Faculty of Engineering - Port Said University

Effect of Airfoil Camber on WIG AerodynamicEfficiency

M. A. Mosaad, M. M. Gaafary, H. El-Kilani & I. A. Amin

Professor, Associate Professor, Associate Professor & Teaching Assistant, in theDepartment of Naval Arch.& Marine Engineering, Faculty of Engineering, Port

Said University, Port-Said, Egypt.

Abstract

The effect of airfoil camber and angle of attack parameterson the improvement of aerodynamic efficiency has beeninvestigated by using numerical method. Various members ofthe NACA 4-digit airfoil family are examined in this work.The influence of changing camber from 0% to 6% of airfoilchord at different angles of attack from (0, 3, 4 and 6degrees) on the aerodynamic efficiency is considered atconstant maximum thickness (12% chord) and fixed location ofthe maximum thickness (40% chord) in ground condition.Navier-Stokes equations for a steady state condition and“Realizable K-ε” turbulence model with enhanced walltreatment are adapted within a CFD code. The present work shows that, increasing the airfoil camberleads to increase in aerodynamic efficiency. Moreover,operating airfoils in ground condition may double theoverall aerodynamic efficiency obtained for unboundedairfoil. The maximum aerodynamic efficiency occurs at 4degrees. Special consideration should be given to symmetricand low camber airfoils in ground condition due to the

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PORT SAID ENGINEERING RESEARCH JOURNAL

suction effect on concave surface. An initial angle ofattack of about 4 degrees would help for the avoidance ofsuction effect and improve the aerodynamic efficiency of allairfoil sections considered.

1. Introduction

Wing in ground (WIG) effectcrafts are unique vehiclesthat operate at altitudesof several meters above thesea surface to takeadvantage of favorableaerodynamic interactionbetween the wing andground. Thus, flying InGround Effect (IGE)potentially offers a veryeconomical way of rapidtransport.

In general, ground effectcan be divided into twodistinct regimes, ram andnormal ground effect [1].Ram ground effect occurswhere the wing is at analtitude of height fromground to chord ratio (h/c)equal to 0.1 or less. Here,the wing is so close toground that trailing edgeof the wing is creating asealed envelope full ofair. As the wing’s altitude

increases, it enters whatis normally considered tobe normal ground effect.This regime extends fromjust above the “ram wing”height (h/c>0.1) toapproximately half the wingspan of ground [2]. Groundeffect phenomenon has beenobserved by manyresearchers. According toCarr and Atkin [3], theground exerts a greatinfluence (suction andstagnation) on pressuredistribution along the wingsurface. The aerodynamiccharacteristics of anairfoil in ground proximitywith bounded airflow areknown to be much differentfrom that of unboundedflow.

The development of the WIGvehicles for possibleapplications in bothoverwater and overlandtransport necessitates a

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thorough investigation ofthe flow characteristicsover the wings and otherlifting surfaces. Someexperimental andtheoretical studies havebeen performed on theinfluence of different wingconfigurations on theaerodynamic characteristics[7-9]. Studies performed byRanzenbach and Barlow [7-9]had demonstrated the groundeffect for a single elementairfoil configuration forracing cars. They performedexperiments and numericalstudies on single elementsymmetrical and camberedairfoils. They had foundout that the lift forcereaches a maximum at aground clearance ofapproximately 0.08c; beyondthis limit the airfoil andground boundary layers werefound to merge. Thisexplains the reduced liftforce at very closedistance to the ground.Although they haddocumented the effect ofground proximity on lift

and drag forces, no otherdata was presented.

Juhee, et al, had been [17]presented a numericalanalysis to investigate theaerodynamic characteristicsand the static heightstability of the endplateand anhedral angle on wingaspect-ratio in wing inground effect. He foundthat, the lift-drag ratioof a wing is not improvedas much with an anhedralangle as it is with anendplate.

Ahmed and Kohama [10]presented results of anexperimental investigationon a tandem wingconfiguration. They hadstudied the influence ofwing spacing in addition tothe effect of angle ofattack for both wings andtheir ground clearances.Zhang, et al [11] reportedthe influence of tip vortexcharacteristics on theaerodynamic performance ofa cambered airfoil. Zerihanand Zhang [12] reportedpressure coefficient, lift

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and drag coefficients on anairfoil provided with endplates. They found that atmoderate clearances,separation of the boundarylayer occurred near thetrailing edge of thesuction surface. The regionof separated flow was foundto increase in size as theairfoil is brought veryclose to the ground.

