Article Aerodynamic study of small scale VAWT in urban circumstance (2015)

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Int. J. Mech. Eng. Autom. 
Volume 2, Number 5, 2015, pp. 199-210 
Received: March 29, 2015; Published: May 25, 2015 
 
 
 
 
International Journal of 
Mechanical Engineering 
and Automation 
Aerodynamic Study of Small Scale Vertical Axis Wind 
Turbine System in Urban Circumstance 
Junhee Han1, Taehyun Jo1, Bonchan Koo1 and Dohyung Lee2 
1. Department of Mechanical Engineering, Hanyang University, Seoul 133-791, Korea 
2. Department of Mechanical Engineering, Hanyang University, Ansan 426-791, Korea 
Corresponding author: Dohyung Lee (dohyung@hanyang.ac.kr) 
 
Abstract: This study presents a 2D computational investigation on the unsteady phenomenon associated with single and multiple 
blades at urban circumstance. The VAWT (vertical axis wind turbine) has advantages over HAWT (horizontal axis wind turbine) that 
it allows less chance to be degraded independent of wind direction and turbine can be operated even at the low wind speed. However, 
VAWT in urban area generally has weak and turbulent wind conditions due to the presence of high-rise building. Therefore, VAWT 
operation under both stalled and unstalled conditions with various effective angles of attack are also in the presence of dynamic stall 
which are particularly significant impact on load and power the operation at low tip-speed ratio. The objective of this study is to 
analyze aerodynamics of the VAWT blade and investigate the ideal shape of blade. The analysis of aerodynamic characteristics with 
various blades has been performed using numerical simulation with CFD software. As the numerical simulation discloses local 
physical features around wind turbine, aerodynamic performance such as lift, drag and torque are computed for single blade rotation 
and multiple blade rotation cases. Through this study, an accurate aerodynamic physic is defined, furthermore, effective blade shape 
is suggested based vortex-blade interaction studies. 
 
Keywords: Vertical axis wind turbine, Darrieus, vortex interaction, camber, TSR (tip speed ratio). 
 
1. Introduction 
The most economical resource of renewable energy, 
i.e., wind energy, has been highlighted not only as a 
driving force of the next-generation industry but for 
the strategic importance in terms of environmental 
countermeasure and energy security reinforcement. 
Major reasons for wind energy growth lie in price 
competitiveness to other clean energy resources and 
its effect on the environment benefits. However, urban 
area has lower average wind resources compared to 
ocean and high mountain. Moreover, designing 
VAWT (vertical axis wind turbine) has technical 
difficulties in the areas of high noise, vibration and 
low power efficiency. Therefore, significant technical 
improvement is needed in terms of aerodynamic 
efficiency and structural stability. 
VAWT has been operated mostly by two mechanisms: 
Darrieus type rotated by its lift force, and Savornius 
type by its drag force. The advantages of VAWTs are 
that it has no degradation regardless of wind direction 
and low cut-in speed which are key factors in the 
usage of a small-sized generator. Darrieus type wind 
turbine has basically two configurations which are 
Tropskien type with a curved wing and straight type 
with a vertical wing. Curved wing type blade allows 
less blade tension that is beneficial in the system 
design. In contrast, straight type blade causes a large 
centrifugal load bending moment. 
Aerodynamic analysis of vertical axis wind turbine 
system is very basic and the most important research 
area. Because aerodynamic of the Darrieus wind 
turbine is rather complex. The reason of complex is 
VAWT is operating under both stalled and unstalled 
conditions with various effective angles of attack. 
Aerodynamic Study of Small Scale Vertical Axis Wind Turbine System in Urban Circumstance 
 
