Showing papers on "Flapping published in 1996"

Leading-edge vortices in insect flight

Abstract: INSECTS cannot fly, according to the conventional laws of aerodynamics: during flapping flight, their wings produce more lift than during steady motion at the same velocities and angles of attack1–5. Measured instantaneous lift forces also show qualitative and quantitative disagreement with the forces predicted by conventional aerodynamic theories6–9. The importance of high-life aerodynamic mechanisms is now widely recognized but, except for the specialized fling mechanism used by some insect species1,10–13, the source of extra lift remains unknown. We have now visualized the airflow around the wings of the hawkmoth Manduca sexta and a 'hovering' large mechanical model—the flapper. An intense leading-edge vortex was found on the down-stroke, of sufficient strength to explain the high-lift forces. The vortex is created by dynamic stall, and not by the rotational lift mechanisms that have been postulated for insect flight14–16. The vortex spirals out towards the wingtip with a spanwise velocity comparable to the flapping velocity. The three-dimensional flow is similar to the conical leading-edge vortex found on delta wings, with the spanwise flow stabilizing the vortex.

Thrust Generation due to Airfoil Flapping

Abstract: Thrust generation on a single flapping airfoil and a flapping/stationary airfoil combination in tandem is studied parametrically. A multiblock Navier-Stokes solver is employed to compute unsteady flowfields. The unsteady flowfield around a single flapping airfoil is also computed by an unsteady potential flow code. The numerical solutions predict thrust generation in flapping airfoils and a significant augmentation of thrust in flapping/stationary airfoil combinations in tandem. The propulsive efficiency is found to be a strong function of reduced frequency and the amplitude of the flapping motion. At a flapping amplitude of 0.40 chord lengths and a reduced frequency of 0.10, the propulsive efficiency of a single NACA 0012 airfoil was computed to be more than 70 %. For the airfoil combination in tandem, the propulsive efficiency was augmented more than 40% at a reduced frequency of 0.75 and a flapping amplitude of 0.20 chord lengths when the airfoils are separated by about two chord lengths.

Simulating moth wing aerodynamics - Towards the development of flapping-wing technology

Abstract: The mechanization of flapping-wing flight is addressed. A tethered moth's flapping wings are simulated using an unsteady aerodynamic panel method and accounts for wing flexibility using a finite element model. The resultant simulation code delineates both the aerodynamic and inertial forces acting on flapping, flexible wings undergoing arbitrary motion in the presence of large-scale vortices and establishes the importance of including the wake in the unsteady analysis of flapping flexible wings. A switching pattern is discovered where the magnitude and direction of the aerodynamic force are decoupled, thereby pointing to a means whereby control is achieved. Overall, important groundwork necessary for the establishment of the principles of flapping-wing flight is laid, leading to the development of a highly agile, alternative flight technology.

Unsteady aerodynamic model of flapping wings

Abstract: Propulsive forces can be generated with flapping or heaving wings traveling through a fluid, as demonstrated in animal flight. To examine the flowfield around variable geometry bodies, with specific application to the unsteady flow associated with flapping wings, an unsteady, three-dimensional, incompressible, potential flow model was developed. The problem is formulated in an inertial reference frame such that the body moves through an otherwise quiescent fluid. Results were compared with the limited experimental results available and analytical methods. In one case the model was applied to a flapping wing in a wind tunnel at high advance ratios (J = 4.31) where the computed average lift and thrust were within the error bounds of the experimental data. The model was also applied to high-frequency flapping flight (J = 0.76) of a pigeon flying in a wind tunnel, where the predicted lift matched the weight of the bird.

The advantages of an unsteady panel method in modelling the aerodynamic forces on rigid flapping wings

Abstract: This paper responds to research into the aerodynamics of flapping wings and to the problem of the lack of an adequate method which accommodates large-scale trailing vortices. A comparative review is provided of prevailing aerodynamic methods, highlighting their respective limitations as well as strengths. The main advantages of an unsteady aerodynamic panel method are then introduced and illustrated by modelling the flapping wings of a tethered sphingid moth and comparing the results with those generated using a quasi-steady method. The improved correlations of the aerodynamic forces and the resultant graphics clearly demonstrate the advantages of the unsteady panel method (namely, its ability to detail the trailing wake and to include dynamic effects in a distributed manner).

