INTRODUCTION
The use of wind power far earlier than the invention of internal combustion
engines winddriven ships were used (Kurtulmus et al.,
2007). Since the environmental problems occurred in the late 20th century,
people began to pay attention to the use of the clean and renewable energies
(Vardar and Eker, 2006). Wind power is one of the fastest
developing and lowcost industry which is a renewable energy utilization way.
However, climate has a great influence on the generation of the wind power (Meng
and Song, 2007). Antarctic, the average wind speed is high and low temperature,
therefore, the worldwide concern about environmental pollution and possible
energy shortage has led to increasing interest in generation of renewable electrical
energy (Babainejad and Keypour, 2010).
Wind turbine can be divided into Horizontal Axis Wind Turbines (HAWTs) and
Vertical Axis Wind Turbin (VAWTs) two categories by different rotor shaft which
are used mainly for electricity generation (Izli et al.,
2007), VAWTs have inherent advantages, the principal advantages of the vertical
axis format are their ability to accept wind from any direction without yawing
and the ability to provide direct rotary drive to a fixed load (Beri
and Yao, 2011). Compares with HAWTs, VAWTs have strong resisting wind ability.
In addition, the noise of VAWTs is much smaller.
This study aims to research and develop a firstfit VAWTs rotor used under Antarctic’s extreme climate which is adopted to provide energy for the Antarctic expedition car’s electronic equipment, on the condition that the wind turbine can work properly to produce electric power, while not affect the normal work of the instrument in the Antarctic’s extreme environment. At the same time, its structure should be light weight, small volume and high efficiency of power generation. Therefore, a kind of VAWTs rotor was tried to be designed.
WIND TURBINE ROTOR
The blades which are mounted on the hub are called the wind rotor which mainly
includes the blade, the hub, the shaft, etc. The wind rotor is the most important
unit to turn wind energy into mechanical energy which directly determine the
success of the wind turbine system (Niu and Zhang, 2007;
Paraschivoiu, 2002).
Main technical indexes: It requires that the wind rotor output energy
with high efficiency to meet 50 W energy supply for the electronic devices in
the mobile platform. Because of extremely low average temperature in the Antarctic,
the wind rotor is required to work normally in cold condition with temperature
from 50 to 50°C. The Antarctic is also an area with the strongest wind
power on the earth, so the wind rotor is required to work high efficiently when
average wind speed is 17 m sec^{1}; The electric generator should work
properly through the mechanical braking adjustment when the wind speed exceeds
17 m sec^{1}, if the wind speed exceeds 40 m sec^{1}, the
wind turbine stopped because of the mechanical brake. Wind generator with heavy
weight will seriously affect the state of motion of the car, so the rotor’s
total weight should not exceed more than 2% of the weight of the car.
Material: The harsh natural environment of the Antarctic requires the
high demands of the wind rotor material, including high strength, high stiffness,
low density, long life, good corrosion resistance, especially low temperature
tolerance. Some researchers compared the properties of the commonly used materials
in wind turbine (Zhang and Ren, 2008), as shown in
Table 1.
As can be seen from the Table 1, the density of the composite wood is the smallest, carbon fiber followed. The tensile strength of aluminum alloy is 0.41 GPa which is similar to the alloy steel and carbon fiber. The fatigue strength of carbon fiber is 100 MPa. Although, the density of the composite wood is small, there is a large gap between the strength, fatigue characteristics with the other three materials. The composite materials such as carbon fiber and aluminum alloys have better performance, they have lower density, better fatigue properties and damping properties.
Blade airfoil, hub and spindle: Blade is the key component of the wind
turbine and the blade airfoil design is especially important which directly
determine the ability to obtain the desired power. So far, spacecraft airfoil
is frequent used on the VAWTs, its technology is mature, with large lift coefficient
and small drag coefficient characteristics, such as the American’s NACA,
NASA series. Hub connect the blade and spindle which is the part that most prone
to cause cyclic loading fatigue. Hub bear the weight of the blade, the circumferential
bending force, plus the tension and pressure force, these forces are very complex,
therefore it needs to focus on analysis of the hub structure and force. The
shaft and the generator directly connected together by the spindle, it is the
main loadbearing component of the entire wind turbine to withstand greater
torque, generally made from highstrength alloy steel.
Table 1: 
The properties of the four materials 

