O escoamento ao redor de cilindros inclinados, ou escoamento oblíquo, é um problema bastante comum em diversas áreas da engenharia. Embora o escoamento ao redor de um cilindro não inclinado consista em um tópico clássico e bastante estudado no contexto da mecânica dos fluídos, os estudos investigativos do escoamento oblíquo existem em menor número. O procedimento mais adotado para o estudo do escoamento ao redor de um cilindro inclinado é o Princípio da Independência, que atesta que todas as características da esteira dependem unicamente da componente da correnteza incidente que é ortogonal ao eixo do cilindro. Visando um melhor entendimento do escoamento ao redor de cilindros rígidos e inclinados, três grupos de experimentos foram conduzidos com cilindros inclinados tanto à montante como à jusante. Cinco ângulos de inclinação, definidos entre o eixo do cilindro e a direção ortogonal à da correnteza foram ensaiados, a saber: teta = 0°, 10°, 20°, 30° e 45°. No primeiro grupo de experimentos, os resultados obtidos com os cilindros estacionários inclinados à montante mostraram que o coeficiente de arrasto médio concorda com os resultados da configuração de referência, desde que a componente da correnteza incidente ortogonal ao eixo do cilindro seja utilizada na normalização da força. No tocante aos experimentos dos cilindros sujeitos ao fenômeno de vibrações induzidas pela emissão de vórtices com um ou dois graus de liberdade, existe um decréscimo da amplitude de oscilação dos cilindros com o aumento do ângulo de inclinação. Em todas as campanhas experimentais, foram verificadas diferenças entre os resultados obtidos com os cilindros inclinados à montante ou à jusante. Essa diferença é tão maior quanto maior for o ângulo de inclinação, e está associada à assimetria nas condições de extremidade do cilindro. / The flow around yawed cylinders, or oblique flow, is a common subject in several engineering applications. Despite the flow around a non-yawed cylinder consists on a classical and extensively investigated problem, there is a considerable lower number of investigation concerning the oblique flow. The most employed approach aiming at investigating the oblique flow is the so called Independence Principle, which states that the flow characteristics depend only on the component of the free-stream that is normal to the cylinder axis. Three groups of experiments were carried out aiming at a better understanding of the flow around yawed cylinders. Five yaw angles defined between the cylinder axis and the direction orthogonal to the free-steam were tested, namely: theta = 0°, 10°, 20°, 30° e 45°. From the first group of experiments, the results obtained with stationary cylinders yawed in the upstream orientation shown that the mean drag coeficient matches the classical results from the non-yawed cylinder, if the component of the free-stream that is orthogonal to the cylinder axis is employed in the normalization of the hydrodynamic force. Concerning vortex-induced vibrations experiments with one and two degrees-of-freedom, a decrease was observed in the maximum oscillation amplitude. For all the experiments, it was observed that the results obtained from the upstream orientation tests can be different from those obtained for the downstream orientation ones. The larger the yaw angle, the larger are the differences, which are associated to the asymmetric end conditions.

A produção de petróleo em ambiente marítimo é feita através de plataformas, em geral flutuantes, conectadas aos reservatórios através de dutos, os chamados risers, responsáveis pelo transporte de petróleo bruto, gás ou água. Do ponto de vista estrutural, estes risers podem ser rígidos ou flexíveis, porém, independente do tipo empregado, com o aumento da profundidade, aumentam-se os esforços de tração sobre essas estruturas. Em águas profundas, essa desvantagem pode ser diminuída com a utilização de configurações mais complexas, dentre as quais se destaca a riser tower, que é foco do presente estudo. Este sistema é composto por vários dutos unidos ao longo de um tubo vertical único, sustentado por uma bóia de subsuperfície. Um grande atrativo desta configuração é a redução da fadiga devido às ondas, uma vez que a bóia e parte significativa dos risers rígidos verticais não sofrem efeitos relevantes das ondas de superfície. No entanto, dois problemas de interação fluido-estrutural persistem: um associado ao fenômeno de VIV Vibração Induzida pela Emissão de Vórtices no riser vertical e outro relativo ao VIM - Movimento Induzido pela Emissão de Vórtices na bóia. Devido ao fato da correnteza ser quase permanente, bem como das altas frequências passíveis de excitação, tem-se um número de ciclos de tensão bastante elevado, associados à flexão, o