Surface structure and dimensional effects on the aerodynamics of an owl-based wing model

European Journal of Mechanics B/Fluids 33 (2012) 58–73

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European Journal of Mechanics B/Fluids

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Surface structure and dimensional effects on the aerodynamics of an owl-basedwing modelS. Klän a,∗, S. Burgmann a, T. Bachmann c, M. Klaas a, H. Wagner b, W. Schröder aa Institute of Aerodynamics, RWTH Aachen University, Wüllnerstr. 5a, 52062 Aachen, Germanyb Institute of Biology II, RWTH Aachen University, Mies-van-der-Rohe-Str. 15, 52056 Aachen, Germanyc Institute for Fluid Mechanics and Aerodynamics, TU Darmstadt, Petersenstr. 30, 64287 Darmstadt, Germany

a r t i c l e i n f o

Article history:Received 19 May 2011Received in revised form29 October 2011Accepted 29 December 2011Available online 8 January 2012

Keywords:Particle-image velocimetryBarn owlSeparationSurface

a b s t r a c t

A model wing based on the geometry of the wing of a barn owl was designed, in which the featherstructure of the barn-owl wing is approximated by a velvet-like surface. The first objective of this paper isto investigate the impact of artificial surface filaments on the overall flow field of a quasi-2D configurationof the model 3D wing. Two velvet-like surfaces are used and the velocity field is measured by particle-image velocimetry in a chord-length based Reynolds number range 20, 000 ≤ Rec ≤ 60, 000 at angles ofattack 0° ≤ α ≤ 6°. An explanation of the mechanism that leads to the change in the near-wall flow fielddue to the surface structures is given. The second objective of the paper is the comparison of the 2D andthe 3D results and the analysis of the impact of the three-dimensionality on the flow field.

The first surface structure (‘‘velvet 1’’) mimics the length and density of the hairs and the softnessof the natural owl-wing surface. It diminishes the size of the separation bubble or completely preventsseparation. However, at three-dimensional flow the effect of the ‘‘velvet 1’’ surface is clearly reduced.The ‘‘velvet 2’’ surface consists of longer and thinner filaments than the ‘‘velvet 1’’ surface. At the lowerReynolds numbers (Rec ≤ 40, 000), the ‘‘velvet 2’’ surface structure does not alter the near-wall flow fieldsignificantly. However, at Rec > 40, 000 the ‘‘velvet 2’’ surface structure serves as a distributed field ofmoving roughness elements such that the size of the separation bubble is reduced and becomes nearlyindependent of the angle of attack.When the three-dimensional flow field at the highest Reynolds number(Rec = 60, 000) is considered it is evident that the ‘‘velvet 2’’ surface yields the aerodynamically morestable flow field.

© 2012 Published by Elsevier Masson SAS.

1. Introduction

This study focuses on the aerodynamics of a model wing basedon the characteristics of an owl wing, i.e., the impact of the feathersurface being approximated by velvet-like surfaces on the velocityfield is analyzed. Due to themuch greaterwing size of the barn owl,the wing loading is approximately only 50% of the wing loading ofthe pigeon in the Reynolds number range Rec = O(104). Togetherwith the low flight speed, the susceptibility to flow phenomenasuch as separation or stall is increased drastically. For technicalapplications, the presence of a separation is undesirable since itdecreases the lift and increases the drag. Additionally, pressurefluctuation due to vortex shedding at the downstream end of theseparation bubble might occur, leading to aerodynamic behaviorwhich is hard to control. In the owl wing, some of its specificfeatures such as the surface structure due to its special feathers [1]

∗ Corresponding author. Tel.: +49 241 8094882.E-mail address: [email protected] (S. Klän).

0997-7546/$ – see front matter© 2012 Published by Elsevier Masson SAS.doi:10.1016/j.euromechflu.2011.12.006

may contribute to an increase in the aerodynamic performance ofthe wing. In other words, the feathers are supposed to decrease oreven suppress the separation bubble or stabilize it such that abruptchanges in the lift distribution are avoided.

Surface structures affect the near-wall flow and as such thecomplete flow field of a wing. For example, passive means of flowcontrol turbulators or boundary layer fences may be attached tothe wing surface to alter the flow macroscopically. To artificiallygenerate a turbulent boundary layer which may resist the adversepressure gradient without any boundary layer separation, trippingdevices like a wire or a zig-zag band are used. Earlier studies [2]showed experimentally that a 2D-surface roughness element hasa destabilizing effect on the boundary layer and may reduce thecritical Reynolds number such that a previously laminar boundarylayer undergoes a transition process into a turbulent boundarylayer. This transition process is governed by a boundary layerinstability, whereas the transition process of a separation bubble isdominated by a free-shear layer instability. This kind of instabilitymay be based on the growth of Tollmien–Schlichting (TS) waves inthe attached and separated shear layer or on a Kelvin–Helmholtz

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S. Klän et al. / European Journal of Mechanics B/Fluids 33 (2012) 58–73 59

(KH) instability of the free-shear layer [3,4]. Investigations [5,6]showed the effect of several surface roughnesses on the transitionprocess of a separation bubble. Using hotwire anemometry, it wasfound that for a stronger roughness the transitionmoves upstreamand the separation point downstream. That is, the amplificationof the TS-instability by surface roughnesses excites the transitionprocess such that transition occurs farther upstream. On the otherhand, riblet structures at a spacing in inner coordinates on theorder of s+ ≈ 16 have been reported to delay transition inzero-pressure gradient boundary layers due to a damping of 3D-structures [7,8]. Recent numerical studies using LES in a channelflow [9] revealed that the riblets accelerate transition by excitingTS-waves. The amplification magnitude depends on the specificgeometry of the riblets. In turbulent flows, riblet structures maycause a reduction of drag due to the damping of the spanwiseand the wall-normal velocity fluctuation, i.e., the damping of themomentum transport as illustrated for instance in [10,11].

The velvet-like structure on the suction side of the owl-wing issimilar to a fur. In other words, the surface roughness is a flexibleelement which may adopt to different flow conditions. The effectof hairy surfaces on the drag-reducing abilities has already beeninvestigated. In a numerical investigation of a viscous flow over aflat plate, the hairs were simulated as strings stretched parallel tothe surface of the plate and in the streamwise direction [12]. Thevelocity profiles and the shear stress distribution were calculatedfor this configuration. It was observed that the change in thelocal velocity profile is similar to that of riblet surfaces. Thatis, the apparent origin of the velocity profile moves away fromthe surface, reducing skin friction. The estimated drag reductionwas higher than for riblet surfaces. However, experimental datacould not verify the numerical drag reduction findings [13]. Theconclusion from the numerical analysis was that the so-calledeffective height of the structure, which is the distance betweenthe apparent origin and the tip of a riblet or the stretched string,is important for the drag reduction. This effective height is greaterfor the simulated hairy surface and thus a higher drag reduction isachieved. Unfortunately, it is not evident whether the reduction oflocal skin friction can be overcompensated by an increased skinfriction due to an increased surface area when the total drag isconsidered.

