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Master's Degree Thesis

ISRN: BTH-AMT-EX--2011/D-09--SE

Supervisor: Ansel Berghuvud, BTH

Dipl.-Ing. Mona Goudarzi, IPH, Hannover

Department of Mechanical Engineering

Blekinge Institute of Technology

Karlskrona, Sweden

2011

Abtin Namiranian

3D Simulation of a 5MW Wind

Turbine


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3D Simulation of a 5MW Wind

Turbine

Abtin Namiranian

Department of Mechanical Engineering

Blekinge Institute of Technology

Karlskrona, Sweden

2011

Thesis submitted for completion of Master of Science in Mechanical

Engineering with emphasis on Structural Mechanics at the Department of

Mechanical Engineering, Blekinge Institute of Technology, Karlskrona,

Sweden.

Abstract:

In the present work, the influence of turbulence and gravity forces on the

tower and the rotor of a 5MW onshore wind turbine has been investigated.

A full geometry of the turbine has been designed and simulated in a virtual

wind farm and then the fluid loads are imported into the structural part by

fluid structure interaction (FSI) method. The final results are shown that the

gravity force is significantly higher than turbulence loads and should be

considered in designing of the large wind turbines.

Keywords:

Wind turbine, Turbulence and gravity loads, Computational Fluid

Dynamics,Finite Element Method, Fluid structure interaction, ANSYS


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Acknowledgements

This thesis is the final project for the Master of Science in Mechanical

Engineering with specialization in structural mechanics at the Departmentof Mechanical Engineering, Blekinge Institute of Technology, Karlskrona,

Sweden.

This work has been performed at Institut fr Integrierte ProduktionHannover gGmbH (IPH) under supervision of Dipl.-Ing. Mona Goudarzi.

I would like to express my sincere appreciation to Dipl.-Ing. MonaGoudarzi for her encouragement, support and guidance through all my

work. I am also grateful to Dr. Ansel Berghuvud at the department of

Mechanical Engineering at Blekinge Institute of Technology for his support

and suggestions.

Finally, special thanks to my parents who have always supported and

inspired me throughout my academic life.

Hannover, August 2011

Abtin Namiranian


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Contents

1. NOTATION 5

2. INTRODUCTION 72.1 BACKGROUND 7

2.2 AIM AND SCOPE 8

3. WIND TURBINE CONCEPTS 9

3.1 MODERN WIND TURBINE 9

3.2 WIND FARMS 11

4. WIND TURBINE COMPONENTS 13

4.1 ROTOR 144.1.1 Rotor Blades 14

4.1.1.1 Aerodynamics of Rotor Blades 14

4.1.1.2 Airfoils 14

4.1.2 Wind Power 18

4.1.3 Rotor Power and Torque 19

4.1.4 Rotor Design 20

4.1.5 Rotor Blades Material 24

4.1.6 Rotor Hub 254.3 NACELLE 25

4.4 TOWER 27

5. WIND TURBINE LOADS 29

5.1 AERODYNAMIC LOADS 30

5.2 GRAVITY LOADS 30

5.3 CENTRIFUGAL LOADS 31

5.4 GYROSCOPIC LOADS 31

5.5 WIND TURBULENCE 325.6 WIND SHEAR 32

6. THEORY 35

6.1 FINITE ELEMENT METHOD (FEM) 35

6.2 COMPUTATIONAL FLUID DYNAMICS (CFD) 36

6.2.1 Governing equation 36

6.2.1.1 Mass conservation 36

6.2.1.2 Newton's second law 37


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6.2.1.3 Energy equation 37

6.2.1.4 Navier-Stokes equations 38

6.2.2 Finite Volume Method 39

6.2.3 Turbulence modeling 41

6.2.3.1 The

model 43

6.2.3.2 The model 436.2.3.3 The Shear-Stress Transport (SST) model 446.3 FLUID STRUCTURE INTERACTION 45

7. SIMULATION 46

7.1 GEOMETRY 46

7.2 CFX 48

7.2.1 Mesh 48

7.2.2 Model set up 49

7.3 STRUCTURAL 52

7.3.1 Mesh 52

7.3.2 Model set up 54

8. RESULTS 56

9. CONCLUSION 64

10. FUTURE WORKS 65

11. REFERENCES 66


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1. Notation

Area Drag force coefficient Lift force coefficient Power coefficient Design power coefficient

Dimensionless constant

Energy Force Acceleration of gravity Height Specific total enthalpy Internal energy

Thermal conductivity Turbulent kinetic energy Mass Rotational speed Pressure

Power

Design power Turbulence production Output power Radius Drag force

Lift force


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Energy source Kinetic energy source Enthalpy energy source Body forces Temperature Torque Velocity vector in Cartesian coordinates Velocity Wind shear coefficient Turbulent dissipation rate Specific dissipation rate Angular rotor speed Air density Angular velocity

Generator efficiency

Mean wind velocity Standard deviation ration Viscous stress Viscous stress components Dynamic viscosity

Turbulent viscosity

Tip speed ratio Viscosity Fluid property Diffusion coefficient


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2. Introduction

2.1 Background

The use of renewable energy sources has increased significantly in the past

decade due to the robust request for the sustainable protection of our

environment. Among these renewable energies, the use of wind power has

become the fastest growing energy technology in the world [17]. This

remarkable energy can be captured and used to generate electricity by

implementing wind turbines which have changed considerably since their

beginnings. Today the commercial size of wind turbines covers from 0.3

MW up to 7.5 MW, as shown in figure 2.1.

Figure 2.1. Growth in size of commercial wind turbine designs


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The size of wind turbines is performing as the main rule for capturing

energy from the wind. In other words, we can produce more utilizable

energy with larger wind turbines due to higher wind speed, lower airflow

frictional resistance and bigger volume of air which flows through the rotor.

Therefore, most of the wind turbine producers in the wind turbine

technology are focused on constructing the larger wind turbines. On theother hand, the cost of labor, maintenance and construction of the wind

turbines increase with the size of each part, specially tower and rotor.

Hence, wind turbine manufacturers are also concentrating on bringing

down the price of the turbines themselves.

