Showing papers on "Flapping published in 2000"

Flexible filaments in a flowing soap film as a model for one-dimensional flags in a two-dimensional wind

Abstract: The dynamics of swimming fish and flapping flags involves a complicated interaction of their deformable shapes with the surrounding fluid flow. Even in the passive case of a flag, the flag exerts forces on the fluid through its own inertia and elastic responses, and is likewise acted on by hydrodynamic pressure and drag. But such couplings are not well understood. Here we study these interactions experimentally, using an analogous system of flexible filaments in flowing soap films. We find that, for a single filament (or 'flag') held at its upstream end and otherwise unconstrained, there are two distinct, stable dynamical states. The first is a stretched-straight state: the filament is immobile and aligned in the flow direction. The existence of this state seems to refute the common belief that a flag is always unstable and will flap. The second is a flapping state: the filament executes a sinuous motion in a manner akin to the flapping of a flag in the wind. We study further the hydrodynamically coupled interaction between two such filaments, and demonstrate the existence of four different dynamical states.

Vortex shedding and frequency selection in flapping flight

Abstract: Motivated by our interest in unsteady aerodynamics of insect flight, we devise a computational tool to solve the Navier–Stokes equation around a two-dimensional moving wing, which mimics biological locomotion. The focus of the present work is frequency selection in forward flapping flight. We investigate the time scales associated with the shedding of the trailing- and leading-edge vortices, as well as the corresponding time-dependent forces. We present a generic mechanism of the frequency selection as a result of unsteady aerodynamics.

Mechanical performance of aquatic rowing and flying.

Abstract: Aquatic flight, performed by rowing or flapping fins, wings or limbs, is a primary locomotor mechanism for many animals. We used a computer simulation to compare the mechanical performance of rowing and flapping appendages across a range of speeds. Flapping appendages proved to be more mechanically efficient than rowing appendages at all swimming speeds, suggesting that animals that frequently engage in locomotor behaviours that require energy conservation should employ a flapping stroke. The lower efficiency of rowing appendages across all speeds begs the question of why rowing occurs at all. One answer lies in the ability of rowing fins to generate more thrust than flapping fins during the power stroke. Large forces are necessary for manoeuvring behaviours such as accelerations, turning and braking, which suggests that rowing should be found in slow-swimming animals that frequently manoeuvre. The predictions of the model are supported by observed patterns of behavioural variation among rowing and flapping vertebrates.

Computational Study of Flapping Airfoil Aerodynamics

Abstract: Unsteady, viscous, low-speed e ows over a NACA 0012 airfoil oscillated in plungeand/orpitch at various reduced frequency,amplitude, andphaseshift arecomputed. Vortical wakeformations, boundary-layere owsat theleading edge, the formation of leading-edge vortices and their downstream convection are presented in terms of unsteady particletraces.Flowseparationcharacteristicsandthrust-producingwakeproe lesareidentie ed.Computedresults compare well with water tunnel e ow visualization and force data and other computational data. The maximum propulsive efe ciency is obtained for cases where the e ow remains mostly attached over the airfoil oscillated in a combined pitch and plunge.

Direct numerical simulation of bifurcating jets

Abstract: The mechanisms leading to bifurcating jets are investigated by means of direct numerical simulation. Two distinct types of jets were obtained by applying a bi-modal perturbation at the nozzle. When forcing simultaneously the counter-rotating helical modes (flapping mode) with the same amplitude and the same frequency, the jet splits into two branches, taking a distinct Y-shape. A different evolution of the jet (Ψ-shape) is observed when superposing the axisymmetric mode at the most amplified unstable frequency on the flapping mode with the same amplitude, but subharmonic frequency. In both cases a spectacular increase of the jet spreading is observed.

Passive aeroelastic tailoring for optimal flapping wings

Abstract: I N AN effort to overcome many of the Reynolds number limitations associated with micro air vehicles (MAVs) much attention has turned to the investigation of flapping flight. Biologists have studied bird and insect flight empirically for quite some time. A good review of this material can be found in Refs. 1, 2, and 3 while some interesting recent work can be found in Refs. 4, 5, and 6. One thing that is clear is that all of these creatures use two specific mechanisms to overcome the small-scale aerodynamic limitations: flexible wings and flapping wings. Birds and insects exploit the coupling between flexible wings and aerodynamic

