Pour des raisons de sécurité, les moteurs aéronautiques doivent pouvoir redémarrer en vol sur toute leur plage d'opération. Mais les contraintes sur les émissions polluantes nécessitent le développement de nouvelles chambres de combustion dont la conception peut détériorer les capacités d'allumage du moteur. Afin d'améliorer la compréhension du processus d'allumage et d'aider à l'optimisation de la conception, les recherches actuelles combinent les études expérimentales de plus en plus complexes et les simulation numériques hautes fidélités. Dans ce travail, l'étude numérique du processus d'allumage des chambres de combustion aéronautiques, de l'étincelle à la propagation azimutale de la flamme, est conduite avec plusieurs objectifs: améliorer la robustesse et la confiance de l'outil LES pour l'étude de l'allumage, étudier les mécanismes qui affectent l'allumage dans des conditions représentatives des conditions réelles et enfin améliorer les méthodes bas-ordre pour la prédiction des performances d'allumage. Dans une première partie, la SGE d'un monobruleur installé au CORIA permet de mettre en évidence les bons résultats de la LES et de construire une base de données pour l'analyses des mécanismes d'extinction. Ces données sont aussi utilisées pour développer une méthodologie permettant de prédire les performances d'allumage à bas coût en utilisant les résultats d'une SGE non-réactive. Dans une seconde partie, la propagation inter-secteur est investiguée par l'étude de deux cas expérimentaux et la SGE est capable de reproduire les modes de propagation mais aussi les temps d'allumage avec précision. Sur la bases de ces bons résultats, une analyse plus fine de la simulation permet d'identifier les mécanismes qui contrôlent la propagation de la flamme. / For safety reasons, in-flight relight of the engine must be guaranteed over a wide range of operating conditions but the increasing stringency of pollutant emission constraints requires the development of new aero-engine combustor whose design might be detrimental to the ignition capability. To improve the knowledge of the ignition process in aeronautical gas turbines and better combine conflicting technological solutions, current research relies on both complex experimental investigation and high fidelity numerical simulations. In this work, numerical study of the ignition process in gas turbines from the energy deposit to the light-around is performed with several objectives: increase the level of confidence of Large Eddy Simulations tool for the analysis of the ignition process, investigate the mechanisms controlling ignition in conditions representative of realistic aeronautical gas turbine flows and improve the low-order methodologies for the prediction of ignition performance. In a first part, LES of the single burner installed at CORIA (France) is carried out and allows to highlight the LES accuracy and to build a database on which the main mechanisms controlling the ignition success are identified. Based on these results, a methodology is developed to predict the ignition performance at a low computational cost using the non-reacting flow statistics only. In a second part, the light-around process is studied on two experimental set-ups and the very good agreement of the LES results with experiments is the starting point from an analysis of the mechanisms driving the flame propagation process.

The colonial era brought about Western civilisation, industrialisation, urbanisation and technology that led to the negation of the validity of the traditional wall decorations and the meaning behind what is understood as merely decorative shapes and parttens. To research as to whether this is true, the aim of this study is to establish whether the symbols applied on the traditional African murals have any linguistic significance. The research investigated the pictography embedded in the traditional murals of three linguistic groups, namely the Balobedu, Vatsonga and Vhavenda in Limpopo Province. The researcher gathered and recorded from the surviving elders the vital codes which unlock the meaning of signs, symbols, colours and geometric shapes before the knowledge is lost forever when they die. Data was collected mainly from Vatsonga female elders in their homes by using unstructured interviews. Traditional huts are decorated with symbols, signs, shapes, lines, and colours on the maguva (walls around the courtyard), mavala ya nghotsa (design and patterns, mainly in repeated half circles (figures 26 to 35)), mabilomu/swiluva/ swiphaswana (calabash/gourd flowers), ku sindza hi makholo (patterned cow dung floor) and ku tsema (coloured bands around the hut), depicted in figures 12 to 15. According to the respondents, the maphapha (calabashes/gourds) is a symbol for the plant that feeds the people. The plant from which maphapha are made, has heart-shaped green leaves and yellow flowers that develop into a calabash/gourd, which are consumed as food (figures 10–11). In this sense, it represents a woman and her responsibilities as carer and life giver. The results affirm that the pictography is not merely a reproduction of common decorative patterns, but is instead the source of an ancient visual expressive language carried down from generation to generation through oratory and visual narratives in the form of symbols, and signs. The study recommends the preservation of the linguistic significance of the traditional South African murals that have long been marginalised, possibly encouraging the younger generations to review their own history. / African Languages / D. Litt. et Phil.(African Languages)

