<div>Aerospace and shipboard power systems, as well as merging terrestrial microgrids, typically include a large ercentage of regulated power-electronic loads. It is well nown that such systems are prone to so-called negative- mpedance instabilities that may lead to deleterious scillations and/or the complete collapse of bus voltage. umerous small-displacement criteria have been developed o ensure dynamic stability for small load perturbations, and echniques for estimating the regions of asymptotic stability bout specic equilibrium points have previously been established. However, these criteria and analysis techniques o not guarantee system stability following large nd/or rapid changes in net load power. More recent research as focused on establishing criteria that ensure arge-displacement stability for arbitrary time varying loads rovided that the net load power is bounded. These yapunov-based techniques and recent advancements in eachability analysis described in this thesis are applied to xample dc and ac microgrids to not only introduce a large- isplacement stability margin, but to demonstrate that the elected systems can be designed to be large-displacement table with practicable constraints and parameters.</div>

<div>Multiscale and multiphysics have been two major challenges in analyzing and designing new emerging engineering devices, materials, circuits, and systems. When simulating a multiscale problem, numerical methods have to overcome the challenges in both space and time to account for the scales spanning many orders of magnitude difference. In the finite-difference time-domain (FDTD) method, subgridding techniques have been developed to address the multiscale challenge. However, the accuracy and stability in existing subgridding algorithms have always been two competing factors. In terms of the analysis of a multiphysics problem, it involves the solution of multiple partial differential equations. Existing partial differential equation solvers require solving a system matrix when handling inhomogeneous materials and irregular geometries discretized into unstructured meshes. When the problem size, and hence the matrix size, is large, existing methods become highly inefficient.</div><div><br></div><div>In this work, a symmetric positive semi-definite FDTD subgridding algorithm in both space and time is developed for fast transient simulations of multiscale problems. This algorithm is stable and accurate by construction. Moreover, the method is further made unconditionally stable, by analytically finding unstable modes, and subsequently deducting them from the system matrix. To address the multiphysics simulation challenge, we develop a matrix-free time domain method for solving thermal diffusion equation, and the combined Maxwell-thermal equations, in arbitrary unstructured meshes. The counterpart of the method in frequency domain is also developed for fast frequency-domain analysis. In addition, a generic time marching scheme is proposed for simulating unsymmetrical systems to guarantee their stability in time domain. </div>

Les instabilités thermoacoustiques, également appelées instabilités de combustion, sont un problème majeur pour la production d’électricité ainsi que dans l’industrie aérospatiale. Ces instabilités sont dues à un transfert d’énergie entre une source chaude, le plus souvent une flamme stabilisée dans un brûleur, et le champ acoustique environnant. Les instabilités de combustion peuvent avoir de nombreuses conséquences délétères telles que l’extinction de la flamme, l’augmentation des flux de chaleur pariétaux, l’émission d’ondes sonores de grande amplitude à certaines fréquences, des vibrations importantes, des dégâts structurels et même l’explosion du moteur dans certains cas. Étant donné les conséquences potentielles de tels phénomènes, d’importants moyens de recherche ont été consacrés à la prédiction de l’apparition d’instabilités de combustion dans les chaudières, les moteurs de fusée et les turbines à gaz ces dernières décennies. Néanmoins, le cadre théorique associé à l’étude de ces instabilités est complexe et nécessite l’emploi de nombreuses disciplines de la physique. De plus, les brûleurs industriels sont constitués de nombreuses cavités tridimensionnelles interagissant entre elles d’un point de vue acoustique. Pour toutes ces raisons, la prédiction de la stabilité thermoacoustique d’un brûleur demeure une tâche ardue à ce jour... (Voir le texte de la thèse pour la suite du résumé) / Thermoacoustic instabilities, also known as combustion instabilities, are a major concern in the aerospace and energy production industries. They are due to an energy transfer that occurs between a heat source, usually a flame stabilized inside a combustor, and the surrounding acoustic field and may lead to undesirable phenomena such as flame extinction, increased heat fluxes, very large sound emissions at certain frequencies, vibration, structural damage and even catastrophic failure in some cases. Given the potential consequences of such phenomena, a large research effort has been devoted to predicting the onset of combustion instabilities in modern boilers, rocket engines and gas turbines during the past few decades. Unfortunately, the theoretical framework associated with the study of thermoacoustic instabilities is complex and multi-physics and the geometry of practical combustors is an intricate arrangement of 3D cavities. As a consequence, predicting the thermoacoustic stability of a combustor at an early design stage is a challenging task to date... (See inside the manuscript for the remainder of the abstract)

