Vortex-induced vibrations of a pivoted circular cylinder and control of these vibrations were investigated experimentally. A novel experimental setup was employed to reproduce orbiting response observed in some engineering applications. An adaptive pendulum tuned-mass damper (TMD) was integrated with the cylindrical structure in order to control the vortex-induced vibrations. All experiments were performed at a constant Reynolds number of 2100 for a range of reduced velocities from 3.4 to 11.3 and damping ratios from 0.004 to 0.018. For the experiments involving TMD, the TMD mass ratio was 0.087 and the TMD damping ratios investigated were 0 and 0.24. The results of the experiments performed without the TMD show that, in the synchronization region, the frequencies of transverse and streamwise vibrations lock onto the natural frequency of the structure. The cylinder is observed to trace elliptic trajectories. A mathematical model is introduced to investigate the mechanism responsible for the occurrence of the observed elliptic trajectories and figure-8 type trajectories reported in previous laboratory investigations. The results show that the occurrence of either elliptic trajectories or figure-8 type trajectories is governed primarily by structural coupling between vibrations in streamwise and transverse directions. Four types of elliptic trajectories were identified. The results show that the occurrence of the different types of elliptic trajectories is linked to phase angle between the streamwise and transverse vibrations of the structure, which depends on structural coupling. The results of the experiments performed to investigate effectiveness of the TMD in controlling vortex-induced vibrations show that tuning the TMD natural frequency to the natural frequency of the structure decreases significantly the amplitudes of transverse and streamwise vibrations of the structure. Specifically, the transverse amplitudes of vibrations are decreased by a factor of ten and streamwise amplitudes of vibrations are decreased by a factor of three. The results show that, depending on the value of the TMD damping ratio, the frequency of transverse vibrations is either characterized by the natural frequency or by two frequencies: one higher and the other lower than the natural frequency of the structure, referred to as fundamental frequencies. Independent of TMD damping and tuning frequency ratios, the frequency of streamwise vibrations matches that of the transverse vibrations in the synchronization region, and the cylinder traces elliptic trajectories. The phase angle between the streamwise and transverse vibrations is nearly constant when the pendulum is restrained. However, with the TMD engaged and tuned to the natural frequency, the phase angle fluctuates significantly with time. A mathematical model was utilized to gain insight into the frequency response of the structure. The results of the modeling show that the frequency of transverse vibrations is characterized by the fundamental frequency or frequencies of the structure and the frequency of streamwise vibrations is characterized by the fundamental frequency or frequencies as well as the first harmonic of the fundamental frequency or frequencies of the structure.

Vortex-induced vibrations of a pivoted circular cylinder and control of these vibrations were investigated experimentally. A novel experimental setup was employed to reproduce orbiting response observed in some engineering applications. An adaptive pendulum tuned-mass damper (TMD) was integrated with the cylindrical structure in order to control the vortex-induced vibrations. All experiments were performed at a constant Reynolds number of 2100 for a range of reduced velocities from 3.4 to 11.3 and damping ratios from 0.004 to 0.018. For the experiments involving TMD, the TMD mass ratio was 0.087 and the TMD damping ratios investigated were 0 and 0.24. The results of the experiments performed without the TMD show that, in the synchronization region, the frequencies of transverse and streamwise vibrations lock onto the natural frequency of the structure. The cylinder is observed to trace elliptic trajectories. A mathematical model is introduced to investigate the mechanism responsible for the occurrence of the observed elliptic trajectories and figure-8 type trajectories reported in previous laboratory investigations. The results show that the occurrence of either elliptic trajectories or figure-8 type trajectories is governed primarily by structural coupling between vibrations in streamwise and transverse directions. Four types of elliptic trajectories were identified. The results show that the occurrence of the different types of elliptic trajectories is linked to phase angle between the streamwise and transverse vibrations of the structure, which depends on structural coupling. The results of the experiments performed to investigate effectiveness of the TMD in controlling vortex-induced vibrations show that tuning the TMD natural frequency to the natural frequency of the structure decreases significantly the amplitudes of transverse and streamwise vibrations of the structure. Specifically, the transverse amplitudes of vibrations are decreased by a factor of ten and streamwise amplitudes of vibrations are decreased by a factor of three. The results show that, depending on the value of the TMD damping ratio, the frequency of transverse vibrations is either characterized by the natural frequency or by two frequencies: one higher and the other lower than the natural frequency of the structure, referred to as fundamental frequencies. Independent of TMD damping and tuning frequency ratios, the frequency of streamwise vibrations matches that of the transverse vibrations in the synchronization region, and the cylinder traces elliptic trajectories. The phase angle between the streamwise and transverse vibrations is nearly constant when the pendulum is restrained. However, with the TMD engaged and tuned to the natural frequency, the phase angle fluctuates significantly with time. A mathematical model was utilized to gain insight into the frequency response of the structure. The results of the modeling show that the frequency of transverse vibrations is characterized by the fundamental frequency or frequencies of the structure and the frequency of streamwise vibrations is characterized by the fundamental frequency or frequencies as well as the first harmonic of the fundamental frequency or frequencies of the structure.

