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Gurney flaps and trailing edge devices for wind turbines Authors: S. Salcedo*, F. Monge*, F. Palacios*, F.Gandía**, A.Rodríguez**, M. Barcala** e-mail: [email protected], [email protected], [email protected], [email protected], [email protected] * INSTITUTO NACIONAL DE TÉCNICA AEROESPACIAL (INTA) National Institute for Aerospace Technology, Fluid Dynamics Department Crtra. Ajalvir, km 4.2 Torrejón de Ardoz 28550 Madrid (SPAIN) Phone: 00 34 91 520 13 62 / 00 34 91 520 20 26 Fax: 00 34 91 520 19 78 ** EUITA-UPM (Universidad Politécnica de Madrid), Spain Summary:
The effect of Gurney flaps has been studied for the range of application of wind turbines. Both two-dimensional and three-dimensional studies have been conducted, and several configurations and variations of trailing edge devices are investigated. Validation of the codes for a wide range of Reynolds number has been performed. By utilization of hybrid meshes, low computation time has been achieved. Stationary 2D computations agree with experimental data both in lift and drag coefficient, which has been achieved by the use of transition prediction. These results, accurate and low cost, make Gurney flaps application to wind turbine more feasible. More complex 3D configurations have been studied, as slotted flap and tabs in the upper side of the airfoil, with good results using meshes under 5·105 cells for three-dimensional complex configurations, which can be used in future work with Detached Eddy Simulation (DES). First attempts have been made to perform wind tunnel tests. 1. Introduction
Gurney flaps are passive devices used to enhance lift, consisting on a small plate located at the trailing edge perpendicular to the trailing edge of the airfoil. Since Dan Gurney used in a race car application for increasing the download, a number of experiments and computations [1,2,3,4] have been performed since then, including wind turbine application [5,6]. They show that Gurney flaps increase the maximum lift coefficient, decrease the angle of attack of zero lift coefficient while the lift curve slope remains relatively constant, and increase the nose down pitching moment, in the same way as when the camber increases, with very little drag penalty. The increase in maximum lift coefficient can be of 20% for wind turbine airfoils.
Liebeck was first reporting airfoil improved performance by Gurney flaps [7]. He explained this improvement in lift with low drag penalty by the increased region of attached flow in the upper side of the airfoil with a recirculation region behind the flap. Computational investigations have also been performed. More effort is needed, both experimental and computational, to further study the behaviour of these devices.
Hypothesized trailing-edge flow conditions of an airfoil with a Gurney flap (from description in[1]
Special requirements for wind turbines can take advantage of the application of Gurney flaps. Typically,
these devices increase lift, drag and moment coefficient. This effect grows with the height of the device, but it has been shown [8] that the penalty in drag is not very high while the Gurney flap remains submerged in the boundary layer. Typically, a Gurney flap size is 1-5% chord. Thickness of the airfoil, boundary layer thickness and turbulence are determinant factors on boundary layer thickness. In the inner part of blades, efficiency is not such a decisive parameter as lift. On the other hand, moment coefficient is not a requirement as important as in aircrafts.
Trailing edge devices are low cost, easy to install and can be used for changing performance or existent blades or to be taken into account in the design of new airfoils and blades. Our study focuses in the prediction at low computational cost of the performance of Gurney flap, mainly in accurate results for drag, and several Reynolds, airfoils and configurations of Gurney flap are explored with a special interest in wind turbine applications. 2. Numerical simulation
The results presented have been computed using FLUENT, a compressible finite volume solver of Navier-Stokes equations. Steady and unsteady calculations were performed. Conditions on simulations presented vary from Re 105 to 5·106, and even lower Reynolds computations were performed, investigating the range of application in
wind turbines (from Kw turbines at low speeds to Mw turbines at maximum power condition). Mach number of validation cases has been taken as M=0.15, and simulations were made at M=0.3. For this range of Mach number, incompressible calculations are an excellent approximation in the lineal zone of the polar, but care must be taken at high angles of attack, where higher Mach can be found near the leading edge. In this study we show compressible computations.
