O escoamento de um fluido ao redor de corpos rombudos configura-se em um dos principais problemas da mecânica dos fluidos. As investigações deste fenômeno são baseadas em técnicas analíticas, numéricas e experimentais. Cada abordagem apresenta suas vantagens e desvantagens, sendo impossível a plena caracterização do escoamento através de uma única técnica para a maioria das geometrias de corpo e condições de escoamento. É neste contexto que se insere esta tese, que compreende um estudo fundamental sobre a dinâmica de formação e desprendimento de vórtices baseado em técnicas experimentais. Primeiramente, realiza-se um estudo de estabilidade hidrodinâmica do escoamento em torno de cilindro circular via modelo de Ginzburg-Landau. Os resultados experimentais deste estudo serviram para validar simulações numéricas e a principal técnica experimental utilizada foi a Velocimetria por Imagem de Partículas (PIV). A decomposição de campos de velocidade em série de Fourier permite verificar a hierarquia sugerida pela solução assintótica da bifurcação de Hopf. Além disso, os resultados também indicam quando a tridimensionalidade passa a ser importante no escoamento. A tridimensionalidade do escoamento está intimamente ligada ao fenômeno de vibração induzida por vórtices (VIV), que exerce efeitos danosos em uma grande quantidade de estruturas sujeitas a escoamentos fluidos. Com o objetivo de suprimir as VIV, realizou-se um estudo paramétrico do supressor do tipo strakes helicoidal, que é bastante empregado na indústria offshore. Verificou-se que os strakes modificam as características tridimensionais da esteira de um cilindro. Apesar da eficiência dos strakes na supressão de VIV, eles possuem algumas limitações inerentes às características hidrodinâmicas do escoamento. Sendo assim, apresenta-se uma técnica que reduz as tridimensionalidades de modo a tornar o escoamento bidimensional, com a possibilidade de controle ativo de malha fechada. O método de controle de camada limite por superfícies móveis (CCLSM) suprime VIV e diminui o arrasto atrasando a separação da camada limite do cilindro através da injeção de quantidade de movimento angular pelos cilindros de controle rotativos. / The fluid flow around bluff bodies is one of the main problems in fluid mechanics. The investigations of this phenomenon are based on analytical, numerical and experimental techniques. Each technique presents advantages and disadvantages, being impossible the comprehensive characterization of the flow though only one technique for the majority of body geometries and flow conditions. Within this context, the present thesis is proposed, which comprehends a fundamental study about the vortex shedding dynamics based on experimental techniques. Firstly, a hydrodynamic stability study of the flow around a circular cylinder is performed using the Ginzburg-Landau model. The experimental results of this study allowed the validation of numerical simulations and the main experimental technique employed was the Particle Image Velocimetry (PIV). The Fourier series decomposition of velocity fields permits to verify the hierarch suggested by the asymptotic solution of the Hopf bifurcation. Additionally, the results also indicate when the three-dimensionalities become important in the flow. The flow three-dimensionality is closely associated to the vortex-induced vibration (VIV) phenomenon, which exerts damaging effects on a great quantity of structures subjected to fluid flows. In order to suppress VIV, it was realized a parametric investigation of the helical strakes, that are commonly employed in the offshore industry. One verifies that the strakes modify the three-dimensional characteristics of the cylinder wake. Despite of the strake efficiency regarding the VIV suppression, the strakes have some limitations that are inherent to the flow hydrodynamic features. Consequently, it is presented a technique that reduces the three-dimensionalities in order to create a two-dimensional flow, with the possibility of closed-loop active control. The moving surface boundary-layer control (MSBC) suppresses VIV and reduces the drag delaying the cylinder boundary-layer separation through the injection of angular momentum by the rotational control cylinders.

