Separation control via self-adaptive hairy flaplet arrays

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1 ERCOFTAC international symposium «Unsteady separation in fluid-structure interaction» Mykonos, Greece, June 17-21, 2013 Separation control via self-adaptive hairy flaplet arrays Christoph BRÜCKER, Christoph WEIDNER Institute of Mechanics and Fluid Dynamics, TU Bergakademie Freiberg, Freiberg, Germany Abstract. It is known for some birds that their feathers on the upper side of the wing pop up under critical flight conditions such as the landing approach, acting like a break on the spreading of flow-separation. To understand the complex fluid-structure interaction and adopt this principle to engineering application the influence of different configurations of self-adaptable flexible flaps (hairy flaps) on the flow around an airfoil (NACA 0020) are investigated experimentally. Therefore, the flow evolution along the airfoil at ramp-up motion with and without hairy flaps was measured and possible stall delay was investigated. The motion of the flaps and the flow field were measured simultaneously with high temporal resolution using high-speed PIV. Correlation between the flap-motion and the velocity distribution shows that backflow induced by vortex structures is prevented by the hairy flaps and the separation bubble does not develop to a larger extend. Mode-locking is achieved between the characteristic wavelength of shear-layer roll-up and spacing between the flap rows. Furthermore interaction of the shear-layer vortices with the flaps in the most downstream row leads to modification of the trailing edge flow in a beneficial way increasing bound circulation. With proper positioning and spacing of the flexible structures we were able to delay stall about a factor of 4 times the timespan of full stall at the plain airfoil after ramp-up procedure. Key words: separation control, stall delay, hairy flaps, mode-locking 1. Introduction If aircraft fly at very high angle of attack stall will occur resulting in dangerous situation due to flow separation on the suction side of the wing and the significant decrease in lift-coefficient (stall). A recent study of the mechanism of stall onset showed that it is promoted by increasing unsteadiness and the mechanism that results in the detachment of the dynamic stall vortex from the airfoil was identified as vortex-induced separation caused by strong viscous interactions (Mulleners and Raffel 2010 [11]). Birds have effective means of dealing with such critical flight conditions. If flow separation starts to develop at the upper side of a bird s wing feathers pop up (Figure 1). These small flexible feathers counteract the backflow and prevent an abrupt lift breakdown. This selfadjusting mechanism has been interpreted as a biological high-lift device assuming that flow separation could be delayed resulting in higher lift at lower flight speeds (Liebe 1979 [8], Liebeck 1978 [9]). Schatz et al. [13] showed that a self-activated spanwise flap near the trailing edge can enhance lift by more than

CH. BRÜCKER & C. WEIDNER 10% at a Reynolds number of Re=1-2 10 6.

2 CH. BRÜCKER & C. WEIDNER 10% at a Reynolds number of Re= Schlüter [15] demonstrated that at low Reynolds numbers (Re= ) lift-breakdown is less developed at increasing angles of attack for an airfoil with passive flaps than without flaps. As a further test of this passive separation control mechanism, Favier et al. [4] investigated numerically a cilia-like hairy coating attached at a two-dimensional circular cylinder at Reynolds number of Re=200. Their results showed that such a coating is capable to reduce the overall drag by 15% and lift fluctuations by 44%. A similar result was obtained at much higher Reynolds-numbers in experiments on a cylinder with flexible flaps similar to those used in the study herein (Kunze & Brücker 2012 [7]). Numerical study of the effect of hairy coatings on a NACA0012 has been performed by Venkataraman & Bottaro [16]. They found coating parameters decreasing drag oscillations by about 11% and increasing lift by about 9% under separated flow conditions at Reynolds number Re=1100 and an angle of attack α=70. Figure 1. A pelican with popped up feathers during gliding flight before landing (picture taken from Favier et al. [4]). Further studies were carried out to concretize the increase of lift with other passive structures (e.g. Neuhart & Pendergraft 1988 [12], Bechert et al [1]). Hu et al. (2008) [6] recently showed that flexibility of the wing is a key factor in contribution to passive separation control. In addition, even near-wall turbulence can be modified considerably by flexible structures [2]. Besides passive self-adaptive mechanisms, research has been conducted on active control which is summarized in a recent review by Choi et al [3]. Among all possibilities, only those working on the trailing edge are discussed here. Trailing edge flaplets has been proven to stabilize flutter (Gerontakos & Lee 2007 [5]). Tang and Dowell (2007) [14] studied the active control of the aerodynamic loading on a NACA 0012 airfoil with an oscillating trailing-edge Gurney flap at Re = The aerodynamic loading was found to be enhanced with increase of the oscillating frequency. The experimental results confirmed the idea that an oscillating small strip located near the trailing edge can be a useful tool for active aerodynamic flow control for a wing. Our work described herein is focussed on the passive control using self-adaptive flexible structures. Therefore, different 2

SEPARATION CONTROL VIA FLEXIBLE FLAPLETS hairy flap configurations were attached to the suction side of an NACA0020 airfoil and the flow during ramp-up motion was investigated experimentally. 2.