200
Simao Ferreira [1] investigated physics and 
appropriate turbulence modeling of rotating blade in 
VAWT through the PIV (particle image velocimetry) 
experimental system and numerical method. 
Moreover, Ferreira investigated accurately 
modeling the separated shed wake resulting from 
dynamic stall through appling DES turbulence model. 
Wang [2] and Tedidiah [3] studied dynamic stall in 
low Reynolds number flow; Fujisawa [4] studied flow 
field around a Darrieus wind turbine blade in dynamic 
stall by flow visualization and PIV measurement in 
stationary and rotating frames of reference. Through 
PIV experimental measurement, Fujisawa described 
the successive shedding of two pairs of stall vortices 
from the blade moving upstream in various tip-speed 
ratios. Yamada [5] evaluated performance of various 
design parameters such as blade camber and thickness. 
Similar with above study, Deshpande [6] analysis 
aerodynamic characteristic using symmetric blade and 
unsymmetric blade with various solidity. However, in 
the previous research, only the final output is 
explained due to change of geometry [7-11]. These 
studies does not offer enough data for understanding 
aerodynamic characteristic of VAWT and 
optimization process. Instead, a more rigorous 
fluid-dynamics analysis should be employed for 
understanding aerodynamic of VAWT [12]. The paper 
is organized as follows : Numberical method and 
blade configuration is explained in Section 2; The 
analysis of location and magnitude of the vortex 
induced by rotating single blade and the vortex 
interaction with rotating multiple blade is also 
investigated in Section 3; The results provide an idea 
of the optimized shape of the blade camber based on 
the torque occurred on the VAWT in Section 4. 
2. Numerical Method and Approach 
2.1 Numerical Method 
Two-dimensional incompressible CFD simulations 
are conducted to comprehend qualitatively the camber 
effect in vertical wind turbine systems. Navier-Stokes 
equations are used for simulation as expressed in the 
followings: 
+ = 0x yU v (1) 
2 1+ = -( ) - ( ) + ( + )
Ret x x y xx yy
U P u uv u u (2) 
2 1+ = -( ) - ( ) + ( + )
Ret y x y xx yy
V P uv v v v (3) 
where u, v represents the velocity component each 
direction and p denotes the pressure. 
DNS (direct numerical simulation), DES (detached 
eddy simulation), and LES (large eddy simulation) has 
been developed those are able to capture the smallest 
eddy movement in viscos sub-layer However, the 
purpose of this study is determine that how each of 
design parameter effect to entire system of VAWT 
through a significant amount of the simulation. 
Therefore, considering Computing time, SST (shear 
stress transport) turbulence model is adopted. An 
immediate benefit of the SST turbulence model is that 
it more accurately predicts the spreading rate of both 
planar and round jets [13]. It is also likely to provide 
superior performance for flows involving rotation, 
boundary layers under strong adverse pressure 
gradients, separation, and recirculation. 
The TSR (tip speed ratio) is the ratio between the 
rotational speed of a blade at the tip and the velocity 
of the wind, defined in Eq. (4) as 
Rωλ=
U
 (4) 
in which R is the radius of the rotation,  is the 
angular velocity of the turbine and U∞ is the free 
stream velocity. 
2.2 Analysis for the Camber Effect 
Darrieus VAWT is operated mainly by lift force 
rather than drag force. Lift over drag ratio is strongly 
affected by its blade camber [14, 15]. In this study, 
five models (NACA0015, NACA2415, NACA4415, 
NACA6415, and NACA8415) are chosen to be 
numerically analyzed. 
Particularly, the relation between the camber and 
the power needs to be quantitatively analyzed. Fig. 1 
explained geometry of blade. 
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Fig. 2 shows the schematic of wind turbinesystem. 
Free stream flows from left to right and the system is 
rotating clockwise. As the blade rotate causes 
increment of angle of attack, the smallest lift force is 
computed at the lowest angle of attack, 0° and the 
biggest lift force is computed at 45°. After the critical 
angle of attack, the lift force will be gradually reduced 
until 135°. However, lift force is main source of 
turbine rotation, and lift is strongly dependent on the 
 
NACA0015 
NACA2415 
NACA4415 
NACA6415 
NACA8415 
Fig. 1 Geometry of blade with various cambers. 
 
 
Fig. 2 Simulation model of VAWT (vertical axis wind 
turbine). 
angle of attack at each location. Thereby, TSR is very 
important in determining the angle of attack and 
resulting power generation performance. 
Fig. 3 shows the computational domain and 
boundary condition [16]. The blade zone is rotated by 
moving mesh function. Hole of geometry divided into 
two parts, such as sub grid1 which is dynamic mesh 
zone and sub-grid 2 what is fixed mesh zone. The 
height of the first row of cells in sub-grid 1 is set as a 
distance to the wall of 10-3 of chord length and this 
corresponds Y+ <2 for the purpose of accurately 
resolving the viscos layer. 1mm grids are generated at 
zone interface and 700,000 grid points are generate 
with 1.05 grow rates. 
There are 450,000 grid points in the computational 
domain which are unstructured grids. Making the 
uniform free stream velocity boundary condition at the 
inlet accurate enough and allowing a full development 
of the wake. Opening condition is set at outlet 
boundary condition for as much as possible to match 
the natural states [17]. Fig. 4 explained grid generation 
in moving zone and boundary of blade, respectively. 
The upper surface of the blade is set facing the hub 
of the wind turbine system. The computation is 
executed under the assumption of total 3 blade and 5 
blade. 
2.3 Vortex Interaction 
The aerodynamic performance of Darrieus VAWT 
such as torque, lift force and drag force is studied for 
appropriate design of the wind turbine system. The 
aerodynamic performance is significantly influenced 
by vortex shedding from the leading edge and trailing 
edge of the rotating blade. Vortex interaction takes 
place as the shedded vortex hits the other following 
blades in multiple blades system. In this study, the 
influence of vortex is analyzed by the rotation of a 
blade or multiple blades system. Three kinds of 
two-dimensional blade system models are suggested 
to be tested. It has been known that the blade camber 
strongly affects the vortex formation, its shedding, and 
Aerodynamic Study of Small Scale Vertical Axis Wind Turbine System in Urban Circumstance 
 
