A crescente utilização de novas tecnologias, as quais necessitam de uma fonte de energia menor e mais eficiente, como os microssensores para monitoramento de sistemas e estruturas nas chamadas cidades inteligentes, torna a captura da energia do ambiente uma opção viável para alimentação de tais dispositivos. Como a energia cinética é uma fonte de energia facilmente encontrada no ambiente, os sistemas que a capturam e convertem em eletricidade têm sido amplamente estudados, especialmente os que utilizam transdutores piezoelétricos. Considerando estruturas aporticadas, como prédios, pontes etc., comumente encontradas nas cidades, este trabalho apresenta um novo modelo de sistema de captura de energia piezoelétrico com base em um pórtico não linear, sob uma excitação não ideal, por meio de uma fonte com potência limitada. Para modelar o acoplamento piezoelétrico, foram consideradas as não linearidades do material piezoelétrico. Por meio das simulações numéricas, pode-se verificar a eficiência e a viabilidade do modelo proposto. Devido ao fato de as vibrações do meio ambiente serem senoidais, aleatórias ou transitórias, surge uma dificuldade na captura de energia de forma eficiente e com um nível contínuo. A utilização de controles passivos pode melhorar a energia capturada, removendo o movimento caótico do sistema e mantendo a oscilação em uma órbita periódica estável. Assim, duas estratégias de controle passivo foram empregadas, a primeira utilizando uma subestrutura com características de absorvedor de energia não linear (NES) e a segunda pela introdução de um pêndulo. Em ambos os casos, as simulações demonstraram que o controle passivo foi eficiente em levar o sistema caótico para uma órbita periódica estável, otimizando a captura de energia do sistema. Uma análise considerando incertezas nos parâmetros foi realizada, para verificar a robustez da estratégia de controle, assim como a sensibilidade do sistema de controle a erros paramétricos. Os resultados mostraram a eficiência do controle passivo e o fenômeno do bombeamento de energia na supressão do comportamento caótico. A principal vantagem do controle passivo é não necessitar de componentes eletrônicos para controlar o sistema, sendo apenas um componente mecânico \"massa\", acoplado à estrutura principal. Uma análise wavelet foi realizada sobre o modelo, para identificar o comportamento oscilatório do sistema e permitir a visualização das frequências de vibração que capturam mais energia. / The increasing use of new technologies, which have the need for smaller and more energy efficient sources, such as micro-sensors for monitoring systems and structures of the so-called smart cities, assigns environmental energy harvesting a viable option to power such devices. As kinetic energy is a source easily found in the environment, the systems that harvest and convert this type of energy into electricity have been widely studied, especially those using piezoelectric transducers. Considering framed structures, such as buildings, bridges, etc., which are commonly found in the cities, this paper presents a new model of piezoelectric energy harvesting system based on a nonlinear portal frame, under a non-ideal excitation by a source with limited power. To model Piezoelectric couplings, they were considered nonlinearities of the piezoelectric material. Through numerical simulations, the effciency and viability of the proposed model can be verified. A difficulty arises in harvesting energy in an efficient manner, and with a continuous level, because the vibrations of the environment are sinusoidal, random or transient. However, the use of passive controls can improve the energy harvested by removing the chaotic motion of the system and maintaining the oscillation at a stable periodic orbit. Thus, two passive control strategies were employed, the first using a substructure with characteristics of nonlinear energy sink (NES), and the second by introducing a pendulum. In both cases, the simulations showed that the passive control was efficient in bringing the chaotic system to a stable periodic orbit, optimizing the energy harvest system. An analysis considering the uncertainties in the parameters was performed to verify the robustness of the control strategy, as well as the sensitivity of the control system of parametric errors. The results showed the efficiency of passive control and the energy pumping phenomenon in the suppression of the chaotic behavior. The main advantage of passive control is not to require any electronic components for controlling the system, only a mechanical component _mass_, attached to the main structure. A Wavelet Analysis was conducted on the model to identify the oscillatory behavior of the system and allowed the viewing of the vibration frequencies that harvest more energy.

