This thesis presents results from experimental and numerical investigations of the hydrodynamic forces on a rigid cylinder moving with prescribed orbits in uniform flow. The hydrodynamic forces are measured in both in-line (IL) and cross- ow (CF) directions. The measurements are processed to nd excitation and added mass coeffcients at discrete frequencies. The numerical simulations are used to illustrate the vortex shedding modes and are compared with the experimental results. The hydrodynamic coeffcients obtained from the harmonic forced motion experiments of a rigid cylinder do not always represent forces on a cross section of a exible beam. The orbits used in the forced motion experiments are therefore extracted from the measured motions of cross sections of a exible pipe under uniform and shear flows. Both periodic and observed orbits within a time window are applied as prescribed motions. Higher order displacement components are present in such orbits. IL response amplitudes from combined IL and CF response are larger than pure IL response amplitudes. The hydrodynamic coefficients obtained from the periodic experiments are often larger than those obtained from the pure IL tests. Higher order displacement components are more common in the IL direction than in the CF direction, and higher order IL displacement components will cause larger hydrodynamic forces in both directions. The hydrodynamic coefficients obtained from periodic motion tests are adequate for representing quasi-periodic observed motions. For chaotic observed motions, periodic orbits will yield hydrodynamic coefficients with larger uncertainties. Results from numerical analyses using large eddy simulation (LES) indicate that this method can be used to identify vortex shedding patterns and predict hydrodynamic forces under certain Re numbers and orbits.

In the recent decades the Fluid-Structure Interaction (FSI) problem has been of great interest to many researchers and a variety of methods have been proposed for its numerical simulation. As FSI simulation is a multi-discipline and a multi-physics problem, its full simulation consists of many details and sub-procedures. On the other hand, reliable FSI simulations are required in various applications ranging from hemo-dynamics and structural engineering to aero-elasticity. In hemo-dynamics an incompressible fluid is coupled with a flexible structure with similar density (e.g. blood in arteries). In aero-elasticity a compressible fluid interacts with a stiff structure (e.g. aircraft wing) or an incompressible flow is coupled with a very light structure (e.g. Parachute or sail), whereas in some other engineering applications an incompressible flow interacts with a flexible structure with large displacement (e.g. oil risers in offshore industries). Therefore, various FSI models are employed to simulate a variety of different applications. An initial vital step to conduct an accurate FSI simulation is to perform a study of the physics of the problem which would be the main criterion on which the full FSI simulation procedure will then be based. In this thesis, interaction of an incompressible fluid flow at low Reynolds number with a flexible circular cylinder in two dimensions has been studied in detail using some of the latest published methods in the literature. The elements of procedures have been chosen in a way to allow further development to simulate the interaction of an incompressible fluid flow with a flexible oil riser with large displacement in three dimensions in future. To achieve this goal, a partitioned approach has been adopted to enable the use of existing structural codes together with an Immersed Boundary (IB) method which would allow the modelling of large displacements. A direct forcing approach, interpolation / reconstruction, type of IB is used to enforce the moving boundary condition and to create sharp interfaces with the possibility of modelling in three dimensions. This provides an advantage over the IB continuous forcing approach which creates a diffused boundary. And also is considered as a preferred method over the cut cell approach which is very complex in three dimensions with moving boundaries. Different reconstruction methods from the literature have been compared with the newly proposed method. The fluid governing equation is solved only in the fluid domain using a Cartesian grid and an Eulerian approach while the structural analysis was performed using Lagrangian methods. This method avoids the creation of secondary fluid domains inside the solid boundary which occurs in some of the IB methods. In the IB methods forces from the Eulerian flow field are transferred onto the Lagrangian marker points on the solid boundary and the displacement and velocities of the moving boundary are interpolated in the flow domain to enforce no-slip boundary conditions. Various coupling methods from the literature were selected and improved to allow modelling the interface and to transfer the data between fluid and structure. In addition, as an alternative method to simulate FSI for a single object in the fluid flow as suggested in the literature, the moving frame of reference method has been applied for the first time in this thesis to simulate Fluid-Structure interaction using an IB reconstruction approach. The flow around a cylinder in two dimensions was selected as a benchmark to validate the simulation results as there are many experimental and analytical results presented in the literature for this specific case.

Elastic cylinder vibration due to different shear parameter in the water flow is investigated experimentally in this research. The water flow ranges from 0.4 m/s to 1.06 m/s. It is found from the experiment that shear parameter has a significant influence on the amplitude of the cylinder vibration. The greater the shear parameter becomes, the later the delaying phenomenon also becomes. The delaying phenomenon will bring about resonant procrastination. Additionally, the greater shear parameter lessens the cylinder¡¦s drag force, but the lift force will be augmented, and the vibration orbit will be asymmetric. At lower flow velocity, cylinder¡¦s displacement is greater. With the enhancement of the shear parameter or the reduced velocity, the flow type and the vortex street behind the cylinder will turn more and more impalpable, and eventually become chaotic.

