2005-iplc- flapping wing paper

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2005-01-3198

Wake Structure Diagnostics of a Flapping Wing MAVManikandan Ramasamy J. Gordon Leishman Beerinder Singh

Department of Aerospace EngineeringGlenn L. Martin Institute of Technology

University of Maryland, College Park, MD 20742

Copyright c 2005 Society of Automotive Engineers, Inc.

SUMMARY

Experiments were performed to better understand the

aerodynamic flow field of a flapping-wing micro air vehicle.High-resolution laser sheet flow visualization and particleimage velocimetry (PIV) analyses have shown the pres-ence of folded vortex filaments that are trailed from the tipand root of the wing, which are combined with a shed dy-namic stall vortex with a strong spanwise flow toward thewing tip. This leading-edge vortex gains strength as thetranslational motion of the wing accelerates through mid-stroke. There is a subsequent shedding of this vortex, butwith the simultaneous formation of another leading-edge

vortex. The generation of the second vortex occurs beforethe first vortex reaches mid-chord, enhancing overall lift.This second vortex moves along the chord during supina-tion, before finally being shed from the trailing-edge of thewing. A starting vortex forms near the trailing-edge as thewing starts to accelerate during the downstroke/upstrokeof the flapping cycle. This starting vortex grows largerin size, gaining energy from further shed vortices, untilthe wing reaches the mid-point of the cycle. The foldedroot and tip vortices that trail from the flapping wing have

been found to be relatively strong, and move inward andaxially downward as the wing moves through its flappingcycle. The close proximity of the starting vortex, as well

as the trailed root and tip vortices, has a large influenceon the downwash over the wing. This suggests that anymodeling techniques used to predict the lift on flappingwings must fundamentally take into account the three-dimensional, unsteady effects associated with its complexvortex wake structure.

INTRODUCTION

Flapping wing based systems are being considered for

application to hovering micro air vehicles (MAVs). De-spite mechanical complications in producing an effective

flapping wing system of high aerodynamic and mechani

cal efficiency with low weight, there are claimed aerody-namic advantages in using flapping wings at low operational Reynolds numbers relative to other hovering MAVconcepts such as rotors. Certainly, nature has found away for insects and some classes of birds to use compli-cated three-dimensional vortical flows and unsteady aerodynamic effects to hover efficiently and perform very de-sirable flight maneuvers without much noise generationall of which are the ideal requirements for MAVs. The successful development of a flapping wing MAV with good ef-ficiency stems, in part, from the better understanding andefficient exploitation of these complex aerodynamic phe-

nomena. Even then, however, the viability of aflappingwing concept for an MAV remains to be seen.

Experiments have been made using living insects to measure and understand their flow field [1,2]. However, thiswork has met with limited success from a fundamentafluid mechanics perspective. This is because of the issuesin making detailed quantitative measurements on flapping

wings and also because of the difficulties in separatingthe aerodynamic forces from the inertial forces on the in-sect [3, 4]. Larger-scale mechanical models that mimicthe kinematics of a flapping wing have been built to allowmeasurements that help more fully understand the under

lying physics of flapping wing-borne flight [58]. Computational fluid dynamic models have also been developedto simulate the aerodynamic flow field surrounding a flapping wing [9,10], but have given limited additional physicainsight into the flow field. This is, in part, because of the

difficulties in predicting three-dimensional wake vorticityto relatively old ages in terms of wing flapping cycles. Inboth cases, a complete understanding of the underlyingwing motion kinematics and a faithful reproduction of theaeroelastic deformation of the wing is essential for suc-cessfully simulating the complex three-dimensional flowfield in the laboratory. Developing the mechanics of a flap

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ping wing that properly emulates insect wing kinematicsand aeroelastics is, in itself, an extremely difficult task.