The ability of soft surfaces to reduce drag in a turbulent flowregime was also demonstrated for seal fur [14]. The hairy surfaceof the seal fur is even more effective in terms of drag reductionthan any technical surface such as riblets. The drag reduction isattributed to the hydrophilic/-phobic properties of the seal fur andsome kind of riblet effect. The hairs of the fur aligned with theflow direction result in a surface structure which is similar to thatof riblets. Whereas a riblet surface is optimized only for a singleReynolds number, the fur can act as an adaptable passive dragreduction mechanism over a wide range of Reynolds numbers.Thus, the drag reduction of the seal fur is even higher than thatof an optimized riblet surface. Since the surface structure of thebarn-owl wing is similar to the hairy surface of a fur and since theReynolds number range [14] is the same as for the barn owl flight, itis assumed that a velvet-like surface structurewill also act as a dragreducing device in a turbulent flow without taking advantage ofany hydrophilic/-phobic properties. Note that even in the Reynoldsnumber regime of Rec = O(104) the flow undergoes transitionwhen separation occurs [15–18].

Previous investigations of an owl-wing based geometry [20] re-vealed that due to the curvature of the airfoil surface downstreamof 20% chord a separation bubble occurs. It has been shown that itis possible to influence the characteristics of a separation bubble,i.e., the point of transition onset and reattachment, by applying avelvet structure to the suction side of a rectangular wing whosegeometry is related to that of an owl wing. Furthermore, video

Fig. 1. Snapshot of a movie of the barn owl in gliding flight at Re ≈ 60 000, takenfrom [19]. Note that the lifted surface feathers in the inner section of the wingindicate an unstable flow condition.

recordings of a natural flying owl substantiate the observation oflocal separation, since in thewing region of high surface curvature,i.e., the inner section, the surface feathers tend to lift and flutter,indicating a reversed flow direction (Fig. 1).

Due to these findings, the first objective of this study is toinvestigate the effect of different velvet-like structures on theformation of a separation bubble on the suction side of theowl-based wing. Especially the influence of the characteristicproperties, e.g. length and flexibility of the filaments, on theeffectiveness of the surface structure will be of interest. Therefore,the results of the previous paper [20] will be compared withthe results of measurements of a quasi-2D wing equipped witha second velvet surface structure. The new velvet surface willvary from the first surface used in [20] in terms of length anddiameter, i.e., in flexibility. The second objective is the analysisof the impact of the three-dimensionality of the flow on theaerodynamic stability of the flow structure. The flow field aroundthe quasi-2D and the 3D owl wing is analyzed and a comparisonof the 2D and 3D flows is drawn. Furthermore, the impact ofthe Reynolds number on the effectiveness of the velvet surfacestructures will be discussed.

The manuscript is organized as follows. First, the experimentalsetup, including a description of the velvet surface structureand the measurement technique, is presented. Subsequently, theresults of the measurement campaign for different surfaces underplanar and spatial flow conditions are depicted and discussedwith respect to previous findings. Then, the final conclusions arepresented.

2. Surface structure and experimental setup

Fig. 2(a) shows the 3D wing model which was designed on thebasis of a real owl wing [20]. The quasi-2D wing model illustratedin Fig. 2(b) was derived from this 3Dwingmodel by averaging overthe inner 40% of the span. Further details are given in [20]. In thefollowing, the applied velvet-like structures are described and theexperimental setups for the investigation of the 2D and 3D owl-based wings are presented.

2.1. Surface structure

To mimic the natural velvet-like surface of the owl, whosecharacteristics such as length, diameter, and density of thefilaments hardly vary over the total wing [1], two artificial,synthetic velvet structures were selected. The natural surfaceconsists of approximately 200 thin hairs of a diameter of d ≈

6 µm per mm2. These hairs tend to stick together due to tiny

60 S. Klän et al. / European Journal of Mechanics B/Fluids 33 (2012) 58–73

2

1

1.5

0.5

0

0

0

2.5

10.80.6

0.20.4

0.1y/c

x/c

z/c

(a) Three-dimensional geometry of the artificial owl wing. The cross-section at2y/b = 0.2 which is almost equivalent to the 2D-wing is highlighted.

0 10.80.60.2 0.4x/c

1

0.5

–0.5

0

(b) Two-dimensional geometry of the artificial owl wing.

Fig. 2. Geometry of the three-dimensional and the two-dimensional wing.

Fig. 3. Photographs showing the natural owl wing surface (left), velvet 1 (center) and velvet 2 (right). The black bar at the bottom of each image illustrates a length of400 µm.

barbs forming bundles of hairs (Fig. 3). This property of the naturalsurface is hard to model by a synthetic structure. Hence, the firstvelvet surface was chosen to mimic the natural surface as well aspossible with respect to the flexibility, length of hairs, and densityof hairs. The hairs tend to return to their vertical orientationwithinapproximately 10 ms after a horizontal deflection. This textile iscalled ‘‘velvet 1’’.

A second velvet which possesses longer filaments and thereforeappears more flexible than ‘‘velvet 1’’ or the natural surfacewas selected. This textile is called ‘‘velvet 2’’. Another specificcharacteristic feature of this velvet is the uniform preferredorientation of the hairs, which coincides in themeasurementswiththe mean freestream direction. The filaments of this velvet tendto stick to the surface if they are deflected horizontally in theirpreferred direction. On the other hand, if they are deflected in the

opposite direction these hairs return to their vertical orientationwithin approximately 10 ms. The characteristics of the differentsurface structures are summarized in Table 1.

Note that the thickness of the coatings changes the maximumthickness of the wing by less than 0.1%. Numerical analyses basedon the Euler and boundary-layer equations showed that onlynegligible variations of the flow field occurred due to the minorgeometric deviations from the reference configuration.

