Membrane wings are used both in nature and small aircraft as lifting surfaces. For these low Reynolds number applications, separated flows are common and are the main sources of unsteadiness. Adaptability of the membrane wing is known to improve the vehicle performance; and membrane compliancy in animal wings such as bats contributes significantly to their astonishing flights. Yet, the aerodynamic characteristics of the membranes are still largely unknown. An experimental study of flexible membranes at low Reynolds numbers was undertaken. Two-dimensional membrane aerofoils were investigated, with particular focus on the unsteady aspects. Membrane deformation, flow fields and fluid-structure interaction were examined over a range of angles of attack and freestream velocities. A comprehensive study of the effect of membrane pre-strain and excess length was carried out. Low aspect ratio membrane wings were investigated for rectangular and nonslender delta wings. The amplitude and mode of membrane vibration are found to be dependent mainly on the location and the unsteadiness level of the shear layer. The results indicate a strong coupling of unsteady flow with the membrane oscillation. With increasing Reynolds number, the separated shear layer becomes more energetic and closer to the surface. The membrane not only has smaller size of the separation region compared to a rigid aerofoil, but also excites the roll-up of large vortices which might lead to delayed stall. The membrane aerofoils with excess length exhibit higher vibration modes, earlier roll-up and smaller separated region, compared to the ones with pre-strain. This smaller separated region delays the onset of membrane vibrations to a larger incidence. For the low aspect ratio membrane wings, the combination of tip vortices and leading-edge vortex shedding results in a mixture of streamwise and spanwise vibrational modes. The flexibility benefits the rectangular wing more than the delta wing by increasing the maximum normal force and the force slope by a larger amount. Similar to the two-dimensional membrane aerofoils, the Strouhal numbers of the oscillations are on the order of unity, and there is a coupling with the wake instabilities in the post-stall region. Stronger tip vortices on membrane wings contribute significantly to total lift enhancement.

Both the need for mixing highly viscous liquids more effectively and the advance of micro-scale applications urge the development of technologies for liquid mixing at low Reynolds numbers. However, currently engineering designs which offer effective jet mixing without structural and operational complexity are still lacking. In this project, the method of enhancing liquid mixing using active pulsation in the laminar flow regime is explored experimentally. This work started by improving the inline pulsation mechanism in an existing confined jet configuration whereby the fluid from a primary planar jet and two surrounding secondary planar jets are pulsated by active fluid injection control via solenoid valves in the out-of-phase mode. The influence of Reynolds number, pulsation modes, frequency, duty cycle on mixing is then investigated using PLIF and PIV experimental techniques. A combination of different mixing mechanisms is found to be at play, including sequential segmentation, shearing and stretching, vortex entrainment and breakup. At a given net flow Reynolds number, an optimal frequency exists which scales approximately with a Strouhal number (Str=fh/Uj) about 1. This optimal frequency reflects the compromise of the vorticity strength and segmentation length. Furthermore, a lower duty cycle is found to produce a better mixing due to a resultant higher instantaneous Reynolds number in the jet flow. Overall, the improvement of the rig has resulted in an excellent mixing being achieved at a net flow Reynolds number of 166 which is at least order of magnitude lower than in the original rig. In order to achieve fast laminar mixing at even lower Reynolds numbers, the active pulsation mechanism using lateral synthetic jet pairs is designed and tested at a net flow Reynolds number ranging from 2 to 166 at which a good mixing is achieved. The influence of actuation frequency and amplitude, and different jet configuration is evaluated using PLIF and PIV experimental techniques. At the mediate to high Reynolds numbers tested in this study, the interaction and subsequent breakup of vortices play a dominant role in provoking mixing. In contrast, at the lower end of Reynolds numbers the strength of vortex rollup is weakened significantly and as a result folding and shearing of sequential segments provide the main mechanism for mixing. Therefore it is essential to use multiple lateral synthetic jet pairs to achieve good mixing in both mixing channel and synthetic jet cavity at this Reynolds number. It is found that an increase in both the actuation magnitude and frequency improves mixing, thereby the velocity ratio represents the relative strength of the pulsation velocity to the mean flow velocity is crucial for mixing enhancement. In order to identify actuation conditions for good mixing, a regression fit is conducted for the correlation between the dimensionless parameters, net flow Reynolds number Ren, stroke length L and Strouhal number Str. Over the tested range of the net flow Reynolds number from 2 to 83, the relationship of parameters is found and the velocity ratio at least above 2.0. Suggested by the comparatively small exponent, net flow Reynolds number is less influential than stroke length and Strouhal number. The success in obtaining excellent mixing using lateral synthetic jet pairs at low Reynolds numbers in the present work has opened up a promising prospect of their applications in various scenarios, including mixing of highly viscous liquids at macro-scale and micro-mixing.

