No description available.

There is currently growing interest in the field of Micro Air Vehicles (MAVs). A MAV is characterized by its low Reynolds numbers flight regime which makes lift and thrust creation a significant challenge. One possible solution inspired by nature is flapping flight, but instead of the large-amplitude low-frequency motion suited to the muscular actuators of nature, small-amplitude high-frequency motion may be more suitable for electrical actuators. In this thesis the effect of small-amplitude high-frequency motion is experimentally investigated focusing on three aspects: general performance improvement, deflected jets, and the effect of geometryResults presented herein demonstrate that using small-amplitude high-frequency plunging motion on a NACA 0012 airfoil at a post-stall angle of attack of 15° can lead to significant thrust production accompanying a 305% increase in lift coefficient. At low Strouhal numbers vortices form at the leading-edge during the downward motion and then convect into the wake. This ‘mode 1’ flow field is associated with high lift but low thrust. The maximum lift enhancement was due to resonance with the natural shedding frequency, its harmonics and subharmonics. At higher Strouhal numbers the vortex remains over the leading-edge area for a larger portion of the cycle and therefore loses its coherency through impingement with the upward moving airfoil. This ‘mode 2’ flowfield is associated with low lift and high thrust. At angles of attack below 12.5° very large force bifurcations are observed. These are associated with the formation of upwards or downwards deflected jets with the direction determined by initial conditions. The upwards deflected jet is associated with the counter-clockwise Trailing Edge Vortex (TEV) loitering over the airfoil and thereby pairing with the clockwise TEV to form a dipole that convects upwards. It therefore draws fluid from the upper surface enhancing the upper surface vortex leading to high lift. The downwards deflected jet is associated with the inverse. Deflected jets were not observed at larger angles of attack as the asymmetry in the strength of the TEVs was too great; nor at smaller amplitudes as the TEV strength was insufficient. To understand the effect of geometry comparable experiments were performed for a flat plate geometry. At zero degrees angle of attack deflected jets would form, as for the NACA 0012 airfoil, however their direction would switch sinusoidally with a period on the order of 100 cycles. The lift coefficient therefore also switched. At 15° angle of attack for Strouhal numbers up to unity the performance of the flat plate was comparable to the NACA 0012 airfoil. Above unity, the upper surface and lower surface leading-edge vortices form a dipole which convects away from the upper surface resulting in increased time-averaged separation and reduced lift.

The present thesis aims at shedding some light for future applications of &amp / #956 / AVs by investigating the hovering mode of flight by flapping motion. In this study, a detailed numerical investigation is performed to investigate the effect of some geometrical parameters, such as the airfoil profile shapes, thickness and camber distributions and as well as the flapping motion kinematics on the aerodynamic force coefficients and vortex formation mechanisms at low Reynolds number. The numerical analysis tool is a DNS code using the moving grid option. Laminar Navier-Stokes computations are done for flapping motion using the prescribed kinematics in the Reynolds number range of 101-103. The flow field for flapping hover flight is investigated for elliptic profiles having thicknesses of 12%, 9% and 1% of their chord lengths and compared with those of NACA 0009, NACA 0012 and SD 7003 airfoil profiles all having chord lengths of 0.01m for numerical computations. Computed aerodynamic force coefficients are compared for these profiles having different centers of rotation and angles of attack. NACA profiles have slightly higher lift coefficients than the ellipses of the same t/c ratio. And one of the most important conclusions is that the use of elliptic and NACA profiles with 9% and 12% thicknesses do not differ much as far as the aerodynamic force coefficients is concerned for this Re number regime. Also, two different sinusoidal flapping motions are analyzed. Force coefficients and vorticity contours obtained from the experiments in the literature and present study are compared. The validation of the present computational results with the experimental results available in the literature encourages us to conclude that present numerical method can be a reliable alternative to experimental techniques.

Flying insects display remarkable maneuverability. Unlike typical airplanes, these insects are able to execute an evasive action, rapidly change their flight speed and direction, or leisurely land on flowers buffeted by wind, exhibiting aerodynamic feats unmatched by any state-of-the-art aircraft. By subtly tuning their wing motions, they generate and manipulate unsteady aerodynamic phenomenon that is the basis of their extraordinary maneuverability. Inspired by these tiny animals, scientists and engineers have pushed the boundaries of technology in many aspects, including meso-scale fabrication, electronics, and artificial intelligence, to develop autonomous millimeter-scale flapping-wing robots. In this thesis, we demonstrate, on real insect-scale robots, that using only an approximate model of the aerodynamics and flight dynamics in combination with conventional tools in nonlinear control, the inherently unstable flapping-wing robot can achieve steady hover. We present the development of flight controllers that gradually enhance the flight precision, allowing the robot to realize increasingly aggressive trajectories, including a highly acrobatic maneuver---perching on a vertical surface, as observed in its natural counterparts. We also demonstrate that these experiments lead to higher fidelity of in-flight aerodynamic models, strengthening our understanding of the dynamics of the robot and real insects. / Engineering and Applied Sciences