3 SEPARATION CONTROL VIA FLEXIBLE FLAPLETS hairy flap configurations were attached to the suction side of an NACA0020 airfoil and the flow during ramp-up motion was investigated experimentally. 2. Experimental set-up 2.1. PREPARATION OF THE AIRFOIL Figure 2: Dimensions of the NACA0020 airfoil and parameters of hairy flap-configuration For the experiments, a NACA0020 profile was chosen which is applied often in vertical axis wind turbines. The wing was equipped with arrays of flexible flaplets made from an elastomer. A typical arrangement of the flaplets is shown in Figure 1 and the corresponding coordinates are given in Table 1. Table 1: Parameters of investigated flap-configurations; No.1: NACA0020 airfoil without flaps; No.2: single solid flap; No.3 No.7: hairy flap-configurations; : mass-density-distribution (nondimensional) π = ((l1+l2+l3) / b)*(ρ flap / ρ water ) The self-adaptive movable flaps consist of a two component silicon rubber (Wacker RT 601 A and B). The two components were mixed, vacuumed and casted to thin foils. Once the curing process was finished the flexible hairy flaps were carved out of the silicon foil and afterwards were attached to the airfoil. In order to realize suitable records of the motion of the hairy flaps during the high-

CH. BRÜCKER & C. WEIDNER speed PIV measurements we coloured the surface of the airfoil in white and painted the front faces of the flaps in black, see Figure 3.

A side-view picture of the flaps during their action in flow is shown in Figure 4 to illustrate the typical elevation at interaction with the flow at the pitched position of the airfoil (α=17.5 ).

2. EXPERIMENTAL METHODS We investigated the influence of self-adaptive flexible flaps on the flow around a symmetric NACA0020 airfoil at Reynolds number of Re = 77000 in a water channel.

4 CH. BRÜCKER & C. WEIDNER speed PIV measurements we coloured the surface of the airfoil in white and painted the front faces of the flaps in black, see Figure 3. Figure 3: NACA0020 airfoil with attached flexible hairy flaps (π = 0.77). A side-view picture of the flaps during their action in flow is shown in Figure 4 to illustrate the typical elevation at interaction with the flow at the pitched position of the airfoil (α=17.5 ). Figure 4: Illustration of the field of view for the hairy flap configuration (π = 0.77) at a constant angle of attack α= EXPERIMENTAL METHODS We investigated the influence of self-adaptive flexible flaps on the flow around a symmetric NACA0020 airfoil at Reynolds number of Re = in a water channel. A sketch of the experimental setup is illustrated in Figure 5. The transparent test section size is 0.4m wide 0.4m high 1.5m long. The airfoil was mounted on the top of the water channel. Figure 4 illustrates one recorded image representing a field of view of FOV= mm 2. First, measurements were carried out at a constant angle of attack (α = 17.5 ) using standard 2D DPIV as a reference case. Thereafter, ramp-up experiments were carried out. Therefore, a linear traverse at the top of the water channel (see figure 4) was used to turn the airfoil at a constant rate (dα/dt=13.3 /sec) of the angle of attack (α= 4

5 SEPARATION CONTROL VIA FLEXIBLE FLAPLETS 0-20 ) until the final state is reached at α=20. Standard DPIV-recordings were taken with camera #1 (PCO 1600 with a resolution of px at a recording frequency of 14Hz) and a pulsed Nd: YAG laser (Continuum Minilite). The processing of the DPIV vector fields was performed using Dynamic Studio V2.30 (Dantec Dynamics) with an adaptive cross-correlation algorithm on a 32 32px grid with an overlap of 75% and peak validation algorithm. Thereafter, the velocity vectors were locally smoothed by a moving average filter of 5 5 kernel size. In another run of experiments, we measured the motion of the flaps and the flow field simultaneously with high temporal resolution (high-speed PIV). Therefore we used two high-speed cameras (Photron Fastcam RS, px resolution, recording frequency 500Hz). Camera #1 was used for high-speed PIV recordings. Camera #2 was looking on the surface mirror 2 (placed in the water channel downstream of the airfoil at a distance of 6L) through a glass plate and was synchronized with a flash lamp (see figure 5), which was flashing in between the pulse pauses of the laser. This ensured that there was no cross-over between the PIV images and reflections from the flaps and vice versa. The light sheet was formed using a 10mJ Nd:YLF high-speed laser with double-lens optics and a mean wavelength of 527nm (Coherent Evolution). The dimension of the field of view for the high-speed PIV measurements was 321 x 321mm. The position of the flaps tips was tracked in all images using image processing and edge detection algorithms. Figure 5: Sketch of the experimental setup for PIV measurements of high-speed recordings of the flow evolution around an airfoil at rump-up motion.