Submitted by RODRIGO FRANCISCO BORGES LOURENÇO null (rodrigoborges@unirv.edu.br) on 2017-02-09T19:40:07Z No. of bitstreams: 1 Rodrigo Borges - Finalizado.pdf: 3257098 bytes, checksum: c3df8350bb0c44864dd0a646a065458f (MD5) / Approved for entry into archive by LUIZA DE MENEZES ROMANETTO (luizamenezes@reitoria.unesp.br) on 2017-02-14T16:25:05Z (GMT) No. of bitstreams: 1 lourenco_rfb_me_ilha.pdf: 3257098 bytes, checksum: c3df8350bb0c44864dd0a646a065458f (MD5) / Made available in DSpace on 2017-02-14T16:25:05Z (GMT). No. of bitstreams: 1 lourenco_rfb_me_ilha.pdf: 3257098 bytes, checksum: c3df8350bb0c44864dd0a646a065458f (MD5) Previous issue date: 2017-01-24 / São diversos os equipamentos de engenharia que apresentam vibrações mecânicas, e estas podem ser observadas em forma de acelerações, deslocamentos e velocidade. Os primeiros estudos envolvendo vibrações foram direcionados aos fenômenos naturais e modelagem matemática de sistemas vibrantes, então, começou a aplicação desses estudos em equipamentos de engenharia. Vibrações mecânicas, na maioria dos sistemas dinâmicos, são consideradas como algo indesejado e podem ser danosos. Porém, existem situações que são utilizadas para melhorar o funcionamento e desempenho de máquinas. São diversas as causas de vibrações em sistemas de engenharia, neste trabalho, destaca-se as vibrações causadas por impacto. Quando componentes destes sistemas impactam entre si, causando ruídos de curta duração, são caracterizados como sistemas tipo vibro - impacto. Podem ser citados diversos equipamentos com essas características, como rolos compactadores de solo, martelos de impacto, perfuratrizes de solo, etc. Neste trabalho, demonstra-se o comportamento dinâmico de um sistema vibro – impactante. Para análise deste sistema, foram desenvolvidos códigos computacionais, através do software Octave. No diagrama de estabilidade de Lyapunov, verificou-se que, pontualmente o sistema se apresenta de forma estável. A partir da variação da frequência de excitação, foi observado através dos históricos no tempo, espectros de frequência, mapas de Poincaré e planos de fase, um comportamento periódico e estável, com situações diversas de respostas. Ao analisar a evolução temporal dos expoentes de Lyapunov, para todas as condições de velocidade e deslocamento impostas, o sistema se apresentou de forma caótica. Implementou-se um controlador linear ótimo ao sistema, afim de atenuar as vibrações nas regiões de operação nas quais o sistema é instável. Comprovou-se que a estratégia de controle linear ótimo (LQR, do inglês Linear Quadratic Regulator) demonstra eficiência para este tipo de situação e pode ser utilizada na redução de danos, evitando prejuízos econômicos, perdas biológicas e materiais. / There are several engineering equipment’s that present mechanical vibrations, and these can be observed in the form of displacement, acceleration, and speed. The first studies involving were directed to the natural phenomena and mathematical modeling of vibrations systems, then the application of these studies began in engineering equipment. In most dynamic systems Mechanical vibrations are considered to be unwanted and can be harmful. However, there are situations that are used to improve the operation and performance of machines. There are several causes of vibrations in engineering systems. In this work, the vibrations caused by impact are highlighted. For components of these systems impacting each other, causing short - term noise, they are characterized as vibro-impact systems. Various equipment with these characteristics can be mentioned, such as soil compacting rollers, impact hammers, soil drills, etc. In this work the dynamic behavior of a vibro-impacting system is demonstrated. For the computational analysis of this system, were implemented codes using the software Octave. In the Lyapunov stability diagram, is was verified that, the system presents is stable. From the variation of the excitation frequency, a periodic and stable behavior was observed through time histories, frequency spectrump, poincaré maps and phase planes, with different situations of responses. When analyzing the time evolution of the Lyapunov is exponents, for all imposed conditions of velocity and displacement, the system appeared chaotic. An optimum linear controller was implemented in the system in order to attenuate the vibrations in the operating regions in which the system is unstable. It was verified that the Linear Quadratic Regulator (LQR) demonstrates efficiency for this type of situation and it can be used to reduce damages, avoiding economic, biological, and material losses.

