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View the table of contents for this issue, or go to the journal homepage for more
2013 IOP Conf. Ser.: Mater. Sci. Eng. 52 052001
(http://iopscience.iop.org/1757-899X/52/5/052001)
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Investigation of the two-element airfoil with flap structure for
the vertical axis wind turbine
Y Wei and C Li
Energy and Power engineering, University for Shanghai Science and Technology,
Shanghai 200093, China
E-mail: lovemozhilanzhang@126.com
Abstract. The aerodynamic performance of Vertical axis wind turbine (VAWT) is not as simple
as its structure because of the large changing range of angle of attack. We have designed a new
kind of two-element airfoil for VAWT on the basis of NACA0012. CFD calculation has been
confirmed to have high accuracy by comparison with the experiment data and Xfoil result.
The aerodynamic parameter of two-element airfoil has been acquired by CFD calculation in
using the Spalart-Allmaras (S-A) turbulence model and the Simple scheme. The
relationship between changings of angle of attack and flap’s tilt angle has been found and
quantified. The analysis will lay the foundation for further research on the control method for
VAWT.
1. Introduction
Generally, wind turbine consist of vertical axis wind tuebine(VAWT) and horizontal axis wind
turbine(HAWT). [1]VAWT can be divided into two categories. One is Darrieus wind turbine that driven by
the lift forces. The other is Savonius wind turbine that driven by the drug forces. Compare with HAWT,
VAWT has a lot of advantages, such as lower blade loading makes it has less fatigue. Lower center of
gravity (CG) makes it more stability. Easy for production and installation[1]. But VAWT also has
disadvantages, such as poor startup performance and hard to get rid of the dynamic stall effect. The paper
has been designed a new kind of two-element airfoil for VAWT on the basis of NACA0012. The
aerodynamic performance of the airfoil is acquired through CFD calculation. It is feasible for two-element
airfoil to be applied in VATW theoretically. Meanwhile, it becomes infinite possible to improve the
performance, stability and efficient for VAWT with flag structure.
2. Model construst and calculation
2.1. Operation principle of VAWT
The azimuthal angle of lift force driven VAWT could be vary from 0 degree to 360 degree, so it always
use the blade that with symmetrical airfoils, such as NACA0012, NACA0015, NACA0018 and so on.
Fig.1 shows the operation principle of VAWT.
The V represents the wind flow velocity, t
V
represents the airfoil’s tangential velocity, w
V
represents the resultant velocity, l
F and d
F represents the lift force and drag force,
represents the
angle of attack,
represents the azimuthal angle,
represents the angular velocity,
represents the
tip speed ratio and R represents the radius.
6th International Conference on Pumps and Fans with Compressors and Wind Turbines IOP Publishing
IOP Conf. Series: Materials Science and Engineering 52 (2013) 052001 doi:10.1088/1757-899X/52/5/052001
Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution
of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
Published under licence by IOP Publishing Ltd 1
Figure 1. Operation principle of VAWT.
The tip speed ratio is defined:
R
V
(1
)
In VAWT, the relative velocity is the vector of tangential veolocity and wind flow velocity. Actually,
the angle of attack is not from -360 degree to 360 degree for VAWT. The angle of attack is a fuction of the
tip speed ratio and the azimuthal angle:
1
cos
tan ( )
sin
(2)
In Fig 2 ,the higher tip speed ratio will correspondence to the small changing range for angle of attack.
The angle of attack changed from the negative angle to the positive angle and distributed in a symmetric
way.
Figure 2. Azimuthal angle vs. Angle of attack.
In VAWT, lift foece provides positive torque in a circle of rotation and is more than the negative
torque caused by drag force. This is the operation principle of VAWT. [2]
6th International Conference on Pumps and Fans with Compressors and Wind Turbines IOP Publishing
IOP Conf. Series: Materials Science and Engineering 52 (2013) 052001 doi:10.1088/1757-899X/52/5/052001
2
2.2. Two-element airfoil construct
The flap could optimize aerodynamic performance. In the stall region, the aerodynamic performance could
be optimized by changing the lift force and the flap derection. Research by Xu Zhang from ZheJiang
University shows that suited airfoil could improve aerodynamic perfoermance and output power of the
wind turbine. [3]
Figure 3. Airfoil with flap.