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Fig. 3 Computational Domain and boundary condition of VAWT. 
 
 
Fig. 4 Mesh structure in blade zone. 
 
its interaction with the following blades. 
The first wind turbine model is given as the single 
blade rotates around a remote rotation axis which is 
similar to flying blade attached to a rotating string. 
The geometric configuration of the simulation model 
is as follows: the chord length is 220 mm; the rotation 
radius is 690 mm and blade angular velocity (ω) is 
195 rpm. The free-stream speed is set as 10 m/s. 
Thereby, the TSR is set as 6. In the study, the 
aerodynamic performances are numerically computed 
in terms of local flow features such as vortex, and 
global characteristics such as torque. Although the 
simulation is carried out by unsteady condition, the 
periodic characteristics such as lift and drag ratios are 
stabilized after 30 rotations. The numerical results of 
lift force coefficient (Cl), drag force coefficient (Cd), 
torque, and power coefficients are obtained in this 
research. 
Secondly, multiple blades wind turbine system (3 
and 5 blade system) is modeled. In this study, the 
aerodynamic performance is computed with a strong 
focus on vortex interaction between the blades. As 
described earlier, the shedded vortex gives the distort 
in aerodynamic quantities like pressure on the surface 
of following blades. The vortex intensities and 
interaction are observed to depend on the camber 
magnitude. In addition, the vortex effect on the global 
performance of the whole wind turbine system is 
attempted to analyze. The geometry and flow 
conditions are given the same as in the first model. 
3. Computational Result and Analysis 
3.1 Vortex Formation and Interaction 
The single blade rotation causes increment of angle 
of attack and thereby the separation starts to take place 
at a certain point. Leading edge vortex begins to 
develop when the angle of attack is increased at 45°. 
Fig. 5 shows vortex occurred at 90°. At the trailing 
edge, the flow is separated and shear region is formed, 
but vortex is not yet developed. The complete vortex 
formation is achieved at the angle of 135°. However, 
the magnitude of the camber determines the strength 
of the vortex. 
The NACA 0015 case allows maximum 170%, The 
NACA 4415 case allows 290% and The NACA8415 
case allows 350% strength of vortex intensity in 
vortex core. While the strength of the vortex is 
dependent on the camber, the size of vortex is 
independent of the camber. 
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Fig. 5 Vortex intensity of three single blades: (a) NACA0015, (b) NACA4415, (c) NACA8415. 
 
Fig. 6 and Table 1 explain the performance 
evaluation of single blade system. As the camber of 
blade increases, max torque also increases. The reason 
of above physics is explained in Fig. 5. An area of 
pressure coefficient (Cp) curve indicates lift force at 
blade location at 90° azimuth angle. As increasing the 
camber, the area of Cp curve is increased as well at 
90°. 
Table 1 Aerodynamic performance by single blade. 
 Mean torque
(N·m) 
Max torque 
(N·m) 
Min torque 
(N·m) 
Standard 
deviation
NACA0015 1.091 17.919 -11.876 6.787 
NACA2415 1.019 18.719 -11.076 7.024 
NACA4415 0.361 22.272 -15.385 8.868 
NACA6415 0.094 24.764 -15.050 9.731 
NACA8415 -0.036 27.255 -15.019 10.595 
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Fig. 6 Torque variation of single blade. 
 
