Large structures exposed to the environment such as the collinear omni and large panel communication antennas in this research suffer damage from cyclic wind, rain, hail, ice load and impacts from birds and stones. Stresses from self-weight, ice loading and wind gusts will produce deformations of the structure that will lead to performance deterioration of the antenna. In order to avoid such a case, it is important to understand the static, dynamic and aerodynamic behavior of these structures and thus optimization can be achieved. In this research the current fluid-structure interaction methods are used to model, simulate and analyze these communication antennas in order to assess whether failure would occur under service loads. The FEA models developed are verified against analytical models and/or experiments. Different antenna configurations are compared based on their capacity to minimize vibration effects, stress-induced deformations and aerodynamic loading effects.

Fluid-structure interactions occur in many engineering and industrial applications. Such interactions may result in undesirable forces acting on the structure that may cause fatigue and degradation of the structural components. The purpose of this research is to develop a solver that simulates the fluid-structure interaction, assess tools that can be used to control the resulting motions and analyze a system that can be used to convert the structure's motion to a useful form of energy. For this purpose, we develop a code which encompasses three-dimensional numerical simulations of a flow interacting with a freely-oscillating cylinder. The solver is based on the accelerated reference frame technique (ARF), in which the momentum equations are directly coupled with the cylinder motion by adding a reference frame acceleration term; the outer boundary conditions of the flow domain are updated using the response of the cylinder. We develop active linear and nonlinear velocity feedback controllers that suppress VIV by directly controlling the cylinder's motion. We assess their effectiveness and compare their performance and required power levels to suppress the motion of the cylinder. Particularly, we determine the most effective control law that requires minimum power to achieve a desired controlled amplitude. Furthermore, we investigate, in detail, the feasibility of using a nonlinear energy sink to control the vortex-induced vibrations of a freely oscillating circular cylinder. It has been postulated that such a system, which consists of a nonlinear spring, can be used to control the motion over a wide range of frequencies. However, introducing an essential nonlinearity of the cubic order to a coupled system could lead to multiple stable solutions depending on the initial conditions, system's characteristics and parameters. Our investigation aims at determining the effects of the sink parameters on the response of the coupled system. We also investigate the extent of drag reduction that can be attained through rotational oscillations of the circular cylinder. An optimization is performed by combining the CFD solver with a global deterministic optimization algorithm. The use of this optimization tool allows for a rapid determination of the rotational amplitude and frequency domains that yield minimum drag. We also perform three-dimensional numerical simulations of an inline-vibrating cylinder over a range of amplitudes and frequencies with the objective of suppressing the lift force. We compare the amplitude-frequency response curves, levels of lift suppression, and synchronization maps for two- and three-dimensional flows. Finally, we evaluate the possibility of converting vortex-induced vibrations into a usable form of electric power. Different transduction mechanisms can be employed for converting these vibrations to electric power, including electrostatic, electromagnetic, and piezoelectric transduction. We consider the piezoelectric option because it can be used to harvest energy over a wide range of frequencies and can be easily implemented. We particularly investigate the conversion of vortex-induced vibrations to electric power under different operating conditions including the Reynolds number and load resistance. / Ph. D.