The airfoil’s main body is constructed on the basis of NACA0015 airfoil showed in Fig.3. The
flap could rotate around its center. Its center is apart from the trailing edge for 1/4 chord’s length. The
rotate angle is range from negative 30 degree to positive 30 degree.
With the need of structure, the
width of the inner seam could be 1/30 chord’s length.
The flap is fixed on the airfoil by upper and lower hinged(showed in Fig.4).
The flap
could rotate around its center. Considering the blade structure, NACA0012 airfoil is used on the upper
and lower blade. So the axis of rotation for the flap could be installed easily.
(a) Overall model
(b) Partial enlarged view on the edge
Figure 4. Three-dimensional model for the blade.
The largest advantage for the airfoil that has flap is optimized aerodynamic performance. When the
VAWT is going to start, the operated condition will very adverse because of the lower tip speed ratio. At
the moment we could adjust the flap to maximize the lift forces so it could quickly get started. When the
6th International Conference on Pumps and Fans with Compressors and Wind Turbines IOP Publishing
IOP Conf. Series: Materials Science and Engineering 52 (2013) 052001 doi:10.1088/1757-899X/52/5/052001
3
VAWT is running in the overloaded situation, the flap can also help the turbine to slow down through
man-made stall or lower the lift force on the airfoil. The method could decreased the fatigue load on the
blade and extend the use age of VAWT.
2.3. Computational domain and meshing
The angle of attact is range from negative 30 degree to positive 30 degree. NACA0012 is symmetrical
airfoil, so flap angle
is symmetrical about
0
.
could be
10
,
20
and
30
in this paper. In the
mashing process, the computational domain has been divided into two parts that is near airfoil part and the
outer flow part. The import comes nine times of chord length from the blade leading edge; export comes
20 times of chord length from the blade trailing edge ; upper and lower surface are 10 times of chord
length from the blade. Assuming the outlet pressure is not the atmospheric pressure, suitable outer flow
should be defined to get the valid boundary conditions.
The air could be the fluid medium, where
=1.225
3
/kg m
,
5
1.7894 10 /kg m s
,
10 /Vms
,
5
Re 6.85 10
and
0.03Ma
. Meshing is the key point in CFD computation. The NACA0012 airfoil with
the flap is meshed in structured grid, showed in Fig.5, the first layer of the grid is 0.0001. The value of
y
alone the blade surface is between 0.5 and 9.
NACA0012
0
Figure 5. Meshing.
2.4. Control equation
Because of the rotation speed is rather slow compare with the wind speed, then the flow around the airfoil
could be seemed as incompressible Navier-Stokes equation. The equation could be express as follow when
the coordinate axis is put on the blade.
0
UV
XY
(3)
22
211
22
22
222
22
1p
2()
1p
2()
uu
UUU
UV XV v
tXY XXY
uu
VVV
VV YU v
tXY YXY
(4)
In the equation:X, Y, U and V are represents the position and velocity in rotating coordinate system.
Their relation in static coordination could be expressed as follow.
cos sinxX Y
cos sinXx y
(5)
sin cosyX Y
sin cosYx y
(6)
cos sinuU V y
cos sinUu v y
(7)
sin cosvU V x
sin cosVu v X
(8)
where:
x
,yrepresents the position and
u
,
v
represents the velocity in static coordinate system
6th International Conference on Pumps and Fans with Compressors and Wind Turbines IOP Publishing
IOP Conf. Series: Materials Science and Engineering 52 (2013) 052001 doi:10.1088/1757-899X/52/5/052001
4
while
represents the rotation angular speed.
3. Result analysis
When
12
, the boundary layer separation haven’t happened. The test data of lift and drag coefficient
could be consistent, and the error could be under 5%. When
12
, the error could be markedly
increased. NACA0012 airfoil stalls when
16
. The airfoil began stalling and separating When
12 16
, the stall vortex appears alone the leading edge.(Showed in Fig.6)
(a)
12
(b)
13
(c)
14
(d)
15
(e)
16
(f)
17
Figure 6. Separation of boundary layer and stall vortex.
Because of the errors in CFD calculation data, qualidative analysis always adopt to the static
computation when the airfoil was static stalling. Errors in both CFD calculation data and Xfoil calculation
data could under 5% before stalling(Showed in Fig.7). But the errors could changed obviously after
stalling. The static stall is the major factor for the misalignment of lift and drag force.