que pode ser importante no estudo de fadiga do material que compõe as referidas estruturas. De maneira mais completa, no que compete à fenomenologia das interações fluidoestruturais mencionadas, o presente trabalho propõe-se a estudar numérica e analiticamente a dinâmica transversal e longitudinal do conjunto formado pelo riser rígido e pela bóia de subsuperfície, particularmente focado nos efeitos da dinâmica sobre a vida útil do riser vertical. / The offshore oil production is performed with platforms, floating in most cases, which are connected to the well through pipes. These pipes are called risers and are responsible for the transport of crude oil, gas and water from seabed to the platform and vice-versa. From the structural standpoint, these risers can be rigid or flexible. However, independently of the type, the topside tension of these structures increases with the water depth. In deep water this disadvantage can be reduced by using more complex configurations, for example the riser tower, which is the focus of the current study. The riser tower is a system composed by pipes bundled around a central steel tube supported by a subsurface buoyancy tank. This configuration is attractive due to the reduction on waves fatigue since the buoy and most of the vertical risers length is not impacted by the effects of the surface waves. Nevertheless, two issues from the fluid-structural interaction still persist: one related to the VIV phenomena vortex-induced vibration on the vertical riser and the other associated to the VIM vortex induced motion on the buoyancy tank. Considering the almost permanent nature of the offshore currents associated to the high frequencies that can be excited, during the operational life, the riser is subjected to a large amount of stress cycles which are important for the evaluation of the structural integrity in terms of fatigue. In a more complete sense, considering the phenomenology of the fluid-structure interactions, the purpose of the present work is studying numerical and analytically the inline and cross-flow dynamics of the riser tower system especially focused on the impact of the dynamics on the vertical riser operational life.

O crescimento da exploração do petróleo e gás em águas ultraprofundas fez com que a demanda por unidades flutuantes crescesse e, com isso, aumentasse a ocorrência do fenômeno de Movimentos Induzidos por Vórtices (ou VIM, de Vortex-Induced Motions) que age em plataformas com casco cilíndrico, como é o caso das monocolunas e spars. Efeito natural desse aumento nas ocorrências foi um maior interesse pelo fenômeno de VIM, simplificadamente investigado em cilindros lisos e curtos como os utilizados nesta pesquisa. A fim de contribuir com o conhecimento dos fundamentos fluido-dinâmicos nesta área, foram investigados quatro diferentes cilindros flutuantes de baixa razão de aspecto, todos caracterizados pelo valor típico de L/D = 2 (comprimento imerso por diâmetro, onde L= 250 mm e D= 125 mm), mas com diferentes condições de arredondamento da extremidade imersa, estas caracterizadas por quatro razões distintas entre o raio de adoçamento e o raio do cilindro, especificamente r/R= 0,00; 0,25; 0,50 e 1,00. Com este objetivo de observar a resposta dos cilindros curtos sob efeito dos vórtices gerados em diferentes formatos de extremidade imersa, experimentos foram realizados no tanque de provas do Instituto de Pesquisas Tecnológicas do Estado de S~ao Paulo (IPT), compreendendo velocidades de reboque entre 0,024 a 0,154 m/s, ou seja, uma faixa de números de Reynolds entre 3.300 e 19.200. Os resultados de amplitude adimensional de resposta observados na direção transversal mostram influência moderada das modificações na extremidade imersa, ligeiramente mais acentuadas no modelo com razão de arredondamento de r/R=0,25, sendo que para este caso, as amplitudes máximas ficaram abaixo daquelas exibidas pelos demais modelos, com uma tendência geral de queda para as velocidades de reboque mais altas. A variação da geometria da extremidade imersa também influenciou moderadamente os resultados de amplitude adimensional de resposta na direção longitudinal, onde o modelo com r/R=1,00 apresentou menores amplitudes nas velocidades mais altas. Por outro lado, a razão entre as frequências de resposta e as frequências naturais de referência mostraram comportamentos típicos, semelhantes aos encontrados em análises de VIV de cilindros longos montados em suportes elásticos com dois graus de liberdade. Além disso, a observação dos movimentos dos modelos no plano da superfície livre não indicaram mudanças significativas nas trajetórias em função do arredondamento da extremidade livre. No tocante aos coeficientes de força (arrasto e sustentação), as comparações