Comparing various aerodynamic aspects of the flappingwing with its counterpart, the rotating wing, can help un-

derstand the inherent difficulties involved in making reli-able measurements in the flow field of a flapping wing.Unlike rotating wings, which have constant angular veloc-ity through out the rotational cycle, the reciprocating mo-

tion in the stroke plane of a flapping wing has varying an-gular velocity (accelerating in the early part of translationfollowed by deceleration). As a result, defining the three-dimensional wake structure for a flapping wing becomesmuch more difficult because of the powerful dependenceon the wing stroke location. For example, the strength of

the leading-edge vortices (LEVs), which are hypothesizedto be mostly responsible for the lift produced by flappingwing insects, will be a function of the stroke position. Also,the location and strength of the vortices that trail from thetip and root of the wing play a significant role in definingthe overall induced flow field (i.e., the non-uniform angle ofattack of the wing), and the convection velocities of these

vortices will be a function of the stroke location. Further-more, the vortical or apparent aerodynamic mass effectsat the start of each stroke (and deceleration at the latter

part of each stroke), as well as the large angular rota-tions at the end of each stroke (pronation and supination),make the predictive analysis of a flapping wing conceptan extremely challenging goal.

Applying the existing knowledge base of conventional,linear, small disturbance quasi-steady aerodynamic the-ory to understand the physics of flapping-wing flight [11]is fundamentally inadequate, despite some recent reat-

tempts and claimed successes [12]. There are also

other problems as well, including the differences in manyaerodynamic aspects of the problem such as the verylow operating Reynolds number of the wings and alsoin effectively characterizing the degree of unsteadinessof the flow. It has been shown that the aerodynamicperformance of stationary airfoils decreases at very lowReynolds numbers [13, 14]. Other than the dominationof viscous forces (a characteristic of low Reynolds num-ber flow), the measure of unsteadiness in flapping wings

(which is usually defined by a form of reduced frequency)is considered to be large. Defining reduced frequencyusing the peak flapping velocity in the stroke (the usualconvention), unfortunately, suggests that the reduced fre-quency is an inverse function of the aspect ratio of thewing [15]. The often used argument that the larger aspectratio wings (such as helicopter rotor blades) have smallerunsteady effects and smaller aspect ratio wings (such asflapping wing) have higher levels unsteadiness is not veryuseful or insightful. Studies have been conducted to un-

derstand the effects of low aspect ratio wings on the per-formance of micro-scale fixed wings that operate at lowReynolds numbers [16, 17]. However, no equivalent sys-tematic studies have been made for flapping wing sys-tems. Consequently, current results for the effects of re-duced frequency remain speculative.

The failure of steady or quasi-steady aerodynamics to explain the enhanced lift produced by the flapping wings[11] has caused various investigators to analyze thisproblem based on unsteady separated flow mechanismsEllington [8] suggested that the insects generate en

hanced lift using a spilled leading-edge vortex (LEV) thatarises from the combination of high angle of attack andlow Reynolds number. The presence of a LEV was identified by performing flow visualization on a hovering hawk

moth Manduca Sexta [1] and also on the mechanical flapper that mimicked the wing movements of the hawkmoth[18,19]. Because the LEV is formed during the translational motion of the flapping wing and not from wing rotation (i.e., not because of pronation or supination), Vanden Berg [18] argued that this vortex is a form of dynamic

stall. A LEV is expected to shed as it gains energy fromthe continuous translation of the wing in the stroke planeHowever, Ellington made an hypothesis that the LEVs stayon top of the wing during most part of the flapping wing cycle because of spanwise flow [8]. It was claimed that thisspanwise flow prevents the LEV from gaining more en-ergy and, as a result, prevents its shedding from the wing

The LEV that is smaller in size and weaker near the rootof the wing increases its size and strength until it reaches75% of the wing span. It was claimed that this results in

a spanwise pressure gradient along the axis of the LEVand is the source of the spanwise flow. Clearly, the continuous presence of LEV on top of the wing reduces thepressure on the top surface of the wing and enhances theoverall lift produced. The spanwise velocity was reportedto be of the order of the tip velocity of the flapping wingVan den Berg & Ellington [18] also proposed a vortex ringstructure in the wake of a flapping wing, which is the resulof combined shed and trailed wake vorticity.

