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DOI: 10.1177/1045389X14521874 2014 25: 786 originally published online 17 February 2014Journal of Intelligent Material Systems and Structures

Yuying Xia, Onur Bilgen and Michael I FriswellThe effect of corrugated skins on aerodynamic performance

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Journal of Intelligent Material Systemsand Structures2014, Vol. 25(7) 786–794� The Author(s) 2014Reprints and permissions:sagepub.co.uk/journalsPermissions.navDOI: 10.1177/1045389X14521874jim.sagepub.com

The effect of corrugated skins onaerodynamic performance

Yuying Xia1,2, Onur Bilgen3 and Michael I Friswell2

AbstractCorrugated skins provide a good solution to morphing wings due to their highly anisotropic behavior. If the low stiffnesscorrugation plane is aligned in the chordwise direction, the airfoil shape change is possible. In contrast to the traditionalsmooth skin of an airfoil, a corrugated skin influences both the local and global aerodynamics of a wing. The aim of thisstudy is to investigate the effect of a corrugated skin on the global aerodynamics of an airfoil, particularly lift and dragcharacteristics. First, the aerodynamic analysis of a NACA 0012 airfoil with a smooth profile is conducted, both in thewind tunnel and by computational fluid dynamics, at different Reynolds numbers and compared to the data in the litera-ture. The lift and drag coefficients at different angles of attack, between 210� and 10�, are considered. Next, twoNACA 0012 airfoils with different sized corrugated profiles are investigated both experimentally and by numerical simu-lation. The effects of corrugation size and Reynolds number are analyzed and quantified relatively to the standardNACA0012 airfoil with a smooth skin. Preliminary numerical results are validated using the experimental data.

KeywordsMorphing, corrugated skin, bio-inspired

Introduction

The concept of morphing aircraft is attracting a lot ofattention as it could yield higher aerodynamic perfor-mance than the conventional fixed wing aircraft.Although there has been significant progress in adap-tive structures for morphing wings (Barbarino et al.,2011), research on morphing skins (Thill et al., 2008b)is not so advanced. Corrugated laminates offer a solu-tion due to their extremely anisotropic behavior.Compliance in the chordwise direction, which is alsoassumed to be the corrugation direction, allows a shapechange and an increase in surface area. In contrast,stiffness in the spanwise direction (transverse to thecorrugation) enables the aerodynamic and inertialloads to be carried. There are many articles on the esti-mation of the equivalent stiffness of corrugated panels(Briassoulis, 1986; Kress and Winkler, 2010, 2011;McFarland, 1967; Thill et al., 2008a, 2010a, 2010b;Winkler and Kress, 2010; Xia et al., 2012; Yokozekiet al., 2006); however, the aerodynamic investigationfor corrugated skins is rarely found in the literature,although the aerodynamic performance plays a key rolein the application of corrugated skins.

It is generally believed that the non-smooth surfaceis not suitable for an aerodynamic profile operating ata high Reynolds (Re) number due to its relatively pooraerodynamic performance, generating low lift and high

drag. However, in nature, some insects, dragonflies,damselflies, and others have corrugated wing profiles(Rees, 1975b), which operate in relatively low Reynoldsnumbers. Rees (1975a) indicated that at very lowReynolds numbers, around 103–104, a well-designedcorrugated wing profile could reach a similar level ofaerodynamic performance compared to traditionalstreamlined airfoils.

Lissaman (1983) reviewed different types of lowReynolds number airfoils and mentioned a criticalReynolds number of about 70,000. Below this number,because of relatively large viscous effects, the maximumlift-to-drag ratio of rough airfoils is higher than theconventional smooth profiles; however, above thisvalue, the roughness of the profiles plays an importantrole, and smooth airfoils have higher values of the lift-to-drag ratio. Although there have been many experi-mental investigations (Kesel, 2000; Sunada et al., 2002;

1School of Engineering, University of the West of Scotland, Paisley, UK2College of Engineering, Swansea University, Swansea, UK3Department of Mechanical and Aerospace Engineering, Old Dominion

University, Norfolk, VA, USA

Corresponding author:

Yuying Xia, School of Engineering, University of West of Scotland, Paisley

PA1 2BE, UK.

Email: Yuying.Xia@uws.ac.uk

Thomas et al., 2004) and computational fluid dynamics(CFD) (Bourdin and Friswell, 2008) simulations for thecorrugated profiles, the majority of previous studieswere conducted at very low Reynolds numbers. Hu andTamai (2008) investigated the aerodynamic perfor-mance of bio-inspired corrugated airfoils using the par-ticle image velocimetry (PIV) technique at Re =34,000. Murphy and Hu (2010) conducted further stud-ies at an Re range of 58,000–125,000. Whitehead et al.(1982) and Thill et al. (2010) performed wind tunneltests and CFD analysis of airfoils with corrugated skinsin the aft one-third of the chordwise section atReynolds numbers between 250,000 and 1,000,000.