In order to catch more power from the wind and supply more electrical

energy, we need to build a bigger rotor which is the most significant part of

the wind turbines. However, this part depends on the size and shape of

tower which provides the safe and reliable performance of turbines under avariety of wind conditions. Therefore, the understanding of forces which

appear in the tower and the suitable selection of the material used are

considered as other main factors in designing the large wind turbines.

2.2 Aim and scope

This work concentrates primarily on the simulation of one of the biggestwind turbines in the world. The purpose of this simulation is calculating the

effect of turbulence loads from the wind on the rotor, nacelle and the tower,

and then computing the gravity force in each part in order to find the

stresses and deflections in the tower and the rotor of the turbine.


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3. Wind Turbine Concepts

One of the significant technologies of the 20th

century is the wind turbine

technology which offers cost-effective solutions for generating the

electrical energy due to eliminate the dependency of the world on the fuel-

based sources such as oil and gas. Therefore, the wind turbine technology is

produced electrical energy without greenhouse effects or deadly pollution

gasses [2]. The wind turbine technology offers electrical energy with lower

installation and maintenance costs unlike the other energy sources.

In this project, a wind turbine is a machine which converts the wind power

into electricity power and does not to be confused with another type of

machine, Windmill which converts the winds power into mechanical

power.

3.1 Modern Wind Turbine

Modern wind turbines can be classified into two configurations depending

on the rotation axis of the rotor blades: horizontal-axis wind turbines

(HAWTs) and vertical-axis wind turbines (VAWTs), as shown in figure

3.1.

Figure 3.1. Types of modern wind turbine

In recent years, most of the commercial wind turbines are the horizontal

axis wind turbines (HAWT) which have their axis of rotation horizontal to

the ground and almost parallel to the wind flow. These types of turbines

have some noticeable advantages such as low cut-in wind speed and easy

furling. In general, the power output of HWAT is higher than vertical-axis


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wind turbines due to the better power coefficient in HWAT. However, the

generator and gearbox of these turbines are to be located over the tower

which makes its design more complex and expensive.

Horizontal axis wind turbines can be classified as single bladed, two

bladed, three bladed and multi bladed, as it is shown in figure 3.2. TheHAWT single bladed are not widely used now, even though they appear to

save the cost of other blades owing to savings materials. In order to balance

the weight of the single blades, they require a counterbalance on the

opposite side of the hub. In addition, they need higher wind speed to

produce the same power output which obtains by the three bladed HAWT.

The two bladed wind turbines almost have the same disadvantage of the

single bladed and they can capture slightly less energy than three bladed.

The multi bladed turbines are mostly used as water pumping windmills

and they do not use for producing the electricity [7]. Therefore, most of thepresent commercial wind turbines have three blades.

Figure 3.2. Classification of wind turbines

The horizontal axis wind turbines base on the orientation of the rotor can

also be classified into upwind and downwind. When the wind flow hits therotor before the tower and makes it to rotate then it is called upwind wind

turbine. The advantage of upwind design is that the blades can be worked in

undistributed air flow but the wind forces turn the rotor in direction of wind

[6]. Thus, they need an extra active mechanism, yaw mechanism, to keep

the rotor (blades) against downwind. On the other side, in downwind wind

turbines, the wind hits the tower first and then hits the rotor. Hence, the

wind itself can keep the rotor in downwind situation without any

supplementary mechanism.


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In all the time, the wind direction is not steady and changes fast, hence the

upwind wind turbine yaws faster than downwind due to having the active

yaw mechanism. Figure 3.3 displays the upwind and downwind wind

turbines.

Figure 3.3. Upwind and downwind turbines

3.2 Wind Farms

Numerous wind farm projects are being constructed around the world with

both offshore and onshore developments in wind turbine technology.

The onshore wind turbines (figure 3.4) are installed frequently in upland in

order to achieve the higher wind speeds. However, the onshore wind

turbines are not growing as fast as offshore wind turbines due to some

restrictions such as turbine noises and limited availability of lands.


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Figure 3.4. Onshore wind turbines

The offshore (figure 3.5) wind turbines give us more power output and

operate more hours in each year compared with the same turbines installedin onshore due to having higher and more constant wind speeds in open

areas [6]. Another advantage of using the offshore wind turbines is having

lower wind turbulence with higher average wind speeds and receiving less

acoustic noises from the turbine [14]. On the other hand, onshore wind

systems have some other advantages which make them also to be

significant such as cheaper substructure, cheaper installation and access

during the construction period, cheaper integration with the electrical-grid

network and cheaper and easier access for operation and maintenance [29].

Figure 3.5. Offshore wind turbines


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4. Wind Turbine Components

Today, most of the commercial wind turbines are horizontal axis wind

turbine with typically three blades [18]. The main subsystems of a

horizontal axis wind turbine, as shown in figure 4.1, can be separated into

the rotor which consists of the blades and the hub; The nacelle which

includes gearbox, drive train, control parts and yaw system; The tower and

the foundation which depends on type of turbine, onshore or offshore and

finally the balance of the electrical system which is including cables,

switchgear, transformers, and possibly electronic power converters [1].

Figure 4.1. Major components of a horizontal axis wind turbine


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4.1 ROTOR

The most important and outstanding part of a wind turbine is the rotor

which is composed of the hub and the blades. The rotor receives kinetic

energy from the wind flow and transforms it into mechanical shaft power.

4.1.1 Rotor Blades

4.1.1.1 Aerodynamics of Rotor Blades

Aerodynamic deals with the influence of gas forces on the bodies when air

or other gases moving through them. During the development of wind

turbine, several research and enquires have been done in aerodynamic filed

in order to find the successful model.

4.1.1.2 Airfoils

The cross-section of a wind turbine blade is an airfoil which is used to

generate mechanical forces due to motion of fluid around the airfoil. The

width and length of the blade depend on the desired aerodynamic

performance and the maximum desired rotor power.