Birch & Dickinson [20], in an alternative hypothesis, suggested that the spanwise flow that prevents the LEV fromshedding is dependent on the Reynolds number (basedon wing chord) of the flow. From the results of an experiment where the spanwise flow of the flapping wingwas restricted using fences and baffles, Birch & Dickinson [20] suggested that the spanwise flow may nostabilize the LEV. However, they hypothesized that at aReynolds number matching the flows relevant for most in

sects (Re =150), the downward flow induced by the tipvortices would be the primary source that limits the growthof the LEV. It should, however, be noted that the experiments performed by Ellington & Usherwood [21] on a ro-tating wing (to eliminate the unsteady effects) at differenReynolds numbers (10,000 to 50,000) suggested that thelift coefficients at higher Reynolds numbers dropped significantly when compared to low Reynolds numbers. Thissuggests the less pronouced formation of LEVs at higherReynolds numbers. The effect of Reynolds number on the

stability of LEVs, or on its mere existence, is still not clearIt is, therefore, essential to have a better understandingof the effects of Reynolds number because flapping wingMAVs will operate in this Reynolds number range (103 to10

5).

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Multiple hypotheses have been put forth by various re-searchers for the enhanced lift produced by flappingwings. While Willmott & Ellington [1] suggested the pres-ence of LEV as the principal source of enhanced lift, Weis-Fogh [22] argued that a mechanism called clap and fling

is responsible for higher lift production. Other variationsof this mechanism include clap and peel and near-clapand fling. However, it is found that most insects rarelyclap in free flight and, as a result this may not explain the

physics behind the overall increased lift shown by flappingwings [23]. Later, Birch & Dickinson [24] (from a PIV anal-ysis) suggested that the enhanced lift has its source notonly from the LEV, but also from wake capture. The totalwing force, based on that proposed by Birch & Dickinson,comes from four sources: (1) the acceleration-reaction

force when the wing starts its motion, (2) force from LEVduring translation, (3) force that results from increased cir-culation as a result of rotation at the end of the strokes,and (4) an unsteady wake-capture when the wing flips andstarts accelerating on the upstroke of its motion.

The aforementioned observations and perhaps contra-

dicting hypotheses suggest the need for much more fun-damental research on flapping wings, including high-resolution quantitative measurements, that can help bet-

ter understand their complex flow structures. Clearly, thesuccessful design of a mature flapping wing MAV conceptlargely depends upon the complete understanding of un-steady aerodynamics that is inherent to its low Reynoldsnumber flight regime. The objectives of the present studywere to develop effective techniques to allow high-fidelitymeasurements in the flow field of a flapping wing and tofurther a fundamental understanding of its overall aerody-namics. The work suggests features of the flow similar to

that found by other investigators, but also several impor-

tant new observations were made, including the role ofthe trailed vortex wake system.

EXPERIMENTAL SETUP

A biomimetic insect based flapping wing MAV has beenbuilt at the University of Maryland [25] that is capable ofemulating insect wing motion kinematics Fig. 1. Themodel, which is similar to the typical motion of an insectwing as shown in Fig. 2, consists of four parts: (1) down-stroke, in which the wing translates with a constant pitchangle; (2) supination, where the wing flips through a large

angle of attack range to produce a positive pitch angle onthe upstroke; (3) upstroke, again a translation motion withconstant pitch angle; (4) pronation at the end of upstroke,where the wing flips back again to have a positive pitchangle for the downstroke.

The required flapping and pitching motion was producedby a brushless motor, which is controlled by a sensorlessspeed controller. This was operated in combination witha microprocessor-based precision pulse generator. Themotor shaft is rigidly attached to a rotating disk, which in

turn is attached to a pin that drives a scotch yoke. As theshaft is actively flapped, pitch actuators, which are rigidly

Figure 1: Insect based flapping wing model developed athe University of Maryland. (Tarascio et al. [25])

SupinationPronation

Net Force

Stroke

Downstroke

Upstroke

Wing Path

SectionWing

Plane

Figure 2: Basic wing kinematics of a flapping insect thaconstitute one full cycle.

attached to the shaft, make contact with Delrin ball endsat the end of each half-stroke. This causes the shaft topitch and, hence, generate the wing flip at the end of eachflapping stroke.

