[pt] No presente trabalho é realizada uma análise do desempenho de três diferentes modelos de turbulência na previsão de escoamentos que apresentam separação e recolamento. Investigou-se o escoamento turbulento através de dutos com obstruções curvilíneas. Os modelos selecionados pertencem a classe de modelos de duas equações diferenciais k-e para baixos números de Reynolds. A análise tem por objetivo verificar se estes modelos são capazes de captar as regiões de recirculação e recuperação de pressão após a obstrução, grandezas fundamentais para a avaliação da distribuição de tensões. Os campos de velocidade, energia cinética turbulenta, taxa de dissipação obtidos pelos diferentes modelos são comparados com dados numéricos e experimentais na literatura, visando identificar qual é o modelo mais adequado para esse tipo de escoamento. Para a determinação numérica do escoamento na geometria com uma obstrução curvilínea, do tipo cossenoidal, utilizou-se o método de volumes finitos com coordenadas curvilíneas não ortogonais que se adaptam à geometria. Os componentes contravariantes da velocidade foram empregadas como variáveis independentes nas equações de conservação de quantidade de movimento e o acoplamento velocidade pressão foi resolvido pelo algoritmo SIMPLEC. Dentre os modelos testados, nenhum conseguiu reproduzir exatamente todos os dados experimentais, porém concluiu-se que o modelo LSH, proposto por Launder e Sharma (1974) com a modificação proposta por Hanjalic e Launder (1980) apresentou o melhor desempenho, o qual pode ser considerado como satisfatório. / [en] In the present work an analysis of the performance of three different turbulence models in the prediction of flows that present separation and reattachment. The turbulent flow was investigated through ducts with curvilinear obstructions. The selected models belong to the class of models with two differential equations k-e for low numbers of Reynolds. The objective of the analysis is to verify if these models are capable of capturing the recirculation areas and pressure recovery after the obstruction, fundamentals for the evaluation of the distribution of tensions. The fields of velocity, turbulent kinetic energy, dissipation rate obtained by the different models are compared with numeric and experiments data found in the literature, seeking to identify which is the model more adapted for those type of flows. The numeric determination of the fluid flows in the geometry with a curvilinear obstruction, described by a sine curve, was accomplished by a finite volume method with non orthogonal curvilinear coordinates which adapt to the geometry. The countervariants velocity components was employed as independent variables in the momentum conservation equation and the velocity-pressure coupling was solved by the SIMPLEC algorithm . Among all the tested models, none was able to reproduce exactly the experimental data. However, the LSH model, proposed by Launder and Sharma (1974) with the modification proposal for Hanjalic and Launder (1980) presented the best performance, which can be considered satisfactory. / [es] En el presente trabajo se analiza el desempeño de tres modelos de turbulencia diferentes para la previsión de flujos con separación y readherencia. Se investigó el flujo turbulento a través de conductos con obstrucciones curvilíneas. Los modelos seleccionados pertenecen a la clase de modelos de dos ecuaciones diferenciales k-y para bajos números de Reynolds. El análisis tiene como objetivo verificar si estos modelos son capaces de captar regiones de recirculación y recuperación de presión después de la obstrucción, cantidades fundamentales para la evaluación de la distribución de tensiones. Se comparan los campos de velocidad, energía cinética turbulenta, tasa de disipación obtenidos por los diferentes modelos con los datos numéricos y experimentales en la literatura, identificando cuál es el modelo más adecuado para ese tipo de flujo. Para la determinación numérica del flujo en la geometría con una obstrucción curvilínea, del tipo cosenoidal, se utilizó el método de volúmenes finitos con coordenadas curvilíneas no ortogonales que se adaptan a la geometría. Los componentes contravariantes de la velocidad fueron utilizados como variables independentes en las ecuaciones de conservación de movimiento y el acoplamiento velocidad-presión fue resuelto a través del algoritmo SIMPLEC. Ninguno de modelos provados consiguió reproducir exactamente todos los datos experimentales, mas se puede concluir que el modelo LSH, propuesto por Launder y Sharma (1974) con la modificación propuesta por Hanjalic y Launder (1980) presentó el mejor desempeño, que puede ser considerado como satisfactorio.