entre os modelos foram semelhantes, mas os coeficientes para o cilindro sem chanfro (r/R=0) mostraram-se maiores para grande parte das velocidades ensaiadas. Para o modelo com r/R=1,00, as força (arrasto e sustentação não se mostraram com a mesma intensidade do que aquelas nos demais modelos, o que, desta forma, se refletiu nos menores valores de coeficiente encontrados. Sob uma ótica geral, os resultados levam a crer que o modelo fluido responsável pelas oscilações nos cilindros com L/D = 2 não tem relação com a existência de vórtices de ponta e aresta, mas com a emissão de vórtices em forma de arco. / The oil and gas exploration in deep and ultra-deep water has increased in the past years, and at the same time, increasing the oating vessel demand, thereby the vortex-induced motion phenomenon (VIM) has been present for cylindrical hull structures, for example, spar and monocolumns oating units. The impact of this, enhance the VIM phenomenon interest with simplify investigations on smooth cylinders, as was used in the present research. In order to expand the knowledge, experiments were made in four floating cylinders with low aspect of ratio, L/D = 2 (Length / Diameter) were tested with different free end corner shape types, namely by the relation between chamfer rounding radius (r) divided by the radius of cylinder (R) (r/R = 0.0, 0.25, 0.5 and 1.0). For the initial case, r/R =0.0 represents at tip and r/R = 1.0 the semi-hemispherical tip. The aims were to understand the effect of different free-end types on VIV behaviour of cylinders. The oating circular cylinders, i.e. unit mass ratio m* = 1(structural mass/displaced uid mass), were elastically supported by a set of linear springs to provide low structural damping on the system and allow six degrees of freedom. The aim is to understand the effects of the free end vortex in the cylinder movements, varying the free end shape. The experiments were carried out in a towing tank of IPT - Institute for Technological Research of the state of São Paulo, Brazil. The range of velocities tested is 0.024 to 0.154 m/s , and Reynolds number covered 3;300 <= Re <= 19;200. The nondimensional amplitudes results for transversal direction, show a moderated effect when the free end shape has changed, more significant in the r/R=0.25 case, where the maximum amplitudes present lower values compare with the others cylinders. For in-line nondimensional amplitudes, model with r/R= 1.00 shown lower values for high velocities. On the other hand, the frequencies ratios between in-line/transverse and natural sistem frequency, presented typical VIV response for long cylinders with two degree of freedom. In addition, the trajectory observations in the XY plane have not illustrated differences for cylinders with rounded edges. Force coeffcients outcome (drag and lift), the no-rounded edge case (r/R=0.0), presented higher coeffcients for the large proportion velocities. Furthermore, the model with a semi-spherical shape in the free end (r/R=1.00), demonstrated the both force coeffcients have not acted with the same severity as was observed for the others cylinders, and thus, the r/R=1.00 models has presented lowest values. In general, the present results suggest the answer for oscillation on L/D=2 models is caused by arc-type vortex shedding, not by the free end vortex effects.

Previous work on rigid splitter plates in the wake of a bluff body has shown that the primary vortex shedding can be suppressed for sufficiently long splitter plates. In the present work, we study the problem of a flexible splitter plate in the wake of a circular cylinder. In this case, the splitter plate can deform due to the fluid forces acting on it, and hence the communication between the two sides of the wake is not totally disrupted like in the rigid splitter plate case. In particular, we study two kinds of flexible splitter plates. In the first case, the splitter plate is rigid but is flexibly mounted (hinged) to the cylinder, while in the second case, the entire splitter plate is flexible. We are interested in both the dynamics of the splitter plate, if they do vibrate at all, and in the wake dynamics downstream of the flexible splitter plates. The main parameters in the problem are the splitter plate length (L) to cylinder diameter (D) ratio, the relative mass of the plate, the Reynolds number, and the stiffness and internal damping associated with the flexible plate. In our study, we investigate this problem in the limit where the stiffness and internal damping of the plate are negligible and hence are not parameters of interest. For the hinged-rigid splitter plate case, experiments show that the splitter plate oscillations increase with Reynolds numbers at low values of Re, and are