Cette thèse porte sur le comportement des élites politiques durant les périodes de crise nationale et plus particulièrement sur leurs réactions aux attentats terroristes. Elle démontre que les crises terroristes sont tout comme les conflits militaires ou diplomatiques propices aux unions nationales et notamment aux ralliements des partis d’opposition auprès du gouvernement. L’analyse statistique d’actes terroristes s’étant produits dans cinq états démocratiques (Allemagne, Espagne, États-Unis d’Amérique, France et Royaume-Uni) entre 1990 et 2006 révèle que l’ampleur d’un attentat en termes de pertes humaines ainsi que la répétition de ces attentats influencent dans une large mesure la réaction des élites politiques. Ainsi plus l’ampleur d’un attentat est élevée, plus la probabilité d’un ralliement est grande. En revanche, la multiplication des attentats augmente la possibilité de dissension entre l’opposition et le gouvernement. Par ailleurs, l’opposition est plus susceptible de se rallier au gouvernement lorsque l’attentat est perpétré par des terroristes provenant de l’étranger. L’analyse quantitative indique également que l’existence d’un accord formel de coopération dans la lutte antiterroriste entre le gouvernement et l’opposition favorise l’union des élites. Enfin, les données analysées suggèrent que la proportion des ralliements dans les cinq pays est plus importante depuis les attentats du 11 septembre 2001. Une analyse qualitative portant exclusivement sur la France et couvrant la période 1980-2006 confirme la validité des variables identifiées dans la partie quantitative, mais suggère que les élites réagissent au nombre total de victimes (morts mais aussi blessés) et que la répétition des actes terroristes a moins d’impact lors des vagues d’attentats. Par ailleurs, les analyses de cas confirment que les élites politiques françaises sont plus susceptibles de se rallier quand un attentat vise un haut-fonctionnaire de l’État. Il apparaît également que les rivalités et rancœurs politiques propre à la France (notamment suite à l’arrivée de la gauche au pouvoir en 1981) ont parfois empêché le ralliement des élites. Enfin, cette analyse qualitative révèle que si l’extrême gauche française soutient généralement le gouvernement, qu’il soit de gauche ou de droite, en période de crise terroriste, l’extrême droite en revanche saisit quasi systématiquement l’opportunité offerte par l’acte terroriste pour critiquer le gouvernement ainsi que les partis de l’establishment. La thèse est divisée en sept chapitres. Le premier chapitre suggère que le comportement des élites politiques en période de crises internationales graves (guerres et conflits diplomatiques) est souvent influencé par la raison d’État et non par l’intérêt électoral qui prédomine lors des périodes plus paisibles. Le second chapitre discute du phénomène terroriste et de la littérature afférente. Le troisième chapitre analyse les causes du phénomène d’union nationale, soumet un cadre pour l’analyse de la réaction des élites aux actes terroristes, et présente une série d’hypothèses. Le quatrième chapitre détaille la méthodologie utilisée au cours de cette recherche. Les chapitres cinq et six présentent respectivement les résultats des analyses quantitatives et qualitatives. Enfin, le chapitre sept conclut cette thèse en résumant la contribution de l’auteur et en suggérant des pistes de recherche. / This thesis focuses on the behaviour of political elites during periods of national crisis and particularly on their reactions to acts of terrorism. It demonstrates that terrorist crises, much like military and diplomatic conflicts, represent a fertile ground for rallies around the flag. The statistical analysis of terrorist events that occurred in five democracies (France, Germany, Spain, the United Kingdom and the United States of America) from 1990 to 2006 indicates that the magnitude of the attack in terms of fatalities and the repetition of attacks influence in large measures the reaction of the political elite. The higher the magnitude is, the more likely the rally. However, the repetition of attacks increases the likelihood of dissention between opposition and government. Moreover, the opposition is more likely to support the government when the attack is perpetrated by terrorists originating from abroad. The quantitative analysis also indicates that the existence of a formal antiterrorist pact between government and opposition increases the likelihood of a rally. Finally, the data suggest that elites are more likely to rally around the flag since the events of 9/11. A qualitative analysis focusing solely on France and examining cases from the period 