The goal of this project is to use inspiration acquired from bird flight to optimize the wing planform of micro-air vehicle wings. Micro-air vehicles are used by the military for surveillance and for search and rescue missions by civilian first-responders. These vehicles fly in the same low Reynolds number regime as birds, and have low aspect ratios similar to the pheasants and grouse of the order Galliformes. Conventional analysis is difficult for low Reynolds numbers, prompting use of biologically inspired methods of optimization. Genetic algorithms, which mimic the process of evolution in nature, were used to define wing shapes that were tested in wind tunnel experiments. In these experiments, lift-drag ratios at various angles of attack were measured on scale model micro-air vehicle wings (with variable length feathers) similar in shape to a bird wing. The planform shape of the scale model wing evolved in the wind tunnel flow over successive generations to ultimately produce superior wings with higher lift-drag ratios. The low angle of attack wings were easily optimized into a wing shape different from and potentially more efficient than the oft-used Zimmerman planform. The process was repeated for a higher angle of attack, near stall conditions, which yielded a different wing planform shape. Chord distributions of the optimized low angle of attack wings were found to closely match the same distributions of birds from the order Galliformes. Results from flow visualization studies meant to illuminate possible physics responsible for the higher lift-drag ratios were also investigated.

Many cells exploit the bending or rotation of flagellar filaments in order to self-propel in viscous fluids. Often swimming occurs in complex, nonlinear fluids, e.g. mucus. Futhermore even in simple Newtonian fluids, if swimming appendages are deformable then locomotion is subject to fluid-structure interactions. The fundamental question addressed in this thesis is how exactly locomotion is impacted, in particular if it is faster or slower, with or without these effects. First we study locomotion in shear-thinning and viscoelastic fluids with rigid swimming appendages. Following the introductory Chapter, in Chapter 2 we propose empirical extensions of the classical Newtonian resistive-force theory to model the waving of slender filaments in non-Newtonian fluids, based on experimental measurements for the motion of rigid rods in non-Newtonian fluids and on the Carreau fluid model. We then use our models to address waving locomotion in shear-thinning fluids, and show that the resulting swimming speeds are systematically lowered a result which we are able to capture asymptotically and to interpret physically. In Chapter 3 we consider swimming using small-amplitude periodic waves in a viscoelastic fluid described by the Oldroyd-B constitutive relationship. Using Taylor’s swimming sheet model, we show that if all travelling waves move in the same direction, the locomotion speed of the organism is systematically decreased. However, if we allow waves to travel in two opposite directions, we show that this can lead to enhancement of the swimming speed, which is physically interpreted as due to asymmetric viscoelastic damping of waves with different frequencies. A change of the swimming direction is also possible. Secondly we consider the affect of fluid-structure interactions. In Chapter 4, we use Taylor’s swimming sheet model to describe an active swimmer immersed in an Oldroyd-B fluid. We solve for the shape of an active swimmer as a balance between the external fluid stresses, the internal driving moments, and the passive elastic resistance. We show that this dynamic balance leads to a generic transition from hindered rigid swimming to enhanced flexible locomotion. The results are physically interpreted as due to a viscoelastic suction increasing the swimming amplitude in a non-Newtonian fluid and overcoming viscoelastic damping. In Chapter 5 we consider peritrichously flagellated bacteria, such as Escherichia coli. The rotation of each motor is transmitted to a flexible rod called the hook which in turns transmits it to a helical filament, leading to swimming. The motors are randomly distributed over the body of the organism, and thus one expects the propulsive forces from the filament to almost cancel out leading to negligible swimming. We show that the transition to swimming is an elasto-hydrodynamic instability arising when the flexibility of the hook is below a critical threshold.

