The aerodynamic performance of low aspect ratio wings at low Reynolds numbers applicable to micro air vehicle design was studied in this thesis. There is an overall lack of data for this low Reynolds number range, particularly concerning details of local flow behavior along the span. Experiments were conducted to measure the local pressure distributions on a wing at various spanwise locations in a Reynolds number range 30000 < Re < 90000. The model wing consisted of numerous wing sections and had a rectangular planform with NACA0012 airfoil shape with aspect ratio of one. One wing section, with pressure ports at various chordwise locations, was placed at different spanwise locations on a wing to effectively obtain the local pressure information. Integration of the pressure distributions yielded the local lift coefficients. Comparison of the local lift distributions to optimal elliptic lift distribution was conducted. This comparison showed a sharply peaked lift distribution near the wing tip resulting in a drastic deviation from the equivalent elliptic lift distributions predicted by the finite wing theory. The local lift distributions were further analyzed to determine the total lift coefficients vs angle of attack curves, span efficiency factors and the induced drag coefficients. Measured span efficiency factors, which were lower than predictions of the elliptic wing theory, can be understood by studying deviations of measured lift from the elliptic lift distribution. We conclude that elliptic wing theory is not sufficient to predict these aerodynamic performance parameters. Overall, these local measurements provided a better understanding of the low Reynolds number aerodynamics of the low aspect ratio wings.

A l’heure actuelle, la microfluidique est une science en plein développement ayant un besoin croissant de dispositifs permettant de générer des écoulements aux échelles micrométriques. Les phénomènes physiques mis en jeu lors du mouvement d’un fluide sont en effet majoritairement gouvernés par la viscosité (bas nombre de Reynolds) contrairement aux écoulements macroscopiques dominés par les effets inertiels.Dans cette thèse, les écoulements engendrés par le mouvement de parois mobiles ont été étudiés en vue d’une application aux micropompes, dispositifs essentiels en microfluidique.Dans une première partie, une étude analytique et numérique évalue la possibilité de générer un écoulement par un cylindre en rotation à proximité de parois mobiles.Les résultats obtenus du régime de Stokes (Re=0) jusqu’à un nombre de Reynolds Re=60 en régime stationnaire témoignent du potentiel notable d’intégration de cette géométrie dans les microsystèmes en tant que micropompes.Dans une seconde partie, une micropompe, basée sur un principe de fonctionnement novateur, est conçue par l’intermédiaire des techniques de microfabrication. Dans cette optique, le procédé de gravure RIE d’un élastomère est entièrement développé. Les performances de la micropompe en terme de pression et débit générés dépassent l’état de l’art des microsystèmes similaires et ceci en utilisant une technologie simple et bas-coût / Currently, microfluidic is a science field in constant development with an increasing need of devices able to generate flows at the micrometer order. At these length scales, physical phenomenons occurring in a moving fluid are mainly governed by its viscosity (low Reynolds number) contrary to macroscale flows dominated by inertial effects.In this thesis, a study on flows engendered by moving walls has been carried to fulfill to micropumps devices.In a first part, an analytical and a numerical study evaluates the possibility to generate a flow for a rotating cylinder close to moving boundaries.The results ranging from Stokes flows (Re=0) up to the low Reynolds number Re=60 in the stationary regime reveals the noticeable potential of integrating this device in microsystems as a micropump. In a second part, a new micropump, based on an innovative principle, is designed thanks to microfabrication technologies. In this perspective, the etching process of an elastomer called Silastic S is developed. Micropump performances in terms of pressure and flow rate are beyond the state of the art for similar microsystems and are achieved by using a simple and low-cost technology

