Effect of Blade Tip Geometry on Vertical Axis Wind Turbine Performance

Benha University
Shoubra Faculty of Engineering
Mechanical Power Department
Graduation Project - 2019
Investigate the effect
of blade tip geometry
on the performance
Vertical Axis Wind
Turbine
By
Nada Gamal Sayed Ali
Sayed Mohamed Ali
Tadrws Makram
Mohamed Sabry
Yosef Hamda
Abanob Adel

ABSTRACT
Fluids always flow from high-pressure region to low pressure region, this is the principle of the
airflow around the airfoil and this can create vortices at the trailing edge which reduce the lift
and increase the induced drag and turbulence around the blade turbine. Winglets are a
different method to change the geometry of the blade tip which in turn can affect the whole
performance of the turbine by reducing the vortices or by enhancing the turbine power
coefficient. This project experimentally and numerical investigates the effect of winglets on the
aerodynamic performance of the vertical axis wind turbine (H-Rotor).

Acknowledgements
Thanks are extended to
Shoubra Faculty of engineering, Benha University, Egypt
We would like to give special thanks to our great supervisors Dr. Samir Ayad Dr. Ibrahim
Shahin have been very generous in providing guidance , keeping us grounded and maintaining
our interests in wind energy.
We would like to thank Eng. Mina Murad for the amazing support in technical and practical
work.

Contents
1. Introduction 1
1.1 Background 2
1.2 Historical Background 4
1.3 Wind turbine 6
1.4 Vertical axis wind turbine (VAWT) 7
1.4.1 Savonius turbines 7
1.4.2 Darrieus turbines 8
1.4.3 Straight-bladed turbines 9
1.5 Theoretical Wind power 10
2. Theoretical Design of the turbine 11
2.1 The chosen wind turbine concept 11
2.2 Design parameters 12
2.2.1 Swept area 12
2.2.2 Tip speed ratio 12
2.2.3 Number of blades 13
2.2.4 Solidity 13
2.2.5 Blade chord 13
2.2.6 Initial angle of attack 14
2.2.7 Reynolds number 15
2.2.8 Aspect ratio 16
3. Aerodynamics of Wind Turbine 17
3.1 General Overview 17
3.2 General Concepts of Aerodynamics 17
3.2.1 Airfoil terminology 17

3.2.2 Lift, drag and non-dimensional parameters 18
3.3 Major aerodynamic challenges of small SB-VAWT 19
3.4 Self-starting problem of SB-VAWT 20
3.5 Desirable aerodynamic characteristic of the airfoil 20
4. Experimental test rig setup 27
4.1 Wind tunnel test 27
4.2 Test Setup 28
4.3 Measurement instruments 29
4.4 Installation 31
4.4.1 Mechanical components 31
5. Computational model 38
5.1 Geometry 38
5.2 Mesh 41
5.3 Setup 43
6. Results 44
6.1Experimental work results 44
6.2CFD Results 46
7. Conclusions 57
8. References 57
Appendix

1
Introduction
Wind power has always been used to sail ships the people have developed their usage of
wind by using it to run the windmills. Then, the electrical energy has used in several aspects of
life and thanks to the low cost machines which operate bye fossil fuel. Researches and scientists
do their best and put their effort to increase the amount of electrical power generated from a
green energy such as solar energy, wind energy Furthermore, green energy will reduce the effect
of fossil fuels on the climate. Another reason to use the green energy is the depression of the
fossil fuel. Recently, the demand of energy continuously grows; therefore the wind power has
become one of the most energy sources that have been being studied thoroughly for years. The
wind energy, with respect to other renewable energies, is usually available everywhere and
requires low investment. During the last few years, different models of wind turbines have been
designed and tests with different axis of rotation.
The current work doesn’t aim to optimizing the rotor dimensions of SB-VAWT, instead of that
we aim to study the effect of modified blade tip (winglets) on the entire turbine performance
specially tip vortex control. So we followed Howell2010 [1] with the following turbine
specification
Airfoil section NACA0022
Blade height 0.4 m
Rotor diameter 0.6 m
Cord length 0.1 m
Solidity 0
Number of blades 3
Blade aspect ratio 4
Rotor aspect ration 1.3
Then compare the experimental and numerical results of the base model with the modified
model to see the effect of changing the blade tip geometry.

2
1.1 Background
The current installed capacity of about 1,190 MW –which considered less than 4% of the
electricity consumption in Egypt [2] the wind power, is not properly exploited in Egypt. But the
government states that the future plane of wind energy in Egypt is to increase the electrical
production from the wind energy to be: 2750 MW in 2020 (6%). To install the wind turbines in
any region, first we should know the wind distribution of this region. The Wind Atlas for Egypt
shows the intensity of wind in deffrient sites. Figure [1] the New and Renewable Energy
Authority (NREA) and the Egyptian Meteorological Authority (EMA) in Cairo published this
wind atlas [3].
Fig [1] the Wind Atlas for Egypt which indicate the meteorological stations

3
The traditional and effective method to estimate the wind resource on different sites is to
determine the wind speed and its direction, this method is applied on several sites and 30
meteorological stations have been showed in the Wind Atlas for Egypt. Figure [1]
The observations measurements of the wind atlas are taken over more than 150 years at different
30 stations. Figure [2] shows the compression of wind climates at the 30 stations.
Fig [2] mean wind speeds versus power densities of 30 stations in the Wind Atlas for Egypt.

4
Figure [2] confirms the existence of high wind resource spreads along the Gulf of Suez: almost
all the stations with mean wind speed above 7 m/s are shown. The Gulf of Suez is the most
recommended site to install the wind turbines and actually, standard MW-size wind turbines are
installed there. Where experience an average annual wind speed of more than 11 m/s and run at
rated power for more than 6000 hours in a year in addition to the capacity factor of almost
70%.[3]
1.2 Historical Background
People have caught the wind to propel their boats for many thousands of years. By skipping that
part of wind power history and jumping forward to the use of wind for mechanical and electrical
purposes.
1st century AD: For the first time in known history, a wind-driven wheel is used to power a
machine. A Greek engineer, Heron of Alexandria, creates this wind wheel.
7th to 9th century: Wind wheels are used for practical purposes in the Sistani region of Iran, near
Afghanistan. The Panemone windmills are used to grind corn, grind flour, and pump water.
1000 AD: Windmills are used for pumping seawater to make salt in China and Sicily.
1888: The first known US wind turbine created for electricity production is built by inventor
Charles Brush to provide electricity for his mansion in Ohio Figure [3].
1900: Approximately 2,500 windmills with a combined peak power capacity of 30MW are being
used across Denmark for mechanical purposes, such as grinding grains and pumping water.
1931: A vertical-axis wind turbine design called the
Darrieus wind turbine is patented by Georges Jean Marie
Darrieus, a French aeronautical engineer. This type of
wind turbine is still used today, but for more niche
applications like on boats, not nearly as widely as
horizontal-axis wind turbines.
Fig [3] the first US wind turbine

5
1931: A horizontal-axis wind turbine similar to the ones we use today is built in Yalta. The wind
turbine has 100 kW of capacity, a 32-meter-high tower, and a 32% load factor (which is actually
similar to what today's wind turbines get).
1957: Johannes Jul., a former student of Poul la Cour, builds a horizontal-axis wind turbine with
a diameter of 24 meters and 3 blades very similar in design to wind turbines still used today.
The wind turbine has a capacity of 200 kW and it employs a new invention, emergency
aerodynamic tip breaks, which are still used in wind turbines today.
1978: The world’s first multi-megawatt wind turbine is produced by Tvind school teachers and
students. The 2-megawatt wind turbine “pioneered many technologies used in modern wind
turbines and allowed Vestas, Siemens and others to get the parts they needed. Especially
important was the novel wing construction using help from German aeronautics specialists.”
(This wind turbine is still running today.)
1995-2000: Commercial wind turbine rotors get up to a diameter of 50 meters and wind turbines
get up to a capacity of 750 kilowatts, 10 times more than approximately 10 years ago.
2004-2011, Siemens grew wind power from 0.5% to 5% of the combined Siemens turnover, with
employees growing from 800 to 7,800.
2016: China becomes the country with the most cumulative installed wind power capacity in the
world. Figure [4]
Fig [4], Charts of new and cumulative wind power capacity by count

