically unstable jet. Hence, the horizontal shear is key to the generation of vortex dipoles. A weak shear does not allow these features to form, instead, frontal eddies are possibly generated, with the predominance of anticyclones. Although the three different model strategies applied here differ in dynamical configurations and approximations, they still kept nearly the same regime for the Brazil Current vortex-dipole formation. Topographic variations such as those associated with the change of coastline orientation near Cape Frio (23ºS) account for those vortex dipoles and streets be dominantly observed within the Santos Bight.
A presente tese examina a natureza das interações entre jato e contorno e o papel de instabilidades barotrópicas no sistema Corrente do Brasil (CB) quando este ainda se encontra fluindo junto à margem continental oeste. A motivação se deu através da frequente observação de feições bipolares associadas ao escoamento da CB ao sul de Cabo Frio (RJ) e ao largo da Bacia de Santos (entre 22º e 28ºS). Tais observações se devem principalmente a imagens termais de temperatura da superfície do mar, a algumas raras observações \"in situ\" através de dados hidrográficos e medições diretas de velocidade. Assim, o principal foco é na formação de feições bipolares associadas ao fluxo médio, sendo a principal hipótese a de que instabilidade barotrópica é responsável pela formação destes dipolos ao longo da corrente quando esta tem que lidar com variações da topografia. Este estudo tenta abordar os problemas em uma perspectiva semi-teórica, e também através de modelagem numérica em uma abordagem de estudos de processo. Assim, inclui estudos semi-teóricos em configurações idealizadas relevantes para o sistema Corrente do Brasil ao largo de Cabo Frio, considerando a teoria quase-geostrófica como a aproximação apropriada para a dinâmica do sistema que queremos avaliar. Adicionalmente, a modelagem numérica é usada através da construção de cenários idealizados onde simplificamos a física a fim de isolar os processos que queremos investigar. Toda a análise dinâmica partiu de um conjunto de dados que compreendeu o sistema Corrente do Brasil ao largo de Cabo Frio. Validamos a teoria quase-geostrófica que estamos considerando e obtivemos as características cinemáticas do jato. Nas análises dinâmicas, começamos a investigação do problema partindo do cenário mais simples utilizado aqui, onde consideramos um campo de vorticidade potencial discretizado em camadas horizontais em um modelo quase-geostrófico de dinâmica de contornos. O modelo incorporou a presença de uma linha de costa retilínea orientada meridionalmente no contorno oeste. Posteriormente, incluímos mais complexidade no sistema, idealizando diferentes cenários de linha de costa e considerando um campo de vorticidade potencial quase-contínuo em um modelo numérico quase-geostrófico. Finalmente, construímos um cenário ainda mais complexo para a Corrente do Brasil, o qual incorporou a topografia real da região e a estratificação da coluna de água em um modelo numérico de equações primitivas. Dentre as principais conclusões, pudemos comprovar que instabilidade barotrópica pode promover a formação de dipolos. Além disso, variações no contorno podem consequentemente ser gatilhos para gerar perturbações no jato e dipolos se formam. Assim, locais de mudança abrupta de batimetria e orientação de linha de costa são preferidos para a formação de dipolos vorticais. Rua de vórtices e trens de instabilidade também podem se desenvolver à juzante de tais locais como consequência de se perturbar um jato potencialmente barotropicamente instável. Consequentemente, um cisalhamento horizontal é chave para a geração de dipolos vorticais. Um cisalhamento relativamente fraco não permite a formação de tais feições, e em vez disso, vórtices frontais são possivelmente gerados, com a predominância de anticiclones. Apesar de as três diferentes estratégias aplicadas aqui diferirem em termos de configurações dinâmicas e aproximações, estas ainda mantiveram aproximadamente o mesmo regime para a formação de dipolos. Variações na topografia tais como aquelas associadas com mudanças de orientação de costa próximo a Cabo Frio (23ºS) contam com o fato de dipolos vorticais e rua de vórtices serem frequentemente observados dentro da Bacia de Santos.

Gli stili APA, Harvard, Vancouver, ISO e altri

33

Armanini, Costanza. "Instabilities and dynamics of elastic rods in the presence of movable constraints". Doctoral thesis, Università degli studi di Trento, 2018. https://hdl.handle.net/11572/367733.

Testo completo

Abstract (sommario):

A new trend in the mechanical design of devices for advanced technologies, such as soft robotics and micro/nano mechanics, is the exploitation of structures undergoing large deflections, in an attempt of achieving superior performances. Within this framework, non-linear modelling becomes a fundamental tool for the design of compliant structures and deformable mechanism. Two structural systems are investigated, both based on the planar elastica and subject to movable and configurational constraints. These two structures disclose unforeseen behaviours when the values of the parameters defining the models are varied. The first structural system is an elastic rod constrained by a slowly rotating clamp, while the other end is loaded with a lumped mass weight. When this weight is lower than that corresponding to buckling, the edge of the rod describes a closed curve, behaving as an elastica compass. Differently, when the load is higher than that of buckling, a release of elastic energy is observed, leading to a snapback of the structure, so that the rod realizes an elastica catapult. The clamp in the above described structure is replaced by a frictionless and fixed sliding sleeve in the second system considered in this thesis. The rod is subject to a sudden release from the underformed configuration, providing dynamic effects on the system. By means of the variational approach, the presence of a configurational force at the exit of the sliding sleeve is proven within the dynamical setting, extending previous results restricted to the quasi-static assumption. The configurational force is found to strongly affect the dynamics of the structure. In particular, two different behaviours are observed, in which the rod may either completely penetrate in ("injection") or be expelled from ("ejection") the sliding sleeve. In both the above problems, the theoretical predictions are corroborated through the experimental validation on physical models, which have been ad hoc invented and designed. A new insight is obtained in the design of flexible devices, paving the way to applications in soft robotics.

Gli stili APA, Harvard, Vancouver, ISO e altri

34

Armanini, Costanza. "Instabilities and dynamics of elastic rods in the presence of movable constraints". Doctoral thesis, University of Trento, 2018. http://eprints-phd.biblio.unitn.it/2853/1/Thesis_Armanini.pdf.

Testo completo

Abstract (sommario):

A new trend in the mechanical design of devices for advanced technologies, such as soft robotics and micro/nano mechanics, is the exploitation of structures undergoing large deflections, in an attempt of achieving superior performances. Within this framework, non-linear modelling becomes a fundamental tool for the design of compliant structures and deformable mechanism. Two structural systems are investigated, both based on the planar elastica and subject to movable and configurational constraints. These two structures disclose unforeseen behaviours when the values of the parameters defining the models are varied. The first structural system is an elastic rod constrained by a slowly rotating clamp, while the other end is loaded with a lumped mass weight. When this weight is lower than that corresponding to buckling, the edge of the rod describes a closed curve, behaving as an elastica compass. Differently, when the load is higher than that of buckling, a release of elastic energy is observed, leading to a snapback of the structure, so that the rod realizes an elastica catapult. The clamp in the above described structure is replaced by a frictionless and fixed sliding sleeve in the second system considered in this thesis. The rod is subject to a sudden release from the underformed configuration, providing dynamic effects on the system. By means of the variational approach, the presence of a configurational force at the exit of the sliding sleeve is proven within the dynamical setting, extending previous results restricted to the quasi-static assumption. The configurational force is found to strongly affect the dynamics of the structure. In particular, two different behaviours are observed, in which the rod may either completely penetrate in ("injection") or be expelled from ("ejection") the sliding sleeve. In both the above problems, the theoretical predictions are corroborated through the experimental validation on physical models, which have been ad hoc invented and designed. A new insight is obtained in the design of flexible devices, paving the way to applications in soft robotics.