Finite element (FE) models serve as the base of many different types of analysis as e.g. dynamic analysis. Hence, obtaining FE models that represent the actual behaviour of real structures with great accuracy is of great importance. However, more often than not, there are differences between FE models and the structures being modelled, which can depend on numerous factors. These factors can consist of uncertainties in material behaviour, geometrical properties and boundary- and continuity conditions. Model validation is therefore an important aspect in obtaining FE models that represents reality to some degree. Furthermore, model verification is also important in terms of verifying theoretical models, other than FE models, in fields such as fatigue-, fracture- and dynamic analysis. In this thesis, two pedestrian steel bridges, the Kallhäll bridge and the Smista bridge, have been modelled in a FE software based on engineering drawings and validated against experimental results with regard to their natural frequencies. Furthermore, in this thesis, a model has been developed in MATLAB based on modal analysis that accounts for pedestrian-induced torsional vibrations, the 3D SDOF model. This model has been verified against the previously mentioned FE models. The aim of this thesis is hence two parted where the first part is to develop three-dimensional FE models of two pedestrian bridges and validate them against measured data regarding the natural frequencies. The second part is to further develop a model for analysing the effect of pedestrian-induced torsional vibrations and to investigate whether the model captures the actual dynamic response of such loading. The results showed that the natural frequencies for the first bending- and torsional mode from the FE models corresponded well to the measured ones with the largest difference of 5 \% obtained for the natural frequency of the first bending mode for the Smista bridge. Furthermore, the 3D SDOF model was able to capture the dynamic response of torsional vibrations with an overall difference of less than 2 \% in comparison to the FE models. The model can be improved by further studying the pedestrian-structure interaction as well as studying the effect of using approximative functions describing the mode shapes. / Finita elementmodeller (FE-modeller) utgör en bas för många olika typer av analyser som exempelvis dynamiska analyser. Därmed är det av stor betydelse att FE-modeller representerar det faktiska beteendet av verkliga strukturer med stor noggrannhet. Ofta är det emellertid skillnader mellan FE-modeller och de verkliga strukturer man modellerar. Dessa skillnader kan bero på en rad faktorer såsom exempelvis osäkerheter i materialbeteende, geometriska egenskaper samt upplag- och randvillkor. Modellvalidering är därför en viktig aspekt i att erhålla FE-modeller som representerar verkligheten i olika omfattningar. Utöver modellvalidering är även modellverifiering viktigt, inte endast för verifiering av FE-modeller utan även för verifiering av andra teoretiska modeller inom områden såsom utmaning-, fraktur- och dynamiska analyser. I detta arbete har två GC-broar, Kallhäll- och Smistabron modellerats i ett FE-program baserat på konstruktionsritningar och validerats mot experimentella resultat med avseende på de naturliga frekvenserna. Vidare har det i detta arbete utvecklats en modell i MATLAB som tar hänsyn till människo-inducerade torsionsvibrationer baserat på modalanalys, benämnd 3D SDOF modellen. Modellen har även verifierats mot de tidigare nämnda FE-modellerna. Målet med detta arbete är således uppdelat i två delar, där den första delen består av att utveckla tredimensionella FE-modeller av två GC-broar samt validera dessa mot mätdata vad gäller de naturliga frekvenserna. Den andra delen består av att utveckla en modell för att analysera effekten av människo-inducerade torsionsvibrationer och undersöka huruvida modellen fångar den dynamiska responsen. Resultaten visade att de naturliga frekvenserna för den första böj- och vridmoden från FE-modellerna motsvarade de uppmätta frekvenserna med en största relativ skillnad på 5 \% för den fösta böjmoden för Smistabron. Vidare visade resultaten att den utvecklade 3D SDOF modellen kunde fånga den dynamiska responsen av torsionsvibrationer med en skillnad på mindre än 2 \% i jämförelse med resultat från de FE-modellerna. Modellen kan förbättras genom att vidare studera interaktionen mellan fotgängare och gångbro samt studera effekten av att använda approximativa funktioner som beskriver modformen.

