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Unsteady airfoil experiments

M.F. Platzer & K.D. JonesAeroHydro Research & Technology Associates, Pebble Beach, CA, USA.

Abstract

This paper describes experiments that elucidate the dynamic stall phenomenon and the generationof thrust by flapping airfoils. To this end, flow visualizations of the vortices shed from a rapidlypitching airfoil and from an oscillating airfoil are presented.Also, wind tunnel tests of two flappingwing models are discussed and thrust measurements on these two models are included.

1 Introduction

In the paper on ‘Steady and unsteady aerodynamics’[1] we referred to two very important unsteadyaerodynamic effects, namely the Kramer and the Katzmayr effects. In this paper we present moredetailed experimental information in order to provide further physical insight.

2 Dynamic airfoil stall

The precise physics of the dynamic stall process has been illustrated in detail by the experimentsof Carr and Chandrasekhara [2] in a special dynamic stall facility at the NASA Ames ResearchCenter. In this wind tunnel the airfoil was mounted between two circular glass windows andthe window–airfoil–window assembly was then rotated in an oscillatory or ramp-type motionso that an unobstructed view of the flow could be achieved. Using a unique interferometrictechnique (point diffraction interferometry), which makes it possible to visualize the changesin flow density, they obtained detailed information about the formation and propagation of thedynamic stall vortex. Figure 1 shows an example of a NACA 0012 airfoil that was pitched rapidlyfrom an incidence angle of 12◦ to 25◦. The development of the leading-edge separation bubbleand its bursting leading to full stall can be seen quite clearly.

Recently, another set of careful measurements of the dynamic stall process on the NACA 0012was published by Lee and Gerontakos [4]; these measurements provide additional valuable infor-mation on the role of the laminar separation bubble and on the initiation, growth and convectionof the dynamic stall vortex in a low-speed flow of Reynolds number 135,000. Consistent with themeasurements of Carr and Chandrasekhara at a significantly larger Reynolds number, a laminar

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Unsteady Airfoil Experiments 699

Figure 1: Dynamic stall development on a transiently pitching airfoil [3].

separation bubble was found and the formation and convection of the dynamic stall vortex couldagain be identified.

3 Experimental studies of the Katzmayr effect

As already pointed out in the paper ‘Steady and Unsteady Aerodynamics’, a sinusoidally plungingairfoil acts like a propeller, generating a jet flow downstream of the airfoil. This phenomenon wasstudied experimentally in considerable detail by Jones et al. [5] and Lai and Platzer [6].

The stationary NACA 0012 airfoil in a water flow of 0.2 m/s sheds the Karman vortex streetshown in Fig. 2 with clockwise upper row vortices and counterclockwise lower row vortices.

Oscillating the airfoil with a frequency of 2.5 Hz and gradually increasing the plunge amplitudeto 10% of the chord from 1.25% produces the changes in vortex shedding shown in Fig. 3. Atfirst, mushroom-like vortices are shed (Fig. 3, top). As the amplitude is increased (Fig. 3, middle),the vortices are not shed alternately one at a time from the upper and lower airfoil surfaces.Instead, two vortices of the same sign are shed from the same side before another two are shedfrom the opposite side. On increasing the amplitude still further, the upper row vortices now rotatecounterclockwise and the lower row vortices rotate clockwise (Fig. 3, bottom). This type of vortex



700 Flow Phenomena in Nature

Figure 2: The Karman vortex street behind a stationary airfoil [6].

Figure 3: Development of a vortex street behind a harmonically plunging airfoil [6].




Figure 4: Time-averaged jet flow behind a harmonically plunging airfoil [9].

configuration induces an increased velocity between the two rows of vortices and therefore resultsin a jet-like flow.

This result can be verified by measuring the time-averaged velocity distribution with a Laser–Doppler velocimeter. Figure 4 shows an example of such a measurement which was taken down-stream of, but very close to, the airfoil trailing edge. It is seen that a distinct jet is generated (whichis predicted quite well with inviscid panel code calculations). With increasing distance from thetrailing edge this jet broadens while the peak velocity decreases [5, 6]. It is also of interest tonote that thrust production occurs even if there is no air or water flow around the airfoil, mak-ing it possible for birds to take off by flapping their wings. This phenomenon was investigatedexperimentally in another investigation by Lai and Platzer [7]. Furthermore, we mention thatthrust is also generated by pitching the airfoil instead of plunging it. However, much higher pitchfrequencies are required to obtain a finite thrust, as shown by Koochesfahani [8].

4 Flapping-wing propulsion

Thrust generation due to wing flapping was measured directly using the wind tunnel modelshown in Fig. 5. In this model two airfoils, with a chord length of 64 mm and an effective span of1200 mm, are allowed to flap with variable pitch and plunge amplitudes. The model was suspendedby four cables from the tunnel ceiling such that it could swing freely in the streamwise direction,as shown in Fig. 6. The thrust was determined by measuring the streamwise deflection of themodel when the wings were flapped. This deflection was measured by bouncing a laser beam offa small notch on the back of the rear nacelle, as shown in Fig. 6. Some of the results are shown inFig. 7. The thrust is seen to increase with increasing frequency and with increasing tunnel speed.Also shown are the comparisons with inviscid panel code calculations. The agreement betweenthe panel code and the experimental data is quite good, roughly 80% at higher velocities andfrequencies [9].




Figure 5: The flapping-wing wind tunnel test model [9].