The artificial velvet-like structures are carefully applied to the2D and the 3D owl-based wings such that the upper surface of thewing is completely covered. That is, an artificial surface structureis applied to the suction side of the wing such that it extends fromthe leading edge to the trailing edge. The surface structure evencovered a small part near the leading edge of the pressure side ofthewing to ensure a smooth surface contour on the suction side. To


Table 1Properties of the natural surface structure of the owl wing and the artificial textiles ‘‘velvet 1’’ and ‘‘velvet 2’’; the reset time of the natural surface could not be measured;the first reset time for ‘‘velvet 2’’ is valid for the preferred hair direction, the second time is valid for a deflection against the preferred direction.

density per mm2 length of hairs (mm) thickness of hairs (µm) reset time

natural ≈200 ≈1.0 ≈6 –velvet 1 ≈190 ≈1.5 ≈18 10 msvelvet 2 ≈160 ≈2.4 ≈10 ∞, 10 ms

fix the velvet on thewing a spray adhesivewhich does not soak thetextile is used. In a first step, the flow of the 2Dwing is investigatedto detect the possible influence of the various velvet structures onthe 2D flow field.

2.2. Experimental setup

The facilities and the measurement techniques are jointlydescribed, first for the two-dimensional and then for the three-dimensional investigations. This structure allows a concise discus-sion of the experimental setups.

2.2.1. 2D flow fieldThe 2D wing flow field was measured in a small low speed

wind tunnel with closed test section (Fig. 4). Based on the chordlength of the wing of c = 0.178 m and the velocity range in thetest section of the wind tunnel of 1.8 ms−1 to 5.4 ms−1 Reynoldsnumbers from 2 · 104 to 6 · 104 could be reached, which are inthe range of natural owl flight. Themeasurements were performedat Rec = 2 · 104, 4 · 104, and 6 · 104. Moderate angles of attack,i.e., α = 0°, 3°, and 6°, which also correspond to those of theowl wing at gliding flight are investigated. The freestream velocitywas adjusted by a hotwire probe which could be mounted in thecenter plane of the test section at a distance of 1.5 c upstreamof the wing. The probe was removed during the measurements.The freestream turbulence level of the wind tunnel is Tu = 0.2%.Since the test section possesses a cross-section of 500× 500 mm2,which leads to an aspect ratio of Λ = 2.8 for the 2D wingand since the boundary layer thickness on the side walls in theleading edge region is in the range of 10 mm, effects from thetest section walls on the flow field in the center of the wingcan be neglected, such that the measurements show meaningfulcenterline characteristics. Furthermore, the maximum blockage ofthewind tunnelwas 6.7% atα = 6°. Thewalls of the test section aremade of transparent plexiglass to ensure an unconditional opticalaccess. The airfoil was fixed on circular rotatable plexiglass platessuch that the angle of attack could be easily adjusted.

The flow field of the suction side was recorded and analyzedusing standard 2D–2C particle-image velocimetry (PIV). Themeasurement plane illuminated from above was normal to thewing surface and oriented in the mean flow direction. Theexperimental setup was similar to that described in [20], sincealso the results of that measurement campaign are taken intoaccount for the analysis of the current investigation. However, forthe measurements of the flow field for the surface ‘‘velvet 2’’ thecamera was exchanged and a PCO.4000 was used. The sensor sizeof this camera is larger, so that the entire chord length of thewing could be recorded. Note that the previousmeasurements [20]required decomposition of the recorded flow field into 3 regions.In other words, the spatial resolution of the former measurementsand the current investigation is comparable. The measurementplane was illuminated by a Nd:YAG double-pulse laser (NewWaveSolo 200XT). An articulated light arm connected the laser andthe light sheet optics. An ILA mini PIV-synchronizer provided thetrigger signal for the laser and the camera with a repetition rateof 2 Hz. The pulse distance was adopted for each case to providea mean particle displacement of ∆xpx ≈ 8 px in the freestream

Fig. 4. Photograph of themeasurement setup of the PIVmeasurement in the closedtest section of the low-speed wind tunnel.

Fig. 5. Photograph of the measurement setup of the PIV measurement in the opentest section of the low-speed wind tunnel.

region, i.e., 200 µs at Rec = 2 · 104, 170 µs at Rec = 4 · 104,and 110 µs at Rec = 6 · 104. In each case, 1500 imageswere recorded which proved to be sufficient to obtain convergingstatistics of the highly unstable flow field. The evaluation of theparticle imageswas performed using VidPIV by ILA. Using adaptivecross-correlation schemes with window deformation and windowshifting a final resolution of 24 × 24 px with 50% overlap wasachieved leading to a vector spacing of ∆xvector ≈ 0.4 mm or∆xvector/c ≈ 0.0022 c.

2.2.2. 3D flow fieldTo analyze the three-dimensional flow field of the 3D artificial

owl wing, a larger test section is needed to provide an undisturbedflow at the wing tip. Hence, this wing is placed in the larger low-speed wind tunnel. This wind tunnel has an open test sectionwith a diameter of 1200 mm (Fig. 5). The maximum freestreamvelocity is ∼60 ms−1. Similar Reynolds numbers as in the formermeasurements of the 2D wing were investigated, i.e., the wind


Fig. 6. Snapshot of the particle laden flow field illustrating the separation location, the separated shear layer, and the unsteady vortex shedding at the rear end of theseparation bubble (Rec = 40 000, α = 3°, ‘‘velvet 2’’).

Fig. 7. Results of the PIV measurements of the clean surface (top), of the velvet 1surface (center), and the velvet 2 surface (bottom) at Rec = 40 000 and α = 0° forthe 2D-case. The color indicates the vorticity distribution.

tunnel was run in the lower operating regime. In this regime,the turbulence level is Tu ≈ 0.3%. Note that in the spanwisedirection the chord length of the three-dimensional model variessignificantly. To obtain comparable results of the quasi-2D andthe fully three-dimensional wing, the chord length of the 2D wingalso was chosen as reference length for the 3D wing. The Reynoldsnumber was adjusted to Rec = 4 ·104, 6 ·104, 1 ·105, and 1.2 ·105.Thewingmodel was fixed on a circular platewhich ismounted in aflat plate, enabling an easy adjustment of the angle of attack. Notethat the 3Dwing is not twisted, i.e., the angle of attack refers to theoverall validwing chord. The same angles of attack as in the 2D casewere investigated. The flat plate was placed in the lower quarter ofthe nozzle of the wind tunnel and possesses an elliptical nose. Theflow on the ground plate was tripped by a 1 mm wire, ensuringa well-defined condition at the root of the wing, i.e., a turbulentboundary layer such that transitional effects of the boundary layerdo not interferewith the flow of the 3D-wing. Thewing possesses ahalf-span of 431mm, i.e., the projected distance between the wingtip and the upper edge of the nozzle is 569 mm. Hence, influencesof the wake of the nozzle contour can be neglected.