Several turbulence model where compared, and finally kω-SST was chosen. Sensitivity to turbulence model was higher in the stall region. After fully turbulent computations, transition was fixed in 2D configurations by using transition location from en envelope method, as implemented in XFOIL and MSES (2D codes, panel method and Euler respectively, coupled with boundary layer). This resulted to be of great importance for drag prediction. Fully turbulent computations give good results in cases in which transition occurred near the leading edge or those which use transition strips near the leading edge, but for laminar airfoils there was a noticeable difference in drag predictions. For cases with laminar bubbles results are even more sensitive to this practice. We used the predictions of XFOIL and MSES for setting transition as the envelope en criterion is still the more commonly used and reliable and as a initial probe. This criterion can be introduced in the code to make more reliable airfoil performance predictions.
Unsteady simulations were performed, with Δt =2·10-5. At low angles of attack, these calculations showed no substantial difference with steady calculations, even with 3D configurations. 3. Computational grids
Several configurations of Gurney flap were studied, and for a plain Gurney flap a study was made to get a mesh not very heavy but still allowing accurate predictions. High demand of calculations presented in literature [9,10] make Gurney flaps difficult to use for industrial applications, so one of the objectives of this work was to diminish the number of cells used without lost of accuracy, even improving the prediction of drag which usually is the main problem in simulations. A special effort was made for achieving a grid which could be used in a later step for DES simulation, fulfilling the requirements of that kind of simulation in both 2D and 3D [11].
Far field of 15 chords around the airfoil was used. Meshes created are hybrid, with a structured mesh near the airfoil and Gurney flap, and unstructured mesh for the rest of the far field. This strategy allows fine meshing near the airfoil, with low stretching (under 1.25) and first cell height under 5· 10-5, allowing to retain y+ ≈ 1. 30÷40 cells have been used in the direction normal to the airfoil, and 400 points around the airfoil. In finer meshes, a conformed structured block has been used to mesh the wake, 0.5% of the chord ( c ) of the airfoil. Gurney flap has been modelled as a flat plate, as a L profile and as a wedge with different angles, finding no relevant differences in total results.
For 3D meshes, spans of length 2%c have been used, with 20 and 40 nodes. This mesh has been created by expanding the 2D mesh along the span. Different volumes were created in structured mesh for the cases of Gurney flaps with slits and tabs. For the creation of meshes, VISCGD2D, Gambit and TGRID were used.
Typical size of meshes and computation time: 2D: ~ 20.000 cells steady: 10 min. unsteady: 5 hours 3D: ~ 400.000 cells steady: 8 hours unsteady: 50 hours
(aprox. time for one point of the drag polar, compressible regime, in a P4 3.6 GHz)
4. Validation
For taking into account the different Reynolds range in that wind turbine works, two different cases are shown. T
SD7037 airfoil, Re=10
his experiments from Selig and Bechert [ 12,13] were chosen because of the amount and quality of the data presented. Low Reynolds: 5
Cd
Cl
0 0.02 0.04 0.06 0.08 0.10.2
0.4
0.6
0.8
1
1.2
1.4
SELIG CLEANSELIG GF 0.4%cCFD CLEANCFD GF 0.4%c
Lift coefficient versus drag coefficient
Pressure coef ack=4º
Results are shown for airfoil with and without Gurney flap. Navier Stokes fully turbulent results show go
Lift coefficient versus angle of attack
ficient versus x chord position, angle of att
a 0.4%c
od agreement with experimental data. This can be explained by looking at the pressure coefficient distribution. This airfoil has a small leading edge radio, which results in a sharp leading edge pick. Transition occurs near the leading edge, which is quite similar to the fully turbulent conditions. Results using MSES are shown in pressure distribution. This method, fast and reliable for the conditions of the experiments with clean airfoil, gives an approximation of the increase in lift coefficient using Gurney flap. It can be used to have a fast prediction of boundary layer thickness for the dimension of Gurney flap, and for an estimation of transition. High Reynolds: HQ17 airfoil, Re=106
AOA(º)
Cl
0 5 10 150.4
0.6
0.8
1
1.2
1.4
1.6
1.8
Exp. Bechert cleanExp. Bechert Gurney Flap 1%cCFD CleanCFD Gurney Flap 1%c
0 0.01 0.02 0.03 0.04Cd
0.4
0.6
0.8
1
1.2
1.4
1.6
Cl
Exp. Bechert CleanExp. Bechert GF 1%cCFD CleanCFD GF 1%c
Lift coefficient versus angle of attack Lift coefficient versus drag coefficient
X
Cp
0 0.2 0.4 0.6 0.8 1
-1.5
-1
-0.5
0
0.5
1
CFD CleanCFD Gurney Flap 0.4%cMSES CleanMSES Gurney Flap 0.4%c
1.4
1.2
1
AOA (º)
Cl
0.8
0.6 Exp. Selig cleanExp. Selig GF 0.4%cCFD CLEANCFD GF 0.4%c
0.4
0.20 5 10 15
Good quantitative prediction of drag is found in these results computed using transition criteria, even in stationary
HQ17, GF1%, Re=106, AOA=4º, path lines coloured by pressure
D studies (different Gurney flap configurations - 1%c)
ffect included different configurations implemented in order to
Experimental and computational results show a slight decrease in lift by using slits or tabs, but smaller with the last
computations. Lift coefficient agrees reasonably, and the increase due to Gurney flap is accurately predicted. In the zone after maximum the lift is overpredicted, which is a well known characteristic of RANS and even URANS, because of the massively separated flow.