The rotation of the shaft or flip at the end of each half-stroke is generated by the pitch assembly, which alsoserves to fix the pitch angle of the shaft during the translational phases of the wing motion [25]. The pitch assemblyconsists of main shaft that is rigidly attached to the camand is, in turn, held by a Delrin slider and compressionspring. In combination with the pitch stop, the entire assembly is bi-stable in that it allows the shaft to rest in only

two positions. The model has 80

flapping stroke in thehorizontal plane.

Aluminum/mylar wings were attached to the flapping wingmechanism, the planform of which is based on a scaled-up fruit fly wing (similar to the Robofly wings in Ref. 7)as shown in Fig. 3. The wing was made using a 0.02 inchthick aluminum frame. The pitch angle of theflapping wingduring the translational stroke was set to 45. The operating Reynolds number based on maximum stroke velocityand mean chord was approximately 19,500.

The flapping wing model was mounted on a test stand (as

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y

x

z

Figure 3: Flapping wing used in the experiment.

Figure 4: Experimental apparatus for flapping wing flowvisualization.

shown in Fig. 4) at about ten effective wing spans from theground. The experimental apparatus was placed insidea flow conditioned test cell of volume 362 m3. Becausethe flapping wing model was tested with a single semi-span wing in the present work, a large image plane wasconstructed to provide nominal symmetry to the flow.

FLOW VISUALIZATION Flow visualization images were

acquired by seeding the flow using a mineral oil fogstrobed with a laser sheet. This light sheet was producedby a dual Nd:YAG laser and was located to the desiredorientation in the flow using an optical arm. Images wereacquired using a Nikon D-70 6.1 mega-pixel digital cam-era that was placed perpendicular to the laser light sheet.The laser was triggered using a once-per-revolution sig-nal, which was obtained from an encoder attached to themechanism. The laser light sheet was placed parallel tothe span of the wing to capture the root and tip vortices,

and was placed along the chord at different span locations

Pressure Chamber

Smoke inflow

Smoke inflow

Laser optics

Laser sheet

CameraWing

Image plane

Honey comb

structure

r

z

x

Figure 5: Schematic of the experimental apparatus for theflapping wing.

to capture the images of the LEVs. A phase delay was introduced so that the laser could be fired at any flappingphase angle. A simple schematic explaining the experimental apparatus used for the flow visualization is shownin Fig. 5.

Seed was produced by vaporizing a mineral oil into adense fog. Oil was broken down into a fine mist by adding

nitrogen under pressure, forced into a heater block andheated to its boiling point, where it became vaporized. Asthe vapor escaped from the heat exchanger nozzle, it wasmixed with ambient air, rapidly cooled, and condensedinto a fog. The fog/air mixture was passed through a se-

ries of ducts and introduced into theflapping wing

flowfield using a plenum and honeycomb structure, as shown

in Fig. 5. The plenum reduces the velocity of the smokethat enters the measurement flow field to such an extenthat the smoke never reaches the region of focus unless

the mechanism is operating. The honeycomb screenshelp ensure that the flow is eddy free.

PIV SYSTEM The important parts of the stereoscopicPIV system included a pulsed laser light sheet for illumi-nating the region of focus in the flow field, a CCD camera to acquire the images, and a calibration grid to obtain

quantitative values from the acquired images. The camera was operated synchronously with the laser in double

exposure mode to acquire two non-interlaced full frameimages during a single frame interval. Images were acquired using a 8-bit CCD camera having a sensor array o1K X 1K (1 MP) pixels.