1980-2006 confirms the validity of the variables identified in the quantitative part but suggests that elites react to the total number of victims (including wounded) and that the repetition has less impact during waves of attacks. Furthermore, the case studies confirm that French political elites are particularly prone to rally when high-ranking representatives of the state fall victim to the terrorists. In addition, it appears that political rivalries and resentment (in particular following the Left’s return to power in 1981) have on occasion thwarted the rallying of elites. Finally, this analysis indicates that whereas the far-left generally supports the government (left-wing or right-wing), the far-right uses almost every opportunity to criticize the government and the parties of the establishment. The thesis is divided into seven chapters. Chapter one suggests that during intense foreign policy crises (military and diplomatic) the behaviour of the political elite is often influenced by the national interest, rather than the electoral interest that predominates in quieter periods. Chapter two discusses the terrorist phenomenon and the corresponding literature. Chapter three analyses the rally-around-the-flag phenomenon, lays out a framework for analysing the reaction of elites, and presents a series of hypotheses. Chapter four details the methodology used. Chapters five and six present respectively the results of the quantitative and qualitative analyses. Finally, chapter seven concludes with a summary of the author’s contribution and suggests avenues of research.

As unidades de conservação (UC) são áreas com características naturais relevantes, com limites definidos e com o objetivo de proteção e preservação da natureza, sendo admitido apenas o uso indireto dos seus recursos naturais, o qual deve compatibilizar a conservação da natureza com o uso sustentável de parcela dos seus recursos naturais. As áreas em volta das UCs são estratégicas para a proteção da biodiversidade e para o desenvolvimento sustentável da população local. Entretanto, é necessário que hajam restrições e limites ao uso e ocupação do solo nos locais da circunvizinhança, para reduzir a influência dos impactos negativos na biodiversidade das áreas protegidas. O objetivo do presente estudo é fornecer embasamento técnico para a determinação de critérios que definam a extensão, o uso e a ocupação da zona de amortecimento (ZA) do Parque Estadual Turístico do Alto Ribeira (PETAR). Para isto foram elaborados critérios que definem: a) a extensão da zona de amortecimento e b) a utilização da zona de amortecimento. A definição da extensão da zona de amortecimento do PETAR baseou-se na delimitação das microbacias hidrográficas cujos cursos de água drenam para dentro do parque e na aproximação dos vetores antrópicos ao limite do parque. Os critérios que definiram o uso e ocupação da paisagem na zona de amortecimento foram baseados na vulnerabilidade à erosão do solo. Conclui-se que a zona de amortecimento de 10 quilômetros contempla os critérios aqui usados, abrangendo o limite das microbacias hidrográficas e os grandes fragmentos florestais. Entretanto a qualidade do parque está ameaçada pela expansão da agricultura, mineração, extração ilegal de palmito e silvicultura. A expansão dessas atividades ocorre de forma irregular em áreas de alta vulnerabilidades à erosão e coloca em risco a manutenção da qualidade do bioma protegido. / Conservation Units (UC) are areas with relevant natural features, with limits defined and with the goal of protecting and preserving nature, being admitted only the indirect use of its natural resources, which should reconcile nature conservation with the use sustainable portion of their natural resources. The areas around the UC are strategic to the protection of biodiversity and the local sustainable development of the population. However, it is necessary that there are restrictions and limits on the use and occupation of land in the surroundings, to reduce the influence of negative impacts on biodiversity in protected areas. The aim of this study is to provide a technical foundation for the determination of criteria that define the extent, use and occupancy of the buffer zone (BZ) of the Alto Ribeira Tourist State Park (PETAR). To this were developed criteria that define: a) the extent of the buffer zone and b) the use of the buffer zone. The definition of the extent of the buffer zone PETAR was based on the delineation of watersheds whose rivers drain into the park and approximation of the antropic vectors to the park boundary. The criteria that defined the use and occupancy of the landscape in the buffer zone were based on the vulnerability to soil erosion. It is concluded that the buffer zone of 10 km covers the criteria used here, covering the source of watersheds and large forest fragments. However the quality of the park is threatened by expanding agriculture, mining, illegal harvesting for palm-heart and forestry. The expansion of these activities occurs irregularly in areas of high vulnerability to erosion and can endanger the maintenance of the quality of the biome protected.

