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Int. J. Mech. Eng. & Rob. Res. 2012 MasoudJahanmorad Nouri et al., 2012

NUMERICAL INVESTIGATION OF AERODYNAMICCHARACTERISTICS OF NACA 23018 AIRFOIL

WITH A GURNEY FLAP

MasoudJahanmorad Nouri1*, Habibollah Sayehvand1 and Abolghasem Mekanik1

*Corresponding Author: MasoudJahanmorad Nouri,[email protected]

A two-dimensional numerical investigation was performed to determine the effect of a Gurneyflap on a NACA 23018 airfoil. An incompressible Navier-Stokes solver with Spalart-Allmarasturbulence model is used to predict the flow field around the airfoil. Gurney flap sizes of 0.5%,1.0%, 1.5%, 2.0%, and 3.0% of the airfoil chord were studied.Computed results have beencompared with available experimental data. Addition of Gurney flap increases the lift coefficientsignificantly with very little drag penalty if proper Gurney flap height is selected. Results showedthat Gurney flaps produced a negative shift in the zero-lift angle of attack. The numerical solutionsshow the details of the flow structure at the trailing edge and provide a possible explanation forthe increased aerodynamic performance.

Keywords: Gurney flap, Lift enhancement, NACA23018 airfoil, Computational fluid dynamics

INTRODUCTIONHigh lift systems play a major role inperformance and economic success ofcommercial, transport and military aircraft. Anefficient high lift system offers manyadvantages like lower take off and landingspeed, greater payload capacity for givenwing, longer range for given gross weight andhigher manoeuvrability. High lift systems aredesired to maintain low drag at take off so asto attain cruise speed faster and high drag atapproach. High lift systems are often quitecomplex consisting of many elements and

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Int. J. Mech. Eng. & Rob. Res. 2012

1 Mechanical Engineering Department, School of Engineering, Bu-Ali Sina University, Hamedan.

multi bar linkages. Therefore there is need tohave simpler high lift systems which arecheaper in terms of manufacturing andmaintenance cost. One such candidate isGurney flap.

The Gurney flap is a simple device,consisting of a short strip, on the order of1-5% of the airfoil chord in height, fittedperpendicular to the pressure surface or thechord-line along the trailing edge of a wing(Figure 1). The most common application ofthis device is in racing-car spoilers, where itis used to increase the down-force.

Research Paper

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Liebeck (1978) conducted wind tunnel testson the effect of a 1.25% C height Gurney flapon a Newman airfoil, which resulted in anincrease in lift and a slight reduction in drag.Larger lift increments were observed forgreater flap heights, but the drag increasednoticeably beyond heights of approximately2% C. Liebeck (1978) also hypothesized onthe changes in the trailing edge flow fieldcaused by the Gurney flap (Figure 2), and thisassumption was based on the trailing edgeflow field for a clean airfoil reported byKuchemann (1967). When a tufted probe wasused by Liebeck (1978), a considerable

turning of the flow over the back side of theflap was observed.

Gurney flaps dramatically increase liftwithout degradation of the airfoil performancewhen the height of Gurney flap is less than theboundary layer thickness. Until now, the flowcontrol effect of Gurney flap has beeninvestigated by several researchers (Stormsand Jang, 1994; and Myose et al., 1998). Theresults showed that compared to a clean airfoil,Gurney flap reduces the pressure on thesuction side and increases the pressure onthe pressure side. These pressure variationsare induced by the enhancement of circulationrelated to the extra downward turning of theflow generated behind the Gurney flap. As aresult of the pressure variations, additionalpressure difference between the upper andlower surfaces occurs, and this causes theincrement of the maximum lift, the decrementof the zero-lift angle of attack, and theincrement of the nose-down pitching moment,similar to the effect of increasing the camberof the airfoil (Myose et al., 1998). The amountof lift increment is proportional to the height ofthe Gurney flap. But when the height exceedsthe boundary layer thickness, the drag alsoincreases. Therefore the flap height should bedetermined in a way that the maximum lift-to-drag ratio can be achieved.