Coherent vortex-dominated structures in the wake are ubiquitous in natural and engineered flows. The well-known 'von Karman street', in which two rows of counter-rotating vortices develop on the leeward side of a solid body immersed in a fluid, is only one such vortex-based structure in the wake. Recent work on fluid-structure interaction has shown that several other types of vortex structures can arise in natural and engineered systems. The production of these vortex structures downstream often mark the onset of qualitative and/or quantitative changes in the forces exerted on the vortex-shedding body upstream, and can be used as diagnostic tools for engineering structures undergoing Vortex-Induced Vibrations. This dissertation presents a two-part study of vortex dynamics in the laminar wakes of bluff bodies. The first part consists of a series of experiments on a transversely oscillating circular cylinder in a uniform flow field at Re≲250. These experiments were carried out in a gravity-driven soap film channel, which provides a `two-dimensional laboratory' for hydrodynamics experiments under certain conditions. In these experiments, we generated a `map' of the vortex patterns that arise in the wake as a function of the (nondimensional) frequency and amplitude of the cylinder's motion. Our results show that the '2P mode' of vortex shedding can robustly occur in the two-dimensional wake of an oscillating cylinder, contrary to what has been reported in the literature. By making small changes to the meniscus region of the soap film, we have explored possible mechanisms that can explain why the `P+S mode' of vortex shedding is usually reported to be more prevalent than the '2P mode' at low Reynolds number, when the flow is two-dimensional. In doing so, we have found that small modifications to the cylinder on the order of the boundary layer thickness can make a significant difference to the vortex shedding process. In the second part, we develop a generalized form of von Karman's drag law for N-vortex streets: periodic wakes in which the vortices are arranged in regularly-repeating patterns with N>2 vortices per period. The original form of von Karman's drag law then reduces to a special case of this generalized form, which has the potential to model several kinds of vortex-dominated wakes that have been reported in the literature. In this work, we show how this generalized drag law can be used to model '2P' and 'P+S' wakes in both `drag' and `thrust' form. As a contribution to the study of three-dimensional wakes, we also studied a periodic array of vortex rings, which are often used to represent the wakes of marine organisms like jellyfish and squid. We described the problem mathematically using a newly-developed Green's function, and comprehensively examine the fluid physics of such an array of vortex rings as a function of the non-dimensional parameters that govern this phenomenon. In the process, we have discovered a new type of topology that arises in this flow, which may have connections with the `optimal vortex formation length' of vortex rings. / Doctor of Philosophy / The interaction of solid objects with fluids such as water and air, often termed Fluid-Structure Interaction (FSI), gives rise to a wide variety of natural phenomena. Understanding FSI is important as an avenue of scientific interest as well as for engineering applications. In this dissertation, we are interested in the subset of FSI phenomena known as wakes: the fluid flow that is left behind when a solid moves rapidly through quiescent fluid, or when water or air flows rapidly past a stationary obstacle. In such situations, the flow is often rapidly rotating, taking the form of vortices or eddies, i.e., concentrated regions of rotating fluid. These eddies, or vortices, can be described mathematically using simple differential equations, and are the subject of the field of vortex dynamics, which is a branch of fluid mechanics. In the first part of this thesis, we have made contributions to the experimental study of FSI and wakes by making use of an experimental technique known as a gravity-driven soap film channel. In these experiments, a 'soap film', i.e., the surface of a soap bubble, is stretched out over a longitudinal channel formed by nylon wires and held taut in a rectangular shape. This rectangular film of soap is only a few micrometers thick, and is continuously fed by soap solution from the top and drained at the bottom, resulting in a steadily-flowing 'channel' of two-dimensional flow. In this experimental setup, we introduce a circular acrylic cylinder to serve as the archetypal 'obstacle' to fluid flow and oscillate it at a range of frequencies and amplitudes while using a high-speed camera to visualize the flow. This gives rise to a fascinating set of qualitatively distinct vortex patterns in the wake, with the structure depending on the selected frequency and amplitude of cylinder oscillation. In the second part of this thesis, we have developed mathematical models of two-dimensional wakes using a system of point vortices and of three-dimensional wakes using a system of circular vortex rings. We show how these idealized mathematical models of rotating flow, i.e., point vortices and vortex rings, can be used as building blocks for physically-plausible models of actually-occurring wakes, including those which were observed in the first part of this work. For two-dimensional wakes, we use Newton's laws applied to a fluid to determine the forces being exerted on a solid body, immersed in a fluid, whose wake takes the form of regularly-repeating vortices known as 'vortex streets'. This allows us to give, for the first time, theoretical predictions of the drag or thrust force associated with vortex streets such as those observed in our experiments.