Analytical models of avian flight have previously been used to predict mechanical and metabolic power consumption during cruise. These models are limited, in that they neglect details of wing kinematics, and model power by assuming a fixed or rotary wing (actuator disk) weight support mechanism. Theoretical methods that incorporate wing kinematics potentially offer more accurate predictions of power consumption by calculating instantaneous aerodynamic loads on the wing. However, the success of these models inherently depends on the availability and accuracy of experimental kinematic data. The predictive simulation approach offers an alternative strategy, whereby kinematics are neither neglected nor measured experimentally, but calculated as part of the solution procedure. This thesis describes the development of a predictive tool for simulating avian wingbeat kinematics and wakes. The tool is designed in a modular format, in order to be extensible for future research in the biomechanics community. The primary simulation module is an inverse dynamic avian wing model that predicts aerodynamic forces and mechanical power consumption for given wing kinematics. The model is constructed from previous experimental studies of avian wing biomechanics. Wing motion is defined through joint kinematic time histories, and aerodynamic forces are predicted using blade element momentum theory. Mechanical power consumption at the shoulder joint is derived from both aerodynamic and inertial torque components associated with the shoulder joint rotation rate. An optimisation module is developed to determine wing kinematics that generate aerodynamic loads for propulsion and weight support in given flight conditions, while minimising mechanical power consumption. For minimum power cruise, optimisation reveals numerous local minima solutions that exhibit large variations in wing kinematics. Validation of the model against wind tunnel data shows that optimised solutions capture qualitative trends in wing kinematics with varying cruise speed. Sensitivity analyses show that the model outputs are most affected by the defined maximum lift coefficient and wing length, whereby perturbations in these parameters lead to significant changes in the predicted amount of upstroke wing retraction. Optimised solutions for allometrically scaled bird models show only small differences in predicted advance ratio, which is consistent with field study observations. Accelerating and climbing flight solutions also show similar qualitative trends in wing kinematics to experimental measurements, including a reduction in stroke plane inclination for increasing acceleration or climb angle. The model predicts that both climb angle and climb speed should be greater for birds with more available instantaneous mechanical power. Simulations of the wake using a discrete vortex model capture fundamental features of the wake geometry that have been observed experimentally. Reconstruction of the velocity field shows that this method overpredicts induced velocity in retracting-wing wakes, and should therefore only be applied to extended-wing phases of an avian wingbeat.

The study of fixed wing aeronautical engineering has matured to the point where years of research result in small performance improvements.  In the past decade, micro air vehicles, or MAVs, have gained attention of the aerospace and robotics communities.  Many researchers have begun investigating aircraft schemes such as ones which use rotary or flapping wings for propulsion.  While the engineering of rotary wing aircraft has seen significant advancement, the complex physics behind flapping wing aircraft remains to be fully understood.  Some studies suggest flapping wing aircraft can be more efficient when the aircraft operates in low Reynolds regimes or requires hovering.  Because of this inherent complexity, the derivation of flapping wing control methodologies remains an area with many open research problems.  This thesis investigates flapping wing vehicles whose design is inspired by avian flight.  The flapping wing system is examined in the cases where the core body is fixed or free in the ground frame.  When the core body is fixed, the Denavit Hartenberg representation is used for the kinematic variables.  An alternative approach is introduced for a free base body case.  The equations of motion are developed using Lagranges equations and a process is developed to derive the aerodynamic contributions using a virtual work principle.  The aerodynamics are modeled using a quasi-steady state formulation where the lift and drag coefficients are treated as unknowns.  A collection of nonlinear controllers are studied, specifically an ideal dynamic inversion controller and two switching dynamic inversion controllers.  A dynamic inversion controller is modified with an adaptive term that learns the aerodynamic effects on the equation of motion.  The dissipative controller with adaptation is developed to improve performance.  A Lyapunov analysis of the two adaptive controllers guarantees boundedness for all error terms.  Asymptotic stability is guaranteed for the derivative error in the dynamic inversion controller and for both the position and derivative error in the dissipative controller.  The controllers are simulated using two dynamic models based on flapping wing prototypes designed at Virginia Tech.  The numerical experiments validate the Lyapunov analysis and illustrate that unknown parameters can be learned if persistently excited. / Master of Science