3
The experiments used for validation of Gurney flap ereduce drag from the solid 1%c Gurney flap configuration. First configuration is a Gurney flap with slits
(wide 0.2%c, separation 2%c between slits). The second one is a solid Gurney flap with tabs in the upper side of the airfoil (0.3%c wide and height, separation 2%c between tabs). This last configuration is a simplification of the dragonfly-like tabs used in the experiments. They have been simulated with rather good results. ( ) 100⋅
−cleanCd
cleanCC dd( ) 100⋅cleanCl
− cleanCC ll
0
5
10
15
20
25
30
35
40
45
50
0
5
10
15
20
25
30
35
40
45
50 GF1% CFD GF1% slits CFD GF1% tab 0.3% CFD GF1% EXP. GF1% slits EXP. GF1% tab 0.3% EXP. AOA=0º AOA=4º AOA=8º AOA=0º AOA=4º AOA=8º
Comparison of different 3D configurations (solid GF, GF with slits and GF with tabs) for percentages of lift and drag coefficients at three angles of attack.
one and a decrease of drag by using slits and tabs. The tendency of finding greater effect with lower angles of attack is only broken by the lift coefficient experimental results at 0º angle of attack.
HQ17 with GF 1%c and slits, contours of vorticity magnitude
HQ17 with GF 1%c and tabs (height 0.3%c), path lines coloured by vorticity
5. Wind turbine airfoils Wind turbine airfoil presents special characteristics in its geometry. NACA 6408 and FFA-W3-241 have been chosen to illustrate the variety of airfoils and work condition. NACA 6408, Gurney flap 0.5- 1%c Re=7·105 (10 KW Wind Turbine)
GF0.5%
Lift, efficiency and moment coefficient versus angle of attack
This computations show the possibility of increase lift of airfoil with a little penalty in stall angle of attack and efficiency. It is shown that this penalty is very small, and that we can decide the height of the Gurney flap to be used depending of the lift need and the importance of efficiency. In this case, the reduction of efficiency is very small, and occurs at angles of attack that may not be our design point (we can change it as our lift has been improved).
Angle of attack 6º 14º Δ cl ↑ 15% ↑ 4%
AOA(º)
Cl/C
d
0 5 10 150
20
40
60
80
CleanGurney Flap 0.5%cGurney Flap 1%c
AOA(º)
Cl
0 5 10 15
0.6
0.8
1
1.2
1.4
1.6
1.8
2
CleanGurney Flap 0.5%cGurney Flap 1%c
AOA(º)
Cm
5 10 150.1
0.15
0.2
0.25
0.3
CleanGurney Flap 0.5%cGurney Flap 1%c
FFA-W3-241, Gurney flap 3%c Re=5·106 (2MW Wind Turbine)
AOA (º)
Cl
0 5 10 150
0.5
1
1.5
2
FFA-W3-211FFA-W3-211 with 3% Gurney Flap
AOA (º)
Cd
0 5 10 150
0.02
0.04
0.06
0.08
0.1
0.12
0.14
FFA-W3-211FFA-W3-211 with 3% Gurney Flap
Lift and drag coefficient versus angle of attack
In this case, a 3%c GF has the increase of lift which can
be achiev
the objective. Examples of application of Gurn
low drag penalty (suitable for airfoils in the inner half of the blade) g speed
ays
s for low winds lected GF
es adapted for higher power with new machines
led wake for acoustic and aeroelastic cons
NACA 0018 wind tunnel
been chosen. This demonstrates the dimension in ed, and that GF are useful too for thick airfoils like this (24%c). Usually, higher Reynolds implies lower
boundary layer thickness and smaller GF, but we have chosen 3%c of height for this application because the position of this airfoil in the blade. We have a greater increase in drag than in NACA 6408 with 0.5%c GF. These results may be not useful for an airfoil which is going to be use in the outer zone of the blade, where drag is an important factor in power. For an airfoil which is in the inner part of the blade, as can be this FFA-W3-241, drag coefficient is not as important in design as lift for power, and other characteristics as starting velocity can be improved (obtaining a lower starting velocity because of the increase in drag and moment coefficient).
