Unsteady airfoil experiments

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1 Unsteady airfoil experiments M.F. Platzer & K.D. Jones AeroHydro Research & Technology Associates, Pebble Beach, CA, USA. Abstract This paper describes experiments that elucidate the dynamic stall phenomenon and the generation of thrust by flapping airfoils. To this end, flow visualizations of the vortices shed from a rapidly pitching airfoil and from an oscillating airfoil are presented. Also, wind tunnel tests of two flapping wing 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 unsteady aerodynamic effects, namely the Kramer and the Katzmayr effects. In this paper we present more detailed 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 experiments of Carr and Chandrasekhara [2] in a special dynamic stall facility at the NASA Ames Research Center. In this wind tunnel the airfoil was mounted between two circular glass windows and the window airfoil window assembly was then rotated in an oscillatory or ramp-type motion so that an unobstructed view of the flow could be achieved. Using a unique interferometric technique (point diffraction interferometry), which makes it possible to visualize the changes in flow density, they obtained detailed information about the formation and propagation of the dynamic stall vortex. Figure 1 shows an example of a NACA 0012 airfoil that was pitched rapidly from an incidence angle of 12 to 25. The development of the leading-edge separation bubble and 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 0012 was published by Lee and Gerontakos [4]; these measurements provide additional valuable information on the role of the laminar separation bubble and on the initiation, growth and convection of the dynamic stall vortex in a low-speed flow of Reynolds number 135,000. Consistent with the measurements of Carr and Chandrasekhara at a significantly larger Reynolds number, a laminar doi: / /7b

2 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 could again be identified. 3 Experimental studies of the Katzmayr effect As already pointed out in the paper Steady and Unsteady Aerodynamics, a sinusoidally plunging airfoil acts like a propeller, generating a jet flow downstream of the airfoil. This phenomenon was studied 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 street shown 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 amplitude to 10% of the chord from 1.25% produces the changes in vortex shedding shown in Fig. 3. At first, 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 shed from the opposite side. On increasing the amplitude still further, the upper row vortices now rotate counterclockwise and the lower row vortices rotate clockwise (Fig. 3, bottom). This type of vortex

3 700 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].

4 Unsteady Airfoil Experiments 701 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 results in 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 downstream of, but very close to, the airfoil trailing edge. It is seen that a distinct jet is generated (which is predicted quite well with inviscid panel code calculations). With increasing distance from the trailing edge this jet broadens while the peak velocity decreases [5, 6]. It is also of interest to note that thrust production occurs even if there is no air or water flow around the airfoil, making it possible for birds to take off by flapping their wings. This phenomenon was investigated experimentally in another investigation by Lai and Platzer [7]. Furthermore, we mention that thrust is also generated by pitching the airfoil instead of plunging it. However, much higher pitch frequencies 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 model shown in Fig. 5. In this model two airfoils, with a chord length of 64 mm and an effective span of 1200 mm, are allowed to flap with variable pitch and plunge amplitudes. The model was suspended by 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 the model when the wings were flapped. This deflection was measured by bouncing a laser beam off a small notch on the back of the rear nacelle, as shown in Fig. 6. Some of the results are shown in Fig. 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 between the panel code and the experimental data is quite good, roughly 80% at higher velocities and frequencies [9].

This model is shown in Fig. 8. In contrast to the model in Fig. 5, the pitch and plunge degrees of freedom and the phasing between the two motions could not be controlled separately.

5 702 Flow Phenomena in Nature 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 its use on a micro air vehicle with flapping wings, as described in the paper Flapping-wing microair vehicles. This model is shown in Fig. 8. In contrast to the model in Fig. 5, the pitch and plunge degrees 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 passively by attaching the flapping wings to the flapping mechanism with a flexible joint so that they were able to pitch aeroelastically. Also, a second model was built, shown in Fig. 9, which incorporated one more degree of freedom, namely flexible wing camber. The measured thrust as a function of flapping frequency is shown for both models in Fig. 10.