<p>Insects and hummingbirds not only can perform long-term stationary hovering but also are capable of acrobatic maneuvers. At their body scale, such extraordinary flight performance remains unmatched by state-of-the-art conventional man-made aerial vehicles with fixed or rotary wings. Insects' and hummingbirds' near maximal performance come from their highly intricate and powerful wing-thorax actuation systems, sophisticated sensory system, and precise neuromotor control. Flapping Wing Micro Air Vehicles (FWMAVs) with bio-inspired flapping flight mechanisms hold great promise in matching the performance gap of their natural counterparts. Developing such autonomous flapping-wing vehicles to achieve animal-like flight, however, is challenging. The difficulties are mainly from the high power density requirements under the stringent constraints of scale, weight, and power, severe system oscillations induced by high-frequency wing motion, high nonlinearity of the system, and lack of miniature navigation sensors, which impede actuation system design, onboard sensing, flight control, and autonomous navigation. </p><p><br></p><p>To address these open issues, in this thesis, we first introduce systematic modeling of a dual-motor hummingbird-scale flapping wing robot. Based upon it, we then present studies of the onboard sensor fusion, flight control, and navigation method. </p><p><br></p><p>By taking the key inspiration from its natural counterparts, the proposed hummingbird robot has a pair of independently controlled wings. Each wing is directly actuated by a dc motor. Motors undergo reciprocating motion. Such a design is a severely underactuated system, namely, it relies on only two actuators (one per wing) to control full six degrees of freedom body motion. As a bio-inspired design, it also requires the vehicle close to its natural counterparts’ size and weight meanwhile provide sufficient lift and control effort for autonomy. Due to stringent payload limitation from severe underactuation and power efficiency challenges caused by motor reciprocating motion, the design and integration of such a system is a challenging task. In this thesis, we present the detailed modeling, optimization, and system integration of onboard power, actuation, sensing, and flight control to address these unique challenges. As a result, we successfully prototyped such dual-motor powered hummingbird robot, either with power tethers or fully untethered. The tethered platform is used for designing onboard sensing, control, and navigation algorithms. Untethered design tackles system optimization and integration challenges. Both tethered/untethered versions demonstrate sustained stable flight. </p><p><br></p><p>For onboard attitude sensing, a real-time sensor fusion algorithm is proposed with model-based adaptive compensation for both sensor reading drift and wing motion induced severe system vibration. With accurate and robust sensing results, a nonlinear robust control law is designed to stabilize the system during flight. Stable hovering and waypoint tracking flight were experimentally conducted to demonstrate the control performance. In order to achieve natural flyers' acrobatic maneuverability, we propose a hybrid control scheme by combining a model-based robust controller with a model-free reinforcement learning maneuver policy to perform aggressive maneuvers. The model-based control is responsible for stabilizing the robot in nominal flight scenarios. The reinforcement learning policy pushes the flight envelope to pilot fierce maneuvers. To demonstrate the effectiveness of the proposed control method, we experimentally show animal-like tight flip maneuver on the proposed hummingbird robot, which is actuated by only two DC motors. These successful results show the promise of such a hybrid control design on severely underactuated systems to achieve high-performance flight.</p><p><br></p><p>In order to navigate confined space while matching the design constraints of such a small robot, we propose to use its wings in dual functions - combining sensing and actuation in one element, which is inspired by animals' multifunctional flapping wings. Based on the interpretation of the motor current feedback which directly indicates wing load changes, the onboard somatosensory-like feedback has been achieved on our hummingbird robot. For navigation purposes, such a method can sense the presence of environmental changes, including grounds, walls, stairs, and obstacles, without the need for any other sensory cues. As long as the robot can fly, it can sense surroundings. To demonstrate this capability, three challenging tasks have been conducted onto the proposed hummingbird robot: terrain following, wall detection and bypass, and navigating a confined corridor. </p><p><br></p><p>Finally, we integrate the proposed methods into the untethered platform, which enables stable untethered flight of such a design in both indoor and outdoor tests. To the best of our knowledge, this result presents the first bio-inspired FWMAV powered by only two actuators and capable of performing sustained stable flight in both indoor and outdoor environment. It is also the first untethered flight of an at-scale tailless hummingbird robot with independently controlled wings, a key inspiration from its natural counterparts.</p><div><div><div><div><div> </div> </div> </div></div></div>