Gli stili APA, Harvard, Vancouver, ISO e altri

35

Sainsbury-Martinez, Felix. "Flows, instabilities, and magnetism in stars and planets". Thesis, University of Exeter, 2017. http://hdl.handle.net/10871/32072.

Testo completo

Abstract (sommario):

Flows, instabilities, and magnetism play significant roles in the internal and atmospheric dynamics of objects ranging from the smallest exoplanets to the largest stars. These phenomena are governed by the equations of magnetohydrodynamics (MHD), which link the flows and magnetic fields, and from which the operational parameters and growth rates of instabilities can be recovered. Here we present an overview of interesting phenomena (such as the internal dynamics of stellar and planetary objects, as well as instabilities which might operate within these environs), as well as computational techniques by which these phenomena might both be understood and analysed (through both ‘simplifications’ of the MHD equations and different numerical/computational approaches). We first present an investigation into the Heat-Flux-Driven Buoyancy Instability (HBI) within stellar and planetary atmospheres, considering both the parameter space it might operate within as well as its non-linear effects during said operation. We find that whilst the HBI may be able to play a role in Solar, stellar and planetary atmospheres, it is likely to be quite limited in scope, only operating within small regions. However, its dramatic consequences for heat transport in the non-linearly evolved state, and the prospects that it may operate outside the narrow regimes that our analytical analysis suggested, suggest that it may merit further study. This is followed with a discussion of a method by which the surface flows of exoplanets might be measured: The Rossiter-Mclaughlin Effect at Secondary Eclipse (RMse). We formulate the effect, showing that the formalism is identical to the traditional Rossiter-Mclaughlin effect, albeit in a different frame (a planet transiting a star becomes a star transiting a planet), and consider its observational implications: the effect should be observable for the brightest planet hosting stars using upcoming 40m-class telescopes (i.e.E-ELT). We finish with a series of 3D anelastic simulations of fully convective stars, designed to investigate how the internal flows are affected by varying stellar parameters, as well as a possible link between residual entropy and differential rotation contours, and a method by which this link can be used (via the thermal wind equation - TWE) to extrapolate the internal rotation. We find a clear transition between ‘solar-like’ and ‘anti-solar’ internal dynamics, characterised in the meridional circulation, differential rotation, residual entropy, and angular momentum flux profiles. Furthermore we find that, whilst the alignment between residual entropy and differential rotation contours is somewhat varied, the resultant extrapolation, via the TWE, produces a generally good fit to the differential rotation contours, suggesting a general robustness to the theory.

Gli stili APA, Harvard, Vancouver, ISO e altri

36

Lamraoui, Ammar. "Acoustique et dynamique de flamme dans un foyer turbulent prémélangé swirlé : application à l'étude du bruit de combustion dans les chambres de turbines à gaz". Phd thesis, Ecole Centrale Paris, 2011. http://tel.archives-ouvertes.fr/tel-00678248.

Testo completo

Abstract (sommario):

La réduction des émissions de polluants et l'augmentation du rendement des moteurs ont conduit à une large utilisation de régimes de combustion pauvres en carburant dans les foyers de type moteurs aéronautiques et turbines à gaz. Des phénomènes de bruit et d'instabilités de combustion peuvent alors apparaître. Des fluctuations cycliques auto-entretenues de la pression au sein d'un foyer peuvent conduire à une limitation des régimes de fonctionnement ou une usure rapide et indésirable des installations et dans certains cas une destruction du système. L'objectif de ce travail de thèse est d'étudier les mécanismes responsables du bruit de combustion et des instabilités dans un foyer turbulent prémélangé swirlé. L'étude repose sur une analyse du champ de pression au sein du foyer, de la dynamique de la combustion et une caractérisation détaillée des conditions limites en amont, aval et dans les lignes d'alimentation en combustible et en comburant. Le banc expérimental CESAM ("Combustion Étagée Swirlée Acoustiquement Maîtrisée") est utilisé au cours de ce travail. Basée sur des observations expérimentales, une étude théorique de l'acoustique du foyer est tout d'abord réalisée grâce à un modèle à deux cavités couplées qui modélisent le tube de prémélange et la chambre de combustion de ce banc. Les fréquences et les structures spatiales des modes propres du foyer sont examinées, et des comparaisons sont menées avec les résultats expérimentaux. La condition limite au fond du tube de prémélange est mesurée, et utilisée comme entrée dans le modèle. L'effet de cette condition sur la prévision des fréquences des modes propres est examiné. Par la suite, le code de calcul AVSP est utilisé pour valider les résultats obtenus avec le modèle couplé. L'interaction entre ces modes acoustiques et la flamme est mise en évidence en caractérisant la dynamique de l'écoulement réactif. La vélocimétrie par images de particules (PIV) à haute cadence est utilisée. Une première étude est menée sur les champs de vitesse moyens et fluctuants puis on s'intéresse à l'analyse spectrale des champs de vitesse instantanés, rendue possible par la haute cadence du diagnostic. Un post-traitement faisant intervenir une méthode de détection des tourbillons est ensuite mis en oeuvre en utilisant le critère _2. Des structures cohérentes sont convectées le long du front de flamme à la fréquence du second mode instable du foyer. Le chapitre précédent ayant permis de montrer que ce mode acoustique était essentiellement associé au tube de prémélange, le mécanisme de couplage est clairement identifié. Par la suite, un traitement en moyenne de phase est appliqué aux champs de vitesse axiale. Des mouvements de battements des bras de la flamme dans les directions longitudinale et transverse sont mis en évidence aux fréquences des modes instables. L'émission naturelle de la flamme est également mesurée avec une caméra rapide. Une analyse spectrale et un traitement en moyenne phase avec transformée d'Abel sont appliqués aux images pour caractériser les régions de la flamme présentant une forte réponse aux fréquences des modes acoustiques du foyer. Les mécanismes à l'origine du bruit sont analysés en corrélant les mesures optiques et acoustiques. Au cours de cette étude, des fonctions de transfert de flamme FTF sont également caractérisées aux fréquences des modes propres du foyer, liant perturbations amont et réponse de flamme. La vitesse acoustique est reconstruite dans le tube de prémélange à partir des mesures des microphones. La FTF est calculée grâce aux mesures de vitesse par PIV, à l'émission des radicaux OH* et CH* et à l'émission naturelle de la flamme obtenue par caméra rapide. La caractérisation et la modélisation du système composé du tube de prémélange et de la chambre de combustion montrent qu'il est nécessaire de s'intéresser à l'influence des conditions aux limites sur les propriétés de la flamme et la stabilité du brûleur.