Since the offshore drilling for oil and gas is venturing into ever greater water depths, drilling risers face problems of vortex-induced vibrations due to the currents. Vortexinduced vibrations are a major fluid load and fatigue component on the long, smooth, and slender bodies placed in a fluid flow and measures must be taken to suppress the shedding of the Karman vortex sheets from its edges. One of the ways to reduce the vibration is to use marine fairings which are attached to the drilling risers with the help of weather vanes. In this thesis, CFD analysis based on solving RANS equations for K-w turbulence model at Reynolds number 10,000 is done for a regular cylinder and one with marine fairing attached. The motivation of such analysis is to compute the efficiency of the fairing arrangement in suppressing the vortex-induced vibrations of the corresponding bluff body.

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.

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.

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.

Understanding static and dynamic nonlinear behavior of pipes and risers is crucial for the design aspects in offshore engineering fields. In this work, we examine two nonlinear problems in offshore engineering field: vortex Induced vibration of straight horizontal pipes, and boundary layer static solution of inclined risers. In the first study, we analyze the effect of the internal velocity of straight horizontal pipe and obtain the vortex induced vibration forces via coupling the pipe equation of motion with the recently modified Van Der Pol oscillator governing the lift coefficient. Our numerical results are obtained for two different pipe configurations: hinged-hinged, and clamped- clamped. The results show that the internal velocity reduces the vibration and the oscillation amplitudes. Also, it is shown that the clamped-clamped pipe configuration offers a wider range of internal velocities before buckling instability occurs. The results also demonstrate the effect of the end condition on the amplitudes of vibration. In the second study, we develop a boundary layer perturbation static solution to govern and simulate the static behavior of inclined risers. In the boundary layer analysis, we take in consideration the effects of the axial stretch, applied tension, and internal velocity. Our numerical simulation results show good agreement with the exact solutions for special cases. In addition, our developed method overcomes the mathematical and numerical limitations of the previous methods used before.