The aim of this study is to investigate the effect of acorrugated skin on the global aerodynamic response ofan airfoil, particularly on the lift and drag coefficients.First, the aerodynamic analysis of a NACA 0012 airfoilwith a smooth profile is conducted, both in the windtunnel and by CFD, at different Reynolds numbersand compared to NACA 0012 data in the literature.The lift and drag coefficients at different angles ofattack, between 210� and 10�, are considered. Next,two NACA 0012 airfoils with different sized corrugatedprofiles are investigated both experimentally and bynumerical simulation, at different Reynolds numbers.The effects of corrugation size and Reynolds numberare analyzed and compared to the standard NACA0012 airfoil with a smooth surface.

Research procedure

Model

The NACA 0012 airfoil is chosen as the base airfoil forthis study. Two NACA 0012 airfoils with differentsized corrugated profiles are chosen for further analysis

of the effect of the corrugated skin, as shown in Figure1. The labels COR01 and COR02 are used to indicatethe two corrugated profiles. These models have a chordlength c of 178 mm and round corrugation shapeswith radius R = 0.5%*c for COR01 and 1%*c forCOR02.

Wind tunnel setup

The aerodynamic experiments were conducted in alow-speed, open-circuit, and closed-test-section windtunnel with octagonal cross test section (Bilgen et al.,2012; Bilgen and Friswell, 2013). At the inlet, an alumi-num honeycomb flow straightener and a fiberglassmesh are used to condition the flow. The flow velocityduring the tests is observed using four static ports atthe inlet of the test section. The test section was con-verted to a 610 mm 3 152 mm rectangular cross sec-tion for two-dimensional (2D) experiments with the useof two removable splitter plates, as shown in Figure 2.

All parameters are controlled and measured auto-matically with a National Instruments compact dataacquisition (NI cDAQ) system and a personal com-puter. A total of 16 channels are monitored using fourNI 9239 four-channel, isolated, 24-bit voltage inputcards. The output signals are generated using two NI9263 16-bit, four-channel voltage output cards. Foreach test point, a 20-s data block is sampled at 100 Hzand then averaged to get the mean value.

Barlow et al. (1999) suggest several corrections dueto the existence of the walls around the airfoil. The solidblockage term esb and the wake blockage terms ewb aredescribed by Barlow et al. (1999). The wind-tunnel walleffects and buoyancy corrections were applied as neces-sary using the standard techniques found in Barlow

(a) (b)

Figure 1. Analysis models of NACA 0012 airfoil with round corrugations (chord normalized airfoil dimensions): (a) COR01,R = 0.5%*c and (b) COR02, R = 1%*c.

Xia et al. 787

et al. (1999). The reported corrected lift and drag coeffi-cients are calculated by

Cl =Clu(1� s � 2esb � 2ewb) ð1Þ

Cd =Cdu(1� 3esb � 2ewb) ð2Þ

The uncorrected lift and drag coefficients, Clu andCdu, are calculated by

Clu =Flift

0:5rcbrv2qc

� � ð3Þ

Cdu =Fdrag

0:5rcbrv2qc

� � ð4Þ

The streamwise turbulence of the flow in the emptytest section is measured by a standard hot-wire anemo-metry technique. The probe is placed at the center ofthe test section (aligned approximately at the quarterchord location along the streamwise direction) for allturbulence tests. After proper conversion of the mea-sured voltages to velocity (V), the turbulence intensity(TI) is calculated by

TI =Vrms

Vmean

3 100, where Vrms =

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1

n

Xn

i= 1

(Vi � Vmean)2

s

ð5Þ

where the index i represents each sample. Turbulenceintensity is measured at several velocities for differentfilter settings. Based on the test results, the turbulenceintensity of the wind tunnel, for the 2D test section, isTI = 0.075% 6 0.01%, which is derived from the

2.5-Hz to 10-kHz band-pass filtered signal for the velo-city range of 5–30 m/s.

The experimental analysis was conducted at chordReynolds numbers of around 120,000, 240,000, and360,000. Each of the three profiles was measured overthe angle of attack range from 210� to 10� with theangle step of 0.5�.