Os materiais inteligentes têm sido utilizados em problemas de controle de vibrações e conversão de energia mecânica em energia elétrica. Apesar das diferentes opções existentes, os piezelétricos têm recebido grande atenção devido a facilidade de instalação, além de possibilidade de uso como sensores ou atuadores. Em termos de sistemas de controle utilizando materiais piezelétricos, dois grandes grupos podem ser encontrados: os controladores passivos e os ativos. Os controladores ativos utilizam o efeito piezelétrico inverso e apresentam bom desempenho na redução de vibrações. Entretanto, apresentam desvantagens relacionadas à complexidades de uma lei de controle, necessidade de equipamentos externos e, potencialmente, exigem elevada potência de atuação. Por outro lado, os controladores passivos utilizam circuitos elétricos simples, compostos somente por elementos elétricos passivos. Apesar de serem de fácil implementação prática, apresentam bom desempenho em faixas restritas de frequências. Os controladores semi-passivos, surgiram como uma alternativa aos pontos negativos dos controladores passivos e ativos. Nestes novos sistemas, o material piezelétrico instalado na estrutura a ser controlada é conectado e desconectado a um circuito shunt de forma sincronizada com as vibrações mecânicas. Em geral, a conversão eletromecânica de energia é amplificada, assim como o efeito shunt damping. Dessa forma, os circuitos chaveados têm sido utilizados tanto como controladores semi-passivos quanto em problemas de coleta piezelétrica de energia. Neste trabalho, o controle piezelétrico semi-passivo de oscilações aeroelásticas lineares e não lineares, assim como a coleta piezelétrica de energia a partir das mesmas condições, são investigados experimentalmente. Uma seção típica com dois graus de liberdade e acoplamento eletromecânico é utilizada nos experimentos. Dois tratamentos não lineares do sinal elétrico proveniente dos piezelétricos são utilizados. Primeiro, o chaveamento a partir da condição de circuito aberto para uma resistência muito baixa, próxima ao curto circuito, e posteriormente, o chaveamento da condição de circuito aberto para um indutor. Um circuito chaveador autônomo (que não depende de fontes externas de energia) é apresentado. O desempenho dos dois sistemas no controle de flutter, e também de oscilações em ciclo limite, são discutidos. Os resultados mostraram um aumento na velocidade de flutter de até 8,8% e 11,5%, com chaveamento em uma resistência e em um indutor, respectivamente. No caso de coleta de energia a partir de oscilações aeroelásticas lineares e não lineares, o desempenho dos circuitos chaveados são comparados entre si, e com o caso em que uma resistência é considerada no domínio elétrico, resultando em um aumento da potência elétrica de até 101%, para chaveamentos em resistência, e 227%, para chaveamentos em um indutor. / Smart materials have been used in vibration control and also in energy harvesting problems. Although different materials are available, piezoelectric one has received most attention due to ease of installation and possibility of use as sensors or actuators. In general, there are two large categories of vibration control techniques using piezoelectric materials: passive and active control. In active control the reverse piezoelectric effect is used. In general, they present good performance in vibration reduction over a range of frequencies. However, active control has the disadvantages of additional complexities of a control law, additional hardware and the potentially large amount of power required. On the other hand, piezoelectric passive controllers use simple electric circuits composed by passive electrical elements. Although they are simple to implement, the performance of the controlled system is sensitive to the exciting frequency. The semi-passive controllers are a recent alternative to the drawbacks of passive and active controllers. In semi-passive systems, the piezoelectric element is switched in and out of a shunt impedance, in a synchronous way with mechanical vibrations. In general, the electromechanical energy conversion is enhanced as well as the shunt effect damping. Therefore, the switching techniques have been used both in semi-passive control problems and in piezoelectric energy harvesting problems. In this work, semi-passive techniques are experimentally investigated in aeroelastic control and piezoaeroelastic energy harvesting cases. An electromechanically coupled aeroelastic typical section is used in the experiments. Two techniques are investigated, the synchronized switching damping on short and the synchronized switching damping on inductor. An autonomous switching circuit (that does not requires external source of energy) is presented resulting in a self-powered flutter controller. The performance of the two semi-passive techniques is discussed for the linear case, flutter control, as well as limit cycle oscillations control. The linear flutter speed is increased by 8.8% and 11.5% when the SSDS and SSDI techniques are used, respectively. In the case of energy harvesting from linear and nonlinear aeroelastic oscillations, the performance of switching techniques is investigated and compared to the case of simple load resistance in the electrical domain. The power output is increased by 101% for the SSDS case and 227% for SSDI case.