Minimum induced power requirements for flapping flight

Abstract: The Betz criterion for minimum induced loss is used to compute the optimal circulation distribution along the span of flapping wings in fast forward flight. In particular, we consider the case where flapping motion is used to generate both lift (weight support) and thrust. The Betz criterion is used to develop two different numerical models of flapping. In the first model, which applies to small-amplitude harmonic flapping motions, the optimality condition is reduced to a one-dimensional integral equation which we solve numerically. In the second model, which applies to large-amplitude periodic flapping motions, the optimal circulation problem is reduced to solving for the flow over an infinitely long wavy sheet translating through an inviscid fluid at rest at infinity. This three-dimensional flow problem is solved using a vortex-lattice technique. Both methods predict that the induced power required to produce thrust decreases with increasing flapping amplitude and frequency. Using the large-amplitude theory, we find that the induced power required to produce lift increases with flapping amplitude and frequency. Therefore, an optimum flapping amplitude exists when the flapping motion of wings must simultaneously produce lift and thrust.

A fringe shadow method for measuring flapping angle and torsional angle of a dragonfly wing

Abstract: A fringe shadow (FS) method was developed for measuring the flapping angle and torsional angle of a dragonfly wing during beating motion. This new method involves two sets of fringe patterns projected onto the wing from orthogonal directions. The torsional angle is determined using the length of the shadow of the wing chord that is cast by the two sets of fringe patterns. The flapping angle is determined using the shadowgraph of the wing projected by a laser. The advantages of the FS method are its capability (i) to measure the flapping angle and torsional angle of a dragonfly wing simultaneously using only one high-speed camera and (ii) to recognize the spanwise position of a section from the number of fringes, without having to use diffuse marks that are common in current methods. The resolution of the FS method can be changed easily by adjusting the fringe spacing. The measurement results for the torsional angle and flapping angle of a dragonfly wing prove the effectiveness of the FS method in studying the flight performance of dragonflies.

Numerical Simulations of Flow Modification of Supersonic Rectangular Jets

Abstract: Numerical simulations have been performed to study the flowfield and near-field noise of a supersonic rectangular jet with two paddles inserted into the flow. The paddles cause a strong flapping motion to develop that enhances mixing of the jet with the surroundings. These simulations have been used to determine the flapping motion's frequency, the mixing enhancement, the near-field noise, and the thrust loss associated with the paddles and to study the acoustic feedback mechanism that modulates the flapping motion. The flapping frequency has been estimated using the pitot pressure distributions at a sequence of times and Fourier analysis of the local pressure and z component of velocity. For paddles located x/h = 7.3 from the nozzle, where h is the narrow dimension of the nozzle, a frequency of 4700 Hz [St(h) = 0.136] with an amplitude of 157.5 dB at the nozzle lip has been predicted and is in agreement with experimental results. The pitot pressure drop, the mass, and the x-momentum fluxes along the flow direction have been used as a measure of jet mixing for jets with and without paddles inserted into the flow. In our numerical simulations, a control volume approach was used to estimate the thrust loss caused by the insertion of the paddles. The computational value of 13% is close to the experimental value of 14.4%, considering that the physical support for the paddles in the experiments is not included in the simulations. A special sequence of local pressure distribution plots, which highlight the acoustic waves, has been used to study the feedback mechanism that modulates the flapping motion.

The fundamentals of wind-driven water pumpers

Abstract: Basic theory aerodynamic theory wind characteristics types and performances of horizontal-axis turbine rotors major factors influencing the design of horizontal-axis machines furling systems for horizontal-axis machines pumps variable delivery pumps flapping vane, savonius and drag type machines structural design.