Computational procedure

The ANSYS/Fluent 14.0 software was used for theCFD analysis, and ANSYS/Gambit 2.4.6 software wasused for the mesh generation. A total of 553,990,1,731,200, and 2,086,400 quadrilateral cells were gener-ated for the NACA 0012, COR01, and COR02 models,respectively. Sufficient numbers of cells were used sothat the mesh was fine enough to ensure convergence.The mesh in the near-wall area, which needs to meetthe y+ constraint based on the turbulence model, isgenerated within the boundary layer of each model.Figure 3 shows the mesh for COR01.

As the Mach numbers in this study are low, incom-pressible flow is considered for the fluid model.Furthermore, steady flow is considered. Velocity inletand pressure outlet boundary conditions are used inthe far field. The k–e turbulence model with enhancedwall treatment was used to estimate the viscous effect.The SIMPLE scheme was used to solve the pressure–velocity coupling. This algorithm uses a relationshipbetween velocity and pressure corrections to enforcemass conservation and to obtain the pressure field(ANSYS, Inc., 2011). For the spatial discretizationscheme, the PRESTO! was used for pressure equation,and a second order upwind scheme is chosen for themomentum equation (ANSYS, Inc., 2011). Only the2D solver was used for the analyses.

The calculations were performed at Reynolds num-bers of around 120,000, 240,000, and 360,000, for com-parison with the experimental study. Each of the threeprofiles was simulated over the angle of attack rangefrom 210� to 10� with a step of 1� for each Reynoldsnumber.

Results and discussion

Validation

The experimental and computational results of theNACA 0012 are compared to the experimental resultspublished in the literature. As there is a limited amountof literature with aerodynamic data at these lowReynolds numbers, we use the drag and lift factor datagiven in Loftin and Smith (1949) at the Reynolds num-ber of 1.0 3 106 for comparison of the trend, which arehigher than the test environment for the current study.Here, the theoretical solution is based on thin airfoiltheory with the lift coefficient given by CI = 2pa.

Figure 4 shows the lift and drag characteristics ofthe NACA 0012 from both wind-tunnel tests and CFD

Figure 2. Wind tunnel experimental setup. Note that theupper and lower walls of the test section and fairing around thesting are omitted.

788 Journal of Intelligent Material Systems and Structures 25(7)

analyses. The experimental lift coefficient results arewell predicted by the CFD results. The slope of the liftcoefficient curves (∂Cl/∂a) are predicted correctly in thetest range, and the lift-to-drag ratio increases withincreasing Re number (Figure 5). These trends matchedwith the literature (Loftin and Smith, 1949).

Effect of corrugation size

Figures 6(a), 7(a), and 8(a) present the lift coefficientresponse of the NACA 0012, COR01, and COR02 air-foils at different Reynolds numbers. For each positiveangle of attack, from smooth airfoil (NACA 0012) tothe small-corrugation airfoil (COR01), then to the

Figure 3. Mesh of COR01 (1,731,200 cells).

−10 −8 −6 −4 −2 0 2 4 6 8 10

−1

−0.5

0

0.5

1

1.5

α (deg)

Cl

Experiment,Re=120,000Experiment,Re=240,000Experiment,Re=360,000CFD,Re=120,000CFD,Re=240,000CFD,Re=360,000Re=1,000,000[25]Theorical solution

−10 −8 −6 −4 −2 0 2 4 6 8 100

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.1

α (deg)

Cd

Experiment,Re=120,000Experiment,Re=240,000Experiment,Re=360,000CFD,Re=120,000CFD,Re=240,000CFD,Re=360,000Re=1,000,000[25]

(a) (b)

Figure 4. 2D lift and drag coefficient comparison of NACA 0012 at different Re.2D: two-dimensional; CFD: computational fluid dynamics.

Xia et al. 789

large-corrugation airfoil (COR02), the lift coefficient isreduced. From these figures, we conclude that the slopeof the lift coefficient curve (∂Cl/∂a) decreases withincreasing the size of the corrugation for the range ofcorrugations that are examined. Figures 6(b), 7(b), and8(b) present the lift-to-drag ratio response of theNACA 0012, COR01, and COR02 airfoils at differentReynolds numbers. For each positive angle of attack,the drag coefficient increases with the increased size ofthe corrugation.

Loftin and Smith (1949) mentioned that the mini-mum drag coefficient of the airfoil with a rough leadingedge is higher than the airfoil with a smooth leadingedge. It is also reported in Loftin and Smith (1949) thatthe lift-curve slope of an airfoil with a smooth surfaceis larger than an airfoil with roughness. In our study, as

−0.4 −0.2 0 0.2 0.4 0.60

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.1

Cl

Cd

Experiment,Re=120,000Experiment,Re=240,000Experiment,Re=360,000CFD,Re=120,000CFD,Re=240,000CFD,Re=360,000

Figure 5. Drag characteristics of NACA 0012 at different Re.CFD: computational fluid dynamics.