Este trabalho aborda o escoamento incompressível ao redor de um cilindro circular. A tese que se quer defender, com base em experimentos numéricos, é: A equação de Landau pode ser obtida a partir das equações de Navier-Stokes por uma análise de estabilidade não-linear global. A teoria produz um procedimento bem-definido para determinação dos coeficientes da equação de Landau, permitindo assim a sua interpretação como um modelo simplificado (equações reduzidas de Navier-Stokes) para a predição das forças aplicadas pelo fluido ao cilindro, que podem ser comparados com resultados experimentais. O modelo não-linear se baseia em uma teoria assintótica que, como se sabe, tem sua faixa de validade no espaço de parâmetros determinada a posteriori, por meio da própria comparação com dados de laboratório. Resultados na faixa 46 <= Re <= 80 são apresentados. Descobriu-se, que a faixa de aplicabilidade da teoria como aqui exposta é restrita, não excedendo em muito o valor crítico do número de Reynolds. Argumentos são expostos para justificar esta afirmação e possíveis maneiras de modificar a teoria para estender esta faixa são apresentadas. São reportados, ainda, teoria e resultados sobre um novo tipo de condição de contorno,denominado impedância fluida, que permite reduzir o tamanho do domínio de cálculo necessário para simulação de escoamentos externos, comparativamente à comumente utilizada condição de outflow. Neste caso, abordou-se a faixa 20 <= Re <= 600. / This work adresses the incompressible flow around a circular cylinder. What we want to prove, based on numerical experiments, reads: The Landau equation can be derived from the Navier-Stokes equations by means of a global nonlinear stability analysis. The theory leads to a procedure for calculating numerically the coefficients of these equation, thus permitting their interpretation as a simplified model - reduced Navier-Stokes equation - for the prediction of the forces applied by the fluid on the cylindrical structure, which can be compared against experimental data. The nonlinear model is based on an asymptotic theory which, as is known, has its validity range in the parameter space determined a posteriori. The focus lies in the range 46 <= Re <= 80. It was found that the theorys applicability range as presented here is restricted to a small neighborhood of Rec. This affirmation in justified and possible means of modifying the theory in order to enlarge this range are proposed. Theory and results concerning a new type of boundary condition called fluid impedance are also reported, permitting the reduction of the domain size necessary for simulating external flows, comparatively to the commonly used outflow condition. In this case, the range 20 <= Re <= 600 was considered.

Made available in DSpace on 2016-03-15T19:37:00Z (GMT). No. of bitstreams: 1 Dionisio Mateo Cardille.pdf: 3138784 bytes, checksum: c27dcdc2765d16f7b30f2b6a5a588e37 (MD5) Previous issue date: 2009-08-18 / This study broaches the diesel engines cylinder liners waste concepts, characterizing abrasive waste, tribo-chemical waste and scuffing, with component external factors, like adultered fuel or with high sulfur level frequently found in Brazil. It presents the modern emissioned engines and the direct causes in the cylinder liners material requirement, with the tribologic evolution of piston, piston rings and cylinder. Some concepts of cylinders internal diameter surface finish and the direct involvement in the cylinder smooth, and the blow by flux besides oil consumption. The cylinder liners are manufactured in perlitic gray casting iron, and it was compared to a bainitic gray casting iron as alternative material, in order to satisfy the modern environment laws for emissions. The experiment technical was based on a dynamometric test in a 146kw diesel engine, during 500hs, with 27,4% overpower. The liners were assembled in the engine block in an intercalated way, avoiding benefits in the cylinders closer to the water pump. After the test, the liners were dimensioned with the respective piston rings. As result, was observed that the proposal bainitic material got a better waste resistance in comparison with the perlitic material, with a better dimensional stability. The piston rings presented similar waste when applied in both materials, respecting the project original tribological pair. / Este estudo aborda os conceitos de desgaste em camisas de cilindro para motores