Gurney flap design depends on airfoil and Reynolds, but also in ey flap for Wind Turbines:
Lift improvement with Improve the starting performance by using GF near the root (↑cl , ↑cm ) by decreasing startin Small wind turbine blades monoairfoil: GF can be used to modify airfoil characteristics in different w
along the span Adapting blade Control loads by deflected/undef In combination with vortex generators, blad Decrease instabilities at the pressure side at low angles of attack
Future work will need to focus in high angles of attack and detaiiderations. Further insight is needed in Gurney flaps wake and high angles of attack by using DES. Short term
activities will include these calculations and apply the capabilities obtained with this work to the multidisciplinary design of airfoils and blades. Besides, some devices as Vortex Generator will be studied to further improve Gurney Flaps benefits. Parallel to these computational activities, further experimental activities will be carried out at INTA and EUITA-UPM wind tunnels.
, Re=163.000, experimental results at EUITA-UPM
Angle of attack 6º 14º Δ cl ↑ 56% ↑ 38%
AOA (º)
Cl
0 5 100
0.5
1
1.5
2 Exp. EUITA cleanExp. EUITA GF1.3%Exp. EUITA GF4.6%Exp. EUITA GF1.3% with slitsExp. EUITA GF4.6% with slits
6. Conclusions
The effect of Gurney flaps has been studied in its range of application for wind turbines. Validations have
eferences
] Neuhart H, Pendergraft OC. A water tunnel study of gurney flaps. NASA 1988, NASA-TM-4071 IAA 1993-0647
, Zhang S. Aerodynamics of Gurney flaps on a single-element high-lift wing. Journal of Aircraft, 2000.
ntfield, JAC. Theoretically and experimentally obtained performances of Gurney flap equipped wind turbines.
eidel.pdf
been shown for both high and low Reynolds number, obtaining good agreement in both lift and drag coefficients. It has been shown that is possible to model 3D complex configurations with meshes bellow 5·105 cells and 2D configurations with meshes around 2·104, which make this calculation feasible for design and optimization. Accurate drag prediction has been achieved with stationary calculations, and increments in total coefficient have been successfully computed for several airfoils and Reynolds conditions. R [1[2] Storms BL, Jang CS, Lift enhancement of an airfoil using a Gurney Flap and Vortex Generators. A[3] Jang CS, Ross JR, Cummings RM. Numerical investigation of an airfoil with a Gurney flap. Aircraft Design, 1998. 75-88 [4] Jeffrey D37(2) [5] KeWind Engineering 1994. Vol. 18, No. 2, pp. 63 – 74. [6] http://www.cpmax.com/donwload/poster_dewek_s (acceded February 13 , 2006)
t., pp.547-561.
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Kroo IM. Computational investigation of wings with miniature Trailing Edege Contol surfaces. AIAA
. Young-person´s guide to detached-eddy simulation grids. NASA 2001. CR-2001-211032 , Soartech
Hage W. Drag reduction of airfoils with miniflaps. Can we learn from dragonflies? AIAA
th
[7] Liebeck RH. Design of subsonic airfoils for high lift. Journal of Aircraft, Vol. 15, Nº 9, Sep[8] Giguère P, Lemay J, Dumas G, Gurney Flap effects and scaling for low-speed airfoils, AIAA 1995-1881 [9] Schatz M, Günter B, Thiele F. Computational modeling of the unsteady wake behind Gurney-Flaps, AIA2004-2417 [10] Lee H,2004-2693 [11] Spalart, PR[12] Lyon CA, Broeren AP, Giguère P, Gopalarathnam A, Selig MS. Summary of low-speed airfoil datapublication, Virginia, 1997 [13] Bechert DW, Meyer R, paper 2000-2315