In more recent studies, articulated Gurneyflap-like devices have been used to tailor thespanwise loading of wings (Bieniawski andKroo, 2003) and rotor blades (Thiel et al.,2006).

The objective of the present study is toprovide quantitative and qualitativecomputational data on the performance of theGurney flap. Computations of a baseline

Figure 1: NACA 23018 Profile Fittedwith a Gurney Flap

Note: h – Gurney Flap Height.

Figure 2: Hypothesized Trailing Edge FlowConditions of an Airfoil with a Gurney Flap

Source: Liebeck (1978)

Airfoil TrailingEdge

Gurney Flap

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NACA 23018 airfoil are compared withexperimental results obtained in the Langleytwo-dimensional low-turbulence pressuretunnel by Abbott et al. (1945). Subsequentcomputations were performed to determinethe effect of various sizes of Gurney flaps onthe lift and the drag of the same airfoil.

MATERIALS AND METHODSNumeric Simulation

Computational Fluid Dynamics (CFD) isgenerally understood to be any numericalmethod used to solve a set of equations tomodel the flow-field of study. For this case,Fluent was used to solve the IncompressibleNavier-Stokes equations (INS).

The continuity equation can be expressedas:

0 u ...(1)

The next set of equations establishes aconservation of momentum in each spatialdimension. In the current 2D study, onlyequations for the x and y direction were solved.The equations can be expressed as:

fuPuut

u

21

...(2)

The equations are solved by thecommercial code Fluent. The nonlinear systemof equation implies the segregated solver, thusis solved sequentially. Standard and SecondOrder Upwind discretization schemes areused for the continuity and the momentumequations respectively.

As the code solves the incompressible flowequations, no equation of state exist for thepressure, and a SIMPLE algorithm is used toenforce pressure-velocity coupling.

Geometry Modeling and GridGeneration

The geometry used for the Gurney flap studyis a NACA 23018 airfoil. Computations wereperformed for Gurney flap sizes ranging from0.5% to 3% chord length, with the flaps locatedon the windward side of the airfoil at the trailingedge. For simplicity, the wind tunnel walls usedfor the experiment were not modeled.

For NACA23018 airfoil, computations wereperformed using a C-grid as shown in Figure3. The top and bottom farfield boundaries aretwelve chord lengths from the airfoil; theupstream and downstream boundaries areeleven and twenty chord lengths away,respectively. The grid was constructed usingthe Gambit software. Figure 4 shows a closerview of the grid in the vicinity of the airfoil. Gridclustering is evident near the surface of theairfoil, as well as near the trailing edge, toobtain reasonable resolution of the boundarylayer and the region around the Gurney flap.Figure 5 shows a grid with a 2% Gurney flapat the airfoil trailing edge.

Figure 3: C-Grid Used for NACA23018Airfoil

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RESULTS AND DISCUSSIONComputations were performed for a NACA23018 airfoil at conditions which match theexperimental data of Abbott et al. (1945). TheReynolds number for the computational casesmatches the experimental Reynolds number,based on wing chord, of Re

c = 3.1 106. The

wind tunnel test was performed in the Langleytwo-dimensional low-turbulence pressuretunnel.

Experimental results are compared withcomputational resultsfor the baseline airfoil inFigure 6 (no Gurney flap simulation). Theresults agree well with the measured data upto 14°. While this is the point of maximumlift for the NACA 23018 airfoil (C

lmax = 1.38),

the Fluent with the Spalart-Allmaras turbulencemodel predicts that the maximum lift coefficientoccurs at = 14° (C

lmax = 1.37). Since the

Gurney flapswill be simulated at angles ofattack below stall, this comparison shows thatthe Navier-Stokes solutions are simulating thepre-stall flowfield quite well. Figure6 alsoshows comparisons between experimentaland computational drag coefficients. There isgeneral agreement with the experimental data.