To measure the flow field in a plane perpendicular to thestroke plane of the flapping wing, the camera was focusedon the laser light sheet located along the wing span. Thecamera covered the region of interest, which was 135 X100 mm. This means that each pixel is a distance of abou

0.1318 mm and 0.0976 mm in the x-axis and y-axis, re

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spectively. A grid of 81 horizontal nodes by 60 verticalnodes was constructed in the aforementioned region ofinterest. This results in 1.6 mm between adjacent nodesin either direction. A laser pulse separation delay of 30 sfor a fluid flow of approximately 8 m/s (tip velocity of theflapping wing) will result in 0.24 mm movement of individ-ual seed particles. This is well within the 1.6 mm distancebetween the adjacent nodes. The cross correlation and allother calculations were performed using commercial PIV

software.

RESULTS

The qualitative and quantitative measurements that wereperformed to understand the flow field surrounding a flap-ping wing MAV are explained in the following categories:(1) flow visualization studies and (2) PIV measurements.

LEADING-EDGE VORTEX Chordwise flow visualiza-tion images, which were acquired at fixed span location(50% of the half-span) through half-stroke of the flapping

wing, are shown in Figs. 6 through 9. Because the lasersheet was projected from the right-hand side of the wing inthis case, a shadow behind the wing appears in all the im-ages. The time period, , in all the images is representedin such a way that the entire flapping stroke (pronation,supination, downstroke and upstroke) is of 360, with 0

representing the mid-point of pronation and 180 repre-senting the mid-point of supination.

It can be observed from Fig. 6 that a LEV does not formuntil the completion of pronation, as also suggested byVan den Berg [18]. The shed vortices that result fromthe wing rotation are clearly visible at the trailing-edge ofthe wing, as shown in Fig. 6(c). As the wing starts toaccelerate after pronation, a small starting vortex formsnear the trailing-edge of the wing, as shown in Fig. 6(e).

As the wing continues its translational motion, the size ofthe starting vortex increases and appears to stay closer tothe trailing-edge see Figs. 6(e) through 7(d). This phe-nomenon has significant effect on the downwash velocityand, hence, on the lift produced by the flapping wing.

Figures 6(f) through 7(e) show that a small LEV forms ataround 60 and it continues to grow with the translationalmotion of the wing. The LEV is then shed and moves aft

over the chord, as shown in Fig. 8. This behavior is awell-known property of a classic dynamic stall vortex [26].However, in this case this shedding is immediately fol-lowed by the formation of another new vortex, therebyproducing multiple vortices at a given time, as shown inFigs. 8(b) and 8(c). It can be observed from images thatcorrespond to 117 through 137 that for some times thereare clearly two vortices over the top surface of the wing.As the wing continues its cycle, the first LEV sheds offthe trailing-edge creating a very unsteady flow field be-hind the wing. Another clean single LEV is produced over

the top of the wing, which is shown in Fig. 8(d). As thewing starts to supinate, the second LEV starts to move aft

towards trailing-edge before being shed see Figs 9(a)through 9(e). The shed vortices that result from supination can be clearly seen in Fig. 9(e).

It is apparent from the foregoing discussion that the LEVis not at all stable on the wing and it continuously shedsfrom the leading-edge as the translational motion of thewing continues, a phenomenon similar to the known classic features of dynamic stall. This observation directly

contradicts the stable LEV concept proposed by severaprevious researchers, who claim a balance between thegeneration of vorticity at the leading-edge and the trans-port of the vorticity into the wake. This difference maybe attributed, in part, to the higher Reynolds number a

which the flapping wing mechanism is operated in the current study (19,500) when compared with the flapping wingmechanisms used by both Ellington, Dickinson and others(150-1,400). Wisdom from helicopter blade studies showsthat an aft movement of the LEV will result in pitching mo-ment stall that will be followed by a significant reduction inlift when the LEV sheds from the trailing-edge. Howeverthe physics is clearly different in the case of flapping wing

because as the LEV moves aft over the chord of the winga new spilled vortex simultaneously forms at the leading

edge. This continuously sustains lift, a novel characteristic that has been shown by flapping wings at low Reynoldsnumber.