Airfoils parameters

The major characteristics of an airfoil are shown in figure 4.2. Different

types of airfoils are used along the blades in order to catch the energy from

the wind. There are many types of airfoils available in designing blades and

they are classified by the numbers which are specified from the NACA

(National Advisory Committee for Aeronautics). Figure 4.3 illustrates the

three classes of the airfoils.


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Figure 4.2. Main parameters of an airfoil

For instance, an airfoil which is specified with four digits, the first number

indicates the maximum camber of the airfoil at the chord line (in per cent ofchord), the second number demonstrates the location of the point of

maximum camber from the leading edge (in tenth of the chord) and the

third and fourth numbers show the maximum thickness (in per cent of the

chord) [4].

Figure 4.3. Sample airfoils

Forces on an airfoil

When an airfoil is located in a wind flow, air passes through both upper and

lower surfaces of the blade which has the typical curved shape. This shape

makes the air to travel more distance per unit time at the upper side than the


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lower side. In other words, the air particles move faster at the upper side of

the airfoil.

According to the Bernoullis theorem, the variety of the speed in upper and

lower side of the blade is made the different pressure on the upper and

lower surfaces of the airfoil. Therefore, these pressure differences in theairfoil will cause a force R (figure 4.4.) which is divided into two main

components in x and y directions as follows:

Lift force is specified as a force which is vertical to the direction of

oncoming airflow. The lift force is outcome of the unequal pressure on the

upper and lower airfoil surfaces. The lift force is given by

coeficient (4.1)Drag force is defined as a force which is 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. The

Drag force () equals to coeficient (4.2)

Figure 4.4. Airfoil lift and drag


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where is the density of air, V is the velocity of undisturbed air flow, A isthe projected airfoil area () and , are the lift and dragcoefficients which can be found under wind tunnel experiments. In the

wind tunnel, the lift forces and the drag forces of the fixed airfoil aremeasured by some transducers which are located in the vertical and

horizontal planes [4].

Lift and drag forces on an airfoil are influenced by the angle of attack,,which is the angle between the undisturbed wind direction and the chord of

the airfoil [4]. For instance, the effect of angle of attack on the lift

coefficient of an airfoil demonstrates in figure 4.5. As it is shown, the lift

force increases with

and reaches to the maximum value at a certain angle

of attack (12in this example). After this specific point, the lift coefficientquickly decreases with further increase in due to entering the airflow inturbulent region which separates the boundary layers from the airfoil.Therefore, the drag force rapidly goes up and lift force goes down at this

region.

Figure 4.5. Effect of angle of attack on airfoil lift


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4.1.2 Wind Power

The kinetic energy of the wind can be written as:

(4.3)where m is mass (kg) and V is speed ( ).The volume of air which flows through the rotor in HWAT turbines is

cylindrical (figure 4.6.) with the amount of mass which passes through the

rotor of turbine per second. Therefore, the energy per second can be defined

as:

4.4

4.5where A is the cross-section area of the cylinder and equals to .

Figure 4.6. Volume of air in front of the rotor

It should be mentioned that power is defined as the rate of wind energy

passes through an area per unit time which means:


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4.6

4.1.3 Rotor Power and Torque

In reality, a wind turbine cannot derive all the wind power from wind

stream when it passes through the rotor of the wind turbine, which means

that some parts of kinetic energy of the wind is transferred to the rotor and

the rest of the energy leaves the rotor. Therefore, the amount of wind

energy which is converted to the mechanical power by the rotor is defined

as the efficiency that is usually termed as the power coefficient, . Thepower coefficient of the rotor can be explained as the ratio of power outputfrom the rotor to the accessible power of the wind in theory [4] which isdefined as:

2 4.7

where is the power output of the wind turbine. The power coefficient ofa turbine relies on many factors such as rotor blades profile, bladearrangement, and blade setting, etc.In order to find the torque of the rotor, we need to define the thrust force by

the rotor which can be expressed as:

12 4.8Hence, the torque of the rotor will be:

12 4.9where R is the radius of the rotor.


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The torque of the rotor, in reality, is less than this value and is expressed in

terms of the torque coefficient which is defined as the ratio between the

actual torque and the theoretical torque; hence this coefficient is explained

by:

2 4.10where is the actual torque developed by the rotor.In order to find the efficiency of interaction between the rotor and the wind

stream, another significant factor should be described as the ratio between

the velocity of the rotor tip and the wind velocity, which is called tip speedratio:

2 4.11whereis the angular velocity and N is the rotational speed of the rotor.

In addition, the torque coefficient and the power coefficient can be changed

by the tip ratio as it is expressed:

4.12

4.1.4 Rotor Design

Some parameters based on the fundamental aerodynamic theories are

needed in order to receive more energy from the wind and produce more

power from the rotor. These parameters can be expressed as follows [4]:

1. Radius of the rotor ()2. Tip speed ratio of the rotor at the design point (

)


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3. Number of blades ()4. Design lift coefficient of the airfoil ()5. Angle of attack of the airfoil lift ()

The first parameter, radius of the rotor, depends on the speed of wind and

the power expected from the turbine which can be expressed at the designpoint as:

12 4.13As we know

, therefore the radius of the rotor can be defined as:

4.14

where is the design power coefficient of the rotor, is the drive trainefficiency, is the generator efficiency and is the design windvelocity [4].

Design tip speed ratio depends on the function of the turbine which is

applied. For instance, wind pump rotors need low tip speed ratio due to the

high starting torque. However, electricity producing wind turbines require

high tip speed ratio due to the fast running rotor.

Number of blades in a rotor is directly related to the tip speed ratio which

means that we need lower blades for the higher tip speed ratio and vice

versa.

Figure 4.7 shows the number of blades based on the design tip speed ratioin order to select the appropriate numbers of the blades.


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Figure 4.7. Number of blades and design tip speed ratio

As it is mentioned before, the lift and drag coefficients are the properties of

an airfoil which can be found under wind tunnel experiments. Thesecoefficients may be available for some standard airfoil sections at different

angles of attack and can be used in rotor design. In order to get more

performance from the rotor, we need to maximize the lift force and

minimize the drag force by reducing the angle of attack that can be obtained

by plotting a tangent to the curve of -as it is shown in figure 4.8.