found to reach a saturation amplitude level at higher Re. This type of saturation amplitude level that appears to continue indefinitely with Re, appears to be related to the fact that there is no structural restoring force in this case, and has been seen previously for elastically-mounted cylinders with no restoring force. In the present case, the saturation tip amplitude level can be up to 0.45D,where D is the cylinder diameter. For this hinged-rigid splitter plate case, it is found that the splitter plate length to cylinder diameter (L/D) ratio is crucial in determining the character and magnitude of the oscillations. For small splitter plate lengths (L/D ≤ 3.0), the oscillations appear to be nearly periodic with tip amplitudes of about 0.45D nearly independent of L/D. The non-dimensional oscillation frequencies (fD/U ) on the other hand are found to continuously vary with L/D from fD/U ≈ 0.2at L/D =1 to fD/U ≈ 0.1 at L/D = 3. As the splitter plate length is further increased beyond L/D ≥ 4.0, the character of the splitter plate oscillations suddenly changes. The oscillations become aperiodic with much smaller amplitudes. In this long splitter plate regime, the spectra of the oscillations become broadband, and are reminiscent of the change in character of the wake oscillations seen in the earlier fixed-rigid splitter plate case for L/D ≥ 5.0. It appears that the vortex shedding is nearly inhibited for L/D ≥ 4.0 in the present case. This is also supported by measurements of the wake vorticity field from Particle-Image Velocimetry (PIV). The phase-averaged PIV vorticity fields show that the strength of the shed vortices decreases rapidly as the splitter plate length increases. For longer splitter plates, L/D ≥ 4.0, the plate oscillations are no longer periodic, and hence it appears that the wake vortices are not synchronized with the splitter plate motions. For the entirely-flexible splitter plate case, the splitter plate deformations appear to be in the form of a travelling wave. In this case, the tip amplitudes are significantly larger of the order of 1.1D, and the non-dimensional oscillation frequency (fD/U )is close to 0.2, approximately the same as the Strouhal number for the bare cylinder. In sharp contrast to the hinged-rigid splitter plate case, the non-dimensional amplitude and frequency appear to be nearly independent of the normalized splitter plate length (L/D)even up to L/D =7.0. PIV measurements of the wake vorticity field indicates that there appears to be a nearly continuous sheet of vorticity on both sides of the flexible splitter plate, and the vortex sheet sheds and forms distinct vortices only at the trailing edge of the plate. The strength of these shed vortices appears to be close to that of the bare cylinder at similar Re. The results appear to suggest that in this entirely-flexible case, the vortices form at the same frequency and are of the same strength as in the bare cylinder case, but their formation is just pushed further downstream. This would suggest that in this case, the base suction and drag could be lower than the bare cylinder. Further, the formation of vortices further downstream of the body could imply that this type of flexible splitter plate could be useful to suppress vortex-induced vibrations (VIV).

Lorsqu’un corps flexible non-profilé est immergé dans un écoulement, les forces instationnaires associées au phénomène de détachement tourbillonnaire peuvent provoquer une réponse vibratoire de la structure. Ces vibrations induites par vortex (VIV) se produisent via un mécanisme de synchronisation entre l’instationnarité du sillage et le déplacement du corps, aussi appelé mécanisme de lock-in. Les VIV ont un effet néfaste sur beaucoup de systèmes industriels, mais elles peuvent également être utilisées comme convertisseur d’énergie mécanique pour l’extraction d’énergie. Dans ce travail, les VIV d’un cylindre circulaire rigide, monté sur un support flexible, sont étudiées dans différentes configurations sur la base de simulations numériques directes. (i) Les mécanismes d’interaction sont d’abord examinés au début du régime turbulent, où les VIV ont rarement été étudiées numériquement. Le nombre de Reynolds, basé sur le diamètre du cylindre et la vitesse incidente, est fixé à 3900. Une analyse conjointe des réponses structurelles et des forces fluides est réalisée sur un intervalle de vitesses réduites (inverse de la fréquence naturelle du corps), dans le cas où le cylindre est libre d’osciller dans les directions longitudinale et transverse, c.a.d les directions parallèle et perpendiculaire à l’écoulement incident. (ii) Le comportement du système lorsque le corps est libre d’osciller dans une direction seulement est également exploré, révélant les interactions