Flap Edge Aeroacoustic Measurements and Predictions

Abstract: An aeroacoustic model test has been conducted to investigate the mechanisms of sound generation on high-lift wing configurations. This paper presents an analysis of flap side-edge noise, which is often the most dominant source. A model of a main element wing section with a half-span flap was tested at low speeds of up to a Mach number of 0.17, corresponding to a wing chord Reynolds number of approximately 1.7 million. Results are presented for flat (or blunt), flanged, and round flap-edge geometries, with and without boundary-layer tripping, deployed at both moderate and high flap angles. The acoustic database is obtained from a Small Aperture Directional Array (SADA) of microphones, which was constructed to electronically steer to different regions of the model and to obtain farfield noise spectra and directivity from these regions. The basic flap-edge aerodynamics is established by static surface pressure data, as well as by Computational Fluid Dynamics (CFD) calculations and simplified edge flow analyses. Distributions of unsteady pressure sensors over the flap allow the noise source regions to be defined and quantified via cross-spectral diagnostics using the SADA output. It is found that shear layer instability and related pressure scatter is the primary noise mechanism. For the flat edge flap, two noise prediction methods based on unsteady-surface-pressure measurements are evaluated and compared to measured noise. One is a new causality spectral approach developed here. The other is a new application of an edge-noise scatter prediction method. The good comparisons for both approaches suggest that much of the physics is captured by the prediction models. Areas of disagreement appear to reveal when the assumed edge noise mechanism does not fully define, the noise production. For the different edge conditions, extensive spectra and directivity are presented. Significantly, for each edge configuration, the spectra for different flow speeds, flap angles, and surface roughness were successfully scaled by utilizing aerodynamic performance and boundary layer scaling method developed herein.

Experimental Simulation of Fish-Inspired Unsteady Vortex Dynamics on a Rigid Cylinder

Abstract: The unsteady hydrodynamics of the tail flapping and head oscillation of a fish, and their phased interaction, are considered in a laboratory simulation. Two experiments are described where the motion of a pair of rigid flapping foils in the tail and the swaying of the forebody are simulated on a rigid cylinder. Two modes of tail flapping are considered: waving and clapping. Waving is similar to the motion of the caudal fin of a fish. The clapping motion of wings is a common mechanism for the production of lift and thrust in the insect world, particularly in butterflies and moths. Measurements carried out include dynamic forces and moments on the entire cylinder-control surface model, phase-matched laser Doppler velocimetry maps of vorticity-velocity vectors in the axial and cross-stream planes of the near-wake, as well as dye flow visualization. The mechanism of flapping foil propulsion and maneuvering is much richer than reported before. They can be classified as natural or forced. This work is of the latter type where discrete vortices are forced to form at the trailing edge of flapping foils via salient edge separation. The transverse wake vortices that are shed, follow a path that is wider than that given by the tangents to the flapping foils

Shear Layer Flapping and Interface Convolution in a Separated Supersonic Flow

Abstract: The steadiness and convolution of the interface between the freestream and recirculating/wake core regions in an axisymmetric, separated supersonic flow were studied using planar imaging. Five regions along the shear layer/wake boundary were investigated in detail to quantify the effects that key phenomena, such as the recompression and reattachment processes, have on the development of large-scale unsteady motions and interfacial convolution. These studies show that flapping motions, when viewed from the side, generally increase in magnitude, in relation to the local shear layer thickness, with downstream distance, except at the mean reattachment point, where they are slightly suppressed. When viewed from the end, the area-based (pulsing) fluctuations increase monotonically downstream as a percentage of the local area, whereas the position-based (flapping) motions show pronounced peaks in magnitude in the recompression region and in the developing wake. The interface convolution increases monotonically with downstream distance in both the side- and end-view orientations

A Computational Study for Biological Flapping Wing Flight

Abstract: A computational model for unsteady aerodynamics of a flapping wing has been developed based on the strip theory, which makes use of the concept of dividing the wing into a number of thin strips. This enables us to study the wing as a set of airfoi1s next to one another by assuming no crossflow between them. Wing kinematics including normal and chordwise force calculations was considered to calculate average lift, thrust, power requirements and propulsive efficiency for a flapping wing in flight. In addition, an optimization procedure was developed for obtaining maximum propulsive efficiency within the range of possible flying conditions. Computations were performed on a mechanical flying Pterosaur replica as wel1 as smal1er biological species including the Corvus monedula, Larus canus and Columba livia, which makes use of the vortex gaits. The effect of aerodynarmic parameters on the performance of these biological flight vehicles was studied. It was found that the propulsive efficiency of al1 species considered were around 65-75% whereas the lift and thrust varied largely depending on the weight, flapping frequency and flight speed of the species. The range of dynamic twist for sustainable flying conditions (Lift＞Weight) also varied for different species, becoming smal1er as the size of a bird increases.