Meanwhile, a reliable connection of the spindle with the hub have a major
impact on the efficiency of the blade transmitted to the torque coefficient
of the spindle.
SIMULATION OF THE AERODYNAMIC PERFORMANCE OF THE WIND ROTOR
Basic theory of wind turbine: Betz theory is the basic theory of the
wind turbine, wind turbine power’s estimate should be compared with its
maximum power by calculation (He, 2006). Betz theory discuss
the maximum wind energy utilization efficiency by applying of the steady flow
momentum equation. It assume that the wind rotor is ideal and it accept all
wind (no hub). The blades are enough and have almost no resistance to the air
flow which is uniformly, continuously and exclude compressible. Consequently,
this is a pure energy converter. It also further assumed that the wind rotor
swept surface of airflow direction in terms of the wind rotor around or through
are perpendicular to the blade sweep surface. On this condition, the impeller
is called the “ideal impeller” which resulting in wind energy utilization
coefficient Cp:
In the formula, ρ is the air density (kg m^{3}), P is wind rotor power (W), S is wind rotor swept area (m^{2}), v_{1} is the wind speed (m sec^{1}). The design constraints of the wind rotor is the pursuit of the proportion of the wind turbine output power obtained from natural wind. Ideal wind rotor wind energy utilization factor Cp_{max} = 16/27 = 0.593, it is the maximum efficiency of the mechanical energy turned out by wind rotor.
Another basic theory of the wind turbine is blade element theory whose basic
starting point is to divide the wind turbine blade into a number of microsegments
and these microsegments is called blade element. Assuming the flow between
each blade element are not interfere, in other words, blade element can be seen
as a twodimensional airfoil (Zhang, 2007).
To study the forces of the blade, a crosssectional view taken on the blade to analyze the force and torque acting on the blade element. The force analysis of blade element shown in Fig. 1:

Fig. 1: 
The force analysis of blade element 

Fig. 2: 
The C_{L} for four kinds of airfoils under the same
wind speed changes 
In the formula, dF_{L} is the airfoil lift, dF_{D} is the airfoil resistance, W is the airflow with respect to the blade element of the relative velocity (m sec^{1}), c is the airfoil chord length, C_{L} C_{D}, represent the airfoil lift coefficient and drag coefficient, respectively. DF_{L}, dF_{D} is the axial direction and the circumferential projection of the wind rotor, according to Eq. 2 and 3 can be obtained as follows:
In the formula, dF_{x} is the shadow of the projection along the reincarnation shaft of the wind turbine, wherein dF_{R} is the urging force of airfoil, dF_{y} is the projection of dF_{R} along the direction of the rotation plane of the wind rotor, I for vapor phase angle, I = I+β, wherein, i is the airfoil angle of attack, β is the blade install angle.
Because blade element theory is to divide wind turbine blade into a finite
number of blade element and then calculate the force and torque on each blade
element, integral can obtain the aerodynamic performance of the entire blade
can be learned by integral.

Fig. 3: 
The C_{D} for four kinds of airfoils under the same
wind speed changes 
For blade element force performance, designers’ most concern is the power
of the blade can be obtained, that is to maximize the utilization efficiency,
so airfoil characteristics should be able to meet the three requirements (Ren,
2010): large lift coefficient, low drag coefficient and in the course of
the angle of attack changes, the airfoil can maintain excellent aerodynamic
performance. Aerodynamics, the theoretical basis in airfoil profile, research
experimental data of airfoil profile aerodynamic performance and theoretical
calculation methods, provide the foundation of wind turbine the aerodynamic
performance research and design.
Airfoil selection: We learned by the blade element theory that the performance
of wind turbine greatly affected by the airfoil. When VAWTs wind rotor in the
rotation, its radial force and cutting force increase or decrease cyclically.
Within one cyclical revolution, the blade torque coefficient is also changing.
Airfoil design is a very complex subject since it involves a lot of knowledge
of fluid dynamics. Four groups of wind turbine with special airfoils used widely
were chosen to study in this study which is published by the American space
agency. They are symmetric airfoil NACA0012, NACA0021 and asymmetrical airfoil
NACA4412, NACA23012. We compare and analyze this four airfoil by the Fluent
simulation software, choosing 525 m sec^{1} wind speed, the rest of
the reference design parameters fixed. Figure 25
compares the lift coefficient (C_{L}), the drag coefficient (C_{D}),
the torque coefficient (C_{m}) and the liftdrag ratio (C_{L}/C_{D})
of four airfoils in different wind speeds.

