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ASME Early Career Technical Journal 2012 ASME Early Career
Technical Conference, ASME ECTC
November 2 3, Atlanta, Georgia USA
THE EFFECT OF VARIOUS GURNEY FLAP SHAPES ON THE PERFORMANCE OF
WIND TURBINE AIRFOILS
Mohammad Mohammadi, Ali Doosttalab Undergraduate, K.N.Toosi
Univ. of Technology
Tehran, Iran
Mehdi Doosttalab R&D Engineer, Nordex Energy GmbH
Hamburg, Germany
ABSTRACT This paper gives an overview on two-dimensional
numerical investigation and comparison of aerodynamic
characteristics of small flaps used to increase lift on wind
turbine airfoils. The small flaps consist of Gurney flaps, trailing
edge wedges and a devised trailing edge curved shape. The
investigations were performed for a diversity of lengths and
heights of these flaps on the TU Delft DU 91-W2-250 airfoil.
Extensive numerical simulations has been done using RANS model
using SST-Transitional turbulence model using a commercial CFD
code, a CFD finite-volume based software, at the Reynolds number of
2 106. The results confirmed advantages of using the trailing edge
curved shape over the Gurney flap, which will be more efficient as
the flap height is increased.
INTRODUCTION The Gurney flap is a small flap utilized to
increase the lift
coefficient of an airfoil. The application of increased lift
coefficient is that for a given airfoil the chord, C, could be
reduced to a comparable amount so that the generated lift still
equals that of the original airfoil [1]. During the last decade due
to the increase of the oil price, the size of the new generation
wind turbines with a higher power production capacity increased
rapidly, and designing such kinds of the wind turbines becomes a
real challenge for designers, because they should be lightweight,
and have low production costs, while maintaining aerodynamic
performance. The benefits of this reduced chord length are that the
weight and the material expenses for building the blades will be
reduced.
The gurney flap was developed and applied to race cars by Robert
Liebeck and Dan Gurney in 1960s [2]. The concept involves of a
small tab located at the trailing edge of an airfoil. The tab was
deployed to a height on the order of the boundary layer thickness
(1-2% of chord length) [3, 5]. It was observed that increasing flap
size over 2% of chord length noticeably increased the drag even
though there was continuing increase in
lift. The aerodynamic force alteration is consequence of a small
region of separated flow directly upstream of the flap with two
counter-rotating vortices downstream of the flap effectively
modifying the trailing edge Kutta condition [4]. Although using
Gurney flap increases the lift coefficient, in return, it also
reduces lift-to-drag (L/D) ratio which will increase drag force
that the wind turbine base has to withstand. There is also a device
called trailing edge wedge that is a wedge located at the trailing
edge of the airfoil [3]. It also increases the lift coefficient not
as much as the Gurney flap but the L/D ratio is better than of a
Gurney flap. In this study the characteristics of a new optimized
curved shape located at the trailing edge were investigated, giving
a divergent trailing edge as a control device, the focus of current
study is to compare lift coefficient and L/D ratio of this new
device to Gurney flap and Trailing edge wedge.
In this paper the steady-state numerical computations using
transitional RANS model for a diversity of trailing edge wedges and
corresponding trailing edge curved shape as well as angle of
attacks at the Reynolds number of 2 106 on the DU 91-W2-250 [6,7]
airfoil were studied. The shape of the airfoil is depicted in
Figure 1. Traditional RANS turbulence models
usually assume that the flow is entirely in a turbulent state.
However, the laminar to turbulent transition may occur on the
Figure 1. The DU 91-W2-250 airfoil
X/C
Y/C
0 0.2 0.4 0.6 0.8 1 1.2
-0.2
0
0.2
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surface of the airfoil. That is to say, considering the
transition can enhance the accuracy of numerical simulations under
certain circumstances.
