International Journal of Engineering Trends and Technology (IJETT) – Volume 32 Number 2- February 2016
Simulating Drag and Lift Coefficient
Variation of the MI VAWT1 airfoil with
Angle of Attack at Low Reynolds Number
Krishpersad Manohar#1, Rikhi Ramkissoon#2,
1
Senior Lecturer, Mechanical and Manufacturing Department, The University of the West Indies, St. Augustine,
Trinidad and Tobago.
2
Ph.D. Candidate, Mechanical and Manufacturing Department, The University of the West Indies, St.
Augustine, Trinidad and Tobago.
Abstract — Due to the complexity of shear layer
development over an airfoil surface, measurements
of time-resolved fluctuating surface pressure within
the separated flow region are difficulty. Modelling
the flow characteristics with computer generated
simulations is an alternative for overcoming this
difficulty. The DesignFoil simulation served as a
tool in analysing the flow regime across the MIVAWT1 airfoil. The results showed that the point at
which transition from laminar to turbulent flow
occurred moved progressively up towards the
leading edge as the angle of attack increased from
0o to 15o. Also, within this range the change in drag
coefficient was small and the lift coefficient
increased linearly up to 10o and then started
decreasing. The results are consistent with published
work.
Keywords — Lift Coefficient, drag coefficient,
airfoil, DesignFoil, vertical axis wind turbine.
I. INTRODUCTION (SIZE 10 & BOLD)
From as early as 5000 B.C wind energy was
used to enhance humankind energy demands. For
centuries wind turbines has served as a practical way
to capture and convert the kinetic energy of the wind
to mechanical and in more recent times directly to
electrical energy [1]. Today, wind-powered
generators operate in every size range, from small
turbines for battery charging at isolated locations to
large, near-gigawatt-size offshore wind farms that
provide electricity to national electric transmission
systems [2].
The straight bladed Darrieus vertical axis wind
turbine (VAWT) is very attractive for its low cost
and simple design [3]. Selection of airfoil was one of
the most critical factors in achieving better
aerodynamic performance. Airfoil selection is also
affected by the optimum dimensions of a fixed-pitch
straight-bladed Darrieus type vertical axis wind
turbine (SB-VAWT), along with solidity, radial arm
parasitic drag and aspect ratio. Airfoil related design
changes also has the potential for increasing the cost
effectiveness of SB-VAWTs. This is one of the
ISSN: 2231-5381
simplest type of wind turbines and has good
potential for diversified urban and rural applications.
It has been shown by Islam etal. [4] that
conventionally-used old NACA 4-digit symmetric
airfoils were not suitable for smaller capacity SBVAWT. Rather, it would be advantageous to utilize
a high-lift and low-drag asymmetric thick airfoil
suitable for low speed operation typically
encountered by SB-VAWT. In this regard a special
purpose airfoil called the MI-VAWT1 airfoil was
designed by Islam etal. from the University of
Windsor, Windsor, Ontario, Canada in 2007 [4, 5].
II. THE MI-VAWT 1 AIRFOIL
To improve the performance of the straightbladed vertical axis wind turbine (SB-VAWT),
sensitivity analyses were conducted with different
geometric features to understand their effects on
performance of SB-VAWT. Subsequently, a special
purpose airfoil named ―MI-VAWT1‖ was designed
by utilizing computer simulations [4, 5]. A
schematic of the MI-VAWT1 airfoil is shown in
Figure 1.
Figure 1. MI-VAWT airfoil [4]
The performance of the MI-VAWT1 was
superior when compared with that of NACA 0015
and LS-0417 at different Reynolds numbers [4. 5].
The Cpnet values of MI-VAWT1 were higher and the
CQnet–λ curves showed superior performance [5].
III. FLOW CHARACTERISTICS DATA
To optimize the use of airfoil in low speed
applications such as wind turbines, flow
characteristics data is a prerequisite. Static surface
pressure data, usually in the form of a pressure
coefficient distribution, is utilized to estimate
locations of flow separation, transition, and
reattachment [6, 7, 8]. Lift coefficients are also
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International Journal of Engineering Trends and Technology (IJETT) – Volume 32 Number 2- February 2016
obtained from the pressure coefficient distributions
[6, 9].
Time-resolved surface pressure fluctuation
measurements can be used for airfoils operating at
low Reynolds numbers as the utility of for
characterizing the separated flow region. However,
due to the complexity of separated shear layer
development, studies employing measurements of
fluctuating surface pressure in the separated flow
region on an airfoil surface at low Reynolds numbers
are limited [10, 11]. Modelling the flow
characteristics with computer generated simulations
is an alternative for overcoming this difficulty.
IV. DESIGNFOIL SIMULATION
Numerical predictions were done for the MIVAWT1 airfoil the using DesignFoil software which
was based on the Hess-Smith panel method [12].