Turbulent wake behind a single-element wing in ground effect

Abstract: A study was performed in order to investigate the flowfield characteristics of a wing in ground effect. A highly cambered single element wing, with the suction surface nearest to the ground, was used to research the effect of changing the operating height from the ground at a single incidence. The results are of direct relevance to both aeronautical and racing car applications. A Laser Doppler Anemometry survey has been used to investigate the ground effect on the mean flow characteristics of the wake of the wing. The size of the wake was found to increase with proximity to the ground. A downward shift of the path of the wake was also observed. Instantaneous Particle Image Velocimetry elucidates the unsteady flow features. Discrete vortex shedding was seen to occur behind the finite trailing edge of the wing (Figure 1). As the ground height is reduced, separation occurs on the suction surface of the wing and the vortex shedding is coupled with a flapping motion of the wake in the transverse direction.

Method of motion of lifting surface in fluid medium and device for realization of this method ("fly" and "fan" versions)

Abstract: FIELD: aviation; wind and hydraulic power engineering; shipbuilding. SUBSTANCE: method of motion of lifting surface in fluid medium is characterized by uniform "flapping" and "angular" motion of lifting surface over entire cycle of rotation. Lifting surface is smoothly rotated at constant angular velocity about its own axis of "angular" rotation. Axis of "angular" rotation is turned at constant angular velocity around axis of "flapping" rotation which is fixed in reference system. Ratio of magnitudes of angular velocities of "angular" and "flapping" rotation is equal to 1:2. Angle between vectors of angular velocities ranges from 90 deg to 180 deg. According to first version of realization of proposed method, each blade of paddle-wheel is rotatable around its own axis which is parallel to axis of wheel and is provided with link-satellite of planetary mechanism; according to second version, each blade of paddle-wheel is rotatable around its own axis which is perpendicular to axis of propeller and is provided with link-satellite of planetary mechanism. Planetary mechanism ensures correspondence of one revolution of blade around axis of wheel or propeller to half revolution of blade around its own axis. Blade of paddle-wheel revolves around its own in opposite direction. EFFECT: improved operational characteristics. 4 cl, 6 dwg

Aerodynamic aspects of flapping wing micro air vehicles

Abstract: The main objective of this paper is to discuss the nature of the little known aerodynamics of the flapping wing micro air vehicles (MAVs), inspired by insect flight. These small (ca 6 inches, or hand-held) reconnaissance vehicles will fly inside buildings, which requires hover for observation and agility at low speeds to move in confined spaces. For this flight envelope insect-like flapping flight seems to be an optimal mode of flying. The investigation of the aerodynamics of the flapping wing MAVs is very challenging. The problem involves complex unsteady, viscous flows (dominantly laminar) with the moving wing generating vortices and interacting with them. At this early stage of research a strategy for numerical modelling has been established. The initial CFD capabilities and their application are described. In particular, scaled up wings of a Bibio fly are used with representative wing kinematics to analyse steady-state, inviscid flow in forward flight.

Unsteady Separated Flowfields Surrounding a Flapping Airfoil: Applications to Micro-Air Vehicles

