Three Dimensional Unsteady Flow and Active Morphing Effect in Flapping Wings

Bumble b ee cannot fly, if we ignore the significant differences b etween flappin

The wake dynamics and flight forces of the fruit fly Drosophila melanogaster

We have used flow visualizations and instantaneous force measurements of tethered fruit flies (Drosophila melanogaster) to study the dynamics of force generation during flight. During each complete stroke cycle, the flies generate one single vortex loop consisting of vorticity shed during the downstroke and ventral flip. This gross pattern of wake structure in Drosophila is similar to those described for hovering birds and some other insects. The wake structure differed from those previously described, however, in that the vortex filaments shed during ventral stroke reversal did not fuse to complete a circular ring, but rather attached temporarily to the body to complete an inverted heart-shaped vortex loop. The attached ventral filaments of the loop subsequently slide along the length of the body and eventually fuse at the tip of the abdomen. We found no evidence for the shedding of wing-tip vorticity during the upstroke, and argue that this is due to an extreme form of the Wagner effect acting at that time. The flow visualizations predicted that maximum flight forces would be generated during the downstroke and ventral reversal, with little or no force generated during the upstroke. The instantaneous force measurements using laser-interferometry verified the periodic nature of force generation. Within each stroke cycle, there was one plateau of high force generation followed by a period of low force, which roughly correlated with the upstroke and downstroke periods. However, the fluctuations in force lagged behind their expected occurrence within the wing-stroke cycle by approximately 1 ms or one-fifth of the complete stroke cycle. This temporal discrepancy exceeds the range of expected inaccuracies and artifacts in the measurements, and we tentatively discuss the potential retarding effects within the underlying fluid mechanics

Plant-pollinator aerodynamics

Interactions between plants and pollinators have adapted over long evolutionary timescales and fill a vital ecological role. For flying pollinators, the same coherent aerodynamic mechanisms are employed across the broad Reynolds number range of 100-10,000. This thesis aims to understand some of the physics involved in plant-pollinator aerodynamics. First, studying the impact of an artificial flower wake on maneuverability revealed emergent simplicity in hawkmoth flower tracking dynamics with increased tracking error at the vortex shedding frequencies of the 3D-printed flower. These results establish that unsteady flow affects complex behaviors as well as steady flight performance. Next, the interplay between steady airflow and wing flexibility was explored in two flow regimes: (1) matching airflow conditions for Manduca sexta flight and (2) matching flow conditions known to produce decoherent leading-edge vortices (LEVs) on rigid wings. Although LEVs still burst on flexible hawkmoth wings, the LEV is decoherent over more of the wingspan as flexibility decreases. Enhanced LEV stability in the hawkmoth flight regime revealed that trade-offs between Coriolis forces (from wing rotation) and inertial forces (from both wing translation and the incoming airflow) influence LEV structure and lift force. Last, the wakes of hawkmoth-pollinated flowers were found to be turbulent but some irregular periodic structures were present downstream of small flowers (diameter less than 40 mm). Like many bluff body flow interactions, flower wakes are dominated by a re-circulation zone downstream and hawkmoths hover-feed within the re-circulation bubble. In addition to characterizing the local flow environment for a hovering hawkmoth, this work showed how flow in the flower wake impacts aerodynamic force (with a blade-element model). Despite the broad diversity in floral environments for pollinators, flapping flight (and the LEV in particular) remains a highly effective strategy. Future work can investigate how insects achieve consistent performance across variable environments from behavioral, neurological, and aerodynamic perspectives.Ph.D

Vortex Dynamics in Near Wake of a Hovering Hawkmoth

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/76398/1/AIAA-2008-583-258.pd

Hovering of a passive body in an oscillating airflow

International audienceSmall flexible bodies are observed to hover in an oscillating air column. The air is driven by a large speaker at frequencies in the range 10–65 Hz at amplitudes 1–5 cm. The bodies are made of stiffened tissue paper, bent to form an array of four wings, symmetric about a vertical axis. The flapping of the wings, driven by the oscillating flow, leads to stable hovering. The hovering position of the body is unstable under free fall in the absence of the airflow. Measurements of the minimum flow amplitude as a function of flow frequency were performed for a range of self-similar bodies of the same material. The optimal frequency for hovering is found to vary inversely with the size. We suggest, on the basis of flow visualization, that hovering of such bodies in an oscillating flow depends upon a process of vortex shedding closely analogous to that of an active flapper in otherwise still air. A simple inviscid model is developed illustrating some of the observed properties of flexible passive hoverers at high Reynolds number

Aerodynamics, kinematics, and ecology of slow flight in birds

The overarching goal of my dissertation is to elucidate the force production mechanisms of slow flight in birds. Slow flight is extremely energetically costly per unit time, yet highly important for takeoff and survival in birds. However, at slow speeds it is presently thought that most birds do not produce beneficial aerodynamic forces during the entire wingbeat: instead, they fold or flex their wings during upstroke, prompting the long-standing prediction that the upstroke produces trivial forces. Here, I examined the kinematics, aerodynamics, skeletal drivers, and potential ecological influences of force production in flight. In chapter one, I establish that wings in upstroke posture are capable of producing beneficial aerodynamic forces. Chapter two illustrates diamond doves that keep their wings extended in a “wingtip-reversal” upstroke (at Re=50,000) produce a kinematic and aerodynamic signature similar to the clap-and-peel mechanism previously reported only in insects (Re=8,000). In contrast, zebra finch use a “flexed-wing” upstroke that is aerodynamically inactive. Integrating an XROMM (X-ray Reconstruction of Moving Morphology) study of pigeons and starlings in chapter three, I demonstrate that both upstroke styles have similar skeletal kinematics (with a few notable exceptions), but the timing and extent of motion differs. Lastly, chapter four faces birds with an ecologically relevant task: transitioning from a compliant substrate reduces initial flight velocities, and birds do not appear to modulate force production to compensate. Collectively, I reveal that the clap and fling mechanism utilized by many species is a wing motion that is aerodynamically beneficial and largely due to an interaction of the skeletal elements. These four chapters illuminate an energetically costly and ecologically relevant period of flight