Zhang and Zerihan [13]investigated the wakebehind a single elementairfoil using laseranemometry. They have founda thicker wake withreducing ground proximity,as a result of boundarylayer separation. Recently,Ahmed and Goonaratne [14]reported the values ofcoefficients of lift anddrag are increasing withthe increase of angle ofattack. They also reportedincreasing lift coefficientvalues and decreasing dragcoefficient values as theground was approached foran angle of attack of 2degrees.

The present paperinvestigates of theinfluence of airfoil camberon aerodynamic efficiencyby numerical methods. Theresults of the numericalinvestigation ofaerodynamic ground effecton four airfoils, (NACA0012, 2412, 4412, and 6412)carried out using a CFDcode are presented andanalyzed. At constantmaximum thickness (12%chord) and location of themaximum thickness (40%chord), the influence ofchange in camber from 0% to6% of airfoil chord on theaerodynamic efficiency isstudied. Turbulent flowaround two-dimensionalairfoil in ground effect isanalyzed withincompressible ReynoldsAveraged Navier-Stokes(RANS) equations which areapproximated by finitevolume methods. The paperinvestigates the two-dimensional ground effectnumerically and the flowcharacteristics due todifferent ground

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conditions. Also, theadvantage of operating inground regime due toaerodynamic efficiency isillustrated. Moving andfixed ground conditions,different angle of attack,and “Realizable K-ε”turbulence model with“Enhanced wall treatment”are adopted.In order to validate thepresent numerical data, thecomputational result ofNACA 4412 (Re=2x10E6) inunbounded flow at differentangles of attack iscompared with experimentaldata [6]. The NACA 4412pressure and velocityfields are then calculatedfor ground clearance of(h/c=0.1) with turbulencemodel.

2. Numerical Modeling ofAirfoil Section

The effect of airfoilcamber on the improvementof aerodynamic efficiencyinvestigates by usingnumerical method. Resultsof the numericalinvestigation of

aerodynamic ground effecton four airfoils, (NACA0012, 2412, 4412, and 6412)carried out using a CFDcode are presented andanalyzed. GAMBIT 2.2.30 isa commercial program usedin building numericalmodel, domain, and mesharound model. The GAMBIT iscompatible with FLUENT 6program which is applied tosolve the flow around themodel as CFD tool.

a) Geometry,computational domainand flow condition

The flow around NACA 4412is to be examined. Theairfoil coordinate isimported in GAMBIT programand the domain with mesh isstructured. The geometry ofthe airfoil is given inFigure (1).

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Figure (1): Geometry ofNACA 4412 airfoil.

Two domains are used toillustrate the change inaerodynamic characteristicsin unbounded and groundeffect flow. The firstdomain is that of unboundedflow, used to model flowaround a wing that extends11.5 chords upsteam of theleading edge of theairfoil, 20 chords behindairfoil model and 12.5chords above and down thepressure and suctionsurfaces as in Figure (2).

Figure (2): Unbounded flowdomain

In the second domain, thedistance below the airfoilis defined with (h/c),where (c) is the chord, and(h) is the ground distanceat the trailing edge

(h/c=0.1) as shown inFigure (3).

In both domains, velocityboundary is appliedupstream (inflow) withspeed (U=50 m/s) andoutflow boundary conditionis defined as pressureoutlet (gauge pressure).Fixed wall boundarycondition is selected forairfoil body according toFirooz [4]. In groundcondition, the groundboundary is taken as“Enhanced wall treatment”.

Figure (3): Ground modeflow domain

An unstructured mesharrangement withquadrilateral elements isadopted to map the flowdomain in unbounded flow.While, in ground effecttriangular elements areadopted to pave the flow.

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Figures (4 and 5) show themesh around model in GAMBITprogram.

Figure (4): NACA 4412 modelin GAMBIT program toconstruct the domain andmesh in ground effect.