As unidades de conservação (UC) são áreas com características naturais relevantes, com limites definidos e com o objetivo de proteção e preservação da natureza, sendo admitido apenas o uso indireto dos seus recursos naturais, o qual deve compatibilizar a conservação da natureza com o uso sustentável de parcela dos seus recursos naturais. As áreas em volta das UCs são estratégicas para a proteção da biodiversidade e para o desenvolvimento sustentável da população local. Entretanto, é necessário que hajam restrições e limites ao uso e ocupação do solo nos locais da circunvizinhança, para reduzir a influência dos impactos negativos na biodiversidade das áreas protegidas. O objetivo do presente estudo é fornecer embasamento técnico para a determinação de critérios que definam a extensão, o uso e a ocupação da zona de amortecimento (ZA) do Parque Estadual Turístico do Alto Ribeira (PETAR). Para isto foram elaborados critérios que definem: a) a extensão da zona de amortecimento e b) a utilização da zona de amortecimento. A definição da extensão da zona de amortecimento do PETAR baseou-se na delimitação das microbacias hidrográficas cujos cursos de água drenam para dentro do parque e na aproximação dos vetores antrópicos ao limite do parque. Os critérios que definiram o uso e ocupação da paisagem na zona de amortecimento foram baseados na vulnerabilidade à erosão do solo. Conclui-se que a zona de amortecimento de 10 quilômetros contempla os critérios aqui usados, abrangendo o limite das microbacias hidrográficas e os grandes fragmentos florestais. Entretanto a qualidade do parque está ameaçada pela expansão da agricultura, mineração, extração ilegal de palmito e silvicultura. A expansão dessas atividades ocorre de forma irregular em áreas de alta vulnerabilidades à erosão e coloca em risco a manutenção da qualidade do bioma protegido. / Conservation Units (UC) are areas with relevant natural features, with limits defined and with the goal of protecting and preserving nature, being admitted only the indirect use of its natural resources, which should reconcile nature conservation with the use sustainable portion of their natural resources. The areas around the UC are strategic to the protection of biodiversity and the local sustainable development of the population. However, it is necessary that there are restrictions and limits on the use and occupation of land in the surroundings, to reduce the influence of negative impacts on biodiversity in protected areas. The aim of this study is to provide a technical foundation for the determination of criteria that define the extent, use and occupancy of the buffer zone (BZ) of the Alto Ribeira Tourist State Park (PETAR). To this were developed criteria that define: a) the extent of the buffer zone and b) the use of the buffer zone. The definition of the extent of the buffer zone PETAR was based on the delineation of watersheds whose rivers drain into the park and approximation of the antropic vectors to the park boundary. The criteria that defined the use and occupancy of the landscape in the buffer zone were based on the vulnerability to soil erosion. It is concluded that the buffer zone of 10 km covers the criteria used here, covering the source of watersheds and large forest fragments. However the quality of the park is threatened by expanding agriculture, mining, illegal harvesting for palm-heart and forestry. The expansion of these activities occurs irregularly in areas of high vulnerability to erosion and can endanger the maintenance of the quality of the biome protected.

Linear springs with variable stiffnesses find some key roles in robotic applications. They are implemented into robotic devices for two main reasons, to increase energy efficiency of walking-running robots and prosthesis, and to get safe human-robot interaction at industrial robots. Being inspired from the human actuation system, a mathematical method to get mechanically adjustable linear springs is noted in the literature / antagonistically working two quadratic springs method. But the proposed solution requires two non-linear springs with quadratic spring characteristics and they are not readily available. Several solutions have been noted in the literature for the acquisition of such non-linear springs. At this thesis work, the solution is realized with a string wrapping around cam mechanism. Two different prototypes were designed and constructed and the second one was physically tested to validate the linear spring behavior. The results displayed good linear spring characteristics with different levels of adjustable spring stiffness. Beside the antagonistically working two quadratic springs method, three novel methods to get mechanically adjustable linear springs are introduced at this thesis. They are based on using hanging weights, an exponential characteristic spring and a linear translation spring respectively. The real prototypes were not manufactured but sample designs using string wrapping around cam mechanisms are made.

