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CFD Analysis of Different Blades in Vertical Axis Wind Turbine 1Dr T.Mothilal,
2P.Harish Krishna,
3G.Jagadeesh Babu,
4Ashwin Suresh,
5K.Baskar
6S.Kaliappan
7M.D.Rajkamal
1Professor Department of Mechanical Engineering, Velammal Institute of Technology, Chennai 601204,
2,3,4,5 UGStudents, Department of Mechanical Engineering, Velammal Institute of Technology, Chennai 601204,
India6Associate Professor, Department of Mechanical Engineering, Velammal Institute of Technology , Chennai-601204, India. 7Assistant Professor, Department of Mechanical Engineering, Velammal Institute of Technology, Chennai-601204, India.
Abstract–The average wind velocity in urban areas
is not sufficient to operate Horizontal Axis Wind Turbine
(HAWT), hence Vertical Axis Wind Turbines (VAWT)
are sought after. Vertical Axis Wind Turbine is of
Savonius (Drag) type and Darrieus (Lift) type. The
present study is focused on the comparison of the
coefficient of performance (COP) of Savonius and
Darrieus types of Vertical Axis Wind Turbine. The above
mentioned VAWTs are numerically analyzed using
ANSYS Fluent- Computational Fluid Dynamics (CFD)
software. The design of the blades of both the turbinesis
chosen such that it is optimized for the best output for the
given input. For the same input parameters, the output
parameters of the wind turbines are obtained separately
and are compared. This comparison provides a basis for
choosing the type of VAWT to be implemented
according to the function.
Keywords – VAWT, Darriues, Savonius, CFD,
Ansys Fluent.
Nomenclature:
HAWT Horizontal Axis Wind
Turbine,
VAWT Vertical Axis Wind
Turbine,
COP Coefficient of
Performance,
CFD Computational Fluid
Dynamics,
NACA National Advisory
Committee of
Aeronautics,
d Diameter of blades
e Gap (eccentricity)
D Rotor diameter
h Rotor height
𝑢 Flow velocity,
𝜐 Kinematic viscosity
w Specific thermodynamic
work (per unit mass)
I. INTRODUCTION:
A Wind turbine is a mechanical device which
converts the wind’s Kinetic energy into the
Electrical energy. Wind turbines are manufactured
widely in vertical and horizontal axis types. Wind
turbines can rotate about either a horizontal or a
vertical direction. Wind energy is the most readily
available and the cleanest source of renewable
energy. Thus wind turbines are popular and widely
used in renewable energy power production.
VAWTs are a type of wind turbine where the main
rotor shaft is set transverse to the wind [1]. VAWT
have the advantages of being omni-directional i.e.
they have the ability to accept winds from all
directions without yawing and the ability to provide
rotary drive to a fixed load [2]. In comparison to
HAWT, VAWT does not require winds at high
speed to rotate and are less noisy. Hence VAWT is
emerging popularly for domestic use in homes
(roof tops) and offices. VAWT are generally
further classified into various types such as
Savonius type, Darrieus type etc. The basic version
of Savonius rotor has an s-shaped cross-section
formed by two semi-circular blades with a small
overlap between them [3]. The Darrieus rotor
consists of a number of curved aerofoil blades
mounted on a vertical rotating shaft or framework.
High fidelity simulation has become the virtual
wind tunnel of today. With the increasing
complexity of problems being analyzed along with
the high costs of experimental setups, high fidelity
numerical analysis tools, such as Computational
Fluid Dynamics (CFD) provide invaluable insight
into the wind turbine flow dynamics. They provide
valuable assistance in both design and analysis
allowing better decisions to be made. CFD has
been used widely to analyze the performance of
VAWTs. [4]
Early assessments of the CFD technology for
Darrieus rotor aerodynamics, aiming at thoroughly
investigating the complex fluid mechanics of these
machines, made use mainly of a two-dimensional
approach. A 2D simulation of H Darrieus can
provide quite accurate estimations of both the
overall performance and the flow field description
around the rotor with reasonable computational
cost, on condition that proper settings are applied.