The present study aims to explore the dynamical behavior in the uniform flow by numerical method. The theoretical model is based on transient of continuity equation and momentum equation in CFD software: Fluent. With User Define Function¡]UDF¡^, we can simulate the Vortex-induced vibration¡]VIV¡^under the uniform flow by numerical method and plot the contour of amplitude and flow field under different Reynolds number. We will identify the accurate and capable of central difference method in UDF by comparing with the previous study. Also, we focus on whether the amplitude and flow situation will effect by uniform flow in different degrees or not. Furthermore, this study shows how the time step size and mesh effect the conclusion so that we could have the best choice on model.

The dissertation presents a CFD approach for 3D simulation of long risers. Long riser VIV simulation is at the frontier of the CFD research area due to its high demand on computational resources and techniques. It also has broad practical application potentials, especially in the oil and gas industry. In this dissertation, I used a time domain simulation program - Finite-Analytic Navier-Stokes (FANS) code to achieve the 3D simulations of riser VIV. First, I developed a riser modal motion solver and a direct integration solver to calculate riser dynamic motions when subject to external forces. The direct integration solver provides good flexibility on inclusion of riser bending stiffness and structural damping coefficients. I also developed a static catenary riser solver based on trial and error iteration technique, which allowed the motion solvers to handle catenary risers and jumpers with arbitrary mass distribution. I then integrated the riser motion solvers to the existing FANS code, and applied the CFD approach to a series of riser VIV problems including a 2D fixed/vibrating riser, a 3D vertical riser in uniform and shear currents, a 3D horizontal riser in uniform and shear current, a hypothetic 3,000 ft marine top tensioned riser in uniform current, a practical 1,100m flexible catenary riser in uniform current, and a hypothetic 265m flexible jumper partially submerged in uniform current. I developed a VIV induced fatigue calculation module based on rain flow counting technique and S-N curve method. I also developed a modal extraction module based on the least squares method. The VIV details, including flow field vorticities, rms a/D, riser motion trajectories, PSDs, modal components, VIV induced stress characteristics, and VIV induced fatigue damages were studied and compared to the published experimental data and results calculated using other commercial software tools. I concluded that the CFD approach is valid for VIV simulations in 3D. I found that the long riser VIV response shows complex behaviors, which suggests further investigation on the lock-in phenomenon, high harmonics response, and sensitivity to the lateral deflections.

Slender marine risers used in deepwater applications often experience vortex-induced vibration (VIV). Fatigue damage associated with VIV is of great concern to offshore engineers; however, it has proven difficult to predict this fatigue damage using existing semi-empirical tools. Similarly, approaches based on theoretical and computational fluid dynamics (CFD) generally rely on simplified assumptions on the fluid flow fields and response characteristics. To gain an understanding of VIV and associated fatigue damage, full-scale field monitoring campaigns as well as reduced-scale laboratory experiments are often carried out, wherein the riser response in the form of strains and/or accelerations is recorded using an array of a limited number of sensors distributed over the length of the riser. Simultaneously, current velocities at a proximate location are also recorded. Such measurements generally reveal complex characteristics of the dynamic response of a riser undergoing VIV, including the presence of multiple vibration harmonics, non-stationary behavior, and the existence of sustained or intermittent traveling wave patterns. Such complex features, often not accounted for in some semi-empirical and theoretical approaches, are critical to take into consideration for accurate fatigue damage estimation. In this study, several empirical methods are employed to first reconstruct the response of an instrumented riser and, then, estimate fatigue damage rates over the entire span of the riser based on a limited number of discrete measurements. The methods presented employ the measured data in different ways. One method, referred to as ``weighted waveform analysis'' relies on expressing the riser response as a summation of several weighted waveforms or riser modes; the mode shapes are ``assumed'' and time-varying weights for each mode are estimated directly from the measurements. The riser response over the entire span is reconstructed based on these assumed mode shapes and estimated modal weights. Other methods presented extract discrete mode shapes from the data directly. With the help of interpolation techniques, continuous mode shapes are formed, and the riser response is again reconstructed. Fatigue damage rates estimated based on the reconstructed strains obtained using the various empirical methods are cross-validated by comparing predictions against direct measurements available at the same locations (but not used in the analyses). Results show that the empirical methods developed here may be employed to accurately estimate fatigue damage rates associated with individual recorded segments of measurements. Finally, a procedure for prediction of long-term fatigue damage rates of an instrumented marine riser is presented that relies on combining (multiplying) the fatigue damage rates associated with short recorded segments for specific current profile types, with the relative likelihood of different incident current profiles, and integration over all current profiles. It should be noted that the empirical approaches to fatigue damage estimation presented in this study are based only on measured data; also, they explicitly account for different riser response characteristics and for site-specific current profiles developed from metocean studies. Importantly, too, such estimation procedures can easily accommodate additional data that become available in any ongoing field monitoring campaign to improve and update long-term fatigue damage prediction. / text