Cette thèse porte sur le comportement des élites politiques durant les périodes de crise nationale et plus particulièrement sur leurs réactions aux attentats terroristes. Elle démontre que les crises terroristes sont tout comme les conflits militaires ou diplomatiques propices aux unions nationales et notamment aux ralliements des partis d’opposition auprès du gouvernement. L’analyse statistique d’actes terroristes s’étant produits dans cinq états démocratiques (Allemagne, Espagne, États-Unis d’Amérique, France et Royaume-Uni) entre 1990 et 2006 révèle que l’ampleur d’un attentat en termes de pertes humaines ainsi que la répétition de ces attentats influencent dans une large mesure la réaction des élites politiques. Ainsi plus l’ampleur d’un attentat est élevée, plus la probabilité d’un ralliement est grande. En revanche, la multiplication des attentats augmente la possibilité de dissension entre l’opposition et le gouvernement. Par ailleurs, l’opposition est plus susceptible de se rallier au gouvernement lorsque l’attentat est perpétré par des terroristes provenant de l’étranger. L’analyse quantitative indique également que l’existence d’un accord formel de coopération dans la lutte antiterroriste entre le gouvernement et l’opposition favorise l’union des élites. Enfin, les données analysées suggèrent que la proportion des ralliements dans les cinq pays est plus importante depuis les attentats du 11 septembre 2001. Une analyse qualitative portant exclusivement sur la France et couvrant la période 1980-2006 confirme la validité des variables identifiées dans la partie quantitative, mais suggère que les élites réagissent au nombre total de victimes (morts mais aussi blessés) et que la répétition des actes terroristes a moins d’impact lors des vagues d’attentats. Par ailleurs, les analyses de cas confirment que les élites politiques françaises sont plus susceptibles de se rallier quand un attentat vise un haut-fonctionnaire de l’État. Il apparaît également que les rivalités et rancœurs politiques propre à la France (notamment suite à l’arrivée de la gauche au pouvoir en 1981) ont parfois empêché le ralliement des élites. Enfin, cette analyse qualitative révèle que si l’extrême gauche française soutient généralement le gouvernement, qu’il soit de gauche ou de droite, en période de crise terroriste, l’extrême droite en revanche saisit quasi systématiquement l’opportunité offerte par l’acte terroriste pour critiquer le gouvernement ainsi que les partis de l’establishment. La thèse est divisée en sept chapitres. Le premier chapitre suggère que le comportement des élites politiques en période de crises internationales graves (guerres et conflits diplomatiques) est souvent influencé par la raison d’État et non par l’intérêt électoral qui prédomine lors des périodes plus paisibles. Le second chapitre discute du phénomène terroriste et de la littérature afférente. Le troisième chapitre analyse les causes du phénomène d’union nationale, soumet un cadre pour l’analyse de la réaction des élites aux actes terroristes, et présente une série d’hypothèses. Le quatrième chapitre détaille la méthodologie utilisée au cours de cette recherche. Les chapitres cinq et six présentent respectivement les résultats des analyses quantitatives et qualitatives. Enfin, le chapitre sept conclut cette thèse en résumant la contribution de l’auteur et en suggérant des pistes de recherche. / This thesis focuses on the behaviour of political elites during periods of national crisis and particularly on their reactions to acts of terrorism. It demonstrates that terrorist crises, much like military and diplomatic conflicts, represent a fertile ground for rallies around the flag. The statistical analysis of terrorist events that occurred in five democracies (France, Germany, Spain, the United Kingdom and the United States of America) from 1990 to 2006 indicates that the magnitude of the attack in terms of fatalities and the repetition of attacks influence in large measures the reaction of the political elite. The higher the magnitude is, the more likely the rally. However, the repetition of attacks increases the likelihood of dissention between opposition and government. Moreover, the opposition is more likely to support the government when the attack is perpetrated by terrorists originating from abroad. The quantitative analysis also indicates that the existence of a formal antiterrorist pact between government and opposition increases the likelihood of a rally. Finally, the data suggest that elites are more likely to rally around the flag since the events of 9/11. A qualitative analysis focusing solely on France and examining cases from the period 1980-2006 confirms the validity of the variables identified in the quantitative part but suggests that elites react to the total number of victims (including wounded) and that the repetition has less impact during waves of attacks. Furthermore, the case studies confirm that French political elites are particularly prone to rally when high-ranking representatives