Quantitative analysis of take-off forces in birds

The increasing interest on Unmanned Air Vehicles (UAV’s) and their several utilities blended with the need of easy carrying and also the stealth, lead to the need to create the concept of Micro Air Vehicles (MAV’s) and the Nano Air Vehicles (NAV’s). Due to the current interest and the present lack of knowledge on the insect’s and bird’s flight, this study was intended to interpret the forces involved on the moment of the take-off of a bird, recurring to an experiment involving a fast data acquisition force sensor and high speed camera, in addition known facts from earlier studies. In order to do that a bibliographic revision was done, to know what was already studied and to find what could yet be studied. That way could be formed a link on the factors involved on the propulsion of a bird at the moment of take-off. The main conclusions obtained by this work is that the bird can produce movements that will enhance the total moment when the bird stretches its neck forward and moving head down followed by stretching even more its neck and moving head up impelling himself into the air, resulting in a main role on the mechanical forces (against perch) for the bird first moments momentum. Columba livia can generate about 4 times its weight worth mechanic force (against perch) and above 8 times its weight during the 2nd downstroke.O interesse crescente nos Veículos Aéreos não Tripulados “Unmanned Air Vehicles (UAV’s)” e suas diversas utilidades em conjunto com a necessidade de seu fácil transporte e furtividade, levaram à necessidade de criar o conceito dos Micro Veículos Aéreos “Micro Air Vehicles (MAV’s)” e os Nano Veículos Aéreos “Nano Air Vehicles (NAV’s)”. Este tipo de veículos tem como fonte inspiradora os insetos e aves devido à necessária produção simultânea de sustentação e propulsão. Tal como no voo convencional, também no voo animal podem ser identificadas as fases de levantamento (descolagem) e aterragem como diferenciadas do voo longe de uma superfície de apoio. Este trabalho é dedicado ao estudo da fase de levantamento de voo de uma ave columba livia. Foram realizadas experiências para medir a força inicial produzida pela ave para iniciar o voo e a respetiva trajetória na zona próxima do ponto de apoio inicial. Estas medidas foram efetuadas com um sensor de força dotado de elevada velocidade de aquisição de dados e uma camara de alta velocidade. As principais conclusões obtidas com a realização deste trabalho é o facto de que a ave consegue produzir movimentos, que aumentar o momento total quando a ave estica o pescoço para a frente e movendo a cabeça para baixo seguido por continuação de esticamento do pescoço e movimento da cabeça para cima impelindo-se para o ar, resultando num papel principal relativamente às forças mecânicas (contra o poleiro) para o momento linear actuante nos primeiros momentos. Columba livia consegue gerar cerca de 4 vezes o seu peso em força mecânica e acima de 8 vezes o seu peso durante o 2º downstroke

Velocity Field around a Rigid Flapping Wing with a Winglet in Quiescent Water

This study investigated the effect of a winglet on the velocity field around a rigid flapping wing. Two-dimensional particle image velocimetry was used to capture the velocity field of asymmetric one-degree-of-freedom flapping motion. A comparison was conducted between wings with and without a winglet at two flapping frequencies, namely 1.5 and 2.0 Hz. The effect of the winglet on the velocity field was determined by systematically comparing the velocity fields for several wing phase angles during the downstroke and upstroke. The presence of a winglet considerably affected the flow field around the wingtip, residual flow, and added mass interaction. The added mass was lower and residual flow was weaker for the wings with a winglet than for the wings without a winglet. The added mass and velocity magnitudes of the flow field increased proportionally with the flapping frequency

Flow Field and Performance Measurements of a Flapping-Wing Device Using Particle Image Velocimetry

A flexible flapping wing was tested using various flow interrogation techniques including particle image velocimetry (PIV) to further the understanding of its complex, unsteady, three-dimensional flow field. The flow field was characterized using high-speed flow visualization (FV) in chordwise and spanwise planes to observe and characterize the evolution of flow structures produced. The formation, growth, convection, and shedding of a leading-edge vortex (LEV) was observed on the upper surface of the wing, which was found to mimic the classical process of dynamic stall. A motion-tracking system was used to characterize the complex wing kinematics and aeroelastic deformations of the flexible wing. These measurements were then used to estimate the noncirculatory forces and moments acting on the wing. Two-dimensional velocity fields around the wing contour and in its wake were obtained using PIV. These velocity fields were used to calculate the circulatory lift as well as the drag produced on the wing. It was found that the process of LEV formation, growth, and convection significantly increased the lift production on the flapping wing. The noncirculatory and circulatory lift measurements were then combined in amplitude and phase to calculate the total lift on the wing. It was shown that the noncirculatory contributions to the airloads were small except near pronation and supination. The flow field results were also used to calculate the lift-to-drag ratio during the wing stroke, where surprisingly it was found that the lift-to-drag ratio increased during the process of LEV formation and shedding. This observation perhaps suggests a reason why flapping-wing flyers intentionally produce LEVs during their wing stroke

Exploring the biofluiddynamics of swimming and flight

cum laude graduation (with distinction