Ce travail de thèse s’inscrit dans la grande problématique de la récupération d’énergie. Il s’agit plus précisément de convertir de petites quantités d’énergie cinétique présentes dans un écoulement d’air en énergie électrique par l’intermédiaire d’un convertisseur électrostatique. L’énergie électrique convertie est ensuite destinée à alimenter des capteurs autonomes communicants pour le monitoring de structures, le suivi environnemental, le monitoring de santé…Le manuscrit comprend une étude des travaux antérieurs en récupération d’énergie des écoulements d’air, la compréhension physique des phénomènes de conversion électrostatique, de mécanique des fluides et d’aérodynamique à très faibles nombres de Reynolds, ainsi qu’une description des prototypes développés et des résultats expérimentaux obtenus.Les récupérateurs que nous avons développés se divisent en deux grandes catégories : (i) les récupérateurs rotatifs qui transforment l’énergie cinétique de l’air en énergie mécanique de rotation et (ii) les récupérateurs aéroélastiques qui puisent l’énergie cinétique du vent pour produire de l’énergie mécanique par oscillations périodiques. Ces deux types de récupérateurs ont été associés à des convertisseurs électrostatiques dédiés, polarisés par l’ajout d’électrets ou auto-polarisés par triboélectricité. Les récupérateurs d'énergie ont été optimisés et nous avons notamment montré l'intérêt de la conversion électrostatique pour des dispositifs de petites dimensions (quelques cm²) fonctionnant à faible vitesse (<3m/s). Les densités surfaciques de puissance atteignent 5µW/cm2@1m/s pour les récupérateurs rotatifs et de l'ordre de 10µW/cm2@10m/s pour les récupérateurs aéroélastiques. Les micro-générateurs ont finalement été combinés à des circuits de gestion d'énergie pour alimenter des capteurs autonomes communicants, validant la chaine complète de récupération d'énergie, montrant par la même occasion l'intérêt des circuits de gestion d'énergie actifs de type SECE (synchronous electric charge extraction) ou MPP (maximum power point). / This work is enshrined in the energy harvesting context. More specifically, the purpose is to convert small amounts of kinetic energy from airflows into electrical energy through an electrostatic converter. The electrical energy produced is then intended to supply low power sensors for structural health monitoring, environmental follow-up, human monitoring…The manuscript includes an overview of the state of the art on airflow energy harvesting, the physical understanding of electrostatic conversion phenomena, fluid mechanics, ultra-low Reynolds number aerodynamics, a description of the prototypes developed and the results obtained.The harvesters we have developed are divided into two families: (i) the rotational harvesters which transform the kinetic energy of airflows into mechanical energy of rotation and (ii) the aeroelastic harvesters which use wind energy to produce mechanical energy of periodical oscillations. These two types of harvesters have been associated to different electrostatic converters, polarized by the addition of electrets or self-polarized by triboelectricity. The energy harvesters have been optimized and we have demonstrated the benefit of the electrostatic conversion for small devices (a few cm2) operating at low speeds (<3m/s). The power densities reach 5µW/cm2@1m/s for rotational devices and in the range of 10µW/cm2@10m/s for aeroelastic devices. The micro-generators were finally combined with power management circuits to supply autonomous and communicating sensors. This last stage completes the energy harvesting chain and also shows the high benefit of active circuits (synchronous electric charge extraction, maximum power point).

The vision driving the work reported herein is to investigate the fluid-structure interac-tion (FSI) effects of the flexible laminated blades for tilt-body micro-air-vehicles (MAV)proprotors in hover and forward flight configurations. This is in order to exploit the po-tential of flexible-bladed proprotor over the rigid-bladed proprotor in the enhancementof proprotor performance during hovering and cruising at a target forward speed. Forthat, the FSI model taking into account the specific problems devoted to MAV-sizedproprotor made of laminate composite was developed. The FSI model combines aerody-namic model adapting Blade Element Momentum (BEM) theory and structural modeladapting Anisotropic Finite Element Beam (AFEM) theory. The aerodynamic model isdeveloped to be capable of adapting in the analysis on low Reynolds number proprotors.In the structural model, the blade is modeled as an elastic beam undergoing deflectionsin flap, lag, and torsion to capture the coupling effects in anisotropic materials, adaptsthe structural analysis on proprotor blades made of laminate composite. The reliabilityof the developed FSI model is verified through a validation on both aerodynamic andstructural models, separately, on several MAV-sized proprotors. As for a direction to theanalysis on passively-adaptive proprotor blades, an optimal design on