A crescente utilização de novas tecnologias, as quais necessitam de uma fonte de energia menor e mais eficiente, como os microssensores para monitoramento de sistemas e estruturas nas chamadas cidades inteligentes, torna a captura da energia do ambiente uma opção viável para alimentação de tais dispositivos. Como a energia cinética é uma fonte de energia facilmente encontrada no ambiente, os sistemas que a capturam e convertem em eletricidade têm sido amplamente estudados, especialmente os que utilizam transdutores piezoelétricos. Considerando estruturas aporticadas, como prédios, pontes etc., comumente encontradas nas cidades, este trabalho apresenta um novo modelo de sistema de captura de energia piezoelétrico com base em um pórtico não linear, sob uma excitação não ideal, por meio de uma fonte com potência limitada. Para modelar o acoplamento piezoelétrico, foram consideradas as não linearidades do material piezoelétrico. Por meio das simulações numéricas, pode-se verificar a eficiência e a viabilidade do modelo proposto. Devido ao fato de as vibrações do meio ambiente serem senoidais, aleatórias ou transitórias, surge uma dificuldade na captura de energia de forma eficiente e com um nível contínuo. A utilização de controles passivos pode melhorar a energia capturada, removendo o movimento caótico do sistema e mantendo a oscilação em uma órbita periódica estável. Assim, duas estratégias de controle passivo foram empregadas, a primeira utilizando uma subestrutura com características de absorvedor de energia não linear (NES) e a segunda pela introdução de um pêndulo. Em ambos os casos, as simulações demonstraram que o controle passivo foi eficiente em levar o sistema caótico para uma órbita periódica estável, otimizando a captura de energia do sistema. Uma análise considerando incertezas nos parâmetros foi realizada, para verificar a robustez da estratégia de controle, assim como a sensibilidade do sistema de controle a erros paramétricos. Os resultados mostraram a eficiência do controle passivo e o fenômeno do bombeamento de energia na supressão do comportamento caótico. A principal vantagem do controle passivo é não necessitar de componentes eletrônicos para controlar o sistema, sendo apenas um componente mecânico \"massa\", acoplado à estrutura principal. Uma análise wavelet foi realizada sobre o modelo, para identificar o comportamento oscilatório do sistema e permitir a visualização das frequências de vibração que capturam mais energia. / The increasing use of new technologies, which have the need for smaller and more energy efficient sources, such as micro-sensors for monitoring systems and structures of the so-called smart cities, assigns environmental energy harvesting a viable option to power such devices. As kinetic energy is a source easily found in the environment, the systems that harvest and convert this type of energy into electricity have been widely studied, especially those using piezoelectric transducers. Considering framed structures, such as buildings, bridges, etc., which are commonly found in the cities, this paper presents a new model of piezoelectric energy harvesting system based on a nonlinear portal frame, under a non-ideal excitation by a source with limited power. To model Piezoelectric couplings, they were considered nonlinearities of the piezoelectric material. Through numerical simulations, the effciency and viability of the proposed model can be verified. A difficulty arises in harvesting energy in an efficient manner, and with a continuous level, because the vibrations of the environment are sinusoidal, random or transient. However, the use of passive controls can improve the energy harvested by removing the chaotic motion of the system and maintaining the oscillation at a stable periodic orbit. Thus, two passive control strategies were employed, the first using a substructure with characteristics of nonlinear energy sink (NES), and the second by introducing a pendulum. In both cases, the simulations showed that the passive control was efficient in bringing the chaotic system to a stable periodic orbit, optimizing the energy harvest system. An analysis considering the uncertainties in the parameters was performed to verify the robustness of the control strategy, as well as the sensitivity of the control system of parametric errors. The results showed the efficiency of passive control and the energy pumping phenomenon in the suppression of the chaotic behavior. The main advantage of passive control is not to require any electronic components for controlling the system, only a mechanical component _mass_, attached to the main structure. A Wavelet Analysis was conducted on the model to identify the oscillatory behavior of the system and allowed the viewing of the vibration frequencies that harvest more energy.

A new seismic isolation system for steel building structures based on the principle of mass isolation is introduced. In this system, isolating interfaces are placed between the lateral-load-resisting sub-system and the gravity-load-resisting sub-system. Because of the virtual decoupling existing between the two structural sub-systems, the gravity-load resisting one is susceptible to instability. Due to the fact that the provided level of isolation from the ground is constrained by the stability requirements of the gravity-load resisting structure, the system is named stability-dependent mass isolation (SDMI). Lyapunov stability and its association with energy principles are used to assess the stable limits of the SDMI system, its equilibrium positions, the stability of the equi-librium positions, and to propose a series of design guidelines and equations that allow the optimal seismic performance of the system while guaranteeing the restoration of its undistorted position. It is mathematically shown that the use of soft elastic interfaces, between the lateral- and gravity-load-resisting sub-systems, can serve the dual role of stability braces and isolators well. The second part of the document is concerned with the analytical evaluation of the seismic performance of the SDMI method. First, a genetic algorithm is used to find optimized SDMI building prototypes and, later, these prototypes are subjected to a series of earthquake records having different hazard levels. This analytical testing program shows that, with the use of SDMI, not only can structural failure be avoided, but a dam-age-free structural performance can also be achieved, accompanied by average reductions in the floor accelerations of ca. 70% when compared to those developed by typical braced-frame structures. Since the SDMI system is to be used in conjunction with viscous energy dissi-paters, the analytical testing program is also used to determine the best places to place the dampers so that they are most effective in minimizing the floor accelerations and controlling the floors’ drift-ratios. Finally, recommendations on continuing research are made.