Hardware-in-the-loop (HIL) modeling is a powerful way of modeling complicated systems. However, some hardware is expensive to use in terms of time or mechanical wear. In cases like these, optimizing using the hardware can be prohibitively expensive because of the number of calls to the hardware that are needed. Variable fidelity optimization can help overcome these problems. Variable fidelity optimization uses less expensive surrogates to optimize an expensive system while calling it fewer times. The surrogates are usually created from performing a design of experiments on the expensive model and fitting a surface to the results. However, some systems are too expensive to create a surrogate from. One such case is that of a flapping flight model. In this thesis, a technique for variable fidelity optimization of HIL has been created that optimizes a system while calling it as few times as possible. This technique is referred to as an intelligent DOE. This intelligent DOE was tested using simple models of various dimension. It was then used to find a flapping wing trajectory that maximizes lift. Through testing, the intelligent DOE was shown to be able to optimize expensive systems with fewer calls than traditional variable fidelity optimization would have needed. Savings as high as 97% were recorded. It was noted that as the number of design variables increased, the intelligent DOE became more effective by comparison because the number of calls needed by a traditional DOE based variable fidelity optimization increased faster than linearly, where the number of hardware calls for the intelligent increased linearly.

The physics of flapping is very important in the design of MAVs. As MAVs cannot have an engine that produces the amount of thrust required for forward flight, and yet be light weight, harnessing thrust and lift from flapping is imperative for its design and development. In this thesis, aerodynamics of pitch and plunge are simulated using a 3-D, free wake, vortex lattice method (VLM), and structural characteristics of the wing are simulated as a membrane supported by a rigid frame. The aerodynamics is validated by comparing the results from the VLM model for constant angle of attack flight, pitching flight and plunging flight with analytical results, existing 2-D VLM and a doublet lattice method. The aeroelasticity is studied by varying parameters affecting the flow as well as parameters affecting the structure. The parametric studies are performed for cases of constant angle of attack, plunge and, pitch and plunge. The response of the aeroelastic model to the changes in the parameters are analyzed and documented. The results show that the aerodynamic loads increase for increased deformation, and vice-versa. For a wing with rigid boundaries supporting a membranous structure with a step change in angle of attack, the membrane oscillates about the steady state deformation and influence the loads. For prescribed oscillations in pitch and plunge, the membrane deformations and loads transition into a periodic steady state. / Master of Science