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Figure 4.8. -relationship of an airfoilIf such data are not simply available for the selected airfoil, the necessary

information should be achieved through wind tunnel experiments, figure

4.9.

Figure 4.9. A low speed wind tunnel


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4.1.5 Rotor Blades Material

The properties of the rotor blades materials depend strongly on the blade

dimensions, operating wind circumstances, and stresses which arise from

bending moments on the hub structure. In order to construct the high size

wind turbines, rotor blades are made from glass-reinforced plastic (GRP),carbon fiber-reinforced plastic (CFRP), steel, and aluminum [3]. On the

other side, the production efficiency and cost of construction for small wind

turbines, which can be categorized by the rotor diameter (smaller than 5 m),

are more important than weight, stiffness, and other design requirements.

Foam, Gel coat, glass fibers, and epoxies are the most used materials in the

composite in order to build long blades with high stiffness and lightweight

characteristics. For instance, Fiberglass offers high stiffness, flexural

strength, and shear resistance under extreme thermal and mechanicalenvironments. Some designers have considered high-performance resins

such as epoxy in production of rotor blades. Therefore, it is important to

evaluate the critical performance characteristics of resins which are

viscosity, exothermic properties, and other factors, in order to be sure that

the blade can be tolerated to the stresses and other causes.


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The active part of the protection system is the brake system which is

consisted of two independent systems: an aerodynamic brake system and a

mechanical brake system. An aerodynamic brake system controls the blade

tips which can be adjusted or the entire rotor blade can be pitched. The

mechanical brakes are usually used as a backup system for the aerodynamic

braking system in the wind turbine [4]. They consist of brake calipers,brake discs and brake pads.

The wind turbine generator transforms the mechanical energy of the shaft

into electric power. While the blades transfer the kinetic energy of the wind

into rotational energy in the transmission system, the generator provides the

next step in the supply of energy from the wind turbine to the electrical

grid.

Yaw mechanism lines up the plane of rotation to be vertical to the direction

of wind. The yaw mechanism is classified into passive yaw and active yaw.

The passive yaw is used for small wind turbines and downwind turbines.

The active yaw, the second type, is used in most of the upwind wind

turbines. This type of yaw mechanism uses an electromechanical drive and

a control system in order to control and monitor the yaw in a wind turbine.

4.4 Tower

The tower of a wind turbine supports the nacelle and the rotor and provides

the safe and reliable operation of turbines under a variety of wind

conditions [6].

In order to capture more energy from the wind and generate more power,

the possible solution is using higher tower. Manufactures prefer to producetaller towers due to utmost safety, optimum performance and better design

flexibility. On the other hand, transportation, assembly and servicing of the

components become more difficult and costly when the height of the tower

increases [4].

The second design parameter of a tower is its stiffness. Determining the

first natural bending frequency in the right way is an important task in the

design [5]. This establishment helps designer to choose the suitable

materials for the tower in order to achieve the required stiffness at the


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5.1 Aerodynamic loads

The aerodynamic loads are caused by airflow when it is passed from the

turbine blades and the tower, thus, the aerodynamic loads can be separated

into aerodynamic loads along the blades, described in section 4.1.1.2 (Liftforce and Drag force), and aerodynamic drag force on tower which is

defined as:

0.5 5.1where

is aerodynamic drag coefficient and A is projected area vertical

to the flow. Furthermore, in high wind speed situations, when a windturbine is stationary, the drag forces are the primary consideration whereasthe lift forces are more concerned when the turbine is operating [1].

5.2 Gravity Loads

The gravity loads is an important source of loads that can be imposed large

fatigue stresses on the rotor and the tower. When the dimension of wind

turbine grows, the structures weight becomes the main problem with

respect to strength. The gravity forces are simply given as:

5.2Where

is the mass of the i-th blade element and

9.82 .

The gravity on the tower equals to:

5.3


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5.3 Centrifugal Loads

The influence of inertial forces on blades mass which causes moments to

pitch the blades is called centrifugal loads. Due to low rotation speed of the

rotors in large wind turbines, centrifugal forces are not very considerable[4]. On some rotors, however, blades are flapped into the out-of-rotating-

plane direction which causes the cone angle between the blades and the

rotational speeds. This angle in some conditions can be reduced the blade-

root bending loads and in contrast, leads to an increase in the mean flap

wise bending. The centrifugal loads on the rotor equal to:

5.4

in which is the mass of the i-th blade element, is the radial position ofthe i-th blade element in a discretisation of the blade into n elements and is the angular rotor speed.

5.4 Gyroscopic Loads

The gyroscopic loads are caused by the rotation of the rotor when it is

yawed into the wind. A fast yawing rate is made a large gyroscopic

moments which reveal as pitching moments on the rotor axis [4]. However,

the yawing rates in horizontal wind turbines are normally low due to using

the active yaw mechanism, whereas the gyroscopic forces in the wind

turbines with passive yawing are the significant problem.

The rotor is needed to also be yawed very quickly when wind directions are

changed rapidly during low wind speeds. Therefore, rotor blades, under thiscondition, are faced to extraordinary bending loads [6]. This is another

reason that passive yaw mechanism are applied in downwind rotor which

are no longer being built.


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5.5 Wind Turbulence

The turbulence of the wind is the fluctuation of the wind speed in short time

scale which acts as a dynamic load in wind turbines. Thus, wind turbine

components can be influenced by fatigue loads, particularly on the rotorblades, due to the large turbulence when the speed of wind is significantly

high. It is helpful to think of the wind as comprise of a mean wind speed

which fluctuates on a time-scale of one or several hours. Wind speed

naturally fluctuates in all three directions, longitudinal, vertical and lateral

directions [6]. The turbulence model, however, is assumed only in onedirection, longitudinal, due to the difficult numerical handling of two

dimensional turbulence models and the influence of longitudinal turbulence

over the rotor-swept area.

The main parameter in turbulence is intensity which measures rapid

changes in wind speed over short intervals:

5.5where

is mean wind velocity

is the standard deviation ration at the

same point and averaged over same period of time.

