Showing papers on "Flapping published in 2008"

Flapping Wing Aerodynamics: Progress and Challenges

Abstract: It is the objective of this paper to review recent developments in the understanding and prediction of flapping-wing aerodynamics. To this end, several flapping-wing configurations are considered. First, the problem of single flapping wings is treated with special emphasis on the dependence of thrust, lift, and propulsive efficiency on flapping mode, amplitude, frequency, and wing shape. Second, the problem of hovering flight is studied for single flapping wings. Third, the aerodynamic phenomena and benefits produced by the flapping-wing interactions on tandem wings or biplane configurations are discussed. Such interactions occur on dragonflies or on a recently developed micro air vehicle. The currently available two- and three-dimensional inviscid and viscous flapping-wing flow solutions are presented. It is shown that the results are strongly dependent on flapping frequency, amplitude, and Reynolds number. These findings are substantiated by comparison with the available experimental data.

Flapping states of a flag in an inviscid fluid: Bistability and the transition to chaos

Abstract: We investigate the "flapping flag" instability through a model for an inextensible flexible sheet in an inviscid 2D flow with a free vortex sheet. We solve the fully-nonlinear dynamics numerically and find a transition from a power spectrum dominated by discrete frequencies to an apparently continuous spectrum of frequencies. We compute the linear stability domain which agrees with previous approximate models in scaling but differs by large multiplicative factors. We also find hysteresis, in agreement with previous experiments.

Transitions in the wake of a flapping foil.

Abstract: We study experimentally the vortex streets produced by a flapping foil in a hydrodynamic tunnel, using two-dimensional particle image velocimetry. An analysis in terms of a flapping frequency-amplitude phase space allows the identification of (i) the transition from the well-known Benard-von Karman (BvK) wake to the reverse BvK vortex street that characterizes propulsive wakes, and (ii) the symmetry breaking of this reverse BvK pattern giving rise to an asymmetric wake. We also show that the transition from a BvK wake to a reverse BvK wake precedes the actual drag-thrust transition and we discuss the significance of the present results in the analysis of flapping systems in nature.

Vortex shedding model of a flapping flag

Abstract: A two-dimensional model for the flapping of an elastic flag under axial flow is described. The vortical wake is accounted for by the shedding of discrete point vortices with unsteady intensity, enforcing the regularity condition at the flag's trailing edge. The stability of the flat state of rest as well as the characteristics of the flapping modes in the periodic regime are compared successfully to existing linear stability and experimental results. An analysis of the flapping regime shows the co-existence of direct kinematic waves, travelling along the flag in the same direction as the imposed flow, and reverse dynamic waves, travelling along the flag upstream from the trailing edge.

Optimal flexibility of a flapping appendage in an inviscid fluid

Abstract: We present a new formulation of the motion of a flexible body with a vortex-sheet wake and use it to study propulsive forces generated by a flexible body pitched periodically at the leading edge in the small-amplitude regime. We find that the thrust power generated by the body has a series of resonant peaks with respect to rigidity, the highest of which corresponds to a body flexed upwards at the trailing edge in an approximately one-quarter-wavelength mode of deflection. The optimal efficiency approaches 1 as rigidity becomes small and decreases to 30–50 % (depending on pitch frequency) as rigidity becomes large. The optimal rigidity for thrust power increases from approximately 60 for large pitching frequency to ∞ for pitching frequency 0.27. Subsequent peaks in response have power-law scalings with respect to rigidity and correspond to higher-wavenumber modes of the body. We derive the power-law scalings by analysing the fin as a damped resonant system. In the limit of small driving frequency, solutions are self-similar at the leading edge. In the limit of large driving frequency, we find that the distribution of resonant rigidities ∼k −5 , corresponding to fin shapes with wavenumber k. The input power and output power are proportional to rigidity (for small-to-moderate rigidity) and to pitching frequency (for moderateto-large frequency). We compare these results with the range of rigidity and flapping frequency for the hawkmoth forewing and the bluegill sunfish pectoral fin.