Converting vibrations to a usable form of energy has been the topic of many recent investigations. The ultimate goal is to convert ambient or aeroelastic vibrations to operate low-power consumption devices, such as microelectromechanical systems, heath monitoring sensors, wireless sensors or replacing small batteries that have a nite life span or would require hard and expensive maintenance. The transduction mechanisms used for transforming vibrations to electric power include: electromagnetic, electrostatic, and piezoelectric mechanisms. Because it can be used to harvest energy over a wide range of frequencies and because of its ease of application, the piezoelectric option has attracted significant interest. In this work, we investigate the performance of different types of piezoelectric energy harvesters. The objective is to design and enhance the performance of these harvesters. To this end, distributed-parameter and phenomenological models of these harvesters are developed. Global analysis of these models is then performed using modern methods of nonlinear dynamics. In the first part of this Dissertation, global nonlinear distributed-parameter models for piezoelectric energy harvesters under direct and parametric excitations are developed. The method of multiple scales is then used to derive nonlinear forms of the governing equations and associated boundary conditions, which are used to evaluate their performance and determine the effects of the nonlinear piezoelectric coefficients on their behavior in terms of softening or hardening. In the second part, we assess the influence of the linear and nonlinear parameters on the dynamic behavior of a wing-based piezoaeroelastic energy harvester. The system is composed of a rigid airfoil that is constrained to pitch and plunge and supported by linear and nonlinear torsional and flexural springs with a piezoelectric coupling attached to the plunge degree of freedom. Linear analysis is performed to determine the effects of the linear spring coefficients and electrical load resistance on the flutter speed. Then, the normal form of the Hopf bifurcation (flutter) is derived to characterize the type of instability and determine the effects of the aerodynamic nonlinearities and the nonlinear coefficients of the springs on the system's stability near the bifurcation. This is useful to characterize the effects of different parameters on the system's output and ensure that subcritical or "catastrophic" bifurcation does not take place. Both linear and nonlinear analyses are then used to design and enhance the performance of these harvesters. In the last part, the concept of energy harvesting from vortex-induced vibrations of a circular cylinder is investigated. The power levels that can be generated from these vibrations and the variations of these levels with the freestream velocity are determined. A mathematical model that accounts for the coupled lift force, cylinder motion and generated voltage is presented. Linear analysis of the electromechanical model is performed to determine the effects of the electrical load resistance on the natural frequency of the rigid cylinder and the onset of the synchronization region. The impacts of the nonlinearities on the cylinder's response and energy harvesting are then investigated. / Ph. D.

<p> Two phase flow induced vibrations is a field that has many inherent modelling difficulties, making research in the area challenging. In order to study the problem more closely, a two phase flow loop using Freon 11 had been designed and commissioned at McMaster University. The initial design required some modifications to make the loop as "user friendly" as possible. The final result meets this desired capability. </p> <p> The loop was designed so that research into vibrations in tube bundles could be carried out. A test section had been designed to facilitate this task. However, this design also required modifications. Additionally, new vibration monitoring instrumentation making use of light was developed to avoid the detrimental effects of Freon 11. The introduction of these items has resulted in a complete facility for the purpose of studying two phase flow induced vibrations. Preliminary experiments revealed a problem relating to tube tuning. Generally, the results are promising and some interesting new phenomena were observed as well. </p> / Thesis / Master of Engineering (ME)

The presented work is intended to develop a geometrically reduced order (homogenized) model for a large antenna space structure with flexible joints. An energy equivalence concept is employed to find the continuum model for the system. The kinetic and strain energy expressions of the fundamental elements are found based on the assumptions of the micropolar elasticity theory. Necessary assumptions are made to reduce the order of the strain variables while retaining the effects of the micro-rotations that are coupled to the primary strain terms. As a result, a micropolar-based continuum model is found for the structure with torsional joints. The vibrations equations of motion for various coordinates of the one dimensional equivalent model are presented. Subsequently, the relations between the physical parameters of the distributed parameter model and the radar structure are introduced. The effect of the asymmetric mass distribution as a result of the addition of the radar panel to the truss system is studied. For the purpose of the experimental validation of the suggested model a planar truss structure with Pratt Girder configuration was built and tested in the laboratory. The results for the experimental frequency response functions are shown to be in good agreement with the theory. Finally, the continuum model is used to quantify the effects of the thermally induced disturbances on the satellite system during the eclipse transition. / Ph. D.