The presence of multiple vortices on top of the wing at agiven time depends not only on the Reynolds number bualso on the position of the wing stroke and the spanwiselocation on the wing. In the present study, because theregion of initial interest was at the mid-span of the wingthe presence of multiple vortices was identified beyond

the mid-point of translation during the stroke cycle. A sim

ilar observation has been reported earlier by Tarascio etal. [25] at the 25% span location for a thin rectangular wingoperated at the same Reynolds number as in the presenexperiment. The presence of multiple axial velocity peaksas observed by Birch et al. [27] for the Robofly operating at a Reynolds number of 1,400 at 65% span mighbe because of the presence of multiple vortices. OngoingPIV analysis of the flow at various stroke locations andspanwise wing sections is now giving a better overall pic

ture about the aerodynamic stability of LEVs on flappingwings.

Chordwise flow visualization images that were acquired adifferent span locations at mid-point of translational stroke(upstroke) are shown in Fig. 10. It can be observed thathe LEV is totally absent at the root of the wing. Theseparated flow region is clearly identified by the eddiesin Fig. 10(a). However, at the 25% span location, the separated flow reattaches at approximately 30% chord result-

ing in a LEV. This point of reattachment moves aft whenmoving towards the wing tip see Figs. 10(b) through (e)Very close to the tip, it can be observed from Fig. 10(f)that the flow separates again. This is because of the increased pitch angle that results from elastic wing bending, which has its source in the inertial forces produced

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(a) (c)(b)

(d) (e) (f)

Figure 6: Flow visualization images obtained during pronation: (a) Schematic of the flow structure during pronation; (b) Midpoint of pronation, =0; (c) During pronation, =13; (d) End of pronation, =25; (e) Accelerating wing, = 40; (f) Duringtranslational motion, =60.

Figure 7: Flow visualization images obtained at during downstroke: (a) =65; (b) =75; (c) =80; (d) Mid-point of downstroke, =90; (e) = 97; (f) Schematic of the flow structure around the flapping wing at = 90.

on the wing. A similar form of flow separation near thetip for a rectangular wing has already been reported byTarascio et al. [25].

WING ROOT AND TIP VORTICES Spanwise visualiza-tion behind the trailing-edge of the wing has shown theexistence of relatively strong root and tip vortices see

Fig. 11. The images are explained in terms of wake agewhich is defined as the time elapsed since the wing reference axis was aligned with the laser light sheet. For ex

ample, a wake age of 0 would mean that the wing shafaxis is in line with the laser light sheet and a wake age of90 means that the wing shaft axis has moved one quar-ter of the flapping wing cycle from the laser light sheet. Itcan be observed from Fig. 12 that there are two sets o

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(a) (b)

(c) (d)

Figure 8: Flow visualization images obtained during downstroke: (a) Aftward movement of LEV, = 107; (b) Formation osecond LEV, =117; (c) Presence of two vortices, =137; (d) Second LEV after first one sheds, =153.

Figure 9: Flow visualization images obtained during supination: (a) Start of supination, =157, (b) During supination=173, (c) Mid-point of supination, =180; (d) During supination, = 188; (e) End of supination, =196; (f) Schematic othe flow structure around the flapping wing during supination.

root and tip vortices. The first set, which is closer to thewing, corresponds to those that are at 90 of wake ageand the second set corresponds to the vortices that were

formed during the wing motion on the previous half-stroke.Because the laser sheet was placed precisely at the mid-cycle of the wing translation, the second set of vorticesare 270 older.

Centrifugal forces, which have their source from thehigher swirl velocities in the flow produced by the root andtip vortices, push the seed particles away from the vortex

centers. This results in a very low density of seed particles near the vortex core axis to reflect the laser light, andso appears as a seed void at the approximate center ofvorticity in all the images. For tracking and targeting, thegeometric center of the seed void can be assumed to be

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Figure 10: Chordwise flow visualization images along the span at mid-point of translation: (a) At the root of the wing; (b)20% span location; (c) 40% span location; (d) 50% span location; (e) 75% span location; (f) 85% span location.