Again, the flow field is analyzed using 2D–2C PIV measure-ments. Like in the closed-test-section tunnel, the measurementplane was normal to the center plane of the wing, i.e., parallel to


the mounting plate of the wind tunnel, and aligned with the flowdirection. Analogous to the quasi-2D wing, only the suction sidewas recorded and analyzed. However, since the 3D flow topologywas of particular interest, several parallel planes along the spanwere investigated. Four spanwise positions at 2y/b = 0.1, 0.2, 0.3,and 0.4 were measured. This selection corresponds to the rangefrom which the quasi-2D wing was derived [20]. Note that eachcross section of the 3D wing differs from the averaged 2D profilesuch that the impact of the three-dimensionality on the flow fieldwas to be investigated. The variation of the cross-section profilesin the near-root region of the three-dimensional wing is depictedin Fig. 15.

The PCO.4000 camera and the NewWave Solo 200XT Nd:YAGdouble-pulse laser and the corresponding peripheral devices wereused. The post-processingwithVidPIV of the 1500 images recordedwith 2 Hz in each case yielded converging statistical data and ledto a final resolution of 24 px × 24 px with 50% overlap, i.e., afinal vector spacing of 0.53 mm or 0.0029 c. In all measurements,Lazkin-nozzle seeders that provide DEHS-particles with a meansize of 1 µm are used. These particles guarantee a sufficiently lowresponse time for unsteady vortical flow fields like in the presentcases.



Note that a flow field at low Reynolds numbers which is likelyto separate may encounter some kind of hysteresis. That is, slightvariations of the angle of attack during themeasurements runmaycause severe changes of the flow field at increasing angle of attackcompared to decreasing angle of attack. Hence, before the angleof attack was changed, the wind tunnel was switched off and theangle of attack was carefully adjusted.

3. Results and discussion

In this section, the results of the PIV measurements are pre-sented. First, the impact of the surface structure is analyzed for thetwo-dimensional flow field. Unlike the two-dimensional investiga-tion in [20], the current analysis focuses on howan alteration of thecharacteristics of the surface structure impacts the near-wall flowand how the freestream conditions, i.e., the Reynolds number, in-fluence its effectiveness. Up to a Reynolds number Rec = 40 000the ‘‘velvet 1’’ surface will be the most promising. However, whenhigher Reynolds numbers are considered and when in the secondpart of the discussion 3D effects are taken into account, it will be-come clear that the properties of the ‘‘velvet 2’’ surface yield anaerodynamically more efficient flow.

3.1. Two-dimensional flow field

In the previous study [20], the pressure distribution of thetwo-dimensional wing was investigated and the lift coefficientat various angles of attack were determined and compared tonumerical results. It was found that the zero-lift angle for Reynoldsnumbers Rec = 40 000 and 60000 is approximately −3°. For thelower Reynolds number, themaximum lift coefficient is at an angleof attack of approximately 3 and close to 6° at the higher Reynoldsnumber.

Table 2Characteristics of the flow field of the clean wing and the ‘‘velvet 1’’ wing and the‘‘velvet 2’’ wing, l.s.: low-Re separation, s.b.: separation bubble, a.f.: attached flow.

clean velvet 1 velvet 2

α\Rec ·103 20 40 60 20 40 60 20 40 60

0° l.s. s.b. s.b. l.s. s.b. a.f. l.s. s.b. s.b.3° l.s. s.b. s.b. l.s. a.f. a.f. l.s. s.b. s.b.6° l.s. l.s. l.s. l.s. l.s. l.s. l.s. l.s. s.b.

In the following, the novel findings of the PIV measurements ofthe 2Dwing, i.e., themean flow is two-dimensional, are presented.In most investigated clean surface cases a separation bubble isformed or complete separation occurs. In the case of a separationbubble, the boundary layer separates from the airfoil surface ina laminar flow regime. The separated shear layer undergoes alaminar–turbulent transition and reattaches. The flow states atvarious freestream conditions are summarized in Table 2.

As has been shown in previous studies [20,21], the character-istic geometric parameters of a separation bubble can be deducedfrom the PIV results, i.e., the point of separation, the transition on-set, and the reattachment of the mean flow. A typical PIV imageis exemplarily depicted in Fig. 6. It clearly shows the flow phe-nomena occurring on the quasi-2D wing such as the separation,the position of the laminar separation bubble, the development ofthe free-shear layer, and the vortex roll-up at the downstream endof the separation bubble due to the KH-instabilities. Downstreamof the bubble, the vortices break up into smaller structures and dis-sipate. The separation point is detected based on the particle im-ages. Ensemble averages based on 15 images were computed andthe particle traces were determined to identify the line of the high-est velocity gradient. The intersection of this line and the surfacedefined the point of separation in the cross section considered. Theseparation point can be detected at an accuracy of approximately3 mm, i.e., ∼0.02 c. The point of reattachment is determined fromthe mean velocity field.

The analysis of the 2D flow field reveals several states of themean flow field, e.g. total separation or formation of a closedseparation bubble. At the lowest Reynolds number (Rec = 20 000),separation occurs without reattachment for all investigated cases.Such a state of the flow field has also been reported earlier [22].Some authors proposed a critical Reynolds number based on thebubble length of Rebubble = 50 000 to be necessary to havereattachment [23]. In other words, if the chord-length-basedReynolds number is less than Rebubble the streamwise extent of theairfoil is simply too small for reattachment. Naturally, the criticalReynolds number at which reattachment can occur depends onthe freestream flow condition, e.g. freestream turbulence, angleof attack, and on the airfoil or wing itself. That is, the surfaceroughness has a great impact on the reattachment. This surfaceroughness has to exceed the viscous sublayer whose heightdepends on the Reynolds number Rec . At Rec = 20 000 thesurface structure does not seem to have any effect on the flowfield, i.e., neither ‘‘velvet 1’’ nor ‘‘velvet 2’’ introduces sufficientdisturbances in the boundary layer upstream of the separationsuch that a turbulent boundary layer emerges that couldwithstandthe pressure gradient.

At higher Reynolds numbers (Rec = 40 000 and 60000), a dif-ferent flow regime develops. At lower angles of attack, a separa-tion bubble is present or the flow stays attached to the surface.Note that at an angle of attack of α = 6° the flow completelyseparates for all Reynolds numbers except for ‘‘velvet 2’’. This sep-aration behavior at increasing angle of attack is typical for stan-dard airfoils with a thick nose. That is, a small separation bubbleoccurs which breaks up at higher angles of attack and the airfoilstalls [24].