However, after 180° azimuth angle, torque is 
sharply decreased and fluctuated with low frequency. 
On the other hands, blade vortex interaction of the 
wakes with the downstream passage of the blade has 
different phenomenon in multiple blades system. 
First of all, the location of vortex interaction is 
different. Leading edge vortex and previous blade’s 
trailing edge vortex is interacted at upper side of blade. 
Naturally, low pressure region has developed at upper 
side of blade. This low pressure core is located on 
straight line of effective angle of attack direction. 
Which means this low pressure core can help to 
generate torque in 3 blade system. However, vortex 
interaction that is coupled by 3 blade is located on 
bottom of blade in 5 blade system. 
Fig. 7 indicates that the vortex from following 
blades overlaps with the vortex from preceding blades. 
As a results, this vortex interaction lead to positive 
effect on 3 blade system. Therefore, torque is 
recovered at 100° azimuth angle where is minimum 
torque generated in W/O vortex interaction. On the 
other hands, Vortex interaction is negative effect in 5 
blade case. Huge turbulence area is take position 
upper side of moving zone. Therefore, mean torque is 
largely lower than W/O vortex interaction case. 
Secondly, the area of turbulence region is different. 
More large turbulence region takes position in upper 
side of rotating area in 5 blade system than 3 blade 
system. As an above explanation,appropriate 
turbulence region leads to wake superpose around 
blade and generate vortex interaction. However, blade 
in overwhelmed turbulence area can not generate 
torque. Therefore, mean torque is largely lower than 
W/O vortex interaction case in 5 blade system. Figs. 8 
and 9 show mean torque compare W/vortex 
interaction with W/O vortex interaction. 
3.2 Power Changes According to the Number of Blade 
The power variation along the camber change in 
case of single-blade is shown in Fig. 6. Numerical 
analysis proves that the maximum peak value of the 
torque arise from the symmetric blade (NACA0015) is 
about 18 N·m which is less than those of the 
asymmetric blade (NACA4415, NACA8415), i.e., 22 
N-m and 27 N-m. This is the evidence of that higher 
L/D leads to better power efficiency at Darrieus 
VAWT. 
3 blade and 5 blade power is explained in Figs. 10 
and 11 respectively. Also detail information of each 
power is written in Tables 2 and 3. 
Fig. 12 shows the pressure distribution at 45°, 
corresponding to the highest power output. At 45°, 
most favorable angle of attack is set and bigger lift 
force is made. 
The Darrieus wind turbine system is operated by lift 
force. Therefore, the torque peak point can be 
observed at the angle. The contour indicates that 
higher camber causes greater pressure on lower surface 
Aerodynamic Study of Small Scale Vertical Axis Wind Turbine System in Urban Circumstance 
 
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Fig. 7 Vortex intensity of multi blades system. 
 
 
Fig. 8 Torque variation of 3 blade vortex interaction and W/O vortex interaction. 
 
 
Fig. 9 Torque variation of 5 blade vortex interaction and W/O vortex interaction. 
 
which could predict greater power output. In the 
meantime, Fig. 13 shows the pressure distribution at 
135°, the lowest power output. There is little 
dependence on the magnitude of the camber. Leading 
edge vortex that is shedded after angle of attack, 90° 
and trailing edge vortex are superposed at 135°. 
Because of the strong vortex, the lift force is affected 
from the vortex interaction and vortex interaction 
causes lower system efficiency. 
 As shown in single blade system, the torque has 
sharply decreased from 60° to 135°. It is mainly due to 
the leading edge vortex formation. In other words, the 
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Fig. 10 Torque variation of 3 blade. 
 
 
Fig. 11 Torque variation of 5 blade. 
 
Table 2 Aerodynamic performance by 3 blade. 
List NACA0015 NACA4415 NACA8415
Mean torque 6.78 N·m 4.84 N·m 3.99 N·m 
Standard deviation 14.34 26.75 28.14 
 
Table 3 Aerodynamic performance by 5 blade. 
List NACA0015 NACA4415 NACA8415
Mean torque 3.4 N·m 0.98 N·m -1.38 N·m 
Standard deviation 2.52 3.74 5.37 
 
vortex formation had negative effect on the power 
efficiency. The torque of VAWT with multiple blade 
system with various camber length is shown in Figs. 8 
and 9. Unlike the single blade rotation, the torque 
curve show more periodic pattern. It is because 3 and 
5 blade are adopted and thereby the frequency is 5 
times higher than in single blade case. 
Bigger torque can be obtained in the symmetric 
blade case (NACA 0015) rather than asymmetric 
blade case (NACA 4415, NACA8415) in both 3 blade 
and 5 blade. Less camber produces positive torque, 
which means overall more power and therefore the 
wind turbine system should rotates more faster (high 
rpm setting is needed). However, more stable periodic 
pattern in torque is allowed in bigger camber, which is 
favorable to stable operation and less fatigue of the 
turbine system. The deviation from perfect periodic 
pattern can be understood as the vortex interacts with 
the blade in a different fashion at different phase. In 
short, the vortex interaction behaves as nonlinear 
pattern in torque computation. 
While 45° is the rotation angle for peak torque value 
in single blade rotation, in multiple blades system, 45°
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207
 
Fig. 12 Pressure distribution at 45° of single blade system: (a) NACA0015, (b) NACA4415, (c) NACA8415. 
 