6
1.3 Wind turbines
The wind turbines are devices which convert the wind power into electricity. The actual
conversion process applies the basic aerodynamic force of lift to produce a net positive torque on
the rotating shaft of rotor, which produces first the mechanical power and then transfers it to
electricity by using a generator. The orientation of the shaft and rotational axis have classified
the wind turbine to two types, vertical axis wind turbine (VAWT) and horizontal axis wind
turbine (HAWT) Figure [5].
The horizontal axis wind turbines are more efficient at converting wind energy into electricity
than the vertical axis wind turbines. However, small axis wind turbines are more suited to urban
areas as they have a low noise level. The general public associates wind turbines with HAWTs
and are unaware of the several other technologies based on the VAWT. The recent attention that
VAWTs have received in several journals arouses interest in making a comparative study
between HAWTs and VAWTs [4–7].
(a) HAWT. (b) VAWT.
Fig.5. visual comparison between the two types of wind turbine

7
1.4 Vertical axis wind turbines(VAWTs)
Vertical axis wind turbines are capable of catching the wind from all directions and do not need
yaw mechanisms, rudders or downwind coning. Their electrical generators can be positions close
to the ground, hence easily deal with it. VAWT can work under unstable wind and low range of
speed which making them suitable for small-scale applications such as house installation. Due to
their particular axial symmetry they can generate energy even there is high turbulence.
A disadvantage is that some types are not self-starting. The vertical axis wind turbines have
many designs over the centuries and currently the vertical axis wind turbines can be clearly
divided into three basic types (1) Savonius type, (2) Darrieus type, and (3) The SB-VAWT. Brief
descriptions of the three VAWT types are given below.
1.4.1 Savonius turbines
The Savonius-type VAWT Figure [6] was invented by S.J. Savonius in 1929 [8]. It is essentially
a drag force driven wind turbine with two cups or half drums fixed to a central rotating shaft.
Each cup/drum holds the wind and then turns the shaft, bringing the opposite cup/drum into the
flow of the wind. This cup/ drum then repeat the process, causing the shaft to rotate further, thus
completing a full rotation. This process continues all the time when the wind blows and makes
the shaft turning which used to drive a pump or a small generator to get electricity. The typical
values of maximum power coefficient for wind turbines vary between30% to 45%, but for the
Savonius turbines are typically not excessed 25% according to most investigators [9]. This type
of turbine is suitable for low-power applications and they are commonly used for wind speed
instruments and usually used for wind velocimetry applications. The utmost advantage of a
Savonius rotor is its ability to self-start in opposite to other VAWTs.
Fig [6] Savonius-type VAWT

8
1.4.2 Darrieus turbines
The modern Darrieus VAWT was created by a French engineer George Jeans Mary Darrieus. He
submitted his patent in 1931 [10] in the USA which included both the ‘Eggbeater (or Curved
Bladed)’’ and ‘‘Straight-bladed’’ VAWTs. Sketches of these two variations of Darrieus concepts
are shown in Figure. [7, 8] The Darrieus-type VAWTs are basically lift force driven wind
turbines. The turbine consists of two or more airfoil-shaped blades which are attached to a
rotating vertical shaft. The wind blowing over the airfoil contours of the blade creates
aerodynamic lift and actually pulls the blades along. Darrieus type offer higher efficiency since
they reduce the losses due to friction. Darrieus-type turbines cannot self-start since,
independently of the wind speed, the start-up torque is invalid: as a consequence, these types of
turbine need an auxiliary device to helping the self-start.
The troposkien shape eggbeater-type Darrieus VAWT, which reduces the bending stress in the
blades, were commercially prevail in California in the past
Fig [7] Curved-blade (or ‘‘Egg-beater’’ type) Darrieus VAWT

9
1.4.3 The SB-VAWT
The SB-VAWT can be thought as a kind of Darrieus VAWT whose curved blades are replaced
by straight blades [12]. Therefore, it always was called as H-Rotor for its rotor outlook often
called giromill or cyclo-turbine. Compared with the Darrieus VAWT, the rotor structure of SB-
VAWT is simpler, and the cost of manufacture is cheaper. Figure [9] shows the structure of a
typical SB-VAWT. Generally, a normal SB-VAWT usually has 2–6 blades. This configuration
falls into two categories: fixed pitch and variable pitch. It has been found out from the previous
research activities that fixed pitch VAWTs provide inadequate starting torque [11].
Contemporary variable pitch blade configuration has potential to overcome the starting torque
problem but it is overly complicated, rendering it impractical for smaller capacity applications.
Majority of the previously conducted research
activities on VAWT focused on straight bladed
VAWTs equipped with symmetric airfoils (like
NACA 4-digit series of 0012, 0015, 0018) which
were unable to self-start. This inability to self-start
is due to several factors (like technical, inadequate
research work & funding), and the most dominant
ones are due to aerodynamic factors. According to
Internet survey, there are handfuls of commercial
straight-bladed VAWTs products, but no reliable
information could be obtained from an independent
source regarding the performance of these products
and the claims made by the manufacturers are yet to
be authentically verified.
Fig [8] Straight-bladed Darrieus VAWT

10
1.5 Theoretical wind power
The power available from wind for the vertical axis wind turbine can be described by equation
1.1
P=
1
2
ρAV ̥3
[1.1]
Where ρ is the air density, A is the swept area of the turbine; Ʋ is the free wind speed.
Wind turbine power production depends on the interaction between the rotor and the wind which
can be described by equation 1.2
P=
1
2
ρACpV ̥3 [1.2]
Where Cp is aerodynamic efficiency (denoted power coefficient):
.
Cp=
𝑝𝑜𝑤𝑒𝑟 𝑐𝑎𝑝𝑡𝑢𝑟𝑒𝑑 𝑏𝑦 𝑡ℎ𝑒 𝑤𝑖𝑛𝑑 𝑡𝑢𝑟𝑏𝑖𝑛𝑒
𝑡ℎ𝑒 𝑎𝑣𝑎𝑖𝑙𝑎𝑏𝑙𝑒 𝑝𝑜𝑤𝑒𝑟 𝑖𝑛 𝑤𝑖𝑛𝑑
=
P
1
2
ρAV ̥3
[1.3]
The Betz limit, Cp= 16
27
⁄ , is the maximum theoretically possible rotor power coefficient can
obtained for the wind turbines [13]. This power coefficient only considers the mechanical energy
converted directly from wind energy and does not consider the mechanical-into-electrical energy
conversion which involves other parameters such as the generator efficiency.
The Betz theory assumes constant linear velocity. Therefore, any rotational forces such as wake
rotation, turbulence caused by drag or vortex shedding (tip losses) will further reduce the
maximum efficiency. Efficiency losses are generally reduced by:
• Reducing low tip speed ratios which increase wake rotation
• Selecting airfoils with high lift to drag ratio
• Specialized tip geometries

11
2. Theoretical Design of the turbine
2.1 The chosen wind turbine concept (H-Rotor)
The main advantage with this wind turbine type is simplicity. The turbine design presented here
is a VAWT with straight blades supported with struts and is named an H-rotor Figure [9]. The
wind turbine consists of few parts and will only have one rotating part. The neglect of the
gearbox, yawing system and pitch system is expected to reduce maintenance [14]. The blades
will be fixed. It will not be possible to turn them out of the wind. Due to the vertical rotational
axis of a VAWT the generator is allowed to be located at the bottom of the tower. The shaft is
directly connected to the generator which eliminates the gearbox. This is expected to simplify
installation and maintenance. The tower can be lighter for a VAWT which reduces structural
loads and problems with making the tower [15]. The generator location increases the efficiency
and minimizing maintenance which affects the cost directly, as the size of the generator is not the
main concern because it does not make load on the tower.
Fig [9] the H-rotor turbine schematic on soildworks