Figure 7. Lift/drag coefficient (
5
Re 6.85 10
). Figure 8. Drag coefficient (
5
Re 6.85 10
).
The difference of the drag coefficient between CFD calculation data and experimental data is less
than 0.3 percent. But the erorr of the actual calculation data is up to 50%(showed in Fig.8). The reason for
the error in the calculation of the drag coefficient should be tested and verified in the further researches.
There is little difference on the lift and drag coefficient between two-element airfoil and NACA0012
-30 -20 -10 0 10 20 30
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
Drag coefficient of experimental
Lift coefficientof experimental
Drag coefficient of CFD
Lift coefficient of CFD
Drag coefficient of Xfoil
Lift coefficient of Xfoil
Angle of attact (°)
Li
f
t/
d
r
ag coe
f
f
icient
C
l
-15 -10 -5 0 5 10 15
0.005
0.010
0.015
0.020
0.025
0.030
CFD
Xfoil
Angle of attact (°)
D
r
ag coe
f
f
icien
t
C
d
6th International Conference on Pumps and Fans with Compressors and Wind Turbines IOP Publishing
IOP Conf. Series: Materials Science and Engineering 52 (2013) 052001 doi:10.1088/1757-899X/52/5/052001
5
airfoil when
0
(showed in Fig.9). It shows that two-element airfoil could keep the aerodynamic
performance of the NACA0012 airfoil when 0
. The physical significance of the offset of the lift
coefficient curve is to change the airfoil aerodynamic performance by changing the tilt angle.
Figure 9. Lift coefficient (
5
Re 6.85 10
). Figure 10. Tilt angle vs. Angle of attack (
0
l
C
).
In this paper, it shows the changes and relationships between angle of swing and angle of attack when
0
l
C. The angle of swing is range from
0
to
30
, and the increment of angle of swing is
5
. It also
provides the basis for controlling of VAWT on high angle of attack(Showed in Fig.10). In other words, the
correspondence could establish the change of angle of attack on the static state with the specific angle of
swing. The control policy of angle of attack could improve the aerodynamic performance theoretically, but
the actual result should be worked out through experimental test.
4. Conclusion
The blade is key technology for VAWT and its airfoil is directly influence it power efficiency. This paper
presents the flap trailing edge on the basis of VAWT’s aerodynamic theory. The CFD calculation for airfoil
with flap has been made. The result proves the airfoil aerodynamic computational accuracy. The result
shows:
1) Both of Xfoil and CFD can get aerodynamic performance of the airfoil accurately before the airfoil
stalling. Calculation accuracy are similar. But if the airfoil stalled, no method can accurately get the
aerodynamic performance.
2) Because of the surface roughness of the airfoil, the assume of the turbulence model and the
round-off error, there are some errors when calculating the drag force. The errors can be ignored for the
overall performance of the airfoil.
3) Flap trailing edge partly changes the attact angle of the airfoil. The paper shows the corresponding
relationship of the flap’s rotating angle and the attack angle. It will provides the basis for the research of
flap controlling scheme.
The work in this article has its meanings in the study for the aerodynamic performance analysis on
two-element airfoil for VAWT. It also lays a solid foundation for the research on the flag control.
References
[1] Wang Q K 2011 Journal of Mechanical Engineering 47(12) 126-32
[2] Liao S X, Li C and Nie J B 2011 Machine Design and Research 27(3) 108-11
[3] Xu Z, Wang Q, Huang-fu K L, Zhong Y J 2011 Journal of Chinese Society of Power Engnineer
30(9) 715-9
[4] Paraschivoiu I 2009 Wind Turbine Design with emphasis: on Darrieus concept. (Canada:
Presses internationales Polytechnique) p 442
[5] Jin K Y 2009 The Analysis and Design of Aerodynamic Performance for Wind Turbine with
Low-speed Airfoil (Shanghai: University for Shanghai Science and Technology)
NACA0012 resul t
Angle of attact (°)
l
6th International Conference on Pumps and Fans with Compressors and Wind Turbines IOP Publishing
IOP Conf. Series: Materials Science and Engineering 52 (2013) 052001 doi:10.1088/1757-899X/52/5/052001
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