Figure (5): NACA 4412 modelin GAMBIT program toconstruct the domain andmesh in unbounded flow.

b) Governing equationsand modelingassumption

The governing equations forthe turbulentincompressible flowencountered in this

research are the steady-state Reynolds-averagedNavier Stokes (RANS)equations. These equationshave the following form intwo-dimensions:

ρ(u ∂u∂x+v ∂v∂y )=F1−

∂P∂x

+μ(∂2u∂x2+∂2u∂y2 )−ρ(u ∂u u∂x

+v ∂uv∂y )

ρ(u ∂u∂x+v ∂v∂y )=F2−

∂P∂y

+μ(∂2v∂x2+∂2v∂y2 )−ρ(u ∂u v∂x

+v ∂vv∂y )

Where, u and v are axialand vertical components ofthe mean velocity vector, Pis the pressure; μ is theviscosity, F volumetricforces. Furthermore, themodel must satisfy thecontinuity equation

(∂u∂x+∂v∂y )=0

The turbulent viscosity iscomputed with “Realizablek-ε” turbulence model.Equations are approximatedby finite volume method,and are solved by“Segregated” method.

The second order upwindmethod is applied for theconvection term according

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to Firooz [4]. For thepressure interpolation,“PRESTO” method is used,and the relation betweenpressure and velocity iscalculated with “SIMPLEC”algorithm according toVandormaal [5].

c) Validation of thenumerical model

The numerical model forairfoil NACA 4412 inunbounded condition isexamined in the sameconditions of anexperimental work carriedout by Abbott and Doenhoff[6].

0 1 2 3 4 5 6 70

0.2

0.4

0.6

0.8

1

1.2

Cl (E...

Angle of Attack in Deg.

Lift

Coe

ffic

ent

Figure (6): Validation ofthe numerical model forNACA 4412

The predicted liftcoefficient for numerical

model NACA 4412 inunbounded flow for angle ofattacks from zero to 6degrees is compared withthe experimental data asshown in Figure (6). Theresults show good agreementat all angles of attack andsupport the adaption of themodel of ground domain.

3. Effect of AirfoilCamber on AerodynamicEfficiency

The influence of airfoilcamber on aerodynamicefficiency is investigatedusing numerical method.Results from a numericalinvestigation ofaerodynamic ground effecton four airfoils, NACA0012, 2412, 4412, 6412,carried out in CFD programare presented. At constantmaximum thickness (12%chord) and location of themaximum thickness (40%chord), the influence ofchanging camber from 0% to6% of airfoil chord on theaerodynamic efficiency isstudied. Turbulent flowaround two-dimensional

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airfoil in ground effect isanalyzed withincompressible ReynoldsAveraged Navier-Stokes(RANS) equations which areapproximated by finitevolume method.

Figures (7-9) show thevariation of aerodynamicefficiency of the airfoilswith the angle of attackfor unbounded condition anddifferent ground clearance(0.05 and 0.1 chordrespectively). For allairfoil sections, theaerodynamic efficiency hasincreased with increaseangle of attack up to 4°.For angle of attacks gratethan 4°, a small dropoccurs in aerodynamicefficiency for all airfoilsespecially those withhigher camber. It is alsonoted that, the aerodynamicefficiency has increasedwith increased camber. Themost efficient airfoil forall angles of attack is theNACA 6412, and the lessefficient is NACA 0012. Ithas been observed that themaximum aerodynamic

efficiency (L/D) inunbounded condition doesnot exceed 26, while inground condition it canreach 50. The increasing inaerodynamic efficiency isdoubled in ground conditionif compared with its valuein the unbounded condition.

The same relation betweenangles of attack and L/Dratio occur in groundcondition except that themagnitude of L/D is moresignificant than forunbounded condition. It isobserved that the maximumvalue of aerodynamicefficiency (L/D) in groundclearance 0.05 and 0.1 forNACA 6412 equal twice itsvalue (about 55) comparedwith unbounded condition(about 27), with theincrease of aerodynamicefficiency about 100%. Asmall increase inaerodynamic efficiency(about 10%) is observed dueto decrease groundclearance, as shown inFigure (11), about 10%increase in aerodynamicefficiency was observed due

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to the decrease of groundclearance from 0.1 to 0.05.