Fig. 4: 
The C_{m} for four kinds of airfoils under the same
wind speed changes 
Table 2: 
Wind rotor model parameters and the optimum range 

As can be seen from Fig. 25, different
airfoil in the same wind speed change of C_{L}, C_{D}, C_{m}
and C_{L}/C_{D} trend is the same and its amount has a tendency
of increase with increasing wind speed. Figure 2 shows that
the wind conditions of the different incoming flow, the four airfoil lift coefficient
of a big difference. Wherein the lift coefficient of the airfoil NACA4412 is
largest and is much higher than the other airfoils. Figure 3
shows that the drag coefficient of four airfoils has almost no difference and
the drag coefficient of airfoil NACA4412 is slightly larger than the other three
airfoils in wind speed of the different incoming flow. Figure
4 shows that under the wind speed of the different incoming flow, the torque
coefficient of airfoil NACA4412 is the largest and the torque coefficient of
airfoil NACA0012 is the smallest. Figure 5 shows that under
the wind speed of the different incoming flow, the lift to drag ratio of the
airfoil NACA4412 is the highest and the lift to drag ratio of the airfoil NACA0012
is the smallest. Therefore aerodynamic performance of the airfoil NACA4412 is
better than the other airfoil, NACA0021 followed, the NACA0012 is the worst.
So choosing NACA4412 asymmetric airfoil as the research subject and provided
the reference basis for future design.
Analysis of the aerodynamic performance of wind rotor
Wind rotor model: Many factors affect wind energy utilization of wind rotor.

Fig. 5: 
The C_{L}/C_{D} for four kinds of airfoils
under the same wind speed changes 
This study focuses on the number of the blades, blade radius, airfoil chord
length and blade installation angle as optimization parameters. The choice of
the number of the blades select 2 to 6 according to wind rotor solidity. Currently,
there is no specific theoretical support for the choice of the airfoil chord
length, so the choice of chord length refer to the mature wind turbine, changes
from 0.08 to 0.19. Wind rotor radius is determined by the mobile platform size,
changing from 0.25 to 0.5. Installation angle changing from 10 to 12° .
The specific parameters of wind rotor as shown in Table 2.
Rated wind speed is set to the average wind speed in the Antarctic, namely 17
m sec^{1}. Using better performance airfoil NACA4412 modeling, Computational
Fluid Dynamics (CFD) simulation for both twodimensional model to obtain the
output of the wind rotor torque under different parameters and wind energy utilization
factor and then find the optimal parameters and is currently the most comprehensive
and the most widely used CFD software.
This article uses Fluent software to simulate and take the secondorder upwind SIMPLEC algorithm solver in 2D unsteady Reynoldsaveraged NaiverStokes equations to calculate and selects the SpalartAllmaras turbulence model. Using moving mesh technology to deal with the rotation of the wind rotor. For a number of different blades, blade chord length, radius of the wind rotor and installation angle, the numerical simulation of wind rotor is completed. At the same time, this article analyzes the flow field characteristics and gets the individual parameter values in the maximum of the wind energy utilization.
Uniform optimization method in the application of the design: This design
considers a number of factors of wind turbine size changes, so there are a lot
of conditions need to calculate, thus the uniform optimization design method
was introduced.
Table 3: 
The VAWTs concrete size and the number of levels given by
the uniform table 