Abstract: A computational study of a periodically pitching and plunging small airfoil in very low Reynolds number flow is presented. The computed vortex structures generated by the airfoil motion are examined, as well as their effects due to pressure distributions around the airfoil. Discussion is presented based upon existing experimental results from open literature, as well as a detailed look at the results of this study. The intended outcome is to produce a computational model that can be utilized in a parametric analysis of flapping modes advantageous to micro-air vehicle development A large variation in both surface pressure distributions and vortex structures are seen, which correspond to small changes in wing motion. The current methods are capable of capturing these differences, thus allowing the future progression of a parametric analysis to identify lifting and propulsive flapping modes of a small airfoil. NOMENCLATURE = airfoil chord, 5cm U = free stream velocity u = kinematic viscosity of air Re = Uc/u, flow Reynolds number 6 = angle between mean camber line of airfoil and horizon, nose-up pitch is positive cop pitching frequency = phase angle between o>p and p/2U, reduced pitching frequency Kj, = ca>h/2U, reduced plunging frequency Po = free stream pressure P = pressure at airfoil surface x = chordwise distance t = time at which a flow condition is shown At = time increment between frames INTRODUCTION Numerous research efforts have recently been dedicated to the development of micro-air vehicles (MAV's) [2,3,7,8,9,16,17]. According to the Defense Advanced Research Projects Agency, the chief driving force behind MAV development, an MAV is a short-range (minimum range one mile) aircraft whose dimensions span no more than * Graduate Research Assistant, Student Member AIAA T Professor, Associate fellow AIAA Copyright © 2000 The American Institute of Aeronautics and Astronautics Inc. All rights Reserved. American Institute of Aeronautics and Astronautics (c)2000 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization. 5cm in any direction. The goal of such an aircraft is to provide low-cost, inconspicuous, unmanned, mission-critical data gathering from the air [7,10]. One of the greatest challenges to MAY technology progression is to develop propulsion for such a small craft. Traditional propulsion methods simply are not readily adaptable to MAV's, due largely to size and efficiency considerations [2,8,9,16]. One possibility for propelling, lifting, and controlling MAVs is based on the flapping wing, which is seen repeatedly in nature. Unsteady, separated flow structures generated by flapping wings can produce vortex pairs with the effect of a jet [4,5,12,13,14]. This jet effect produces forces and moments on the airfoil, which, if controlled properly, can produce flight mode characteristics similar to birds and insects [11,15,18]. Although computational work in the Micro-Air Vehicle realm is still coming of age, there have been experimental works well-suited as precursors and the data from these works is available in literature. It is known that propulsive lifting vortex structures can be produced by airfoils undergoing either pitching and/or plunging oscillations [1,4,5,12]. The nature of these vortices is highly dependent on the frequency, phase angle, and amplitude of flapping motion, and the magnitude of the relative wind [1,4]. Figure 1 shows a sketch of potentially desirable propulsive vortex structures. According to Anderson (et al.) [1,18], the most efficient propulsive modes are seen when no leading-edge separation occurs. In order to produce lift from the same wing, however, such separation (termed dynamic-stall vortex separation) is deemed necessary [1]. If the parameters of the airfoil motion producing these structures are controlled properly, the lift and thrust modes have the potential of being very efficient. Ideally, the same wing mechanism used for forward flight could also be controlled to allow hovering flight and therefore greater maneuverability. This could theoretically be accomplished by simply altering the pitching/plunging motion [6]. Figure 1. a) Initial leadingand trailing-edge vortex formation b) Shedding of initial vortices and formation of new vortices c) Shed vortices form a propulsive jet pair As shown in Figure 1, thrust is produced, according to the conclusion of Freymuth [4] by the formation of unique vortex pairs. These vortices, which are shed from the trailing edge of the airfoil, propel the airfoil forward due to their jet effect. In physical situations, these propulsive signatures can be seen and have been called reverse-von Karman vortex streets. In Figure 2, the flow developments described above, as observed by Freymuth [4], are shown. Note that the vortex structure shown creates a jet-type street rather than a commmon Von Karman street The end goal of this study is to define an effective range of flapping frequencies which may be used to provide lift and propulsion for MAV's. To this end, it becomes necessary to examine the pressure distributions over the wing. With this in mind, a study of unsteady, separated flowfields around a small ornithopter airfoil is conducted. The airfoil is small in size and is studied while both pitching and plunging in an oscillatory manner, at a low Reynolds number. American Institute of Aeronautics and Astronautics (c)2000 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization. Figure 2. Closeup of a vortex street generated behind the trailing edge of an airfoil pitching periodically about the quarter chord at low Reynolds number [Taken from reference 4], PARAMETRIC CONSIDERATIONS

3 – Rotor aerodynamics and dynamics in forward flight

Abstract: Publisher Summary For determination of the blade lift, drag and flapping moment, and ultimately, rotor performance, it is necessary to determine the induced velocity in forward flight. This chapter discusses the various formulas to calculate induced velocity: Glauert's formula and the Mangler and Squire expression. A series of pioneering attempts to measure the induced velocity in forward flight were made, such as flow patterns revealed by smoke filaments and the tunnel test. The dynamics of the blades and rotor are equally susceptible to simple approaches based on straightforward ideas of induced velocity, aerofoil characteristics, and blade modeling.

Manpower flapping wing flight vehicle

Abstract: The utility model discloses a flying vehicle flapping wings by manpower. The upper end of an air bag of the flying vehicle is connected with a fender rod, the fender rod and the air bag are connected between a left wing and a right wing, and slits are arranged on the left wing and the right wing. The slits imitate an outer half wing of a glede on utility, and the slits can help a pilot to generate more backward thrust in the process of flapping. The air bag is extended to the back part of two feet of the pilot, and the barycenter of a body is shifted forward, which is convenient for keeping the equilibrium of the body in a horizontal direction in the process of flying. The utility model has the advantages of simple structure and high safety.

1 – Basic mechanics of rotor systems and helicopter flight

Abstract: Publisher Summary This chapter discusses the fundamental mechanisms of rotor systems from the mechanical system, the kinematics motion and dynamics view. Adoption of blade hinges was an important step in the evolution of the helicopter. Problems are posed by the presence of hinges and the dampers, which are fitted to restrain lagging motion. The rotor hinge system leads to a study of the blade motion, rotor forces, and moments. Improvements in blade design and construction enabled rotors to be developed that dispensed with the flapping and lagging hinges. These “hinge-less,” or “semi-rigid,” rotor blades allow the flapping and lagging freedoms. In spite of the flexibility of rotor blades, much of helicopter theory can be affected by regarding the blade as rigid, with obvious simplifications in the analysis. The simple rotor system analysis allows the whole helicopter trimmed flight equilibrium equations to be derived.