Thesis (M.S.)--State University of New York at Binghamton, Thomas J. Watson School of Engineering and Applied Science, Department of Electrical and Computer Engineering, 2008. / Includes bibliographical references.

Thesis (Ph. D.)--University of Texas at Austin, 2006. / Vita. Includes bibliographical references.

Orientador: Katia Lucchesi Cavalca Dedini / Tese (doutorado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecânica / Made available in DSpace on 2018-08-23T01:22:57Z (GMT). No. of bitstreams: 1 Nonato_Fabio_D.pdf: 15501749 bytes, checksum: e33de59a100f07bcab576880e3654db0 (MD5) Previous issue date: 2013 / Resumo: A representação dinâmica dos contatos mecânicos em modelos computacionais de mancais de elementos rolantes baseia-se comumente no clássico modelo de Hertz para o contato seco, mesmo sendo a presença de um meio lubrificante imperativa para evitar o desgaste prematuro destes componentes. Desta maneira, os efeitos dinâmicos da presença de um filme fluídico nas interfaces de contato são desconsiderados, quando tais mancais são lubrificados por óleo. Com o intuito de introduzir tais efeitos em um modelo de mancal de elementos rolantes de esferas, este trabalho propõe um método baseado na aproximação dinâmica das características de cada contato lubrificado por vínculos de rigidez não linear e amortecimento viscoso. Estes vínculos dinâmicos são caracterizados a partir da resposta dinâmica da simulação completa do contato elastohidrodinâmico (EHD) elíptico, através da implementação de um integrador temporal que utiliza o método de diferenças finitas em múltiplos níveis para solução da parcela hidrodinâmica do problema. Posteriormente, aplica-se a aproximação do contato EHD por vínculos não lineares a um modelo do mancal radial de esferas, de dois graus de liberdade, com o intuito de analisar vibrações transversais no mancal, em modelos computacionais de máquinas rotativas. A comparação entre os resultados obtidos pelo modelo proposto e os valores obtidos pelo equacionamento completo por diferenças finitas mostrou ser adequada à representação do comportamento estacionário e dinâmico do contato. Observa-se a presença do enrijecimento do mancal devido à presença do filme EHD, quando analisando a resposta do mancal linearizado, efeito observável anteriormente apenas em investigações experimentais. Por fim, a integração temporal de um modelo de elementos finitos de um rotor apoiado por mancais lubrificados representou adequadamente o comportamento em frequência de uma montagem experimental de rotor. Conclui-se, portanto, que o modelo não linear proposto para o mancal de elementos rolantes radiais de esfera, sob influência da lubrificação EHD, é promissor para descrever o comportamento destes componentes quando em aplicação / Abstract: The dynamic representation of mechanical contacts in computational models of rolling element bearings is commonly based on the classic Hertz's model for the dry contact, even though the presence of a lubricant medium being essential to avoid premature wear of such components. Thus, the dynamic effects of the fluid film on the contacting interfaces are overlooked when such bearings are under oil lubrication. With the intent of introducing such effects in a radial deep groove ball bearing model, this work proposes a method based on the dynamic approximation of each lubricated contact by dynamic links of non-linear stiffness and viscous damping. Such dynamic links are characterized from the response of a complete dynamic simulation of the elastohydrodynamic (EHD) elliptic contact, through an implementation of a numerical integrator, which uses the multi-level finite difference method for the solution of the hydrodynamic portion of the problem. Afterwards, the approximation of each contact by dynamic links is applied to a complete rolling element bearing model, with two degrees of freedom, with the intent of analyzing transverse vibrations on the bearings applied to rotating machinery computational models. The comparison between the results attained through the proposed model and the values simulated using the complete finite difference model showed an adequate representation of the stationary and dynamic behaviors of the contact. When evaluating the linearized bearing model, the presence of fluid stiffening is observed, which was previously only observed through experimental investigations. Lastly, the time integration of a finite element model of a rotor supported by lubricated bearings portrayed the frequency content of an experimental set-up of the rotor adequately. It is concluded that the proposed nonlinear model for the deep grove ball bearing, under the influence of the EHD lubrication, is promising to describe the behavior of such components when on application / Doutorado / Mecanica dos Sólidos e Projeto Mecanico / Doutor em Engenharia Mecânica