12
2.2 Design parameters
There are a list of parameters must be varied in the design process to achieve a suitable wind
turbine for any specific application.
2.2.1 Swept area
The swept area is the section of air that encloses the turbine in its movement, the shape of the
swept area depends on the rotor configuration, this way the swept area of an HAWT is circular
shaped while for a straight-bladed vertical axis wind turbine the swept area has a rectangular
shape and is calculated using:
S = 2 R L [2.1]
Where S is the swept area [m2
], R is the rotor radius [m], and L is the blade length [m].
The swept area limits the volume of air passing by the turbine. The rotor converts the energy
contained in the wind in rotational movement so as bigger the area, bigger power output in the
same wind conditions.
2.2.2 Tip speed ratio
The power coefficient is strongly dependent on tip speed ratio, defined as the ratio between the
tangential speed at blade tip and the actual wind speed.
TSR (λ) =
Tangential speed at blade tip
Actual wind speed
=
𝑅𝜔
V ̥
[2.2]
Where ω is the angular speed [rad/s], R the rotor radius [m] and V ̥ the ambient wind speed
[m/s]. Each rotor design has an optimal tip speed ratio at which the maximum power extraction
is achieved. This optimal TSR variation depending on ambient wind speed

13
2.2.3 Number of blades
The number of blades has a direct effect in the smoothness of rotor operation, the stability of the
wind turbine and the optimum power efficiency increase with the number of blades for turbine
with the same radius and the inlet wind speed.
The choice of three blades is mainly motivated by the reduction in complexity. Results of
reference [16] show that favorable load variations on the turbine may be achieved with more
than three blades but a higher manufacturing cost.
2.2.4 Solidity
The solidity σ is defined as the ratio between the total blade area and the projected turbine area.
It is an important non dimensional parameter which affects self-starting capabilities and for
straight bladed VAWTs is calculated with:
σ =
N c
𝑅
[2.3]
Where N is the number of blades, c is the blade chord, L is the blade length and S is the swept
area; it is considered that each blade sweeps the area twice. This formula is not applicable for
HAWT as they have different shape of swept area.
2.2.5 Blade chord
The chord is the length between leading edge and trailing edge of the blade profile. The blade
thickness and shape is determined by the airfoil used, in this case it will be a NACA airfoil,
where the blade curvature and maximum thickness are defined as percentage of the chord.
Chord c can be expressed as a function of solidity, rotor radius and blade number N,
c =
N σ
𝑅
[2.4]

14
2.2.6 Initial angle of attack (α ̥)
The angle of attack, figure [10], specifies the angle
between the chord line and the vector representing
the relative motion between the body and the fluid
through which it is moving. The effect of the initial
angle of attack in overall performance will be
discussed.
Fig [10] the angle of attack of an airfoil
2.2.7 Reynolds number
The local Reynolds number is:
Re =
𝐶 𝑊
ѵ
[2.5]
Where c is the chord from Eq. [2.4], ѵ is the kinematic
air viscosity, and W is the air speed relative to the
airfoil as Figure [11], shows. Adopting a mathematical
approximation, to evaluate the Reynolds number, W
can be substituted by ωR, then simply calculate ωR
directly from TSR .Equation [2.2]
The Reynolds number strongly influences the power
coefficient of a vertical-axis wind turbine.
Furthermore, it changes as the main dimensions of the
turbine rotor change. Increasing rotor diameter raises
the Reynolds number of the blade. [17]
Fig [11] Wind rotor rotational plane

15
2.2.8 Aspect ratio
The turbine’s aspect ratio (AR) is the ratio between blade height and rotor radius (AR = h/R). Figure [12]
The factors which influence the Reynolds number [17], it was found that the ratio between blade
height and rotor radius (aspect ratio) influences the Reynolds number and as a consequence the
power coefficient. It has been highlighted that a turbine with a lower aspect ratio has several
advantages over one with a higher value. The advantages of a turbine with a lower aspect ratio
are: higher power coefficients, a structural advantage by having a thicker blade (less height and
greater chord) and greater in-service stability from the greater inertia moment of the turbine
rotor.
Fig [12] Wind turbines with different aspect ratios

16
3. Aerodynamics of Wind Turbine
3.1 General Overview
The power which obtained from the wind turbine depends on the interaction between the rotor
and the wind around it. The wind has been considered to be a combination of the average wind
and turbulent fluctuations about that flow. By the experience of experimental tests, the major
aspects of wind turbine performance (mean power output and mean loads) are determined by the
aerodynamic forces generated by the mean wind. The aerodynamic forces caused by wind shear
(off-axis winds), rotation of rotor, randomly fluctuating forces induced by turbulence of wind
flow and dynamic effects are the source of stress loads in addition to be a factor in the peak loads
experienced by a wind turbine.
3.2 General Concepts of Aerodynamics
The airfoils are used wind turbine blades to develop mechanical power. The cross-sections of the
turbine blades have the special shape of airfoils. The blade dimensions are functions of the
desired aerodynamic performance, the optimum desired rotor power, the selected airfoil
properties and strength considerations. Aerodynamic concepts related to airfoils will be
discussed.
3.2.1 Airfoil terminology
The following terms are used to characterize an airfoil, figure [13].
• The mean camber line - is the locus of points halfway between the upper and lower
surfaces of the airfoil.
• The camber - is the distance between the mean camber line and the chord line, measured
perpendicular to the chord line.
• The chord line - is the straight line connecting the leading and trailing edges is of the
airfoil.
• The chord – is the distance from the leading to the trailing edge measured along the
chord line.

17
• The thickness - is the distance between the upper and lower surfaces, also measured
perpendicular to the chord line.
• The angle of attack - is defined as the angle between the relative wind and the chord line
• The span - is the length of the airfoil perpendicular to its cross-section.
The geometric parameters that have an effect on the aerodynamic performance of an airfoil
include: the leading edge radius, mean camber line, maximum thickness and thickness
distribution of the profile and the trailing edge angle.
Fig [13] Airfoil nomenclatures
3.2.2 Lift, drag and non-dimensional parameters
Airflow over an airfoil produces a distribution of forces over the airfoil surface. The flow
velocity over airfoils increases over the convex surface resulting in lower average pressure on the
‘suction’ side of the airfoil compared with the concave or ‘pressure’ side of the airfoil.
Meanwhile, viscous friction between the air and the airfoil surface slows the airflow to some
extent next to the surface.

18
As shown in Figure [14], the resultant of all of these pressure and friction forces is usually
resolved into two forces and a moment that act along the chord at a distance of c/4 from the
leading edge (at the ‘quarter chord’):
• Lift force – defined to be perpendicular to direction of the oncoming airflow. The lift
force is a consequence of the unequal pressure on the upper and lower airfoil surfaces.
• Drag force – defined to be parallel to the direction of oncoming airflow. The drag force is
due both to viscous friction forces at the surface of the airfoil and to unequal pressure on
the airfoil surfaces facing toward and away from the oncoming flow.
• Pitching moment – defined to be about an axis perpendicular to the airfoil cross-section
Theory and research having shown that many flow problems can be characterized by
nondimensional parameters. The most important non-dimensional parameter for defining
the characteristics of fluid flow conditions is the Reynolds number. Equation [2.5]
Fig [14] Drag and lift forces on stationary airfoil; α, angle of attack; c, chord
Force and moment coefficients, which are a function of Reynolds number, can be defined for
two- or three-dimensional objects. Force and moment coefficients for flow around two
dimensional objects are usually designated with a lower case subscript, as in Cd for the two-
dimensional drag coefficient. In that case, the forces measured are forces per unit span. Lift and
drag coefficients that are measured for flow around three dimensional objects is usually
designated with an upper case subscript, as in CD. Rotor design usually uses two-dimensional