Gli stili APA, Harvard, Vancouver, ISO e altri

37

Selçuk, Savas Can. "Numerical study of helical vortices and their instabilities". Thesis, Paris 6, 2016. http://www.theses.fr/2016PA066138/document.

Testo completo

Abstract (sommario):

Le travail présenté dans ce mémoire est une contribution à l'étude numérique des systèmes tourbillonnaires hélicoïdaux qui sont émis dans le sillage des rotors (éoliennes, hélicoptères,...) et de leurs instabilités. Ici, ces écoulements sont localement modélisés par un ensemble de tourbillons à symétrie hélicoïdale. À l'aide d'un code de simulation numérique directe dédié, des solutions de base quasi-stationnaires sont obtenues pour différents systèmes tourbillonnaires. Une caractérisation précise et détaillée de ces solutions est ensuite effectuée : vitesse de rotation, taille et ellipticité du cœur, structure des champs de vitesse et de vorticité... À l'aide d'un algorithme d'Arnoldi couplé à une version linéarisée du code, on détermine les modes dominants d'instabilité ayant la même symétrie que l'écoulement de base, en fonction des paramètres du système: nombre de vortex, pas hélicoïdal, taille de cœur, nombre de Reynolds et présence d'un vortex de moyeu. En dessous d'un certain pas hélicoïdal critique, l'instabilité est dominée par un mode de déplacement global analogue au mode d’appariement d'une allé infinie de points vortex ou d'anneaux tourbillonnaires. En régime non linéaire, ce mode est à l'origine d'une dynamique complexe du système: dépassements, saute-mouton et fusion. On utilise un autre code linéarisé pour déterminer les modes instables qui brisent la symétrie hélicoïdale de l'état de base, caractérisés par une longueur suivant l'axe. À faible nombre d'onde, ces modes induisent localement des rapprochements entre portions de spires voisines. À grand nombre d'onde, on observe un autre type de mode qui déforme les cœurs tourbillonnaires via l'instabilité elliptique
The work presented in this manuscript is a contribution to the numerical study of helical vortex systems and their instabilities, as encountered in the near wake of rotors (wind turbines, helicopters,~...). In this work, such flows are locally modelled within the framework of helical symmetry. Using a dedicated DNS code, helical quasi-stationary basic state solutions are obtained for several configurations, and accurate tools for their characterisation are developed: angular velocity, core size and ellipticity, structure of the velocity and vorticity fields... An Arnoldi algorithm is then coupled to a linearised version of the code. The dominant instability modes with the same symmetry as the base flow are extracted as a function of the system parameters: number of vortices, helical pitch, core size, Reynolds number, presence of a central hub vortex. Under a critical helical pitch, the instability is dominated by a global displacement mode analogous to the pairing mode of an infinite array of point vortices or vortex rings. In the nonlinear regime, this mode gives rise to complex dynamics: overtaking events, leapfrogging and merging. Another linearised code is then used to extract modes characterised by a wavelength along the helix, which break the helical symmetry of the base flow. At low wavenumbers, these modes induce local displacements of the vortices and bring together portions of neighbouring coils. At large wavenumbers, another type of mode is found, which deforms the vortex cores through the elliptical instability mechanism

Gli stili APA, Harvard, Vancouver, ISO e altri

38

Dubois, Thibault. "Electron dynamics in crossed-field discharges". Electronic Thesis or Diss., Orléans, 2023. http://www.theses.fr/2023ORLE1069.

Testo completo

Abstract (sommario):