Fig. 10. Geometry of the owl-based 2D-airfoil at α = 0°, 3° and 6°. The zero-inclination point is indicated in each case by an x.

Fig. 11. Triangle-like distribution of the shear stresses−u′v′/u2∞

of the clean surface (top) and of the surface equippedwith velvet 2 (center) at Rec = 40 000 andα = 0°. Thediagram (bottom) shows the growth of the shear stresses on the line of maximum shear-stresses (indicated by the dashed line in the illustrations above) and the definitionof the transition onset as the significant rise of the shear stresses.

The velvet-like surface may significantly influence the flowfield when the flow is likely to undergo separation. To show theinfluence of the surface structure on the flow field, the results atRec = 40 000 and α = 0°, Rec = 60 000 and α = 3°, andRec = 60 000 and α = 6° are presented in Figs. 7–9 for the cleanwing, ‘‘velvet 1’’, and ‘‘velvet 2’’, respectively. It is obvious that thevarious velvet-like surfaces affect the flow field in different flowregimes. For instance, the separation bubble is strongly reducedor completely suppressed if a structure like ‘‘velvet 1’’ is appliedon the suction side of the airfoil (Table 2). On the other hand, thesofter surface ‘‘velvet 2’’ does not really seem to affect the meanflow field at Rec = 40 000, α = 0° but reduces the bubble size atRec = 60 000, α = 3° and prevents, unlike ‘‘velvet 1’’, completeseparation at Rec = 60 000, α = 6° (Fig. 9).

Interestingly, if separation occurs, the separation point is notaltered by the surface structure. It only depends on the angleof attack, i.e., the pressure gradient. This indicates that the flow

state upstream of the separation point is not changed due tothe application of the surface structure. To be more precise, theupstream impact of the surface structure as to the separationlocation is negligible. That is, the roughness height does not forcethe laminar–turbulent transition and laminar separation occurs inall cases. The corresponding values of the chordwise coordinate aresummarized in Table 3.

It can be deduced from the shape of the airfoil that theseparation point coincides with the zero-inclination point whichhas been determinedwith respect to the freestream direction. Thispoint is illustrated in Fig. 10. Hence, the boundary layer upstreamof the zero-inclination point, i.e., the separation point, seems to benot sufficiently disturbed by the surface structure. In other words,the velvet-like surface does not provide a sufficiently increasingmomentum transfer into the boundary layer to delay or preventseparation at Re ≤ 40 000 (Fig. 7). As discussed above, the point ofseparation is expected to move downstream only for large surface


Fig. 12. Velocity distribution for the clean surface at Rec = 20 000 (top), Rec = 40 000 (center), and Rec = 60 000 (bottom) indicating the strength of the backflow in therecirculation region.

Fig. 13. Sketch of the interaction of the free-shear layer and the surface structure; depending on the surface structure smaller eddies occur, i.e., the distance l is reducedresulting in a more intense transition.

Table 3Location of the separation point of the flow field x/c of the clean wing, the ‘‘velvet 1’’ wing, and the ‘‘velvet 2’’ wing.


α \ Rec · 103 20 40 60 20 40 60 20 40 60

0° 0.19 0.19 0.20 0.19 0.19 – 0.19 0.20 0.213° 0.16 0.16 0.17 0.16 – – 0.16 0.16 0.186° 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.14 0.15

roughness heights [5,6]. It can be surmised that due to the softnessof the velvet-like surface such a critical surface structure height isnot achieved in the investigated cases, since the separation pointdoes not move downstream. However, the reattachment of theflow is strongly affected by the application of the velvet surface ascan be clearly seen in Figs. 7–9 since the bubble size decreases inlength and thickness or a bubble instead of a completely separatedflow is formed.

This statement is quantified in Table 4, which summarizesthe distance between the point of separation and the pointof reattachment. The ∞-symbol denotes cases in which totalseparation occurs. In some cases using ‘‘velvet 1’’, separation isprevented entirely. In other words, the velvet structure forces thetransition process in the attached boundary layer. On the otherhand, it becomes evident from Table 4 that even when the overallflow state is not changed the reattachment of the mean flow


(a) clean. (b) velvet 1. (c) velvet 2.

(d) clean. (e) velvet 1. (f) velvet 2.

Fig. 14. Velocity fields and contours of the spatial two-point correlation of the normal velocity fluctuation v′(x)v′(x + ∆x)/v′(x)2 for the surface structure ‘‘velvet 1’’ and‘‘velvet 2’’, respectively, at Rec = 40 000 (left) and at Rec = 60 000 (right). The angle of attack is α = 3° for all cases. The quantity l represents the distance between twoconsecutive vortices.

Table 4Distance from separation to reattachment of the flow of the clean wing, the‘‘velvet 1’’ wing, and the ‘‘velvet 2’’ wing.∞: complete separation, — : no separation.


α\Rec ·103 20 40 60 20 40 60 20 40 60

0° ∞ 0.47 0.35 ∞ 0.3 – ∞ 0.47 0.233° ∞ 0.50 0.34 ∞ – – ∞ 0.53 0.176° ∞ ∞ ∞ ∞ ∞ ∞ ∞ ∞ 0.15

seems to be affected by the velvet structure. However, althoughthe transition mechanism yields a fuller velocity profile in theboundary layer, e.g. by amplifying TS-waves or a KH-instability, itwill be indicated by the following investigation of the transitionbehavior that especially for the ‘‘velvet 2’’ surface the separation isnot only determined by the transition phenomenon.

To analyze the transition process the Reynolds shear stressdistribution −u′v′/u2

∞was calculated for each case. The shear

stress distributions describe the transfer of momentum into theboundary layer [25]. Typically, the planar presentation of the shearstresses shows a triangle-like shape [15,21,26]. Such a pattern isillustrated in Fig. 11 for the Rec = 40 000 flow. Analyzing thedevelopment of the local maximum of the shear stress distributionalong the chord, a significant rise can be observed. The point wherea significant rise of the shear stresses above the noise level can bedetected determines the transition onset [15]. To be more precise,

the transition onset is defined at the location where the rate ofchange of the best-fit lines of the shear stress with respect to thestreamwise coordinate is at least doubled compared to the slopeupstream.