In general, the highest turbulence intensities occur at the lowest wind

speeds [6], but the lower limiting value at a given location will depend on

the specific land features and surface conditions at the location.

5.6 Wind Shear

Wind shear is meteorological phenomenon which describes the changes in

wind speed as a function of height [6]. The velocity of wind around the

ground surface is theoretically considered to be zero due to the frictional

resistance of the airflow around the ground. Thus, the rate of wind velocity

increases with the height which is an important factor in designing of wind

energy plant. Figure 5.2 displays the wind speed changes respect to height.


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Figure 5.2. Wind flow in boundary layer

The power law is the most common method to describe the average wind

speed as a function of height above the ground which is defined as [12]:

5.6in which are wind speeds at heights , and is the windshear coefficient which depends on some factors, such as height, time andlocations. This coefficient is normally considered between 0.15-0.3., as

shown in figure 5.3.


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Figure 5.3. wind speed respect to height for different values of shear, the

average wind speed is given 8 m/s at 50 m


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6.

6.1

The

part

Thi

diff

fini

con

sol

ele

maall

sol

The

Theo

Finite

FEM is one

ial differentia

method appl

rential equati

e number o

ected to eac

ed by defini

ent. These f

be linear, qulements are

tion for the e

Figure 6

major steps i

1. Find the

system.

2. Convert t

y

lement

f the most p

l equations w

ies a variation

on over the

sub domai

other at the

g approxima

nctions shou

dratic or higknown then

tire region [2

.1. Typical fin

FEM are [13

trong form o

e strong form

35

ethod (F

werful nume

hich are appe

al problem w

odel domain.

s called ele

oints called

e interpolatio

d be a compl

er order. Whhey are plac

1].

ite element m

]:

the governi

of the equati

M)

ical methods

ared in engin

ich involves

This domain

ments and t

odes.The su

or shape fu

ete set of pol

n the polynod together in

sh (two dime

g differential

n to the weak

for resolving

eering proble

n integral of

is divided int

e elements

domains can

nctions for e

ynomials, wh

ial functions order to fin

sional)

equation of

form.

the

s.

the

o a

are

be

ch

ich

ofa

the


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36

3. Choose suitable interpolation (shape) functions.

4. Choose the weight functions and set up the algebraic equations for

each element.

5. Obtain the global matrix system of the equations through the

assembly of all elements.

6.

Impose boundary conditions.7. Solve the system of algebraic equations.

8. Postprocess the results.

6.2 Computational Fluid Dynamics (CFD)

CFD is a numerical method which can be used to predict fluid flow, heat

transfer and chemical reactions in complex systems. CFD has been appliedmost widely in industrial and non-industrial application areas due to less

times and costs requirement in designing models [10]. In order to analyze a

fluid problem with CFD, we need to obtain the mathematical equations

which describe the behavior of the fluid flow.

6.2.1 Governing equation

All fluid dynamics are based on three fundamental physical principles:Mass conservation, Newtons second law (Momentum) and energy

conservation.

6.2.1.1 Mass conservation

The conservation of mass means that the rate of mass flow into a fluid

element (volume) equals to the rate of increase of mass in the fluid element

(volume) [10], therefore for a compressible fluid:

div 0 6.1where is density of the fluid and is the velocity vector in Cartesiancoordinates.


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And the density of an incompressible fluid, such as liquid, is constant

( 0, so:

div 0

0 6.2

where , are the velocity components of .6.2.1.2 Newton's second law

Newton's second law declares that the rate of change of momentum equals

to forces summation on the fluid particles. The forces can be divided to

surface forces as separate terms and body forces as source term [10]. Then,the momentum equations in three directions can be obtained by considering

the stresses in terms of the pressures on a control volume. Therefore, the

momentum equation in x, y and z components equals to:

div

6.3a

div 6.3b div 6.3cwhere , are body forces (source term), for example thevalue of body forces due to the gravity will be [10]:

0 , 0 . The stress components are obtained by Navier-Stokesequations.6.2.1.3 Energy equation

The energy equation is obtained by the first law of thermodynamics which

describes the rate of change of energy of a fluid particle is equal to the rate


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of heat addition to the fluid particle plus the rate of work done on the

particle [10]. Hence, the energy equation equals to:

div div div 6.4

where is the internal energy, is the temperature, is the thermalconductivity, p is the pressure, u, v and w are the velocity components of uand is a new source term which is a source of energyand is a Mechanical (Kinetic) energy source.Therefore, the equation of energy for compressible fluids will be:

div div

6.5

where

is the specific total enthalpy and

is an enthalpy energy source.

6.2.1.4 Navier-Stokes equations

There still some unknown variables, the viscous stress components, areremained in previous equations. These values can be obtained by presenting

the suitable model which is represented as functions of the local rate of

deformation for most of the fluid flows. The local rate of deformation is

made of the linear deformation rate and the volumetric deformation rate in


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three-dimensional flows [10]. The Newton's law of viscosity for

compressible flows composed of two constant viscosities, dynamic

viscosity, , which is related to linear deformations and the secondviscosity, , which is related to the volumetric deformation. Therefore, thesix viscous stress components are constant and three of them are variable.

These components are explained as:

2 , 2 , 2 , 6.6By substituting the equations 6.6 to equations 6.3a, b, c we will reach to the

Navier-Stokes equations:

div 6.7a

div 6.7b div 6.7c

6.2.2 Finite Volume Method

Finite Volume Method (FVM) or Finite Control Volume Method is the

most popular discretization method in CFD. This method divides the main

domain into control volumes and then integrates the equations over each

control volume. The FVM method in some features is similar to finite

different method while the descritization form is connected to the finite

element method. The computational effort of this method is greater than

finite difference method and less than the finite element method for a


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similar accur

value which

(mass, mom

secondly, the

Figure 6.2.