Insect-Based Hover-Capable Flapping Wings for Micro Air Vehicles: Experiments and Analysis

Abstract: This paper addresses the aerodynamics of insect-based, biomimetic, flapping wings in hover. An experimental apparatus, with a biomimetic flapping mechanism, was used to measure the thrust generated by a number of wing designs at different wing pitch settings. To quantify the large inertial loads acting on the wings, vacuum chamber tests were conducted. Results were obtained for several high-frequency tests conducted on lightweight aluminum and composite wings. The wing mass was found to have a significant influence on the maximum frequency of the mechanism because of a high inertial power requirement. All the wings tested showed a decrease in thrust at high frequencies. In contrast, for a wing held at 90-deg pitch angle, flapping in a horizontal stroke plane with passive pitching caused by aerodynamic and inertial forces, the thrust was found to be larger. To study the effect of passive pitching, the biomimetic flapping mechanism was modified with a passive torsion spring on the flapping shaft. Results of some tests conducted with different wings and different torsion spring stiffnesses are shown. A soft torsion spring led to a greater range of pitch variation and produced more thrust at slightly lower power than with the stiff torsion spring. The lightweight and highly flexible wings used in this study had significant aeroelastic effects which need to be investigated. A finite element based structural analysis of the wing is described, along with an unsteady aerodynamic analysis based on indicial functions. The analysis was validated with experimental data available in literature, and also with experimental tests conducted on the biomimetic flapping-pitching mechanism. Results for both elastic and rigid wing analyses are compared with the thrust measured on the biomimetic flapping-pitching mechanism.

Effects of Reynolds Number and Flapping Kinematics on Hovering Aerodynamics

Abstract: Motivated by our interest in micro and biological air vehicles, Navier-Stokes simulations for fluid flow around a hovering elliptic airfoil have been conducted to investigate the effects of Reynolds number, reduced frequency, and flapping kinematics on the flow structure and aerodynamics. The Reynolds number investigated ranges from 75 to 1700, and the reduced frequency from 0.36 to 2.0. Two flapping modes are studied, namely, the "water-treading" hovering mode, and the normal hovering mode. Although the delayed-stall mechanism is found to be responsible for generating the maximum lift peaks in both hovering modes, the wake-capturing mechanism is identified only in the normal hovering mode. In addition to the strong role played by the kinematics, the Reynolds number's role has also been clearly identified. In the low Reynolds number regime, 0(100), the viscosity dissipates the vortex structures quickly and leads to essentially symmetric flow structure and aerodynamics force between the forward stroke and backward strokes. At higher Reynolds numbers (300 and larger), the history effect is influential, resulting in distinctly asymmetric phenomena between the forward and backward strokes.

Three-dimensional flow structures and evolution of the leading-edge vortices on a flapping wing.

Abstract: SUMMARY Following the identification and confirmation of the substructures of the leading-edge vortex (LEV) system on flapping wings, it is apparent that the actual LEV structures could be more complex than had been estimated in previous investigations. In this experimental study, we reveal for the first time the detailed three-dimensional (3-D) flow structures and evolution of the LEVs on a flapping wing in the hovering condition at high Reynolds number ( Re =1624). This was accomplished by utilizing an electromechanical model dragonfly wing flapping in a water tank (mid-stroke angle of attack=60°) and applying phase-lock based multi-slice digital stereoscopic particle image velocimetry (DSPIV) to measure the target flow fields at three typical stroke phases: at 0.125 T ( T =stroke period), when the wing was accelerating; at 0.25 T , when the wing had maximum speed; and at 0.375 T , when the wing was decelerating. The result shows that the LEV system is a collection of four vortical elements: one primary vortex and three minor vortices, instead of a single conical or tube-like vortex as reported or hypothesized in previous studies. These vortical elements are highly time-dependent in structure and show distinct `stay properties9 at different spanwise sections. The spanwise flows are also time-dependent, not only in the velocity magnitude but also in direction.