actively-adaptiveproprotor was carried out. For this, a program for designing the optimum rigid blades atsingle-point (for either isolated cruise-point or isolated hover-point) and multiple-point(combined cruise and hover point) for proprotors have been developed. The proceduresin the optimal design program employs the numerical iterative inverse design method,based upon the minimum thrust induced losses (MIL). Even if the work in this thesiswas directed primarily towards the proprotor, however, the propulsion system from themotor part was not neglected since the propulsion efficiency is a crucial factor to the suc-cess of MAVs. A cheap and time-effective method of proposing the best motor from theselected commercial motors was developed, based on Taguchi’s method. The sensitivityof the total power consumption to the variation of value of each motor design variableswas also studied. The benefit of the use of tip body in the blade and the effect of bendingon the induced twist and on the thrust degradation, respectively, were also analyzed andidentified. Finally, the systematically designed passively-adaptive composite proprotors were evaluated under steady operating conditions. Hovering and cruise propulsive performance, characterized by total power Ptotal, were compared between the rigid-bladed and flexible-bladed proprotors. As a result of the comparison, the flexible-bladed proprotor with fixed system is found to be capable of slightly enhancing the performance through the reduction in Ptotal over its optimal rigid-bladed proprotor. / L’idée principale du travail rapporté ici est d’étudier les effets de l’intéraction fluide-structure (FSI) de pales laminées flexibles pour les proprotors de micro véhicules aériens(MAV) de type tilt-body dans les configurations de vol stationnaire et en avant. Eneffet, le but est d’exploiter les possibilités offertes par les proprotors à pales flexiblespar rapport aux proprotors à pales rigides pour améliorer leur performance dans cesphases de vol. Le modèle FSI a été développé à cet effet. Ce modèle tient compte desproblèmes spécifiques liés aux proprotors de MAV faits de composite laminé. Il com-bine l’adaptation de modèle aérodynamique par la théorie d’élement de pale (BEM) etl’adaptation de modèle structurel par la théorie des éléments finis de poutre anisotropes(AFEM). Le modèle aérodynamique est développé pour être capable de s’adapter àl’analyse des proprotors à bas nombres de Reynolds. Dans le modèle structural, la paleest modélisée comme une poutre élastique subissant des déviations dans la flexion, latraction et la torsion afin de capturer les effets de couplage de matériaux anisotropes.Il adapte l’analyse structurale des pales du proprotor faites de composite laminé. Lafiabilité du modèle FSI développé est vérifiée à travers une validation par modèles aéro-dynamique et structural, séparément, sur plusieurs proprotors de MAV. Afin de se dirigervers une analyse de pales de proprotors à adaptation passive , une recherche de designoptimal a été effectuée pour des proprotor à adaptation active. Pour cela, un programmepour la conception de pales rigides optimales à un unique point de fonctionnement (soitle vol de croisière soit le vol stationnaire) et à plusieurs points (combinant croisière etvol stationnaire) ont été développés. Les procédures du programme de design optimalemploient les mèthodes de design inverse par itération numérique, sur la base de pertesde poussée induites minimales (MIL). Même si le travail dans cette thèse a été dirigéprincipalement vers le proprotor, la partie moteur du système de propulsion n’a pasété négligée puisque l’efficacité de la propulsion est un facteur crucial pour le succès desMAVs. Une méthode simple et rapide de sélection du meilleur moteur parmi les moteurscommerciaux choisis est élaborée sur la base de la méthode de Taguchi. La sensibilitéde la consommation d’énergie totale à la variation de la valeur de chaque variable deconception du moteur a été étudiée. Le bénéfice de l’utilisation de la charge à la pointe de la pale et l’effet de la flexion sur la torsion induite et sur la dégradation de la poussée respectivement ont aussi été analysés et identifiés. Enfin, les proprotors à pales flexibles conçues systématiquement ont été évalués dans des conditions de fonctionnement stables. Performances en vol stationnaire et performances croisière propulsive, caractérisées par la puissance totale Ptotal ont été comparées entre les proprotors à pales rigides et à pales flexibles. En tant que résultat de la comparaison, les proprotors à pales flexibles s’avère capable d’améliorer légèrement les performances par la réduction de la Ptotal surson optimal proprotors à pales rigides.