0 1 2 3 4 5 6051015202530

NACA0012NACA2412NACA4412

Angle of attacks, degees

L/D

Figure (7): Variationbetween angle of attack and(L/D) in unboundedcondition.

0 1 2 3 4 5 6-40-20020406080 NACA0012

NACA2412NACA4412

Angle of attacks, degrees

L/D

Figure (8): Variationbetween angle of attack and(L/D) in ground condition(h/c = 0.05).

0 1 2 3 4 5 6-20

020406080

NACA 2412

Angle of attack, degree

L/D

Figure (9): Variationbetween angle of attack and(L/D) in ground condition(h/c = 0.1).

1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.50

10

20

30

40

50

60

h/c=0.05

Camber in %

L/D

Figure (10): Variationbetween camber and (L/D)for different 3 NACAsection at constant angleof attack 0 degrees.

Figures (10, 12 and 13)show the variation ofaerodynamic efficiency ofthe airfoils with camberfor unbounded condition anddifferent ground clearance.Figure (10) shows thevariation in aerodynamicefficiency on differentairfoils section and camberpercent at constant angleof attack zero. It isinteresting to prove that,aerodynamic efficiency ofall airfoil sections

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increase with camberincrease. A clear increasein aerodynamic efficiencywith decrease groundclearance especially athigh camber airfoils.Symmetric and small camberairfoils suffer fromsuction effect in the lowersurface due to concaveshape and flow accelerationbetween airfoil and ground,as shown in Figure (11).Negative results areobserved for symmetric andsmall camber airfoils inground condition comparedwith unbounded conditiondue to suction effect.Therefore the use of NACA0012 and 2412 is notrecommended in groundeffect for zero or verysmall angle of attack.

Figure (11): Pressure andvelocity contourrespectively for NACA 2412at zero angle of attack.

0 1 2 3 4 5 60

10203040506070

h/c=0.05

Camber in %

L/D

Figure (12): Variationbetween camber and (L/D) atconstant angle of attack 3degrees.

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0 1 2 3 4 5 605

101520253035404550

h/c=0.05h/c=0.1

Camber in %

L/D

Figure (13): Variationbetween camber and (L/D) atconstant angle of attack 6degrees.

Figures (12 and 13) showthat the variation inaerodynamic efficiency ondifferent airfoils sectionsand camber present atconstant angles of attack 3and 6 respectively. Theresults show that for allangles of attack, byincreasing the camber, theaerodynamic efficiencyincreases. The groundeffect may double theaerodynamic efficiencycompared with unboundedeffect. A small drop inaerodynamic efficiency isobserved for large anglesof attack in groundcondition in NACA 6412.

4. Conclusions

The shape of airfoilsection in ground andunbounded conditions hasthe major effect onaerodynamic efficiency(L/D). The most importantshape parameter is airfoilcamber. The aim of thepresent paper is to studythe effect of camberparameter on the airfoilaerodynamic efficiency innumerical investigation.Various members of the NACA4-digit airfoil family areused in this paper and theresults are discussed.Another goal in this paper,to illustrate the favorableeffect of ground effectcompared with unboundedcondition. The paperresults are summarized inthe following points:

1- The numerical modelpresented in this papermay successfully simulatedifferent ground effectcondition.

2- The model has beenvalidated using the

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published experimentalresults obtained for NACA4412.

3- The application of NACA4412 in and out of groundconditions shows thefavorable effect ofground effect inimproving the overallaerodynamic efficiency.The results show thatthere is about (30%)increase in liftcoefficient in groundcondition compared withunbounded condition andabout (100%) increase inaerodynamic efficiency.

4- It is observed that, forall airfoil sections theaerodynamic efficiencyincreases with increaseof the angle of attack upto 4 degrees. Above 4degree, a small dropoccurs in aerodynamicefficiency (L/D) in allairfoils especially inthe airfoils with highercamber. This results islogically because theairfoil shape became semiflat when airfoil takeinitial angle of attack

about 4 degree, whichlead to conclude thatflat lower surface forairfoil section operatein ground effect mostrecommended. When theangle of attack increasesmore than 4 degree, thefavorable ground effectdecrease due to theunclosed lower surface toground and decreases thepotential pressure underthe airfoil.