The thesis develops singly-periodic mathematical models for complex laminar wakes which are formed behind vortex-shedding bluff bodies. These wake structures exhibit a variety of patterns as the bodies oscillate or are in close proximity of one another. The most well-known formation comprises two counter-rotating vortices in each shedding cycle and is popularly known as the vk vortex street. Of the more complex configurations, as a specific example, this thesis investigates one of the most commonly occurring wake arrangements, which consists of two pairs of vortices in each shedding period. The paired vortices are, in general, counter-rotating and belong to a more general definition of the 2P mode, which involves periodic release of four vortices into the flow. The 2P arrangement can, primarily, be sub-classed into two types: one with a symmetric orientation of the two vortex pairs about the streamwise direction in a periodic domain and the other in which the two vortex pairs per period are placed in a staggered geometry about the wake centerline. The thesis explores the governing dynamics of such wakes and characterizes the corresponding relative vortex motion. In general, for both the symmetric as well as the staggered four vortex periodic arrangements, the thesis develops two-dimensional potential flow models (consisting of an integrable Hamiltonian system of point vortices) that consider spatially periodic arrays of four vortices with their strengths being +/-1 and +/-2. Vortex formations observed in the experiments inspire the assumed spatial symmetry. The models demonstrate a number of dynamic modes that are classified using a bifurcation analysis of the phase space topology, consisting of level curves of the Hamiltonian. Despite the vortex strengths in each pair being unequal in magnitude, some initial conditions lead to relative equilibrium when the vortex configuration moves with invariant size and shape. The scaled comparisons of the model results with experiments conducted in a flowing soap film with an airfoil, which was imparted with forced oscillations, are satisfactory and validate the reduced order modeling framework. The experiments have been performed by a collaborator group at the Department of Physics and Fluid Dynamics at the Technical University of Denmark (DTU), led by Dr. Anders Andersen. Similar experiments have also been run at Virginia Tech as part of this dissertation and the preliminary results are included in this treatise. The thesis also employs the same dynamical systems techniques, which have been applied to study the 2P regime dynamics, to develop a mathematical model for the P+S mode vortex wakes, with three vortices present in each shedding cycle. The model results have also been compared favorably with an experiment and the predictions regarding the vortex circulation data match well with the previous results from literature. Finally, the thesis introduces a novel concept of clean and renewable energy extraction from vortex-induced vibrations of bluff bodies. The slow-moving currents in the off-shore marine environments and riverine flows are beyond the operational capabilities of the more established hydrokinetic energy converters and the discussed technology promises to be a significant tool to generate useful power from these copiously available but previously untapped sources. / Ph. D.

Esta Dissertação de Mestrado apresenta um estudo experimental sobre os efeitos de interferência das vibrações induzidas pelo escoamento ao redor de cilindros rígidos livres para oscilar transversalmente ao escoamento fluido. Primeiramente, justifica a necessidade de pesquisas experimentais dentro do contexto prático da engenharia offshore, que motiva este trabalho. Apresenta uma revisão bibliográfica sobre escoamentos ao redor de corpos rombudos seguida de uma síntese sobre vibrações induzidas pelo escoamento. Destacam-se as oscilações causadas por VIV – Vibrações Induzidas por Vórtices e “galloping" como os fenômenos até o momento atribuídos às excitações de um cilindro isolado e um par de cilindros alinhados com o escoamento. Apresenta a metodologia experimental empregada, descrevendo a aplicação de bases elásticas fletoras com um grau de liberdade. Introduz as técnicas experimentais e de tratamento de sinais utilizadas e faz rápidas considerações sobre a realização de ensaios nos dois canais de água circulante onde os experimentos foram conduzidos. Os resultados apresentados, discutidos e comparados envolvem: medições da resposta dinâmica em amplitude e freqüência dominante de oscilação; medição instantânea do ângulo de fase entre a força fluida e o deslocamento do cilindro e do ângulo de fase entre as oscilações de dois cilindros; e medições da dinâmica da esteira com PIV. Os arranjos de cilindros estudados são: cilindro isolado; par alinhado com cilindro à montante oscilando; par alinhado com cilindro à jusante oscilando; e par alinhado com ambos os cilindro oscilando. Todas as configurações analisadas em detalhes possuem baixo parâmetro de massa e baixíssimo parâmetro de amortecimento . Os espaçamentos entre os centros dos cilindros alinhados variam entre . A faixa de velocidade reduzida analisada está entre. Um cilindro isolado apresentou resposta típica de VIV com os três ramos de resposta (inicial, superior e inferior) identificados. O fenômeno de intermitência do ângulo de fase instantâneo foi verificado nas regiões de transição. Os modos 2S e 2P de emissão de vórtices foram identificados com PIV. Estes dados mostraram boa concordância com outros experimentos da literatura e serviram de referência para as demais comparações deste texto. A resposta de um