6 CH. BRÜCKER & C. WEIDNER 3. Results 3.1. REFERENCE CASE AT CONSTANT ANGLE OF ATTACK Figure 6 illustrates the velocity contour plot for the plain airfoil (without hairy flaps) at Re = as the reference case for an angle of attack of α=17.5. The flow field represents the average over a number of 100 sample images. Positive values of the velocity are depicted as solid lines and dashed lines represent negative velocities. In order to evaluate the wall-normal thickness of the separation bubble for each flap configuration we placed evaluation lines (E1 and E2) perpendicular to the chord length where we compare the data of different experiments. The first evaluation line (E1) was placed near the leading edge (y/l = 0, x/l = 0.275) and the second one (y/l = 0.8, x/l = -0.2) near the trailing edge. We defined the parameter ξ characterizing the area of negative values of U x as an indication of the wall-normal thickness of the separation bubble. With this approach we were able to make conclusions of the dimension of the separation region on the suction side of the airfoil. Figure 6: Contour lines of the time averaged stream-wise velocity component Ux normalized with free stream velocity U 0 at Re = and a constant angle of attack α= 17.5 for the NACA0020 airfoil without hairy flaps and the evaluation lines (E1 and E2). Solid lines represent positive velocities and dashed ones negative velocities. ξ is defined as the thickness of the separation bubble. A comparison for the case without flaps and the modified airfoil is shown below in Figure 7. It is well seen that the hairy flap coating has largely reduced the region of backflow and has prevented stall. Flow still remains attached at α=17.5 for the airfoil covered with hairy flaps at π=0.77. The latter was the most effective of all configurations tested and shown in Table 1. Note, that complete stall appears for the modified airfoil at a larger angle of attack α=20, 6

7 SEPARATION CONTROL VIA FLEXIBLE FLAPLETS too. Such a terminal angle of attack was chosen for the ramp-up experiments to force the airfoil flow going into stall conditions. Figure 7: Comparison of mean flow properties for the NACA0020 airfoil at α=17.5 angle of attack by means of contour lines of stream-wise velocity component Ux normalized with free stream velocity U 0 at Re = a) plain airfoil, b) airfoil covered with hairy flaps in configuration π= RAMP-UP RESULTS The magnitude of separation delay was estimated by the use of velocity information along the evaluation line E1. Therefore we evaluated the time-span from the end of ramp-up motion at an angle of attack α= 20 up to the time when positive values of the velocity component Ux turn to negative ones and remain negative across E1. This procedure gives information about the time when flow separation is fully developed over the suction side of the airfoil. The results are illustrated in Figure 8. Figure 8: Relative time-span to achieve fully developed flow separation on the whole suction side of the airfoil after ramp-up procedure with a final angle of attack α= 20. Iπ represents the corresponding time-span for the flap configurations and I 0 the one for the reference airfoil as a result of the evaluation at line E1.

CH. BRÜCKER & C. WEIDNER The results show that it is possible to delay flow separation by a maximum factor of 4.4 with the flap configuration π = 0.77 compared to the reference airfoil without flaps.