L'utilisation de micronageurs hélicoidaux capables de se mouvoir dans des liquides à faible nombre de Reynolds trouve son intérêt dans beaucoup d'applications: de tâches in-vitro dans des laboratoires sur puce (transport et tri de micro-objets; assemblage de micro-composants...), à des applications in-vivo en médecine mini-invasive (livraison interne et ciblée de médicaments, curiethérapie, thermothérapie...); grâce à leur dimensions microscopiques et agilité permettant l'accès à des endroits normalement très restreints. Plusieurs types de nageurs hélicoïdaux actionnés magnétiquement possédant divers paramètres géométriques, formes de tête et positions de la partie magnétique ont été proposés dans de précédents travaux. Cependant, l'influence de tous ces paramètres n'a pas clairement été étudiée. À notre connaissance, les micronageurs hélicoïdaux dans l'état de l'art sont principalement contrôlés en boucle ouverte, en raison de la complexité de la commande du champ magnétique actionnant la propulsion, et du nombre limité de retours ayant des critères satisfaisants. Cette thèse vise à comparer les performances de déplacement de nageurs hélicoïdaux avec des conceptions différentes a n d'améliorer leur design et de les caractériser et réaliser un asservissement visuel de nageur hélicoïdal. Pour se faire, des nageurs hélicoïdaux de tailles millimétriques ont été conçus et sont mis en conditions à faible nombre de Reynolds. La conception de ces "millinageurs" servira de base à la conception de micronageurs. Une commande boucle fermée par retour visuel de l'orientation d'un micronageur hélicoïdal dans un espace 3D, et un suivi de trajectoires sur plan horizontal ont été effectués. Cette méthode de commande sera par la suite appliquée à des micronageurs hélicoïdaux. / Helical microswimmers capable of propulsion at low Reynolds numbers have been proposed for numerous applications, ranging from in vitro tasks on lab-on-a-chip (e.g. transporting and sorting micro objects; mechanical components micro assembly...) to in vivo applications for minimally invasive medicine (e.g. targeted drug delivery; brachytherapy; hyperthermia...), due to their micro sizes and accessibility to tiny and clustered environments. Several kinds of magnetically actuated helical swimmers with di erent geometry parameters, head shapes, and magnetic positioning have been proposed in prior works. However, the in uence of the geometry parameters, the head shape and the magnetic positioning (head, coated tail...) has not been clearly studied. As far as we know, the existing helical microswimmers are primarily open-loop controlled, due to the complexity of the control of the magnetic eld actuating the helical propulsion, and the limited number of feedback options processing the required precision. This thesis aims to compare the swimming performances of helical swimmers with di erent designs to further improve their design and to characterize their swimming properties and realize a visual servo control of helical swimmers. Scaled-up helical microswimmers at the millimeter scale are designed and swim at low Reynolds numbers. The design of these scaled-up helical microswimmers can be a guideline for the micro-fabrication of helical microswimmers. A visual servo control of the scaled-up helical microswimmer orientation in the 3D space, and a path following on the horizontal plane have been realized. The control method will be applied on helical microswimmers in future works.