cilindro oscilando à montante de outro fixo também apresentou comportamento típico de VIV. Conclui-se que o primeiro cilindro não sofre efeito de interferência do cilindro à jusante para os espaçamento e velocidades analisados. Os principais efeitos de interferência ocorrem para um cilindro oscilando à jusante de outro fixo. Este arranjo, foco deste estudo, não apresenta resposta típica de VIV, uma vez que a amplitude apresenta um ramo crescente após a faixa típica de sincronização. Também não apresenta resposta típica de excitação pura por galloping, já que a força fluida não está em fase com a velocidade do cilindro. Assim, denomina-se um novo fenômeno responsável por estas excitações que combina: captura da freqüência de emissão e faixa de sincronização de VIV; e amplitude assintoticamente crescente típica da resposta de galloping. Trata-se das WIV – Vibrações por Interferência da Esteira. Quando ambos os cilindros estão livres para oscilar, o cilindro à montante continua apresentando resposta típica de VIV, enquanto o segundo cilindro responde com ramos descontínuos também excitados por WIV. Por fim, conclui-se que não é conveniente chamar de “excitações por galloping" os ramos crescentes nas configurações de interferência. Entende-se que esta resposta esteja sendo excitada pelos efeitos de interferência das esteiras formada entre os cilindros e desprendida no cilindro à jusante. Sugere-se que a nomenclatura Vibrações por Interferência da Esteira seja mais adequada à natureza dos fenômenos. As visualizações de PIV foram importantes para a verificação da redução do comprimento de formação de vórtices com o aumento do número de Reynolds, mostrando a necessidade de experimentos de interferência com Reynolds constante. Encerra-se o texto apresentando propostas para trabalhos futuros que continuem nesta linha de pesquisa. / This MSc Thesis presents an experimental study on flow-induced vibrations and interference effects around rigid circular cylinders free to oscillate transversally to the flow. Firstly, it justifies the real needs of an experimental approach within the context of offshore engineering, which motivates this project. After that, presents a review over bluff-bodies flows followed by some consideration concerning flow-induced vibrations. Special attention is found over the oscillations caused by VIV – Vortex-Induced Vibrations and galloping phenomena, which are attributed to excite either a single isolated cylinder or a pair of tandem interfering cylinders. The experimental methodology is shown, describing the applications of elastic bases with one degree of freedom. Experimental techniques and signal analysis procedures are discussed considering the executions of these experiments in two water channel facilities. Presented, discussed and compared results involve: dynamic responses in amplitude and dominant oscillation frequency; instantaneous phase angle between fluid forces and cylinder displacement and phase angle betweens the two cylinders oscillations; and wake dynamics measurements and visualizations employing PIV technique. Cylinders are arranged as follow: single isolated cylinder; tandem pair with upstream one free to oscillate; tandem pair with downstream one free to oscillate; tandem pair with both cylinders free to oscillate. All carefully analyzed arrangements present low-mass parameter and very low damping . Gaps between cylinder centers vary through . Reduced velocity range is comprised in. The isolated cylinder case presented a typical VIV response, with three identified branches (initial, upper and lower). The instantaneous phase angle intermittency phenomenon was observed in transition regions. The 2S and 2P vortex modes were verified by PIV technique. These data showed to be in accordance to other literature measurements and are employed as reference results for comparisons throughout this text. The dynamic response of a cylinder oscillating upstream a fixed one showed a typical VIV behavior. From this can be concluded that the downstream cylinder does not imply any interference phenomenon for analyzed gaps and velocities. On the other hand, major interference effects occur when a downstream cylinder is oscillating in the wake of another fixed one. This configuration, which is the focus of this study, does not show a typical VIV response, since amplitude curves present a crescent branch after the typical synchronization regime. It does not either present a typical galloping excitation response, since the fluid forces are not in phase with the cylinder velocity. Thus, a new suggested name, WIV – Wake-Interference Vibrations, describes the phenomenon responsible for these excitations, which combines: lock-in of shedding frequency from VIV; and asymptotically crescent response from galloping-like excitations. When both cylinders are free to oscillate, the upstream one presents the typical VIV response, while the downstream one passes through three branches with discontinuities, excited by WIV. Finally, it can be concluded that is not convenient to call “galloping excitation" these crescent branches for interference arrangements. It can be understood that the dynamic response is being excited by the interference effects from upstream wake and downstream cylinder vortices. The nomenclature Wake-Interference Vibrations is suggested to be more adequate to the nature of this phenomenon. In addition, PIV visualizations showed to be very important to certify that the formation length decreases while increasing Reynolds number, requiring interference experiments with constant Reynolds numbers. To conclude, some suggestions for future work in this research field are presented.