Um dos grandes desafios enfrentados pelos fabricantes de turbinas hidráulicas é prevenir o aparecimento de vibrações induzidas pelo escoamento nas travessas do pré-distribuidor e pás do rotor. Considerando apenas as travessas, e atribuídos a tais vibrações, foram relatados 28 casos de trincas ou ruídos anormais nas últimas décadas, que acarretaram enormes prejuízos associados a reparos, atrasos e perda de geração. O estado da arte na prevenção destes problemas baseia-se na utilização de sofisticados, e caros, programas comerciais de dinâmica dos fluidos computacional para o cálculo transiente do fenômeno. Este trabalho faz uma ampla revisão bibliográfica e levantamento de eventos de trincas ou ruídos ocorridos em travessas nos últimos 50 anos. Propõe, então, um enfoque alternativo, baseado exclusivamente em ferramentas de código aberto. A partir de hipóteses simplificadoras devidamente justificadas, o problema é formulado matematicamente de forma bidimensional, no plano da seção transversal da travessa, levando em conta a interação fluido-estrutura. Nesta estratégia, as equações de Navier-Stokes são resolvidas pelo método dos elementos finitos por meio da biblioteca gratuita oomph-lib. Um código especial em C++ é desenvolvido para o problema de interação fluido-estrutura, no qual o fenômeno de turbulência é levado em consideração por meio de um algoritmo baseado no modelo de Baldwin-Lomax. O método proposto é validado por meio da comparação dos resultados obtidos com referências e medições disponíveis na literatura, que tratam de problemas de barras retangulares suportadas elasticamente. O trabalho finaliza com a aplicação do método a um estudo de caso envolvendo uma travessa particular. / One of the biggest challenges for hydraulic turbine manufacturers is to prevent vortex-induced vibration on the stay vanes and runner blades. Only regarding stay vanes, 28 cases of cracks or unusual noises attributed to such vibrations were reported in the past decades leading to huge costs due to repair, delays and lack of generation. The state of the art today is to use powerful and expensive commercial computational fluid dynamics software to address the required transient phenomena. The present work carries out a comprehensive survey on occurred events in stay vanes during the last 50 years. Then, an alternative approach, based only on free open-source tools, is proposed. From due justified simplifying assumptions, the problem is formulated two-dimensionally, in the stay vane cross section plane, taking the fluid-structure interaction into account. In such a strategy, the Navier-Stokes equations are solved using oomph-lib, an object-oriented, finite-element library. A special C++ computational code is developed to deal with the fluid-structure interaction problem, in which turbulence is considered through a special algorithm, based on the Baldwin-Lomax model. The proposed method is validated through comparisons with an aerodynamics benchmark and an experimental measurement of oscillating rectangular bars both available in the literature. The method is finally applied to a case study of a particular stay vane. Keywords: Hydraulic turbine. Fluid-structure interaction. Vortex-induced vibration.