(a) (b)

−10 −8 −6 −4 −2 0 2 4 6 8 10

−0.4

−0.2

0

0.2

0.4

0.6

α (deg)

C l (Re=

120,

000)

Experiment,NACA0012Experiment,COR01Experiment,COR02CFD,NACA0012CFD,COR01CFD,COR02

−10 −8 −6 −4 −2 0 2 4 6 8 10−15

−10

−5

0

5

10

15

α (deg)

Cl/C

d (Re=

120,

000)

Experiment,NACA0012Experiment,COR01Experiment,COR02CFD,NACA0012CFD,COR01CFD,COR02

Figure 6. 2D lift coefficient and lift-to-drag ratio comparison at Re = 120,000: (a) lift coefficient and (b) lift-to-drag ratio.CFD: computational fluid dynamics; 2D: two-dimensional.

(a) (b)

−10 −8 −6 −4 −2 0 2 4 6 8 10

−0.4

−0.2

0

0.2

0.4

0.6

α (deg)

C l (Re=

240,

000)

Experiment,NACA0012Experiment,COR01Experiment,COR02CFD,NACA0012CFD,COR01CFD,COR02

−10 −8 −6 −4 −2 0 2 4 6 8 10−15

−10

−5

0

5

10

15

α (deg)

Cl/C

d (Re=

240,

000)

Experiment,NACA0012Experiment,COR01Experiment,COR02CFD,NACA0012CFD,COR01CFD,COR02

Figure 7. 2D lift coefficient and lift-to-drag ratio comparison at Re = 240,000: (a) lift coefficient and (b) lift-to-drag ratio.CFD: computational fluid dynamics; 2D: two-dimensional.

790 Journal of Intelligent Material Systems and Structures 25(7)

expected, the corrugations reduced the lift-curve slope(∂Cl/∂a) and lift-to-drag ratio (∂Cl/∂Cd).

Based on the validation mentioned above, the corru-gation effect is identified by further CFD analysis.Figure 9 shows the aerodynamic characteristics as thecorrugation size varies. From these results, we concludethat both the lift-curve slope (∂Cl/∂a) and the lift-to-drag ratio (∂Cl/∂Cd) have reduced due to the increase inthe size of the corrugation.

Effect of Reynolds number

Figures 10 and 11 present the comparison of the liftcoefficient and the lift-to-drag ratio for the COR01 and

COR02 airfoils at different Reynolds numbers. Theresults suggest that a slightly larger lift-curve slope(∂Cl/∂a) is obtained by increasing the Reynolds numberfrom 120,000 to 360,000. Unlike smooth airfoils, theaerodynamic performance of corrugated airfoils at lowangles of attack, quantified in terms of lift-to-dragratio, has only a small variation as the Reynolds num-ber is increased.

Flow behavior of corrugated airfoils

Figure 12 shows the static pressure coefficient distribu-tion around the corrugated airfoil (COR01) at an angleof attack of 5� and at Re = 240,000. Figure 13

−10 −8 −6 −4 −2 0 2 4 6 8 10

−0.4

−0.2

0

0.2

0.4

0.6

α (deg)

C l (Re=

360,

000)

Experiment,NACA0012Experiment,COR01Experiment,COR02CFD,NACA0012CFD,COR01CFD,COR02

−10 −8 −6 −4 −2 0 2 4 6 8 10−15

−10

−5

0

5

10

15

α (deg)

Cl/C

d (Re=

360,

000)

Experiment,NACA0012Experiment,COR01Experiment,COR02CFD,NACA0012CFD,COR01CFD,COR02

(a) (b)

Figure 8. 2D lift coefficient and lift-to-drag ratio comparison at 360,000: (a) lift coefficient and (b) lift-to-drag ratio.CFD: computational fluid dynamics; 2D: two-dimensional.

0 0.25 0.5 0.75 10.17

0.18

0.19

0.2

0.21

0.22

0.23

0.24

0.25

0.26

0.27

R/c (%)

Cl

CFD,Re=120,000CFD,Re=240,000CFD,Re=360,000

0 0.25 0.5 0.75 12

4

6

8

10

12

14

R/c (%)

Cl/C

d

CFD,Re=120,000CFD,Re=240,000CFD,Re=360,000

(a) (b)

Figure 9. Variation of the 2D lift coefficient and the lift-to-drag ratio with the size of the corrugation (angle of attack (AoA) = 5�):(a) lift coefficient and (b) lift-to-drag ratio.CFD: computational fluid dynamics; 2D: two-dimensional.