Esta Dissertação de Mestrado apresenta um estudo experimental sobre os efeitos de interferência das vibrações induzidas pelo escoamento ao redor de cilindros rígidos livres para oscilar transversalmente ao escoamento fluido. Primeiramente, justifica a necessidade de pesquisas experimentais dentro do contexto prático da engenharia offshore, que motiva este trabalho. Apresenta uma revisão bibliográfica sobre escoamentos ao redor de corpos rombudos seguida de uma síntese sobre vibrações induzidas pelo escoamento. Destacam-se as oscilações causadas por VIV – Vibrações Induzidas por Vórtices e “galloping” como os fenômenos até o momento atribuídos às excitações de um cilindro isolado e um par de cilindros alinhados com o escoamento. Apresenta a metodologia experimental empregada, descrevendo a aplicação de bases elásticas fletoras com um grau de liberdade. Introduz as técnicas experimentais e de tratamento de sinais utilizadas e faz rápidas considerações sobre a realização de ensaios nos dois canais de água circulante onde os experimentos foram conduzidos. Os resultados apresentados, discutidos e comparados envolvem: medições da resposta dinâmica em amplitude e freqüência dominante de oscilação; medição instantânea do ângulo de fase entre a força fluida e o deslocamento do cilindro e do ângulo de fase entre as oscilações de dois cilindros; e medições da dinâmica da esteira com PIV. Os arranjos de cilindros estudados são: cilindro isolado; par alinhado com cilindro à montante oscilando; par alinhado com cilindro à jusante oscilando; e par alinhado com ambos os cilindro oscilando. Todas as configurações analisadas em detalhes possuem baixo parâmetro de massa e baixíssimo parâmetro de amortecimento . Os espaçamentos entre os centros dos cilindros alinhados variam entre . A faixa de velocidade reduzida analisada está entre. Um cilindro isolado apresentou resposta típica de VIV com os três ramos de resposta (inicial, superior e inferior) identificados. O fenômeno de intermitência do ângulo de fase instantâneo foi verificado nas regiões de transição. Os modos 2S e 2P de emissão de vórtices foram identificados com PIV. Estes dados mostraram boa concordância com outros experimentos da literatura e serviram de referência para as demais comparações deste texto. A resposta de um cilindro oscilando à montante de outro fixo também apresentou comportamento típico de VIV. Conclui-se que o primeiro cilindro não sofre efeito de interferência do cilindro à jusante para os espaçamento e velocidades analisados. Os principais efeitos de interferência ocorrem para um cilindro oscilando à jusante de outro fixo. Este arranjo, foco deste estudo, não apresenta resposta típica de VIV, uma vez que a amplitude apresenta um ramo crescente após a faixa típica de sincronização. Também não apresenta resposta típica de excitação pura por galloping, já que a força fluida não está em fase com a velocidade do cilindro. Assim, denomina-se um novo fenômeno responsável por estas excitações que combina: captura da freqüência de emissão e faixa de sincronização de VIV; e amplitude assintoticamente crescente típica da resposta de galloping. Trata-se das WIV – Vibrações por Interferência da Esteira. Quando ambos os cilindros estão livres para oscilar, o cilindro à montante continua apresentando resposta típica de VIV, enquanto o segundo cilindro responde com ramos descontínuos também excitados por WIV. Por fim, conclui-se que não é conveniente chamar de “excitações por galloping” os ramos crescentes nas configurações de interferência. Entende-se que esta resposta esteja sendo excitada pelos efeitos de interferência das esteiras formada entre os cilindros e desprendida no cilindro à jusante. Sugere-se que a nomenclatura Vibrações por Interferência da Esteira seja mais adequada à natureza dos fenômenos. As visualizações de PIV foram importantes para a verificação da redução do comprimento de formação de vórtices com o aumento do número de Reynolds, mostrando a necessidade de experimentos de interferência com Reynolds constante. Encerra-se o texto apresentando propostas para trabalhos futuros que continuem nesta linha de pesquisa. / This MSc Thesis presents an experimental study on flow-induced vibrations and interference effects around rigid circular cylinders free to oscillate transversally to the flow. Firstly, it justifies the real needs of an experimental approach within the context of offshore engineering, which motivates this project. After that, presents a review over bluff-bodies flows followed by some consideration concerning flow-induced vibrations. Special attention is