Uniform design theory is proposed by Fang Kaitai, The theory is a design method
which just considers the test points uniformly distributed within the test range.
It is a test method that applies to multifactor and multilevel as well. Compared
with the traditional method that test points were evenly spread regardless of
neat comparable orthogonal design, the uniform design method can greatly reduce
the number of trials. For a selected number of tests, factor in the number and
level number, uniform design table already exists and can be used directly.
Table 3 is the VAWTs concrete size and the number of levels
given by the uniform table, By Fluent software to numerical simulation of various
conditions, select the parameter values of the largest wind energy utilization.
From the Table 3, we can get the initial the wind energy
utilizing rate increases significantly with increasing radius of the wind rotor,
this also applies to the chord length. Within a certain range, the installation
angle of a relatively small impact on the wind energy utilization. With the
increase of blades, lower utilization of wind energy. This design adopt uniform
optimal design method, only 12 times of the numerical simulation was taken and
the various parameters of the wind rotor of wind energy utilization can be found.
The wind energy utilization close to literature (Zhou, 2009)
but significantly reduced the number of trials. The statistical results in Table
3 can be found in the airfoil NACA4412 in the case of an optimal set of
design parameters, i.e. The radius is 0.5 m, the blade chord length is 0.14
m, the installation angle is 6°, the number of the blades is 3, wind utilization
efficiency is 36.1%. Subsequent work requires the test data for statistical
processing, so as to extract more useful information. It provide a basis for
building a physical model which is of great significance to further wind turbine
design.
HUB MATERIAL SELECTION AND HUB STRUCTURE
Plan selection: VAWTs’ high torque fluctuations with each revolution,
no selfstarting capability are the drawbacks (Islam
et al., 2008). The worlds of David Darling (2009).
Table 4: 
Triangular structure hub of maximum displacement and stress
simulation results 

Table 5: 
Square structure hub of maximum displacement and stress simulation
results 

The stability requirement of hub structure is particularly important in the
bad environment of the Antarctic. The design chooses two common hub structures
in market, they are the triangular structure (Fig. 6) and
square structure (Fig. 7). Figure 6 is the
three dimensional model created by Pro/E 3D software. By using ANYSY software
to get static analysis and modal analysis. Then to select the best hub material
and structure theoretically.
The static analysis of hub structure: The average wind speed of Antarctic
is 17 m sec^{1}, depending on the wind formula WP = v^{2}/1600
(kN m^{2}), the wind pressure WP = 180 Pa, Select the wind pressure
direction perpendicular to a blade and static analysis of the wind turbine.
Assume that the wind turbine blade material is a carbon fiber, the density ρ
= 1.4 g cm^{3}, its gravity approximately G = 20 N. According to these
two programs, the wind turbine model draw in the Pro/E software, corresponding
load and fixed constraints applied to the model at the same time. Four materials
chosen for the wind turbine hub, carbon fiber, aluminum, stainless steel and
structural were compared and analyzed. The four materials total deformation
simulation cloud picture shown in Fig. 811.
The triangular structure hub maximum displacement and stress simulation results
are shown in Table 4. According to the chart data, we known
the maximum stress is much less than the respective yield strength, not exceeding
the allowable stress. The amount of deformation of the carbon fiber is 0.2146
mm and it is the smallest in the four materials.
The four materials total deformation simulation cloud picture shown in Fig.
1215. Square structure of the hub maximum displacement
and stress simulation results are shown in Table 5. According
to the chart data, the maximum stress is much less than the respective yield
strength. The amount of deformation of the carbon fiber is 0.34 mm and it is
the smallest in the four materials.

Fig. 6: 
The model of triangle hub structure selected in the market 

Fig. 7: 
The model of square hub structure selected in the market 

Fig. 8: 
The displacement simulation results of triangular structure
hub by using carbon fiber 

Fig. 9: 
The displacement simulation results of triangular structure
hub by using aluminum alloy 

Fig. 10: 
The displacement simulation results of triangular structure
hub by using stainless steel 

Fig. 11: 
The displacement simulation results of triangular structure
hub by using structural steel 

Fig. 12: 
The displacement simulation results of square structure hub
by using carbon fiber 

Fig. 13: 
The displacement simulation results of square structure hub
by using aluminum alloy 
Analysis by the above simulation, compared the displacement of the two different materials under the triangular structure of the hub and the square structure of the hub, when the material is carbon fiber, the deformation of the displacement value to the minimum, so the material of carbon fiber used for the wind turbine hub is more appropriate in the theoretically. In the case of the four different materials, the deformation volume of the triangular structure is smaller than the square structure, so the wind turbine hub structure using the triangular structure is more appropriate.
Modal analysis of the blades and hub: When the wind generator is running,
the blade’s displacement occurred under the force produced by the wind,
the selfgravity, centripetal force and so on. So it is necessary to verify
the natural frequency of the wind hub (Divya and Nagendra
Rao, 2006).