Nous avons tou(te)s été témoins des nuages d'étourneaux dans le ciel ou de la formation de bancs de poissons dans l'océan. Ce type d'organisation chez les êtres vivant se produit aussi à des échelles parfois invisibles pour l'oeil humain: celles des micro-organismes. Les suspensions de micro-nageurs présentent une dynamique riche. Elles peuvent former des structures cohérentes résultant d'un mouvement collectif, mélanger le fluides environnant et/ou modifier ses propriétés rhéologiques. Leurs comportements peuvent jouer un rôle important dans la survie, l'équilibre des espèces, leur stratégie trophique et même pour la fertilité animale. La diversité des phénomènes observés résulte de l'interaction complexe entre mécanismes de nage, processus physiologiques, processus chimiques et interactions hydrodynamiques. Comprendre et maîtriser les mécanismes impliqués fait nécessairement appel la Mécanique des Fluides. Les études expérimentales permettent de mettre en exergue certains phénomènes et parfois de les expliquer. Cependant la modélisation s'avère indispensable. Or, inclure une description fine des mécanismes de nages dans une suspension contenant des milliers (voire des millions) d'individus, implique de considérer une vaste gamme d'échelles couplées (typiquement du micron 10^-6m au millimètre 10^-3m). Décrire une physique multi-échelles pour ce type problème reste un défi majeur pour la modélisation numérique actuelle. Ainsi, dans le cadre de cette thèse nous nous proposons d'apporter une contribution dans cette direction. Nous montrerons dans une premiere partie qu'il est possible de reproduire les mécanismes de nage de façon satisfaisante à l'échelle du micro-organisme avec des modèles de différentes complexités. Nous présenterons ensuite nos développements pour étendre ces modèles a l'échelle de la suspension. Nous montrerons comment inclure simultanément les effets Browniens qui agissent sur les plus petite particules (10^-6m). Enfin, nous exploiterons l'outil mis en place pour simuler des suspensions actives. Sa capacité à reproduire certains résultats de la littérature à précision égale, à moindre coût et à plus grande échelle, permet de combler le fossé entre modèles individuels, travaux expérimentaux et modèles continus issus de la théorie cinétique. Forts de cet outil, nous tenterons de répondre à deux questions ouvertes dans la littérature expérimentale : l'origine des corrélations d'orientation dans les suspensions de microgouttes auto-propulsées et les mécanismes en jeu dans la diffusion des particules Browniennes dans les suspensions actives. / We have all witnessed the flocking of starlings in the sky and the schools of fish that form in the ocean. This kind of organization of living creatures is not limited to those that we see, but also occurs for those that we don’t : swimming microorganisms. Suspen- sions of micro-swimmers exhibit a rich dynamics. Their behaviors can play an important role in the survival of the group, its development, the balance between species, their trophic strategies and even animal fertility. They can form coherent structures due to collective motion, mix the surrounding fluid or modify its rheological properties. Such diversity results from the complex interplay between swimming strategies, physiological processes, chemical reactions and hydrodynamic interactions. Fluid Mechanics is there- fore essential to understand and master the mechanisms involved in these phenomena. While experimental studies bring out new findings and, sometimes, provide physical ex- planations, modeling remains essential. Yet, including an accurate description of the micro-swimmers in a suspension containing thousands (nay millions) individuals, requires considering a wide range of coupled scales (from one micron 10^−6m to several millimeters 10^−3m). What happens on large scales depends on sophisticated mechanisms occurring two or three orders of magnitude below. Therefore, the multiscale modeling of such phenomena is still a major challenge for the state-of-the-art numerical methods. This thesis aims at providing a contribution in that direction. In a first part, we will show that reproducing swimming mechanisms at the scale of the micro-swimmer can be achieved with various models spanning different levels of complexity. We will then present our developments to incorporate these models in an efficient framework for large scale simulations. We will show how to simultaneously account for the Brownian motion of the smallest particles (10^−6m). Our code reproduces known results from the literature with the same accuracy, but at lower cost and at larger scales, thus bridging a gap between particle-based models, experiments and continuum formulations from kinetic theory. Using the capabilities afforded by our method, we eventually address two open problems in the experimental literature : the origins of orientational correla- tions between interacting self-propelled micro-droplets and the mechanisms at play in the nonlinear enhancement of Brownian particle diffusion in active suspensions.