Figure 6: The wind tunnel arrangement of a flapping-wing model [9].

Because of these encouraging results a much smaller model was built in preparation for itsuse on a micro air vehicle with flapping wings, as described in the paper ‘Flapping-wing micro-air vehicles’. This model is shown in Fig. 8. In contrast to the model in Fig. 5, the pitch and plungedegrees of freedom and the phasing between the two motions could not be controlled separately.Instead, to keep things mechanically simple, the pitch degree of freedom was obtained passivelyby attaching the flapping wings to the flapping mechanism with a flexible joint so that they wereable to pitch aeroelastically. Also, a second model was built, shown in Fig. 9, which incorporatedone more degree of freedom, namely flexible wing camber. The measured thrust as a function offlapping frequency is shown for both models in Fig. 10.




Figure 7: Thrust measurements for a flapping-wing model [9].

Figure 8: Micro air vehicle model with flapping wings [9].

Figure 9: Micro air vehicle model with flexible cambering wings [10].




Figure 10: Static thrust measurements as a function of flapping frequency [10].

5 Flapping-wing aerodynamics in hover

The dragonfly and many other insects have the ability to reduce their flight speeds to zero, i.e. tohover, followed by rapid maneuvers. The kinematics of the wings of these insects consists of twotranslational phases during which the wings sweep through the air with relatively slow changes inthe incidence angle, followed by rapid rotations at the end of each stroke. The motion is periodicand is composed of two half-cycles that, in hover, are mirror images of each other. At the endof each half-cycle the wing flips so that the leading edge points backwards and the wing’s lowersurface becomes its upper side. These wing flips allow the insects to maintain a positive incidenceangle and thus to generate lift during both forward and reverse strokes. However, this type of wingmotion involves strong unsteady aerodynamic effects because the wing is being decelerated at theend of the stroke and then reaccelerated again, thus causing the shedding of stopping and startingvortices. In addition, the flipping of the wing at the end of the stroke causes the shedding of a strongdynamic stall vortex. The shedding and interactions between these vortices involve complicatednonlinear aerodynamic phenomena that are insufficiently understood at the present time. We referto Freymuth’s paper on ‘Applications of the unsteady two-dimensional aerodynamic model tocommon dragonfly maneuvers’ for a more detailed discussion and especially his flow visualiza-tion shown in Fig. 4. Dickinson and Goetz [11] contributed flow visualization and quantitativeforce data for an airfoil that was accelerated from rest to a constant velocity, thus simulating onepart of the total motion of an insect in hover. This experiment was expanded by Dickinson [12] toinclude the airfoil rotation. Very recently, Kurtulus et al. [13] presented Navier–Stokes computa-tions which provide further insight into the intricate vortex shedding process caused by flappingairfoils in hover.

References

[1] Platzer, M.F. & Jones, K.D., Steady and unsteady aerodynamics. Flow Phenomena inNature, Vol. 2, pp. 531–541, 2006.




[2] Carr, L.W. & Chandrasekhara, M.S., Compressibility effects on dynamic stall. Progress inAerospace Science, 32, pp. 523–573, 1999.

[3] Lee, T. & Gerontakos, P., Investigation of flow over an oscillating airfoil. Journal of FluidMechanics, 512, pp. 313–341, 2004.

[4] Chandrasekhara, M.S., Carr, L.W. & Wilder, M.C., Interferometric investigationsof compressible dynamic stall over a transiently pitching airfoil. AIAA Journal, 32(3),pp. 586–593, 1994.

[5] Jones, K.D., Dohring, C.M. & Platzer, M.F., Experimental and computational investigationof the Knoller-Betz effect. AIAA Journal, 36(7), pp. 1240–1246, 1998.

[6] Lai, J.C.S. & Platzer, M.F., Jet characteristics of a plunging airfoil. AIAA Journal, 37(12),pp. 1529–1537, 1999.

[7] Lai, J.C.S. & Platzer, M.F., Characteristics of a plunging airfoil at zero free-stream velocity.AIAA Journal, 39(3), pp. 531–534, 2001.

[8] Koochesfahani, M.M., Vortical patterns in the wake of an oscillating airfoil. AIAA Journal,27(9), pp. 1200–1205, 1989.

[9] Jones, K.D., Lund, T.C. & Platzer, M.F., Experimental and computational investigation offlapping wing propulsion for micro air vehicles (Chapter 16). Progress in Astronautics andAeronautics, 195, American Institute of Aeronautics and Astronautics, pp. 307–339, 2001.

[10] Jones, K.D., Bradshaw, C.J., Papadopoulos, J. & Platzer, M.F., Improved performance andcontrol of flapping-wing propelled micro air vehicles. AIAA 2004-0399, 42nd AerospaceSciences Meeting and Exhibit, Reno, NV, 2004.

[11] Dickinson, M.H. & Goetz, K.G., Unsteady aerodynamic performance of model wings atlow Reynolds numbers. Journal of Experimental Biology, 174, pp. 45–64, 1993.

[12] Dickinson, M.H., The effects of wing rotation on unsteady aerodynamic performance atlow Reynolds numbers. Journal of Experimental Biology, 192, pp. 179–206, 1994.

[13] Kurtulus, D.F., Farcy, A. & Alemdaroglu, N., Unsteady aerodynamics of flapping airfoilin hovering flight at low Reynolds numbers. AIAA 2005-1356, 43rd Aerospace SciencesMeeting and Exhibit, Reno, NV, 10–13 January 2005.



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