8 CH. BRÜCKER & C. WEIDNER The results show that it is possible to delay flow separation by a maximum factor of 4.4 with the flap configuration π = 0.77 compared to the reference airfoil without flaps. In order to verify these results we repeated the experiments several times producing the same magnitude of outcome. Note that in this configuration with π=0.77 the flaps are positioned in three successive arrays along the second half of the chord of the wing. In addition, the typical length-to-width ratio of the flaps is l/h=4 which make the flaps bend more easily. This is a remarkable outcome of the tests which shows that hairy coatings are capable to delay flow separation in a very effective way. Based on these results high temporal resolution measurements were carried out with the flap configuration α = 0.77 and the reference airfoil to investigate the interaction between flap motion and the flow around the airfoil in more detail. A characteristic sequence of flap motion is illustrated in Figure 9. The red arrow indicates three successive flaps along the streamwise direction where the lightsheet is positioned for the flow measurements. a b Figure 9: a) Instantaneous picture of the black-colored trailing edges of the hairy flaps in the three successive rows along the chord of the wing as seen by camera #2 from downstream. Note the strong zig-zag type variation of elevation of the individual flaps along the array. b) spatial-temporal reconstruction of trailing edge vertical position of the hairy flaps along the red arrow as a function of dimensionless time t* (time is made dimensionless with free stream velocity and chord length in the form t* = t U 0 /L). A large event appears at t* 4 at the downstream row. The flaps of type π=0.77 are very sensitive and respond to appearing backflow by pop-up as the natural feathers do. Furthermore, the flaps are rapidly pushed back to lower elevation if flow reattaches to the wall. Events with large amplitudes of elevations in Figure 9 are signatures of local flow structures moving over the flaps and inducing stronger changes in the pressure distribution. For correlation of the flap motion events to the flow, additional information is given in Figure 10 about the vortex dynamics in the flow along the light-sheet position. In order to identify vortices within the 2D-PIV velocity fields we used the Q-criterion to detect the vortex cores. 8

9 SEPARATION CONTROL VIA FLEXIBLE FLAPLETS Figure 10: Sequence ( t*=0.2) of vortex motion for the flap configuration π=0.77 (left) and the reference airfoil (right) at Re = after ramp-up procedure. The pictures in the first row represent the instant t*=3.9 after ramp-up, compare the large event in Figure 9.

10 CH. BRÜCKER & C. WEIDNER Local maxima of positive Q represent the vortex cores in the shear layer. The sequence for the modified airfoil shown in Figure 10 left displays a more regular roll-up of the vortices in the shear layer which forms rows of vortices with rather constant spacing along the chord of the wing. In contrast, the reference case obeys a more irregular distribution of the vortices. Formation of stronger vortices is seen already near the leading edge in both cases. However while for the modified airfoil these vortices seem to be transported downstream without mutual interaction (transportation velocity is U C 0.57 U 0 ), the plain airfoil shows vortices which interact together already at an early time of the roll-up process. Figure 10a-c show a vortex pair interaction at a streamwise location x/l 0.3. The consequence of the vortex pair interaction is the overtaking of the leading one by the trailing one in a type of roller motion. As a result, a strong wall-normal motion is induced which lifts the vortex off from the wall. Therefore, the region of flow deceleration grows in wall-normal direction. In contrast, such type of vortex interaction in the shear layer is not observed for the modified airfoil. Another striking observation is the change of characteristic frequencies of flap motion along the chord. Figure 9 shows that the first row of flaps is excited with a rather regular frequency corresponding to the frequency of the shear-layer vortices passing the structures. The corresponding Strouhal number of the excitation is Sr 1.78 (Sr = f L/U 0 ). Further downstream, the flap motion amplitude increases largely while the frequency content shows that energy is redistributed from the higher to the lower range. This hints on a stronger interaction of the vortices with the flaps in the 2 nd and 3 rd row as documented in Figure 10 left by means of the dashed line along the path of one larger vortex. Initially, the vortex #1 is formed in the shear layer near the leading edge and is transported downstream with nearly constant velocity (U C 0.57 U 0 ). After passing the 2 nd row it starts to split up in two parts as a consequence of interaction with the flap in the last row. The more downstream part vortex #1 moves unaffected while the other part of the vortex #1 has stopped its motion and remains near the flap edge. The splitting process is accompanied with the growth of a starting vortex at the trailing edge of the wing that by itself indicates a temporal increase of bound circulation around the wing. This starting vortex is then shed into the wake as depicted in Figure Conclusions The present study is a continuation of our works on fluid-structure interaction with flexible hairs and flaps (see Kunze & Brücker 2011), adapted to the application of passive separation control on airfoils. A detailed investigation of the interaction of the flaps and the flow field is carried out by simultaneous recordings of flap motion and flow field using two high-speed cameras. 10