Insectlike Flapping Wings in the Hover Part II: Effect of Wing Geometry

Abstract: The aerodynamic design of a flapping-wing micro air vehicle requires a careful study of the wing design space to ascertainthebestcombinationofparameters.Anonlinearunsteadyaerodynamic modeldevelopedbytheauthorsis used to make such a study for hovering insectlike flapping wings. The work is characterized, in particular, by the insights it provides into flapping-wing flows and the use of these insights for aerodynamic design. The effects of wing geometry on the aerodynamic performance of such flapping wings are investigated by comparing the influence on a numberofsyntheticplanformshapeswhilevaryingonlyoneparameteratatime.Bestperformanceappearstobefor wingshapesthathavenearlystraight leadingedges andmoreareaoutboard,where flowvelocities arehigher.Other important trends are also identified and practical considerations are noted. When possible, comparisons are also drawn with quasi-steady expectations and discrepancies are explained.

Numerical and experimental study of the fluid dynamics of a flapping wing with low order flexibility

Abstract: A simple canonical problem for understanding the role of flexibility in flapping wing flight is investigated numerically and experimentally. The problem consists of a two-dimensional two-component wing structure connected by a single hinge with a damped torsion spring. One component of the wing is driven with hovering flapping wing kinematics, while the other component responds passively to the fluid dynamic and inertial/elastic forces. Numerical simulations are carried out with the viscous vortex particle method with strongly coupled body dynamics. The experiments are conducted in a water tank with suspended particles for flow visualization. The system is analyzed in several different kinematic test cases that are designed to span a broad parametric range of flapping. Hinge deflection is used as the primary metric for comparison; the agreement between computation and experiment is very good in all cases. The trajectories of shed vortices are also compared, again with favorable agreement. Fluid forces and...

Insectlike Flapping Wings in the Hover Part I: Effect of Wing Kinematics

Abstract: A GILE flight inside buildings, caves, and tunnels is of significant military and civilian value and is an attractive application for micro air vehicles (MAVs), defined here as flying machines of the order of 150 mm in size Indoor flight imposes particular design and performance requirements, including small size, low speed, hovering capability, high maneuverability at low speeds, and (for covert operations) small acoustic signature, among other things As discussed elsewhere [1–4], insectlike flapping is a solution thatmeets these requirements and is proven in nature Although a number of elements characterize the design of a flapping-wing MAV, the focus here is on its wing aerodynamic design This is crucial because for a flapping-wing MAV (FMAV) the wings are not only responsible for lift, but also for propulsion and maneuvers Although insect flapping wings offer a proven solution and are abundant in nature (there are over 170,000 species of flying insects), little is known about the optimality of their wing design Unlike forfixed or rotarywings, the parametric space associatedwith flapping wings is largely unexplored A study that addresses the effects of both wing kinematics and wing geometry on the aerodynamic performance of flapping wings is required, and the former forms the underlying theme of this paper The effect of wing geometry is considered elsewhere [5] This work also provides insights into flapping-wing flow physics and uses these insights for aerodynamic design Although Ellington’s [6] seminal work rejuvenated interest in insect flight, it is only recently that attention has been directed toward the design of vehicles that use insectlike flapping wings, particularly at the MAV scale [1–3] In a later study, Ellington [7] proposed design guidelines based on scaling fromnature, but this does not give physical insight or allow design optimization Dickinson et al [8] investigated the effect of advancing or delaying pitch rotation of the flapping wing with respect to its translational motion, using experiments on Dickinson’s Robofly: a scaled-up mechanical model of the fruit fly Drosophila Ramamurti and Sandberg [9] used a computational fluid dynamics (CFD) method to demonstrate this effect and presented some useful flow visualization Sun and Tang [10] also used a CFD code to investigate the effect of advancing and delaying pitch rotation on insectlike flapping flight and the effect of varying the duration of stroke reversals [11] In an earlier study [12], they investigated the effect of Reynolds number and the duration of wing stroke reversal They also studied the effect of advance ratio (the ratio of flight speed to wing mean tip speed) in forward flapping flight [13] Yu and Tong [14] used an aerodynamic modeling approach [15] to study forward flapping flight at various advance ratios by varying asymmetries between upand downstrokes However, none of the preceding studies aimed to produce an optimized wing aerodynamic design Milano and Gharib [16] made probably the only study thus far aimed at optimizing wing kinematics They used a genetic algorithm paired with digital particle-image velocimetry experiments on a flapping wing in a water-filled towing tank By using insectlike kinematics, they optimized for average lift over four flapping cycles and found a number of convergent solutions in the parameter space They noted that the optimally efficient solutions all tended to generate leading-edge vortices ofmaximum strength However, their Received 26 October 2007; revision received 11 June 2008; accepted for publication 13 June 2008 Copyright © 2008 by SalmanA Ansari Published by the American Institute of Aeronautics and Astronautics, Inc, with permission Copies of this paper may be made for personal or internal use, on condition that the copier pay the $1000 per-copy fee to the Copyright Clearance Center, Inc, 222 Rosewood Drive, Danvers, MA 01923; include the code 0021-8669/08 $1000 in correspondence with the CCC ResearchOfficer, Department ofAerospace, Power and SensorsMember AIAA Professor of Aeromechanical Systems, Department of Aerospace, Power and Sensors Associate Fellow AIAA Reader in Control Engineering, Department of Aerospace, Power and Sensors Member AIAA JOURNAL OF AIRCRAFT Vol 45, No 6, November–December 2008