Cette thèse est consacrée à l'étude de la dynamique et de la rhéologie des fluides complexes. Nous utilisons une méthode de simulation numérique à trois dimensions. Les systèmes que nous étudions ici sont des suspensions de micro-nageurs actifs, des suspensions de particules sphériques rigides en présence d'un champ externe auquel elles sont sensibles et de la dynamique de suspensions de particules sphériques et confinées en cisaillement. Les Micro-nageurs sont les objets microscopiques qui se propulsent dans un fluide et ils sont omniprésents dans la nature. Un exemple commun de micro-nageurs est la micro-algue textit{Chlamydomonas} . Un des buts principaux de cette thèse est de comprendre l'effet de la motilité de ces micro-organismes sur les propriétés macroscopiques globales de la suspension, telles que la viscosité effective pour expliquer les observations expérimentales. Nous avons élaboré différents modèles de suspensions de textit{Chlamydomonas} et effectué des simulations numériques utilisant la version 3D de la dynamique des particules fluides (FPD) (méthode expliquée dans cette thèse). Les résultats de nos simulations numériques ont été présentés et discutés à la lumière des observations expérimentales. Un des modèles proposés intègre tous les phénomènes observés expérimentalement et sont applicables à d'autres types de suspensions de micro-nageurs. Cette thèse consacre également un chapitre sur les effets du confinement sur la dynamique de cisaillement des suspensions diluées de particules sphériques. Nous avons constaté que dans la géométrie confinée, la vitesse angulaire des particules diminue par rapport à celle imposée par l'écoulement de cisaillement. La vitesse angulaire des particules diminue également lorsque la particule est proche d'une paroi unique et la vitesse de translation de la particule par rapport à la vitesse de la paroi diminue. Un autre objectif de cette thèse est d'étudier les suspensions à viscosité effective ajustable. Nous avons mené une étude numérique sur des suspensions de particules sphériques en présence d'un couple externe. Nous avons montré que le changement de vitesse angulaire des particules due à l'application d'un couple externe est suffisante pour modifier fortement la viscosité de la suspension. Basée sur des simulations numériques, une formule semi-empirique a été proposée pour la viscosité des suspensions de particules sphériques valables jusqu'à 40% de concentration. Nous avons également montré que la 2ème loi de Faxén peut être étendue par une expression empirique pour de grandes concentrations. / This work is dedicated to the study of dynamics and rheology of the complex fluids. We use three dimensional numerical simulations. The systems we study here are: suspensions of biological active micro-swimmers, suspensions of rigid spherical particles in presence of an external field and the dynamics of sheared confined spherical particles. Micro-swimmers are the microscopic objects that propel themselves through a fluid and they are ubiquitous in nature. A common example of micro-swimmers is the textit{Chlamydomonas} . One of the main goal of this thesis is to understand the effect of self-motility of these micro-organisms on the global macroscopic properties of the fluid, such as the effective viscosity to explain experimental observations. We elaborated different models for textit{Chlamydomonas} suspensions and conducted numerical simulations using the 3D version of the Fluid Particle Dynamics method (explained in this thesis). The results of our numerical simulations has been shown and discussed in light of the experimental observations. One of the proposed models incorporates all experimentally observed phenomena and is expendable for other types of micro-swimmer suspensions. This thesis is also dealing with the effects of confinement on the dynamics of sheared spherical particles. We found that in confined geometry, angular velocity of sheared particles decreases compared to the one imposed by the shear flow. The angular velocity of the particles decreases also when the particle are close to a single wall and the translational velocity of the particles changes so that the difference between velocity of the particle and the velocity of the wall decreases. Another objective of this work is to study suspensions with tunable effective viscosity. We conducted a numerical investigation of sheared spherical particle suspensions in presence of an external torque. We showed that the change of particle angular velocity with an external torque is sufficient to strongly change the effective viscosity of the suspension. Based on numerical simulations, a semi-empirical formula has been proposed for the effective viscosity of spherical particles suspensions valid up to 40% concentration. We also showed that a modified second Faxén law can be equivalently established for large concentrations.