Les décharges plasma en champs croisés ont une grande variété d'applications, y compris la propulsion spatiale (les propulseurs de Hall) et le traitement des matériaux (magnétrons planaires). La configuration croisée des champs magnétiques et électriques crée des phénomènes tels que la turbulence plasma et des anisotropies dans les propriétés des particules, dont la compréhension nécessite des informations détaillées sur les électrons. L'étude expérimentale directe de ces caractéristiques contribuerait au développement de simulations numériques de haute fidélité de ces plasmas et à une meilleure compréhension de leur fonctionnement. Un effort de recherche intense a été consacré ces dernières années à l'étude de phénomènes tels que le transport "anormal" d'électrons à travers les lignes de champ, qui dépasse de plusieurs ordres de grandeur le transport attendu par les collisions. L'incompréhension de ce phénomène limite la capacité de prédiction des codes numériques pour les propulseurs et autres dispositifs. Des travaux récents ont cependant établi un lien entre ce transport et les instabilités à haute fréquence dans le plasma. La complexité de ce comportement (propagation 3D, effets de couplage) nécessite le développement de codes numériques performants à trois dimensions, mais aussi des diagnostics avancés pour sonder pour le comportement et les propriétés des électrons. La nature de ces décharges (électrons énergétiques et magnétisés, leur architecture) rend les diagnostics invasifs tels que les sondes de Langmuir limités dans leur capacité à sonder les caractéristiques des électrons. Dans ce travail, deux diagnostics basés sur la diffusion Thomson ont été appliqués : THETIS (diffusion Thomson incohérente), pour mesurer les propriétés individuelles des électrons telles que la température (plus largement, les fonctions de distribution de l'énergie des électrons), la densité et la vitesse de dérive, et PRAXIS (diffusion Thomson cohérente), pour mesurer les fluctuations à petite échelle de la densité des électrons (associées aux ondes MHz). Dans un propulseur à effet Hall de 1,5 kW, l'évolution des propriétés des électrons le long de la direction radiale a été directement mesurée. Une variation radiale, plus marquée pour la température, a été observée. Un modèle de théorie cinétique linéaire a été utilisé pour évaluer la forme de la relation de dispersion correspondant aux conditions de plasma mesurées, et celle-ci a été comparée aux mesures antérieures effectuées avec PRAXIS. Cette analyse a révélé un effet de lissage de la relation de dispersion qui peut être expliqué par la variation des propriétés des électrons le long du volume de mesure. Dans un magnétron planaire HiPIMS, plusieurs conditions de décharge ont été étudiées, avec de l'argon et de l'hélium, pour des courants de pointe allant de 40 A à 600 A. Des caractéristiques résolues en temps, telles que l'anisotropie de la vitesse de dérive, ont été mesurées, et une analyse des contributions aux dérives des particules a été réalisée dans différentes conditions. La théorie cinétique linéaire a été adaptée pour le magnétron. L'application de PRAXIS aux mesures dans le magnétron planaire a révélé la présence de deux instabilités dans le mode HiPIMS, l'une identifiée comme l'instabilité de dérive cyclotronique électronique (ECDI), étudiée dans des travaux antérieurs, et le second mode se propageant à un angle de 45° par rapport à la surface de la cible. Une analyse préliminaire de la perturbation par une simple sonde de Langmuir dans le plasma a également été effectuée sur le magnétron planaire, et il a été observé qu'une augmentation systématique de la vitesse de groupe de l'instabilité se produisait en présence de la sonde. L'application de diagnostics avancés dans le cadre de ce travail a permis une meilleure caractérisation (y compris avec une résolution temporelle élevée) des propriétés et de la dynamique des électrons de ces dispositifs en champs croisés
Crossed-field plasma discharges have a wide variety of applications, including space propulsion (e.g. Hall thrusters) and materials processing (planar magnetrons). The crossed magnetic and electric field configuration creates features such as plasma turbulence and anisotropies in particle properties, the understanding of which require detailed information on electrons. Direct experimental study of these features would contribute to the development of high-fidelity numerical simulations of such plasmas, and to an improved understanding of their operation. An intense research effort has been devoted in recent years to the study of phenomena such as so-called "anomalous" electron transport across the field lines, which exceeds by several orders of magnitude that expected from collisions. The lack of complete understanding of this phenomenon limits the predictive capability of numerical codes for thrusters and other devices. Recent work has, however, established a link between this transport and high-frequency instabilities within the plasma. The complexity of this behavior (3D propagation, coupling effects) requires not only the development of high-performance, full-dimensional numerical codes, but also advanced diagnostics for electron behavior and properties.The nature of these discharges (such as high degrees of electron magnetization and energy, architecture) render invasive diagnostics such as Langmuir probes limited in their capacity to probe electron features. In this work, two diagnostics based on Thomson scattering were applied to the study of such features: THETIS (incoherent Thomson scattering), for measurement of individual electron properties such as temperature (more broadly, electron energy distribution functions), density and drift velocity, while PRAXIS (coherent Thomson scattering), for measurement of small-scale electron density fluctuations (associated with MHz-frequency waves). In a 1.5 kW Hall-effect thruster, the evolution of electron properties along the radial direction was directly measured. A radial variation in the electron properties, most marked for electron temperature, was observed. A linear kinetic theory model was used to evaluate the form of the dispersion relation corresponding to the measured plasma conditions, and this was compared with previous measurements made using PRAXIS. This analysis revealed a smoothing effect of the dispersion relation which could be accounted for due to the variation of electron properties along the characteristic measurement volume dimensions. In a HiPIMS planar magnetron, several discharge conditions were studied, with argon and helium, for peak currents ranging from 40 A to 600 A. Time-resolved features such as anisotropy of the drift velocity were measured, and an analysis of the contributions to the particle drifts was performed under varying conditions. The linear kinetic theory was adapted to the magnetron. The application of PRAXIS to measurements in the planar magnetron revealed the presence of two instabilities in the HiPIMS mode, one identified as the electron cyclotron drift instability (ECDI), studied in previous work, and the second mode propagating at an angle of 45° to the target surface. A preliminary analysis of the perturbative influence of a simple Langmuir probe in the plasma was also carried out on the planar magnetron, and it was observed that a systematic increase in the instability group velocity was occurred in the presence of such a probe.The application of advanced electron diagnostics to measurements in this work have enabled an improved characterization (including with high temporal resolution) of the electron properties and dynamics of these crossed-field devices

Gli stili APA, Harvard, Vancouver, ISO e altri

39

Peacock, Thomas. "Global dynamics in a liquid crystal flow". Thesis, University of Oxford, 1997. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.299157.

Testo completo

Gli stili APA, Harvard, Vancouver, ISO e altri

40

Koutsogiannakis, Panagiotis. "Dynamics and instability of flexible structures with sliding constraints". Doctoral thesis, Università degli studi di Trento, 2022. https://hdl.handle.net/11572/362124.

Testo completo

Abstract (sommario):

Although instabilities and large oscillations are traditionally considered as conditions to be avoided in structures, a new design philosophy based on their exploitation towards the achievement of innovative mechanical features has been initiated in the last decade. In this spirit, instabilities are exploited towards the development of systems that can yield designed responses in the post-critical state. Further, the presence of oscillating constraints may allow for a stabilization of the dynamic response. These subjects entail a rich number of phenomena due to the non-linearity, so that the study of such mechanical systems becomes particularly complex, from both points of view of the mechanical modeling and of the computational tools. Two elastic structures are studied. The first consists of a flexible and extensible rod that is clamped at one end and constrained to slide along a given profile at the other. This feature allows one to study the effect of the axial stiffness of the rod on the tensile buckling of the system and on the compressive restabilization. A very interesting effect is that in a region of parameters double restabilization is found to occur, involving four critical compressive loads. Also, the mechanical system is shown to work as a novel force limiter that does not depend on sacrificial mechanical elements. Further, it is shown that the system can be designed to be multi-stable and suitable for integration in metamaterials. The second analyzed structure is a flexible but inextensible rod that is partially inserted into a movable rigid sliding sleeve which is kept vertical in a gravitational field. The system is analytically solved and numerically and experimentally investigated, when a horizontal sinusoidal input is prescribed at the sliding sleeve. In order to model the system, novel computational tools are developed, implementing the fully nonlinear inextensibility and kinematic constraints. It is shown that the mathematical model of the system agrees with the experimental data. Further, a study of the inclusion of dissipative terms is developed, to show that a steady motion of the rod can be accomplished by tuning the amplitude or the frequency of the sliding sleeve motion, in contrast with the situation in which a complete injection of the rod inside the sleeve occurs. A special discovery is that by slowly decreasing the frequency of the sleeve motion, the length of the rod outside the sleeve can be increased significantly, paving the way to control the rod’s end trajectory through frequency modulation.

Gli stili APA, Harvard, Vancouver, ISO e altri

41

Colinet, Pierre. "Amplitude equations and nonlinear dynamics of surface-tension and buoyancy-driven convective instabilities". Doctoral thesis, Universite Libre de Bruxelles, 1997. http://hdl.handle.net/2013/ULB-DIPOT:oai:dipot.ulb.ac.be:2013/212204.

Testo completo

Abstract (sommario):

This work is a theoretical contribution to the study of thermo-hydrodynamic instabilities in fluids submitted to surface-tension (Marangoni) and buoyancy (Rayleigh) effects in layered (Benard) configurations. The driving constraint consists in a thermal (or a concentrational) gradient orthogonal to the plane of the layer(s).

Linear, weakly nonlinear as well as strongly nonlinear analyses are carried out, with emphasis on high Prandtl (or Schmidt) number fluids, although some results are also given for low-Prandtl number liquid metals. Attention is mostly devoted to the mechanisms responsible for the onset of complex spatio-temporal behaviours in these systems, as well as to the theoretical explanation of some existing experimental results.