Un micro-drone à voilure tournante est l’appareil aérien optimal pour assister un rover d’exploration à lanavigation sur la planète Mars. Toutefois, les écoulements qu’il rencontre sont compressibles à très faiblenombre de Reynolds, ce qui constitue un domaine de l’aérodynamique inédit et quasiment inexploré à cejour. L’objectif de la thèse est de comprendre, simuler et recréer expérimentalement les phénomènes aérodynamiquesliés au régime inédit des écoulements martiens pour concevoir un système propulsif performant.Après avoir validé les outils de simulation numérique, le comportement instationnaire des écoulements estétudié sur des géométries 2D et 3D. L’écoulement est dominé par la viscosité : les couches limites laminairessont épaisses et le décollement a beaucoup d’influence sur son comportement très instationnaire.Par la suite, plusieurs millions de géométries de profil sont évaluées par un processus d’optimisation basésur un code 2D stationnaire. Les profils optimisés sont fortement cambrés (entre 5.5% et 7%) et de faibleépaisseur relative (e/c ∼ 2%). Le bord d’attaque et le bord de fuite sont très cambrés pour permettrerespectivement l’adaptation à l’écoulement incident et la fixation du point de décollement de la couchelimite. À partir des géométries de profils, l’ensemble du système propulsif est optimisé par intégration despolaires 2D. La théorie des éléments de pale permet de déterminer rapidement les configurations les plusperformantes aérodynamiquement. Et une méthode de sillage libre permet l’optimisation de rotors isoléset de systèmes propulsifs coaxiaux. Les rotors ont des solidités et des vrillages importants, ce qui rappelleles formes d’hélices marines. Les simulations Navier-Stokes 3D mettent en évidence la tridimensionnalitédes écoulements sur la pale, elle est fortement corrélée avec la solidité du rotor et le vrillage de bout depale. La rotation stabilise la couche limite et donne lieu à un décollement stable au bord d’attaque pourcertaines géométries. Le dévrillage en bout de pale permet de stabiliser le tourbillon et de diminuer la perteinduite. Un banc de mesure est placé dans un caisson dépressurisé pour estimer les efforts de poussée et decouple générés par les rotors optimisés en conditions aérodynamiques martiennes. Les essais permettentde valider les tendances d’estimation des codes de simulation ainsi que les processus d’optimisation. Laconfiguration bi-rotors coaxiaux, en comparaison avec une configuration à deux rotors adjacents, permetun gain d’encombrement de moitié pour une perte sur la puissance de seulement 15%. C’est la configurationla plus adaptée pour un micro-drone en atmosphère ténue. Un système propulsif coaxial optimisé (dediamètre 30 cm) permettrait de sustenter un micro-drone d’environ 400 grammes en conditions nominalessur la planète Mars. / A micro-rotorcraft is the most suited aerial vehicle for rover navigation assistance on Mars. The martianatmosphere’s density, being hundred times lower than on Earth, requires the micro-drone to hover at highrotational speed. Hence, flows on the blade are both compressible and at very low Reynolds number (fewthousands). It constitutes a new aerodynamic domain to be explored. The purpose of the dissertation isto understand, simulate and experimentally duplicate the aerodynamic phenomena in a view to design anefficient propulsion system. After a phase of validation of the simulation tools, the flows’ unsteady behavioris studied on 2D and 3D geometries. Wall flow is highly viscous : laminar boundary layers are thick andtheir separation has a huge influence on its unsteadiness. Then, several millions of airfoil geometries areevaluated by an optimization process based on a steady 2D solver. As final result, the optimized airfoildisplays a highly cambered shape (between 5.5% and 7%) with low relative thickness (t/c ∼ 2%). Leadingand trailing edges are strongly cambered, allowing proper incoming flow adaptation and late boundarylayer separation. Based on this airfoil geometry, rotor shapes are optimized by two methods. Blade elementtheory provides a quick investigation of the most aerodynamically efficient configurations. And a free wakesolver is applied for the final design of isolated rotors and coaxial dual-rotors. Optimized shapes exhibitimportant twist and solidity, evoking marine propellers. 3D Navier-Stokes simulations highlight the flow’sthree-dimensional mechanisms on the blade, which are highly correlated to the rotor’s solidity and twist.Rotation stabilizes the blade’s boundary layer and a stable leading edge separation is observed in somecases. Blade tip twist reduction diminishes the tip vortex and the induced loss. A thrust and torquemeasurement setup is placed in a depressurized tank for the evaluation of optimized rotors in martianatmospheric conditions. Performed experiments confirm the solvers’ trend and validate the design process.Therefore, chosen coaxial dual-rotors configuration provides an important size gain for a low correspondingpower loss (∼ 15%) compared to adjacent dual-rotors. Coaxial dual-rotors are the optimal configuration fora micro-rotorcraft in low-pressure atmosphere flight conditions. Such propulsion system (with a diameterof 30 cm) could lift a 400 grams micro-rotorcraft in hover on the planet Mars.