diesel, caracterizando desgaste abrasivo, tribo-químico e "scuffing", juntamente com fatores externos ao componente como a questão dos combustíveis adulterados ou com alto teor de enxofre, freqüentemente encontrados no Brasil. Apresenta o enquadramento dos motores nas modernas leis de emissões, e suas causas diretas na exigência de materiais para camisas de cilindro, juntamente com a evolução tribológica do conjunto pistão, anéis e camisa. Definem-se também alguns conceitos de rugosidade e acabamento superficial dos cilindros e suas implicações diretas no amaciamento neste componente, além do tema passagem de gases da câmara de combustão para o cárter (blow by) e consumo de óleo lubrificante. As camisas de cilindro são em sua maioria confeccionadas em liga de ferro fundido cinzento com matriz perlítica, a qual foi objeto de comparação com uma liga proposta de matriz bainítica, como material alternativo, visando satisfazer as condições impostas pela moderna legislação ambiental. A técnica experimental empregada baseou-se em um ensaio dinamométrico em um motor diesel de 146kw, porém submetido a plena carga durante 500hs, com 27,4% de acréscimo de potência. As camisas foram montadas no motor de forma a estarem intercaladas e evitando favorecimentos em cilindros mais quentes do bloco. Após o teste, as camisas de cilindro foram dimensionadas, juntamente com os respectivos anéis de pistão. Como resultado, observou-se que o material proposto em liga com matriz bainítica teve uma melhor resposta ao desgaste em comparação com o material de matriz perlítica, inclusive com uma melhor estabilidade dimensional. Os anéis de pistão tiveram desgaste similar em ambos os materiais, respeitando o par tribológico original de projeto.

The constant pursuit in the automotive industry to increase the engines performance, new solutions are always developed and tested to reduce the friction and increase the efficiency in the engine. One component that contributes to friction losses is the piston ring pack where the top compression stands for up to 40 %. This master thesis collaborated with Scania’s material science department Basic engine and covers the friction and wear of four different materials on the cylinder liner surface against the top compression ring.The four tested materials were grey cast iron with different honing quality and three atmospheric plasma sprayed coatings with titanium oxide, chromium oxide and Metco’s mixture F2071 which is a stainless steel mixed with a ceramic. A martensitic steel piston ring with a chromium coated sliding surface was used for all the testing in the Cameron-Plint TE77 test-rig. This is a pin-on-disc test method and the parameters used for testing is set to replicate the environment the ring is exposed to at the top dead centre.The test-rig has been in Scania’s possession for a long time and has not always given a satisfying result. An uneven contact between the ring and liner has been a problem resulting in only worn edges of the liner specimen. The piston ring holder was therefore redesigned to be able to adjust the radius of the ring. This allowed a good conformability between the ring and liner to be obtained.The tested materials were evaluated according to friction and wear. Friction was measured with the test-rig and the wear was calculated with surface profiles that were measured before and after testing. Worn surfaces were studied in a SEM to verify which wear mechanism that was active. The changes of the surfaces was studied with the use of following surface parameters Ra, Rk, Rpk, Rvk and if there was a connection between these parameters and friction and wear coefficient.Independent of honing quality showed the grey cast iron lowest friction coefficient just under 0.13, the F2071 liner showed a friction coefficient just above 0.13. Both oxide layers showed similar friction where the chromium oxide had a friction just below 0.15 and the titanium oxide lay just above 0.15. Lowest wear coefficient had the chromium oxide followed by F2071, titanium oxide and the bad honed grey cast iron. These three liners showed almost the exact same wear coefficient. Worst wear coefficient had the grey cast iron with a good honing quality. A mild abrasive wear mechanism was active during the wear test and vague wear marks was found on the surface. There is no connection between wear coefficient and friction and the change in surface roughness during the test does not affect the friction.