possibles entre les déplacements longitudinal et transverse dans le cas à deux degrés de liberté (2-ddl) ; l’analyse montre par exemple comment des oscillations longitudinales de grandes amplitudes peuvent être induites par un déplacement transverse du corps. (iii) Le sillage tri-dimensionnel se développant en aval du cylindre est analysé dans le cas 2-ddl et dans le cas du corps fixe. Les structures d’écoulement dans la direction axiale (parallèle à l’axe du cylindre) sont analysées quantitativement à partir des amplitudes et longueurs d’onde des fluctuations de vorticité dans cette direction ; l’altération de ces structures lorsque le corps oscille diffère dans les couches cisaillées et plus en aval dans le sillage. (iv) La dernière configuration étudiée concerne un cylindre immergé dans un écoulement linéairement cisaillé dans la direction transverse. L’impact de la brisure de symétrie, induite par le cisaillement, sur le comportement du système fluide-structure, est exploré. Différents régimes d’interaction sont mis en évidence dans l’espace paramétrique défini par le taux de cisaillement et la vitesse réduite. Certains d’entre eux sont associés à une profonde reconfiguration du sillage et à une altération majeure des forces fluides. / When a flexible body with bluff cross-section is immersed in a flow, the unsteady fluid forces accompanying the vortex-shedding phenomenon may lead to structural vibrations. These vortexinduced vibrations (VIV) occur through a mechanism of synchronization between flow unsteadiness and body displacement, referred to as lock-in. VIV are detrimental to many industrial systems, but may also be used as mechanical energy converter in the context of flow energy harvesting. In the present work, the VIV of a rigid circular cylinder mounted on a elastic support are investigated in various configurations on the basis of direct numerical simulations. Four aspects are studied. (i) The interaction mechanisms are first examined in the early turbulent regime, where VIV have been rarely studied numerically. The Reynolds number, based on the cylinder diameter and oncoming flow velocity, is set to 3900. A combined analysis of the structural responses and fluid forcing in the case where the cylinder is free to oscillate in the in-line and cross-flow directions, i.e. the directions parallel and perpendicular to the oncoming flow, is performed over a range of the reduced velocity (inverse of the oscillator natural frequency). (ii) The system behavior when the body is free to oscillate in a single direction only is also explored, shedding some light on the possible interactions between in-line and cross-flow motions in the two-degree-of-freedom (2-dof) case; the analysis shows for instance how large-amplitude in-line oscillations may be induced by cross-flow motion. (iii) The three-dimensional wake developing downstream of the oscillating body is analyzed in the 2-dof case as well as in the fixed body case. The spanwise flow patterns, which are analyzed quantitatively in terms of wavelength and amplitude of vorticity fluctuations, are differently altered in the shear-layer and wake regions, when the body oscillates. (iv) The last physical configuration involves a cylinder immersed in a flow linearly sheared in the cross-flow direction. The impact of the symmetry breaking induced by the shear, on the flow-structure system behavior, is explored. Different interaction regimes are uncovered in the shear rate - reduced velocity domain. Some of them are associated with a profound reconfiguration of the wake and a major alteration of the fluid forces.

O crescimento da exploração do petróleo e gás em águas ultraprofundas fez com que a demanda por unidades flutuantes crescesse e, com isso, aumentasse a ocorrência do fenômeno de Movimentos Induzidos por Vórtices (ou VIM, de Vortex-Induced Motions) que age em plataformas com casco cilíndrico, como é o caso das monocolunas e spars. Efeito natural desse aumento nas ocorrências foi um maior interesse pelo fenômeno de VIM, simplificadamente investigado em cilindros lisos e curtos como os utilizados nesta pesquisa. A fim de contribuir com o conhecimento dos fundamentos fluido-dinâmicos nesta área, foram investigados quatro diferentes cilindros flutuantes de baixa razão de aspecto, todos caracterizados pelo valor típico de L/D = 2 (comprimento imerso por diâmetro, onde L= 250 mm e D= 125 mm), mas com diferentes condições de arredondamento da extremidade imersa, estas caracterizadas por quatro razões distintas entre o raio de adoçamento e o raio do cilindro, especificamente r/R= 0,00; 0,25; 0,50 e 1,00. Com este objetivo de observar a resposta dos cilindros curtos sob efeito dos vórtices gerados em diferentes formatos de extremidade