Figure 4: Close Up of Grid in the Vicinityof NACA 23018 Airfoil

Figure 5: Details of Grid in the Vicinityof the Trailing Edge NACA 23018 Airfoil

with a 2% C Gurney Flap

Figure 6: Comparison of Computed Liftand Drag Coefficients with Experimental

Data; NACA 23018 Airfoil (Base Line),Rec = 3.1 106, = 0°, 2°, 4°, …, 18°

Figure 7 shows the lift coefficient results forthe NACA 23018 airfoil with different size

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Gurney flaps. In general, the lift coefficientincreases as the Gurney flap size increasesfor a given angle of attack. As an example, a1% chord Gurney flap shifts the lift curve bymore than 3°, but the relationship between theGurney flap size and lift-curve shift does notappear to be linear. Specifically, the increasein lift coefficient due to changing the Gurneyflap size from 0% to 0.5% chord is greater thanthe change found by changing the flap from1.5% to 2% chord. The figure also shows thatthe zero lift angle of attack appears to becomeincreasingly more negative as a larger Gurneyflap is utilized. These results suggest that theeffect of the Gurney flap is to increase theeffective camber of the airfoil.

Shown in Figure 8 is the drag polar for thesame configurations. The significant increasein lift coefficient for the 3% height Gurney flapcomes at the price of substantially increaseddrag as shown in Figure 8.

Figure 9 presents the polar of lift-to-dragratio versus lift coefficient. At low-to-moderatelift coefficients (C

l < 1.2), there is a drag

Figure 7: Effect of Gurney Flap Size on LiftCoefficient; NACA 23018 Airfoil (Base Line

Case and with Gurney Flap Size of h =0.5% C, 1% C, 1.5% C, 2% C and 3% C),

Rec = 3.1 106, = 0°, 2°, 4°, …, 16°

Figure 8: Effect of Gurney Flap Size onDrag Coefficient; NACA 23018 Airfoil (BaseLine Case and with Gurney Flap Size of h =0.5% C, 1% C, 1.5% C, 2% C and 3% C),

Rec = 3.1 106, = 0°, 2°, 4°, …, 16°

Figure 9: Lift-to-Drag Ratio vs. LiftCoefficient NACA 23018 Airfoil (Base Line

Case and with Gurney Flap Size of h =0.5% C, 1% C, 1.5% C, 2% C and 3% C),

Rec = 3.1 106, = 0°, 2°, 4°, …, 16°

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penalty associated with the Gurney flap whichincreased with flap height. At higher liftcoefficients (C

l 1.2), however, the lift-to-drag

ratio is significantly increased. When the liftcoefficient is about 1.37, the increment of lift-to-drag ratio, approximate 43%, was obtainedby the 2% C Gurney flap. As a result, the effect

on the maximum lift-to-drag ratio is small, butthe lift coefficient for a given lift-to-drag ratio issignificantly increased.

Pressure distributions for the clean airfoiland the 2% C height Gurney flap at threedifferent angles of attack are shown in Figure10. Using the Gurney flap, Figure 10 shows

Figure 10: Comparison of Pressure Coefficient NACA 23018 Airfoil (Base Line Case andwith Gurney Flap Size of h = 2% C), Rec = 3.1 106

(a) = 0°

(b) = 6°

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that the increased suction is evidenteverywhere on the upper surface while thelower surface experiences increasedpressure. This results in the substantiallyincreased lift coefficient with the Gurney flapwhich was discussed earlier. Note the adversepressure surface due to the presence of theGurney flap. Such an adverse pressure regionis to be expected in front of the flap, and was

found in all previous studies with pressuredistribution measurements (Katz and Dykstra,1989; Giguère et al., 1995; and Myose et al.,1996). Liebeck (1978) has theorized that arecirculating vortex may be associated with thisadverse pressure region just upstream of theflap on the lower surface, as shown in Figure 2.

The computed flowfieldin the vicinity of thetrailing edge is shown in Figure11 for = 6°

Figure 10 (Cont.)