Tip vortex

Root vortex

Wing

Laser light sheet

Root vortex

LEV

Figure 11: Root and tip vortices behind the flapping wingat 0 wake age.

Tip vortex= 90 deg

Root vortex = 270 deg

Root vortex= 90 deg

Tip vortex = 270 deg

Figure 12: Root and tip vortices behind the flapping wingat 90 wake age.

Figure 13: Spatial locations of the root and tip vorticestrailing the flapping wing in the mid-stroke plane at variouswake ages.

the center of the vortex. The location of the root and tip

vortices relative to the flapping wing at various wake ageswere determined by using the location of the center of the

seed void relative to a calibration grid. Because there isalways an inherent amount of aperiodicity associated withthis type of unsteady flow, both the average and the standard deviation of these trailed vortex locations is shown inFig. 13. These vortices are of sufficient proximity to eachother that they produce mutually induced effects. Noticethat the root and tip vortices move radially towards eachother and axially downward with increasing time, suggesting a contracting wake structure. The location of the roovortices could be observed until 300, and for the tip vor

tices only until 150 of age. This is because of the spin

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(a)

(d)

(b)

(c)

Figure 14: PIV analysis for 0 wake age (both laser light sheet and the wing are at the mid-point of downstroke: (aRepresentative image used for PIV analysis; (b) Ensemble averaged velocity plot; (c) Vector plot before ensemble averagingshowing all unsteady wake interactions; (d) Close-in view of root and tip vortices.

down of the vortex cores through viscous and turbulenteffects, which results in less distinct seed voids at older

wake ages.

QUANTITATIVE ANALYSIS The results from the PIVmeasurements that were made in a plane perpendicularto the stroke plane of the flapping wing are explained in

this section. The measurements shown here were madeat two wake ages: (1) when both the wing shaft axis andlaser light sheet are at 90 (mid-point of the translationstroke), and (2) when the wing is at 160 and the laser

light sheet at 90, i.e., essentially at 70 wake age. Thelaser light sheet in both the cases was centered along anaxis perpendicular to the stroke plane.

A representative image from which the flow velocity vec-tors across the flow field were obtained is shown inFig. 14(a). Two such images were taken with 30 s phasedelay and were cross-correlated to obtain the velocity

field. Eighty such pairs were used to obtain an ensemble average of the velocity field. The results for the firs

case are shown in Fig. 14(b). It can be seen that the av-erage velocity field, as shown in Fig. 14(b), eliminates althe aperiodic and turbulent wake fluctuations. The velocity field obtained before performing ensemble averaging isshown in Fig. 14(c). A closer view of root and tip vorticesfor the ensemble phase-averaged vector plot is shown in

Fig. 14(d).

The significant spanwise velocity can be seen in Fig. 14which Ellington has previously hypothesized to be the

source of the stability of the LEV. The root to tip flow measured here unambigously proves the presence of a spanwise flow. The spanwise flow in the LEV was found to besignificant, and of approximately the same magnitude asthe maximum wing tip velocity during the flapping cycleThe vortex-like velocity vectors at approximately mid-spanof the wing suggests that the LEV is considerably threedimensional and is slightly inclined to the stroke plane.

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(a) (b)

(d)(c)

Figure 15: PIV analysis for 70 wake age (laser light sheet at the mid-point of downstroke and the wing is at 70 away fromthe mid-point of downstroke: (a) Representative image used for PIV analysis; (b) Ensemble averaged velocity plot; (c) Vectoplot before ensemble averaging showing all unsteady wake interactions; (d) Closer view of root and tip vortices.

