Advances in composite wind turbine blades: A comparative study
©2014
Textbook
83 Pages
Summary
In the wind industry, the current trend is towards building larger and larger turbines. This presents additional structural challenges and requires blade materials that are both lighter and stiffer than the ones presently used. This study is aimed to aid the work of designing new wind turbine blades by providing a comparative study of different composite materials. A coupled Finite-Element-Method (FEM) - Blade Element Momentum (BEM) code was used to simulate the aerodynamic forces subjected on the blade. For this study, the finite element study was conducted on the Static Structural Workbench of ANSYS, as for the geometry of the blade it was imported from a previous study prepared by Cornell University. Confirmation of the performance analysis of the chosen wind turbine blade is presented and discussed including the generated power, tip deflection, thrust and tangential force for a steady flow of 8m/s.<br>A homogenization method was applied to derive the mechanical properties and ultimate strengths of the composites. The Tsai-Hill and Hoffman failure criterions were both conducted to the resulting stresses and shears for each blade composite material structure to determine the presence of static rupture. A progressive fatigue damage model was conducted to simulate the fatigue behavior of laminated composite materials, an algorithm developed by Shokrieh.
Excerpt
Table Of Contents
9
ACKNOWLEDGMENTS
May be the last few month were exhausting and full of challenges, but what I find
most difficult at this time is writing the few words to come that has to summarize me
acknowledgement.
May be the best way, is to name the names of few that their presence was
indispensable and their efforts must be marked. My professor and coordinator, Dean Rafic
Younes, who has been an inspiration, a leader and a researcher colleague that has supported
me since the first day till present.
Dr Mazen Ghandour, I would not have been in this current position today without
your continuous contributions and all of your support will never be forgotten.
To Dr Hussein Ibrahim and the team of TechnoCentre Éolien in Gaspé Québec a
special appreciation, they have been threw out my research internship a second family that
not only offered professional assistance, but also made room for me in their life.
To all my colleagues, classmates, academic staff at the Faculty of Engineering and
the EDST at the Lebanese University, the best of success and much love.
Lastly, all of my achievements derive from one source, my loving family, to whom I
owe all of my success, overcoming many struggles and prevailing over many cynics.
"Parce qu'aimer c'est renoncer à la force"
Milan Kundera
Copyright © 2013 Adam Rafic Chehouri
All rights reserved.
No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form
or by any means, electronic, mechanical, photocopying, recording or otherwise, without prior written
permission from the author.