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.

O presente trabalho apresenta uma metodologia de simulação numérica de perfis aerodinâmicos bidimensionais com foco na utilização para o projeto e otimização de pás e rotores de pequenas turbinas eólicas de eixo horizontal, bem como o emprego desses métodos em simulações nas quais efeitos de alta solidez do rotor e baixos números de Reynolds são avaliados. Essa metodologia inclui geração de malhas, seleção de métodos numéricos e validação, tendo as escolhas sido guiadas pelas práticas mais bem sucedidas na simulação de perfis aerodinâmicos, e foi aplicada na simulação dos aerofólios NACA 0012, S809 e SD7062. O código comercial ANSYS Fluent foi utilizado em todas as simulações. Na simulação de aerofólios isolados a altos números de Reynolds dos perfis NACA 0012 e S809, o modelo Transition SST (γ-Reθ) apresentou resultados mais próximos a dados experimentais do que aqueles apresentados pelo modelo k-ω SST para CL e CD, além de produzir resultados para CP que mostraram boa precisão quando comparados aos mesmos dados experimentais. Resultados de CL, CD, CF e CP são apresentados para 20 diferentes condições de operação às quais o perfil SD7062 foi submetido, com números de Reynolds variando entre 25.000 e 125.000. As distribuições dos dois últimos coeficientes sobre os dorsos do aerofólio evidenciam com clareza a presença e magnitude da bolha de separação laminar. Os coeficientes de sustentação e arrasto mostram o impacto negativo da presença da bolha nessa faixa de números de Reynolds. Além disso, nos casos simulados, o arrasto aumenta em função da diminuição do Re. Um design de pá produzido com o auxílio do código de otimização SWRDC, baseado em algoritmos genéticos, é apresentado. Três seções ao longo da envergadura dessa pá foram simuladas em uma bateria de 45 simulações, sob diversas condições de operação em função de solidez, ângulo de ataque e razão de velocidade de ponta de pá. Esses resultados mostram que a bolha de separação laminar se move na direção do bordo de ataque com o aumento da solidez, do ângulo de ataque e da TSR. Além disso, distribuições do CP mostram aumento de pressão em ambos os dorsos do perfil quando submetido aos efeitos da solidez, embora esses efeitos tenham sido responsáveis por um aumento na relação CL/CD nos casos estudados. / This thesis presents a methodology of two-dimensional airfoil simulation focusing on its application on the design and optimization of blades and rotors of small horizontal axis wind turbines, and its application in a set of numerical simulations involving high rotor solidity and low-Re effects. This methodology includes grid generation, selection of numerical methods and validation, reflecting the most successful practices in airfoil simulation, and was applied in the simulation of the NACA 0012, S809 and SD7062 airfoils. The ANSYS Fluent commercial code was used in all simulations. Results for the isolated NACA 0012 and S809 airfoils at high Reynolds numbers show that the Transition SST (γ-Reθ) turbulence model produces results closer to experimental data than those yielded by the SST k-ω model for CL and CD, having also produced CP plots that show good agreement to the same experimental data. Plots of CL, CD, CF and CP for the SD7062 airfoil are presented, for simulations at 20 different operating conditions. The CF and CP distributions evidence the negative impact of the laminar separation bubble in the range of Reynolds numbers evaluated. Results show that, for Re between 25,000 and 125,000, drag increases with decreasing Re. A blade design generated using the SWRDC optimization code, based on genetic algorithms, is presented. Three sections of the resulting blade shape were selected and were tested in a set of 45 simulations, under an array of operating conditions defined by solidity, angle of attack and TSR. Results show that the laminar separation bubble moves towards the leading edge with increasing solidity, angle of attack and TSR. Furthermore, CP plots show an increase in pressure on both surfaces when the airfoil is subject to solidity effects, although these effects show an increase in the lift-to-drag ratio at the conditions evaluated.