Parallel Optimization of Flapping Airfoils in a Biplane Configuration for Maximum Thrust

Abstract: Publisher Summary This chapter discusses parallel optimization of flapping airfoils in a biplane configuration for maximum thrust. Flapping airfoils in a biplane configuration are optimized for a maximum thrust production. The unsteady viscous flowfields around flapping airfoils in biplane configuration are computed by solving the Navier–Stokes equations on overset grids. Computations on each subgrid are performed in parallel. PVM message passing library routines are used in the parallel solution algorithm. The computed flowfields are analyzed in terms of aerodynamic loads, instantaneous distribution of flow variables, and unsteady particle traces. The computational domain is discretized with overset grids. The periodic flapping motion of airfoils in a plane configuration is described in a combined pitch and plunge. The pitch and plunge amplitudes and the phase shift between them are optimized for a range of flapping frequencies. At low flapping frequencies, flapping airfoils in a biplane configuration produce more thrust than a single flapping airfoil. However, at high flapping frequencies the pitch amplitude tends to go to zero, which promotes an early leading edge vortex formation and limits the thrust production.

Miss distance for rotor blade-vortex interaction noise reduction

Abstract: Blade-vortex interaction noise generated by helicopter main rotor blades is one of the most important noise mechanisms both in military aspects and community acceptance. Noise generating mechanisms and controlling concepts are studied, particularly in terms of a blade-vortex miss distance. A comprehensive code (CAMRAD-JA) coupled with a finite difference code(FPR) has been used to assess airloads and blade aeroelastic deformations. The results are compared with existing BO-105 test results. Potential methods to control the blade-vortex miss distance are studied by exploiting the blade flapping and torsional characteristics through active blade control technology. Introduction Rotor blade-vortex interaction(BVI) noise is generated from unsteady pressure fluctuations on the blade's leading edge due to interactions with previously generated tip vortices during descent or maneuvering flight conditions. This BVI noise is loud and impulsive in nature and it plays an important role both in military aspects and community annoyance. This paper is work of the US Government and is not subject to copyright protection in the United States. * Associate Fellow Understanding of the noise generating mechanisms has advanced substantially in recent years, but the progress of controlling this noise has been limited. Major controlling parameters for rotor BVI noise have been analytically investigated (ref.l) and several parameters were identified, such as tip vortex strength, blade-vortex miss distance, and blade pitch. Among many efforts to control these parameters, active blade control concepts have recently achieved some success in reducing BVI noise. In particular, the higher harmonic blade pitch control (HHC) and individual blade control (IBC) concepts have been tested and significant mid-frequency noise reductions of 5-6 dB have been reported (refs. 2-10). Meanwhile, several analyses (refs. 2,4,6, and 11) reported that the HHC concept with a proper blade pitch control schedule changes blade-vortex miss distances, vortex geometry, and blade circulation. Results also indicated that pitch control would not only modify the pitch, but would also modify the strengths of the shed vortices as well as possibly the interaction locations. Decreased blade loading in specific azimuthal regions has been identified to reduce BVI noise (ref.5); in addition, blade-vortex miss distances are definitely affected by HHC, and they evidently represent a dominant factor in the BVI noise reductions (refs.4 and 6). A vibration suppression mechanism by HHC was also analytically investigated through the flapping moment at the blade root (ref.6). Results showed that a vibration suppression can be achieved by reducing the blade flapping motion, thus decreasing the blade-vortex miss distance and, in turn, increasing the BVI noise. An international cooperative program, called Higher-Harmonic Control Aeroacoustics Rotor Test(HART), was carried out with a 40%-geometricalIy and dynamically scaled model of a BO-105 main rotor in the DNW in order to investigate the effects of HHC inputs on BVI noise reduction(refs.!2-18). This program concluded that a blade-vortex miss distance is strongly influenced by both a tip vortex trajectory and blade aeroelastic characteristics. In this paper, results of a study of the effect of a blade-vortex miss distance on rotor BVI noise reduction are presented. Potential concepts using active blade control technology to control the miss distance are also discussed. For this study, the in-house CAMRAD-JA/FPR codes (ref. 19) were used; the results are compared with the existing HART test data (ref.18). Blade Deformation and Airload From a blade-vortex interaction geometry and acoustic radiation, it has been known that the major contribution to acoustic radiation comes from near-parallel interactions between a blade and a vortex, which happen in both the advancing and the retreating sides. For example, a fourbladed model rotor of BO-105 shows that the major contribution of noise comes from the interaction of a blade at the azimuthal angle(y) of 50° with a vortex generated at y of 130° in the advancing side. In the retreating side, it comes from the interaction at 310° of the blade position with the vortex generated at y of approximately 230° (refs.14 and 15). Blade tip deflection, blade-vortex miss distance, airload, and vortex circulation will be discussed at these two azimuthal angles of 50° and 130° for the following three flight conditions: baseline case(run 140, without HHC), low noise case (run 138, with HHC inputs of 0.8° amplitude and 300° phase angle) and low vibration case (run 133, with HHC inputs of 0.8° amplitude and 180° phase angle) from the HART test (ref.18). Other common parameters are: advance ratio=0.15, tip Mach number =0.64, shaft angle=5.3° and thrust coefficient(Cr) = 0.0044. Blade tip deflection: Blade tip deflections along azimuthal angles play an important role in determining blade-vortex miss distances during interactions. In particular, blade positions at the time of both vortex generation and blade-vortex interaction are important to determine the miss distance. Figure 1 shows a blade tip deflection history over azimuthal angles from the HART test, along with results from the AFDD in-house analysis for rigid and soft blades. In this figure, measured blade tip deflec-tions show I/rev characteristics for the baseline case and 3/rev characteristics for cases with HHC 3P inputs(run 138 and 133). In the analytical results, the tip deflection history for a rigid blade shows substantially different characteristics compared to the experimental data. However, the results with a CAMRAD-JA elastic blade model show characteristics similar to those with the measured one in mode shapes, but not in amplitudes. This result has a serious implication in determining a miss distance. The measured blade tip positions at v=130° and \|/ = 50° show an interesting phenomenon. For the low noise case(run 138), the blade tip position is 3.5 cm at \y=130°, the point at which the vortex is generated, and the blade tip position is 9 cm at y=50°, the point at which the blade interacts with the vortex. The relative separation distance of blade tip positions at the two azimuthal angles is about 5.5 cm, which is much larger compared to 1.5 cm for the baseline case. This large separation distance of the blade tip positions directly contributes to the large bladevortex miss distance. For the low vibration case (run 133), the relative separation distance of blade tip positions at v = 130° and 50° is much smaller compared to those of the baseline case. The relative separation distance can also be obtained through the blade flapping deformations at y = 50° and Y = 130° as shown in figure 2. These figures suggest that blade tip deflections and flapping deformations can be controlled by adjusting HHC inputs.