 
Fig. 13 Pressure distribution at 135° of single blade system: (a) NACA0015, (b) NACA4415, (c) NACA8415. 
 
is not any more peak point of torque. In addition, 
contrast to the single blade case finding that bigger 
camber yields bigger torque, the multiple blades 
system show opposite pattern. All these phenomena 
related to camber, can be explained as nonlinear 
interaction between vortex and following blades. This 
vortex interaction is shown in Fig. 14. The extent and 
strength of the vortex are strong enough to degrade the 
power, which allows significant deviation from the 
single blade case. 
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Fig. 15 shows the pressure distribution and vortex 
intensity at 45°, lowest power output by multi blades 
system. Bigger pressure effects on the turbine system 
and bigger lift force occurs at the bottom of cambered 
blade than symmetric. 
Therefore, bigger torque is generated by cambered 
blade. But, the power is degenerated on account of the 
vortex intensity. Fig. 16 shows the vortex intensity at 
45°. The vortex is stronger according to the height of 
camber. 
 
 
Fig. 14 Vortex intensity at 120° of 3 blade system: (a) NACA0015, (b) NACA4415, (c) NACA8415. 
 
 
Fig. 15 Pressure distrubution at 45° of 5 blade system: (a) NACA0015, (b) NACA4415, (c) NACA8415. 
Aerodynamic Study of Small Scale Vertical Axis Wind Turbine System in Urban Circumstance 
 
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Fig. 16 Vortex intensity at 45° of 5 blade system: (a) NACA0015, (b) NACA4415, (c) NACA8415. 
 
4. Conclusions 
In this study, five cases of blade models and three 
types of blade number are applied in Darrieus type 
wind turbine. Some new aerodynamic features are 
observed as follows: 
(1) The results of the spinning single blade indicate 
that small camber produces weak vortex. Bigger 
camber increase lift force in the front rotation (less 
than 90°), however, after 90° rotation, the lift rather 
decays more compared to less camber. 
(2) In the result of single blade rotation system, the 
higher camber provides greater power output. The 
larger L/D ratio leads to bigger power in Darrieus 
wind turbine. The vortex induced from single blade 
rotation does not affect the power output since the 
vortex moves out without any interaction. 
(3) In multiple blades system, the induced vortex 
influences the aerodynamic performances such as 
torque and power in a different fashion to single blade 
case. In this system, a vortex shedded from prior blade 
is superposed with another vortex on the following 
blades, leading to the formation of stronger vortex. 
However, bigger camber yields stronger vortex 
strength however, as the increased maximum torque, 
standard deviation is also increased therefore, total 
mean torque is decreased. Conclusively the total the 
power is decreased. In contrast, smaller camber allows 
the steady periodic pattern in torque and bigger power 
can be obtained. 
(4) As increasing the number of blade, the strength 
of secondary vortex that is generated at 330° is getting 
weaker cause of slip stream from the previous blade. 
Therefore, amplitude of Mean Torque is getting 
decreased. 
(5) Vortex interaction effects are taken different 
position in various numbers of blades. Appropriate 
vortex area and vortex interaction have positive effect 
to generate torque. However, as increase number of 
blades, vortex area and interaction is getting increased. 
This vortex interaction influences on the degrading 
system output. Accordingly,the most important 
variation is vortex interaction in multiple blades wind 
turbine system. However, more study needs to be 
done for more accurate analysis of wind turbine 
system with special interest in TSR and periodic 
vortex formation pattern. 
Aerodynamic Study of Small Scale Vertical Axis Wind Turbine System in Urban Circumstance 
 
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Acknowledgments 
This research was supported by the New & 
Renewable Energy of the KETEP (Korea Institute of 
Energy Technology Evaluation and Planning) grant 
funded by the Korea Government Ministry of 
Knowledge Economy (Grant No. 20133010031751). 
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