For the last two decades heart disease has been the highest single cause of death for the human population. With an alarming number of patients requiring heart transplant, and donations not able to satisfy the demand, treatment looks to mechanical alternatives. Rotary Ventricular Assist Devices, VADs, are miniature pumps which can be implanted alongside the heart to assist its pumping function. These constant flow devices are smaller, more efficient and promise a longer operational life than more traditional pulsatile VADs. The development of rotary VADs has focused on single pumps assisting the left ventricle only to supply blood for the body. In many patients however, failure of both ventricles demands that an additional pulsatile device be used to support the failing right ventricle. This condition renders them hospital bound while they wait for an unlikely heart donation. Reported attempts to use two rotary pumps to support both ventricles concurrently have warned of inherent haemodynamic instability. Poor balancing of the pumps’ flow rates quickly leads to vascular congestion increasing the risk of oedema and ventricular ‘suckdown’ occluding the inlet to the pump. This thesis introduces a novel Bi-Ventricular Assist Device (BiVAD) configuration where the pump outputs are passively balanced by vascular pressure. The BiVAD consists of two rotary pumps straddling the mechanical passive controller. Fluctuations in vascular pressure induce small deflections within both pumps adjusting their outputs allowing them to maintain arterial pressure. To optimise the passive controller’s interaction with the circulation, the controller’s dynamic response is optimised with a spring, mass, damper arrangement. This two part study presents a comprehensive assessment of the prototype’s ‘viability’ as a support device. Its ‘viability’ was considered based on its sensitivity to pathogenic haemodynamics and the ability of the passive response to maintain healthy circulation. The first part of the study is an experimental investigation where a prototype device was designed and built, and then tested in a pulsatile mock circulation loop. The BiVAD was subjected to a range of haemodynamic imbalances as well as a dynamic analysis to assess the functionality of the mechanical damper. The second part introduces the development of a numerical program to simulate human circulation supported by the passively controlled BiVAD. Both investigations showed that the prototype was able to mimic the native baroreceptor response. Simulating hypertension, poor flow balancing and subsequent ventricular failure during BiVAD support allowed the passive controller’s response to be assessed. Triggered by the resulting pressure imbalance, the controller responded by passively adjusting the VAD outputs in order to maintain healthy arterial pressures. This baroreceptor-like response demonstrated the inherent stability of the auto regulating BiVAD prototype. Simulating pulmonary hypertension in the more observable numerical model, however, revealed a serious issue with the passive response. The subsequent decrease in venous return into the left heart went unnoticed by the passive controller. Meanwhile the coupled nature of the passive response not only decreased RVAD output to reduce pulmonary arterial pressure, but it also increased LVAD output. Consequently, the LVAD increased fluid evacuation from the left ventricle, LV, and so actually accelerated the onset of LV collapse. It was concluded that despite the inherently stable baroreceptor-like response of the passive controller, its lack of sensitivity to venous return made it unviable in its present configuration. The study revealed a number of other important findings. Perhaps the most significant was that the reduced pulse experienced during constant flow support unbalanced the ratio of effective resistances of both vascular circuits. Even during steady rotary support therefore, the resulting ventricle volume imbalance increased the likelihood of suckdown. Additionally, mechanical damping of the passive controller’s response successfully filtered out pressure fluctuations from residual ventricular function. Finally, the importance of recognising inertial contributions to blood flow in the atria and ventricles in a numerical simulation were highlighted. This thesis documents the first attempt to create a fully auto regulated rotary cardiac assist device. Initial results encourage development of an inlet configuration sensitive to low flow such as collapsible inlet cannulae. Combining this with the existing baroreceptor-like response of the passive controller will render a highly stable passively controlled BiVAD configuration. The prototype controller’s passive interaction with the vasculature is a significant step towards a highly stable new generation of artificial heart.