Orientador: Marcos Silveira / Banca: Paulo José Paupitz Gonçalves / Banca: Fabricio Cesar Lobato de Almeida / Banca: Angelo Marcelo Tusset / Banca: Maíra Martins da Silva / Resumo: O objetivo desse trabalho é realizar um estudo do comportamento dinâmico de sistemas de suspensão veicular que empregam amortecimento assimétrico incluindo não linearidade geométrica, com foco na melhoria do conforto para os passageiros, considerando o perfil da estrada harmônico e estocástico. Os veículos terrestres estão sujeitos às vibrações induzidas pela superfície em que se deslocam e o conforto para os passageiros e o contato entre a estrada e o pneu depende desses fatores. Na prática, os veículos precisam ser equipados com sistemas de suspensão para minimizar os efeitos das imperfeições da estrada, aumentando o seu conforto. Nesse contexto, a eficácia do sistema de suspensão pode ser avaliada observando a aceleração da massa suspensa, que está relacionada diretamente com o nível de conforto para os passageiros. O perfil vertical da estrada pode ser representado de diferentes formas e um perfil harmônico simples ou um perfil estocástico realista pode ser utilizado dependendo do objetivo. A norma ISO 8608:1995 especifica um método uniforme para caracterizar o perfil das ruas, rodovias e estradas não pavimentadas baseado na densidade espectral de potência. O perfil da estrada pode ser classificado dentro de uma classe de rugosidade de A a H, onde A representa uma estrada muito boa e H uma estrada muito acidentada. Neste trabalho, é apresentada a modelagem do perfil da estrada de acordo com a ISO 8608:1995 para diferentes classes de rugosidade. O perfil vertical é aproxim... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: The objective of this work is to perform a study of the dynamic behavior of vehicle suspension systems that employ asymmetrical damping including geometrical nonlinearity, focusing on the improvement of comfort for the passenger, considering harmonic and stochastic road profiles. Ground vehicles are subject to the vibration induced by the surface on which they are moving and the passenger comfort and road-tire contact depend on these factors. In practice, vehicles need to be equipped with suspension systems to minimize the effects of road imperfections, increasing their comfort. In this context, the effectiveness of the suspension system can be measured by observing the acceleration of the sprung mass, which is directly related to the comfort level for the passengers. The vertical profile of the road can be represented in different ways and a simple harmonic or a realistic stochastic profile can be used depending on the purpose. ISO 8608:1995 specifies a uniform method for characterizing the profile of streets, highways and unpaved roads based on power spectral density. The road profile can be classified within a roughness class from A to H, where A represents a very good road and there is a very rough road. This work presents the modeling of the road profile according to ISO 8608:1995 for different roughness classes. The vertical profile is approximated by a harmonic function with overlap in the wavenumber and the phase angle is generated randomly. The classic quarter-car mo... (Complete abstract click electronic access below) / Doutor

This dissertation focuses on nonlinear modal interactions in plates. Our first investigation involved the activation of a two-to-one internal resonance in the response of a metallic cantilever plate. Although the plate was excited around the frequency of its second bending mode, its response contained a contribution from its first torsional mode. The frequency ratio between the bending and torsional modes was nearly two-to-one. Next, we investigated the energy transfer from high-frequency to low-frequency modes in a cantilever graphite-epoxy composite plate (90/30/ — 30/ — 30/30/90)_s. The plate was excited around the natural frequency of its seventh (third torsional) mode. For some excitation amplitudes and frequencies, we observed the activation of a low-frequency (first bending) mode accompanied by an amplitude and phase modulation of the seventh mode. We studied combination resonances in the responses of cantilever composite plates with the layups (90/30/ — 30/ — 30/30/90)_s and (—75/75/75/ — 75/75/ — 75)_s to harmonic base excitations. We activated the combination resonance f_e≈ ω₂ + ω₇ in the (90/30/ — 30/ — 30/30/90)_s plate, where the w; are the natural frequencies of the plate and f<sub>e<sub> is the excitation frequency. In the (—75/75/75/ — 75/75/ — 75)_s plate, we activated the external combination resonance f<sub>e<sub>≈ 1/2(ω₂+ω₅) and the combination internal resonance f_e≈1/2(ω₂+ω₁₃) ≈ ω₈. We carried out an experimental-modal analysis (EMA) of a nonclassically supported plate with and without a constrained-layer damping (CLD) patch attached on its upper left-hand side surface. The natural frequencies and mode shapes were used to ascertain the effect of the CLD patch. / Ph. D.

No description available.

Approved for public release, distribution is unlimited / Ensign, United States Navy

This research is aimed at investigating and analyzing the rain-windinduced cable vibration phenomena experienced in cables of cable-stayed bridges and also the countermeasures employed by engineers to mitigate the large-amplitude vibration problem reported by various researchers around the world. In order to investigate the problem of the water rivulet creation at the top of the cable surface, a single-degree-of-freedom (SDOF) analytical model was developed and analyzed. This thesis studies the aerodynamic instability of cables in cable-stayed bridges by doing literature review of a typical in-situ test, developing a single degree-of-freedom (SDOF) analytical model, and an ANSYS finite element model. Furthermore, a linear viscous damper that acts as a countermeasure to the large amplitudes of vibration is reported and analyzed. The suppression characteristics and damper effectiveness of such countermeasure are summarized. / Includes bibliography. / Thesis (M.S.)--Florida Atlantic University, 2014. / FAU Electronic Theses and Dissertations Collection