In case of the medium-size rotor and low tip-speed
ratios, the use of a transitional model forturbulence
closure is suggested by Alessandro Bianchini. [5]
Savonius type of wind turbine is also called as
S-rotor wind turbine. It was invented and patented
by Finnish engineer, Sigurd J. Savonius in 1931.
International Journal of Pure and Applied MathematicsVolume 119 No. 12 2018, 13545-13551ISSN: 1314-3395 (on-line version)url: http://www.ijpam.euSpecial Issue ijpam.eu
13545
The turbine made of two halves cylinder and then
moving the two semi cylinder surfaces sideways
along the cutting plane like the letter ‘S’[6].It is
suggested that a Savonius rotor should not be
referred to solely as a drag-driven rotor, but as a
VAWT whose torque is generated by the combined
effects of lateral and longitudinal forces [7]. A lot
of performance improvement studies have been
done on Savonius rotor in the past by various
researchers as the efficiency of a conventional
Savonius rotor is poor. The importance of reducing
greenhouse gases leads to research more
sustainable energy resource and to investigate more
efficient technologies.
In this article, a dynamic study was conducted
to simulate the transient behavior of the Savonius
and darrieus rotor. -
II. DESIGN OF BLADES
A. Darrieus Rotor
The Darrieus wind turbine consists of a
number of curved aero foil blades mounted on a
vertical rotating shaft. The curvature of the blades
allows the blade to be stressed only in tension at
high rotating speeds. When the Darrieus rotor is
spinning, the aero foils areadvancing through the
air in a circular path.The energy is taken from the
wind by a component of the lift force working in
the direction of rotation. Aerodynamic modelling is
designed using software tools by considering
NACA0012 aero foil whose chord length is 0.12
m.[8]
Fig. 1. NACA0012 air foil
B. Savonius Rotor
The Savonius turbine is one of the simplest
turbines. Aerodynamically, it is a drag-type device,
consisting of two or three blades. Because of
the curvature, the blades experience less drag when
moving against the wind than when moving with
the wind. The differential drag causes the Savonius
turbine to spin. A number of blades will influence
the rotation of therotor of wind turbine models. The
three blades wind turbineproduces higher rotational
speed and tip speed ratio than that two and four
blades. [6]
Dimensions of the blades of Savonius wind
turbine model are diameter of blades (d)= 200 mm;
gap ( e ) = 0.15 x r = 0.15 x 100 =15 mm; rotor
diameter ( D ) = 200 + 200 – 30= 370 mm; [6].
Fig 2. Savonius 3 cup rotor
III. NUMERICAL ANALYSIS
The process of this study is categorized into 3
parts. For simulation of blades, CFD is used which
includes 3 stages pre-processing, simulation, and
post processing.
For an isothermal, 3D incompressible flow,
the governing equations are the conservation
ofmass and the conservation of momentum given
by equations (1) and (2) respectively. Since the
flow velocities in the domain are much smaller than
sound velocity,it can be assumedthat the density
remains constant throughout the flow field.
∇. 𝑢 = 0 …(1)
𝜕𝑢
𝜕𝑡+ 𝑢. ∇ 𝑢 − 𝜈∇2𝑢 = −∇𝑤 + 𝑔 … (2)
Where,𝑢is the flow velocity,𝜐 = 𝜇
𝜌0 is the
kinematic viscosity, w is the specific
thermodynamic work (per unit mass), 𝑔is
acceleration due to gravity.
Creo 3.0 is used for creating a2-Dmodel of
blades. This is used as pre-processorto run the
simulation in ansys fluent.Fluent is used to analyse
the fluid flow properties such as distribution and
separation of the velocity through and around
turbine blade, and variables to describe the fluid
flow.Results of theanalysis are post-processed in a
quantitative and qualitative manner.