As far as linear stability analyses (of the diffusive reference state) are concerned, a number of different effects are studied, such as Benard convection in two layers coupled at an interface (for which a general classification of instability modes is proposed), surface deformation effects and phase-change effects (non-equilibrium evaporation). Moreover, a number of different monotonous and oscillatory instability modes (leading respectively to patterns and waves in the nonlinear regime) are identified. In the case of oscillatory modes in a liquid layer with deformable interface heated from above, our analysis generalises and clarifies earlier works on the subject. A new Rayleigh-Marangoni oscillatory mode is also described for a liquid layer with an undeformable interface heated from above (coupling between internal and surface waves).

Weakly nonlinear analyses are then presented, first for monotonous modes in a 3D system. Emphasis is placed on the derivation of amplitude (Ginzburg-Landau) equations, with universal structure determined by the general symmetry properties of the physical system considered. These equations are thus valid outside the context of hydrodynamic instabilities, although they generally depend on a certain number of numerical coefficients which are calculated for the specific convective systems studied. The nonlinear competitions of patterns such as convective rolls, hexagons and squares is studied, showing the preference for hexagons with upflow at the centre in the surface-tension-driven case (and moderate Prandtl number), and of rolls in the buoyancy-induced case.

A transition to square patterns recently observed in experiments is also explained by amplitude equation analysis. The role of several fluid properties and of heat transfer conditions at the free interface is examined, for one-layer and two-layer systems. We also analyse modulation effects (spatial variation of the envelope of the patterns) in hexagonal patterns, leading to the description of secondary instabilities of supercritical hexagons (Busse balloon) in terms of phase diffusion equations, and of pentagon-heptagon defects in the hexagonal structures. In the frame of a general non-variational system of amplitude equations, we show that the pentagon-heptagon defects are generally not motionless, and may even lead to complex spatio-temporal dynamics (via a process of multiplication of defects in hexagonal structures).

The onset of waves is also studied in weakly nonlinear 2D situations. The competition between travelling and standing waves is first analysed in a two-layer Rayleigh-Benard system (competition between thermal and mechanical coupling of the layers), in the vicinity of special values of the parameters for which a multiple (Takens-Bogdanov) bifurcation occurs. The behaviours in the vicinity of this point are numerically explored. Then, the interaction between waves and steady patterns with different wavenumbers is analysed. Spatially quasiperiodic (mixed) states are found to be stable in some range when the interaction between waves and patterns is non-resonant, while several transitions to chaotic dynamics (among which an infinite sequence of homoclinic bifurcations) occur when it is resonant. Some of these results have quite general validity, because they are shown to be entirely determined by quadratic interactions in amplitude equations.

Finally, models of strongly nonlinear surface-tension-driven convection are derived and analysed, which are thought to be representative of the transitions to thermal turbulence occurring at very high driving gradient. The role of the fastest growing modes (intrinsic length scale) is discussed, as well as scalings of steady regimes and their secondary instabilities (due to instability of the thermal boundary layer), leading to chaotic spatio-temporal dynamics whose preliminary analysis (energy spectrum) reveals features characteristic of hydrodynamic turbulence. Some of the (2D and 3D) results presented are in qualitative agreement with experiments (interfacial turbulence).

Doctorat en sciences appliquées
info:eu-repo/semantics/nonPublished

Gli stili APA, Harvard, Vancouver, ISO e altri

42

Ranalli, Joseph Allen. "Spatially Resolved Analysis of Flame Dynamics for the Prediction of Thermoacoustic Combustion Instabilities". Diss., Virginia Tech, 2009. http://hdl.handle.net/10919/27657.

Testo completo

Abstract (sommario):

Increasingly stringent emissions regulations have led combustion system designers to look for more environmentally combustion strategies. For gas turbine combustion, one promising technology is lean premixed combustion, which results in lower flame temperatures and therefore the possibility of significantly reduced nitric oxide emissions. While lean premixed combustion offers reduced environmental impacts, it has been observed to experience increased possibility of the occurrence of combustion instabilities, which may damage hardware and reduce efficiency. Thermoacoustic combustion instabilities occur when oscillations in the combustor acoustics and oscillations in the flame heat release rate form a closed feedback loop, through one of two possible mechanisms. The first is direct coupling which occurs due to the mean mass flow oscillations induced by the acoustic velocity. Secondly, the acoustics may couple with the flame due to acoustic interactions with fuel/air mixing, resulting in an oscillating equivalence ratio. Only velocity coupling was considered in this study. The methodology used in this study is analysis of instabilities through linear systems theory, requiring knowledge of the individual transfer functions making up the closed-loop system. Methods already exist by which combustor acoustics may be found. However, significant gaps still remain in knowledge of the nature of flame dynamics. Prior knowledge in literature about the flame transfer function suggests that the flame behaves as a low-pass filter, with cutoff frequency on the order of hundreds of hertz. Nondimensionalization of the frequency by flame length scales has been observed to result in a convenient scaling for the flame transfer function, suggesting that the flame dynamics may be dominated by spatial effects. This work was proposed in two parts to extend and apply the body of knowledge on flame dynamics. The phase one goal of this study was to further understand this relationship between the flame heat release rate dynamics and the dynamics of the reaction zone size. The second goal of this work was to apply this flame transfer function knowledge to predictions of instability, validated against measurements in an unstable combustor. Both of these goals meet an existing practical need, providing a design tool for prediction of potential thermoacoustic instabilities in a combustor at the design stage.Measurements of the flame transfer function were made in a swirl-stabilized, lean-premixed combustor. The novel portion of these measurements was the inclusion of spatial resolution of the heat release rate dynamics. By using a speaker, a sine dwell excitation to the velocity was introduced over the range of 10-400Hz. Measurements were then made of the input (inlet velocity) and output (heat release rate, or flame size) resulting in the flame transfer function. The spatial dynamics measurement was approached through several measures of the flame size: the volume and offset distance to the center of the heat release. Each was obtained from deconvoluted, phase averaged images of the flame, referenced to the speaker excitation signal. The results of these measurements showed that the spatial dynamics for each of these three measures were virtually identical to the heat release rate dynamics. This suggests a quite important result, namely that the flame heat release rate dynamics are completely determined by the dynamics of the flame structure. Therefore, prediction of flow structure interaction with the flame distribution is crucial to predict the dynamics of the flame. These spatially resolved transfer function measurements were used in conjunction with the linear closed-loop model to make predictions of instability. These predictions were made by applying the Bode stability criterion to the open-loop system transfer function. This criterion states that instabilities may occur at frequencies where the heat release rate and acoustic oscillations occur in phase and the system gain has a value greater than unity. Performing this analysis on the combined system transfer function yielded results that agreed quite well with actual instability measurements made in the combustor. Closed-loop predictions identified two possible modes for instability, both of which were observed experimentally. One mode resulted from an acoustic peak around 160 Hz, and occurred at lean equivalence ratios. A second mode occurred at lower frequencies (100-150 Hz) and was associated with the increase in flame transfer function gain at increasing equivalence ratios. These are some of the first successful predictions of combustion instability based on linear systems theory. When multiple modes were predicted, it was assumed that if non-linear effects were to be considered, the lower frequency mode would become the dominant mode at these operating conditions due to its higher gain margin. Also of note is that in the practical system, high frequency oscillations are observed, but not predicted, associated with harmonics of the low frequency mode due to the linear nature of the predictions. While these non-linear effects are not captured, the linear predictive capability is thought to be most important, as from a practical perspective, instabilities should be avoided altogether. The primary findings of this study have significant applications to modeling and prediction of combustion dynamics. The classic heat release rate flame transfer function was observed to coincide almost exactly with the flame size transfer functions. The time scales observed in these transfer functions correspond to convective length scales in the combustor, suggesting a fluid mechanical basis of the heat release rate response. Additionally, linear systems theory predictions of instability based on the measured flame transfer functions were proved capable of capturing the stability of the actual combustor with a reasonable degree of accuracy. These predictions should have considerable application to design level avoidance of combustion instability in practical systems.
Ph. D.

