piv investigation of an airfoil with a gurney...

ISTP-16, 2005, PRAGUE 16TH INTERNATIONAL SYMPOSIUM ON TRANSPORT PHENOMENA

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Abstract This paper reviews the research on NACA 23012 airfoil profile equipped with Gurney flap. It investigates airfoil performances for different angles of attack at Re ≈ 5.105, based on chord, both with clean configuration and various Gurney flap sizes. Measuring device 2D-PIV (Two-dimensional Particle Image Velocimetry) system was used. With the aid of 2D-PIV system was thoroughly measured velocities in the whole area around the airfoil NACA 23012. The contribution of Gurney flap to the aerodynamic forces on the aerofoil was calculated from the data by using Kutta-Joukowski Lift Theorem and is in very good agreement with force measurements results.

1 Introduction The Gurney flap is a vertical short strip added to the trailing edge on the pressure side of a wing. It can have relatively powerful effect on the aerodynamics of a wing, increasing lift with only small change of drag penalties.

The most common application of this device is in racing-car spoilers, where it is used to increase the down-force. Gurney flap was applied to some helicopters, where the flaps are fitted on the horizontal inverted wing to improve performance during the high-powered climb or on vertical tails, to increase the force produced by a horizontal stabilizer. Gurney

flaps were also tested on delta wings and wind turbines.

Fig. 1: Airfoil Fitted with a Gurney Flap (c –

chord length, h – Gurney flap height).

A number of measurements was carried out in the Gurney flap investigation. But only two basic methods were used: pressure and force measurements. This report brings new method in Gurney flap research, using 2D-PIV system and Kutta-Joukowski Lift Theorem to evaluate airfoil performance.

2 Experimental Equipment

2.1 Wind Tunnel All measurements were done in the closed-

circuit wind tunnel with an open test section with dimensions 900 x 600 mm.

PIV INVESTIGATION OF AN AIRFOIL WITH A GURNEY FLAP

Pavel Bárta, Jan Novotný, Jiří Nožička, Ivo Sedlář

Czech Technical University in Prague, Faculty of Mechanical Engineering, Department of Fluid Dynamics and Power Engineering, Division of Fluid Dynamics and Thermodynamics

Address: Technická 4, 166 07 Praha, Czech Republic Corresponding author: [email protected], +420 224352580 phone,

Fax +420 224310292 Keywords: Gurney flap, PIV measurement, Kutta-Joukowski Lift Theorem, wind tunnel tests

c

h Gurney flap

mailto:[email protected]


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Fig. 2: Scheme of the Wind Tunnel.

2.2 2D-PIV (Particle Image Velocimetry) For PIV measurement a DANTEC

apparatus in following configuration has been used: • a Nd-YAG pulse LASER New Wave Gemini

15 Hz PIV, 120 mJ, 2 cavities • CCD camera Dantec HiSense PIV-PLIF,

1280 x 1024 pixel – 12 bit, CCD chip of type progressive scan interline, max. 9Hz – single frame mode, 4,5 Hz – double frame mode

• PIV processor Dantec FlowMap 1500, 1GB buffer

• PC – 2 x XEON 3 GHz, HDD 300Gb, Windows XP

• Software FlowManager v. 4.3 + FlowManager Advanced measurement synchronization (80S32) + FlowManager MatLab Links (80S36) + other modules

2.3 Profile NACA 23012 Model The model was made of polyester core coated with balsa and plastic film. The chord length is c = 400 mm and the width b = 400 mm. The profile is equipped with end plates to simulate two-dimensional flow well. The Gurney flap is interchangeable or fully removable.

Fig. 3: Removable and Interchangeable Gurney

Flap.

Angle of Attack γ

Laser

Mirror

End Plates

Approaching Flow

Model of NACA 23012 (400 x 400) mm

CCD Camera

Fig. 4: Experimental Setup.