19
Coefficients, determined for a range of angles of attack and Reynolds numbers, in wind tunnel
tests. The two-dimensional lift coefficient is defined as:
CL =
𝐿𝑖𝑓𝑡 𝑓𝑜𝑟𝑐𝑒
𝐷𝑦𝑛𝑎𝑚𝑖𝑐 𝑓𝑜𝑟𝑐𝑒
=
𝐿
1
2
ꝭ𝑉2𝐶
[3.1]
The two-dimensional drag coefficient is defined as:
Cd =
𝐷𝑟𝑎𝑔 𝑓𝑜𝑟𝑐𝑒
𝑑𝑦𝑛𝑎𝑚𝑖𝑐 𝑓𝑜𝑟𝑐𝑒
=
𝐷
1
2
ꝭ𝑉2𝐶
[3.2]
The pitching moment coefficient is:
CM =
𝑃𝑖𝑡𝑐ℎ𝑖𝑛𝑔 𝑚𝑜𝑚𝑒𝑛𝑡
𝐷𝑦𝑛𝑎𝑚𝑖𝑐 𝑚𝑜𝑚𝑒𝑛𝑡
=
𝑀
1
2
ꝭ𝑉2𝐶𝐴
[3.3]
Where: ꝭ is the density of air, V is the velocity of undisturbed airflow, A is the projected airfoil
area, c is the airfoil chord length.
3.3 Major aerodynamic challenges of small SB-VAWT
The smaller-capacity Fixed-pitch SB-VAWT is a lift force driven wind turbine consisting of two
or more airfoil shaped blades which are attached to a rotating vertical shaft. The wind blowing
over the airfoil contours of the blade creates aerodynamic lift and pulls the blades along.
In Figure [15], a SB-VAWT blade’s circular path is shown with both the 3D & 2D
representations. As this blade rotates, it experiences a varying relative flow velocity (W), which
is the vector sum of the local induced wind speed and the blade speed. The lift force generated
by the blade has a tangential component in the direction of rotation. If the contribution of the
drag force is smaller than that of the lift force, the blade contributes positive torque which drives
a load connected with the central rotating shaft. The orbital position of the blade is called
azimuth (θ) and both relative flow velocity and the local angle of attack (α) vary with θ. The
amplitude of this variation is related to a non-dimensional parameter known as

20
(a) 3D Model (b) 2D Plan View
Fig [15] Flow Velocities around SB-VAWT
Tip speed ratio (λ = (Rω)/V ̥)). The angle of attack (α) changes through positive and negative
values in each revolution of the blade at different λ and as the turbine speed increases, more
energy is extracted and the flow is decelerated. Though SB-VAWT has very simple structural
features, its rotor aerodynamics is very complex. The major aerodynamic challenges faced by
smaller capacity fixed-pitch SB-VAWT are:
• They operate in the low Reynolds number (RN) regime which is a highly sensitive unstable
region with high probability of separation. Considering the chord length as the characteristic
length, the operating chordal RN (ρVoc/μ) of interest is restricted between 100,000 to
500,000, which are considered as Low RN. In this range of RN, very complex flow
phenomena take place within a short distance of the leading edge on the upper surface of the
blade and the laminar separation bubble that commonly forms in this range plays an
important role in determining the boundary layer behavior and the stalling characteristics of
the blade.
• Because of the oscillating α, the blades always produce a fluctuating force, even in steady
conditions. Consequently, undesirable dynamic stall, vibrations, gust response etc. may

21
result, where the most important one is the dynamic stall effect which is dependent on
parameters including airfoil shape, amplitude and oscillation frequency of angle of attack,
type of motion, turbulence-level and three-dimensional effects.
• They operate in circular motion and thereby encounter flow-curvature effects if the
chord/radius ratio is high which can have a detrimental influence on the performance of a
SB-VAWT, especially at higher λ [18, 19].
• Unlike the conventional aerodynamic applications, VAWTs encounter a wide range of angle
of attack, especially at low tip speed ratios. When the turbine starts and as the rotational
speed increases, the blades can even experience back flow [20]. In this situation, the effective
angle of attack can be greater than the stall angle and thus the air flow detaches from the low
pressure side of the blade airfoil. As α over the blade is increased, at some point the air will
separate, which usually starts at the trailing edge and shifts forward with increasing α. This
phenomenon is called deep stall and has a strong detrimental influence on the performance of
SB-VAWT blade mainly because of a hysteresis loop where the lift is small while drag
remains large [20].
• The blades on the downstream-pass operate in their own wakes shed by blades on the
upstream pass; for all the VAWTs, the blade/blade wake interaction presents the most
fundamentals modelling problem [21]. It has been found out that most of the power is
extracted from the wind during the upstream side of a VAWT.
• They suffer from parasitic drag losses due to radial arms which support the blades and
transform the kinetic energy of wind into the rotating shaft. The parasitic drag losses reduce
the overall power
• They mostly operate in turbulent atmospheric conditions. As per a detailed computational
analysis, their performance can be reduced significantly due to atmospheric turbulence [22].
All these factors collectively make the thorough analysis of straight-bladed Darrieus VAWT a
challenging undertaking. One of the most problematic aspects of the complex aerodynamics of
SB-VAWT is that they produce very little starting torque when conventional airfoils are used.

22
3.4 Self-starting problem of SB-VAWT
The main concept of the small-capacity SB-VAWT is completely unable to self-start properly.
This concept is true for the initial designs which were designed by using old symmetric NACA
airfoils. According to Watson [23], the inability of Darrieus type VAWTs to self-start is due to a
strap of tip speed ratios cannot satisfy the operating condition at which the net amount of energy
collected by each blade in each revolution is negative. To solve self-starting problem, SB-
VAWT must be modify so that a net positive amount of torque is produced at all tip speed ratios
up to the operating point, at a low value of λ, the blades of SB-VAWT encounter a wide range of
α and thus fluctuate between stalled and unstalled conditions. At low λ, the blade is stalled at
almost all azimuth angles. As a result, most fixed pitch Darrieus turbines do not self-start reliably
and it is only at higher values of λ, above about 3, that the blades remain ‘unstalled’ and the
turbine can achieve high efficiency.
The problem of self-starting can be alleviated by (i) using high-lift low-drag special purpose
airfoil; and (ii) by incorporating a Savonius rotor or torque tube [24]. Self-starting problems can
be overcome electrically if the SB-VAWT is coupled with a synchronous alternator or a DC
generator which can function as a motor and can be used to drive the turbine up to operating
speed. Although motor starting is used in grid-connected wind turbines and could be done with
smaller capacity wind turbines, it is not a common practice.
3.4 Desirable aerodynamic characteristic of the airfoil
In this section, an attempt will be made to short list the desirable aerodynamic characteristics of a
self-starting and optimum performing SB-VAWT based on previous researches and
computational analysis. These performance indices are utilized for considering the following
desirable aerodynamic characteristics:

23
I. Large Stall Angle at Small Reynolds Number
Behavior of lift and drag coefficients in the post-stall conditions at low λ has serious
consequence for fixed-pitch SB-VAWT and their lack of starting torque is due to the cyclical
change in α with θ. As at low λ, α exceeds the stall angle for much of the blades’ travel path.
Thus stalled blades generally contribute negatively to the driving torque so that the network
output per revolution may be negative for some values of λ. If the stall angle can be increased,
this situation will be improved as the blades are stalled for a smaller proportion of their travel.
So, it is clear that the stall angle of the airfoil sections for fixed-pitch SB-VAWT should be as
large as possible in the low RN operation, SB VAWT airfoils encounter negative incidences in
the downstream side (180o≤θ≤360o) where lesser amount of energy is available for harnessing
as most of the power is extracted from the wind during the upstream side of a VAWT, The
vortex flow model of Duremberg [25] finds that the velocity difference across a three bladed
turbine is about 0.55,i.e. the downstream blades receive wind at a speed of 0.55 times of the
upstream side. About 90 or 95% of the energy is extracted from the upstream pass and this
proportion depends on various factors including α and λ, Loth and McCoy [26] have shown that
a trade-off exists between energy extraction on the upstream and downstream passes.
(a) Concave-out Configuration (b) Concave-in Configuration
Fig [16] two types of configurations for attaching cambered airfoils with the supporting struts
The power is extracted on the upstream pass; the less energy is available on the downstream
pass. So, though asymmetric airfoils produces lesser amount of lift in comparison to the

