This thesis focuses on what stability is, and what interventions have supported stability in four communities in Afghanistan and Nepal. It is the author’s view that this is the first in-depth village level assessment of how populations conceive of stability and stabilisation and thus presents a challenge to existing analysis and research about how to foster stability in contexts in extreme tension and often violent conflict. The thesis argues that international, particularly Western, notions of stability and stabilisation processes have failed to grasp the importance of local political legitimacy formation, which is a vital aspect of contemporary statebuilding of a ‘non-Westphalian’ nature. The interventions, across defence, diplomatic and defence lines, have also at times undermined one another and in some cases contributed to instability. This is particularly acute when the interventions have been motivated by the conflicting demands of statebuilding, counter-insurgency (COIN) and development theories. The thesis argues that the nature of the interventions, their conception of stability and exogenously-driven goals limit the ability to promote stability. Research findings indicate that that local processes of stabilisation have, at times, proven to be more enduring but only in circumstances where a combination of local and national political processes have allowed political legitimacy to be formed and maintained. Research findings also suggest that the more successful stability interventions have been critically supported by humanitarian and security activities which have provided for the immediate needs of the population. Longer term stability has only been embedded in contexts which have also been able to exploit economic opportunities.

Diss.--Wirtschaft--München--Ludwig-Maximilians-Universität, 1995. / Bibliogr. p. 225-237. Index.

Dans ce travail, on montre des nouveaux résultats pour l’analyse et la synthèse des systèmes stables en temps fini et fixe. Ce genre des systèmes convergent exactement à un point d’équilibre dans une quantité du temps qui est fini et, dans le cas de systèmes stables en temps fixe, dans un temps maximal constant qui ne dépend pas des conditions initiales du système.Les chapitres 2 et 3 portent sur des résultats d’analyse ; ce premier present des conditions nécessaires et suffisants pour la stabilité en temps fixe des systèmes autonomes continues tandis que ce dernier combine l’approche de la fonction implicite de Lyapunov avec des résultats de stabilisation ISS pour étudier la robustesse de ce genre de systèmes.Les chapitres 4 et 5 présentent des résultats pratiques liés á la procédure de synthèse des contrôleurs et des observateurs. Le chapitre 4 emploie la méthode de la fonction de Lyapunov implicite afin d’obtenir des observateurs convergents en temps fini et fixe pour les systèmes linéaires MIMO. Le chapitre 5 utilise des propriétés d’homogénéité et des fonctions de Lyapunov implicites pour synthétiser un contrôleur de sortie en temps fixe pour une chaîne d’intégrateurs. Les résultats obtenus ont été validés par des simulations numériques et le chapitre 4 contient des tests de performance sur un pendule rotatif. / This work presents new results on analysis and synthesis of finite-time and fixed-time stable systems, a type of dynamical systems where exact convergence to an equilibrium point is guaranteed in a finite amount of time. In the case of fixed-time stable system, this is moreover achieved with an upper bound on the settling-time that does not depend on the system’s initial condition.Chapters 2 and 3 focus on theoretical contributions; the former presents necessary and sufficient conditions for fixed-time stability of continuous autonomous systems whereas the latter introduces a framework that gathers ISS Lyapunov functions, finite-time and fixed-time stability analysis and the implicit Lyapunov function approach in order to study and determine the robustness of this type of systems.Chapters 4 and 5 deal with more practical aspects, more precisely, the synthesis of finite-time and fixed-time controllers and observers. In Chapter 4, finite-time and fixed-time convergent observers are designed for linear MIMO systems using the implicit approach. In Chapter 5, homogeneity properties and the implicit approach are used to design a fixed-time output controller for the chain of integrators. The results obtained were verified by numerical simulations and Chapter 4 includes performance tests on a rotary pendulum.