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3 Data Evaluation Method

3.1 Piling of Velocity Field Images It is necessary to map thoroughly the velocity field around the whole airfoil. The CCD camera resolution and particle size are limiting factors for using PIV system. Resolution is increased by piling of images, in our case 8 images.

Fig. 5: PIV Images. Each image has the same dimensions and consists of the same amount of commensurate subregions. FlowManager calculates one velocity vector for each subregion from its displacement. Therefore it is important for piling images and results to connect images following their subregions. The images piling is complicated by these factors:

• the images are shifted in x and y direction

• each image is divided into subregions (each represents one vector)

• subregions can overlap (25 %, 50 % and 75 %)

New more effective method was made up to pile images. This method of piling images modifies the images before FlowManager evaluation. Comparing to the standard post evaluation piling, has this method following advantages:

• shorter evaluation time

• modified image is returned back to the FlowManager and standardly analyzed

• resulted values (vectors) are evaluated directly from measured data, they are not interpolated

3.2 Description of New Piling Method

3.2.1 Calibration

There is found calibration point in the neighboring images.

Fig. 6: Calibration. Shift (y direction) and overlap (x direction) is calculated from the calibration point, size of images, size of subregions (eventually subregion overlap). These values are used for modification of velocity field images, images analyzed with FlowManager, described in next paragraph and for piling of resultant images and matrixes (3.2.3).

Fig. 7: Calibration.

x y

Calibration point

Image 1 Image 2

Image 2 Image 1

Calibration point

x y


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3.2.2 Modification of Velocity Field Images (VFI) It is cut the left margin of the second image and added the same size region on the right size of the second image, so that the second image subregion boundaries align the first image subregion boundaries in horizontal direction. It is filled and cut the second image according to the first image boundaries. Added and filled regions are “white” to interfere in the results the least. Modified and original images have the same size.

Fig. 8: Modification of a VFI.

3.2.3 Piling of Resultant Images and Matrixes Exported images of a velocity field or resultant matrixes are simply concatenated according to the calibration data (3.2.1).

Fig. 9: Resultant Image.

3.3 Evaluation of Airfoil Lift Using Kutta-Joukowski Lift Theorem The airflow is considered to be potential two-dimensional flow sufficiently far from the airfoil (outside the boundary layer). There rules Kutta-Joukowski Lift Theorem in such an idealized flow:

Γ⋅⋅= aFy ρ , (1) where

yF ………lift [N] ρ ……….density of approaching flow [kg/m³] a ………..velocity of approaching flow [m/s] Γ ……….circulation around the profile [m²/s]

dsvc

⋅⋅=Γ ∫ αcos (2)

c…………closed path v…………velocity vector [m/s] α ………..angle between v and ds

Fig. 10: Total Circulation. Thorough knowledge of velocity field around the profile is the base for evaluating of the total circulation integral (eq. 2) and subsequently for calculating the profile lift (eq. 1).

αv

ds

c

Image 2 Image 1

Calibration point

Cut and add

Fill and cut

Closed path


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4 Results There are measured 8 areas around the profile (fig. 5). Each resultant value is averaged from 60 instantaneous values. Image of the size 1024x1280 pixels contains 123x155 subregions with size 32x32 pixels and overlap 75 %. Measured and calculated values:

3/2928,1 mkg=ρ sm /104078,1 25−⋅=υ …kinematic viscosity

sma /7.16= °= 7γ …angle of attack

5

5mod 1075,4104078,1

4,07,16Re ⋅=⋅⋅

=⋅

= −υca

el

4.1 Configuration without GF

sm /1299,1 2=Γ

mNaccaF yyP /39,242

2

=⋅⋅⋅=Γ⋅⋅= ρρ

3383,0=yc … lift coefficient

Fig. 11: Circulation Values along the Closed

Path – no GF.

4.2 Configuration with 4 % GF

smGK /5049,2 2=Γ

mNaFyPGK /08,54=Γ⋅⋅= ρ 75,0=yc


Path – 4 % GF.