NUMERICAL METHODS AND GRID To simulate the flow field, a
commercial finite volume
CFD code was used as a flow solver. The two-dimensional
incompressible RANS (Reynolds-Averaged Navier-Stokes)
turbulence model, which was used in this investigation, is the
four equations SST-Transitional RANS model to simulate the
transition of the flow over airfoil. The discretization scheme for
all equations was the second-order upwind scheme. Moreover, a
commercial grid generator was used to create highly accurate
structured mesh around the airfoils. In this paper O-type mesh was
used and the domain of O-type mesh had a radius of 40 chord lengths
to avoid boundary reflections; furthermore, far field flow boundary
condition was applied to the border of the domain. The length of
the numerical airfoil was 1m. Grid contains about 100,000 cells
with around 400 grid points on airfoil surface. The height of the
first row of cells around the airfoil is set to around 0.00005 of
cord length to ensure acceptable value of Y+ for utilized
SST-Transitional model so that the boundary layer flow can be
appropriately resolved. In Figure 2 the grids around airfoil can be
seen.
GEOMETRY For Gurney flaps, a series of Gurney flaps of 1% and
2%
of cord length with thickness of 0.33% of cord length were used
at the trailing edge perpendicular to the chord line on the DU
91-W2-250 airfoil with geometric parameter as shown in Figure 3.
Figure 4 shows the geometric parameters for the trailing edge wedge
and the curved shape attached to the trailing edge. H and L are
height and length of the trailing edge wedge respectively. In this
study different L/H ratios (L/H
ratio=length of L/length of H) at two different values of 1% and
2% of cord length for H were considered. Furthermore, L/H = 1.5,
2.1 and 3 were used, as depicted in Figure 5 to compare performance
of the curved shape to that of Gurney flap and the trailing edge
wedge. For all of the curved shape devices, a curve was obtained
from trial and error for the best performance of this device. The
curve can be expressed by Figure 4. As it can be seen, first, the
longest edge of the wedge
Figure 2. Grid distribution around the airfoil
Figure 3. DU 91-W2-250 airfoil with 1% of cord length Gurney
flap
X/C
Y/C
0.975 0.98 0.985 0.99 0.995 1 1.005
-0.02
-0.015
-0.01
-0.005
0
0.005
0.01
H
Figure 4. DU 91-W2-250 airfoil with trailing edge wedge and
curve geometric parameters
X/C
Y/C
0.975 0.98 0.985 0.99 0.995 1 1.005
-0.02
-0.015
-0.01
-0.005
0
0.005
0.01
0.395
0.6050.535
0.465
H
L
Wedge
Curve
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is split by the ratio of 0.395/0.605 to obtain a new point on
that edge, and then from that point the line perpendicular to the
airfoil chord is drawn to the airfoil surface to create a new line
connecting the airfoil to the wedge longest edge. Then this new
line is split by the ratio of 0.465/0.535 to create a new point and
the created point is the shoulder point of the curve.
RESULTS AND DISCUSSION For numerical validation as depicted in
Figure 6 the
computed lift coefficients (CL) of the DU 91-W2-250 airfoil at 0
to 10 degrees of angle of attack at Reynolds number of 2 106 were
compared to the experimental data performed at the Delft University
wind tunnel (LST) [3]. As it can be seen, the numerical results
agree well with the experimental data at the mentioned angles of
attack range.
Figure 5. DU 91-W2-250 airfoil with a diversity of trailing edge
curved shapes
X/C
Y/C
0.97 0.975 0.98 0.985 0.99 0.995 1
-0.015
-0.01
-0.005
0
0.005
0.01
L/H=1.5L/H=2.1L/H=3
Figure 6. Experimental and numerical lift coefficient
comparision for DU 91-W2-250 airfoil
Angle of attack
CL
0 2 4 6 8 100.2
0.4
0.6
0.8
1
1.2
1.4
ExperimentalNumerical
Figure 7. Lift coefficient for DU 91-W2-250 airfoil with Gurney
flap
Angle of attack
CL
0 2 4 6 8 10
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
1%C Gurney flap2%C Gurney flapDU 91-W2-250 airfoil
Figure 8. Lift/Drag ratio for DU 91-W2-250 airfoil with
Gurney flap
Angle of attack
Lift/
Dra
g
0 2 4 6 8 1045
50
55
60
65
70
75
80
85
90
95
100
1%C Gurney flap2%C Gurney flapDU 91-W2-250 airfoil
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The effects of two Gurney flaps with heights of 1% and 2% of
cord length were compared for the DU 91-W2-250 airfoil in Figure 7
and Figure 8. As it can be seen, the 1% and 2% of cord length
Gurney flaps have increased lift coefficient 0.275 and 0.386
respectively; however, the maximum lift to drag ratio decreased
from 99 to 90 and 77.