of the state fall victim to the terrorists. In addition, it appears that political rivalries and resentment (in particular following the Left’s return to power in 1981) have on occasion thwarted the rallying of elites. Finally, this analysis indicates that whereas the far-left generally supports the government (left-wing or right-wing), the far-right uses almost every opportunity to criticize the government and the parties of the establishment. The thesis is divided into seven chapters. Chapter one suggests that during intense foreign policy crises (military and diplomatic) the behaviour of the political elite is often influenced by the national interest, rather than the electoral interest that predominates in quieter periods. Chapter two discusses the terrorist phenomenon and the corresponding literature. Chapter three analyses the rally-around-the-flag phenomenon, lays out a framework for analysing the reaction of elites, and presents a series of hypotheses. Chapter four details the methodology used. Chapters five and six present respectively the results of the quantitative and qualitative analyses. Finally, chapter seven concludes with a summary of the author’s contribution and suggests avenues of research.

O fenômeno de vibração induzida por vórtices (VIV) é particularmente danoso para estruturas submarinas como risers de exploração de petróleo. A maneira mais usual de se atenuarem os efeitos de VIV é instalar um supressor, como por exemplo strakes ou fairings. Dentre esses, foi desenvolvido por All Brow Universal Components um supressor chamado Ventilated Trousers (VT), que consiste em uma malha permeável feita de uma rede flexível e dezenas de bobbins. Através de um estudo experimental, procurou-se investigar os mecanismos hidrodinâmicos pelos quais o supressor V T funciona. Foram construídos três modelos diferentes de supressores: um modelo idêntico ao V T e duas malhas dele derivadas, alterando-se a geometria dos bobbins e a distribuição destes ao redor da malha. Foram realizados ensaios com o modelo xo e ensaios de VIV em um grau de liberdade alterando-se o amortecimento estrutural. Foram medidos deslocamento e forças de sustentação e arrasto. Os resultados mostraram que o supressor do tipo V T reduz as amplitudes de vibração, força de sustentação e arrasto quando comparados com um cilindro oscilando. No entanto, aumenta a força de arrasto quando comparado com o cilindro xo. A geometria da malha mostrou-se de grande importância para a supressão de VIV. Modelos que possuem o disco externo no bobbin impedem o surgimento de folga entre o modelo e o cilindro, além de aumentar o amortecimento hidrodinâmico. Três hipóteses foram levantadas para explicar o funcionamento do supressor V T. A primeira diz que a supressão é provocada pelo aumento do amortecimento hidrodinâmico. Os ensaios mostraram que, de fato, o supressor V T aumenta o amortecimento e, consequentemente, diminui as VIV. No entanto, somente esse efeito não explica toda a supressão obtida. As outras hipóteses, relacionadas à alterações bi e tridimensionais da esteira, foram avaliadas, porém não se pode afirmar que alguma delas seja isoladamente responsável por produzir o mecanismo hidrodinâmico de supressão. / The phenomenon of vortex-induced vibration (VIV) is particularly harmful to submarine structures such as risers used for oil extraction. The most usual way to attenuate the effects of VIV is the installation of suppressors, like strakes or fairings. Among them, All Brow Universal Components developed a VIV suppressor called Ventilated Trousers (VT), which consist of a permeable mesh made of a flexible net and tens of bobbins. Three different models of suppressors based on permeable meshes have been assembled with the objective to understand the hydrodynamic mechanism behind the suppression: one model identical to the VT and two meshes with different bobbin geometries and distribution. Tests were carried out with xed models and models free to oscillate in one degree of freedom varying the structural damping. Displacements, drag and lift forces were measured. Results showed that the VT suppressor reduced vibration amplitudes, lift and drag forces when compared to an oscillating circular cylinder. However, it increased drag force when compared to a fixed circular cylinder. The mesh geometry proved to be important to VIV suppression. Models that had an external disc on the bobbins avoided the appearance of a gap between the model and the cylinder. Three hypotheses were formulated to explain how the VT suppressor works. The first one says that the increase on hydrodynamic damping is responsible for suppression. In fact, tests showed that the VT increased hydrodynamic damping and, consequently, reduced the VIV response. However, this effect alone does not explain the suppression as a whole. The other two hypotheses related to two-dimensional and three-dimensional wake changes were evaluated, but it cannot be stated that any of them, on its own, is responsible for the whole of the suppression mechanism.