At low Reynolds number flows, the effect of inertia becomes negligible and the fluid motion is dominated by the effect of viscous forces. Understanding of the behaviour of low Reynolds number flows underpins the prediction of the motion of microorganisms and particle sedimentation as well as the development of micro-robots that could potentially swim inside the human body to perform targeted drug/cell delivery and non-invasive microsurgery. The work in this thesis focuses on developing an understanding in the mathematical analysis of objects moving at low Reynolds numbers. A boundary element implementation of the Method of regularized Stokeslets (MRS) is applied to analyse the low Reynolds number flow field around an object of simple shape (sphere and cube). It also showed that the results obtained by a boundary element implementation for an unbounded cube, where singularities are presented in the corners of the cube, agrees with more complex solutions methods such as a GBEM and FEM.A methodology for analysing the effect of walls by locating collocation points on the surface of the walls and the object is presented. First at all, this methodology is validated with a boundary element implementation of the method of images for a sphere at different locations. Then, the method is extended when more than one wall is presented. This methodology is applied to predict the velocity filed of a cube moving in a tow tank at low Reynolds numbers for two different cases with a supporting rod similar to an experimental set-up, and without the supporting rod as in the CFD simulations based on the FVM. The results indicate a good match between CFD and the MRS, and an excellent approximation between the MRS and experimental data from PIV measurements. The drag, thrust and torque generated by helices moving at low Reynolds numbers in an unbounded medium is analysed by the resistive force theory, a slender body theory, and a boundary element method of the MRS. The results show that the resistive force theory predict accurately the drag, thrust and torque of moving helices when the resistive force coefficients are calculated from a slender body theory approximation by calculating independently the resistive force coefficients for translation and rotation, because it is observed that the resistive force coefficients depend also of the nature of motion. Moreover, the thrust generated by helices of different pitch angles is analysed calculated by a CFD numerical simulation based on the FVM and a boundary element implementation, an compared with experimental data. The results also show an excellent prediction between the boundary element implementation, the CFD results and the experimental data. Finally, a boundary element implementation of the MRS is applied to predict swimming of a biomimetic swimmer that mimics the motion of E.coli bacteria in an unbounded medium. The results are compared with the propulsive velocity and induced angular velocity measurement by recording the motion of the biomimetic swimmer in a square tank. It is observed that special care needs to be taken when the biomimetic swimmer is modelled inside the tank, as there is an apparent increment in the calculate thrust propulsion which does not represent a real situation of the biometic swimmer which propels by a power supply. However, this increment does not represent the condition of the biomimetic swimmer and a suggested methodology based on the solution from an unbounded case and when the swimmer is moving inside the tank is presented. In addition, the prediction of the free-swimming velocity for the biomimetic swimmer agrees with the results obtained by the MRS when the resistive force coefficients are calculated from a SBT implementation. The results obtained in this work have showed that a boundary element implementation of the MRS produces results comparable with more complex numerical implementations such as GBEM, FEM, FVM, and also an excellent agreement with results obtained from experimentation. Therefore, it is a suitable and easy to apply methodology to analyse the motion of swimmers at low Reynolds numbers, such as the biomimetic swimmer modelled in this work.