Le travail de thèse s’appuie sur un recueil exhaustif des travaux effectués dans le domaine du contrôle des vibrations auxquels sont soumis les ouvrages génie civil. Une contribution innovante et originale est apportée et permet de classifier, de généraliser et d’optimiser certains critères dans le but d’assurer une conception optimale de divers dispositifs d’atténuation des vibrations, et ce, selon leur application. L’un des objectifs de la thèse a donc consisté à traiter ces critères de manière originale. Après avoir résolu le problème dit « direct » s’appuyant sur la modélisation des systèmes dotés d’un ou de plusieurs AMAs, nous nous sommes intéressés au problème dit « indirect » en envisageant divers critères d’optimisation. Ainsi, plusieurs critères d’optimisation des paramètres mécaniques de plusieurs AMAs appliqués à un système principal comportant 1 ou plusieurs degrés de liberté peuvent être utilisés. L’excitation du système principal est envisagée de deux manières, soit à sa base (en vue d’une application à la sismique), soit au niveau de la structure (en vue d’une application aux effets du vent).Des simulations numériques sont réalisées dans le but d’étudier la performance de chaque modèle optimisé en se fondant sur des approches fréquentielles et temporelles. La robustesse de chaque critère d’optimisation face aux incertitudes liées au changement des paramètres physiques de la structure principale a été examinée. Nous avons également étudié la sensibilité des critères par rapport aux incertitudes des paramètres optimisés des AMAs. La conception et l’optimisation de p AMAs placés en parallèle pour un système principal à Nddl, a constitué une nouvelle contribution originale dans le cadre de cette thèse. Dans ce contexte, lors d’une sollicitation sismique, nous estimons la contribution de chaque mode dans la structure principale et nous conservons seulement les modes de vibrations qui ont un rapport de masses modales cumulées supérieur à 90%. Le choix du critère d’optimisation s’appuie seulement sur les étages les plus sensibles aux modes conservés et permet ainsi de tenir compte des modes élevés de la structure principale. Dans le but de limiter les dommages subis par les constructions du génie civil lors de sollicitations sismiques, nous cherchons à évaluer l’efficacité des AMAs afin d’atténuer les réponses temporelles sismiques. Ainsi une étude comparative est réalisée en appliquant quatre séismes réels sur les modèles optimisés. Afin d’illustrer les résultats obtenus, des tests de caractérisation d’un AMA utilisant un amortissement par courants de Foucault et un ajustement de la rigidité, ont été menés. Ils ont permis d’obtenir une validation expérimentale du modèle et du critère d’optimisation adopté / The architectural demand and the desire to reduce costs permit the construction of light structures with innovating shapes. The great flexibility of these structures makes them increasingly sensitive to the external dynamic loads such as traffic, wind and earthquakes. Vibration control techniques allow to construct modern buildings increasingly slender, and, whether they are economic or architecturally audacious. Instead of modifying the geometrical and mechanical characteristics of a structure, vibration control consists in producing reaction forces which are opposed to the negative effects of the external excitations when they appear. This technological advance has the great advantage to not influencing planners and architects’ work and it provides them with additional creative options in both geometrical and mechanical characteristics of buildings. We restrict our focus to passive vibration control. Among available passive vibration absorber systems, Tuned Mass Dampers (TMDs) were selected for their simplicity and reliability. A TMD consists of a mass, a dashpot, and a spring, and is commonly attached to a vibrating primary system to suppress undesirable vibrations. The performance of TMDs is strongly affected by the adjustment of their parameters. The problem is the optimization of the mechanical parameters of TMD and their location in order to attenuate vibrations of the main structure. This thesis is based on understanding the dynamic characteristics of TMD. It aims to make an innovative and original contribution to classify, generalize and optimize some criteria in order to ensure an optimal design of TMDs, depending on their application. Our work consisted to treat these criteria in an original way. After solving the direct problem based on the modelling of systems with one or several TMD, we tackled the