Modeling turbulence seen by multibladed rotors for predicting rotorcraft response with three-dimensional wake

Abstract: A turbulence-input matrix for multibladed rotors is developed in the time domain (correlation matrix) and in the frequency-time domain (spectral density matrix), and earlier-studied turbulence models are recovered as special cases. The matrix is then applied to isolated rotors with one and three blades executing flapping motion; a linear airfoil theory with a 3D finite-state wake model is used. The second-order response statistics of flapping and hub shear are predicted. The statistics show that while the station-to-station cross-correlation effects on response are negligible, the effects of blade-to-blade cross-correlation and dynamic wake are appreciable. Moreover, wake modeling with at least three harmonics is required for converged results. (Author)

Damping force control device of vehicle

Abstract: PURPOSE:To optimally control rolling, pitching and flapping by disturbance of a road surface by detecting vertical directional vibrations of horizontal directional two positions, controlling a damping force changing mechanism when the vibrations become large, and changing control sensitivity of a damping force control means by the correlation between both detecting values. CONSTITUTION:Damping force of shock absorbers 10A to 10D arranged between a lower arm and a car body is switched in two stages of hard and soft by switching opening of valves 14a to 14d. Detecting signals of vertical acceleration sensors 21 and 22 arranged on the car body in the vicinity of left and right front wheels are inputted to a microcomputer 25 through band-pass filters 23 and 24, and damping force is controlled by driving the valves 14a to 14d through driving circuits 26a to 26d. The vertical acceleration sensors 21 and 22 judge that in which detected vibration is put among the in-phase and antiphase relationships, and changes control sensitivity of the damping force according to this. Therefore, optimal damping force control fitted for an occupant can be performed on flapping, pitching and rolling of the car body.