The general c

where is aBy integratio

becomes

cy [21]. In

has some a

ntum, energ

complex geo

ypical finite

onservation e

luid property

n of equatio

divCV

40

ddition, the

vantages, fir

) remains c

etries can be

olume grid t

uation is equ

div

and is the d

6.8 over a

VM is based

st the conser

nserved at t

meshed.

o dimension

l to:

ffusion coeffi

ontrol volum

on the cell

vation of qu

he local scal

l (rectangula

cient.

e [10], the e

verage

antities

es and

grid)

6.8

uation

6.9


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41

By applying Gauss' divergence theorem, equation (6.9) can be written asfollow [10]:

. A . 6.10

The change rating term of (6.9) for the steady state problems is equal to

zero, therefore,

. A

.

6.11And for the transient problems, the equation will be [10]:

. A

.

6.12

6.2.3 Turbulence modeling

Turbulent flow is a type of fluid (gas or liquid) flow when the velocity and

other properties of the fluid fluctuate in all directions. Using the Navier

Stokes equations for a turbulent flow is extremely difficult due to the time-


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42

dependent, nonlinear and three-dimensional equations. Hence, the Reynolds

Averaged Navier-Stokes Equation (RANS) is the most widely used for

calculating industrial flows [10]. The RANS can be obtained from Navier

Stokes equations by considering the mean properties for the flow such as

mean velocities, mean pressures and mean stresses etc., therefore the

equations 6.7a, b, c become [10]

div

6.13a

div

6.13b

div

6.13c

The additional parts of equation on the mean velocity components U, V and

W are turbulent stresses which calledReynolds stresses.

where:

; ; ; ; u, v, w are the velocity components of u ; U,V, W are the mean velocity

components of U; , , are the fluctuating velocity components of and

are mean and fluctuating component of pressure.


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43

The Reynolds stresses could be obtained from mean rates of deformation

which is equal to [10]:

6.14

where is the turbulent viscosity which is determined by different models.6.2.3.1 The modelThe considered model for the free stream fluid is the model whichhas one equation for the turbulent kinetic energy, k, and one equation for

the turbulent dissipation rate, . Hence, the turbulence viscosity of thismodel can be obtained by the below equation: 6.15where is a dimensionless constant, k and are determined by thefollowing equations:

div 6.16 div 6.17

is the turbulence production and

, , are constants and

equal to [10]: 1 ; 1.3 ; 1.44 ; 1.926.2.3.2 The modelThis model is suitable for calculating the turbulence near the wall. The

model is based on model transport equations for the turbulence


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44

kinetic energy, k, and the specific dissipation rate, . These values arederived from the equations below:

div

6.18 div 6.19

, , , are constants and equal to:

1 ; 2 ; 59 ; 0.075 ; 0.09Therefore, the turbulence viscosity of this model will be:

6.20

The advantage of this method is using the low-Reynolds number near the

wall and easier modeling which gives us more accurate and more robust

result but the disadvantage of this method is high sensitivity to the free-

stream conditions [25].

6.2.3.3 The Shear-Stress Transport (SST) model

The SST model is based on the model and has the same automaticwall treatment. This model is mixing the best properties of modeland model which means around the near-wall region is using the model and in free-stream flow is using the model in order toget better results [25].


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45

6.3 Fluid structure interaction

When the system is a coupled of fluid and solid structure then it is called

fluid structure interaction (FSI) which means that the fluid effects on

deformation of solid geometry and the deformed geometry is changing thefluid variables, too. The FSI is an example of a multiphysics problem which

the fluid and solid have interacted with each other. Two types of FSI have

supported by mechanical application in ANSYS, one way FSI and two

ways FSI. The result from CFD analysis is applied as a load to the

mechanical analysis and if the result has passed back to CFD then it iscalled tow way FSI otherwise is called one way FSI [25].

The one way FSI allows computational fluid dynamics (CFD) and finite

element analysis (FEA) solvers to be run independently whereas in two

ways FSI, both solvers have to be run at the same time because the results

are transferred between CFD and FEA for each step until overall

equilibrium is reached among the Mechanical application solution and

ANSYS CFX solution. In this research, we are considered the one-way FSI

for our simulation.


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46

7. Simulation

7.1 Geometry

The structural model contains the full geometry of wind turbine which is

tower, nacelle and rotor. The 3D model has been created in

ANSYS/DesignModeler based upon published information of 5MW wind

turbine from Repower [27], as shown in table below:

Table 7.1. 5MW wind turbine specifications

Rotor Nacelle Tower

Diameter 126 m Length 19 m Hub height 120 m

Blade length 61.5 m Width 6.8 m material Steel

Max. bladewidth

4.6 m Height 6 m weight 540 tone

Blade materialGFRP with

EpoxyWeight

316

toneUp diameter 5.5 m

Weight of eachblade

18-19.5 tone Down diameter 6 m

Rotor weight125 129.5

tone

Maximumrotation speed

12.1 rev/min

Figure 7.1 illustrates the final 3D structural model which has been designed

and the real model from Repower. In the next step, the solid model has

been inserted in a proper wind farm which has been recommended by the

manufacturer, as shown in figure 7.2.


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6

Figure 7.1.

Figure

756

m

47

umerical and

.2. Wind far

real models

model

4D 504 m

1.5D

189m


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48

7.2 CFX

7.2.1 Mesh

The aim of CFX-mesh is producing high quality meshes which can resolveboundary layer phenomena and fulfill severe quality criteria [25]. CFX-

Mesh makes meshes including tetrahedral, prisms, and pyramids in

standard 3D meshing model and also can be contained hexahedra in the 2D

meshing mode. In this project, tetrahedral mesh has been chosen with

advance size function in ANSYS due to the complexity of model. This

function can be effective for creating high quality meshes around the solid

walls. In addition, Mapped face meshing has been used for the faces of the

nacelle, rotor and the tower in order to provide more uniform meshes.

According to the ANSYS help, different types of analyses have differentmeshing requirements which mean that we should create finer mesh with

slow transition in CFD problems whereas coarser mesh with higher order

elements and faster transition should be used in mechanical problems [25].

Figure 7.3 shows the final mesh that has been created in the CFX part. The

total number of mesh which has been used in the CFX part equals to 2.5

million.