IV. MODELING OF VAWT BLADES
A. Darrieus rotor
After selecting the airfoil, its co-ordinates are
imported to creo software for designing the air foil
structure.
International Journal of Pure and Applied Mathematics Special Issue
13546
Parameter Value with limit
Chord length 120mm
Rotating diameter 500mm
Table 1. Design parameters of darrieus rotor
The modelled Darrieus VAWT rotor NACA 0012
is shown in fig 3.
Figure 3. Darrieus rotor
B. Savonius rotor
The model of savonius wind rotor was
designed with following parameters.
Parameter Value with units
Diameter of blade(d) 200mm
Gap (e) 15 mm
Rotor diameter (D) 370 mm
Table 2. Design parameters of Savonius rotor
The modeled Savonius rotor with above
mentioned dimensions is shown in figure 4.
Fig.4. Savonius Rotor
V. COMPUTATIONAL MODELING AND
ANALYSIS
The two dimensional computational domain of
the two bladed Darrieus and three buckets Savonius
rotor along with the boundary conditions is shown
in Fig. 5(a). Fig. 5(b). The 2D computational model
of rotors was generated in Ansys fluent of the CFD
software such that the dimensions of the rotor are
exactly same as those of the dimensions mentioned
above.
Velocity inlet is considered on the face AB,
pressure outflow is considered on the outlet CD,
symmetry condition is considered for the free slip
(faces AC & BD). And the freeslip condition is
considered on the blades and buckets. The inlet
velocity considered is 6 m/s, and the solver
calculates the pressure at the outlet by default due
to the pressure outlet condition specified on the
outlet face. Any wind speeds up to the value of 10
m/s can be called low wind speed. [9]
Fig. 5(a). Blade arrangement of Savonius rotor
International Journal of Pure and Applied Mathematics Special Issue
13547
Fig. 5(b). Blade arrangement of Darrieus Rotor
The Darrieus rotor is arranged in the form of 2
blades at 180 deg.,opposite in direction. The
savonius is arranged in the form of 3 cup rotor at an
angle of 120 deg between each blade.
For both darriues and savonius rotor, fine
meshing is done in Ansys with aqaud mesh having
10layer inflation near the bladesand domain.
Table 3. Meshing values
Fig.6 (a). Darrieus blade mesh
Fig.6 (b). Savonius blade mesh
The pressure based solve is used to the steady
state condition. The analysis is run by K – omega
SST turbulence model, the material chosen for the
blades is aluminium the boundary conditions are
varied by varying the inlet velocity of the air. The
rotating domain is given moving wall condition
with an angular velocity respect to the inlet
velocity.
The coupled method is used as the pressure
velocity relation equation. The solution is
initialized by the hybrid method. Thepressure
momentum, turbulent kinetic energy,and specific
dissipation rate are got by second order upwind
equation.
The flow courant number is 200 and explicit
relaxation factor for momentum and pressure is 0.5.
The following are the obtained contours,
Fig. 7. Darrieus Velocity Contour
Fig. 8. Darriues pressure contour
Type of
blade Darrieus
Savonius
Domain Nodes Element
s Nodes
Element
s
Rotating
domain 5770 2801
3812 1802
Surface
body
28029
2 139333
28094
2
139555
All
Domain
s
28606
2 142134
28475
4
141357
International Journal of Pure and Applied Mathematics Special Issue
13548
Fig. 9. Savonius rotor velocity contour
Fig. 10. Savonious rotor Pressure contour
VI. RESULTS AND DISCUSSION
From the analysis, various results are obtained
and according to that graphs are plotted.
Graph 1. TSR vs COP
Graph 2. Velocity vs COP
The following are the discussions made from the
above graph and obtained results,
1. The Savonius type VAWT shows better
starting torque at low wind speed.
2. The increasing wind speed and tip speed
ratio are better for the Darrieus type of
VAWT.
3. For the Domestic purpose, Savonius type
VAWT shows the better result as the
average wind speed will produce regular
power supply.
4. For regions with higher wind velocity,
Darrieus type VAWT proves to be more
efficient.