O presente trabalho apresenta uma metodologia de simulação numérica de perfis aerodinâmicos bidimensionais com foco na utilização para o projeto e otimização de pás e rotores de pequenas turbinas eólicas de eixo horizontal, bem como o emprego desses métodos em simulações nas quais efeitos de alta solidez do rotor e baixos números de Reynolds são avaliados. Essa metodologia inclui geração de malhas, seleção de métodos numéricos e validação, tendo as escolhas sido guiadas pelas práticas mais bem sucedidas na simulação de perfis aerodinâmicos, e foi aplicada na simulação dos aerofólios NACA 0012, S809 e SD7062. O código comercial ANSYS Fluent foi utilizado em todas as simulações. Na simulação de aerofólios isolados a altos números de Reynolds dos perfis NACA 0012 e S809, o modelo Transition SST (γ-Reθ) apresentou resultados mais próximos a dados experimentais do que aqueles apresentados pelo modelo k-ω SST para CL e CD, além de produzir resultados para CP que mostraram boa precisão quando comparados aos mesmos dados experimentais. Resultados de CL, CD, CF e CP são apresentados para 20 diferentes condições de operação às quais o perfil SD7062 foi submetido, com números de Reynolds variando entre 25.000 e 125.000. As distribuições dos dois últimos coeficientes sobre os dorsos do aerofólio evidenciam com clareza a presença e magnitude da bolha de separação laminar. Os coeficientes de sustentação e arrasto mostram o impacto negativo da presença da bolha nessa faixa de números de Reynolds. Além disso, nos casos simulados, o arrasto aumenta em função da diminuição do Re. Um design de pá produzido com o auxílio do código de otimização SWRDC, baseado em algoritmos genéticos, é apresentado. Três seções ao longo da envergadura dessa pá foram simuladas em uma bateria de 45 simulações, sob diversas condições de operação em função de solidez, ângulo de ataque e razão de velocidade de ponta de pá. Esses resultados mostram que a bolha de separação laminar se move na direção do bordo de ataque com o aumento da solidez, do ângulo de ataque e da TSR. Além disso, distribuições do CP mostram aumento de pressão em ambos os dorsos do perfil quando submetido aos efeitos da solidez, embora esses efeitos tenham sido responsáveis por um aumento na relação CL/CD nos casos estudados. / This thesis presents a methodology of two-dimensional airfoil simulation focusing on its application on the design and optimization of blades and rotors of small horizontal axis wind turbines, and its application in a set of numerical simulations involving high rotor solidity and low-Re effects. This methodology includes grid generation, selection of numerical methods and validation, reflecting the most successful practices in airfoil simulation, and was applied in the simulation of the NACA 0012, S809 and SD7062 airfoils. The ANSYS Fluent commercial code was used in all simulations. Results for the isolated NACA 0012 and S809 airfoils at high Reynolds numbers show that the Transition SST (γ-Reθ) turbulence model produces results closer to experimental data than those yielded by the SST k-ω model for CL and CD, having also produced CP plots that show good agreement to the same experimental data. Plots of CL, CD, CF and CP for the SD7062 airfoil are presented, for simulations at 20 different operating conditions. The CF and CP distributions evidence the negative impact of the laminar separation bubble in the range of Reynolds numbers evaluated. Results show that, for Re between 25,000 and 125,000, drag increases with decreasing Re. A blade design generated using the SWRDC optimization code, based on genetic algorithms, is presented. Three sections of the resulting blade shape were selected and were tested in a set of 45 simulations, under an array of operating conditions defined by solidity, angle of attack and TSR. Results show that the laminar separation bubble moves towards the leading edge with increasing solidity, angle of attack and TSR. Furthermore, CP plots show an increase in pressure on both surfaces when the airfoil is subject to solidity effects, although these effects show an increase in the lift-to-drag ratio at the conditions evaluated.

O presente trabalho apresenta uma metodologia de simulação numérica de perfis aerodinâmicos bidimensionais com foco na utilização para o projeto e otimização de pás e rotores de pequenas turbinas eólicas de eixo horizontal, bem como o emprego desses métodos em simulações nas quais efeitos de alta solidez do rotor e baixos números de Reynolds são avaliados. Essa metodologia inclui geração de malhas, seleção de métodos numéricos e validação, tendo as escolhas sido guiadas pelas práticas mais bem sucedidas na simulação de perfis aerodinâmicos, e foi aplicada na simulação dos aerofólios NACA 0012, S809 e SD7062. O código comercial ANSYS Fluent foi utilizado em todas as simulações. Na simulação de aerofólios isolados a altos números de Reynolds dos perfis NACA 0012 e S809, o modelo Transition SST (γ-Reθ) apresentou resultados mais próximos a dados experimentais do que aqueles apresentados pelo modelo k-ω SST para CL e CD, além de produzir resultados para CP que mostraram boa precisão quando comparados aos mesmos dados experimentais. Resultados de CL, CD, CF e CP são apresentados para 20 diferentes condições de operação às quais o perfil SD7062 foi submetido, com números de Reynolds variando entre 25.000 e 125.000. As distribuições dos dois últimos coeficientes sobre os dorsos do aerofólio evidenciam com clareza