The environmental and monetary cost of energy has renewed interest in horizontal-axis wind turbines (HAWT). One problem with HAWT design is turbulent winds, which cause cyclic loading and reduced life. Controlling short-term aerodynamic fluctuations with blade pitching or mechanical flaps is limited by the speed of actuation. The objective was to investigate using jet-flap-like fluidic actuators on the 'suction surface' of an aerofoil for rapid aerodynamic control. A NACA 0025 aerofoil was constructed for wind-tunnel experiments. The low Reynolds number (Re) flow was measured non-intrusively with particle image velocimetry (PIV). The jet showed limited effect compared to published work. The sharp trailing edge and distance to the jet were determined to be critical factors. At Re≈100000 the 'suction surface' jet sheet is less useful for control than the conventional 'pressure surface' sheet. The experiment suggests usage near the blade root on truncated aerofoils.

The environmental and monetary cost of energy has renewed interest in horizontal-axis wind turbines (HAWT). One problem with HAWT design is turbulent winds, which cause cyclic loading and reduced life. Controlling short-term aerodynamic fluctuations with blade pitching or mechanical flaps is limited by the speed of actuation. The objective was to investigate using jet-flap-like fluidic actuators on the 'suction surface' of an aerofoil for rapid aerodynamic control. A NACA 0025 aerofoil was constructed for wind-tunnel experiments. The low Reynolds number (Re) flow was measured non-intrusively with particle image velocimetry (PIV). The jet showed limited effect compared to published work. The sharp trailing edge and distance to the jet were determined to be critical factors. At Re≈100000 the 'suction surface' jet sheet is less useful for control than the conventional 'pressure surface' sheet. The experiment suggests usage near the blade root on truncated aerofoils.

The aerodynamics of two-dimensional and three-dimensional flapping motion in hover is analyzed in incompressible, laminar flow at low Reynolds number regime. The aim of this study is to understand the physics and the underlying mechanisms of the flapping motion using both numerical tools (Direct Numerical Simulation) and experimental tools (Particle Image Velocimetry PIV technique). Numerical analyses cover both two-dimensional and three-dimensional configurations for different parameters using two different flow solvers. The obtained results are then analyzed in terms of aerodynamic force coefficients and vortex dynamics. Both symmetric and cambered airfoil sections are investigated at different starting angle of attacks. Both numerical and experimental simulations are carried out at Reynolds number 1000. The experimental analysis is carried out using Particle Image Velocimetry (PIV) technique in parallel with the numerical tools. Experimental measurements are taken for both two-dimensional and three-dimensional wing configurations using stereoscopic PIV technique.

This work is focused on the aerodynamics for a proprietary laminar flow airfoil for Unmanned Aerial Vehicle (UAV) applications. The two main focuses are (1) aerodynamic performance at Reynolds number on the order of 10,000, (2) the effect of a conventional hot-wire probe on laminar separation bubbles. For aerodynamic performance, pressure and wake velocity distributions were measured at Re = 40,000 and 60,000 for a range of angles of attack. The airfoil performed poorly for these Reynolds numbers due to laminar boundary layer separation. 2-D boundary layer trips significantly improved the lift-to-drag ratio. For probe effects, three Reynolds numbers were investigated (Re = 100,000, 150,000, and 200,000), with three angles of attack for each. Pressure and surface shear distributions were measured. Flow upstream of the probe tip was not affected. Transition was promoted downstream due to the additional disturbances in the separated shear layer.

This work is focused on the aerodynamics for a proprietary laminar flow airfoil for Unmanned Aerial Vehicle (UAV) applications. The two main focuses are (1) aerodynamic performance at Reynolds number on the order of 10,000, (2) the effect of a conventional hot-wire probe on laminar separation bubbles. For aerodynamic performance, pressure and wake velocity distributions were measured at Re = 40,000 and 60,000 for a range of angles of attack. The airfoil performed poorly for these Reynolds numbers due to laminar boundary layer separation. 2-D boundary layer trips significantly improved the lift-to-drag ratio. For probe effects, three Reynolds numbers were investigated (Re = 100,000, 150,000, and 200,000), with three angles of attack for each. Pressure and surface shear distributions were measured. Flow upstream of the probe tip was not affected. Transition was promoted downstream due to the additional disturbances in the separated shear layer.