indirect problem by considering various optimization criteria. Thus, several optimization criteria of the mechanical parameters of TMDs applied to a main system (single (SDOF) or multiple degrees of freedom (MDOF)) are used. The excitation of the main system can be done in two different ways; either on the base (for seismic application) or on the structure (for wind effects).Numerical simulations based on a time and frequency approach are used to examine the performance of each optimized model. The robustness of each optimization criterion is assessed by taken into account the uncertainties related to the change of the physical parameters of the main structure. Such problems can be discussed by considering sensitivity analysis for criteria under uncertainty of the optimum TMD parameters. A new and original contribution of this thesis is the design and optimization of multiple TMDs in parallel with a MDOF main structure. In this context, during seismic loads, modes in the main structure with relatively high effective masses can be readily excited by base excitation. Afterwards, optimization criterion can be developed based on the most sensitive storeys to vibration modes which are a cumulative modal effective mass fraction exceeding 90%. To protect structures under earthquake loads, we seek to assess the effectiveness of TMDs in mitigating the response of structure under different real earthquakes. A comparative study is then achieved with four real earthquakes applied on systems with TMD optimized parameters. To illustrate the results obtained, characterization tests are conducted on a TMD with damping by eddy currents effect and adjustable stiffness. They allow the validation of the model and optimization criterion adopted

Apresentam-se modelos matemáticos de poucos graus de liberdade para o estudo de vibrações estruturais não lineares excitadas por fonte não-ideal de energia, amortecidas por impacto, para duas aplicações: um pórtico que serve de apoio para um motor elétrico e uma torre de suporte a uma turbina eólica. Considera-se a existência de interação entre o fornecimento de energia e o movimento da estrutura de suporte. Se a potência fornecida pela fonte de energia não é suficiente, a rotação do rotor pode ficar estagnada à freqüência de ressonância da estrutura, impossibilitando o mesmo de alcançar regimes de rotação mais altos. Isso é uma manifestação do chamado Efeito Sommerfeld. No primeiro modelo, somente dois graus de liberdade são considerados: o movimento horizontal da estrutura, na direção perpendicular ao eixo do rotor, e a rotação do rotor. Depois, adiciona-se outro grau de liberdade, representando o movimento de uma pequena massa que se desloca livremente dentro de uma câmara (amortecedor por impacto). As equações de movimento desses modelos são obtidas via formulação Lagrangiana. Por intermédio de simulações numéricas, procurou-se estudar os parâmetros do amortecedor por impacto que produzem a melhor eficiência do dispositivo. Nota-se que o impacto da massa com as paredes do recipiente fornece controle da amplitude de vibração da estrutura e da largura da banda de freqüências em que o Efeito Sommerfeld ocorre. / We present mathematical models with few degrees of freedom for the study of nonlinear structural vibrations excited by a non ideal energy source, with impact damping, for two applications: a portal frame that supports an electric motor and a tower structure supporting an aeolian turbine. We consider that there is interaction between the energy supply and the motion of the supporting structure. If the power supplied by the energy source is not enough, the rotation of the engine may get stuck at a resonance frequency of the structure, disabling the engine to reach higher regimes of rotation. This is a manifestation of the so-called Sommerfeld Effect. In the first model, only two degrees of freedom are considered: the horizontal motion of the structure, in the transverse direction to the axis of the rotor, and the rotation of the rotor. Next, another degree of freedom is added to the model, representing the motion of a rolling small mass, free to bounce back and forth inside a chamber (impact damper). The equations of motion of these models are obtained via a Lagrangian approach. The parameters that produce the greatest effectiveness of the impact damper were studied through numerical simulations. One notices that the impact of the mass with the walls of the container supplies control of the vibration amplitude of the structure and the width of the band of frequencies where the Sommerfeld Effect occurs.

