vortex formation and force generation mechanisms of the delfly ii in hovering...

8/10/2019 Vortex Formation and Force Generation Mechanisms of the DelFly II in Hovering Flight_imav2014_proceedings

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Vortex Formation and Force Generation Mechanisms of the DelFly II in Hovering FlightA. Tenaglia, M. Percin , B.W. van Oudheusden, S. Deng and B. Remes

Delft University of Technology, Delft, the Netherlands

A BSTRACT

T his paper addresses the unsteady aerody-namic mechanisms in the hovering ight of theDelFly II apping-wing Micro Aerial Vehicle(MAV). Stereoscopic Particle Image Velocime-try (Stereo-PIV) were carried out around thewings at a high framing rate. Thrust-force wasmeasured to investigate the relation between thevortex dynamics and the aerodynamic force gen-eration. The results reveal that the Leading-Edge-Vortex (LEV), as well as the high exibil-

ity of the wings, have a major effect on thrustgeneration. Comparison of three wing pairs withdifferent aspect ratio (AR) is also reported, yield-ing signicant differences in both vortical struc-tures and thrust generation.

1 I NTRODUCTION

The interest for Micro Air Vehicles (MAVs) is motivatedby their versatility and adaptability that make them attrac-tive for a wide range of operations. To be able to carry outthese missions, MAVs require great agility and enhanced ma-neuverability. In this respect, apping-wing concepts are re-garded as promising systems at low Reynolds numbers, con-sidering the abundance of apping animal species in nature.Over the years, both biologists and engineers have investi-gated the mechanisms involved in insect and bird ight thatmay lead to yer-based solutions for new, innovative MAVdesigns. In view of the inadequacy of a quasi-steady ap-proach for the analysis of a apping wing ight, experimen-tal and computational research have brought new insights.There is currently consensus that the dominant aerodynamicmechanisms that govern the performance of a apping-wingyer are: 1) [1, 2, 3, 4] delayed stall and the presence of a Leading-Edge Vortex (LEV) which, entailing lower pressureon the wing surface, produces very high lift coefcients; 2)

[5, 6, 7] rotational circulation and wake capture , mechanismsthat are able to produce high peaks in force production dur-ing stroke reversal; 3)[1, 3] clap-and-ing , a lift-enhancingeffect, which takes advantage of the wing-wing interaction atthe end of the strokes.

Delft University of Technology (DUT) is represented inthis research area with the DelFly, a bio-inspired small-scale

Email address(es): [email protected]

ornithoper. The rst version of the DelFly was presented dur-ing EMAV 05 competition in the summer of 2005. Since thattime, research and development of apping-wing MAVs hascontinued at DUT.

Figure 1: DelFly II in ight.

The DelFly project follows a top-down approach, whichmeans that by studying a large-scale model, insights can begained to develop an improved, even smaller, version. The

DelFly II (g. 1) is the object of the current research. It has abiplane wing conguration with the wing structure composedof Mylar foil and carbon rods; the wing span is 280 mm andit weighs 17 grams. A brushless motor with a motor con-troller, a gear system and a crank-shaft mechanism, drive thewings in their motion. The DelFly is capable of forward andbackward ight, as well as stable hovering. It is equippedwith an on-board camera, which enables autonomous ight[8]. Investigation on the DelFly, has addressed different tech-nical aspects: aerodynamical [9, 10, 11], structural [12], andthe development of an analytical ight-model. Here, we con-centrate on the aerodynamic mechanisms of the DelFly. Aprominent feature of the aerodynamics of the DelFly is theaforementioned clap-and-ing. However because of the ex-ibility of the Mylar foil, the ing occurs as a peeling motion,where the wings curve along the chords and smoothly sepa-rate until the separation point reaches the trailing edge (TE).Similarly, the clap can be seen as a reverse peel. De Clercq[9] rst examined the aerodynamic mechanisms involved inthe hovering ight of the DelFly by means of Stereo-PIV inthe vicinity of the wings in combination with simultaneous

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force measurements. She stated that the peel contributes sub-stantially to the lift generation, due to the LEV formation atthe onset of the ing. In particular, the leading edge motionduring the outstroke is thought to reinforce the LEV gener-ation by increasing the velocity of the uid moving into theopening gap. Subsequent experiments, carried out by Groen[10], conrmed that the two most important mechanisms re-sponsible for the lift generation on the DelFly are the LEVdevelopment and the clap-and-peel effect. The instantaneousthree-dimensional wake structure of the DelFly in forwardight regime was investigated by Percin and Eisma [13]. Itwas revealed that the wake structure is a complex interactionbetween tip vortices (TVs), TEVs and root vortices (RVs):a U-shaped vortical structure can be clearly seen at severalinstants during the apping cycle, and its intensity increasesas the reduced frequency ( k = 2 fc/U ref ) increases. This isconsistent with the research reported by Muijres et al. [14],who revealed the existence of similar structures in the wakeof bats.

Following this approach, the current research considersthe generation and behaviour of the vortices involved in theDelFly motion, for a hovering ight regime. Using time-resolved Stereo-PIV and force measurements, it will be in-vestigated how the vortical structures evolve and relate this tothe force generation by the wings.