La microgéométrie de la surface de la chemise joue un rôle très important dans les pertes par frottement et dans la consommation de l’huile dans un moteur à combustion interne. Une des texturations classiques de cette surface est celle créée par pierrage. Elle se compose de plateaux plus ou moins lisses et de profondes stries croisées. L’´épaisseur du film d’huile est influencée fortement par cette texturation. Un modèle simplifié du contact segment chemise a été créé en la présence de la microgéométrie. Puis, un code de calcul basé sur la méthode numérique multigrille a été développé. Ce code a été utilisé pour des études paramétriques avec des jeux de paramètres très variés. Les calculs quantifient l’influence de cette microgéométrie particulière sur la relation film d’huile - portance. Les résultats mettent en évidence deux mécanismes distincts de génération de portance selon le type du segment. Le segment parabolique porte par son convergent et les stries ne font que diminuer cette portance. Inversement, le segment plat ne porte pas et ce sont les stries qui génèrent la portance. Deux modèles de prédiction ont finalement été déduits, un pour les segments paraboliques et un pour les segments plats. Ces deux prédictions ont ensuite été validées par des calculs sur des surfaces mesurées; / The liner microgeometry influences the friction losses and the oil consumption in IC engines. The cross-hatched texturing, created by the honing process, plays an essential part in the load carrying capacity - film thickness relation. This work studies the influence of the crosshatched groove parameters on the piston ring load carrying capacity. First, a simplified model of the hydrodynamic contact has been created. Thus a Multigrid based code was developed. Calculations with different sets of microgeometric parameters have been performed. These calculations quantify the load carrying capacity for both parabolic and flat rings. The results show that the load carrying capacity is generated differently in both cases. For the parabolic case, the pressure is build up by the wedge in the inlet zone and the grooves decrease the global load carrying capacity. In the flat case the grooves generate the total load carrying capacity. Finally, two prediction models were deduced for the parabolic ring and for the flat ring. These predictions were validated by measured surface calculations.

Les travaux portent sur l’étude de la formation de motifs sédimentaires à l’aval d’un cylindre vertical soumis à un courant. Le cylindre simule une fondation monopieu d’éolienne en mer, ou une pile de pont. Une modélisation théorique est développée. Les résultats obtenus sont en bon accord avec les résultats expérimentaux acquis au cours de la thèse. Une étude préliminaire expérimentale et théorique sur le tri sédimentaire au voisinage du cylindre est également effectuée. / This work concerns the study of sediment patterns formation downstream a vertical cylinder under a steady current. The cylinder simulates an offshore monopile foundation, or a bridge pile. A theoretical modeling is developed. The results are in good agreement with the experimental results obtained in the framework of this PhD. An experimental and theoretical preliminary study on sediment segregation in the vicinity of the cylinder is also performed.

Face à la complexité croissante des trajectoires et des conditions opérationnelles des forages pétroliers et géothermiques, le phénomène de fatigue est devenu la cause principale de rupture des garnitures de forage. La fatigue des tiges est essentiellement liée à leur flexion cyclique due à leur rotation dans une section courbe du puits. L'objectif de ce travail est d'élaborer une méthodologie ainsi que les modèles numériques nécessaires pour évaluer la fatigue des tiges au cours du forage de puits à trajectoire complexe. Pour ce faire, nous proposons d'abord de choisir parmi les approches existantes de prévision de la durée de vie en fatigue d'une structure celles qui nous ont semblé les plus pertinentes pour le problème de fatigue des systèmes de forage. Puis, ces approches (qui comprennent les théories de la fatigue et de la rupture ainsi que des lois empiriques), et des logiciels de calcul de structures sont ensuite intégrés dans des algorithmes de calcul incrémental de la fatigue d'un système en fonction de l'évolution de l'opération du forage. Du fait que les contraintes dans les tiges restent souvent dans le domaine élastique, deux modèles de fatigue des tiges sont développés : un premier est basé sur le calcul du cumul de fatigue et un second sur le calcul de la propagation de fissure par fatigue. Ces deux modèles peuvent être utilisés dans la phase de conception de la trajectoire du puits et de la garniture pour le forer, de même qu'en opération pour prédire les risques de rupture par fatigue du train de tiges. Ceci permet à l'opérateur de planifier la gestion des tiges et leurs inspections en fonction de l'historique de leur utilisation. / Facing the growing complexity of trajectories and operating conditions of oil and geothermal drillings, the fatigue phenomenon has become the main cause of drill-string failure. The fatigue of drill-pipes is essentially due to their cyclic bending caused by their rotation in a curved section of the well. The objective of this work is to develop a methodology and the necessary numerical models to assess the fatigue of drill-pipes during drilling operations of complex trajectory wells. For this purpose, we propose firstly to choose among the available approaches for structure fatigue life prediction those that seem most relevant to drill-string fatigue problem. Then, these approaches (which include the fatigue and fracture theories as well as empirical laws), and structural calculation software are then integrated into incremental computation algorithms of drill-pipe fatigue in function of drilling operation evolution. Since the stresses in drill-pipes remain often within the elastic domain, two fatigue models for drill-pipes are developed: the first one is based on the cumulative fatigue calculation and the second one on the fatigue crack growth calculation. These models can be used in the well and drill-string design, or in real time during drilling to predict the fatigue failure in the drill-string. This allows the drill operator to plan the management of drill-pipes and their inspections depending on their usage history.