(a)


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49

(b)

(c)

Figure 7.3. CFX mesh details (a) whole domain, (b) half of the whole

domain, and (c) rotor, nacelle and tower

7.2.2 Model set up

The next step after creating the appropriate and suitable meshes for all the

parts in CFX is specifying domains, boundary conditions, type of analysis,

interfaces, etc. In order to solve the problem, it is necessary to create at

least two types of domains which are a stationary and a rotating. The

nacelle and the tower are considered inside the stationary domain and the

rotor is inserted in the rotating domain. Then, three interfaces have been


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defined betw

changes in re

In order to r

instead of ste

changing witthe analysis

significant p

small enough

each domain

discussed in t

Figure 7.4 r

simulation.

The next step

below:

Inlet

According to

is changing

input wind s

assumed to b

Stationary dom

een the stati

erence frame

tate the roto

ady state anal

time. Therefsection. It sh

rameter in th

to resolve t

is another

e next part.

epresents the

Fig

is to define t

the wind shea

ith the height

eed in the i

air at 25 .

ain

50

nary domain

.

r in ANSYS,

ysis which m

ore, total timeould be note

e transient si

e problems a

actor in the

domains w

re 7.4. Dom

e boundary c

r principle, th

. Thus, the eq

nlet boundar

and the rot

we should u

eans that the

and time step that the si

ulations and

ppropriately.

transient ana

ich have be

ins in CFX

nditions for t

speed of wi

ation 5.5 is

layer. The

ting domain

se transient

ariables of fl

should be dee of time st

must be con

The initial v

lysis which

en supposed

he whole do

d is not const

efined and se

ype of fluid

Rotating do

due to

nalysis

ow are

ined inp is a

sidered

lue for

ill be

in the

ains as

ant and

as the

is also

ain


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Ou

The

exit

pre

turbFig

Tur

The

hav

mo

cha

let

type of outl

ed and enter

sure is also s

ine.re 7.5 shows

bulence mod

turbulence

been chosen

el respect to

ter.

Outlet (open

t has been c

d through th

pposed to be

the assumed

el

odel for the

Shear Stress

the k- m

Figure 7.5.

round

ng)

51

nsidered ope

boundary s

zero due to u

oundary cond

stationary do

Transport (SS

del which

oundary con

ing class w

rfaces. The

bounded area

itions in the s

ain and the

T) due to the

as described

itions in CF

ere fluid can

alue of relat

around the w

mulation.

rotating dom

dvantage of t

in the previ

Inlet (Speci

Interface

be

ive

ind

ain

his

us

ied velocity)

s


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52

Initial conditions

In order to solve the transient problems, we have to specify the initial

conditions for each domain since the data describes the state at the

simulation start time [25]. In stationary domain initialization, the type of

velocity has been considered in Cartesian coordinate with the zero values,whereas Cylindrical coordinate has been chosen with the same values for

the rotating domain. The relative pressure between inlet and outlet

boundary layers is also assumed to be zero in both domains.

According to the specification of the wind turbine, the angular velocity of

the rotating domain should be set at 12.1 which is the maximumrotation speed of the rotor. Therefore, the rotor speed has been fixed at the

constant value in the software.

7.3 Structural

7.3.1 Mesh

In structural section, different types of mesh have been carried out for all

the parts. Hexahedron and tetrahedron are the suitable meshes for thenacelle and tower. The appropriate mesh, however, for the rotor is only

tetrahedron due to the complexity of the shape. Figure 7.6 shows the final

meshes which have been created for all the parts.


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53

Figure 7.6. Different types of meshes for mechanical part

The number of elements and nodes for each type of mesh can be seen in

table below:


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54

Table 7.2. Different meshes statics

Mesh Nodes Elements

Tetrahedrons 1434062 726347

Tetrahedrons + Hexahedrons1128165 412554

The simulation has been done with the second type of mesh (hexahedron

and tetrahedron meshes).

7.3.2 Model set up

In this section, the boundaries and loads are defined for the model in order

to calculate the stresses and deflections in the tower and the rotor. The

significant feature in structural part is importing the effective forces from

the CFX into the mechanical. Therefore, ANSYS is mapping the forces on

each node in CFX into the mechanical node which depends on some factors

such as quality of the mesh, element size, etc [25]. It should be noted that

we should check the percentage of mapping and the value of forces after

importing the loads in order to be sure that all the forces are properlymapped. Figure 7.7 illustrates the imported loads on the rotor, nacelle and

the tower from CFX into the Mechanical.


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Figure 7.7. Imported loads on the tower, nacelle and the rotor


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8. Results

The simulation has been carried out with transient analysis for 80 seconds

with time step size of 0.1. Figure 8.1 represents the different snapshots of

the wind velocity at 80 second in the stationary domain. It should be

mentioned that the rotating domain has not been considered in these figures

due to high velocity changes around the rotor. As can be seen from the

figures, the speed of the wind has been changed around the tower and the

nacelle due to the rotation of the rotor.

(a)


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57

(b)

Figure 8.1. Velocity of the wind in the stationary domain in different

snapshots at 80 sec, (a) in XY plane (left view), and (b) in XZ & XY plane

When the energy of the wind is extracted by a wind turbine, then the air

leaves the turbine with less speed, less energy and higher turbulence level,

which is called the wake of a wind turbine. Another wind turbine operating

in this wake will therefore produce less energy and tolerate greater

structural loading than a turbine operating in the free stream. Figure 8.2

shows the shape of the wake turbulence behind the wind turbine when the

air passes through the turbine.


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58

(a)

(b)

Figure 8.2. Wake Turbulence behind the wind turbine, (a) Front view, and

(b) Isometric view


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59

In order to import the loads from the CFX into the Mechanical, first we

should obtain the influence of wind forces on the nacelle, tower and the

rotor. In figure 8.3, the value of wind forces on the rotor has been caculated

in all directions. As can be seen from the figure, the maximum force on the

rotor is in direction of the wind which is increased sharply and then

gradually reach to the maximum value. On the other hand, the forces valuesin other directions are almost equal to zero. Figure 8.4 shows the impact of

wind forces on the nacelle and the tower which is not constant and

changing periodically due to influence of the turbulence. These changes in

scale of turbulence increase the structural vibrations of the wind turbine,

which cause increased fatigue loads.