Xia et al. 791

illustrates the local streamlines around the corruga-tions. These figures show that the local flow sustains anattached flow (outside the corrugation troughs). Theeddies fill the troughs of the corrugations and ‘‘smooth’’the shape of the corrugated structure so that the flowoutside the corrugation is similar to that aroundstreamlined airfoils.

Conclusion

This article investigated the 2D aerodynamic effect ofchordwise corrugations in a 2D NACA 0012 airfoil interms of lift and drag coefficients using both experimen-tal and computational methods. It was found that thelift-curve slope (∂Cl/∂a) decreased, and the minimumdrag coefficient (Cd0) increased with the increasing size

of corrugation (i.e. the roughness of the skin). Theresults for the corrugated airfoils suggest that a slightlylarger lift-curve slope (∂Cl/∂a) is obtained by increasingthe Reynolds number from 120,000 to 360,000. Unlikesmooth airfoils, the aerodynamic performance of corru-gated airfoils at low angles of attack, quantified interms of lift-to-drag ratio, only reduces slightly as theReynolds number is increased. The CFD flow-fieldplots show that the local flow sustains an attached flow(outside the corrugation troughs). The eddies fill thetroughs of the corrugations and ‘‘smooth’’ the shape ofthe corrugated structure so that the flow outside thecorrugation is similar to that around streamlinedairfoils.

This article highlights that the aerodynamic effectsof the corrugated skin should be considered in the

(a) (b)

−10 −8 −6 −4 −2 0 2 4 6 8 10−0.4

−0.3

−0.2

−0.1

0

0.1

0.2

0.3

0.4

0.5

0.6

α (deg)

Cl

Experiment,Re=120,000Experiment,Re=240,000Experiment,Re=360,000CFD,Re=120,000CFD,Re=240,000CFD,Re=360,000

−10 −8 −6 −4 −2 0 2 4 6 8 10−15

−10

−5

0

5

10

15

α (deg)

Cl/C

d

Experiment,Re=120,000Experiment,Re=240,000Experiment,Re=360,000CFD,Re=120,000CFD,Re=240,000CFD,Re=360,000

Figure 10. Computational and experimental 2D lift coefficient comparison: (a) COR01 and (b) COR02.CFD: computational fluid dynamics; 2D: two-dimensional.

−10 −8 −6 −4 −2 0 2 4 6 8 10−0.4

−0.3

−0.2

−0.1

0

0.1

0.2

0.3

0.4

0.5

0.6

α (deg)

Cl

Experiment,Re=120,000Experiment,Re=240,000Experiment,Re=360,000CFD,Re=120,000CFD,Re=240,000CFD,Re=360,000

−10 −8 −6 −4 −2 0 2 4 6 8 10−8

−6

−4

−2

0

2

4

6

8

α (deg)

Cl/C

d

Experiment,Re=120,000Experiment,Re=240,000Experiment,Re=360,000CFD,Re=120,000CFD,Re=240,000CFD,Re=360,000

(a) (b)

Figure 11. Computational and experimental 2D lift-to-drag ratio comparison: (a) COR01 and (b) COR02.CFD: computational fluid dynamics; 2D: two-dimensional.

792 Journal of Intelligent Material Systems and Structures 25(7)

design of a morphing wing. In particular, for a deform-able trailing edge control surface, the increased dragfrom the skin will counteract the reduced drag obtainedfrom avoiding the sharp changes in camber at the hingelines, leading to flow separation. The optimization of amorphing wing system needs an estimate of theincreased drag and the changes in lift as a function ofthe corrugation geometry; much more work needs tobe undertaken to derive these relationships. Thus,based on this study, simulations and wind tunnelexperiments should be conducted to further understandthe effect of corrugation size and geometry, Reynoldsnumber, and turbulence. The aerodynamic perfor-mance should also be assessed at large angles of attackand for a range of morphing deformations. The full

CFD model requires too much computation to inte-grate into the optimization procedure, and hence, avalidated meta-model needs to be derived. This articleinitiates the research required to derive such a meta-model.

Declaration of conflicting interests

The authors declare that there is no conflict of interest.

Funding

The research leading to these results has received fundingfrom the European Research Council under the EuropeanUnion’s Seventh Framework Programme (FP/2007-2013)/ERC Grant Agreement No. (247045).

Figure 12. Static pressure coefficient distribution (COR01) at the angle of attack of 5� and Re = 240,000.

Figure 13. Local streamlines for COR01 at the angle of attack of 5� and Re = 240,000.

Xia et al. 793

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