Vortex shedding is an important mechanism, by which the flow around bluff bodies create forces that excite vibratory motion. Vortex-induced vibration (VIV) is studied for a single circular cylinder by means of Computational Fluid Dynamics (CFD) simulations. An arbitrary Lagrangian Eulerian (ALE) formulation is used to achieve the grid deformation needed for VIV. In this thesis, a multifaceted approach is undertaken by which response dynamics and wake interaction are addressed. Four major aspects are considered in the study: the Reynolds number (Re); the mass and damping; the degree-of-freedom for VIV; and the mutual effect between VIV and heat transfer.As attention is paid towards high pre-critical Re flow, the turbulent flow around the cylinder is treated by two turbulence modelling approaches: unsteady Reynolds Averaged Navier Stokes (uRANS), and Large Eddy Simulation (LES). The wake-VIV interaction is analyzed by looking at mean velocities and Reynolds stresses, where decomposition of flow scales is undertaken to explore the evolution of coherent eddy structures, downstream of the cylinder. Conversely, the VIV response is analyzed by considering oscillation amplitude and frequency, in addition to the excitation and inertial dynamics.High turbulence in the separated shear layers disorders the cylinder's VIV response and induces higher amplitudes. The sensitivity for Re is found more pronounced in cylinders with low mass and damping. Meanwhile, VIV is found to enhance wake mixing, and to significantly change the near wake Reynolds stresses. It is suggested that the increase in Re brings a change to the wake patterns, which are known in VIV at lower Re. The kinetic energy production, of near wake eddy structures, is qualitatively altered with the presence of VIV. Furthermore, the surface heat flux is found to cause a noticeable increase in VIV amplitude, as long as it does not disorder the wake correlation. The cylinder's oscillation increases the average value of the Nusselt number (Nu), while the local variance of Nu rises markedly post-separation.

Thss thesis presents the results of an experimental programme on the static mono-tonic response of cohesive-frictional granular materials. The purpose of this experimental programme was to gain insight into the mechanical behaviour of uncemented sands, and sands with small percentages of cementation. With this objective in sight, the research involved understanding and delineating the e ects of four variables: the intermediate principal stress, stress inclination, cohesion (or cementation), and particle morphology. The hollow cylinder torsion (HCT) apparatus, which allows control over both the magnitude and direction of principal stresses, was used in this study to carry out a series of elemental tests on the model materials. The test results were analysed in a plasticity theory based framework of critical state soil mechanics. Drained and undrained HCT tests were conducted on a model angular sand to understand the combined influence of intermediate principal stress ratio (b) and principal stress inclination ( ). Sand specimens were reconstituted to a given density and confining pressure, and were sheared to large strains towards a critical state. The stresses at the critical state with varying `b' were mapped on an octahedral plane to obtain a critical state locus. The shape of this locus closely resembles a curved triangle. Also these specimens showed increased non-coaxiality between the stress and strain increment directions at lower strains. This non-coaxiality decreased significantly, and the response at the critical state was by and large coaxial. The effect of `b' and ` ' on the flow potential, phase transformation, and critical state was also investigated. At phase transformation, ` ' plays a more dominant role in determining the flow potential than `b'. The shape and size of the critical state locus remained the same immaterial of the drainage conditions. Next, small amounts of cohesion (using ordinary Portland cement) was added to this sand ensemble to study the mechanical behaviour of weakly cemented sands. The peak in the stress strain curve was used to signal the breakdown of cohesion further leading to a complete destructuring of the sand at the critical state. The response of the cemented sand changes from brittle to ductile with increase in confining pressure, while reverses with increase in density and `b'. Stress-dilatancy response for the weakly cemented materials shows the non coincidence of peak stress ratio and maximum value of dilation unlike