imersa, experimentos foram realizados no tanque de provas do Instituto de Pesquisas Tecnológicas do Estado de S~ao Paulo (IPT), compreendendo velocidades de reboque entre 0,024 a 0,154 m/s, ou seja, uma faixa de números de Reynolds entre 3.300 e 19.200. Os resultados de amplitude adimensional de resposta observados na direção transversal mostram influência moderada das modificações na extremidade imersa, ligeiramente mais acentuadas no modelo com razão de arredondamento de r/R=0,25, sendo que para este caso, as amplitudes máximas ficaram abaixo daquelas exibidas pelos demais modelos, com uma tendência geral de queda para as velocidades de reboque mais altas. A variação da geometria da extremidade imersa também influenciou moderadamente os resultados de amplitude adimensional de resposta na direção longitudinal, onde o modelo com r/R=1,00 apresentou menores amplitudes nas velocidades mais altas. Por outro lado, a razão entre as frequências de resposta e as frequências naturais de referência mostraram comportamentos típicos, semelhantes aos encontrados em análises de VIV de cilindros longos montados em suportes elásticos com dois graus de liberdade. Além disso, a observação dos movimentos dos modelos no plano da superfície livre não indicaram mudanças significativas nas trajetórias em função do arredondamento da extremidade livre. No tocante aos coeficientes de força (arrasto e sustentação), as comparações entre os modelos foram semelhantes, mas os coeficientes para o cilindro sem chanfro (r/R=0) mostraram-se maiores para grande parte das velocidades ensaiadas. Para o modelo com r/R=1,00, as força (arrasto e sustentação não se mostraram com a mesma intensidade do que aquelas nos demais modelos, o que, desta forma, se refletiu nos menores valores de coeficiente encontrados. Sob uma ótica geral, os resultados levam a crer que o modelo fluido responsável pelas oscilações nos cilindros com L/D = 2 não tem relação com a existência de vórtices de ponta e aresta, mas com a emissão de vórtices em forma de arco. / The oil and gas exploration in deep and ultra-deep water has increased in the past years, and at the same time, increasing the oating vessel demand, thereby the vortex-induced motion phenomenon (VIM) has been present for cylindrical hull structures, for example, spar and monocolumns oating units. The impact of this, enhance the VIM phenomenon interest with simplify investigations on smooth cylinders, as was used in the present research. In order to expand the knowledge, experiments were made in four floating cylinders with low aspect of ratio, L/D = 2 (Length / Diameter) were tested with different free end corner shape types, namely by the relation between chamfer rounding radius (r) divided by the radius of cylinder (R) (r/R = 0.0, 0.25, 0.5 and 1.0). For the initial case, r/R =0.0 represents at tip and r/R = 1.0 the semi-hemispherical tip. The aims were to understand the effect of different free-end types on VIV behaviour of cylinders. The oating circular cylinders, i.e. unit mass ratio m* = 1(structural mass/displaced uid mass), were elastically supported by a set of linear springs to provide low structural damping on the system and allow six degrees of freedom. The aim is to understand the effects of the free end vortex in the cylinder movements, varying the free end shape. The experiments were carried out in a towing tank of IPT - Institute for Technological Research of the state of São Paulo, Brazil. The range of velocities tested is 0.024 to 0.154 m/s , and Reynolds number covered 3;300 <= Re <= 19;200. The nondimensional amplitudes results for transversal direction, show a moderated effect when the free end shape has changed, more significant in the r/R=0.25 case, where the maximum amplitudes present lower values compare with the others cylinders. For in-line nondimensional amplitudes, model with r/R= 1.00 shown lower values for high velocities. On the other hand, the frequencies ratios between in-line/transverse and natural sistem frequency, presented typical VIV response for long cylinders with two degree of freedom. In addition, the trajectory observations in the XY plane have not illustrated differences for cylinders with rounded edges. Force coeffcients outcome (drag and lift), the no-rounded edge case (r/R=0.0), presented higher coeffcients for the large proportion velocities. Furthermore, the model with a semi-spherical shape in the free end (r/R=1.00), demonstrated the both force coeffcients have not acted with the same severity as was observed for the others cylinders, and thus, the r/R=1.00 models has presented lowest values. In general, the present results suggest the answer for oscillation on L/D=2 models is caused by arc-type vortex shedding, not by the free end vortex effects.