[ES] La Interacción Fluido Estructura consiste en un problema físico en el que dos materiales, gobernados por conjuntos de ecuaciones distintas, se acoplan de diferentes formas. La investigación en el campo de la Interacción Fluido Esructura experimentó un importante desarrollo desde principios del siglo XX, de la mano del campo de la aeroelasticdad. Durante el desarrollo de la industria aeroespacial en el contexto de las guerras mundiales, el uso de materiales más ligeros (y flexibles) comenzó a hacerse obligatorio para la obtención de aeronaves con un comportamiento (y costes) aceptable. A lo largo de los últimos años, el uso de materiales de construcción cada vez más ligeros, se ha extendido al resto de campos de la industria. A modo de ejemplo, podría servir el desarrollo de trackers en la producción de energia solar; la utilización de materiales ligeros en ingeniería civil o el desarrollo de elementos constructivos de plástico en la industria del automóvil. Como consecuencia, la predicción con exactitud de las deformaciones inducidas por un fluido y, si aplica, la influencia de estas deformaciones en el propio flujo, ha adquirido una importancia vital. Este documento intenta porporcionar, en primer lugar, una profunda revisión de los métodos experimentales y computacionales que se han utilizado en este contexto en la bibliografía, así como los análisis en problemas de este tipo realizados por otros investigadores de cara a presentar una primera aproximación a la Interacción Fluido Estructura. Se verá cómo existe una importante cantidad de herramientas y metodologías aplicables a cualquier tipo de problema y para cualquier combinación de flujos y estructuras. Sin embargo, no existe una aproximación general que, en función de valores de números adimensionales, permita establecer cuáles de ellos son los de mayor importancia en este tipo de problemas. En este sentido, se llevará a cabo un completo análisis paramétrico durante el desarrollo del Capítulo 2 para establecer cuáles de ellos son de mayor importancia. Una vez se establezca la importancia de estos parámetros, se analizará un caso que es de especial interés en la industria: la aerovibroacústica. Éste es un caso particular de Interacción Fluido Estructura en el que, debido a la combinación de parámetros adimensionales, la interacción se puede considerar como prácticamente unidireccional, permitiendo extender estudios mediante un conste computacional relativamente acotado. La Aerovibroacústica y la vibroacústica se analizarán mediante la presentación de dos casos de referencia, permitiendo proponer una metodología que se podrá extender a otros problemas similares. / [CA] La Interacció Fluid Estructura consisteix en un problema físic en què dos materials, governats per conjunts d'equacions diferents, s'acoblen de diferents formes. La investigació en el camp de la Interacció Fluid Esructura va experimentar un important desenvolupament des de principis del segle XX, de la mà del camp de la aeroelasticdad. Durant el desenvolupament de la indústria aeroespacial en el context de les guerres mundials, l'ús de materials més lleugers (i flexibles) va començar a fer-se obligatori per a l'obtenció d'aeronaus amb un comportament (i costos) acceptable. Al llarg dels últims anys, l'ús de materials de construcció cada vegada més lleugers, s'ha estès a la resta de camps de la indústria. A tall d'exemple, podria servir el desenvolupament de textit trackers en la producció d'energia solar; la utilització de materials lleugers en enginyeria civil, el desenvolupament d'elements constructius de plàstic a la indústria de l'automòbil. Com a conseqüència, la predicció amb exactitud de les deformacions induïdes per un fluid i, si escau, la influència d'aquestes deformacions en el propi flux, ha adquirit una importància vital. Aquest document intenta porporcionar, en primer lloc, una profunda revisió dels mètodes experimentals i computacionals que s'han utilitzat en aquest context en la bibliografia, així com les anàlisis en problemes d'aquest tipus realitzats per altres investigadors de cara a presentar una primera aproximació a la Interacció Fluid Estructura. Es veurà com, encara que existeix una important quantitat d'eines i metodologies aplicables a qualsevol tipus de problema i per a qualsevol combinació de fluxos i estructures, no hi ha una aproximació general que, en funció de valors de nombres adimensionals, permeti establir quins d'ells són els de major importància en aquest tipus de problemes. En aquest sentit, es durà a terme una completa anàlisi paramètric durant el desenvolupament del Capítol 2 per a establir quins d'ells són de major importància. Un cop s'estableixi la importància d'aquests paràmetres, s'analitzarà un cas que és d'especial interès en la indústria: la aerovibroacústica. Això és un cas particular d'Interacció Fluid Estructura en què, a causa de la combinació de paràmetres adimensionals, la interacció es pot considerar com pràcticament unidireccional, permetent estendre estudis mitjançant un consti computacional relativament acotat. La Aerovibroacústica i la vibroacústica s'analitzaran mitjançant la presentació de dos casos de referència, permetent proposar una metodologia que es podrà estendre a altres problemes similars. / [EN] Fluid Structure Interaction is a physical problem where two different materials, governed by different set of fundamental equation, are coupled on different ways. The research on the field of Fluid Structure Interaction experienced a noticeable growth since the beginnings of the XXth century, by means of the field of aeroelasticity. During the development of the aerospace industry in the context of first and second Wolrd War, as the use of lighter (and softer) materials became mandatory for the correct behavior (and cost savings) of the produced aircrafts. During these past years, the use of use of increasingly lighter construction materials has extended to the rest of fields of the industry. As an example, it could be mentioned the use of solar trackers on the solar energy sector; the use of light materials on civil engineering or the use of plastic for some constructive elements in the context of the automotive field. As a consequence, the accurate prediction of the deformations induced to a fluid flow over a structure and, if needed, the influence of this deformation on the fluid flow itself is becoming of primal importance. This document intends to provide with a deep review of the computational and experimental reported methodologies already available on the literature and the previous works performed by other researches in order to infer a first approximation to the Fluid Structure Interaction Problem. It will be observed how an important amount of solving methodologies is available in order to face these problems regarding with the strength of the interaction. However, a general approximation allowing to predict this strength as a function of a set of dimensional number is rarely known. In this sense, a full parametric study will be performed during the development of Chapter 2 showing which of them are of higher importance. Once the influence of these parameters is determined, a case of special interest will be analyzed: aerovibroacoustics. This, is a particular case of Fluid Structure Interaction where, due to the combination of its nondimensional parameters, one directional coupling can be supposed for most of the cases. Aerovibroacoustics and vibroacoustics will be analyzed by means of two reference cases, allowing finally to propose a methodology which could be extended for other related problems. / Quintero Igeño, PM. (2019). Characterization of Fluid Structure Interaction mechanisms and its application to vibroacoustic phenomena [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/128412