Figure 8.3. Wind forces on the rotor

1.0E+05

0.0E+00

1.0E+05

2.0E+05

3.0E+05

4.0E+05

5.0E+05

0 10 20 30 40 50 60 70 80

Fo

rce[N]

Time[s]

Force(X) Force(Y) Force

(Z)


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60

(a)

(b)

Figure 8.4. Wind forces on the (a) nacelle and the (b) tower

4.0E+03

3.0E+03

2.0E+03

1.0E+03

0.0E+00

1.0E+03

2.0E+03

3.0E+03

0 20 40 60 80Force[N]

Time[s]

Force(X) Force(Y) Force(Z)

5.0E+04

4.0E+043.0E+04

2.0E+04

1.0E+04

0.0E+00

1.0E+04

2.0E+04

3.0E+044.0E+04

5.0E+04

0 20 40 60 80Force[N]

Time[s]

Force(X) Force(Y) Force(Z)


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61

The stresses and deflections in each part can be estimated after importing

the loads from the CFX into the Mechanical part. Figure 8.5 and 8.6

illustrate the stresses and deflections in the tower and the rotor,

respectively. As it can be seen in these figures, the maximum stress in the

tower is located at the lowest point of the tower.

Figure 8.5. Stresses in the tower and the rotor

(scale factor=62)


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62

Figure 8.6. Deformations in the tower and the rotor

(scale factor=62)

And finally, the influence of gravity forces on all the parts in structural has

been investigated. As we can see from the figure 8.7, the stresses in the

tower and the rotor are siginifantly higher than the case which is turbulence


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63

loads effect on these parts. Therefore, the gravity forces are very important

loads and should be considered in designing of the large wind turbines.

Figure 8.7. Gravity loads on tower and rotor (logarithm scale with a scale

factor of 82)


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64

9. Conclusion

The purpose of this research has been to investigate the effect of turbulence

and gravity loads on the tower and the rotor of a 5MW wind turbine in

order to calculate the stresses and deflections in these parts.

For this goal, a full 3D-model of a 5MW onshore wind turbine was

simulated by using the commercial software ANSYS. The simulation was

set with fairly realistic domains and boundaries which had considerable

influences on the outcomes.

The results represented that the turbulence after the rotor was influenced on

the wind turbine as dynamic loads which cause increased fatigue loads on

the structural parts. The fatigue loads decrease the lifetime of componentsand might be brought them to failure. In addition, the gravitational load of

each part must be taken into account in designing of a wind turbine. As it

was shown in the result section, the gravitational loads were higher than

turbulence loads in the large wind turbine and both loads with together

become dominant factor for the fatigue load in each component in wind

turbines.

Wind turbines are generally grouped together in wind farms and some of

them operate partly or fully in the wake turbulence of upstream turbines.

These downstream turbines are faced to the wind with higher turbulenceand less energy which increases the fatigue loading in the components.

Therefore, the influences of turbulence loads in downstream wind turbines

are higher than upstream wind turbines. However, when the downstream

turbines are far enough from the upstream turbines, the wake wind speed

will recover to the free stream value and then the effect of turbulence loads

will be normal and not so high. The distances between wind turbines are

thus have an important factor in the value of turbulence loads on the

turbines in a wind farm.


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10. Future Works

This research tried to investigate the effect of two important types of loads

on an enormous wind turbine. Other explorations can be executed with this

3D-model in the future:

Implementing different types of turbulence models in simulation

and comparing the results.

Calculating the fatigue loads on the wind turbine components.

Modifying the blade shape and calculating the aerodynamic loads in

the wind turbine.

Calculating the effect of turbulences and wind forces on the

components with extreme wind speed condition which is exceeded

only once within a period of 50 years.

Investigating the load effects of upstream on downstream wind

turbines in a wind farm.

Study of the turbulent wake behind a wind turbine.


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EXPLAINED: Theory, Design and Application (2nd

Edition), John

Wiley & Sons Ltd.

2. A.R. Jha, (2008), Wind Turbine Technology, CRC Press.

3. Martin O. L. Hansen, (2008), Aerodynamics of Wind Turbines (2nd

Edition), Earthscan.

4. Sathyajith Mathew, (2006), Wind Energy: Fundamentals, Resource

Analysis and Economics, Springer.

5. Tony Burton, David Sharpe, Nick Jenkins, Ervin Bossanyi, (2001),

WIND ENERGY Handbook, JOHN WILEY & SONS Ltd.

6. Wei Tong, (2010), Wind Power Generation and Wind Turbine Design,

WIT Press.

7. Jashua Earnest, Tore Wizelius, (2011), Wind power plants and project

development, PHI Learning Pvt. Ltd.

8. Martin O. L. Hansen, (2008), Aerodynamics of Wind Turbines (2nd

Edition), Earthscan.

9.

DNV/Ris, (2002), Guidelines for Design of Wind Turbines (2nd

Edition), Det Norske Veritas.

10.H. K. VERSTEEG and W. MALALASEKERA, (1995), An

introduction to computational fluid dynamics, Longman Group Ltd.

11.Tore Wizelius, (2007), Developing wind power projects: theory and

practice, Earthscan.

12.Pramod Jain, (2010), Wind Energy Engineering, McGraw-Hill

13.

Broman, Gran, (2003), Computational Engineering, Bleking Instituteof Technology.

14.Olimpo Anaya-Lara, Nick Jenkins, Janaka Ekanayake, Phill Cartwright,

Mike Hughes, (2009), Wind Energy Generation: Modelling and

Control, John Wiley & Sons, Ltd.;

15.European Wind Energy Association (EWEA), (2009), Wind Energy

The Facts: A Guide to the Technology, Economics and Future of Wind

Power, Earthscan.


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16.Thomas Ackermann , (2005), Wind Power in Power Systems, John

Wiley & Sons, Ltd.

17.Mathew Sathyajith, Geeta Susan Philip, (2011), Advances in Wind

Energy Conversion Technology, Springer.

18.

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