Design improvements to the ESI-80 wind turbine

Abstract: This paper describes two investigations related to improvements to an ESI-80 wind turbine. One of them involved modeling the tip flaps during braking. The other was a study of the turbine behavior with various delta-3 angles. These topics are of interest since the turbine is a two-bladed, teetered, free-yaw machine with tip flaps and an adjustable delta-3 angle. Tip flaps are used for slowing the turbine during shutdown and as an emergency system to insure that the rotor does not go into an overspeed condition in the event of failure of other parts of the system. Upon deployment, the tip flaps are exposed to a number of varying forces including aerodynamic, damper, spring, centripetal, and gravitational forces and forces at the hinged connection to the blades. For maximum braking the angle of tip flap deployment needs to be as large as possible without striking the blades in overspeed conditions and when covered with ice. To investigate tip flap design tradeoffs, a dynamic model of the tip flaps on the modified ESI-80 turbine was developed. Results include a determination of the effect of the addition of weight to the flap, overspeed conditions, and changes in damping coefficient. Changes in the delta-3more » angle can be used to couple pitching and flapping motions, affecting both teeter and yaw behavior. These effects have been investigated using a modified version of YawDyn. The effects of changes in the delta-3 angle on the teeter and yaw behavior of the modified ESI-80 wind turbine were investigated. Results show that increased teeter excursions in steady high winds can be reduced by increasing the delta-3 angle. Increasing the delta-3 angle may also increase yaw motion in low wind speeds. Results suggest that the optimum delta-3 angle for improved performance may be substantially greater than the presently used angle of zero degrees. 8 refs., 16 figs.« less

Control issues for rigid-in-plane helicopter rotors

Abstract: This article addresses an adverse control effect that arises in helicopter rotors when the following conditions occur: each blade is attached to the shaft through a single flapping hinge, there is no offset of the flapping hinges, the blades are constrained from leading or lagging, and the shaft maintains a strictly constant rate of rotation. When the rotor disk is tilted from the shaft, a bending moment is induced in the shaft that tends to further misalign the shaft and body from the rotor. The effect, which can amount to a control reversal, also applies to teetering rotors. The effect is derived analytically and corroborated by computer simulation. Two-blade rotors relieve the adverse effect by a two-per-rev fluctuation in the rate of rotation. The lesson for flight simulation is that a variable rate of rotation that responds to engine and rotor torque must be allowed in modeling rotor blade dynamics. In multiblade rotors the adverse effect cannot be relieved without lead/lag hinges. It can be overcome by offset flapping hinges.

Analysis of attitude change of space satellite by flapping motion

Abstract: This paper analyzes the attitude change of a space flapping robot (SFR) by flapping motion. The SFR consists of a satellite body and two wings, which are used for attitude control of the satellite body and each of which can be moved about two axes in each joint connected with the satellite. For developing the control algorithm of the SFR body, the attitude change of the body after one cyclic motion of wing flapping is analytically examined using Green's theorem, where a specified wing motion is assumed, and its value can be described by a simple algebraic equation. The validity of this analysis is confirmed by computer simulation.

Numerical Simulation of the Flow Field about a Multi-Element Airfoil with Oscillating Flap.

Abstract: : Investigation of steady and unsteady flowfields over airfoils is an active area of current computational and experimental research. In this study, the compressible, viscous, flow over a single and multi-element airfoil is numerically simulated by solving the Navier Stokes equations. The motivation for this work includes interest in studying the effects of a stationary/flapping airfoil combination in tandem configuration. A single-block Navier-Stokes (NS) solver is employed to compute unsteady flowfields. Turbulence is treated using the Baldwin-Lomax turbulence model. A single C-grid is generated and it is partially distorted to simulate the flapping motion. Numerical solutions are obtained for flows at a fixed angle of attack and for unsteady flows over flapping airfoils. The numerical solutions agree well with the experimental data The difficulties faced during the study are discussed and future improvements are suggested.