(c) = 10°

Figure 11: Computed Stream Lines Around Trailing Edge of the NACA 23018 Airfoil;Rec = 3.1 106, = 6°

(a) Airfoil with No Gurney Flap (b) Airfoil with 2% Chord Gurney Flap

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with and without a 2% chord Gurney flap. Arecirculation region can be seen in front of theflap and a strong clockwise vortex is apparenton the upper backside of the flap. It is observedfrom the flow fields comparison (a) and (b) thatGurney flap causes flow to turn downwardbeyond the flap, which indicates thatincreased lift is being generated by the Gurneyflap. This is in agreement with Liebeck’s (1978)wind tunnel test, where tufted probe indicatedsignificant turning of the flow downstream ofthe flap. The adverse pressure gradientobserved upstream of the flap on the lowersurface may be attributed to the formation ofrecirculating cove vortex.

CONCLUSIONA computational study of the flowfieldpastNACA 23018 airfoil with Gurney flap has beenconductedin detail using the commercial codeFluentwith the one-equation turbulence modelof Spalart-Allmaras. Computational results arefound to agree reasonably well with availableexperimental data. Results showed that Gurneyflaps produced a positive increment in liftcoefficient, a negative shift in the zero-lift angleof attack and a drag increment compared tothose obtained for clean airfoil, however theseincrements are nonlinear with respect to flapheight. There is no significant increase in dragif Gurney flap height is kept within boundarylayer, however beyond this limit drag incrementis significant. Airfoil pressure distributionresults show that the Gurney flap increases theupper surface suction and the lower surfacehigh pressure, this is the reason why the liftcan be enhanced. The flowfield in the vicinityof the trailing edge shows that the addition ofthe Gurney flap results in a downward turningof the flow behind the airfoil, so that the Gurney

flap increases the effective camber of theairfoil.

REFERENCES1. Abbott Ira H, Von Doenhoff Albert E and

Stivers Louis S Jr. (1945), “Summary ofAirfoil Data”, NACA Rep. No. 824.

2. Bieniawski S and Kroo I (January 2003),“Development and Testing of anExperimental Aeroelastic Model withMicro-Trailing Edge Effectors”, AmericanInstitute of Aeronautics and Astronautics(AIAA) 2003-220.

3. Giguère P, Lemay J and Dumas G (1995),“Gurney Flap Effects Scaling for Low-Speed Airfoils”, American Institute ofAeronautics and Astronautics (AIAA)Paper 95-1881.

4. Katz J and Dykstra L (1989), “Study of anOpen-Wheel Racing Car’s Rear-WingAerodynamics”, Paper Presented at theSAE International Congress andExposition, February 27-March 3, SAEPaper 890600, Detroit, MI.

5. Kuchemann D (1967), “Inviscid ShearFlow Near the Trailing Edge of an Airfoil”,Z. Flugwiss, Vol. 15, pp. 292-294.

6. Liebeck R H (1978), “Design of SubsonicAirfoils for High Lift”, J. Aircraft, Vol. 15,pp. 547-561.

7. Myose R, Heron I and Papadakis M(1996), “The Post-Stall Effect of GurneyFlaps on a NACA-0011 Airfoil”,Aerospace Atlantic Conference, May22-23, SAE Paper 961316, Dayton, Ohio.

8. Myose R, Papadakis M and Heron I(1998), “Gurney Flap Experiments on

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Airfoils, Wings and Reflection PlaneModel”, J. Aircraft, Vol. 35, No. 2,pp. 206-211.

9. Storms B L and Jang C S (1994), “LiftEnhancement of an Airfoil Using a GurneyFlap and Vortex Generators”, J. Aircraft,Vol. 31, No. 3, pp. 542-547.

10. Thiel M R, Lesieutre G A, Maughmer M Dand Koopmann G H (May 2006),“Actuation of an Active Gurney Flap forRotorcraft Application”, American Instituteof Aeronautics and Astronautics (AIAA)2006-2181.

C Chord length

Cd

Drag coefficient

Cl

Lift coefficient

Cp

Pressure coefficient

f Body force

P Pressure

Re Reynolds number

t Time

u Velocity

Angle of attack

Kinematic viscosity

Density

Divergence

Nomenclature

APPENDIX

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