Flapping motion and force generation in a viscoelastic fluid.

Abstract: In a variety of biological situations, swimming cells have to move through complex fluids. Similarly, mucociliary clearance involves the transport of polymeric fluids by beating cilia. Here, we consider the extent to which complex fluids could be exploited for force generation on small scales. We consider a prototypical reciprocal motion (i.e., identical under time-reversal symmetry): the periodic flapping of a tethered semi-infinite plane. In the Newtonian limit, such motion cannot be used for force generation according to Purcell's scallop theorem. In a polymeric fluid (Oldroyd-B, and its generalization), we show that this is not the case and calculate explicitly the forces on the flapper for small-amplitude sinusoidal motion. Three setups are considered: a flapper near a wall, a flapper in a wedge, and a two-dimensional scalloplike flapper. In all cases, we show that at quadratic order in the oscillation amplitude, the tethered flapping motion induces net forces, but no average flow. Our results demonstrate therefore that the scallop theorem is not valid in polymeric fluids. The reciprocal component of the movement of biological appendages such as cilia can thus generate nontrivial forces in polymeric fluid such as mucus, and normal-stress differences can be exploited as a pure viscoelastic force generation and propulsion method.

Aerodynamic efficiency of flapping flight: analysis of a two-stroke model.

Abstract: SUMMARY To seek the simplest efficient flapping wing motions and understand their relation to steady flight, a two-stroke model in the quasi-steady limit was analyzed. It was found that a family of two-stroke flapping motions have aerodynamic efficiency close to, but slightly lower than, the optimal steady flight. These two-stroke motions share two common features: the downstroke is a gliding motion and the upstroke has an angle of attack close to the optimal of the steady flight of the same wing. With the reduced number of parameters, the aerodynamic cost function in the parameter space can be visualized. This was examined for wings of different lift and drag characteristics at Reynolds numbers between 10 2 and 10 6 . The iso-surfaces of the cost function have a tube-like structure, implying that the solution is insensitive to a specific direction in the parameter space. Related questions in insect flight that motivated this work are discussed.

Experimental Investigation on the Aerodynamic Characteristics of a Bio-mimetic Flapping Wing with Macro-fiber Composites:

Abstract: This study describes the development of a bio-mimetic flapping wing and the aerodynamic characteristics of a flexible flapping wing. First, the flapping wing is designed to produce flapping, twisting, and camber motions by using a bio-mimetic design approach. A structural model for a macro-fiber composite (MFC) actuator is established, and structural analysis of a smart flapping wing with the actuator is performed to determine the wing configuration for maximum camber motion. The analysis model is verified with the experimental data of the smart flapping wing. Second, aerodynamic tests are performed for the smart flapping wing in a subsonic wind tunnel, and the aerodynamic forces are measured for various test conditions. Additionally, the effects of camber and chordwise wing flexibility on unsteady and quasi-steady aerodynamic characteristics are discussed. The experimental results demonstrate that the effect of the camber generated by the MFC produces sufficient aerodynamic benefit. It is further found that chordwise wing flexibility is an important parameter in terms of affecting aerodynamic performance, and that lift produced in a quasi-steady flow condition is mostly affected by the forward speed and effective angle of attack.