VII. CONCLUSION
From the results obtained, it can be concluded
that both the type of VAWT depends on the
application, wind characteristics, etc. Savonius
works under drag forces. Darrieus operates because
of lift forces. Savonius is able to self-start but
darrieus cannot start. [10]
A Darrieus is a high speed, low torque
machine suitable for generating alternating current
(AC) electricity. Darrieus type requiresa manual
push, therefore, some external power source is
required to start turning as the starting torque is
very low.
The Savonius uses drag and therefore cannot
rotate faster than the approaching wind speed. An
area that has strong and gusting winds or when you
need a unit that self-starts, this is the best type [11].
A. Ghosh et.al found that the combined darrieus
and savonius had increased concentration of
vortices. There is a velocity shortage in the
savonius rotor in comparison to darrieus for which
darrieus be able to extract more energy
compensating each other [12]. Therefore a
combination of both is a good decision for low
power application.
REFERENCE
0
0.1
0.2
0.3
0.4
0.5 1 1.5 2 2.5 3 3.5 4
CO
P
TSR
COP VS TSR
DARRIEUS SAVONIUS
0
0.1
0.2
0.3
0.4
5 6 7 8 9 10 11
CO
P
VELOCITY m/s
COP VS VELOCITY
DARRIEUS SAVONIUS
International Journal of Pure and Applied Mathematics Special Issue
13549
1. Wind turbine. Retrieved from
https://en.wikipedia.org/wiki/Wind_turbine
2. Zhai Yuyi, Zeng Decai, Liu Liang, Tang
Wenbin and Luo Jun, 2013. The Design of
Vertical Axis Wind Turbine Rotor for
Antarctic. Information Technology Journal, 12:
604-613.
3. Ivan Dobreva, Fawaz Massouha, ‘CFD and
PIV investigation of unsteady flow through
Savonius wind turbine’, Energy Procedia 6
(2011) 711–720.
4. Sayyad Basim Qamar, Isam Janajreh ‘A
comprehensive analysis of solidity for
cambered darrieus VAWTs’, international
journal of hydrogen energy (2017) 1 – 2.
5. V. D’Alessandro*, S. Montelpare, R. Ricci, A.
Secchiaroli, ‘Unsteady Aerodynamics of a
Savonius wind rotor: a new computational
approach for the simulation of energy
performance’, Energy 35 (2010) 3349 – 3363.
6. Frederikus Wenehenubuna, Andy Saputraa,
Hadi Sutantoa, ‘An experimental study on the
performance of Savonius wind turbines related
with the number of blades’, Energy Procedia
68 ( 2015 ) 297 – 304
7. Placide Jaohindy, Sean McTavish, François
Garde, Alain Bastide ‘An analysis of the
transient forces acting on Savonius rotors with
different aspect ratios’, Renewable Energy 55
(2013) 286 – 295
8. Kalakanda Alfred Sunny and Nallapaneni
Manoj Kumar ‘Vertical axis wind turbine:
Aerodynamic modelling and its testing in wind
tunnel’,Procedia Computer Science 93 (2016)
1017 – 1023.
9. Ghosh, A., Biswas, A., Sharma, K. K., &
Gupta, R. (2015). Computational analysis of
flow physics of a combined three bladed
Darrieus Savonius wind rotor. Journal of the
Energy Institute, 88(4), 425–437.
https://doi.org/10.1016/j.joei.2014.11.001
10. https://www.researchgate.net/post/which_kind
_of_vertical_axis_wind_turbine_is_better_Sav
onius_or_Darieus
11. https://www.conserve-energy-
future.com/verticalaxiswindturbines.php
12. Ghosh, A., Biswas, A., Sharma, K. K., &
Gupta, R. (2015). Computational analysis of
flow physics of a combined three bladed
Darrieus Savonius wind rotor. Journal of the
Energy Institute, 88(4), 425–437.
https://doi.org/10.1016/j.joei.2014.11.001.
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