5- Many precautions for usesymmetrical and non-flatbottom airfoils in groundapplication, due to thesuction effect which cancause undesirable effectfor aerodynamicefficiency in case ofzero angle of attack.

6- The airfoil camber hasthe major effect onaerodynamic efficiency.Increasing the camberpresent from chord from0% to 6% lead to increaseaerodynamic efficiencyfor all small angle ofattack. A small drop inaerodynamic efficiency atangle of attack 6 degree

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for NACA 6412 due toescape the dynamicpressure under airfoil.

7- The favorable effect ofground effect shows thatwider application of WIGcrafts has a greatpotential. The resultspresented here can beapplied at preliminarydesign stage for theinitial analysis of a WIGcraft moving in theground effect mode.

5. References

1- Husa B, “WIGConfiguration Developmentfrom Component Matrix”,Orion Technologies, 2008.

2- Moore M, Wilson P A, andPeters A J,”AnInvestigation into Wingin Ground Effect AirfoilGeometry”, University ofSouthampton, UK, 2008.

3- G. W. Carr and P. D.Atkin, “influence ofmoving Belt Dimension onVehicle AerodynamicForce”, Proc. of WindTunnels and Wind Tunnel

Test Techniques, RoyalAerodynamic Society,London, pp.37.1-37.16,1997.

4- A. Firooz, M.Gadami,”Turbulence Flowfor NACA 4412 inUnbounded Flow and GroundEffect with DifferentTurbulence Models and TwoGround Condition: Fixedand Moving GroundConditions”, int.Conference on Boundaryand Interior Layers,2006.

5- J. P. Vandoormaal, and G.D. Raithby, “Enhancementsof the SIMPLE Method forPredicting IncompressibleFluid Flows”, Numer HeatTransfer, 1984.

6- I. H. Abbott and A. E.von Doenhoff, “Theory ofWing Sections”, Dover,New York, 1959.

7- Ranzenbach, R. andBarlow, J. “Two-dimensional airfoil inground effect, anexperimental andcomputational study” SAEPaper No. 94-2509, 1994.

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8- Ranzenbach, R. andBarlow, J., “Camberedaerofoil in groundeffect: wind tunnel androad conditions” AIAAPaper 95-1909, 1995.

9- Ranzenbach, R. andBarlow, J., “Camberedaerofoil in groundeffect, an experimentaland computational study”SAE Paper No. 96-0909, 1996.

10- Ahmed, M. and Kohama,Y., “Experimentalinvestigation on theaerodynamiccharacteristic of atandem wing configurationin close groundproximity” JSMEinternational Symposiumon Applications of LaserTechniques to FluidMechanics, Portugal,2002.

11- Zhang, X., Zerihan,J., Ruhrmann, A. andDeviese, M. “Tip vorticesgenerated by a wing inground effect” Proceedingsof the 11th InternationalSymposium on Applications ofLaser Techniques to FluidMechanics, Portugal, 2002.

12- Zerihan, J. and Zhang,X. “Aerodynamics of asingle element wing inground effect”” Journal ofAircraft, Vol. 37, No. 6,pp. 1058-1064, 2000.

13- Zhang, X. and Zerihan,J. “Turbulent wake behinda single element wing inground effect” Proceedingsof the 11th internationalsymposium on applications oflaser techniques to fluidmechanics, Portugal, 2002.

14- Ahmed, N. andGoonaratne, J. “Liftaugmentation of a low-aspect-ratio thick wingin ground effect” Journalof Aircraft, Vol. 39, No. 2,pp. 381-384, 2002.

15- Anderson, J. Jr.“Fundamentals ofaerodynamics”. 3rd edition,McGraw-Hill, 2001.

16- Barlow, J., Rae, W.and Pope, A. “Low-speedwind tunnel testing”. 3rdedition, John Wiley,1999.

17- Juhee Lee, Chang-sukHan, and Chang-Hwan Bae“Influnce of WingConfigrations on

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AerodynamicCharacteristics of Wingsin Ground Effect” Journalof Aircraft, Vol. 47, No.3, May-June 2010.

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