The chordwise distance of these points from the separationpoint is listed in Table 5 for all investigated cases. It can be deducedfrom these values that the two different velvet structures seem tohave an opposing effect on the flow field in most cases. Whereas‘‘velvet 1’’ has an amplifying effect on the flow disturbances, sincethe transition onset is shifted upstream compared to the clean-wing case, ‘‘velvet 2’’ seems to damp flow-inherent disturbancesbelow a critical Reynolds number Recrit . For the ‘‘velvet 2’’-surface,the transition onset is significantly delayed at Rec = 20 000 andRec = 40 000. At Rec = 60 000 the transition onset is in the sameregion as for the clean-wing case. In other words, the dampingeffect of ‘‘velvet 2’’ compared to ‘‘velvet 1’’ does not play a roleat higher Reynolds numbers. The disturbance-damping effect of‘‘velvet 2’’ may be based on some kind of natural riblet structuresimilar to the description of the seal fur [14]. The surface structureof ‘‘velvet 2’’ ismuch softer than that of ‘‘velvet 1’’, the filaments arelonger, align with the flow direction, and bend toward the surface.In this Reynolds number regime these filaments form a riblet-likestructure and seem to damp disturbances which are generated inthe free-shear layer. The damping effect is reduced at higher anglesof attack α when the shear stresses in the free-shear layer are also

Table 5Distance from separation to the transition point of the flow of the clean wing, the ‘‘velvet 1’’ wing, and the ‘‘velvet 2’’ wing.


α \ Rec · 103 20 40 60 20 40 60 20 40 60

0° 0.34 0.20 0.14 0.25 0.05 – 0.47 0.24 0.133° 0.26 0.16 0.11 0.22 – – 0.40 0.17 0.096° 0.23 0.10 0.04 0.18 0.03 0.01 0.35 0.15 0.03


Fig. 15. Geometry of the variousmeasurement cross sections at spanwise positions2y/b = 0.1 (black), 0.2 (blue), 0.3 (green), and 0.4 (red). The position of the leadingedge has been set to y/c = 0 for all profiles. (For interpretation of the references tocolour in this figure legend, the reader is referred to the web version of this article.)

increased. That is, when α is larger, transition onset occurs furtherupstream.

Hardly any damping effect is observed for ‘‘velvet 2’’ at Rec =

60 000. The separation bubble is much smaller in these casesthan for the clean wing and even complete laminar separationis prevented. It may be conjectured that the preferred directionand the flexibility of the filaments play a role in the developmentof the overall flow structure. Since the filaments are oriented inthe freestream direction, they bend to the surface if the fluidmoves in the same direction. However, vortical structures at thedownstream end of a separation bubble lead to a low-frequencyoscillation of the flow and provide some kind of backflow due tothe rotation of the vortices. The stronger the vortices, the higher

the local backflow velocity. That is, for a higher freestreamvelocity,which generates higher vorticity, the reverse flow is increased.This is illustrated in Fig. 12 for the clean wing problem. Note thathigher negative values of the velocity occur at the higher Reynoldsnumbers. At Reynolds number Rec = 20 000 and Rec = 40 000,the flow field of the clean wing and the wing equipped withthe ‘‘velvet 2’’-surface is similar (Fig. 7). At the higher Reynoldsnumber (Rec = 60 000), a clear difference is observed (Figs. 8and 9). It is assumed that this difference in the flow field is dueto the higher backflow velocity which tends to lift the hairs ofthe ‘‘velvet 2’’-surface structure such that these filaments workas movable elements similar to the effect of movable flaps forseparation control [13]. This effect is sketched in Fig. 13.

The effectiveness of this separation control is a function ofthe freestream conditions such as Reynolds number and angleof attack. Figs. 7–9 clearly show the differences between thethree surface structures. Since the freestream conditions are alike,the observed change in the flow field is merely a result of thechange in the surface structure. Measurements of the separationand recirculation region at high spatial resolution in Fig. 14show that in the clean surface case a laminar backflow develops.When the velvet structures are applied to the wing, this backflowis altered significantly. That is, velocity fluctuations are excitedwhich are finally transported into the laminar free-shear layer

Fig. 16. Flow field of the clean 2D-wing and the clean 3D-wing at 2y/b = 0.2 for α = 0°, 3°, and 6° at Rec = 40 000 and 60000.


x/c0 0.2 0.4 0.6

0.1

0.2

2D airfoil

α=0°, Rec=40,000

0 0.2 0.4 0.60

0.1

0.2

3D wing

x/c0 0.2 0.4 0.6

0.1

0.2

2D airfoil

α=0°, Rec=60,000

0 0.2 0.4 0.60

0.1

0.2

3D wing

x/c0 0.2 0.4 0.6

0.1

0.2

2D airfoil

α=3°, Rec=60,000

x/c0 0.2 0.4 0.6

0.1

0.2 α=3°, Rec=40,000

2D airfoil

x/c0 0.2 0.4 0.6

0.1

0.2 α=6°, Rec=40,000

2D airfoil

0 0.2 0.4 0.60

0.1

0.2

3D wing

x/c0 0.2 0.4 0.6

0.1

0.2 α=6°, Rec=60,000

2D airfoil

0 0.2 0.4 0.60

0.1

0.2

3D wing

0 0.2 0.4 0.60

0.1

0.2

3D wing

0 0.2 0.4 0.60

0.1

0.2

3D wing

y/c

y/c

y/c

y/c

y/c

y/c

y/c

y/c

y/c

y/c

y/c

y/c

x/cx/c

x/c x/c

x/c x/c

Fig. 17. Flow field of the ‘‘velvet 1’’ 2D-wing and the ‘‘velvet 1’’ 3D-wing at 2y/b = 0.2 for α = 0°, 3°, and 6° at Rec = 40 000 and 60000.

exciting the KH-instabilities. Therefore, due to the fluctuations inthe backflow the free-shear layer is destabilized and undergoes alaminar–turbulent transition. In contrast to the clean wing case,this destabilization moves the point of transition onset furtherupstream.