11
TABLE OF CONTENTS
ABSTRACT ... 7
ACKNOWLEDGMENTS ... 9
TABLE OF CONTENTS ... 11
NOMENCLATURE ... 12
LIST OF FIGURES ... 14
LIST OF TABLES ... 16
CHAPTER I: LITERATURE REVIEW ... 17
1.1 Background ... 17
1.2 Scopes and Aims
... 18
CHAPTER II: AERODYNAMIC MODELING... 20
2.1 Methods for Calculating the Aerodynamic Forces ... 20
2.2 BEM Model
... 20
2.2.1 Introduction
... 20
2.2.2 BEM Theory ... 21
CHAPTER III: STRUCTURAL MODELING ... 31
3.1 Blade Design ... 31
3.2. Blade Model ...
... 31
3.3. Load Application ... 32
3.3.1 Chord Length, Aerodynamic Centre and Twist Angle ... 33
3.3.2 Load Application and Moment Correction ... 33
3.4 Material Elastic Properties ...
35
3.5 Static Failure Criteria's ... 38
CHAPTER IV: RESULTS ... 40
4.1 Static Failure: Interlock Textures ... 40
4.2 Static Failure: Orthogonal Laminates ... 47
4.3. Static Failure: Braided Textures...55
5.2 Progressive Fatigue Damage Model ... 66
CONCLUSION & FUTURE WORK... 72
ANNEX ... 74
REFERENCES ... 81
12
NOMENCLATURE
Latin Symbols
a
- Induction factor
[-]
a'
- Tangential induction factor
[-]
A
- Area
[m
2
]
c
- Chord
[m]
C
D
- 2-D Drag coefficient
[-]
C
L
- 2-D Lift coefficient
[-]
C
N
- 2-D normal direction coefficient
[-]
C
Tang
- 2-D tangential direction coefficient
[-]
C
T
- Coefficient of thrust
[-]
D
- Drag force
[N]
E
- Stiffness
[N/m
2
]
f
- Glauert correction
[-]
F
tip
- Tip loss correction factor
[-]
F
hub
- Hub loss correction factor
[-]
H
- Total head
[N/m
2
]
L
- Lift force
[N]
m
- Mass
[kg]
m
aero
- Aerodynamic moment per unit of length
[N]
M
corr
- Correction moment per unit of length
[N]
M
- Induced moment
[N.m]
n
- Number of cycles
[-]
N
- Number of blades
[-]
P - Pressure
[N/m
2
]
P
N
- Resultant in the normal direction
[N]
P
T
- Resultant in the tangential direction
[N]
r
- Local radius position
[m]
R
- Tip blade radius position
[m]
R
- Stress ratio
[-]
R
- Residual strength
[N/m
2
]
R
hub
- Hub position
[m]
13
T
- Thrust
[N]
V
0
- Undisturbed
air stream
[m/s]
v
- Velocity
[m/s]
v
f
- Fiber content
[-]
Greek Symbols
- Angle of attack
[rd]
- Comparative parameter
[-]
- Material nonlinearity parameter
[-]
- Pitch angle
[rd]
- Air density
[kg.m
3
]
- Stress
[N/m
2
]
` - Local solidity
[-]
- Average of f( )
[-]
Ø - Relative flow angle
[rd]
- Rotational velocity
[rd/s]
- Rotation of the wind near the blade
[rd/s]
- Standard deviation
[-]
Abbreviations
ADAMS/WT
- Automatic Dynamic Analysis of Mechanical Systems - Wind
Turbine
BEM
- Blade Element Momentum Theory
CFD
- Computational Fluid Dynamics
FAST
- Fatigue, Aerodynamics, Structures, and Turbulence
FEM
- Finite Element Method
HAWC
- Horizontal Axis Wind Turbine Code
NACA
- National Advisory Committee for Aeronautics
NREL
- National Renewable Energy Laboratory
REV
- Representative Elementary Volume
14
LIST OF FIGURES
Figure 9.1: Comparison between two method solving strategies; FEM-BEM and
FEM-BEM ... 18
Figure 10.1: Schematic of blade elements; c, airfoil chord length; dr, radial length of
element; r, radius; R, rotor radius; , angular velocity of rotor ... 21
Figure 2.2: Actuator disk model ... 22
Figure 2.3: Velocities at the rotorplane ... 24
Figure 2.4: The local forces on a cross section of a blade ... 24
Figure 2.5: The numerical approach when using BEM ... 26
Figure 2.6: Terms used for representing displacements, loads and stresses on the rotor.
Reproduced from [16] ... 27
Figure 11.2: Geometry of the blade ... 31
Figure 3.3: A zero total displacement constraint at the ring ... 32
Figure 3.12: Actual and discretized system of BEM loading on profile [19] ... 32
Figure 3.13: Determining aerodynamic centre, chord length and twist angle for the
ANSYS model ... 33
Figure 3.14: Aerodynamic loading in ANSYS (loading point) and its relation with the
aerodynamic centre ... 34
Figure 3.15: Modeling of the aerodynamic loads ... 35
Figure 3.8: Total deformation for Interlock 71 ... 37
Figure 4.1: Hoffman vs. Hill: Interlock 71 ... 40
Figure 4.2: Hoffman vs. Hill: Interlock H2 ... 41