found over the oscillations caused by VIV – Vortex-Induced Vibrations and galloping phenomena, which are attributed to excite either a single isolated cylinder or a pair of tandem interfering cylinders. The experimental methodology is shown, describing the applications of elastic bases with one degree of freedom. Experimental techniques and signal analysis procedures are discussed considering the executions of these experiments in two water channel facilities. Presented, discussed and compared results involve: dynamic responses in amplitude and dominant oscillation frequency; instantaneous phase angle between fluid forces and cylinder displacement and phase angle betweens the two cylinders oscillations; and wake dynamics measurements and visualizations employing PIV technique. Cylinders are arranged as follow: single isolated cylinder; tandem pair with upstream one free to oscillate; tandem pair with downstream one free to oscillate; tandem pair with both cylinders free to oscillate. All carefully analyzed arrangements present low-mass parameter and very low damping . Gaps between cylinder centers vary through . Reduced velocity range is comprised in. The isolated cylinder case presented a typical VIV response, with three identified branches (initial, upper and lower). The instantaneous phase angle intermittency phenomenon was observed in transition regions. The 2S and 2P vortex modes were verified by PIV technique. These data showed to be in accordance to other literature measurements and are employed as reference results for comparisons throughout this text. The dynamic response of a cylinder oscillating upstream a fixed one showed a typical VIV behavior. From this can be concluded that the downstream cylinder does not imply any interference phenomenon for analyzed gaps and velocities. On the other hand, major interference effects occur when a downstream cylinder is oscillating in the wake of another fixed one. This configuration, which is the focus of this study, does not show a typical VIV response, since amplitude curves present a crescent branch after the typical synchronization regime. It does not either present a typical galloping excitation response, since the fluid forces are not in phase with the cylinder velocity. Thus, a new suggested name, WIV – Wake-Interference Vibrations, describes the phenomenon responsible for these excitations, which combines: lock-in of shedding frequency from VIV; and asymptotically crescent response from galloping-like excitations. When both cylinders are free to oscillate, the upstream one presents the typical VIV response, while the downstream one passes through three branches with discontinuities, excited by WIV. Finally, it can be concluded that is not convenient to call “galloping excitation” these crescent branches for interference arrangements. It can be understood that the dynamic response is being excited by the interference effects from upstream wake and downstream cylinder vortices. The nomenclature Wake-Interference Vibrations is suggested to be more adequate to the nature of this phenomenon. In addition, PIV visualizations showed to be very important to certify that the formation length decreases while increasing Reynolds number, requiring interference experiments with constant Reynolds numbers. To conclude, some suggestions for future work in this research field are presented.

The goal of this project was to develop a wind generator that utilizes the collective oscillating motion of multiple piezoelectric devices. These devices would be an alternative to rotating turbine designs for low power generation, for use in applications such as remote power generation. A series of inexpensive devices were developed that harvested energy from vortex shedding, both as independent and cooperative devices. The behavior of single devices was studied, but more interestingly, the way that multiple devices arranged together can increase power output was studied. It was shown that individual devices could harvest more energy if they were placed as specific positions relative to the vortices shed by devices upstream. Through investigating the behavior of these devices, fundamental principles of the phenomenon of vortex induced vibrations were explored. Methods were developed to measure the amplitude and frequency of these vibrations in a wind tunnel, through high speed video and correlations that were found between oscillation and power output from the piezoelectric transducers. Similarly, vortex induced vibrations were explored in an approximation of a two dimensional system in a flowing soap film.