24
symmetric airfoils at negative incidences and have a lower stall angle, their better performance at
positive incidence can result in superior overall performance at low Re if the concave out
configuration.
II. Wide Drag Bucket
Generally, airfoils exhibit the lowest drag over a narrow range of angle of attack called the “drag
bucket”, figure [17], and the airfoil shape solely determines the shape and position of the drag
bucket, Klimas [27] identified that wide drag bucket is one of the desirable characteristics of
VAWTs.
Fig [17] Drag bucket
III. Small Zero-Lift-Drag Coefficient
The desired airfoil should have the least amount of drag, especially the zero-lift-drag coefficient
(Cd ̥) should be very low, and it also suggested that zero-lift angle and minimum drag coefficient
angles of a better performing airfoil should coincide or the difference should be small. Basically,
the zero-lift drag coefficient is reflective of parasitic drag which makes it very useful in
understanding how “clean” or streamlined a VAWT’s aerodynamics is.
IV. Large Cl/Cd ratio
Drag is the price paid to obtain lift. The lift to drag ratio (L/D) is the amount of lift generated by
a wing or airfoil compared to its drag. The lift/drag ratio is used to express the relation between
lift and drag and is determined by dividing the lift coefficient by the drag coefficient, CL/CD. A

25
ratio of L/D indicates airfoil efficiency. Aircraft with higher L/D ratios are more efficient than
those with lower L/D ratios.
The shape of an airfoil and other lift producing devices affect the production of lift which will
vary with changes in the angle of attack. The maximum lift/drag ratio occurs at one specific CL
and α. If the aircraft is operated in steady flight at Lift/Drag maximum ratio, the total drag is at a
minimum. Any α lower or higher than that producing the maximum Lift/Drag ratio reduces the
Lift/Drag ratio and consequently increases the total drag for a given aircraft’s lift.
V. Large Maximum Lift-Coefficient
If the airfoil shape of a smaller-capacity SB-VAWT has higher CLmax, more positive torque will
be generated in the pre-stall regime. This feature will also enhance the starting torque. Both high
lift and stall angle are therefore desirable for VAWTs. However, CLmax should be relatively
insensitive to the changes of Re. it suggested that the slope of the lift curve of SB-VAWT airfoil
should be steeper for improved efficiency.
VI. Delayed Deep-stall Property
Deep stall has negative influence on the performance of SBVAWT. According to Claessens [28],
deep stall characteristics of airfoil are important for VAWTs and he suggested that:
(i) Deep stall should be postponed to a larger angle of attack.
(ii) Hysteresis loop of the deep stall should be as small as possible.
(iii) The drop of lift coefficient should be as small as possible at deep stall.
The angle at which deep stall occurs depends on Re and the nose radius.
VII. Small Roughness Sensitivity
The airfoil should have least amount of roughness sensitivity as wind turbines operate at
diversified climatic conditions and its maintainability and performance deteriorate with surface
roughness due to dust, dirt, rain or insect debris. Because of surface roughness the boundary
layer of the blades will turn turbulent at the nose, which results in a turbulent boundary layer

26
over the airfoil. Surface roughness generally decreases Clmax and increases Cd ̥ and these effects
become more pronounced as Re increases.
VIII. Small Trailing Edge Noise Generation
The laminar separation bubbles that extend over the trailing edge of the airfoil cause the blades
to vibrate and are a source of noise, noise emission should be kept as low as possible. According
to Claassen’s [28], an airfoil with smooth stall characteristics is desirable to reduce the trailing
edge noise.

27
4. Experimental test rig setup
Experimental methods can be thought as the most direct and effective method if the tests are well
done. Usually, for wind turbine aerodynamic performance, the experiments can be categorized as
a wind tunnel test, visualization test, and field test. The wind tunnel test is applied to our turbine
to evaluate its performance.
4.1 Wind tunnel test
Wind tunnel usually uses a powerful fan to move the air stream through the tube section. The
turbine being tested is placed inside the tunnel or at the tunnel exit. Usually, the wind tunnel can
be categorized as a closed layout type and open-ended type. Both of them can be used for
aerodynamic characteristics test of wind turbines. Furthermore, according to the section shape of
the test part, it can be divided into rounded shape section and square shape section. For SB-
VAWT, the wind tunnel with round shape section is used in our test. Figure.18. Considering the
blockage effect, blockage ration defined as the ration of the swept area of the rotor against
section area of wind tunnel should be less than 35%. Usually, the power can be calculated by
measuring the torque and revolution of rotor. The aerodynamic performance evaluation
indicators include power coefficient (Cp) from equation (1.3), and tip-speed ratio (λ) from
equation (2.2), torque coefficient (Cm) from equation (3.3).
Fig [18] the wind tunnel with an axial fan and straighteners at the middle.

28
4.2 Test Setup
To study the performance of SB-VAWT rotor, the low-speed low turbulence wind tunnel with an
open test section has been designed as round tube, developed and fabricated with a metal sheet
and use circular straighteners in the middle of the section are arranged as a honeycomb to
produce a laminar stream. The rotor axis is placed at a distance of 3 m from the tunnel exit
having a cross-sectional area of 1x 2m as shown in Figure 18. The wind tunnel is capable of
providing wind speeds up to 15.5m/s. All the tests have been conducted in the range of air
velocity of 8 - 9 m/s.
The 5.5 KW axial fan is used to move big quantities of airflows into wind tunnels. The hot-dip
galvanized casing prevents corrosion. The impeller has been realized with seven aluminum
blades, galvanized steel hub, to limit the weight and reduce energy consumption. Moreover, the
special shape of the winglet-profile blades allows an even higher energy-efficiency. The impeller
is dynamically balanced.
The open loop wind tunnel uses inverter to control a 3 – phase electricity supply. The inverter
LS-Starvert-IG5A, figure [19] regulates the current according to the revolving speed and allows
changing its carrier frequency from 10 to 50 HZ. Appendix 5.Furthermore the performance of the
wind tunnel is measured as illustrated in Table 1.
Fig.19, SV-IG5A inverter which control the rotational speed of the fan

29
Power Supply (HZ) Wind Speed (m/s)
10 2.9
15 4.7
20 6.3
25 7.7
30 9.2
35 11
40 12.7
45 15
50 15.5
Table 1, shows the wind speed at the tunnel exit which change with the frequency range 10-50
Hz. The wind speeds clearly increase as frequency increase.
4.3Measurement instruments
(a) Anemometer
Precision vane type anemometer provides fast, accurate air velocity measurements for use in
balancing HVAC systems or
determining CFM calculations. A
convenient TYPE K thermocouple is
built-in to the remote vane providing
quick measurement of grille/duct
outflow temperatures from 32 to 122 F /
0 to 50 C. Included is an easy-to-read
laminated conversion chart that allows
the user to quickly convert air velocity
measurements into CFM calculations at
a glance. Fig.20, The Anemometer which measure air velocity

30
(b) Tachometer
Built-in infrared for non-contact measurements Narrow beam for accurate non-contact RPM
measurements Palm-size, light weight, easy to carry around.
• Display: 5digital, 18mm LCD
• Accuracy: ± (0.05%+1digital) of reading
• Sampling Time: 0.8second (over 60RPM)
• Memory: Max. Value, Min. Value, Last
value. Can store 96s data.
• Range Select: Auto-range
• Time Base: 6MHz Quartz crystal
• Detecting Distance: 50mm-500mm
• Dimension: 155*70*35mm
Fig.21, the Tachometer which measures the RPM
(c) Voltmeter
Digital Mustimeter Voltmeter avometer Suoer Sd-9205A
• Maximum voltage between: 1000VDC or 700VAC.
• Terminals and earth ground. Fuse protection: F
200mA/250V.
• Power: 9V battery,
• NEDA 1604 or 6F22 Display: LCD, 1999 counts,
updates 2-3/sec.
• Measuring method: Dual-slope integration A/D
converter Over Operating Environment: 0 to 40°C.
• Storage temperature: -10°C to 50°C. Size:
31.5*91*189mm.
Fig. 22, Digital Avometer