2 S ET -U P

The experiments were performed in quiescent environ-ment. Before the start of each experiment, the test roomwas lled with water-glycol-based particles (mean diameterof 1 m), by means of a SAFEX generator. A simpliedDelFly model without tail, servo-controls, batteries and cam-era, was positioned on a balance structure which incorporates

a 6 component force sensor (Nano 17 Titanium ATI Indus-trial Automation). The force measurements were recordedat a frequency of 10 kHz. Three wing pairs with the sameMylar foil thickness (10 m ) but with different aspect ratio(AR = b2 /S ) were tested: 1) a standard wing pair (AR=1.75) with a wing span (R) of 280 mm and a max chord length(c) of 88 mm, 2) a high aspect ratio wing pair (AR=2) witha wing span of 320 mm and the same max chord length, 3) alow aspect ratio wing pair (AR=1.5) with a wing span of 240mm and the same max chord length. Henceforth they will bereferred to as Std , HARDA , and LARDA , respectively. Std wasscaled up and down in the spanwise direction for the design of HARDA and LARDA , respectively. The stiffeners were placedwith their rear tips at the corners of the wing taper and theirfront tips at the same relative spanwise position. Since previ-ous experiments near the wings [9, 10] had suffered from thereection caused by Mylar foils, the wings were sprayed witha mat black paint. This indeed improved the quality of the rawacquired images, although it did not completely eliminate thereection problem. Two different apping frequencies (9 and11.2 Hz) were investigated, but in the current paper only the

11.2 Hz case will be discussed.

Figure 2: DelFly wing layout.

High-speed Stereo-PIV measurements were carried out atseveral chordwise-aligned planes around the wing (g. 3).The rst plane was placed at a distance of 40 mm outboardfrom the fuselage (normal to the laser sheet) and after every

recording the complete balance-DelFly system was shifted bysteps of 20 mm with respect to the laser sheet. Three CMOS

Figure 3: Schematic side view (left) and front view (right) of the measurement conguration.

cameras (High Speed Camera 6 LaVision) with a resolutionof 1024x1024 pixels and equipped with a Nikon 60 mm fo-cal objective, were used to record images. The rst cam-era was viewing normal to the object plane, while the oth-ers were placed on the same horizontal plane, symmetricallywith respect to the focal axis of the rst camera, with an an-gle of 80 degrees between them (thus 40 degrees betweeneach of them and the rst camera). These two cameras werealso equipped with Scheimpug adapters. The choice of us-ing three cameras for a stereoscopic approach provided thepossibility of capturing both the structure within the wingsand outside them during the ing phase, which otherwisewould be impossible using only two cameras due to the shad-ows and visual obstruction by the wings. A eld of view of 210x210 mm was captured with a magnication factor of ap-proximately 0.09. Double frame image sequences of tracerparticles were recorded with a frequency of 200 Hz, with atime separation between frames varying between 250 and 350s . The ow was illuminated with a double pulse Nd:YLFlaser (Quantronix Darwin Duo) with a wavelength of 527 nm;thanks to an appropriate setting of lens and mirrors the laser

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[4] W. Shyy and H. Liu. Flapping wings and aerodynamiclift: the role of leading-edge vortices. AIAA Journal , .

[5] S.P. Sane M.H. Dickinson, F.O. Lehmann. Wing rota-tion and the aerodynamic basis of insect ight. Science ,1999.

[6] F-O. Lehmann. The mechanism of lift enhancement ininsect ight. Naturwissenschaften , 2004.

[7] F-O. Lehmann, S.P. Sane, and M. Dickinson. The aero-dynamic effects of wingwing interaction in apping in-sect wings. The journal of Experimental Biology , 2005.

[8] G.C.H.E. de Croon, K.M.E. de Clercq, R. Ruijsink,B. Remes, and C. de Wagter. Design, aerodynamics,and vision-based control of the dely. Internation Jour-nal of Micro Air Vehicles , 2009.

[9] K.M.E. De Clercq. Flow visualization and force mea-surements on a hovering apping-wing mav dely ii.Masters thesis, DUT, 2009.

[10] M.A. Groen. Piv and force measurements on theapping-wing mav dely ii. Masters thesis, DUT,2010.

[11] J. Eisma. Flow visualization and force measurementson a apping-wing mav dely ii in forward ight con-guration. Masters thesis, DUT, 2012.

[12] B.Bruggeman. Improving ight performance of delyii in hover by improving wing design and driving mech-anism. Masters thesis, DUT, 2010.

[13] M. Percin, H.E. Eisma, J.H.S. de Baar, B.W. van Oud-heusden, B. Remes, R. Ruijsink, and C. de Wagter.

Wake reconstruction of apping-wing mav dely ii inforward ight. In International Micro-Air-Vehicle Con- ference (IMAV) , 2012.

[14] F.T. Muijres, L.C. Johansson, R. Bareld, M. Wolf, andA. Hedenstrom G.R. Spedding. Leading edge vorteximproves lift in slow-ying bats. Science , 2008.

A PPENDIX A E FFECT OF THE DIHEDRAL ANGLE .

For all the measurements, the laser sheet is normal tothe horizontal plane (see Fig:3) and its position is calculatedas the distance from the root on a line (black dotted line inFig:14) parallel to the horizontal plane. As a consequenceof the apping motion, the laser sheet does not illuminatethe wing at the same position throughout the apping cy-cle. Hence, in order to calculate the exact position, it mustbe taken into account the offset due to the stroke angle ( ).Also, because of the dihedral angle ( ), there will be a dispar-ity of the laser position between the two wings. This disparitycan be calculated as follows:

x 1 x 2 = X

cos ( + ) X

cos ( ) (1)

Figure 14: Effect of the dihedral angle on the effective lasersheet position on the wings.

where X is the nominal laser position, x1 is the laserposition on the upper wing, x2 is the laser position on thelower wing and x 1 x 2 is the disparity of the laser positionbetween the two wings.

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vortex formation and force generation mechanisms of the delfly ii in hovering...

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