Gli stili APA, Harvard, Vancouver, ISO e altri

43

Prieur, Kevin. "Dynamique de la combustion dans un foyer annulaire multi-injecteurs diphasique". Thesis, Université Paris-Saclay (ComUE), 2017. http://www.theses.fr/2017SACLC070/document.

Testo completo

Abstract (sommario):

Ces dernières décennies ont vu apparaître de nombreuses innovations dans le domaine de la combustion afin de réduire la consommation et les émissions polluantes. De nouveaux types d'injecteur, de type LPP - Lean Premixed Prevaporized, ont été mis au point permettant de diminuer le rapport combustible/air et visent à pré-vaporiser le carburant en amont de la combustion afin de mieux le mélanger à l'air issu du compresseur. Cette architecture permet une amélioration de la consommation et des émissions polluantes, mais rend les foyers annulaires plus sensibles à des phénomènes instationnaires qui perturbent le fonctionnement du système, accroissent les flux de chaleur vers les parois de la chambre, induisent des vibrations de structures, entrainent une fatigue cyclique des pièces mécaniques et dans des cas extrêmes conduisent à des dommages irréversibles. L'objectif est de poursuivre l'effort engagé au laboratoire EM2C sur ce thème et plus particulièrement sur celui de la dynamique de la combustion dans les chambres annulaires. La thèse concerne plus spécialement le cas où l'injection du combustible s'effectue sous forme liquide. La configuration reproduit sous forme idéalisée celle que l'on trouve en pratique dans les moteurs aéronautiques. La chambre, désignée sous le nom de MICCA-Spray, est équipée de 16 injecteurs swirlés pouvant être alimentés par un combustible liquide ou gazeux, permettant ainsi une combustion diphasique ou prémélangée. Le système possède des parois en quartz donnant un accès optique à la zone de flamme. Il est aussi équipé d'un ensemble de diagnostics tels des microphones, des photomultiplicateurs ainsi que des systèmes d'imagerie à haute cadence
These last decades have seen many innovations in the field of combustion to reduce fuel consumption and pollutant emissions. New types of injector, for example LPP - Lean Premixed Prevaporized, have then been developed to reduce the fuel / air ratio and aim to pre-vaporize the fuel upstream of the combustion in order to mix it better with the air coming from the compressor. Unfortunately this architecture makes annular chambers more sensitive to unsteady phenomena which disturb the functioning of the system, increase the heat flows towards the walls of the chamber, induce vibrations of structures, cause cyclic fatigue of mechanical parts and in extreme cases lead to irreversible damage. The objective of this thesis is to continue the effort undertaken at the EM2C laboratory on this topic and more particularly on the dynamics of combustion in annular chambers comprising a set of injectors. The thesis concerns more particularly the case where the injection of the fuel takes place in liquid form. This configuration reproduces, in idealized form, what can be found in practice in aeronautical engines. It is also a configuration studied at the fundamental level. The chamber, known as MICCA-Spray, is equipped with 16 swirled injectors that can be powered by liquid or gaseous fuel, thus enabling two-phase or fully premixed combustion. The system has quartz walls giving optical access to the flame zone. It is also equipped with a set of diagnostics such as microphones, photomultipliers and high-speed imaging systems

Gli stili APA, Harvard, Vancouver, ISO e altri

44

Hermeth, Sébastian. "Mechanisms affecting the dynamic response of swirled flames in gas turbines". Thesis, Toulouse, INPT, 2012. http://www.theses.fr/2012INPT0064/document.

Testo completo

Abstract (sommario):

Les réglementations toujours plus drastiques sur les émissions de polluants ont conduit au développement de systèmes de combustion opérant en régimes pauvres qui sont malheureusement sujet aux instabilités thermo acoustiques. La capacité de la Simulation aux Grandes Echelles (SGE) à simuler des turbines à gaz industrielles complexes de grande puissance est mise en évidence au cours de ce travail de thèse. Tout d’abord, la SGE est appliquée à un brûleur académique et validée par comparaison à des mesures effectuées à l’Université de Berlin ainsi qu’à des simulations SGE effectuées avec OpenFOAM chez Siemens. Afin de déterminer la stabilité de ce bruleur le couplage entre l’acoustique et la combustion est modélisé par l’approche de type fonction de transfert de flamme (FTF). Suite à ces calcules et l’évaluation de la FTF les fluctuations du nombre de swirl sont identifiées comme un paramètre à même de modifier cette réponse de flamme. Après cette première étape de validation, une turbine à gaz industrielle est simulée en SGE pour deux géométries différentes du brûleur et pour deux points de fonctionnement. La FTF issue de ces calculs est peu influencée par les deux points de fonctionnement. A l’inverse, des légères modifications de la géométrie du swirler modifient les caractéristiques de la FTF montrant que plusieurs mécanismes sont en jeu. Ces mécanismes sont identifiés comme étant la vitesse d’entrée, les fluctuations de swirl et les fluctuations de fraction de mélange. Cette dernière est causée par: 1) la pulsation du débit de carburant injecté et 2) la trajectoire fluctuante des jets de carburant. Bien que le swirler soit conçu pour fournir un mélange le plus homogène possible, d’importantes hétérogénéités de mélange à l’entrée de la chambre de combustion sont présentes. Les perturbations de mélange se combinent avec les fluctuations de vitesse (et donc avec les fluctuations de swirl) aboutissant à des résultats de FTF différents. Un modèle étendu pour la FTF reliant le dégagement de chaleur à la vitesse d’entrée et à la fluctuation de fraction de mélange (modèle MISO) se révèle être une bonne solution pour ces systèmes complexes. Une analyse non linéaire montre en outre que l’amplitude de forçage conduit non seulement à une saturation de la flamme, mais aussi à un changement de la réponse de flamme. La saturation de la flamme n’est vérifiée que pour la FTF globale et le gain augmente localement avec une amplitude croissante. Pour ce système on notera enfin que la flamme linéaire, comme la flamme non linéaire, ne sont pas compactes: certaines zones pourtant situées l’une à coté de l’autre, ont des différences significatives de délai de FTF, montrant que certaines parties de la flamme amortissent l’excitation alors que d’autres l’amplifient
Modern pollutant regulation have led to a trend towards lean combustion systems which are prone to thermo-acoustic instabilities. The ability of Large Eddy Simulation (LES) to handle complex industrial heavy-duty gas turbines is evidenced during this thesis work. First, LES is applied to an academic single burner in order to validate the modeling against measurements performed at TU Berlin and against OpenFoam LES simulations done at Siemens. The coupling between acoustic and combustion is modeled with the Flame Transfer Function (FTF) approach and swirl number fluctuations are identified changing the FTF amplitude response of the flame. Then, an industrial gas turbine is analyzed for two different burner geometries and operating conditions. The FTF is only slightly influenced for the two operating points but slight modifications of the swirler geometry do modify the characteristics of the FTF showing that a simple model taking only into account the flight time is not appropriate and additional mechanisms are at play. Those mechanisms are identified being the inlet velocity, the swirl and the inlet mixture fraction fluctuations. The latter is caused by two mechanisms: 1) the pulsating injected fuel flow rate and 2) the fluctuating trajectory of the fuel jets. Although the diagonal swirler is designed to provide good mixing, effects of mixing heterogeneities at the combustion chamber inlet occur. Mixture perturbations phase with velocity (and hence with swirl) fluctuations and combine with them to lead to different FTF results. Another FTF approach linking heat release to inlet velocity and mixture fraction fluctuation (MISO model) shows further to be a good solution for complex systems. A nonlinear analysis shows that the forcing amplitude not only leads to a saturation of the flame but also to changes of the delay response. Flame saturation is only true for the global FTF and the gain increases locally with increasing forcing amplitude. Both, the linear and the nonlinear flames, are not compact: flame regions located right next to each other exhibited significant differences in delay meaning that at the same instant certain parts of the flame damp the excitation while others feed it