Since the velvet structures are not solid but flexible thefilaments can move in response to the forces excited by the flow(Fig. 13). On the other hand, the flow structure reacts to themotionof the filaments generating different vortex sizes for varioussurfaces. The differences between ‘‘velvet 1’’ and ‘‘velvet 2’’ are thegeometric properties of the filaments (see Table 1). The greaterlength and the smaller diameter of the filaments of ‘‘velvet 2’’lead to a higher flexibility of the surface structure and, thus, to adifferent interaction with the near-wall flow field which resultsin smaller vortical structures. Fig. 14 shows the spatial two-pointcorrelations for the normal velocity component

rvv =v′(x)v′(x + ∆x)

v′(x)2(1)

in the separation region for the clean, the ‘‘velvet 1’’, and the‘‘velvet 2’’ surface at Rec = 40 000 and at Rec = 60 000,respectively. The angle of attack is α = 3° for all depicted cases.The analysis was performed in two steps. First, the v′-field was

computed to determine the location of the maximum value andthen the two-point correlation distribution was calculated. Notethat the spatial two-point correlations at various angles of attackpossess the same tendency. The distribution of the local maximaand minima indicates the average streamwise extent of a shedvortex and the distance between two consecutive vortices [27].This average length scale l, which is presented in Fig. 14 andalso illustrated in Fig. 13, is clearly diminished by ‘‘velvet 2’’.Due to the smaller eddies, the kinetic energy in the reversed-flow region is redistributed on a smaller scale, which enhancesthe mixing between the backflow region and the free-shear layer.The increased mixing destabilizes the free-shear layer, resultingin a laminar–turbulent transition and thus a reattached flowat ‘‘velvet 2’’, whereas at ‘‘velvet 1’’ this destabilization effectis delayed. The higher the Reynolds number, the stronger thismechanism is.

From the analysis of the quasi-2D flow field of the clean, the‘‘velvet 1’’, and the ‘‘velvet 2’’ airfoil it is evident that the surfacestructure plays an important role. Even though the separationpoint for any surface is not altered by the surface structure, thetransition onset is clearly shifted (Table 5) for the ‘‘velvet 1’’ surfacein comparison to the clean surface, i.e., transition occurs furtherupstream. At the lower Reynolds numbers (Rec = 20 000 and40000), the effect of the ‘‘velvet 2’’ surface on the flow field is not


Fig. 18. Flow field of the ‘‘velvet 2’’ 2D-wing and the ‘‘velvet 2’’ 3D-wing at 2y/b = 0.2 for α = 0°, 3°, and 6° at Rec = 40 000 and 60000.

very pronounced. However, at Rec = 60 000 the ‘‘velvet 2’’ surfacereduces the size of the separation bubble drastically. Especiallyat high angles of attack, this surface structure is very effective.Due to this Reynolds number dependence of the effectiveness of‘‘velvet 2’’, which is also observed in the three-dimensional case(Figs. 19–21), it is conjectured that some kind of critical Reynoldsnumber or freestream velocity has to be exceeded for the surfacestructures to be effective. In the ‘‘velvet 2’’-case, this critical valueseems to be between Rec = 40 000 and Rec = 60 000.

In the following, how the three-dimensional flow structuresinfluence the near-wall flow will be discussed.

3.2. Three-dimensional flow field

Besides the 2D wing, a fully three-dimensional wing shown inFig. 2(a) which mimics an owl-wing geometry was manufactured[20]. The geometry of the cross-section profiles changes along thespan, i.e. the camber line, the thickness etc. vary in each crosssection. Thewingmodel is produced from a plastic material whoseelasticity is very low. It is mounted on two metal bolts whichensure a defined position with respect to the freestream direction.Additionally, the wing loading and hence the forces exerted on thewing model, are very low, such that a deformation of the wing canbe neglected. Moreover, due to the low wing loading the strengthof thewing tip vortex is reduced compared towings at higherwing

loading. That is, the induced angle of attack can expected to bemuch smaller than the geometric angle of attack. However, Fig. 15shows that each cross section in the spanwise direction possessesa different shape causing a weak spanwise velocity component.Although this component is small compared to the streamwisecomponent, the findings indicate that this component influencesthe separation, since adverse pressure gradients can be balancedand additional velocity fluctuations in the spanwise direction alsoaffect the transition process.

It is obvious from Fig. 16, which shows a comparison of theflow fields of the clean 2D and 3D wing at the spanwise crosssection 2y/b = 0.2, whose overall geometry is very similar tothat of the quasi-2D airfoil, that the velocity distribution is alteredsignificantly due to the three-dimensionality of the wing. Theadverse pressure gradient which causes severe separation in the2D case seems to be almost completely eliminated for the α =

0°-cases. Additionally, at higher angles of attack the size of theseparation bubble is strongly reduced. Moreover, it is obvious thatthe three-dimensionality of the flow field does not completelychange the pressure distribution of the 3D wing in comparison tothe 2Dwing, since in the near-root section a separation bubble stilloccurs. That is, similar flow conditions as for the quasi-2Dwing areobserved for the 3Dwing at higher angles of attack. This differencebetween the 2D and the 3D case is greater at higher Reynoldsnumbers.


Fig. 19. Flow field of the clean 3D-wing for α = 0°, 3°, and 6° at Rec = 40 000 and 60000. The flow field in the investigated measurement planes is shown by streamlines.The approximate size of the separation bubble is sketched by white lines.

In Fig. 17, the findings of the 2D and 3D analyses are juxtaposedfor the ‘‘velvet 1’’ surface structure at Rec = 40 000 and Rec =

60 000 for various angles of attack. Overall, the result for theclean surface is corroborated. The larger the angle of attack,the more different is the flow field on the suction side. It isobserved that in all depicted cases a laminar separation bubbleis present on the 3D wing. The location and the size of theseparation bubble, however, do not change significantly at higherReynolds numbers as is observed in the quasi-2D case. That is, theweak spanwise velocity component balances the adverse pressuregradient which stabilizes the separation bubble independentlyfrom the freestream conditions (Rec , α). Moreover, the comparisonwith the clean-surface results in Fig. 16 shows the 3D ‘‘velvet 1’’separation to be also reduced against the 3D clean-surface findings.

Unlike the ‘‘velvet 1’’ findings, the flow field of the ‘‘velvet 2’’surface, which is illustrated in Fig. 18, shows a Reynolds number

dependence. At Rec = 40 000 the tendency observed for the‘‘velvet 1’’ surface also holds. That is, independent of the angleof attack the size of the separation region is diminished in the3D flow compared to the 2D flow. However, at Rec = 60 000the development of the separation behavior seems to be differentwhen the 2D results and flow field at spanwise position 2y/b = 0.2are juxtaposed. In contrast to the Rec = 40 000 case where thesize of the separation bubble is diminished due to the influence ofthe three-dimensionality of the flow field the size of the separationincreases at this particular spanwise position. That is, in the three-dimensional case the ‘‘velvet 2’’ surface seems to be less effectivethan the ‘‘velvet 1’’ surface which, in the first place, contradicts thefindings of the quasi-2D airfoil.