As vibrações induzidas pela emissão de vórtices (VIV) representam um problema de interação fluido-estrutura presente em diversas áreas da engenharia. Em particular, na engenharia oceânica, esse fenômeno é um tópico importante na análise de risers. Diante do seu impacto na vida útil das estruturas por questões de fadiga estrutural, a supressão desse fenômeno ressonante tem demandado diversos esforços de pesquisa. Uma maneira de reduzir as oscilações estruturais causadas pelo VIV é através do supressor passivo denominado absorvedor não linear de vibração (NVA). Esta pesquisa visa investigar numericamente a eficácia de uma classe de NVA na mitigação do VIV em cilindros rígidos montados em apoios elásticos com um e dois graus de liberdade. A força hidrodinâmica é considerada por meio de modelos fenomenológicos. As equações de movimento desenvolvidas são numericamente integradas com o intuito de analisar a influência dos parâmetros de massa, raio e amortecimento do supressor na resposta do cilindro. As curvas de amplitude de resposta como funções da velocidade reduzida representam a principal contribuição deste trabalho. De forma complementar, o comportamento do sistema ao longo da faixa de velocidades reduzidas característica do lock-in é explorado por meio das séries temporais do cilindro e do NVA. Para quantificar a eficiência do NVA, um critério de supressão baseado na amplitude de oscilação do cilindro é avaliado em função da velocidade reduzida e na forma de mapas de cores definidos em um espaço de parâmetros de controle para velocidades reduzidas específicas. Com base no estudo paramétrico feito, é possível verificar que o parâmetro de massa do NVA é o mais influente na supressão do VIV. Em geral, a supressão é maior no sistema em que o cilindro oscila apenas na direção transversal ao escoamento. Na condição em que o cilindro oscila nas duas direções do plano horizontal, constata-se que o NVA rotativo pode levar a uma amplificação da oscilação na direção do escoamento. / Vortex-induced vibrations (VIV) represent a fluid-structure interaction problem commonly found in several engineering areas. In the offshore engineering scenario, VIV plays an important role in risers dynamics. The suppression of this resonant phenomenon has required several efforts due to its impact on the reduction of the lifespan due to structural fatigue. Among other suppression solutions, it can be highlighted the use of non-linear vibration absorbers (NVAs). This research aims at numerically investigating the effectiveness of a particular class of NVAs as a passive suppressor of rigid cylinders mounted on elastic supports with one or two degrees of freedom subjected to VIV. The wake dynamics is studied through phenomenological models. The equations of motion are obtained and numerically integrated focusing on the identification of the influence of the NVA parameters on the response of the cylinder. Oscillation amplitude curves as functions of the reduced velocity are the main contribution of this work. Complementarily, response time series allow discussing the dynamics of the coupled system throughout the lock-in range of reduced velocities. In order to quantify the efficiency of the NVA, a suppression criterion based on the oscillation amplitude of the cylinder is evaluated as a function of reduced velocities and in the form of color maps defined in a space of control parameters for specific reduced speeds. The parametric study herein described shows that the mass of the suppressor has more influence on the VIV suppression than the other parameters for both 1-dof VIV (only cross-wise oscillations are allowed) and 2-dof VIV (concomitant in-line and cross-wise oscillations). In general, the suppression has proved to be greater in the 1-dof VIV condition. In 2-dof VIV, the rotative NVA may cause amplification of the in-line response.