Os materiais inteligentes têm sido utilizados em problemas de controle de vibrações e conversão de energia mecânica em energia elétrica. Apesar das diferentes opções existentes, os piezelétricos têm recebido grande atenção devido a facilidade de instalação, além de possibilidade de uso como sensores ou atuadores. Em termos de sistemas de controle utilizando materiais piezelétricos, dois grandes grupos podem ser encontrados: os controladores passivos e os ativos. Os controladores ativos utilizam o efeito piezelétrico inverso e apresentam bom desempenho na redução de vibrações. Entretanto, apresentam desvantagens relacionadas à complexidades de uma lei de controle, necessidade de equipamentos externos e, potencialmente, exigem elevada potência de atuação. Por outro lado, os controladores passivos utilizam circuitos elétricos simples, compostos somente por elementos elétricos passivos. Apesar de serem de fácil implementação prática, apresentam bom desempenho em faixas restritas de frequências. Os controladores semi-passivos, surgiram como uma alternativa aos pontos negativos dos controladores passivos e ativos. Nestes novos sistemas, o material piezelétrico instalado na estrutura a ser controlada é conectado e desconectado a um circuito shunt de forma sincronizada com as vibrações mecânicas. Em geral, a conversão eletromecânica de energia é amplificada, assim como o efeito shunt damping. Dessa forma, os circuitos chaveados têm sido utilizados tanto como controladores semi-passivos quanto em problemas de coleta piezelétrica de energia. Neste trabalho, o controle piezelétrico semi-passivo de oscilações aeroelásticas lineares e não lineares, assim como a coleta piezelétrica de energia a partir das mesmas condições, são investigados experimentalmente. Uma seção típica com dois graus de liberdade e acoplamento eletromecânico é utilizada nos experimentos. Dois tratamentos não lineares do sinal elétrico proveniente dos piezelétricos são utilizados. Primeiro, o chaveamento a partir da condição de circuito aberto para uma resistência muito baixa, próxima ao curto circuito, e posteriormente, o chaveamento da condição de circuito aberto para um indutor. Um circuito chaveador autônomo (que não depende de fontes externas de energia) é apresentado. O desempenho dos dois sistemas no controle de flutter, e também de oscilações em ciclo limite, são discutidos. Os resultados mostraram um aumento na velocidade de flutter de até 8,8% e 11,5%, com chaveamento em uma resistência e em um indutor, respectivamente. No caso de coleta de energia a partir de oscilações aeroelásticas lineares e não lineares, o desempenho dos circuitos chaveados são comparados entre si, e com o caso em que uma resistência é considerada no domínio elétrico, resultando em um aumento da potência elétrica de até 101%, para chaveamentos em resistência, e 227%, para chaveamentos em um indutor. / Smart materials have been used in vibration control and also in energy harvesting problems. Although different materials are available, piezoelectric one has received most attention due to ease of installation and possibility of use as sensors or actuators. In general, there are two large categories of vibration control techniques using piezoelectric materials: passive and active control. In active control the reverse piezoelectric effect is used. In general, they present good performance in vibration reduction over a range of frequencies. However, active control has the disadvantages of additional complexities of a control law, additional hardware and the potentially large amount of power required. On the other hand, piezoelectric passive controllers use simple electric circuits composed by passive electrical elements. Although they are simple to implement, the performance of the controlled system is sensitive to the exciting frequency. The semi-passive controllers are a recent alternative to the drawbacks of passive and active controllers. In semi-passive systems, the piezoelectric element is switched in and out of a shunt impedance, in a synchronous way with mechanical vibrations. In general, the electromechanical energy conversion is enhanced as well as the shunt effect damping. Therefore, the switching techniques have been used both in semi-passive control problems and in piezoelectric energy harvesting problems. In this work, semi-passive techniques are experimentally investigated in aeroelastic control and piezoaeroelastic energy harvesting cases. An electromechanically coupled aeroelastic typical section is used in the experiments. Two techniques are investigated, the synchronized switching damping on short and the synchronized switching damping on inductor. An autonomous switching circuit (that does not requires external source of energy) is presented resulting in a self-powered flutter controller. The performance of the two semi-passive techniques is discussed for the linear case, flutter control, as well as limit cycle oscillations control. The linear flutter speed is increased by 8.8% and 11.5% when the SSDS and SSDI techniques are used, respectively. In the case of energy harvesting from linear and nonlinear aeroelastic oscillations, the performance of switching techniques is investigated and compared to the case of simple load resistance in the electrical domain. The power output is increased by 101% for the SSDS case and 227% for SSDI case.