Figure 8: Micro air vehicle model with flapping wings

6 Unsteady Airfoil Experiments 703 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].

7 704 Flow Phenomena in Nature 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. to hover, followed by rapid maneuvers. The kinematics of the wings of these insects consists of two translational phases during which the wings sweep through the air with relatively slow changes in the incidence angle, followed by rapid rotations at the end of each stroke. The motion is periodic and is composed of two half-cycles that, in hover, are mirror images of each other. At the end of each half-cycle the wing flips so that the leading edge points backwards and the wing s lower surface becomes its upper side. These wing flips allow the insects to maintain a positive incidence angle and thus to generate lift during both forward and reverse strokes. However, this type of wing motion involves strong unsteady aerodynamic effects because the wing is being decelerated at the end of the stroke and then reaccelerated again, thus causing the shedding of stopping and starting vortices. In addition, the flipping of the wing at the end of the stroke causes the shedding of a strong dynamic stall vortex. The shedding and interactions between these vortices involve complicated nonlinear aerodynamic phenomena that are insufficiently understood at the present time. We refer to Freymuth s paper on Applications of the unsteady two-dimensional aerodynamic model to common dragonfly maneuvers for a more detailed discussion and especially his flow visualization shown in Fig. 4. Dickinson and Goetz [11] contributed flow visualization and quantitative force data for an airfoil that was accelerated from rest to a constant velocity, thus simulating one part of the total motion of an insect in hover. This experiment was expanded by Dickinson [12] to include the airfoil rotation. Very recently, Kurtulus et al. [13] presented Navier Stokes computations which provide further insight into the intricate vortex shedding process caused by flapping airfoils in hover. References [1] Platzer, M.F. & Jones, K.D., Steady and unsteady aerodynamics. Flow Phenomena in Nature, Vol. 2, pp , 2006.

8 Unsteady Airfoil Experiments 705 [2] Carr, L.W. & Chandrasekhara, M.S., Compressibility effects on dynamic stall. Progress in Aerospace Science, 32, pp , [3] Lee, T. & Gerontakos, P., Investigation of flow over an oscillating airfoil. Journal of Fluid Mechanics, 512, pp , [4] Chandrasekhara, M.S., Carr, L.W. & Wilder, M.C., Interferometric investigations of compressible dynamic stall over a transiently pitching airfoil. AIAA Journal, 32(3), pp , [5] Jones, K.D., Dohring, C.M. & Platzer, M.F., Experimental and computational investigation of the Knoller-Betz effect. AIAA Journal, 36(7), pp , [6] Lai, J.C.S. & Platzer, M.F., Jet characteristics of a plunging airfoil. AIAA Journal, 37(12), pp , [7] Lai, J.C.S. & Platzer, M.F., Characteristics of a plunging airfoil at zero free-stream velocity. AIAA Journal, 39(3), pp , [8] Koochesfahani, M.M., Vortical patterns in the wake of an oscillating airfoil. AIAA Journal, 27(9), pp , [9] Jones, K.D., Lund, T.C. & Platzer, M.F., Experimental and computational investigation of flapping wing propulsion for micro air vehicles (Chapter 16). Progress in Astronautics and Aeronautics, 195, American Institute of Aeronautics and Astronautics, pp , [10] Jones, K.D., Bradshaw, C.J., Papadopoulos, J. & Platzer, M.F., Improved performance and control of flapping-wing propelled micro air vehicles. AIAA , 42nd Aerospace Sciences Meeting and Exhibit, Reno, NV, [11] Dickinson, M.H. & Goetz, K.G., Unsteady aerodynamic performance of model wings at low Reynolds numbers. Journal of Experimental Biology, 174, pp , [12] Dickinson, M.H., The effects of wing rotation on unsteady aerodynamic performance at low Reynolds numbers. Journal of Experimental Biology, 192, pp , [13] Kurtulus, D.F., Farcy, A. & Alemdaroglu, N., Unsteady aerodynamics of flapping airfoil in hovering flight at low Reynolds numbers. AIAA , 43rd Aerospace Sciences Meeting and Exhibit, Reno, NV, January 2005.

JOURNAL PUBLICATIONS

1 JOURNAL PUBLICATIONS 71. Lee, T., Mageed, A., Siddiqui, B. and Ko, L.S., (2016) Impact of ground proximity on aerodynamic properties of an unsteady NACA 0012 airfoil, submitted to Journal of Aerospace