Vortex-wake interactions of a flapping foil that models animal swimming and flight

Abstract: The fluid dynamics of many swimming and flying animals involves the generation and shedding of vortices into the wake. Here we studied the dynamics of similar vortices shed by a simple two-dimensional flapping foil in a soap-film tunnel. The flapping foil models an animal wing, fin or tail in forward locomotion. The vortical flow induced by the foil is correlated to (the resulting) thickness variations in the soap film. We visualized these thickness variations through light diffraction and recorded it with a digital high speed camera. This set-up enabled us to study the influence of foil kinematics on vortex-wake interactions. We varied the dimensionless wavelength of the foil (lambda*=4-24) at a constant dimensionless flapping amplitude (A*=1.5) and geometric angle of attack amplitude (A(alpha,geo)=15 degrees ). The corresponding Reynolds number was of the order of 1000. Such values are relevant for animal swimming and flight. We found that a significant leading edge vortex (LEV) was generated by the foil at low dimensionless wavelengths (lambda*<10). The LEV separated from the foil for all dimensionless wavelengths. The relative time (compared with the flapping period) that the unstable LEV stayed above the flapping foil increased for decreasing dimensionless wavelengths. As the dimensionless wavelength decreased, the wake dynamics evolved from a wavy von Karman-like vortex wake shed along the sinusoidal path of the foil into a wake densely packed with large interacting vortices. We found that strongly interacting vortices could change the wake topology abruptly. This occurred when vortices were close enough to merge or tear each other apart. Our experiments show that relatively small changes in the kinematics of a flapping foil can alter the topology of the vortex wake drastically.

Propulsive performance of biologically inspired flapping foils at high Reynolds numbers.

Abstract: SUMMARY Propulsion and maneuvering underwater by flapping foil motion, optimized through years of evolution, is ubiquitous in nature, yet marine propulsors inspired by examples of highly maneuverable marine life or aquatic birds are not widely implemented in engineering Performance data from flapping foils, moving in a rolling and pitching motion, are presented at high Reynolds numbers, Re= Uc/ ν, or O(10 4 ), where U is the relative inflow velocity, c is the chord length of the foil, and ν is the kinematic viscosity of the fluid, from water tunnel experiments using a foil actuator module designed after an aquatic penguin or turtle fin The average thrust coefficients and efficiency measurements are recorded over a range of kinematic flapping amplitudes and frequencies Results reveal a maximum thrust coefficient of 209, and for low values of angle of attack the thrust generally increases with Strouhal number, without much penalty to efficiency Strouhal number is defined as St =2 h 0 f/U , where f is the frequency of flapping, and 2 h 0 is the peak-to-peak amplitude of flapping The thrust and efficiency contour plots also present a useful performance trend where, at low angles of attack, high thrust and efficiency can be gained at sufficiently high Strouhal numbers Understanding the motion of aquatic penguins and turtle wings and emulating these motions mechanically can yield insight into the hydrodynamics of how these animals swim and also improve performance of biologically inspired propulsive devices

Effects of unsteady deformation of flapping wing on its aerodynamic forces

Abstract: Effects of unsteady deformation of a flapping model insect wing on its aerodynamic force production are studied by solving the Navier-Stokes equations on a dynamically deforming grid. Aerodynamic forces on the flapping wing are not much affected by considerable twist, but affected by camber deformation. The effect of combined camber and twist deformation is similar to that of camber deformation. With a deformation of 6% camber and 20° twist (typical values observed for wings of many insects), lift is increased by 10% ∼ 20% and lift-to-drag ratio by around 10% compared with the case of a rigid flat-plate wing. As a result, the deformation can increase the maximum lift coefficient of an insect, and reduce its power requirement for flight. For example, for a hovering bumblebee with dynamically deforming wings (6% camber and 20° twist), aerodynamic power required is reduced by about 16% compared with the case of rigid wings.