Gli stili APA, Harvard, Vancouver, ISO e altri

45

Moisy, Frederic. "Instabilités, ondes, et turbulence en rotation". Habilitation à diriger des recherches, Université Paris Sud - Paris XI, 2010. http://tel.archives-ouvertes.fr/tel-00519073.

Testo completo

Abstract (sommario):

Ce mémoire présente une synthèse de mes travaux de recherche effectués depuis mon recrutement au laboratoire FAST en septembre 2000. Mes travaux sont essentiellement organisés autour de 4 axes~: les instabilités dans les écoulements de von Kármán, les ondes d'inertie, la turbulence en rotation, et les ondes de surface. Ces 4 axes de recherche ont entre eux un certain nombre de connections, comme l'influence de la rotation sur la nature d'un écoulement laminaire ou turbulent, ou encore les systèmes d'ondes dispersives en interaction, qu'il s'agisse des ondes capillaires ou des ondes d'inertie en référentiel tournant. D'un point de vue expérimental, ces axes de recherche ont également en commun l'utilisation de méthodes de mesure optiques, qu'il s'agisse de la Vélocimétrie par Images de Particules (PIV), méthode aujourd'hui standard en mécanique des fluides, ou encore du ``Synthetic Schlieren'', que nous avons étendu au cas des ondes de surface.

Gli stili APA, Harvard, Vancouver, ISO e altri

46

Martin, Christopher Reed. "Systematic Prediction and Parametric Characterization of Thermo-Acoustic Instabilities in Premixed Gas Turbine Combustors". Thesis, Virginia Tech, 2006. http://hdl.handle.net/10919/30889.

Testo completo

Abstract (sommario):

This thesis describes the coincident prediction and observation of thermo-acoustic instabilities in a turbulent, swirl-stabilized research combustor using a stability model constructed from validated reduced-order component models. The component models included the acoustic response to flame heat release rate at various locations in the combustor, the turbulent diffusion of uneven fuel-air mixing, and the flame's response to perturbations in both inlet velocity and equivalence ratio. These elements are closed in a system-level model to reflect their natural dynamic coupling and assessed with linear stability criteria. The results include the empirical validation of each of the component models and limited validation of the total closed-loop model with a lean premixed gaseous fuel combustor not dissimilar to an industrial burner. The degree of agreement between the predictions and the measurements encourages the conclusion that the reduced-order technique described herein not only includes the relevant physics, but has characterized them with sufficient acuracy to be the basis for design techniques for the passive avoidance of thermo-acoustic instabilities.
Master of Science

Gli stili APA, Harvard, Vancouver, ISO e altri

47

Arnould, Maëlis. "Some surface expressions of mantle convective instabilities". Thesis, Lyon, 2018. http://www.theses.fr/2018LYSEN036/document.

Testo completo

Abstract (sommario):

Constituant la couche limite supérieure de la convection mantellique, la lithosphère terrestre est à l'interface entre les enveloppes externes et internes de notre Planète. Les interactions multiples entre celle-ci et le manteau sont à l'origine de déformations latérales (tectonique des plaques) et verticales (topographie dynamique) de la surface terrestre. Comprendre comment la formation et l'évolution d'instabilités convectives mantelliques renouvellent sans-cesse la surface est donc primordial pour améliorer nos interprétations d'un grand nombre d'observations de surface, telles que la formation de bassins sédimentaires, le mouvement des continents, la localisation des points chauds, la formation d'anomalies gravimétriques ou encore les variations du niveau marin.Cette thèse propose de développer des modèles numériques de convection mantellique générant defaçon auto-organisée de la tectonique des plaques en surface an d'étudier la façon dont le développement et la dynamique d'instabilités convectives telles que les panneaux de subduction ou les panaches mantelliques modifient la surface, dans un contexte de tectonique de surface approchant le régime terrestre.Dans une première partie, je m'intéresse à l'influence du couplage des mouvements de convection mantellique et de tectonique des plaques sur le développement de topographie dynamique (i.e. les mouvements verticaux de la lithosphère induits par la convection mantellique) à différentes échelles spatio-temporelles. Mes résultats suggèrent que la surface terrestre peut se déformer à toutes les échelles spatiales, du fait de mouvements convectifs de grande ampleur faisant intervenir le manteau entier (> 104 km) ou encore de convection à petite échelle sub-lithosphérique (< 500 km). Les variations temporelles de topographie dynamique s‘étendent de cinq à plusieurs centaines de millions d'années selon la nature des processus convectifs dont elles dérivent. En particulier, la dynamique d'initiation ou d'arrêt des zones de subduction contrôle l'existence d'échelles intermédiaires de topographie dynamique (longueurs d'onde variant entre 500 et 104 km). Ces résultats montrent donc que les interactions entre la dynamique de la lithosphère et la convection mantellique génèrent des motifs spatio-temporels de topographie dynamique complexes et cohérents par rapport aux observations terrestres.Dans un deuxième temps, cette thèse se focalise sur la dynamique des panaches mantelliques, et leurs interactions avec la surface. Je caractérise d'abord précisement le comportement des panaches générés dans nos modèles de convection à la lumière d'observations de surface. Puis, j'étudie la façon dont leurs interactions avec la tectonique de surface et les différentes échelles convectives modifient leurs mouvements latéraux. Enfin, la compréhension de la signature thermique des interactions entre panaches et rides océaniques me permet de proposer une reconstitution des mouvements relatifs entre le panache des Açores et la ride médio-Atlantique
Earth's lithosphere, which is the upper boundary layer of mantle convection, represents the interface between the external and internal envelopes of our Planet. The multiple interactions between the mantle and lithosphere generate lateral (plate tectonics) and vertical (dynamic topography) deformations of Earth's surface. Understanding the influence of the dynamics of mantle convective instabilities on the surface is fundamental to improve our interpretations of a large range of surface observations, such as the formation of sedimentary basins, continental motions, the location of hotspots, the presence of gravity anomalies or sea-level variations.This thesis aims at developing numerical models of whole-mantle convection self-generating plate-like tectonics in order to study the impacts of the development and the dynamics of mantle convective instabilities (such as slabs or mantle plumes) on the continuous reshaping of the surface.First, I focus on the influence of the coupling between mantle convective motions and plate tectonics on the development of dynamic topography (i.e. surface vertical deformations induced by mantle convection) at different spatial and temporal scales. The results suggest that Earth's surface can deform over large spatial scales (> 104 km) induced by whole-mantle convection to small-scales (< 500 km) arising from small-scale sub-lithospheric convection. The temporal variations of dynamic topography range between five and several hundreds of millions of years depending on the convective instabilities from which they originate. In particular, subduction initiation and slab break-off events control the existence of intermediate scales of dynamic topography (between 500 and 104 km). This reflects that the interplay between mantle convection and lithosphere dynamics generates complex spatial and temporal patterns of dynamic topography consistent with constraints for Earth.A second aim of this thesis is to understand the dynamics of mantle plumes and their interactionswith surface. I first characterize in detail the behaviour of mantle plumes arising in models ofwhole-mantle convection self-generating plate-like tectonics, in light of surface observations. Then, I study how the interactions between surface plate tectonics and mantle convection affect plume motions. Finally, I use observations of the thermal signature of plume/ridge interactions to propose a reconstruction of the relative motions between the Azores mantle plume and the Mid-Atlantic Ridge