The signatures of the root and tip vortices that wereformed during the previous half stroke can also be seenin Fig. 14. The approximate peak swirl velocity of the root

and tip vortices at 180 wake age at mid-point of down-stroke was found to be 28% and 36% of the maximum tip

velocity of the flapping wing at its mid-stroke, respectively.This is a relatively substantial velocity and will have sig-nificant effect on the induced velocity distribution over thewing. The term approximate peak swirl velocity is usedhere because of various issues that are not addressedhere in accurately determining the core size and peakswirl velocity of the root and/or tip vortices, such as theinherent aperiodicity of the flow. Correcting the spatial lo-

cations of the vortices for effects of aperiodicity in the flowwould generally result in a reduced vortex core size andan increase in peak swirl velocity [28].

The ensemble average obtained for 70 wake age isshown in Fig. 15(b). It can now be observed that the rooand tip vortices are 36% and 42% of the maximum tip

velocity, respectively, which is large enough to have sig-nificant effect on the angle of attack of the flapping wing

a mechanism Dickinson hypothesized for the stability ofLEVs. Despite having such a large induced velocity thais comparable to the magnitude of the tip velocity of theflapping wing, the LEVs were found to shed. As explainedearlier, one notable difference between the current experiment and other experiments is the higher Reynolds number at which the present experiment was conducted.

The velocity vectors measured along the chord to estimate the magnitude of leading-edge vortex is shown inFig. 16. The image on the left shows the original image used to find the velocity vectors and the velocity vec

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(a) Raw PIV image (b) PIV vectors

Figure 16: PIV diagnostics of the dynamic stall vortex on top of the flapping wing at its mid-point during the translationastroke. (a) Raw PIV image, (b) PIV velocity vectors.

Spilled lead-ing edge

vortex

Tip vortex

Separatedflow

Spanwiseflow

Separatedflow

Rootvortex

y

z

x

Figure 17: Schematic of the three-dimensional flow over

the top of the flapping wing at its mid-point during the

translational stroke.

tors are shown on the right. The image was obtained atthe mid point of upstroke. This suggests that the LEV ispresent on either side of the translation (both downstrokeand upstroke) and is of a magnitude that is comparable tothe maximum tip velocity of the flapping wing.

A schematic of the overall flow structure on top of the flap-ping wing, which is based on interpretations of both theflow visualization and PIV analysis, is shown in Fig. 17.Despite the presence of such a significant spanwise flow,

the LEVs were still in the process of shedding from the

wing. This would mean either that the spanwise flow isnot large enough to prevent the shedding at this Reynoldsnumber or that the spanwise flow is not responsible for thecontinuous attachment of the LEVs.

CONCLUSIONS

High-resolution flow visualization and PIV analysis havebeen performed on a flapping wing MAV concept. The

following are the conclusions that have been derived fromthe analysis of the measured results:

1. A strong starting vortex forms during the early par

of translation that stays close to the wing for mostof the flapping stroke. The strength of the startingvortex increases during the flapping stroke and has asubstantial influence on the induced velocity field athe wing.

2. A LEV forms and gains strength during the transla

tional motion of the wing. This is followed by the

shedding of the LEV. A new LEV is formed beforethe first LEV sheds from the trailing-edge. The presence of at least one LEV over the wing throughouthe translation stroke helps explain the sustained lifcharacteristics shown by flapping wings.

3. The presence of a spanwise flow that has similar magnitude as the wing flapping velocity is nolarge enough to prevent the shedding of LEVs at thisReynolds number. This LEV is three-dimensional innature and its axis of vorticity lies out of the strokeplane.

4. A strong root and tip vortex pair has been observed

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to trail behind the flapping wing. The peak swirl ve-locities of these trailed vortices were measured to becomparable in magnitude to the the wing tip velocity.The root and tip vortices move radially towards eachother and axially downward for increasing time, with a

contracting wake structure suggesting substantial liftgeneration.

5. The entire flapping wing wake structure, and the in-terconnectivity between the LEVs and the tip and rootvortices, is still not fully understood and still remainsthe subject of ongoing research.

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2005-iplc- flapping wing paper

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