Fig. 14: 
The displacement simulation results of square structure hub
by using stainless steel 

Fig. 15: 
The displacement simulation results of square structure hub
by using structural steel 
Modal analysis of triangular structure: When the material is carbon
fiber and the hub is a triangular structure, it is the natural frequency of
the six modes as shown in Fig. 1621.
From that we can know that the six orders of the natural frequency are 34.942,
34.985, 35.549, 79.772, 115.06 and 115.19, respectively. The 1, 2 and 3 order
are similar. 5 order natural frequency and 6 order natural frequency are similar.
This is due to the three groups of blades and the blade strut distribution are
120° and the first 3 order of their respective vibration modes in the XZ
plane.
RESULTS AND DISCUSSION
In the fixedspeed wind turbine design, the wind rotor rotation frequency is
the most important factor. This frequency is often defined as "1P", it may induce
dynamic load increase, as a result of the wind rotor imbalance.

Fig. 16: 
First order modal vibration diagram 

Fig. 17: 
Second order modal vibration diagram 

Fig. 18: 
Third order modal vibration diagram 

Fig. 19: 
Fourth order modal vibration diagram 

Fig. 20: 
Fifth order modal vibration diagram 

Fig. 21: 
Sixth order modal vibration diagram 
In addition, the order of "P" is also very important, as the "2P" and "3P"
which correspond to the rotational frequency of the wind turbine blade of the
two blades and three blades. In the variable speed wind turbine design, it must
make sure that the speed of blades does not close to in the first natural frequency
range of the blade model.
In this study, the rated speed of the wind generator model is 400 r min^{1}. According to the frequency formula:
From the above, the rotational frequency of the wind rotor the f = 20/3 Hz, a single blade passing frequency, f_{0} = 3f = 20 Hz. Since the natural frequency of the wind model must be outside of ±10% of the wind hub rotational frequency and the individual blade frequencies, it will not cause resonance. By the formula:
In the formula, f_{1} is the first natural frequency. As can be seen, the natural frequency of the triangular structure wind hub model of the carbon fiber is 42.8%. Namely this structure of wind hub model is outside ±10% of the wind hub rotational frequency and the individual blade frequencies. So it is can guarantee the problem of load amplitude will not be caused by the natural frequency.
CONCLUSION
A kind of rotor for VAWTs was introduced in this study. It briefly describe the material, blade, hub and spindle of wind rotor and introduce the basic theory of wind turbine, including Betz theory and blade element theory. Four dedicated wind turbine blade airfoil was studied by the commercial software Fluent, compared with the four blade airfoils of C_{L}, C_{D}, C_{m} and C_{L}/C_{D}, then it can be concluded that the aerodynamic performance of NACA4412 airfoil is better than other airfoils. A model of VAWTs rotor is established which considering four parameters of the radius, the number of the blades, the blade chord, the blade install angle of the wind rotor and the impact of the changes on the aerodynamic performance of the wind rotor. It can greatly reduce the number of trials to find a set of optimal design parameters from orthogonal optimization results. Under the optimal design parameters, the wind energy utilization of the wind turbine can up to 36.1%.
By analyzing the small vertical axis wind rotor of four different materials,
two different structure of the wind hub static analysis by the finite element
software analyzes, it calculate out the carbon fiber is more suitable for wind
hub. In four different materials, the hub structure is more appropriate to use
triangular structure. Calculating the frequency of wind hub’s first six
order, We learn that it will not produce resonance and therefore the additional
load caused by resonance problem does not occur in normal circumstances, the
system is secure.
ACKNOWLEDGMENTS
This study is supported by National 863 Program of China (Grant No. 2011AA04Z0202) and Innovation Fund of Shanghai University (Grant SHUCX120080).