a presença e magnitude da bolha de separação laminar. Os coeficientes de sustentação e arrasto mostram o impacto negativo da presença da bolha nessa faixa de números de Reynolds. Além disso, nos casos simulados, o arrasto aumenta em função da diminuição do Re. Um design de pá produzido com o auxílio do código de otimização SWRDC, baseado em algoritmos genéticos, é apresentado. Três seções ao longo da envergadura dessa pá foram simuladas em uma bateria de 45 simulações, sob diversas condições de operação em função de solidez, ângulo de ataque e razão de velocidade de ponta de pá. Esses resultados mostram que a bolha de separação laminar se move na direção do bordo de ataque com o aumento da solidez, do ângulo de ataque e da TSR. Além disso, distribuições do CP mostram aumento de pressão em ambos os dorsos do perfil quando submetido aos efeitos da solidez, embora esses efeitos tenham sido responsáveis por um aumento na relação CL/CD nos casos estudados. / This thesis presents a methodology of two-dimensional airfoil simulation focusing on its application on the design and optimization of blades and rotors of small horizontal axis wind turbines, and its application in a set of numerical simulations involving high rotor solidity and low-Re effects. This methodology includes grid generation, selection of numerical methods and validation, reflecting the most successful practices in airfoil simulation, and was applied in the simulation of the NACA 0012, S809 and SD7062 airfoils. The ANSYS Fluent commercial code was used in all simulations. Results for the isolated NACA 0012 and S809 airfoils at high Reynolds numbers show that the Transition SST (γ-Reθ) turbulence model produces results closer to experimental data than those yielded by the SST k-ω model for CL and CD, having also produced CP plots that show good agreement to the same experimental data. Plots of CL, CD, CF and CP for the SD7062 airfoil are presented, for simulations at 20 different operating conditions. The CF and CP distributions evidence the negative impact of the laminar separation bubble in the range of Reynolds numbers evaluated. Results show that, for Re between 25,000 and 125,000, drag increases with decreasing Re. A blade design generated using the SWRDC optimization code, based on genetic algorithms, is presented. Three sections of the resulting blade shape were selected and were tested in a set of 45 simulations, under an array of operating conditions defined by solidity, angle of attack and TSR. Results show that the laminar separation bubble moves towards the leading edge with increasing solidity, angle of attack and TSR. Furthermore, CP plots show an increase in pressure on both surfaces when the airfoil is subject to solidity effects, although these effects show an increase in the lift-to-drag ratio at the conditions evaluated.

Highly-loaded turbomachine blades can stall under off-design conditions. In this regime, the flow can separate close to the leading edge of the blade in a periodic manner that can lead to blade vibrations, commonly referred to as stall flutter. Prior experimental studies on stall flutter have been at large Re (Re ~ 106). In the present work, motivated by applications in Unmanned Air Vehicles (UAV) and Micro Air Vehicles (MAV), we study experimentally the forces and flow fields around an oscillating blade at low Re (Re ~ 3 x 104). At these low Re, the flow even over the stationary blade can be quite different. We experimentally study the propensity of an isolated symmetric and cambered blade (with chord c) to undergo self-excited oscillations at high angles of attack and at low Reynolds numbers (Re ~ 30, 000). We force the blade, placed at large mean angle of attack, to undergo small amplitude pitch oscillations and measure the unsteady loads on the blade. From the measured loads, the direction and magnitude of energy transfer to/from the blade is calculated. Systematic measurements have been made for varying mean blade incidence angles and for different excitation amplitudes and frequencies (f). These measurements indicate that post stall there is a possibility of excitation of the blade over a range of Strouhal Numbers (St = fc/U) with the magnitude of the exciting energy varying with amplitude, frequency and mean incidence angles. In particular, the curves for the magnitude of the exciting energy against Strouhal number (St) are found to shift to higher St values as the mean angle of attack is increased. We perform the same set of experiments on two different blade shapes, namely NACA 0012 and a compressor blade profile, SC10. Both blade profiles show qualitatively similar phenomena. The flow around both the stationary and oscillating blades is studied through Particle Image Velocimetry (PIV). PIV measurements on the stationary blade show the gradual shift of the flow separation point towards the leading edge with increasing angle of attack, which occurs at these low Re. From PIV measurements on an oscillating blade near stall, we present the flow field around the blade at different phases of the blade oscillation. These show that the boundary layer separates from the leading edge forming a shear layer, which flaps with respect to the blade. As the Strouhal number is varied, the phase between the flapping shear layer and the blade appears to change. This is likely to be the reason for the observed change in the sign of the energy transfer between the flow and the blade that is responsible for stall flutter.