11 SEPARATION CONTROL VIA FLEXIBLE FLAPLETS Figure 11: Illustration of vortex-pair interaction at the trailing edge of the airfoil for the flap configuration π=0.77. Depicted is a sequence ( t*=0.06) of vortex motion (Q-criterion left) and the corresponding velocity-vector-field (a streamwise velocity of 40% of free stream velocity U 0 is subtracted). The picture in the first row represent the instant of time of t*=5.25 sec after ramp-up procedure. A special emphasis is devoted to the analysis of the ramp-up procedure and the possibility to delay stall using different type of arrays of flexible flap structures. It has been shown that structures with slender flaps are indeed able to delay stall about a factor of 4 in comparison to the characteristic time-scales until full stall is established on a plain airfoil after ramp-up. The most dramatic effect was observed for a configuration of three rows of slender flaps of size l w=0.1l 0.025L with a streamwise spacing of 0.15L. Other configurations did not show considerable stall delay, therefore it is concluded that the size and the inter-spacing of the flaps are important parameters. The observation of a more regular roll-up of the shear-layer vortices for the modified airfoil hints on a possible mode-locking of these instabilities with the motion of the flaps, compare also [16]. While the vortices are being transported across the rows of the flaps, the excitation of the flaps moves downstream in a wave-like motion with increasing amplitude of flap elevation events with streamwise position. Higher frequency contributions are damped out while energy is converted to large-scale motion, compare to [2]. Finally, strong vortex interaction is observed at the final row, where part of the vorticity is captured at the lee side of the flap and is accumulated there. This is counterbalanced by the growth of a starting vortex at the trailing edge of the wing. From Kutta condition it is argued, that bound

12 CH. BRÜCKER & C. WEIDNER circulation is increased at the same time. Both vortices are then shed together as a pair into the wake. Acknowledgements This work has been carried out at the water tunnel installed at IMFD which has been funded by the DFG. It is based on the collaborative efforts of Ch. Weidner and S. Kunze in the context of their diploma and PhD thesis. References 1. Bechert, D.W., Bruse, M., Hage, W., and Meyer, R., Biological Surfaces and their Technological Application Laboratory and Flight Experiments on Drag Reduction and Separation Control, AIAA Paper (1997) Snowmass Village, CO. 2. Brücker Ch., Interaction of near-wall turbulence with flexible hairs, J. of Physics: Condensed Matter. 23 (2011) Choi, H., Jeon, W.P., and Kim, J., Control of Flow over a Bluff Body, Annual Review of Fluid Mechanics 40 (2008) Favier J., Dauptain A., Basso D, and Bottaro, A. Passive separation control using a selfadaptive hairy coating, J. Fluid Mech. 627 (2009) Gerontakos, P., Lee, T., Dynamic stall flow control via a trailing-edge flap. AIAA Journal 44(3) (2006) Hu, H., Tamai, M., and Murphy, J. T., Flexible Membrane Airfoils at Low Reynolds Numbers, Journal of Aircraft 45(5) (2008) Kunze, S., Brücker, Ch., Control of vortex shedding on a circular cylinder using selfadaptive hairy-flaps, Comptes Rendus Mécanique 340(1-2) (2012) Liebe, A., Der Auftrieb am Tragflügel: Entstehung und Zusammenbruch, Aerokurier 12, (1979) Liebeck, R.H., Design of Subsonic Airfoils for High Lift, Journal of Aircraft 15(9) (1978) Meyer, R., Hage, W., Bechert, D. W., Schatz, M., Knacke, T., and Thiele, F., Separation Control by Self-Activated Movable Flaps, AIAA Journal 45(1) (2007) Mulleners, K., Raffel, M., The onset of dynamic stall revisited, Exp. Fluids 52(3) (2010) Neuhart, D.H. and O.C. Pendergraft, A water tunnel study of Gurney flaps, NASA Technical Memorandum, 4071 (1988). 13. Schatz, M., Knack. T., Thiele, F., Mey, R., Hage, W., Bechert, D.W., Separation Control by Self-Activated Movable Flaps. AIAA paper (2004). 14. Tang, D., and Dowell, E.H., Aerodynamic loading for an airfoil with an oscillating gurney flap, Journal Of Aircraft 44(4) (2007) Schlüter, J. U., 2009 Lift Enhancement at Low Reynolds Numbers using Pop-up Feathers. AIAA Paper (2009). 16. Venkataraman D. and Bottaro A., Numerical modeling of flow control on a symmetric aerofoil via a porous, compliant coating, Physics of Fluids 24(9) (2012) 12

Tim Lee s journal publications

Tim Lee s journal publications 82. Lee, T., and Tremblay-Dionne, V., (2018) Impact of wavelength and amplitude of a wavy ground on a static NACA 0012 airfoil submitted to Journal of Aircraft (paper in