O escoamento ao redor de cilindros inclinados, ou escoamento oblíquo, é um problema bastante comum em diversas áreas da engenharia. Embora o escoamento ao redor de um cilindro não inclinado consista em um tópico clássico e bastante estudado no contexto da mecânica dos fluídos, os estudos investigativos do escoamento oblíquo existem em menor número. O procedimento mais adotado para o estudo do escoamento ao redor de um cilindro inclinado é o Princípio da Independência, que atesta que todas as características da esteira dependem unicamente da componente da correnteza incidente que é ortogonal ao eixo do cilindro. Visando um melhor entendimento do escoamento ao redor de cilindros rígidos e inclinados, três grupos de experimentos foram conduzidos com cilindros inclinados tanto à montante como à jusante. Cinco ângulos de inclinação, definidos entre o eixo do cilindro e a direção ortogonal à da correnteza foram ensaiados, a saber: teta = 0°, 10°, 20°, 30° e 45°. No primeiro grupo de experimentos, os resultados obtidos com os cilindros estacionários inclinados à montante mostraram que o coeficiente de arrasto médio concorda com os resultados da configuração de referência, desde que a componente da correnteza incidente ortogonal ao eixo do cilindro seja utilizada na normalização da força. No tocante aos experimentos dos cilindros sujeitos ao fenômeno de vibrações induzidas pela emissão de vórtices com um ou dois graus de liberdade, existe um decréscimo da amplitude de oscilação dos cilindros com o aumento do ângulo de inclinação. Em todas as campanhas experimentais, foram verificadas diferenças entre os resultados obtidos com os cilindros inclinados à montante ou à jusante. Essa diferença é tão maior quanto maior for o ângulo de inclinação, e está associada à assimetria nas condições de extremidade do cilindro. / The flow around yawed cylinders, or oblique flow, is a common subject in several engineering applications. Despite the flow around a non-yawed cylinder consists on a classical and extensively investigated problem, there is a considerable lower number of investigation concerning the oblique flow. The most employed approach aiming at investigating the oblique flow is the so called Independence Principle, which states that the flow characteristics depend only on the component of the free-stream that is normal to the cylinder axis. Three groups of experiments were carried out aiming at a better understanding of the flow around yawed cylinders. Five yaw angles defined between the cylinder axis and the direction orthogonal to the free-steam were tested, namely: theta = 0°, 10°, 20°, 30° e 45°. From the first group of experiments, the results obtained with stationary cylinders yawed in the upstream orientation shown that the mean drag coeficient matches the classical results from the non-yawed cylinder, if the component of the free-stream that is orthogonal to the cylinder axis is employed in the normalization of the hydrodynamic force. Concerning vortex-induced vibrations experiments with one and two degrees-of-freedom, a decrease was observed in the maximum oscillation amplitude. For all the experiments, it was observed that the results obtained from the upstream orientation tests can be different from those obtained for the downstream orientation ones. The larger the yaw angle, the larger are the differences, which are associated to the asymmetric end conditions.

A produção de petróleo em ambiente marítimo é feita através de plataformas, em geral flutuantes, conectadas aos reservatórios através de dutos, os chamados risers, responsáveis pelo transporte de petróleo bruto, gás ou água. Do ponto de vista estrutural, estes risers podem ser rígidos ou flexíveis, porém, independente do tipo empregado, com o aumento da profundidade, aumentam-se os esforços de tração sobre essas estruturas. Em águas profundas, essa desvantagem pode ser diminuída com a utilização de configurações mais complexas, dentre as quais se destaca a riser tower, que é foco do presente estudo. Este sistema é composto por vários dutos unidos ao longo de um tubo vertical único, sustentado por uma bóia de subsuperfície. Um grande atrativo desta configuração é a redução da fadiga devido às ondas, uma vez que a bóia e parte significativa dos risers rígidos verticais não sofrem efeitos relevantes das ondas de superfície. No entanto, dois problemas de interação fluido-estrutural persistem: um associado ao fenômeno de VIV Vibração Induzida pela Emissão de Vórtices no riser vertical e outro relativo ao VIM - Movimento Induzido pela Emissão de Vórtices na bóia. Devido ao fato da correnteza ser quase permanente, bem como das altas frequências passíveis de excitação, tem-se um número de ciclos de tensão bastante elevado, associados à flexão, o que pode ser importante no estudo de fadiga do material que compõe as referidas estruturas. De maneira mais completa, no que compete à fenomenologia das interações fluidoestruturais mencionadas, o presente trabalho propõe-se a estudar numérica e analiticamente a dinâmica transversal e longitudinal do conjunto formado pelo riser rígido e pela bóia de subsuperfície, particularmente