Figures 1 to 4 show the DesignFoil simulation at 0 o,
5o, 10o and 15o angle of attack and the respective
values for the lift coefficient and the boundary layer
transition at a Reynolds number of 332000.
Fig. 2 DesignFoil Simulation at Reynolds number of 332000 and
Angle of Attack 5o.
The simulation in Figure 2 gave a lift coefficient
of 0.422 and a drag coefficient of 0.00456. The
transition from laminar to turbulent flow started at
12.86 % along the lifting side and turbulent flow
started at 15.71 %. On the non-lifting side of the
airfoil the transition from laminar to turbulent flow
started at 27.14 % and turbulent flow started at
30.00 %.
Fig. 1 DesignFoil Simulation at Reynolds number of 332000 and
Angle of Attack 0o.
The simulation in Figure 1 gave a lift coefficient
of -0.215 and a drag coefficient of 0.0043. The
transition from laminar to turbulent flow started at
15.71 % along the lift side and turbulent flow started
at 18.57 %. On the non-lifting side of the airfoil the
transition from laminar to turbulent flow started at
15.71 % and turbulent flow started at 18.57 %.
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Fig. 3 DesignFoil Simulation at Reynolds number of 332000 and
Angle of Attack 10o.
The simulation in Figure 3 gave a lift coefficient
of 1.053 and a drag coefficient of 0.0101. Both the
laminar and turbulent flow started at 12.86 % along
the lifting side. On the non-lifting side of the airfoil
the transition from laminar to turbulent flow started
at 50.00 % and turbulent flow started at 52.86 %.
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International Journal of Engineering Trends and Technology (IJETT) – Volume 32 Number 2- February 2016
Fig. 4 DesignFoil Simulation at Reynolds number of 332000 and
Angle of Attack 15o.
The simulation in Figure 4 gave a lift coefficient
of 1.238 and a drag coefficient of 0.0207. The
transition from laminar to turbulent flow started at
10.00 % along the lifting side and turbulent flow
started at 12.86 %. On the non-lifting side of the
airfoil the transition from laminar to turbulent flow
started at 52.86 % and turbulent flow started at
52.86 %.
V. RESULTS
The DesignFoil simulation was conducted for
Reynolds Number ranging from 332000 to 964000.
This corresponded to the relatively low wind
velocity range of operation for wind turbines.
Figure 5 is a plot of lift and drag coefficient
variation with angle of attack at a Reynolds number
of 332000. Table 1 shows the simulated lift
coefficient and drag coefficient variation with
Reynolds Number and angle of attack.
Fig. 5 Lift and Drag Coefficient Variation with Angle of Attack at
Reynolds number 332000
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TABLE 1
DESIGNFOIL SIMULATION RESULTS AT
DIFFERENT REYNOLDS NUMBER AND ANGLE
OF ATTACK
Reynolds Angle of Lift
Drag
Number
Attack
Coefficient Coefficient
332000
0
-0.215
0.0043
5
0.422
0.0056
10
1.053
0.0101
15
1.238
0.0207
532000
0
-0.215
0.0039
5
0.422
0.0051
10
1.053
0.0095
15
1.245
0.0206
731000
0
-0.215
0.0037
5
0.422
0.0048
10
1.053
0.0092
15
1.251
0.0201
964000
0
-0.215
0.0035
5
0.422
0.0046
10
1.053
0.0089
15
1.259
0.0198
VI. DISCUSSION AND CONCLUSIONS
Due to the complexity of separated shear layer
development over an airfoil surface, measurements
of time-resolved fluctuating surface pressure within
the separated flow region were less common than
those performed with separated flow region forming
on simpler geometries. For an airfoil the flow
conditions changes the location and extent of the
separated flow region. As a consequence, it was
difficult to design and instrument an airfoil with an
array of pressure transducers to adequately resolve
and cover the separated flow region for various flow
conditions.
The DesignFoil simulation served as a tool in
analysing the flow regime across the MI-VAWT1
airfoil. The results shown in Figures 1 to 4 indicated
that the point at which transition from laminar to
turbulent flow occurred moved progressively up
towards the leading edge as the angle of attack
increased from 0o to 15o. This result was consistent
with published work [12, 13, 14].
Figure 5 shows a plot of lift and drag coefficient
variation with angle of attack between 0o to 15o.
Within this range the change in drag coefficient was
small, however, the lift coefficient increased linearly
up to 10o and then started decreasing. The results are
consistent with published work that showed a region
of conventional linear growth in the lift coefficient
with the angle of attack at low angles [14, 15]. The
change in lift and drag coefficients with increasing
Reynolds over the range simulated was negligible.
This can be attributed to the low Reynolds number
which is associated with the low wind velocity for
wind turbines.
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International Journal of Engineering Trends and Technology (IJETT) – Volume 32 Number 2- February 2016
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