Figure 4.3: Comparison between interlock 71 & H2 under the Tsai Hill criteria ... 41
Figure 4.4: Comparison between interlock 71 & H2 under the Hoffman criteria ... 42
Figure 4.5: Tsai-Hill for LTL1 ... 43
Figure 4.6: Hoffman for LTL1 ... 43
Figure 4.7: Comparison between Tsai Hill and Hoffman for LTL1 ... 44
Figure 4.16: Comparison for the interlocks under the Hoffman criteria ... 45
Figure 4.9: Comparison for the interlocks under the Tsai-Hill criteria ... 46
Figure 4.10: Tsai-Hill for the 0-90 laminate texture ... 47
Figure 4.11: Hoffman for the 0-90 laminate texture ... 48
Figure 4.12: Comparison between Tsai-Hill and Hoffman criteria's for 0-90... 48
Figure 4.13: Tsai-Hill for the 0-90-0 laminate texture ... 49
Figure 4.14: Hoffman criteria for the 0-90-0 laminate texture ... 49
Figure 4.15: Comparison between Tsai-Hill and Hoffman criteria for the 0-90-0 texture ... 50
Figure 4.16: Tsai-Hill for the 90-0-90 laminate texture ... 50
Figure 4.17: Hoffman criteria for the 90-0-90 laminate texture ... 51
15
Figure 4.18: Comparison between Tsai-Hill and Hoffman criteria for the 90-0-90 texture . 52
Figure 4.19: Comparison between all three laminates under Tsai-Hill ... 53
Figure 4.20: Comparison between all three laminates under Hoffman ... 54
Figure 4.21: Tsai-Hill for the Br30 braded texture ... 55
Figure 4.22: Hoffman for the Br30 braded texture ... 56
Figure 4.23: Tsai Hill vs. Hoffman: Br 30 ... 56
Figure 4.24: Tsai-Hill for the Br45a braded texture ... 57
Figure 4.25: Hoffman for the Br45a braded texture ... 58
Figure 4.26: Tsai Hill vs. Hoffman: Br 45a ... 59
Figure 4.27: Tsai-Hill for the Br 60 braded texture ... 59
Figure 4.28: Hoffman for the Br 60 braded texture ... 60
Figure 4.29: Tsai-Hill vs. Hoffman: Br 60 ... 61
Figure 4.30: Comparison between Br 30, Br 60 and Br 45a; Tsai-Hill ... 62
Figure 4.31: Comparison between Br 30, Br 60 and Br 45a; Hoffman ... 62
Figure 4.32: Comparison between Beta values of all composite textures ... 63
Figure 5.1: Flowchart of the progressive model ... 67
Figure A.1: Lift coefficient for NACA S821 ... 74
Figure A.5: Drag coefficient for NACA S821 ... 75
Figure A. 6: Moment coefficient for NACA S821 ... 76
Figure B.3: BEM performance results using LabView ... 78
Figure B.4: Block diagram of the BEM code ... 78
Figure C.1: The user interface for the progressive fatigue damage model ... 79
Figure C.2: Block diagram of the progressive damage model ... 79
16
LIST OF TABLES
Table 3.2: Fiber and matrix properties ... 36
Table 3.4: Strength properties for the composite materials ... 39
Table A.4: Blade properties ... 74
Table B.1: Parameters of the WP1.5MW machine ... 77
Table B.2: WP1.5MW Structural Blade Definition ... 77
17
CHAPTER I:
LITERATURE REVIEW
1.1 Background
Until recently, wind turbine blades had a relative high rigidity and small
deformations. This allowed for modeling techniques which assumed a simplified aeroelastic
response.
Recent reports have shown that an aeroelastic optimized flexible blade can offer a
number of advantages over the more rigid variant: higher energy yield and/or shedding loads
(increasing fatigue life) [5].
Consequently, there is a trend towards lighter and more flexible
wind turbines, which makes design and dimensioning even more demanding and important
[6].
Wind turbines operate in a hostile environment where strong flow fluctuations, due to the
nature of the wind, can excite high loads. The varying loads, together with an elastic
structure, create a perfect breeding ground for induced vibration and resonance problems
[6]. Many manufactured items are designed to a reference "design point". This corresponds
to an operating condition such that, if met it will perform adequately to any other set of
conditions. A single design point is not adequate, but rather the wind turbine must be able to
withstand other unusual conditions with no significant damage. The most important
considerations are [7]:
1.
Expect event during normal operation
2.
Extreme events
3.