The purpose of this thesis is to use automation to create appropriate models in PSS/E with the data from Hardware-in-Loop real-time simulations. With the increase in technology of power electronics, the use of High Voltage Direct Current Technology and Flexible Alternating Current Transmission System devices in the electrical power system have increased tremendously. Static Var Compensators are widely used and it is important to have accurate and reliable models for studies relating to power systems planning and interaction. An automation method is proposed to find the parameters of an SVC model in PSS/E with the data from the Hardware-in- loop real-time simulation of the SVC physical controller using Hypersim. The effect of the SVC on the system under steady state and fault conditions are analyzed with HIL simulation of an SVC physical controller in Hypersim and its corresponding model in PSS/E in the IEEE 14 bus system. The parameters of the SVC model in PSS/E can be effectively varied to bring its response closer to that of the response from HIL simulations in Hypersim. An error function is used as a measure to understand the extent of difference between the model and the physical controller.

With the growth of renewable energy usage and energy storage adoption in recent decades, people have started to reevaluate the possible roles of dc systems in current and future electrical systems. The dc voltage distribution has been applied in various applications, such as data centers and aircraft industry, for high efficiency and power density. However, for some applications such as subsea gas and oil fields, and ocean observatory systems, the dc current distribution is preferred over dc voltage distribution for its low cost and robustness against cable faults. Design and control of dc power distribution systems for different applications is an emerging research area with complex technical challenges. This dissertation solves the technical challenges in analysis, design, modeling, control and protection of series resonant converters (SRCs) for dc current distribution applications. An optimum design that has high efficiency, high reliability, and minimum required control efforts for the SRC with constant input current has been achieved and demonstrated by applying the analysis and design procedures developed in this dissertation. The modeling and analysis presented in this dissertation represents an operating condition that has not been studied in the literature and could be easily extended to other resonant converter topologies. Explicit analytical expressions have been provided for all key transfer functions, including input impedance and control-to-output, offering valuable resources to design feed-back regulation and to evaluate system stability. Based on the control strategies and control design presented in this dissertation, stable and reliable operation of dc current distribution systems with long distance cable has been achieved and demonstrated. The proposed analysis, design procedure, stability evaluation, control strategy and protection techniques in this dissertation can be applied to a wide range of similar scenarios as well, which greatly increases their value.