The offshore oil industry is engaged in the development of new floating platforms, such as Spar, semi-submersible, tension-leg, FPSO and monocolumn for the exploration of deep and ultra-deep waters. Some of these floating systems have circular cross sections (or cross sections of other bluff geometries) being susceptible to vortex-induced vibrations (VIV). Vortex shedding behind a bluff body can be altered, suppressed or controlled over a limited range of Reynolds numbers. Various flow-control techniques, which result in the reduction of drag and unsteady forces, have been suggested and tested in simple geometries. One such method is the moving-surface boundary layer control (MSBC), in which smaller control rotating cylinders are placed close to the bluff body. This method is considered as an inspiration for the present experimental investigation of VIV suppression for omni-directional flows. In this context, three different configurations have been assembled to compare the effect of suppression on a plain cylinder surrounded by two, four and eight control cylinders distributed symmetrically around it. Experiments were carried out with static models and models free to oscillate in one-degree-of-freedom with fixed and rotating control cylinders. Experiments with a plain cylinder were performed to serve as reference. Displacements, drag and lift forces were measured. The position of the control cylinders proved to be an important parameter to VIV suppression. Configurations with two control cylinders increased lift and drag forces. In contrast, configurations of four and eight control cylinders showed to be more effective to suppress VIV. Furthermore the results for all the cases of the configuration of eight fixed control cylinders presented a reduction of displacement amplitude, lift and drag forces when compared to a plain cylinder. However, when the control cylinders were actuated, the two cases with rotating control cylinders increased drag force when compared to fixed control cylinders. / A indústria offshore está envolvida no desenvolvimento de novas plataformas flutuantes como Spar, semi-submersível, TLP, FPSO e monocoluna para a exploração de águas profundas e ultra-profundas. Alguns destes sistemas flutuantes têm seções transversais circulares (ou de outras seções rombudas) sendo susceptíveis à vibrações induzidas por vórtices (VIV). A esteira de vórtices desprendida de um corpo rombudo pode ser alterada ou suprimida ao longo de uma faixa de número de Reynolds. Várias técnicas de controle do escoamento foram sugeridas e testadas em geometrias simples, resultando na redução de forças de sustentação e arrasto. Um desses métodos é o controle de camada limite por superfícies móveis (CCLSM), no qual cilindrinhos rotativos de controle são colocados próximos ao corpo rombudo. Neste trabalho, este método foi abordado através de uma investigação experimental como um supressor de VIV para o escoamento omnidirecional. Neste escopo três diferentes configurações foram montadas para comparar o efeito de supressão sobre um cilindro liso rodeado por dois, quatro e oito cilindros de controle, distribuídos simetricamente em torno dele. Foram realizados ensaios com o modelo estático, ensaios de VIV em um grau de liberdade com cilindros de controle fixos e rotativos. Foram medidos deslocamento e forças de sustentação e arrasto. Os resultados mostraram que a posição dos cilindros de controle é um parâmetro importante para a supressão de VIV. A configuração com dois cilindros de controle aumentou as forças de sustentação e arrasto. Diferentemente, as configurações de quatro e oito cilindros de controle mostraram-se mais eficazes para suprimir VIV. Além disso, todos os casos da configuração de oito cilindros de controle fixos apresentaram redução nas amplitudes de vibração e nas forças de sustentação e arrasto, quando comparados com um cilindro liso. No entanto, quando os cilindros de controle foram acionados para rotacionar, mostrou-se um aumento na força de arrasto em relação aos cilindros de controle fixos.

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.