Computational Study of Flexible Wing Ornithopter Flight

Abstract: This paper presents the development and evaluation of a computational fluid dynamics based methodology to predict the aerodynamic forces produced by a flexible flapping wing. The computational fluid dynamics analysis code solves the compressible Reynolds-averaged form of the Navier-Stokes equations on structured curvilinear grids. A grid deformation algorithm is devised that deforms the body-conforming volume grid at each time step consistent with the measured wing motions. This algorithm is based on geometric considerations and is both computationally efficient and capable of handling very large deformations. This methodology is validated using experimental data obtained from a test on an ornithopter with flexible wings. Test data include measurements of the wing surface deformations as well as the generated forces in the horizontal and vertical directions. Correlation with test data shows good agreement with measured vertical force and satisfactory agreement with measured horizontal force at low flapping frequencies. However, the prediction accuracy degrades with an increase in Happing frequency. Evidence of resonance in the vehicle system was detected from the analysis of the experimental data. Unmodeled inertial effects from the vehicle body and support mounts may be one of the contributors to disagreement between the data and analysis.

An aeroelastic analysis of a flexible flapping wing using modified strip theory

Abstract: The present study proposed a coupling method for the fluid-structural interaction analysis of a flexible flapping wing. An efficient numerical aerodynamic model was suggested, which was based on the modified strip theory and further improved to take into account a high relative angle of attack and dynamic stall effects induced by pitching and plunging motions. The aerodynamic model was verified with experimental data of rigid wings. A reduced structural model of a rectangular flapping wing was also established by using flexible multibody dynamics, so as to consider large flapping motions and local elastic deformations. Then, the aeroelastic analysis method was developed by coupling these aerodynamic and structural modules. To measure the aerodynamic forces of the rectangular flapping wing, static and dynamic tests were performed in a low speed wind-tunnel for various flapping pitch angles, flapping frequencies and the airspeeds. Finally, the aerodynamic forces predicted by the aeroelastic analysis method showed good agreement with the experimental data of the rectangular flapping wing.

Lift from Spanwise Flow in Simple Flapping Wings

Abstract: Spanwise flow contributes to lift in thin flat-plate zero-pitch-angle flapping wings in quiescent air. It is reasonable to maintain only the kinematics and mechanical complexity absolutely necessary in developing flapping-wing micro air vehicles. This study continues the quantification of the lift generated from a flapping motion of absolute minimum complexity thought to be capable of generating lift. A flapping-wing micro air vehicle with rectangular planform wings fabricated in-house (semispan aspect ratios from 1.5 to 4.0) was used to quantify the contributions to lift from flow along the span of wings at numerous points throughout the flapping cycle under a variety of operating conditions (3-6 Hz and Reynolds numbers of 6000-15,000). These experiments were performed for several aspect ratios for flat-plate and spanwise-cambered wings. The lift force was quantified experimentally using a force transducer and a high-speed camera. Digital particle image velocimetry was used to determine the lift contributions of spanwise flow to the total measured lift. Additionally, the presence of spanwise camber was shown to affect the transient lift behavior.

Characteristics of an Insect-mimicking Flapping System Actuated by a Unimorph Piezoceramic Actuator

Abstract: This study introduces an insect-mimicking flapping-wing system, where the rotation, corrugation, and clapping of insect wings have been mimicked. Unlike most motor-driven flapping systems, the flapping in this system is actuated by a unimorph piezoceramic actuator. The artificial wings are first made of a thin polyethylene sheet and then corrugated. As the wings are assembled through a pitching hinge, they can passively rotate about the hinge during the flapping motion due to the resultant aerodynamic force. The effects of the rotation, corrugation, and clapping of the wings are experimentally explored with respect to the vertical force produced by the flapping system. A smoke-wire flow visualization is also conducted to confirm whether the flapping-wing system can generate leading edge and trailing edge vortices, which are essential for generating lift and thrust in insect flight.