The ability to reduce flexural vibrations of lightweight structures has been a goal for many researchers. A type of transducer-controller system that accomplishes this is a piezoelectric patch connected to an electrical impedance, or shunt. The piezoelectric patch converts the vibrational strain energy of the structure to which it is bonded into electrical energy. This converted electrical energy is then modified by the shunt to influence to mechanical response. There are many types of shunt circuits which have demonstrated effective control of flexural systems. Of interest in this work is the negative capacitance shunt, which has been shown to produce significant reduction in vibration over a broad frequency range. A negative capacitance circuit produces a current that is 180̊ out of phase from a traditional, passive capacitor. In other words, the voltage of the capacitor decreases as charge is added. The negative capacitance shunt consists of a resistor and an active negative capacitance element. By adding a resistor and negative capacitor to the electrical domain, the shunt acts as a damper and negative spring in the mechanical domain. The performance of the negative capacitance shunt can be increased through proper selection of the shunt's electrical components. Three aspects of component selection are investigated: shunt efficiency, maximum suppression, and stability. First, through electrical modeling of the shunt-patch system, the components can be chosen to increase the efficiency of the shunt for a given impedance. Second, a method is developed that could be utilized to adaptively tune the magnitude of resistance and negative capacitance for maximum control at a given frequency. Third, with regard to stability, as the control gain of the circuit is increased, by adjusting the circuit parameters, there is a point when the shunt will become unstable. A method to predict the stability of the shunt is developed to aid in suppression prediction. The negative capacitance shunt is also combined with a periodic piezoelectric patch array to modify the propagating wave behavior of a vibrating structure. A finite element method is utilized to create models to predict both the propagation constant, which characterizes the reduction in propagating waves, and the velocity frequency response of a full system. Analytical predictions are verified with experimental results for both a 1- and 2-D periodic array. Results show significant attenuation can be achieved with a negative capacitance shunt applied to a piezoelectric patch array. Three electromechanical aspects are developed: design for maximum suppression, more accurate stability prediction, and increased power-output efficiency. First, a method is developed that may be used to adaptively tune the magnitude of resistance and negative capacitance for maximum suppression. Second, with regard to stability, a method is developed to predict the circuit components at which the circuit will obtain a stable output. Third, through electrical modeling of the shunt-patch system, the components are chosen to increase the power output efficiency of the shunt circuit for a given impedance. The negative capacitance shunt is also combined with a periodic piezoelectric patch array to modify the propagating wave behavior of a vibrating structure. Analytical predictions are verified with experimental results for both a 1- and 2-D periodic array. Results show significant attenuation can be achieved with a negative capacitance shunt applied to a piezoelectric patch array.

Manufacturers of commercial, power-generating, gas turbine engines continue to develop combustors that produce lower emissions of nitrogen oxides (NOx) in order to meet the environmental standards of governments around the world. Lean, premixed combustion technology is one technique used to reduce NOx emissions in many current power and energy generating systems. However, lean, premixed combustors are susceptible to thermo-acoustic oscillations, which are pressure and heat-release fluctuations that occur because of a coupling between the combustion process and the natural acoustic modes of the system. These pressure oscillations lead to premature failure of system components, resulting in very costly maintenance and downtime. Therefore, a great deal of work has gone into developing methods to prevent or eliminate these combustion instabilities. This dissertation presents the results of a theoretical and experimental investigation of a novel Fuel System Tuner (FST) used to damp detrimental combustion oscillations in a gas turbine combustor by changing the fuel supply system impedance, which controls the amplitude and phase of the fuel flowrate. When the FST is properly tuned, the heat release oscillations resulting from the fuel-air ratio oscillations damp, rather than drive, the combustor acoustic pressure oscillations. A feasibility study was conducted to prove the validity of the basic idea and to develop some basic guidelines for designing the FST. Acoustic models for the subcomponents of the FST were developed, and these models were experimentally verified using a two-microphone impedance tube. Models useful for designing, analyzing, and predicting the performance of the FST were developed and used to demonstrate the effectiveness of the FST. Experimental tests showed that the FST reduced the acoustic pressure amplitude of an unstable, model, gas-turbine combustor over a wide range of operating conditions and combustor configurations. Finally, combustor acoustic pressure amplitude measurements made in using the model combustor were used in conjunction with model predicted fuel system impedances to verify the developed design rules. The FST concept and design methodology presented in this dissertation can be used to design fuel system tuners for new and existing gas turbine combustors to reduce, or eliminate altogether, thermo-acoustic oscillations.