O presente trabalho tem como objetivo investigar o fenômeno de vibração induzida por vórtices (VIV) em cilindros com razão de aspecto elevada. Para alcançar tal objetivo, utilizou-se o código computacional denominado ANFLEX-CFD devidamente validado através de comparações com resultados experimentais. O foco do estudo é voltado para a análise do fenômeno de VIV, causado pelo desprendimento de vórtices ao redor de cilindros, e seu efeito no comportamento estrutural desses corpos rombudos. O programa computacional utiliza o Método dos Vórtices Discretos (Discrete Vortex Method - DVM) para o cálculo das forças hidrodinâmicas, acoplado ao modelo estrutural do ANFLEX que utiliza o método dos elementos finitos para resolver a estrutura e determinar seus deslocamentos. Os ensaios que serviram para comparação dos resultados foram os realizados no tanque da IFREMER, na França. Os resultados numéricos apresentaram uma boa concordância com o experimento para as maiores velocidades de corrente, o que pode ser justificado pela menor influência da tração perante o arrasto. A partir dos resultados pode-se afirmar que o ANFLEX-CFD é uma ferramenta útil na predição do fenômeno de VIV e pode se tornar uma ferramenta fundamental na determinação da vida útil à fadiga de elementos estruturais como os risers da indústria petrolífera offshore. / This thesis aims to instigate vortex-induced vibration on circular cylinders with high aspect ratio. In order to do so, the computer code named ANFLEX-CFD is employed, and this code is validated using experimental tests results. The focus of the study is to analyze VIV phenomenon caused by the vortex shedding around circular cylinders, and its effect on the structural behavior of these bluff bodies. The computer program uses the Discrete Vortex Method (DVM) for the assessment of the hydrodynamic forces, coupled with the structural model of ANFLEX that uses the finite element method to solve the structure displacements. The tests used in the comparison were the ones carried out at IFREMER tank in France. The numerical results have shown a good agreement with the experimental results at the highest current velocity as a result of the decreasing influence of the tension on the drag. As conclusion, it can be said that the ANFLEX-CFD can become a useful tool for predicting the VIV phenomenon and therefore in a near future become a fundamental tool in determining the fatigue life of structural components such as the offshore oil industry risers.

O presente trabalho tem como objetivo investigar o fenômeno de vibração induzida por vórtices (VIV) em cilindros com razão de aspecto elevada. Para alcançar tal objetivo, utilizou-se o código computacional denominado ANFLEX-CFD devidamente validado através de comparações com resultados experimentais. O foco do estudo é voltado para a análise do fenômeno de VIV, causado pelo desprendimento de vórtices ao redor de cilindros, e seu efeito no comportamento estrutural desses corpos rombudos. O programa computacional utiliza o Método dos Vórtices Discretos (Discrete Vortex Method - DVM) para o cálculo das forças hidrodinâmicas, acoplado ao modelo estrutural do ANFLEX que utiliza o método dos elementos finitos para resolver a estrutura e determinar seus deslocamentos. Os ensaios que serviram para comparação dos resultados foram os realizados no tanque da IFREMER, na França. Os resultados numéricos apresentaram uma boa concordância com o experimento para as maiores velocidades de corrente, o que pode ser justificado pela menor influência da tração perante o arrasto. A partir dos resultados pode-se afirmar que o ANFLEX-CFD é uma ferramenta útil na predição do fenômeno de VIV e pode se tornar uma ferramenta fundamental na determinação da vida útil à fadiga de elementos estruturais como os risers da indústria petrolífera offshore. / This thesis aims to instigate vortex-induced vibration on circular cylinders with high aspect ratio. In order to do so, the computer code named ANFLEX-CFD is employed, and this code is validated using experimental tests results. The focus of the study is to analyze VIV phenomenon caused by the vortex shedding around circular cylinders, and its effect on the structural behavior of these bluff bodies. The computer program uses the Discrete Vortex Method (DVM) for the assessment of the hydrodynamic forces, coupled with the structural model of ANFLEX that uses the finite element method to solve the structure displacements. The tests used in the comparison were the ones carried out at IFREMER tank in France. The numerical results have shown a good agreement with the experimental results at the highest current velocity as a result of the decreasing influence of the tension on the drag. As conclusion, it can be said that the ANFLEX-CFD can become a useful tool for predicting the VIV phenomenon and therefore in a near future become a fundamental tool in determining the fatigue life of structural components such as the offshore oil industry risers.

The present study is aimed to explore dynamical behavior of the fluid- elastic vibration of cylindrical arrays and single cylinder in shear flow by numerical simulations .The effects of the shear parameter, spacing(P/D) ¡Bmass ratio and arrangement of cylinders on fluid-elastic vibration of the cylinders are investigated Continuity and momentum equations are solved alternatively by using a CFD package, Fluent 6.3.26. Dynamic meshing techniques together with the cylinder motion equations are employed in the simulation. Under different flow conditions, flow types and cylinder motion models, lock-in and vortex-induced vibration are studied. According to the research the motion and flow types of a single cylinder in uniform flow are in good agreement with the previous studies in literatures. In shear flow, however, as the shear parameter increases, the fluid vortex-induced vibration of the cylinder is induced, and thus amplitude of the cylinder increases considerably. Further, cylindrical arrays in the shear flow are studied. Cylindrical arrays arrangements (rectangle and rhombus) ¡B the distance between cylinders and regulate shear parameter are the factors to cause fluid-elastic vibration. Compared with the single cylinder motion, cylindrical arrays motion¡¦s critical flow velocity is smaller than the single cylinder motion, which means cylindrical arrays motion are more subject to fluid-elastic vibration.