Fatigue
As is commonly used in mechanics, the loads are the externally applied forces or moments
to the entire turbine or to any of the components considered separately. Wind turbines are
usually designed for two types of loads (1) ultimate loads and (2) fatigue loads. Ultimate
loads refer to likely maximum loads, multiplied by a safety factor. Fatigue loads refer to the
component's ability to withstand an expected number of cycles of possibly varying
magnitude [7]. Most Materials can withstand a load of a certain magnitude when applied
once, but cannot withstand the same load when applied in a cyclic pattern. The decreasing
ability to survive repeated loads is called fatigue.
18
1.2 Scopes
and
Aims
The goal of this project is to develop a comparative study of different composite
material structures, a study that will be based on their quasi- static and fatigue behavior
subjected to the same aerodynamic load. The majority of the aeroelasticity models are based
on a modal formulation or finite element (FE) representation. However a coupled FEM-
BEM method was used in this work to calculate the aeroelastic response and compare the
static failure performance knowing the ultimate strengths of each material. The use of
computation fluid dynamics (CFD) rather than BEM is due to the fact that a computational
fluid dynamic simulation is time consuming and hence considered to be impractical for the
purpose of our study (see figure 1.1). The BEM offers the advantage of having short
computation time and the model can be simulated without difficulty.
-
Figure 9.1: Comparison between two method solving strategies; FEM-BEM and FEM-BEM
A number of design codes have been used over to model the wind turbines dynamic
behavior, or to carry out design calculations. Listed below are some of the most common
design codes:
· ADAMS/WT (Automatic Dynamic Analysis of Mehhhhhhhchanical Systems
Wind Turbine). ADAMS/WT is designed as an application-specific add-on to
19
ADAMS/SOLVER and ADAMS/View and it is a toolkit for analyzing wind-turbine
aeromechanics
[8].
· FAST (Fatigue, Aerodynamics, Structures, and Turbulence). The FAST code is
being developed through a subcontract between National Renewable Energy
Laboratory (NREL) and Oregon State University. NREL has modified FAST to use
the AeroDyn subroutine package developed at the University of Utah to generate
aerodynamic forces along the blade [9].
· HAWC (Horizontal Axis Wind Turbine Code). HAWC is developed at Risø in
Denmark. The model is based on the FE method using the substructure approach.
The code predicts the response of horizontal axis two- or three bladed machines in
time domain [10]
· YawDyn. YawDyn is developed at the Mechanical Engineering Department
University of Utah, with support of the National Renewable Energy Laboratory
(NREL), National Wind Technology Center. YawDyn simulates e.g. the yaw
motions or loads of a horizontal axis wind turbine, with a rigid or teetering hub [11].
Finally, this research will serve as an aid and a step towards the design of a more lightweight
blade and hope that it will serve as a tool that will aid the design of new wind turbine blade
composite material. This tool can be used to evaluate the pros and cons of using more
lightweight material and their behavior for different operating condition
20
CHAPTER II:
AERODYNAMIC MODELING
2.1
Methods for Calculating the Aerodynamic Forces
As mentioned earlier the aerodynamic forces used in this study are calculated using
the Blade Element Momentum (BEM) method, which is described in this chapter. The BEM
theory is the most commonly used method for calculating aerodynamic loads in the wind-
power industry [1].
Other methods such as the Helical Vortex Method (HVM) and the Free Vortex Method
(FVM) are not much used for wind turbines yet, but find great application in the helicopter
industry and in the propeller industry.
The most advanced ones are numerical methods
solving the Navier-Stokes equations for the global compressible flow as well as the flow
near the blades [6]. These methods may see increasing use in the wind-power industry as
well.
2.2
BEM Model
2.2.1 Introduction
BEM is a very common tool for wind turbine applications; it offers the advantage of
having a very short computational time and good accuracy, at least for the cases for which
BEM is suitable for. In short, the benefits of BEM are:
· Very fast.
· Accurate.
The disadvantages are:
· No way to define the geometry in flap or edge wise direction, (for example pre-bend
or a curved blade).
· Engineering models needed.
BEM can accurately be used when the blade is straight (no complicated shapes in either
direction), and the analysis is done assuming a steady state. The actuator disc model used to
derive the momentum equations assumes an infinite number of blades but in reality wind
turbines will have only two or three blades, therefore not every air particle passing through
21
the rotor swept area will be strongly affected by the pressure fields of the blades of the wind
turbine. To compensate for this fact, so-called tip-loss corrections can be used. These
corrections will reduce the induction factor in the outer annuli and therefore the
aerodynamic forces acting near the tip [12].