O estudo do comportamento de um perfil de uma seção aerolástica típica, com Reynolds na faixa de microaeronaves, constitui o principal foco deste trabalho, tomando como objetivo a estimativa de parâmetros do fenômeno de flutter. A pesquisa analisa o escoamento de um fluido sobre um corpo (cilindro e perfil de aerofólio) em estado estacionário e oscilante, em escoamento de velocidade constante, e, especificamente, o fenômeno de flutter. As equações de Navier-Stokes, com termo de força, são resolvidas pelo método da fronteira virtual para modelagem da interface escoamento/estrutura, representada pela geometria de um corpo de geometria complexa. Na discretização das equações governantes foi utilizado o método de diferenças finitas, sobre malhas deslocadas, com avanço temporal das velocidades do escoamento por meio de um esquema de Runge-Kutta de ordem 4. Os códigos computacionais, para as simulações das diretrizes e a lógica de cálculo, foram criados no contexto deste trabalho. A verificação foi feita através do método da solução manufaturada por meio de um problema fictício, que tem uma solução analítica conhecida, e que preenche as condições de contorno implementadas no código. O modelo da fronteira virtual é testado para os casos de escoamento sobre cilindro de base quadrada, cilindro de base circular e perfil de aerofólio tipo NACA0012, com malhas regular e não regular, e para condições estacionária e sob oscilação forçada. Foi estudado o comportamento de formação de vórtices, provocados por escoamento uniforme sobre o perfil de aerofólio, através dos coeficientes de arrasto, sustentação e pressão com visualização por meio da vorticidade e linhas de corrente, para vários ângulos de ataque e oscilação forçada com elevação e rotação em torno de um pivô posicionado no centro geométrico do perfil (50% da corda). Finalmente, é apresentada uma determinação numérica das características aeroelásticas para o perfil de aerofólio NACA0012, em escoamento de número de Reynolds ultra baixo (Re = 1.000), e parâmetros de flutter para um caso de baixa frequência de oscilação. / The behavior study of a profile of a typical aerolastic section, with Reynolds in range of micro aerial vehicle, is the main focus of this work, taking as objective the estimation of parameters of flutter phenomenon. The research analyzes of the flow of a incompressible fluid on a body (cylinder and airfoil profile) at steady state and oscillating with constant speed and, specifically, the flutter phenomenon. The Navier-Stokes equations, with force term, are solved by virtual boundary method for modeling interface flow/structure, represented by the geometry of a body of complex geometry. In discretization of the governing equations, the method of finite differences on staggered grid, with temporal advancement of discharge velocity through a Runge-Kutta of order 4. The computer codes, for simulations guidelines and logic calculation, were created in the context of this work. The verification was done by method ofmanufactured solution through a fictional problem, which has a known analytical solution, and satisfies the boundary conditions implemented in code. The model of the virtual boundary is tested for cases of flow over a square cylinder, circular cylinder and profile of a NACA0012 airfoil type, with regular and non-regular meshes, over stationary and forced oscillation conditions. We studied the behavior of vortex formation, caused by uniform flow over the airfoil profile, by the drag, lift and pressure coefficients with view through the vorticity and streamlines for various attack angles and forced oscillation with plunge and pich around a pivot witch was positioned at the geometric airfoil profile (half chord). Finally, it is presented a numerical determination of aeroelastic characteristics for the NACA0012 airfoil profile, flow under ultra low Reynolds number, and flutter parameters for a case of low oscillation frequency.

O escoamento em torno de corpos tem sido objeto de estudo de muitos pesquisadores e é muito explorado experimental e computacionalmente, devido a sua grande aplicabilidade na engenharia. No entanto, simular computacionalmente este tipo de escoamento requer uma atenção especial ao escolher o tipo malha a ser utilizado. Em muitos casos faz-se necessário o uso de uma malha que se adapte ao contorno do obstáculo, o que pode ocasionar um aumento no esforço computacional. Um maneira de contornar este problema é a utilização do Método das Fronteiras Imersas, que possibilita o uso de malha cartesiana na simulação computacional do escoamento em torno de obstáculos. Isso é possível através da adição de um termo forçante nas equações que modelam o escoamento, e assim as forças que agem sobre o contorno do corpo são transferidas diretamente para a malha. O objetivo deste trabalho de mestrado foi implementar o método das Fronteiras Imersas e simular o escoamento em torno de um cilindro circular em repouso, movimentando-se na mesma direção do escoamento, na direção perpendicular ao escoamento, ou rotacionando em torno do próprio eixo. As simulações computacionais possibilitaram a captura do fenômeno de Atrelagem Síncrona, caracterizado pela sincronia entre a frequência de desprendimento natural de vórtices e a frequência de oscilação do mesmo. O Método das Fronteiras Imersas mostrou um ótimo desempenho quando comparado