31
0
5
10
15
20
25
30
35
40
45
7.52 8.16 8.5 8.69 9.82
Hz
average wind speed
HZ vs average wind speed
4.4 the installation of the turbine
Our small vertical axis wind turbine with 0.3m radius and 0.4m height is placed at a distance of
3m from the wind tunnel exit where the average wind velocity versus the Hz is showed at the
next graph:
Fig [23] the average wind velocities at 3m from the tunnel versus the Hzs
4.1.1 mechanical components
a) the shaft of the rotor
One of the most important and critical part in turbine is the rotor so we use Thomson steel shaft
with 20 mm diameter and 160 cm Height the main advantages of Thomson steel is that it can
withstand shear stress, normal stress and environmental condition.
Fig [24] section of Thomson steel shaft

32
b) coupling
Coupling is used to Absorbing incidental misalignment, shock loads and small amplitude
vibrations, fanner jaw coupling 070 is used to connect the rotor shaft with 20mm and motor shaft
with 15 mm together. Jaw couplings offer a low cost flexible solution for most applications;
some of its specifications are listed below:
• Ease of alignment
• Fail-safe shaft connection
• Range of element materials available including nitrile, urethane and Hytrel
• Pump spacer variant available
• Quick-fit wrap around element available
• Design powers up to 42.2kW available at 1440rpm
Fig [25] fanner jaw coupling
c) Supports struts
Supports are used to connect between the rotor blade and the rotor shaft. Steel sheet Supports
with 30cm length, 2mm thickness and 5cm wide are used.
Fig [26] the supports (sleeves) between the blades and the shaft

33
d) Bearing
Bearing is a mechanical component which used to reduce the friction of rotation. At the first
design of turbine base a couple of UFL20 bearing are used with a 40 cm distance between it. Due
to misalignment between the two bearing the vibration increased and it affected the rotation of
the turbine, so the new bearing of the deep-groove ball type is installed in hollow column with
80cm length and 55mm outer diameter the deep-groove ball bearing has 20mm inner diameter.
Then the misalignment problem is solved and the turbine vibration is reduced.
Fig [27] the old base with the ufl20 bearing
Fig [28] the new base with the ball bearing and the vertical column
Ball bearing
UFL20 bearing

34
e) Generator
To calculate the efficiency of the turbine we should calculate the power output of the turbine.
There are a several methods to measure the power output such as mechanical method by
measuring the torque output by a torque meter or electrical by installing a generator and measure
output values of volt and ampere. The electrical method selected to calculate the power output by
installing electric motor DC 280W 24Vand reverse the connection to be a generator for more
information see appendix (3).
Fig [29] Zhejiang Unite Electric Motor Co., Ltd.
The motor Zhejiang Unite Electric Motor has rated speed 2800 rpm and our turbine has
maximum speed 500 rpm so it cannot sense the low speed correctly so the output power from our
turbine is incorrect then we decide to change the motor with another motor have low range of
rpm. The Ametek Permanent-Magnet 40-volt DC Servo Motor / DC Electric Wind Turbine
Generator are installed for more information see appendix (4).
Fig [30] DC Electric Wind Turbine Generator

35
f) Installation turbine base
The installation base has to be stable to prevent the vibration due to the turbine rotation which
causes the turbine to stall. Therefore the design and the used material are important to control the
stability level of the turbine. With our first installation base, when the rotor rotate the whole
turbine vibrates and the rotation stopped, therefore we make a lot of changes to achieve the
stability of the turbine with L-section steel with dimensions 4x4 cm and 4 mm thickness.
Fig [31] the L-section steel used in the base installation design
a) The old base design b) the modified base design
Fig [32] the installation turbine base before and after the modification.

36
During initial testing at some conditions, the turbine would suddenly reduce its rotation speed
and eventually stop despite no changes to the applied torque. It transpired that the turbine blades
were slowly rotating about an axis centered through the bolts that fix them to the support struts.
This was caused by the center of rotor lift not being aligned properly with the fixing bolts. It is
clear that the center of lift for VAWT rotor blades is constantly changing throughout every
rotation so it was not a simple process to determine the best location of the fixing holes on the
rotors. The maximum change in the rotor blade fixing angle was measured to be less than 5_ and
this small angle change caused a complete loss of lift and hence the eventual complete loss of
power from the turbine, therefore, we changed the support struts to overcome the reduction of
the rotation speed of the turbine.
Fig [33] the first turbine design with six support struts

37
Fig [34] the final design of the turbine from three different views with three support struts

38
5. Computational model
In the present work ANSYS FLUENT is used to validate the experimental and numerical results
which were done by Howell 2010 [1], then using this model to investigate the effect of winglet
on the turbine performance and validate the experimental results of this project.
5.1 Geometry
3D model of the turbine rotor was made using Solidworks software with the same dimensions
discussed in the previous sections. Figure [35] the flow domains. Figure [36] were created by
FLUENT design modeler, as we are going to use sliding mesh method in the solver so two flow
domains were created with an interface boundary condition in between, the sides of the outer
domain were set at a proper distance to include the effect of walls to correctly simulate the actual
wind tunnel case but the inlet and outlet were set further away as recommended by Howell 2010
[1], the outer ( fixed ) domain has the dimensions of ( 5m ) for +X, ( 3m ) for –X and ( .75m ) for
the rest directions, while the inner ( rotating ) domain is a cylinder of diameter ( .675m ) and a
height of ( .42m ), also it should be mentioned that the turbine shaft and the supporting arms are
not included in the geometry, just to reduce the simulation time by simplifying the 3D model, as
a result it is expected that the simulation results would be slightly higher than the experimental as
there is no existence to the shaft or supporting arms drag forces.
Fig [35] 3D model of turbine rotor blades

39
Fig [36] isometric view of flow domains
The winglet is a modified blade tip which has the same airfoil section NACA 0022 for both
extended and grooved parts, three different cases of blade tip modifications will be investigated
their effect on the turbine performance and tip vortex control.
Case 1: the blades were extended (5mm) in both directions with the same chord length of the
base model blade, and a groove of (3mm) with a (7cm) chord length Figure [37]
Fig [37] modified blade tip case 1

40
Case 2: the blades were extended (5mm) in both directions with a (13 cm) chord length without
any grooves Figure [38]
Fig [38] Modified blade tip case 2
Case 3 : the blades were extended ( 5mm ) in both directions with a ( 13cm ) chord length, and a
groove of ( 3mm ) with a ( 7cm ) chord length Figure. [39].
Fig [39] modified blade tip case 3

41
5.2 Mesh
The mesh was generated using the mesh tool in Fluent; the mesh independence was achieved by using the
following mesh details:
Element Meshing
General size function Proximity and curvature
Num cells across gap 5
Proximity size function source Faces and edges
Min size 50 mm
Proximity min size 50 mm
Max Tet size 150 mm
Max face size 150 mm
Contact region 35 mm element sizing
Blades surface 2mm face sizing + face meshing
Trailing edge 1 mm edge sizing
Rotating domain 15 mm element size
Fixed domain sides 20 inflation layers with 2 mm max thickness
This mesh sizing produced around 1.7 million cells Figure [40], the mesh is then converted to
polyhedral to much saving the simulation time, polyhedral mesh shows a good accuracy besides
it highly reduces the solving time as it merges two or more neighboring tetrahedral cells together
to form one polyhedral cell, so the number of nodes increase but the number of cells highly
decrease.
Fig [40] Full domain meshing and section view for blade surface mesh

42
5.3 Setup
Realizable k-ε will be the turbulence model for the current simulation as it`s known by a good
prediction of flow separation [29], however k- ɯ family is still recommended for near wall
situations as it`s more sensitive to flow properties near wall specially SST k- ɯ as it`s a
combination between k-ε and k- ɯ models [30], but k- ɯ models in general need high wall mesh
resolution which means more simulation time.
The inlet of the outer flow domain is set as velocity inlet with ( 4.31m/s ) flow velocity in x-
direction, the turbulence intensity and length scale are set as Howell 2010 [1] ( 1% and .01 )
respectively, the outlet is outflow while the walls are non-slip walls, the blades are rotating walls
with zero relative rotational velocity to adjacent cell zone.
Unsteady simulation with ( .005s ) time step and ( 50 ) max iterations per time step, however
larger time step maybe also appropriate, the solution will be initialized with first order till a
stable solution is obtained ( almost after 120 time steps ) then the solver will be switched to
second order for higher accuracy results.