The effects of a diversity of trailing edge wedges on the
airfoil for L/H = 1.5, 2.1 and 3 were compared for H=1% of chord
length in Figure 9 and Figure 10. The computed results showed that
with increasing upstream length (L) the lift
coefficient decreased while the maximum lift-to-drag ratio (L/D)
increased and compared to the Gurney flaps of same height there was
a tradeoff, a decrease in lift coefficient for an increase in L/D
ratio. As for the new trailing edge curved flaps, the aerodynamic
performances of different amounts of L/H = 1.5, 2.1 and 3 for H=1%
of chord length were compared and the same results were obtained
for it, as shown in Figure 11 and Figure 12. The effect of H= 1%
and 2% of cord length for Trailing edge curved flap at L/H = 2.1 on
the airfoil were compared as illustrated in Figure 13 and Figure
14.
Figure 9. Lift coefficient for DU 91-W2-250 airfoil with a
diversity of trailing edge wedges for H=1%
Angle of attack
CL
0 2 4 6 8 10
0.6
0.8
1
1.2
1.4
1.6
DU 91-W2-250 airfoilL/H=1.5L/H=2.1L/H=3
Figure 10. Lift/Drag ratio for DU 91-W2-250 airfoil with a
diversity of trailing edge wedges for H=1%
Angle of attack
Lift/
Dra
g
0 2 4 6 8 10
50
60
70
80
90
100
DU 91-W2-250 airfoilL/H=1.5L/H=2.1L/H=3
Figure 11. Lift coefficient for DU 91-W2-250 airfoil with a
diversity of trailing edge curves H=1%
Angle of attack
CL
0 2 4 6 8 10
0.6
0.8
1
1.2
1.4
1.6
1.8
DU 91-W2-250 airfoilL/H=1.5L/H=2.1L/H=3
Figure 12. Lift/Drag ratio for DU 91-W2-250 airfoil with a
diversity of trailing edge curves H=1%
Angle of attack
Lift/
Dra
g
0 2 4 6 8 10
50
60
70
80
90
100
DU 91-W2-250 airfoilL/H=1.5L/H=2.1L/H=3
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As can be seen, going from H=1% to H=2% of cord length, the lift
coefficient increased about 0.12, though, the maximum lift over
drag ratio has decreased from 92 to 81.
And at last, the results between the Gurney flap, trailing edge
wedge and the new trailing edge curved flap at H=1% of cord length
and L/H =2.1 were compared, as depicted in Figure 15 and Figure 16.
Clearly, the trailing edge curved flap compared to the Gurney flap,
lift coefficient reduced about 0.02 (1.5 %) while the maximum lift
over drag ratio increased from 90.2 to 92.1 (2.1%) compared to the
trailing edge wedge that reduced lift coefficient about 0.06 (4.5%)
while the maximum lift over drag ratio increased from 90.2 to 93.6
(3.7%). The simulated data showed that in H=2% of cord length and
L/H = 2.1 the lift coefficient decreased about 1% while maximum
lift over drag ratio increased about 5.4% in the curved flap
compared to the Gurney flap.
The streamlines over the DU 91-W2-250 airfoil with trailing edge
curved flap at 7 degree angle of attack is depicted in Figure 17.
Figure 18 shows the streamline over the trailing edge curved flap
at H=1% of cord length and L/H =2.1 for DU 91-W2-250 airfoil at 6
degree angle of attack. As presented here, there is no separation
bubble upstream of the flap unlike the Gurney flap that has a
separation bubble upstream of it. Furthermore, there is no change
to two counter-rotating vortices downstream of the flap. It is
clearly visible in Figure 18 that the flap curve filled the area
otherwise filled by separation bubble upstream of the Gurney flap.
Streamlines over Gurney flap and trailing edge wedge are also
depicted in Figure 19 and Figure 20 respectively. As there is no
separation bubble upstream of
the trailing edge curved flap and trailing edge wedge, the drag
force is lower, thus, lift/drag ratio is increased.