a resultados experimentais e numéricos encontrados na literatura / The flow around bodies have been studied by many researchers. Both experimental and computational approaches have been extensively explored in researches on flow around bodies and have been applied in many engeneering problems. However, to choose an appropriate type of mesh to perform computational simulations of this type of problem requires special attention. In many cases, it is necessary to use a mesh that is able to conform to the boundary if a given obstacle. The need to perform this adaptation may increase the computational effort. The Immersed Boundary Method enables the use of cartesian meshes to perform computational simulations of flows around obstacles. The idea of this method is to add a forcing term in the equations that model the flow. Thus, the forces applied on the body boundaries are directly transfered to the mesh. The aim of this work was to perform a computational implementation of the Immersed Boundary Method to simulate the flow over a oscilating circular cylinder. This oscilation may be inline with the flow, cross-flow, or rotating. The computational simulations enabled the capture of the lock-in phenomena, which consists of the syncronization between the vortex shedding frequency and the cylinder oscilation frequency. The results obtained from the computational simulations using the Immersed Boundary Method were in good agreement with the numerical and experimental results found in the literature

This study focuses on the investigation of flow behavior at low Reynolds numbers by the experimental and numerical performance testing of micro-channel heat exchangers. An experimental study of the heat transfers and pressure drop of compact heat exchangers with louvered fins and flat tubes was conducted within a low air-side Reynolds number range of 20 < ReLp < 225. Using an existing low-speed wind tunnel, 26 sample heat exchangers of corrugated louver fin type, were tested. New correlations for Colburn j and Fanning friction f factor have been developed in terms of non-dimensional parameters. Within the investigated parameter ranges, it seems that both the j and f factors are better represented by two correlations in two flow regimes (one for ReLp = 20 – 80 and one for ReLp = 80 – 200) than a single regime correlation in the power-law format. The results support the conclusion that airflow and heat transfer at very low Reynolds numbers behaves differently from that at higher Reynolds numbers. The effect of the geometrical parameters on the heat exchanger performance was investigated. The numerical investigation was conducted for further understanding of the flow behavior at the range of experimentally tested Reynolds number. Ten different heat exchanger geometries with varied geometrical parameters obtained for the experimental studies were considered for the numerical investigation. The variations in the louver angle were the basis of the selection. The heat transfer and pressure drop performance was numerically investigated and the effect of the geometrical parameters was evaluated. Numerical results were compared against the experimental results. From the comparison, it is found that the current numerical viscous laminar models do not reflect experimentally observed transitional two regime flow behavior from fin directed to the louver directed at very low Reynolds number ranging from 20 to 200. The flow distribution through the fin and the louver region was quantified in terms of flow efficiency. The flow regime change was observed at very low Reynolds number similar to the experimental observations. However, the effect of two regime flow change does not reflect on the thermal hydraulic performance of numerical models. New correlations for the flow efficiency � have developed in terms of non-dimensional parameters.

Le sujet de cette thèse est l'hydrodynamique de corps minces (feuilles) et élancés (filamenteux) de fluide visqueux immergés dans un second fluide ayant une viscosité différente. Nous nous concentrons sur deux exemples : la subduction de la lithosphère océanique et le flambage de fils visqueux dans microcanaux divergents, les deux ont un nombre de Reynolds caractéristique Re<<1. Pour le cas de la subduction d'une feuille mince, nous proposons une hybride méthode «boundary integral & thin sheet» (BITS). Après la validation en comparant ses prévisions avec celles de la boundary-element méthode, deux solutions instantanées et dépendant du temps sont effectués pour analyser la subduction avec la méthode BITS. L'analyse à l'échelle de la vitesse d'immersion normalisée en fonction de «la rigidité en flexion» de la feuille est confirmée par nos prédictions numériques. Pour des rapports de viscosité modérée (≈100), la feuille amincit sensiblement quand elle coule, mais pas assez pour conduire à la «rupture de la dalle» que l'on observe dans plusieurs zones de subduction sur Terre. Ensuite, le code BLEU parallèle pour écoulements polyphasiques est utilisé à simuler pliage visqueux tridimensionnel dans des microcanaux divergent. Nous avons réalisé une étude paramétrique comprenant cinq simulations dans lequel le rapport de débit volumétrique, le rapport de viscosité, le nombre de Reynolds, et la forme de la chaîne ont été modifiées par rapport à un modèle de référence. Le fil devient instable à