O chatter é o principal problema de instabilidade nos processos de usinagem, e é causado pelas ondulações deixadas na superfície durante cortes sucessivos, ou chamado processo de regeneração, e é caracterizado pelo ruído e qualidade superficial ruim nas superfícies usinadas. Portanto, a comunidade científica tem se preocupado em desenvolver ações, tanto para a predição do fenômeno, como para desenvolver estratégias para sua redução. Neste trabalho avalia-se numérica e experimentalmente, a utilização de pastilhas piezelétricas acopladas no suporte da ferramenta, e aplicando estratégias de controle passivo e ativo, procurando a redução do chatter em processos de torneamento. A solução passiva consiste em conectar os terminais das pastilhas piezelétricas a um circuito elétrico dissipador de energia. A solução ativa propõe usar uma das pastilhas como sensor e a outra como atuador para aplicar leis de controle de realimentação. Na avaliação numérica foi considerado um modelo eletromecânico de parâmetros distribuídos usando a teoria de viga engastada de Euler-Bernoulli, e as equações constitutivas elétricas e mecânicas do material piezelétrico. A comparação das funções de resposta em frequência (FRFs) do sistema, obtidas numericamente, mostra uma adição de amortecimento ao sistema quando é usado um circuito de dissipação com uma resistência e uma indutância como solução passiva. A avaliação numérica da solução ativa indica que o controle de realimentação de velocidade adiciona amortecimento do sistema. A melhora da estabilidade no processo de torneamento destas duas estratégias é comprovada num diagrama de lóbulos de estabilidade. Na parte experimental foram obtidas as funções de resposta em frequência do sistema suporte da ferramenta, usando um sistema de aquisição de dados, com o fim de comparar as magnitudes da resposta, e foram feitos testes de torneamento com o fim de comparar qualitativamente as qualidades superficiais obtidas nas peças usinadas. A medição das FRFs com circuitos de dissipação indicou uma atenuação da resposta para um sistema com circuito em série, estratégia que foi avaliada em testes de torneamento, e mostrando uma melhora no acabamento superficial. / Chatter is the main problem of instability in machining processes, caused by the modulations left on the surface during the successive cuts, called regeneration process, and is characterized by violent vibrations, noise and poor surface quality on machined surfaces. Thus, the scientific community has been concerned with developing actions for both the phenomenom prediction, and developing strategies to reduce them. This work evaluates numerically and experimentally the use of piezoelectric layers embedded to the tool-holder, and applying active and passive strategies trying to reduce the chatter in turning processes. For the passive case, the conductive electrode pairs of the piezoelectric layers are connected to a shunt circuit which modifies the system dynamics. The active solution proposes to use one of the piezoelectric layers as sensor an the other one as actuator, in order to apply feedback control laws. A numerical evaluation considers an electromechanical distributed parameter model based on Euler- Bernoulli cantilever beam theory, and the electrical and mechanical constitutive equations of the piezoelectric material. A comparison of the system frequency response functions (FRFs), numerically obtained, shows an increase of system damping when a resistive-inductive shunt circuit is used as a passive solution. The numerical evaluation of the active solution shows that the velocity feedback control increases the system damping. The turning process stability improvement using both strategies, is shown in a stability lobe diagram. Frequency response functions of the tool-holder system were obtained experimentally using a data acquisition system, in order to compare the response amplitudes. Turning tests were performed in order to compare surface qualities obtained of machined parts. Measurement of FRFs using series resistive-inductive shunt circuits shows a system response attenuation, later this strategy was evaluated in turning tests, showing an improvement in surface finish.

Large-scale unsteady flow structures play an influential role in the dynamics of many practical flows, such as those found in gas turbine combustion chambers. This thesis is concerned primarily with large-scale unsteady structures that arise due to self-sustained hydrodynamic oscillations, also known as global hydrodynamic instability. Direct numerical simulation (DNS) of the Navier--Stokes equations in the low Mach number limit is used to obtain a steady base flow, and the most unstable direct and adjoint global modes. These are combined, using a structural sensitivity framework, to identify the region of the flow and the feedback mechanisms that are responsible for causing the global instability. Using a Lagrangian framework, the direct and adjoint global modes are also used to identify the regions of the flow where steady and unsteady control, such as a drag force or heat input, can suppress or promote the global instability. These tools are used to study a variety of reacting and non-reacting flows to build an understanding of the physical mechanisms that are responsible for global hydrodynamic instability in swirling diffusion flames. In a non-swirling lifted jet diffusion flame, two modes of global instability are found. The first mode is a high-frequency mode caused by the instability of the low-density jet shear layer in the premixing zone. The second mode is a low-frequency mode caused by an instability of the outer shear layer of the flame. Two types of swirling diffusion flames with vortex breakdown bubbles are considered. They show qualitatively similar behaviour to the lifted jet diffusion flames. The first type of flame is unstable to a low-frequency mode, with wavemaker located at the flame base. The second type of flame is unstable to a high-frequency mode, with wavemaker located at the upstream edge of the vortex breakdown bubble. Feedback from density perturbations is found to have a strong influence on the unstable modes in the reacting flows. The wavemaker of the high-frequency mode in the reacting flows is very similar to its non-reacting counterpart. The low-frequency mode, however, is only observed in the reacting flows. The presence of reaction increases the influence of changes in the base flow mixture fraction profiles on the eigenmode. This increased influence acts through the heat release term. These results emphasize the possibility that non-reacting simulations and experiments may not always capture the important instability mechanisms of reacting flows, and highlight the importance of including heat release terms in stability analyses of reacting flows.