Figure 15. Lift coefficient for DU 91-W2-250 airfoil for H=1%C
and L/H=2.1
Angle of attack
CL
0 2 4 6 8 10
0.6
0.8
1
1.2
1.4
1.6
1.8
Gurney flap 1%CTrailing edge curved shapeTrailing edge wedge
Figure 13. Lift coefficient for DU 91-W2-250 airfoil with
trailing edge curved flap at L/H = 2.1
Angle of attack
CL
0 2 4 6 8 100.6
0.8
1
1.2
1.4
1.6
1.8
2
H=1%CH=2%C
Figure 14. Lift/Drag ratio for DU 91-W2-250 airfoil with
trailing edge curved flap at L/H = 2.1
Angle of attack
Lift/
Dra
g
0 2 4 6 8 10
60
65
70
75
80
85
90
95
H=1%CH=2%C
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CONCLUSION In the present study the steady CFD simulations
are
performed on DU 91-W2-250 airfoil with the Gurney flap, trailing
edge wedge and the innovative trailing edge curved flap with a
variety of length over height ratio of L/H = 1.5, 2.1 and 3 and
heights of H= 1% and 2% of cord length at Reynolds number of 2 106.
Results for the different length over height ratios suggested that
with increasing L/H ratio the lift
coefficient decreased, while maximum L/D ratio increased, but
these changes are under 2% when comparing L/H of 1.5 and 3.
Finally, results showed that the innovative trailing edge curved
flap has the benefits of the Gurney flaps and trailing edge wedges
simultaneously, which means, having a lift coefficient relatively
closer to the Gurney flap while retaining a maximum lift over drag
ratio close to the trailing edge wedge. Note that all of the
numerical simulation in this investigation was
Figure 16. Lift/Drag ratio for DU 91-W2-250 airfoil for H=1%c
and L/H=2.1
Angle of attack
Lift/
Dra
g
0 2 4 6 8 10
60
65
70
75
80
85
90
95
Gurney flap 1%CTrailing edge curved shapeTrailing edge wedge
Figure 18. Streamlines around the trailing edge curved flap
X/C
Y/C
0.97 0.98 0.99 1 1.01 1.02 1.03
-0.04
-0.03
-0.02
-0.01
0
0.01
0.02
Figure 19. Streamlines around the Gurney flap
X
Y
0.95 0.96 0.97 0.98 0.99 1 1.01 1.02 1.03 1.04 1.05-0.06
-0.05
-0.04
-0.03
-0.02
-0.01
0
0.01
0.02
0.03
Figure 17. Streamlines around the airfoil with trailing edge
curved flap
X/C
Y/C
0 0.5 1
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
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performed in two-dimensional domain, which is valid for thin
airfoils that are located at the outboard of the wind turbine
blade, where the flow is two-dimensional. However, further
investigations with rotating flow field for thick airfoils at the
inboard section of the blade where the flow is three dimensional is
recommended.
REFERENCES [1] Scott J. Johnson, C.P. Case van Dam and Dale E.
Berg; Active Load Control Techniques for Wind Turbines; Sandia
National Laboratories; pages 33-38. [2] Liebeck, R. H.; Design of
Subsonic Airfoils for High Lift; Journal of Aircraft; Vol. 15,
No.9; pages 547-561; 1978. [3] W.A. Timmer and R.P.J.O.M. van
Rooij; Numerical investigation of an airfoil with a Gurney flap;
AIAA-2003-0352; 2003. [4] Hak-Tae Lee, Ilan M. Kroo; Computational
Investigation of Airfoils with Miniature Trailing Edge Control
Surfaces; AIAA-2004-1051; 2004. [5] Cory S. Jang, James C. Ross,
Russell M. Cummings; Numerical investigation of an airfoil with a
Gurney flap; Aircraft Design; Volume 1, Issue 2, June 1; pages
75-88; 1998. [6] W.A. Timmer and R.P.J.O.M. van Rooij; Summary of
the Delft University Wind Turbine Dedicated Airfoils; J. Sol.
Energy Eng.125; pages 488-497; 2003. [7] Airfoil coordinates was
provided by Delft University upon request.
Figure 20. Streamlines around the trailing edge wedge
X/C
Y/C
0.96 0.98 1 1.02 1.04
-0.04
-0.02
0
0.02
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