focado nos efeitos da dinâmica sobre a vida útil do riser vertical. / The offshore oil production is performed with platforms, floating in most cases, which are connected to the well through pipes. These pipes are called risers and are responsible for the transport of crude oil, gas and water from seabed to the platform and vice-versa. From the structural standpoint, these risers can be rigid or flexible. However, independently of the type, the topside tension of these structures increases with the water depth. In deep water this disadvantage can be reduced by using more complex configurations, for example the riser tower, which is the focus of the current study. The riser tower is a system composed by pipes bundled around a central steel tube supported by a subsurface buoyancy tank. This configuration is attractive due to the reduction on waves fatigue since the buoy and most of the vertical risers length is not impacted by the effects of the surface waves. Nevertheless, two issues from the fluid-structural interaction still persist: one related to the VIV phenomena vortex-induced vibration on the vertical riser and the other associated to the VIM vortex induced motion on the buoyancy tank. Considering the almost permanent nature of the offshore currents associated to the high frequencies that can be excited, during the operational life, the riser is subjected to a large amount of stress cycles which are important for the evaluation of the structural integrity in terms of fatigue. In a more complete sense, considering the phenomenology of the fluid-structure interactions, the purpose of the present work is studying numerical and analytically the inline and cross-flow dynamics of the riser tower system especially focused on the impact of the dynamics on the vertical riser operational life.

No description available.

La déflection statique d'une structure flexible exposée à un écoulement transverse permet classiquement de réduire la traînée à laquelle elle est soumise. Dans le domaine de la biomécanique, la déformation induite par l'écoulement d'éléments végétaux flexibles conduisant à une réduction du chargement est désignée par le terme `reconfiguration' pour souligner le caractère avantageux de ce processus adaptatif. Dans cette thèse, nous examinons les mécanismes qui sous-tendent le processus de reconfiguration, dans des systèmes fluide-structure présentant une variabilité spatiale, ou de la dynamique provenant au choix de l'instationnarité de l'écoulement de base, d'un couplage fluide-structure conduisant à une instabilité, ou de vibrations induites par vortex. Nous montrons que l'aptitude des structures flexibles à réduire l'intensité du chargement imposé par l'écoulement est preservée en présence de non-uniformités ou de dynamique, à condition que le design de la structure soit tel que la traînée résistive domine les forces inertiels. Nous montrons de plus que la capacité à se déformer présente l'avantage supplémentaire de permettre la réduction des vibrations induites par vortex. Notre travail indique également que des structures légères et élancées sont les mieux adaptées pour supporter les chargements induits par l'écoulement en se reconfigurant, et que l'efficacité de la réduction du chargement par reconfiguration élastique dépend faiblement de la distribution spatiale des propriétés du système. Finalement, la réduction des chargements résulte toujours, indépendamment du régime de reconfiguration, de la concentration de la déformation sur une longueur caractéristique inférieure à la longueur réelle de la structure. / The static deflection of a flexible structure exposed to a transverse flow is classically known to reduce the drag it has to withstand. In the field of biomechanics, the flow-induced deformation of flexible plant elements leading to a reduction of the loads is referred to as `reconfiguration', in order to highlight the alleged benefits of such adaptive process. In this thesis, we investigate the mechanisms underpinning the reconfiguration in flow-structure systems featuring some spatial variability, or some dynamics arising either from the unsteadiness of the free-stream, from a flow-structure coupling leading to an instability, or from vortex-induced vibrations. We show that the ability of flexible structures to reduce the magnitude of the flow-induced loads is preserved in the presence of non-uniformities or dynamics, provided that the design of the structure is such that resistive drag dominates over inertial forces. We also show that the ability to deform has the added benefit of reducing the magnitude of the vortex-induced vibrations. Our work further indicates that light, slender structures are better suited to accommodate the flow-induced loads by reconfiguring, and that the efficiency of the process of load reduction by elastic reconfiguration is weakly sensitive to the spatial distribution of the system properties. Finally, regardless of the regime of reconfiguration, the reduction of the load always results from the concentration of the deformation on a characteristic bending length smaller than the actual length of the structure.