43
6. Results
6.1 Experimental work results
The target of the project not to make a turbine with maximum Cp put to investigate the effect
of winglet on power output and TSR so that that curves present the relation between the Cp
and TSR at different wind speed vales.
We use in this case Amatek motor (first motor we use and did not gave us the required power)
and take the volt and amber reading after 2 sec after the startup values to take a correct
ampere and volt values.
Base model Groove tip Winglet tip Winglet with
groove
Hz 32 32 32 32
TSR 1.582826797 1.532802288 1.514844771 1.609763072
Cp 0.080043548 0.07548369 0.072454151 0.08966486
groove
Hz 35 35 35 35
TSR 1.851984314 1.754705882 1.806423529 1.746086275
Cp 0.09104153 0.078415918 0.089564848 1.746086275
groove
Hz 38 38 38 38
TSR 1.880729167 1.874888393 1.88656994 1.920446429
Cp 0.090576727 0.079818585 0.088691381 0.093544945

44
0.05
0.055
0.06
0.065
0.07
0.075
0.08
0.085
0.09
0.095
0.1
1.4 1.45 1.5 1.55 1.6 1.65 1.7 1.75 1.8 1.85 1.9 1.95
Cp
TSR
Power Curve
Base Model Groove Tip Winglet Tip Winglet With Groove
That result shows that there is no improve in performance in our range of sensitivity in full load
of generator at deferent air velocity so that we make other experiment with deferent generator
load we use variable resistance to achieve that and that table and curves show that
Base Case
hz Vo RPM volt ampere ohm power TSR Cp
38 8.96 453 14.4 0.25 40 3.6 1.587522 0.034755
38 8.96 465 14.1 0.32 44 4.512 1.629576 0.043559
38 8.96 473 12.8 0.3 48 3.84 1.657612 0.037072
38 8.96 481 12.6 0.24 52 3.024 1.685647 0.029194
38 8.96 488 12.1 0.23 56 2.783 1.710179 0.026867

45
38 8.96 496 12.1 0.23 60 2.783 1.738214 0.026867

46
Winglet case (groove tip)
hz Vo RPM volt ampere ohm power TSR Cp
38 8.96 450 14.3 0.26 40 3.718 1.577009 0.035894
38 8.96 466 13.6 0.3 44 4.08 1.63308 0.039389
38 8.96 475 12.6 0.28 48 3.528 1.664621 0.03406
38 8.96 483 12.4 0.26 52 3.224 1.692656 0.031125
38 8.96 488 12.4 0.25 56 3.1 1.710179 0.029928
38 8.96 492 12.2 0.26 60 3.172 1.724196 0.030623
The result from this measurement method is that there is no improve will happen
in the performance of the turbine

47
6.2 CFD results
6.2.1 Convergence
The simulation was completed by obtaining a periodical residuals Figure [6.1] curve and also
torque coefficient (Cm) curve Figure [6.2] after about 1.2s of rotation.
Fig [6.1] residuals convergence curve
Fig [6.2] Cm convergence curve
Then the values of Cm were averaged and multiplied by tip speed ratio (TSR) to get the power
coefficient (Cp) according to the following relations: [30]

48
6.2.2 Power Coefficient
Figure [6.3] represents the comparison between the current work outcomes for base model and
the outcomes of Howell 2010 [1].
Fig [6.3] performance curve for base model at wind velocity 4.31 m/s

49
As shown the current CFD produces a good approach at small rotation velocities of the turbine,
however at high rotation speeds ( TSR > 2 ) the results are almost out of range, this may be due
to the mesh at the rotating zone and blades are not smooth enough and a further refinement is
still recommended to allow the solver sense the flow at high speeds with a more accuracy, but
due to the used computer limitations that refinement was hard to be done, so the results will be
accepted for ( TSR < 2 ).
The modified blade tip study were made at two different TSR ( 1.35 ) and ( 1.85 ) to compare
their Cp with the base model, the results didn`t show any improvement to the power coefficient ,
on the contrary it reduced the power coefficient by around 2.4% at high TSR ( 1.85 ), the
following table shows the Cp comparison between base and modified cases :
TSR Base case Cp Case 1 Cp Case 2 Cp Case 3 Cp
1.35 .162 .161 .162 .159
1.85 .231 .2257 .2257 .2275
6.2.3 Contours
For more analysis on the effect of modified blade tip on the flow characteristics around the
blades, many contours were taken at two sections, one at upper tip and the second is a at a
vertical plane at mid chord in the direction of blade height, all contours represent the flow
characteristics after 1.28s ( ɵ = 27.77 ° ), all following figures are arranged as base model ( top
left ), groove tip ( top right ), winglet with groove ( bottom left ), winglet tip ( bottom right ).
Base model tip shows low turbulence kinetic energy at blade tip comparing to modified blade tip
geometries Figure[6.4] Figure [6.6], especially groove tip model highly increased the T.K.E at
the tip.

50
Fig [6.4] turbulence kinetic energy at mid chord vertical plane
Fig [6.5] turbulence kinetic energy at upper tip plane

51
Fig [6.6] turbulence kinetic energy for vortex core region isosurface swirling strength level .005
Fig [6.7] turbulence kinetic energy for vortex core region isosurface swirling strength level .005 top view
The region of high velocity in Y-direction at the tip is much smaller at base model comparing to
modified blade tip geometries which means increasing the low pressure region in modified blade
tip geometries Figure [6.8]

52
Fig [6.8] velocity in Y-direction contour at mid chord vertical plane
Fig [6.9] mean velocity contour at mid span

53
Fig [6.10] mean velocity contour at upper tip
Fig [6.11] pressure contour at mid span

54
Fig [6.12] pressure contour at upper tip
Fig [6.13] RMS velocity at mid span

55
Fig [6.14] RMS velocity contour at upper tip
Fig [6.15] RMS static pressure contour at mid span

56
Fig [6.16] RMS static pressure contour at upper tip
Fig [6.17] mean pressure contour at mid span

57
Fig [6.18] mean pressure contour at upper tip
Fig [6.19] stn frame velocity vectors at mid span

58
Fig [6.20] stn frame velocity vectors at upper tip
7. Conclusion
Experimental and numerical tests were carried out to investigate the effect of modified blade tip
geometries on SB-VAWT performance and flow characteristics around the tip, a circular cross
section wind tunnel with 1m diameter and 2m length attached with a 5.5 kw axial fan as air supplier
was used for the experimental test, unsteady sliding mesh technique numerical model was solved by
using ANSYS FLUENT Realizable k-ε turbulence model to verify the experimental results and
study the flow properties around the blades, both the experimental and numerical results for power
coefficient showed that the used winglets shapes and dimensions had a negative effect on the turbine
performance through most of the working range of TSR, however winglet with groove showed a
slightly increasing in power coefficient during the experimental test, also the numerical analysis
contours proved that the winglets had increased the tip vortex by increasing the turbulence kinetic
energy at the tip and increasing the low pressure region.
8. References
[1] R. Howell et al. / Renewable Energy 35 (2010) 412–422