une instabilité de pliage en raison de la contrainte de compression longitudinale. L'axe de pliage initial peut être parallèle ou perpendiculaire à la dimension étroite de la chambre. Dans le premier cas, le pliage transforme lentement au pliage perpendiculaire au moyen d'une torsion, ou peut disparaître totalement. / The hydrodynamics of thin (sheet-like) and slender (filamentary) bodies of viscous fluid immersed in a second fluid with a different viscosity is studied. Here we focuses on two examples: the subduction of oceanic lithosphere and the buckling of viscous threads in diverging microchannels, both have a characteristic Reynolds number Re<<1. A hybrid boundary integral & thin sheet method (BITS) is build for the subduction of 2D immersed sheet. After the validation by comparing with the results of full boundary elements method, both instantaneous and time-dependant soloutions are done to analyze the subduction with the BITS method. The scaling analysis of the normalized sinking speed V/V_Stokes as a function of the sheet's 'flexural stiffness' is confirmed by our numerical predictions. For moderate viscosity ratios (≈100), the sheet thins substantially as it sinks, but not enough to lead to the ‘slab breakoff’ that is observed in several subduction zones on Earth. Next, the parallel code BLUE for multi-phases flows is used to simulate the 3-dimensional viscous folding in diverging microchannels. We performed a parameter study comprising five simulations in which the flow rate ratio, the viscosity ratio, the Reynolds number, and the shape of the channel were varied relative to a reference model. The thread becomes unstable to a folding instability due to the longitudinal compressive stress. The initial folding axis can be either parallel or perpendicular to the narrow dimension of the chamber. In the former case, the folding slowly transforms via twisting to perpendicular folding , or may disappear totally.

Small-scale wind turbines were not considered viable in the past due to their poor efficiencies, mainly because of their aerodynamic effects around the irfoil shape. Recently researchers have renewed interest in enhancing the aerodynamic performances of the blades’ designs inspired by the aerodynamic pattern of biological characteristics of insects and marine mammals such as locusts, dragonflies, damselflies, Humpback Whales etc. Bioinspired wing designs have advantages compared to conventional smooth irfoil blades as they can counter the bending forces that the wings experience during flapping. Bio-inspired corrugated airfoil based on dragonfly wing geometries have been reported to perform well compared to conventional airfoil at low Reynolds numbers. Corrugated airfoils reduce flow separation and enhance aerodynamic performance by trapping vortices in the corrugations thus drawing flow towards the airfoil’s surface. This results in the higher lift whilst incurring only marginally higher drag. Such airfoils also have an advantage when it comes to span-wise structural stiffness due to the corrugated cross-sections. Replacing conventional turbine blades by tubercles or corrugated blades could enhance turbine performance by reducing the pressure gradient along the leading edge; however, the aerodynamic effects at the leading edge will depend on the variations of wavelength and amplitude. In this study, two types of computational studies were investigated: Optimising a corrugated airfoil and investigating the aerodynamic effects of a sinusoidal shape at the leading edge of a blade. Previous studies used an idealized geometry based on the dragonfly wing cross-section profile but did not attempt to optimize the geometry. In the present study: a two-dimensional CFD model is constructed using ANSYS Fluent Workbench-Design Explorer to determine the optimal corrugated blade profile for four angles of attack (AOA) from 5° to 20° corresponding to typical AOA of small-scale wind turbine blades. Two modified blades with variations of wavelength and amplitude at the leading edge were studied to investigate the aerodynamic effects. Three-dimensional models were constructed using Qblade software and 3D points were exported to AutoCAD Inventor to generate the CAD model. The governing equations used are continuity and Navier-Stokes equations written in a frame reference rotating with the blade. The CFD package used is ANSYS FLUENT 19.0. The simulation was run under steady-state, using SST-k omega turbulence model. The modifications have improved the aerodynamic performance. The optimised corrugated blade produced a maximum increase of CL and L/D. Both modified blades (1 and 2) had their performances measured separately and compared to that of baseline blade SG6042 (Conventional blade). Modified blade 1 had a lower wavelength and amplitude at the leading edge of 14.3 % and 4 % respectively of the chord. It was noted that the aerodynamic performance decreased by 6%. Modified model 2, on the other hand had a higher wavelength and amplitude at the leading edge. of 40.4 % and 11.9 % respectively of the chord. It was also noted the aerodynamic performance increased by 6%. From the empirical evidence highlighted above, it can be observed that there is a direct correlation between wavelength, amplitude, and aerodynamic performance of the blade. / Electrical and Mining Engineering / M. Tech. (Engineering)