Magnetic double gradient mechanism for flapping oscillations of a current sheet

Abstract: [1] A new kind of magnetohydrodynamic waves are analyzed for a current sheet in a presence of a small normal magnetic field component varying along the sheet. As a background, two simplified models of a current sheet are considered with a uniform and nonuniform current distributions in the current sheet. On a basis of these two models, the flapping-type waves are obtained which are related to a coexistence of two gradients of the tangential and normal magnetic field components along the normal and tangential directions with respect to the current sheet. A stable situation for the current sheet is associated with a positive result of the multiplication of the two magnetic gradients, and unstable (wave growth) condition corresponds to a negative result of the product. In the stable region, the “kink”-like wave mode is interpreted as so called flapping waves observed in the Earth's magnetotail current sheet.

Simulation and Parameter Variation of Flapping-Wing Motion Based on Dragonfly Hovering

Abstract: The flapping motion of a wing based on the hind wing of the Aeschna juncea dragonfly is simulated using a three-dimensional incompressible Navier-Stokes solver. The performance of the wing is investigated by variation of a number of kinematic parameters. Flapping amplitudes of between 10 and 60 deg (half-angle) and frequencies of 1 to 300 Hz are considered, resulting in a Reynolds number range of 100 to 50,000. The flapping amplitude observed for Aeschna juncea is shown to maximize the ratio of mean vertical force produced to power required.

Aerodynamic forces and flow fields of a two-dimensional hovering wing

Abstract: This paper reports the results of an experimental investigation on a two-dimensional (2-D) wing undergoing symmetric simple harmonic flapping motion. The purpose of this investigation is to study how flapping frequency (or Reynolds number) and angular amplitude affect aerodynamic force generation and the associated flow field during flapping for Reynolds number (Re) ranging from 663 to 2652, and angular amplitudes (αA) of 30°, 45° and 60°. Our results support the findings of earlier studies that fluid inertia and leading edge vortices play dominant roles in the generation of aerodynamic forces. More importantly, time-resolved force coefficients during flapping are found to be more sensitive to changes in αA than in Re. In fact, a subtle change in αA may lead to considerable changes in the lift and drag coefficients, and there appears to be an optimal mean lift coefficient \( \left( {\overline {C_{{\text{l}}} } } \right) \) around αA = 45°, at least for the range of flow parameters considered here. This optimal condition coincides with the development a reverse Karman Vortex street in the wake, which has a higher jet stream than a vortex dipole at αA = 30° and a neutral wake structure at αA = 60°. Although Re has less effect on temporal force coefficients and the associated wake structures, increasing Re tends to equalize mean lift coefficients (and also mean drag coefficients) during downstroke and upstroke, thus suggesting an increasing symmetry in the mean force generation between these strokes. Although the current study deals with a 2-D hovering motion only, the unique force characteristics observed here, particularly their strong dependence on αA, may also occur in a three-dimensional hovering motion, and flying insects may well have taken advantage of these characteristics to help them to stay aloft and maneuver.

Energy Extraction Through Flapping Foils

Abstract: We demonstrate experimentally that flapping foils within an oncoming stream can efficiently extract energy from the flow, thus offering an attractive, alternative way for energy production. The greatest promise for flapping foils is to use them in unsteady and turbulent flow, where their own unsteady motion can be controlled to maximize energy extraction. The foils in this study perform a sinusoidal linear motion (sway, or heave) in combination with a sinusoidal angular motion (yaw or pitch); the effect of three principal parameters is studied systematically, yaw amplitude, the Strouhal number, and the phase angle between sway and yaw. The foils are made of aluminum, in the shape of NACA 0012 airfoils, using three different aspect ratios, 4.1, 5.9, and 7.9; they were tested at Reynolds numbers around 14,000. Efficiencies of up to 52 ± 3% are achieved with simple sinusoidal motions, thus demonstrating that foils can efficiently extract energy from unsteady flows.Copyright © 2008 by ASME