59
[2] Niels G. Mortensen1, Jens Carsten Hansen, Jake Badger, Bo H. Jørgensen, Charlotte B.
Hasager, Uwe S. Paulsen, Ole F. Hansen, Karen Enevoldsen Wind Energy Department,
Risø National Laboratory, Roskilde, Denmark
[3] Mortensen, N.G., J.C. Hansen, J. Badger, B.H. Jørgensen, C.B. Hasager, Uwe S.Paulsen,
Ole F. Hansen, Karen Enevoldsen, L. Georgy Youssef, U. Said Said, A.Abd El-Salam
Moussa, M. Akmal Mahmoud, A. El Sayed Yousef, A. MahmoudAwad, M. Abd-El Raheem
Ahmed, M. A.M. Sayed, M. Hussein Korany, M. Abd-El Baky Tarad (2006). Wind Atlas
for Egypt: Measurements, micro- and mesoscalemodelling. Proceedings of the 2006
European Wind Energy Conference and Exhibition, Athens, Greece, February 27 to March
2.Griffith University, Australia, 1998.
[4] Anon. 1. Turning wind power on its side. Economist 2006;378(8468):3–4.
[5] Peace S. Another approach to wind (cover story). Mech Eng 2004;126(6):28–31.
[6] Riegler H. HAWT versus VAWT: small VAWTs find a clear niche. Refocus
2003;4(4):44–6.
[7] Knight J. Urban wind power: breezing into town. Nature 2004;430(6995):12–3.
[8] Savonius SJ. The S-Rotor and its applications. Mech Eng 1931;53(5):333–8.
[9] Kirke BK. Evaluation of self-starting vertical axis wind turbines for stand-alone
applications. PhD thesis,
[10] Darrieus GJM. Turbine Having its rotating shaft transverse to the flow of the current.
US Patent No. 1835081, 1931.
[11] Kirke BK. Evaluation of self-starting vertical axis wind turbines for stand-alone
applications. PhD thesis, Griffith University, Australia, 1998.
[12] Paraschivoiu I. Wind Turbine Design with Emphasis on Darrieus Concept. Montreal,
Quebec, Canada: Polytechnic International Press; 2002
[13] Wind energy explained : theory, design, and application / James Manwell, Jon
McGowan, Anthony Rogers.
[14] J. Ribrant and L.M. Bertling. Survey of failures in wind power systems with focus on
Swedish wind power plants during 1997-2005. IEEE Transactions on Energy Conversion,
22(1):167 – 173, 2007.
[15] C. Brothers. Vertical axis wind turbines for cold climate applications. Montreal,
Canada. Renewable Energy Technologies in Cold Climates ’98 International Conference.

60
[16] P. Deglaire. Swedish Centre for Renewable Electric Energy Conversion, Division for
Electricity and Lightning. The Ångström Laboratory, Uppsala.
[17] Design of a vertical-axis wind turbine: how the aspect ratio affects the turbine’s
performance S. Brusca • R. Lanzafame • M. Messina
[18]. Mandal, A.C. and J.D. Burton., The Effects of Dynamic Stall and Flow Curvature on
the Aerodynamics of Darrieus Turbines Applying the Cascade Model, Wind
Engineering,1994, 18(6), 267-282.
[19]. Hirsch, C. and Mandal, A.C., Flow Curvature Effect on Vertical Axis Darrieus Wind
Turbine Having High Chord-Radius Ratio, Proceedings of European Wind Energy
Conference, Hamburg, 22-26. October, 1984, 405-410.
[20]. Claessens, M.C., The Design and Testing of Airfoils for Application in Small Vertical
Axis Wind Turbines, Master of Science Thesis, Faculty of Aerospace Engineering, Delft
University of Technology, The Netherlands, November. 2006.
[21] Klimas, P.C.. Darrieus rotor aerodynamics. ASME Transactions, Journal of Solar
Energy Engineering, 1982, 104, 102-105.
[22].Pawsey, N.C.K., Development and Evaluation of Passive Variable-pitch Vertical Axis
Wind Turbines, Doctoral Thesis, The University of New South Wales. November, 2002.
[23] Watson, G. R., The Self Starting Capabilities of Low Solidity Fixed Pitch Darrieus
Rotor, 1st British Wind Energy Association Workshop paper, 1979, 32-39.
[24]. Islam, M., Ting, D. S-K. and Fartaj, A. Assessment of Small-Capacity Straight-bladed
VAWT for Sustainable Development of Canada. International Journal of Environment
Studies: Special Canadian Issue on Energy Conversion and the Environment. 2007,64(4),
489-500.
[25] Duremberg, C. J., Unsteady aerodynamics of Vertical Axis Wind Turbines. 1st British
Wind Energy Assiciation workshop, 1979.
[26] Loth, J.L. and McCoy, H., Optimization of Darrieus turbines with an upwind and
downwind momentum model. Journal of Energy, 1983, 7(4), 313-318.
[27] Klimas, P.C., Airfoil treatments for vertical axis wind turbines. WINDPOWER `85,
SanFrancisco, CA, USA. 27 August, 1985.

61
[28] Claessens, M.C., The Design and Testing of Airfoils for Application in Small Vertical
Axis Wind Turbines, Master of Science Thesis, Faculty of Aerospace Engineering, Delft
University of Technology, The Netherlands, November. 2006
[29] CFD analysis for H-rotor Darrieus turbine as a low speed wind energy converter, M.H.
Mohamed*, A.M. Ali, A.A. Hafiz
[30] Simulation Verification and Optimization of a Vertical Axis Wind Turbine using CFD,
Transport Phenomena,Department of Chemical Engineering,Delft University of Technology

62
APPENDIXES APPENDIX 1
Symbols
P wind turbine power
Cp power coefficient
H the height of the blade
R the radius of the blade
C blade chord
Cd drag coefficient
Cl lift coefficient
α angle of attack/angular acceleration
αo initial angle of attack
θ angle between Vo and the position of the blade in the rotor
ꝭ air density
σ solidity
ω rotor angular speed
Re Reynolds number
Ѵ kinematic air viscosity
W air speed relative to the airfoil m/s
AR aspect ratio
S swept area

63
APPENDIX 2
GLOSSARY
Cut-in windspeed: The minimum windspeed at which the wind turbine starts producing
energy.
Cut-out windspeed: The maximum windspeed at which the wind turbine stops operating,
mainly for safety reasons.
Rated windspeed: windspeed at which the rated power is produced, this value defines the
shape of the power curve.
NACA-series Airfoil: an airfoil which section and aerodynamic properties are available in
reference books like Abbott & Von Doenhoff, 1959, these airfoil data are the result of the
research conducted at the National Advisory Committee for Aeronautics (NACA).

66
APPENDIX 4
Ametek Permanent-Magnet 40-volt DC Servo Motor / DC
Electric Wind Turbine Generator
Product features
• Near-new, run only 70 hours for quality-control purposes. Not salvaged from a
mainframe tape drive.
• Built for long life: 21-bar commutator, 4 pure carbon brushes, and sealed 1.625" radial
ball bearings.
• Welded steel motor case (1/6-inch thick). Cold-rolled carbon steel shaft, black oxide
coating.
• Voltage constant 32.5 volts/rpm 218 oz-in peak 113 oz-in sustained torque. 60 volt max
continuous.
• Use it as a wind-turbine DC electric generator or as a low-rpm high-torque servo motor.
Product description
This Ametek 40V PMDC motor is near-new and not scavenged from an open-reel
mainframe tape drive. Never used or installed, merely run 70-hours to produce data for a
statistical quality-control database. . . . . It was originally designed to precisely position
magnetic tape at slow or fast speed, so it has a 21-bar commutator, minimal starting
voltage, and a linear torque curve. Magnetically shielded, RFI suppressed, and UL
recognized. This "7980" series Ametek motor has four carbon brushes instead of the usual
two. This results in lower resistance at the commutator, generates less waste heat, and
prolongs brush life to 15,000 hours. . . . . Use it as a DC generator in a bicycle-style
emergency power generator, a water-wheel electric generator, or a wind-electric generator.
The simplest and quickest way to get into small-scale electrical generation is with an
Ametek PMDC motor. This is a good motor for use as a direct-drive windmill electric
generator because of its powerful magnets and high torque constant. It produces higher
voltages and more current at a lower rpm than regular DC permanent magnet motors. . . . .
This Ametek motor has a reputation for doing a decent job of generating electrical power
from wind to charge 12V battery systems. This Ametek will generate 12+ volts (enabling
it to charge 12 volt batteries) at about 360-rpm and higher. Under a full load this dynamo
will max-out at roughly 420-rpm because as more torque input energy is applied to rotate
the shaft, that rotational force is mostly converted into more electrical current rather than

67
higher rpm. Maximum generated power output is roughly 400-watts under load. This load
can be something such as a battery charger, a hydrolytic cell producing hydrogen fuel
from water, or a "grid-intertie inverter" which synchronizes the power output of the wind
electric generator and dumps it back into your house AC current line.

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