The flow field depicted in Fig. 18 is, however, merely a two-dimensional cross-section of a three-dimensional flow field and,thus, does not show the real three-dimensional structure of the


Fig. 20. Flow field of the ‘‘velvet 1’’ 3D-wing for α = 0°, 3°, and 6° at Rec = 40 000 and 60000. The flow field in the investigated measurement planes is shown bystreamlines. The approximate size of the separation bubble is sketched by white lines.

separation bubble. Figs. 19–21 present a three-dimensional viewof the wing and the flow field at spanwise positions 2y/b =

0.1, 0.2, 0.3, and 0.4 at various freestream conditions. Consideringthe flow field at various spanwise positions gives an idea of thespanwise distribution of flow phenomena such as a separationbubble. The separation region is indicated by the white solid lines.Since the separation region must be closed or extends to thetrailing edge, the shape of the separation can be extrapolated.Those regions are indicated by the white dashed lines. Comparingthe clean case (Fig. 19) and the ‘‘velvet 1’’ case (Fig. 20) the findingsof the 2D analysis are confirmed. However, the analysis of the‘‘velvet 2’’ case clearly shows that the discussion of only one singledistinct cross-section of a three-dimensional flow field can bemisleading. That is, the comparison of the 2D results and the flowfield at 2y/b = 0.2 leads to the assumption that the ‘‘velvet 2’’surface structure is detrimental to the aerodynamic performance,

i.e., the size of the separation bubble seems to be increased. Theanalysis of the three-dimensional flow field in Fig. 21, however,shows that this interpretation is wrong and that the overallsize of the separation bubble is reduced drastically. Especially atRec = 60 000 it is evident that the ‘‘velvet 2’’ surface yields thesmaller separation region independent of the angle of attack.

As discussed before, a Reynolds number dependence is ob-served. That is, the size of the separation bubble at a Reynolds num-ber of Rec = 40 000 is the same size as for the clean or ‘‘velvet 1’’surface. At a Reynolds number Rec = 60 000, however, the sep-aration region is much smaller, especially compared to the cleanwing. It is also evident that the size of the separation region doesnot change significantly at changing angle of attack. That is, atRec = 60 000, unlike ‘‘velvet 1’’, the ‘‘velvet 2’’ surface structureallows for an interaction of the surface filaments and the near-wall


Fig. 21. Flow field of the ‘‘velvet 2’’ 3D-wing for α = 0°, 3°, and 6° at Rec = 40 000 and 60000. The flow field in the investigated measurement planes is shown bystreamlines. The approximate size of the separation bubble is sketched by white lines.

flow resulting in a separation bubble which is hardly affected bythe angle of attack in the range of 0° ≤ α ≤ 6°.

From Figs. 19–21 it becomes evident that the structure ofthe separation bubble is three-dimensional and that a two-dimensional view does not yield enough information to describethe overall shape of the separation bubble. The size of the bubblechanges significantly in the spanwise direction in all cases. Sincethe flow field is a direct consequence of the geometry and thepressure distribution and since the geometry is unaltered due tothe application of the velvet surfaces, it is conjectured that it isthe pressure distribution which is changed due to the roughnessand flexibility of the surface structures. Primarily at Rec = 60 000the impact of the velvet surfaces becomes evident. The overallsize of the separation region is clearly reduced for the ‘‘velvet 1’’and the ‘‘velvet 2’’ surface compared to the clean surface. Again,as discussed for the quasi-2D airfoil, a clear dependence on the

Reynolds number is observed for ‘‘velvet 2’’. That is, at Rec =

40 000 the reduction of the separation region due to ‘‘velvet 1’’is greater than due to ‘‘velvet 2’’. At the higher Reynolds number(Rec = 60 000) this is inverted and the size of the separation regionis smallest for the ‘‘velvet 2’’ surface structure. This may lead toan increase in the aerodynamic performance since the separationbubble is stabilized.

4. Conclusion

The effect of hairy surface structures on the flow field of abarn-owl-based 2D wing model and 3D wing mode is analyzedby applying two velvet-like surfaces to the wing models. Thesevelvets differ in stiffness, length, diameter, and density of theirfilaments. The first velvet (‘‘velvet 1’’) mimics the characteristics ofthe natural owl-wing surface regarding the length and the density


of the filaments. The secondvelvet (‘‘velvet 2’’) simulates the abilityof the filaments of the owl-wing surface to form soft bundleswhichadapt to the local flow condition.

It is found that the ‘‘velvet 1’’ surface has an amplifying effect onflow disturbances. The size of the separation bubble is drasticallyreduced or flow separation is even avoided. That is, the ‘‘velvet 1’’surface serves as some kind of roughness.

The analysis of the flow field over the ‘‘velvet 2’’ surfaceshows two effects. First, a Reynolds number effect regarding theeffectiveness in influencing the near-wall flow field is determined.Whereas the transition onset for the ‘‘velvet 1’’ surface structureat a Reynolds number of Rec = 40 000 is excited furtherupstream, the onset is delayed by the surface structure ‘‘velvet 2’’.At Rec = 60 000 the ‘‘velvet 2’’ surface decreases the size of the2D separation bubble drastically. The filaments of the ‘‘velvet 2’’split the large vortical structure into several small vortices whichenhance the mixing with the free-shear layer and as such theseparation is reduced.

The second effect is found in the 3D-case. Comparing the2D-case and the 3D-case at 2y/b = 0.2, the separation bub-ble seems to increase in the 3D case. Considering the three-dimensional structure of the separation bubble, however, itbecomes evident that the overall size of the separation is reduceddrastically. Furthermore, the Reynolds number dependence of theinfluence of the surface ‘‘velvet 2’’ on the near-wall flow field isevident in the 3D-case. At a Reynolds number of Rec = 60 000, asignificant decrease in the size of the separation bubble even com-pared to the ‘‘velvet 1’’ flow is present.

In conclusion, the unique shape of the owl wing leads to apressure distribution, where separation is likely to occur at 2D and3D flow conditions. This flow phenomenon can also be observedfor the barn owl in gliding flight. The artificial wings investigatedin the present measurement campaign also show this kind offlow phenomenon. The application of some kind of velvet-likesurface structure similar to the structure which can be found onthe natural owl wing influences the flow such that the size of theseparation bubble is either reduced or stabilized, i.e., it does notchange depending on the angle of attack, and thus enhances theaerodynamic performance of the wing. Considering the complete3D flow field, a flexible surface like the ‘‘velvet 2’’ surface possessesthe smallest andmost stable separation bubble. Thismay lead to animprovement of the aerodynamic performance of the barn owl atReynolds numbers in the range of Rec ≥ 60 000. This, however,has to be verified in future experiments investigating the impactof flexible surface structures on lift, drag, and pitching moment.

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Documents

Surface structure and dimensional effects on the aerodynamics of an owl-based wing model