This work takes place in the framework of Knowledge Discovery in Databases (KDD), often called "Data Mining". This domain is both a main research topic and an application ¯eld in companies. KDD aims at discovering previously unknown and useful knowledge in large databases. In the last decade many researches have been published about association rules, which are frequently used in data mining. Association rules, which are implicative tendencies in data, have the advantage to be an unsupervised model. But, in counter part, they often deliver a large number of rules. As a consequence, a postprocessing task is required by the user to help him understand the results. One way to reduce the number of rules - to validate or to select the most interesting ones - is to use interestingness measures adapted to both his/her goals and the dataset studied. Selecting the right interestingness measures is an open problem in KDD. A lot of measures have been proposed to extract the knowledge from large databases and many authors have introduced the interestingness properties for selecting a suitable measure for a given application. Some measures are adequate for some applications but the others are not. In our thesis, we propose to study the set of interestingness measure available in the literature, in order to evaluate their behavior according to the nature of data and the preferences of the user. The ¯nal objective is to guide the user's choice towards the measures best adapted to its needs and in ¯ne to select the most interesting rules. For this purpose, we propose a new approach implemented in a new tool, ARQAT (Association Rule Quality Analysis Tool), in order to facilitate the analysis of the behavior about 40 interest- ingness measures. In addition to elementary statistics, the tool allows a thorough analysis of the correlations between measures using correlation graphs based on the coe±cients suggested by Pear- son, Spearman and Kendall. These graphs are also used to identify the clusters of similar measures. Moreover, we proposed a series of comparative studies on the correlations between interestingness measures on several datasets. We discovered a set of correlations not very sensitive to the nature of the data used, and which we called stable correlations. Finally, 14 graphical and complementary views structured on 5 levels of analysis: ruleset anal- ysis, correlation and clustering analysis, most interesting rules analysis, sensitivity analysis, and comparative analysis are illustrated in order to show the interest of both the exploratory approach and the use of complementary views.

Current climate models have a limited ability to increase spatial resolution because numerical stability requires the time step to decrease. I describe initial experiments with two independent but complementary strategies for attacking this "time barrier". First I describe computational experiments exploring the performance improvements from overlapping computation and communication on hybrid parallel computers. My test case is explicit time integration of linear advection with constant uniform velocity in a three-dimensional periodic domain. I present results for Fortran implementations using various combinations of MPI, OpenMP, and CUDA, with and without overlap of computation and communication. Second I describe a semi-Lagrangian method for tracer transport that is stable for arbitrary Courant numbers, along with a parallel implementation discretized on the cubed sphere. It shows optimal accuracy at Courant numbers of 10-20, more than an order of magnitude higher than explicit methods. Finally I describe the development and stability analyses of the time integrators and advection methods I used for my experiments. I develop explicit single-step methods with stability up to Courant numbers of one in each dimension, hybrid explicit-implict methods with stability for arbitrary Courant numbers, and interpolation operators that enable the arbitrary stability of semi-Lagrangian methods.

Rotordynamic and thermal analyses of compliant flexure pivot tilting pad gas bearings were performed. First, compliant flexure pivot tilting pad gas bearings with pad radial compliance (CFTPBs) were introduced and designed for high-speed oil-free micro turbomachinery. The pad radial compliance was for accommodation of large rotor growth at high speeds. Parametric studies on pivot offset, preload, and tilting stiffness were performed using non-linear orbit simulations and coast-down simulations for an optimum design. Second, coast-down tests for imbalance response and stability of typical rotor-bearing system with a rigid rotor and two CFTPBs designed from the above design studies were conducted over operating speeds up to 55 krpm. Prediction of synchronous rotordynamic responses was made in terms of critical speed for various imbalance modes by using a rotordynamic analysis software (XLTRC), combined with dynamic force coefficients from the perturbation analysis. For stability analyses, a generalized orbit simulation program was developed considering both the translational and angular rotor motions with two different bearings. Linear stability analyses for the conical vibration mode were also performed by using XLTRC and the perturbation analysis based on the Lund method. Predictions of whirl speed showed good agreement to the tests, but the estimated onset speed of instability appeared lower than the measured instability. Finally, a new thermo-hydrodynamic analysis model of a typical rotor-bearing system with CFTPBs was presented, accompanying linear perturbation analyses to investigate thermal effects on the rotordynamic performance. A numerical procedure was established for solving the generalized Reynolds equation, the 3-D energy equation, and the associated boundary conditions at the pad inlet flow and solid walls (rotor and pad) simultaneously. Parametric studies were conducted on nominal clearance and external load. Nominal clearance showed significant influence on temperature fields, and external load had uneven thermal effects among pads. Case studies with heat flux and temperature boundary conditions on the rotor end surface were performed to simulate various working conditions of the bearing. Large rotor thermal growth due to the high rotor temperature showed noticeable influence on rotordynamic performance by increasing direct stiffness and damping coefficients.

No description available.