Gli stili APA, Harvard, Vancouver, ISO e altri

48

Benton, Stuart Ira. "Capitalizing on Convective Instabilities in a Streamwise Vortex-Wall Interaction". The Ohio State University, 2015. http://rave.ohiolink.edu/etdc/view?acc_num=osu1437664298.

Testo completo

Gli stili APA, Harvard, Vancouver, ISO e altri

49

Zimmermann, Oliver. "The influence of the dynamic ergodic divertor at TEXTOR on MHD instabilities and microturbulences". [S.l.] : [s.n.], 2004. http://deposit.ddb.de/cgi-bin/dokserv?idn=973540486.

Testo completo

Gli stili APA, Harvard, Vancouver, ISO e altri

50

Amorim, David. "Etude de l'instabilité de couplage des modes transverses dans le Grand Collisionneur de Hadrons du CERN". Thesis, Université Grenoble Alpes (ComUE), 2019. http://www.theses.fr/2019GREAY041.

Testo completo

Abstract (sommario):

Le projet de LHC Haute Luminosité sera une amélioration majeure de l'accélérateur visant à accroître ses performances et son potentiel de découvertes.L'intensité du faisceau sera multipliée par deux pour augmenter la luminosité du collisionneur.À de telles intensités, les effets collectifs et en particulier l'impédance de couplage du faisceau sont une limitation possible des performances du LHC.Après une introduction à la physique des accélérateurs et aux effets des champs électromagnétiques induits, deux codes de simulation des effets cohérents du faisceau seront détaillés : PyHEADTAIL, un code macro-particulaire temporel, et DELPHI, un solveur de l'équation de Vlasov.Ces derniers sont important pour estimer les marges de stabilité dans les accélérateurs du CERN, c'est pourquoi une comparaison détaillée pour différentes impédances, y compris celle du LHC, sera présenté.% Les résultats seront également comparés à des formules souvent utilisées comme premières approximations dans les études de stabilité.Les limites de stabilité actuelles du LHC seront ensuite étudiées à l'aide de simulations DELPHI.En particulier, l'instabilité de couplage des modes transversaux, une instabilité rapide se produisant dans les faisceaux de haute intensité pour des chromaticités proches de zéro, sera étudiée.Les résultats seront ensuite comparés à des mesures réalisées dans l'accélérateur.Des mesures basées sur le faisceau de plusieurs collimateurs seront également présentées et comparées aux prédictions du modèle d'impédance.L'association de ces différentes mesures et leur comparaison avec les simulations nous permettra d'estimer l'incertitude sur le modèle d'impédance du LHC.Pour faire face à l'augmentation de l'intensité du faisceau, une réduction d'impédance est prévue pour le projet de LHC Haute Luminosité.L'impact de différents scénarios sera étudié du point de vue de l'instabilité de couplage des modes transversaux.L'effet bénéfique d'une réduction d'impédance sera démontré par des mesures sur le LHC.Les mesures effectuées dans le LHC sur un prototype de collimateur d'impédance réduite, un élément clé du projet, seront également présentées.Ces simulations et mesures confirmeront l'augmentation du seuil de couplage des modes à une valeur trois fois supérieure à l'intensité nominale des paquets
The High-Luminosity upgrade of the CERN LHC will increase its performance and the potential physics discoveries.The beam intensity will be multiplied by two to increase the collider luminosity.With such high intensities, collective effects and in particular beam coupling impedance are a possible performance limitation for the accelerator.After an introduction to accelerator physics and wake-fields, two collective effects codes will be detailed: PyHEADTAIL, a time-domain macro-particle code, and DELPHI, a Vlasov equation solver.Both are important to estimate coherent beam stability margins in the CERN accelerator complex, therefore a detailed comparison for different wakes and impedances, including the LHC model, will be presented.The current LHC stability limits will then be investigated with DELPHI simulations.In particular the Transverse Mode Coupling Instability, a fast instability occurring for high intensity beams with chromaticities close to zero, will be studied.The results will then be compared to measurements performed in the accelerator.Beam based measurements of several collimators will also be presented and compared to predictions from the impedance model.Combining these measurements and their comparison to simulations we will estimate the uncertainty on the LHC impedance model.To cope with the increased beam intensity, the impedance of the High Luminosity upgrade of the LHC will be reduced.The impact of different upgrade scenarios will be studied from the Transverse Mode Coupling Instability perspective.The potential benefits of an impedance reduction will be demonstrated through measurements in the LHC.Measurements performed in the LHC on a low impedance prototype collimator, the key component to the impedance reduction, will also be presented.These simulations and measurements will confirm the increase in the mode coupling threshold towards a value three times higher than the nominal bunch intensity

Gli stili APA, Harvard, Vancouver, ISO e altri

Tesi sul tema "Dynamical instabilities"

Cita una fonte nei formati APA, MLA, Chicago, Harvard e in molti altri stili