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Pas de résumé

Controllability and stabilisability are two fundamental properties of control systems and it is intuitively appealing to conjecture that the former should imply the latter; especially so when the state of a control system is assumed to be known at every time instant. Such an implication can, indeed, be proven for certain types of controllability and stabilisability, and certain classes of control systems. In the present thesis, we consider real analytic control systems of the form $\Sgr:\dot{x}=f(x,u)$, with $x$ in a real analytic manifold and $u$ in a separable metric space, and we show that, under mild technical assumptions, small-time local controllability from an equilibrium $p$ of \Sgr\ implies the existence of a piecewise analytic feedback \Fscr\ that asymptotically stabilises \Sgr\ at $p$. As a corollary to this result, we show that nonlinear control systems with controllable unstable dynamics and stable uncontrollable dynamics are feedback stabilisable, extending, thus, a classical result of linear control theory. Next, we modify the proof of the existence of \Fscr\ to show stabilisability of small-time locally controllable systems in finite time, at the expense of obtaining a closed-loop system that may not be Lyapunov stable. Having established stabilisability in finite time, we proceed to prove a converse-Lyapunov theorem. If \Fscr\ is a piecewise analytic feedback that stabilises a small-time locally controllable system \mbox{$\Sgr:\dot{x}=f(x,u)$} in finite time, then the Lyapunov function we construct has the interesting property of being differentiable along every trajectory of the closed-loop system obtained by ``applying" \Fscr\ to \Sgr. We conclude this thesis with a number of open problems related to the stabilisability of nonlinear control systems, along with a number of examples from the literature that hint at potentially fruitful lines of future research in the area. / Thesis (Ph.D, Mathematics & Statistics) -- Queen's University, 2012-09-24 10:24:22.51

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Thèse numérisée par la Direction des bibliothèques de l'Université de Montréal.

Les super-statoréacteurs sont des systèmes de propulsion aérobie à grande vitesse qui ne nécessitent pas d’éléments rotatifs pour comprimer l’écoulement d’air. Celui-ci est comprimé dynamiquement par un système d’admission intégré dans le véhicule, atteignant la pression et la température requises pour que la combustion puisse s’opérer dans la chambre de combustion. La chambre de combustion est traversée par un écoulement supersonique dans ce type de moteur, ce qui limite considérablement le temps disponible pour injecter le carburant, le mélanger avec un oxydant, enflammer le mélange obtenu et parvenir à une combustion complète. Les cavités peuvent être utilisées pour augmenter le temps de séjour sans perte excessive de pression totale et sont donc utilisées comme éléments de stabilisation dans les chambres de combustion supersonique. Cette thèse se concentre sur l’étude du mécanisme de stabilisation et des interactions chimie-turbulence dans le cas d’une injection pariétale de combustible dans un écoulement supersonique d’air vicié en amont d’une cavité carrée. Les conditions d’écoulement réactif à grande vitesse correspondantes sont examinées sur la base de simulations numériques d’un modèle de scramjet représentatif d’expériences effectuées précédemment à l’Université du Michigan. Les calculs sont effectués avec le solveur CREAMS, développé pour effectuer la simulation numérique d’écoulements multi-espèces réactifs compressibles sur des architectures massivement parallèles. Le solveur utilise des schémas numériques d’ordre élevé appliqués sur des maillages structurées et la géométrie de la chambre de combustion est modélisée à l’aide d’une méthode de frontières immergées (IBM). Les simulations LES font usage du modèle wall-adapting local eddy (WALE). Deux températures distinctes sont considérées dans l’écoulement entrant d’air vicié pour étudier la stabilisation de la combustion.Une attention particulière est accordée à l’analyse de la topologie et de la structure des écoulements réactifs, les régimes de combustion sont analysés sur la base de diagrammes standards de combustion turbulente. / Scramjet engines are high-speed air breathing propulsion systems that do not require rotating elements to compress the air inlet stream. The flow is compressed dynamically through a supersonic intake system integrated in the aircraft’s forebody, reaching the required pressure and temperature for combustion to proceed within the combustor in this kind of engine. The combustion chamber is crossed by a supersonic flow, which limits severely the time available to inject fuel, mix it with oxidizer, ignite the resulting mixture and reach complete combustion. Cavities can be used to increase the residence time without excessive total pressure loss and are therefore used as flame holders in supersonic combustors.This thesis focuses in studying the flame stabilization mechanism and turbulence-chemistry interactions for a jet in a supersonic crossflow (JISCF) of vitiated air with hydrogen injection upstream of a wall-mounted squared cavity. The corresponding reactive high-speed flow conditions are scrutinized on the basis of numerical simulations of a scramjet model representative of experiments previously conducted at the University of Michigan. The computations are performed with the high-performance computational solver CREAMS, developed to perform the numerical simulation of compressible reactive multi-component flows on massively-parallel architectures. The solver makes use of high-order precision numerical schemes applied on structured meshes and the combustion chamber geometry is modeled by using the Immersed Boundary Method (IBM) algorithm. The present set of computations is conducted within the LES framework and the subgrid viscosity is treated with the wall-adapting local eddy (WALE)model. Two distinct temperatures are considered in the inlet vitiated airstream to study combustion stabilization. Special emphasis is placed on the analysis of the reactive flow topology and structure,and the combustion regimes are analyzed on the basis of standard turbulent combustion diagrams.