L’atomisation d’un jet ou d’une nappe liquide assistée par un courant gazeux rapide est couramment utilisée dans l’industrie ainsi qu’en propulsion aéronautique (turboréacteur) et spatiale (moteur-fusée cryotechnique). Plusieurs processus permettent la fragmentation de la structure cohérente liquide en gouttes. L’épluchage, qui intervient à courte distance en aval de l’injection, a été assez largement étudié (Marmottant et Villermaux 2004, Hong et al 2004) et les mécanismes sont assez bien décrits. En revanche, l’origine des instabilités large échelle – ou « flapping » - intervenant plus loin en aval, instabilités qui sont à l’origine de la production de large gouttes, reste mal comprise. Ceci est particulièrement vrai pour des jets cylindriques qui, contrairement au cas de nappes, ont fait l’objet de très peu d’études. Nous nous sommes donc attachés à comprendre l’origine du « flapping », à analyser ses liens avec les instabilités interfaciales de cisaillement, et à quantifier son impact sur la structure du jet ainsi que sur les gouttes produites. Pour cela, des expériences ont été menées en eau/air sur de larges plages de paramètres, aussi bien en termes de vitesses phasiques que des dimensions des veines gaz et liquide. Un soin particulier a été apporté au contrôle des écoulements internes.Pour l’ensemble des géométries, nous avons montré que la longueur du dard liquide est pilotée par le battement large échelle et non par le processus d’épluchage. Par ailleurs, la longueur de brisure présente une décroissance marquée avec la vitesse gaz, puis reste constante au delà d’une vitesse gaz critique. Un modèle a été proposé pour ce comportement asymptotique dans lequel la longueur de brisure est pilotée par le rapport de la vitesse liquide d’injection à une vitesse capillaire construite sur le diamètre liquide.La technique de mesure de la fréquence du battement large échelle mise en œuvre à partir d’images acquises par ombroscopie s’est avérée opérationnelle sur toute la plage de vitesses gaz considérées. Cette fréquence, qui ne varie pas spatialement, présente deux comportements : un premier où elle augmente avec la vitesse gaz, et un second où elle reste indépendante de la vitesse gaz. Ce second régime n’est pas mentionné dans la littérature. Pour le premier régime, le lien entre flapping et instabilité de cisaillement a été démontré en s’appuyant notamment sur des analyses de stabilité. Le nombre de Strouhal associé est piloté par le cisaillement côté gaz. La dépendance de la fréquence de battement à l’épaisseur de vorticité côté gaz est ainsi établie lorsque l’instabilité de cisaillement est pilotée par un mécanisme inviscide. Pour le second régime, le caractère opportuniste du flapping a été démontré l’aide d’une expérience de forçage : le flapping amplifie des structures liquides de longueur d’onde plus grande que celle associée à l’instabilité de cisaillement. Un nombre de Strouhal construit sur le diamètre liquide du jet et la vitesse du jet liquide à la distance de brisure a été proposé. Enfin, le rapport du diamètre du jet liquide à la longueur d’onde de l’instabilité de cisaillement semble pertinent pour définir la frontière entre ces deux régimes.Les tailles des gouttes produites sur l’axe de symétrie ont été mesurées à l’aide d’une sonde optique. Il apparaît que la distribution granulométrique évolue fortement avec la vitesse gaz, et qu’elle est multi-modale, ce qui traduit la présence de plusieurs mécanismes de brisure. La taille moyenne des gouttes décroit globalement comme UG-2, dans la limite de forts nombres de Weber aérodynamique. Cette taille moyenne s’avère aussi très sensible à la géométrie : elle diminue lorsque l’épaisseur gaz augmente jusqu’à atteindre une valeur plancher, et elle croît avec le diamètre liquide. / Jet or sheet atomized by a fast coaxial gas jet is currently used in industry, like aeronautical propulsion (turbofan) or spatial propulsion (cryotechnic rocket engine). Many physical processes allows liquid coherent structure fragmentation into drops. Stripping, which appears downstream near injector, has been largely studied (Marmottant et Villermaux 2004, Hong & al 2004), mecanisms has been correctly described.However, the origin of large scale - or 'flapping' instabilities - intervening further downstream, instabilities that are causing the production of large drops, remains poorly understood. This is particularly true for cylindrical jets which, unlike the case of sheets, have been the subject of very few studies. We are therefore committed to understand the origin of the "flapping", to analyze its relationship with interfacial shear instabilities, and to quantify its impact on the structure of the jet as well as on the drops produced. For this, experiments were carried out in water/air on wide set of parameters, both in terms of phasic speed than the dimensions of the gas gap and liquid diameter. Special care were made to the internal flow control.For all the geometries, we showed that the length of the liquid cone is driven by the large scale displacements and not by the stripping process. Furthermore, the length of brokenness jet presents a decline marked with the gas speed, then remains constant beyond a critical gas speed. A model was proposed for this asymptotic behavior in which the break-up length is driven by the report of the liquid injection speed to a capillary speed built on the liquid diameter.Measurement of the frequency of large scale displacement technology has been implemented from images acquired by shadowgraphy proved operational over the gas velocity range considered. This frequency, which varies not spatially, present two behaviors: a first where it increases with the speed of the gas, and a second where it remains independent of the gas speed. This second scheme is not mentioned in the literature. For the original plan, the link between flapping and shear instability has been demonstrated based on analyses of stability. The associated Strouhal number is controlled by the shear gas side. The dependence of the frequency of heartbeat to the thickness of vorticity gas side is thus established when shear instability is driven by an inviscide mechanism. For the second scheme, the opportunistic nature of the flapping has been demonstrated using forcing experience: the flapping amplifies liquid structures of wavelength greater than those associated with shear instability. A Strouhal number built on liquid jet diameter and the speed of the liquid jet at break distance has been proposed. Finally, the ratio of the diameter of the liquid jet at the wavelength of the shear instability seems relevant to define the border between these two regimes.Sizes drops produced on the symmetry axis were measured using an optical probe. It appears that granulometric distribution is evolving strongly with speed gas, and it is multi-modal, reflecting the presence of several mechanisms of brokenness. The average size of the drops decreases overall as UG - 2, in the limit of strong numbers of aerodynamic Weber. This medium size is also very sensitive to geometry: it decreases when the thickness of the gas increases until it reaches a floor value, and it grows with the liquid diameter. Finally, by forcing large amplitude lateral displacement, the average radial distribution of sizes of drops has been made much more homogeneous, and the average size of the drops on the axis has been reduced by a factor of 2. These results therefore open opportunities in terms of control of atomization.