2.2.2 BEM
Theory
The Blade Element Momentum (BEM) theory
1
is a very widely used method for
calculating the forces on a wind turbine [1]. It is actually the combination of blade element
theory (also known as strip theory) and momentum theory.
Blade element theory divides the blade into discrete 2D sections, for which the aerodynamic
lift and drag forces per unit length are calculated based on local values of pitch angle, angle
of attack, chord length, airfoil section lift/drag coefficients, induction and wind speed. Note
that the wind speed is the vectorial sum of the free stream velocity and the rotational
induced velocity. Further, the aerodynamic coefficients of the 2D airfoil section have to be
known as function of angle of attack. See figure 2.1.
Figure 10.1: Schematic of blade elements; c, airfoil chord length; dr, radial length of element; r, radius;
R, rotor radius; , angular velocity of rotor
The momentum theory relates rotor thrust to the induction over the rotor plane. The
induction could be interpreted as the change in wind speed conditions due to the presence of
the lift and drag generating rotor blades [5].
By using the actuator disk theory where the disk changes the pressure and the rotation of the
fluid, and couple it with blade theory a
1
The derivations shown in this chapter have been extracted from [9] and [10]
very f
by a c
figure
Figure
By the
and th
where
assum
calcula
before
after it
where
also
fast tool can
circular plan
2.2.
2.2: Actuator
e actuator d
he induced m
p is the p
ming that the
ate p
2
and p
e it (between
t (between 4
:
n be created
ne that chan
r disk model
disk theory
moment can
pressure drop
e flow is inc
p
3
. This is d
n 1 and 2) a
4 and 3).
d [1]. The ac
nges the pre
the thrust c
n be calculat
p and A is t
compressib
done by cal
and calculat
22
ctuator disk
essure, and
can be calcu
ted as:
the area of t
le and stati
lculating the
ting the stat
k theory assu
creates a ro
ulated as th
the disk i.e.
onary Bern
e state far u
te for far do
umes that th
otational for
he pressure
noulli's equa
upstream of
ownstream o
he blade is
rce on the f
drop over t
ation can be
f the blade,
of the blade
(
replaced
fluid, see
the disk.
(2.1)
(2.2)
(2.3)
(2.4)
e used to
and just
and just
(2.5)
(2.6)
(2.7)
23
and
(2.8)
adding equations these equations and you get:
(2.9)
(2.10)
by combining these equations the head drop can be calculated as:
(2.11)
where the total pressure head also can be calculated as:
(2.12)
due to the fact that H = 0, the pressure drop over the blade can be written as:
(2.13)
where is the rotation of the wind close to the blade.
By combining these equations the total pressure drop can be calculated as:
(2.14)
since the angular velocity omega is supposed to be small, the term
2
can be neglected. By
applying these assumptions, on the actuator disk model the thrust and the moment can be
calculated as:
(2.15)
Similarly the momentum can be calculated as
(2.16)
where m is the mass of the fluid.
By substituting some variables, these equations can be written as:
(2.17)
(2.18)
By expressing u as
these two equations can be written as:
(2.19)
(2.20)
The axial induction factor and the tangential induction factor can be defined respectively by:
and the angle of attack can be calculated as:
(2.21)
where:
is the relative flow angle
24
is the pitch angle (see figure 2.3)
Figure 2.3: Velocities at the rotorplane
where:
(2.22)
which makes it possible to calculated the lift and the drag:
(2.23)
(2.24)
where C
L
and C
D
can be gathered from tabulated data (see figure 2.4).
Figure 2.4: The local forces on a cross section of a blade
Details
- Pages
- Type of Edition
- Erstausgabe
- Year
- 2014
- ISBN (eBook)
- 9783954897308
- ISBN (Softcover)
- 9783954892303
- File size
- 5.9 MB
- Language
- English
- Publication date
- 2014 (March)
- Keywords
- Coupled FEM-BEM turbine blades Orthotropic Properties