purely frictional materials. This mismatch in peak stress ratio and maximum dilation diminishes with increase in confining pressure. The peak stress (cemented structured sand) locus and the critical state (destructured) locus were constructed on the octahedral plane from these HCT tests. The critical state locus of the cemented sand when it is completely destructured almost coincides with the critical state locus of the clean sand. Using this experimental data set, some important stress-dilatancy relationships (like Zhang and Salgado) and failure criteria (Lade's isotropic single hardening failure criteria and SMP failure criteria) were benchmarked and their prediction capabilities of such models were discussed in detail. The effect of particle morphology was also investigated in this testing programme. Rounded glass ballotini and angular quartzitic sand which occupy two extreme shapes were selected, and a series of HCT tests at different `b' values were con-ducted. A larger sized CS locus was obtained for angular particles and it encompassed the critical state locus of the spherical glass ballotini. Spherical particles exhibit a predominantly dilative behaviour, however present a lower strength at the critical state. The mobilization of strength as a result of rearrangement of angular particles and the consequent interlocking is higher. Even with contractive behaviour which is reflected in the higher values of critical state friction angle and the larger size of the yield locus for sand. Finally, a series of unconfined compression tests were performed to understand if there exists a scale separation in cohesive frictional materials. Specimens were reconstituted to a range of sizes while maintaining a constant aspect ratio and density. As the specimen size increased, the peak strength also increases, counter to an idea of a generalized continuum for all model systems. The observed secondary length scale (in addition to the continuum length scale) is obverse to the one observed in quasi-brittle materials such as concrete, rock. In order to ascertain the reason behind this phenomenon, a series of tomography studies were carried out on these contact-bound ensembles. The presence of cohesion between the grains brings about an \entanglement" between the grains, which contributes to increase in strength, with increase in the size of the sample. This in e ect bringing forth a second length scale that controls the behaviour of these cohesive frictional granular materials. This experimental data set provides quantification of various aspects of the me-chanical response of both cemented and uncemented granular materials under myriad stress conditions. This data set is also extremely useful in developing and bench-marking constitutive models and simulations.

Swirling flows abound in nature and numerous engineering applications. Under conditions which are not completely understood, the swirling cores could undergo a sudden enlargement of their vortex core, leading to a ’vortex breakdown’. The physics of vortex breakdown and strategies to control it have been active areas of research for nearly half a century. There are many competing theories of vortex breakdown in the literature; broadly, these are surmised on similarities to flow separation, hydrodynamic instability or transition from a supercritical to a subcritical state. However, a rational criterion for vortex breakdown continues to be elusive. One of the most well known criteria in the literature is the one due to Brown and Lopez (1990) based on an inviscid vortex dynamics model which suggests that the helix angle of the velocity vector should enclose the helix angle of the vorticity vector. However it appears that this only suggests that the stream surface would diverge and not necessarily constitute a condition for breakdown. In this work, we propose a new criterion based on helicity (scalar product of velocity and vorticity vectors) for characterizing breakdown since it has fundamental topological interpretations relating to change in linkages of vortex lines. In particular, it is suggested that the breakdown location corresponds to the location where helicity becomes zero. We study the problem of vortex breakdown in a cylindrical container with a rotating top lid in order to clarify and elucidate our hypothesis. We present results from Direct Numerical Simulation of this problem for three different Reynolds numbers and evaluate the utility of our proposed helicity criterion. Our studies indicate that helicity is indeed a better choice for characterizing vortex breakdown.