Path (blue - no GF, red – 4 % GF).


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4.3 Velocity Field around the Profile

Fig. 14: Piled Image of X-component Velocity Field around the Profile with Closed Path – no GF (results in m/s).

Fig. 15: Piled Image of X-component Velocity Field around the Profile with Closed Path – 4 % GF(results in m/s).

Fig. 16: Piled Image of Y-component Velocity Field around the Profile with Closed Path – no GF (results in m/s).

x

y

x

y


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4.4 Comparison with Other results

5 Conclusion New method of piling images proved its advantages – shorter evaluation time, simpler analyzing of results. Results are evaluated only by FlowManager, there is not used any other interpolation method. Hence there was possible to calculate the total circulation and evaluate lift of the profile with and without Gurney flap by using Kutta-Joukowski Lift Theorem.

There is increase of the lift coefficient of the 2D profile NACA 23012 equipped with GF 114%. This increase is in good agreement with other available results. Absolute values of the lift coefficient are lower than the measurement results. This could be caused by piling images, selecting of closed path, selecting of integration step, or invalidation of potentiality condition in wake region. Mentioned issues are subject of our next investigation. Assessed results proved applicability of this method to evaluate lift of an airfoil.

1 2 3 4 5 6 7 Airfoil NACA 0012 0011 632-215 23012 References [6] [9] [5] [3] [10] [1] This

report Angle of attack [˚] 7 7 6 6 6 6 7 CL 1.2 0.7 0.5 0.6 - 0.7 0.34 CL with GF 1.7 1.2 0.9 1.25 - 1.05 0.75 Increase [%] 41 71 80 108 47 50 114 Note Force m. 3 %

GF CFD cLmax

increase

Table 1: Comparison of Lift Coefficient Results.

Fig. 17: Piled Image of Y-component Velocity Field around the Profile with Closed Path – 4 % GF (results in m/s).


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References [1] Bao, N., Ma, H. and Ye, Y. Experimental study of

wind turbine power augmentation using airfoil flaps, including the Gurney flap. Wind Engineering, 24, (1), pp 25-34, January 2000.

[2] Brown, L., Filippone, A. Aerofoil at low speeds with Gurney flaps. Aeronautical Journal, September 2003, vol. 107, nr. 1075, pp 539-546.

[3] Date, J.C., Stephen, R.T. Computational evaluation of the periodic performance of a NACA 0012 fitted with a Gurney flap. ASME, March 2002, vol. 124, pp 227-234.

[4] Gai, S.L. and Palfrey, R. Influence of trailing-edge flow control on airfoil performance. J Aircraft, 40, (2), March 2003, pp 332-337.

[5] Jeffrey, D.R.M. An Investigation into the Aerodynamics of Gurney Flaps. Ph.D. thesis, University of Southampton, Southampton, UK.

[6] Jeffrey, D., Zhang, X. , Hurst, D.W. Aerodynamics of Gurney Flaps on a Single-element High-lift Wing. J Aircraft, 37, (2), March 2000.

[7] Ježek, J., Váradiová, B., Adamec, J. Mechanika tekutin. Vydavatelství ČVUT, červen 1998.

[8] Li, Y., Wang, J., Zhang, P. Influence of mounting angles and locations on the effect of Gurney flaps. Journal of Aircraft, May 2003, vol. 40, No. 3, pp 494-498.

[9] Li, Y., Wang, J., Zhang, P. Effects of Gurney Flaps on a NACA0012 Airfoil. Flow, Turbulence and Combustion 68, 2002, pp 27-39.

[10] Sedlář I. Komplexní výzkum aerodynamického profilu s Gurneyho klapkou, Gradient 2001.

[11] Myose, R., Papadakis, M. and Heron, I. Gurney flap

experiments on airfoils, wings, and reflection plane models. J Aicr. March 1998, 35, (2), pp 206-211.

piv investigation of an airfoil with a gurney...

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