A Comparative Study of Static and Fatigue Behaviors for Various Composite Orthotropic Properties for a Wind Turbine Using a Coupled FEM-BEM Method

Master's Thesis 2013 69 Pages

Engineering - Mechanical Engineering








CHAPTER I: Literature Review
1.1 Background
1.2 Scopes and Aims

CHAPTER II: Aerodynamic Modeling
2.1 Methods for Calculating Aerodynamic Forces
2.2 BEM Model
2.2.1 Introduction
2.2.2 BEM Theory
2.2.3 Correction Models

CHAPTER III: Structural Modeling
3.1 Blade Design
3.2 Blade Model
3.3 Load Application
3.3.1 Chord Length, Aerodynamic Centre and Twist Angle
3.3.2 Load Application and Moment Correction
3.4 Material Elastic Properties
3.5 Static Failure Criteria’s

4.1 Static Failure: Interlock Textures
4.2 Static Failure: Orthogonal Laminates
4.3 Static Failure: Braded Textures

CHAPTER V: Fatigue Model
5.1 Overview
5.2 Progressive Fatigue Damage Model

A. Aerodynamic Blade Data
B. BEM Model Verification
C. Progressive Fatigue Damage Model



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.[1] This work 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. The developed BEM code was written using LabView allowing an iterative numerical approach solver taking into the consideration the unsteady aerodynamic effects and off -design performance issues such as Tip Loss, Hub Loss and Turbulent Wake State therefore developing a more rational aerodynamic model. For this thesis, 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[2]. Confirmation of the performance analysis of the chosen wind turbine blade are presented and discussed blade including the generated power, tip deflection, thrust and tangential force for a steady flow of8m/s.

The elastic and ultimate strength properties were provided by Hallal et al[3]. 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 oflaminated composite materials, an algorithm developed by Shokrieh[4].

It is concluded that with respect to a material blade design cycle, the coupling between a finite element package and blade element and momentum code under steady and static conditions can be useful. Especially when an integration between this coupled approach and a dynamic simulation tool could be established, a more advanced flexible blade design can be then analyzed for a novel generation of more flexible wind turbine blades.

Keywords: wind turbine blade - BEM - FEM - aerodynamic - orthotropic - static - fatigue


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 master thesis 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 for my thesis but also made room for me in there 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 laforce”

Milan Kundera

Copyright © 2013 Adam Rafie Chehouri All rights reserved.

No part of this publieation may be reprodueed, stored in a retrieval system, or transmitted in any form or by any means, eleetronie, meehanieal, photoeopying, reeording or otherwise, without prior written permission from the author.


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Figure 1.1. Comparison between two method solving strategies; FEM-BEM and FEM-BEM

Figure 2.1. Schematicof bladeelements;c,airfoil chordlength; <7r,radiallength of element; r, radius; R, rotor radius; Ω, angular velocity of rotor

Figure 2.2. Actuator disk model

Figure 2.3. Velocities at the rotorplane

Figure 2.4. The local forces on a cross section of a blade

Figure 2.5. The numerical approach when using BEM

Figure 2.6. Terms used for representing displacements, loads and stresses on the rotor

Figure 2.7. Flowchart BEM code

Figure 3.1. Cross section of the blade

Figure 3.2. Geometry of the blade

Figure 3.3. A zero total displacement constraint at the ring

Figure 3.4. Actual and discretized system of BEM loading on profile

Figure 3.5. Determining aerodynamic centre, chord length and twist angle for the ANSYS model

Figure 3.6. Aerodynamic loading in ANSYS and its relation with the aerodynamic centre

Figure 3.7. Modeling of the aerodynamic loads

Figure 3.8. Total deformation for Interlock 71

Figure 3.9. The General work procedure

Figure 4.1. Hoffman vs. Hill: Interlock 71

Figure 4.2. Hoffman vs. Hill: Interlock H2

Figure 4.3. Comparison between interlock 71 & H2 under the Tsai Hill criteria

Figure 4.4. Comparison between interlock 71& H2 under the Hoffman criteria

Figure 4.5. Tsai-Hill for LTL1

Figure 4.6. Hoffman for LTL1

Figure 4.7. Comparison between Tsai Hill and Hoffman for LTL1

Figure 4.8. Comparison for the interlocks under the Hoffman criteria

Figure 4.9. Comparison for the interlocks under the Tsai-Hill criteria

Figure 4.10. Tsai-Hill for the 0-90 laminate texture

Figure 4.11. Hoffman for the 0-90 laminate texture

Figure 4.12. Comparison between Tsai-Hill and Hoffman criteria’s for 0-90

Figure 4.13. Tsai-Hill for the 0-90-0 laminate texture

Figure 4.14. Hoffman criteria for the 0-90-0 laminate texture

Figure 4.15. Comparison between Tsai-Hill and Hoffman criteria for the 0-90-0 texture

Figure 4.16. Tsai-Hill for the 90-0-90 laminate texture

Figure 4.17. Hoffman criteria for the 90-0-90 laminate texture

Figure 4.18. Comparison between Tsai-Hill and Hoffman criteria for the 90-0-90 texture

Figure 4.19. Comparison between all three laminates under Tsai-Hill

Figure 4.20. Comparison between all three laminates under Hoffman

Figure 4.21. Tsai-Hill for the Br30 braded texture

Figure 4.22. Hoffman for the Br30 braded texture

Figure 4.23. Tsai Hill vs. Hoffman: Br 30

Figure 4.24. Tsai-Hill for the Br45a braded texture

Figure 4.25. Hoffman for the Br45a braded texture

Figure 4.26. Tsai Hill vs. Hoffman: Br 45a

Figure 4.27. Tsai-Hill for the Br 60 braded texture

Figure 4.28. Hoffman for the Br 60 braded texture

Figure 4.29. Tsai-Hill vs. Hoffman: Br 60

Figure 4.30. Comparison between Br 30, Br 60 and Br 45a; Tsai-Hill

Figure 4.31. Comparison between Br 30, Br 60 and Br 45a; Hoffman

Figure 4.32. Comparison between Beta values of all composite textures

Figure 5.1. Flowchart of the progressive model

Figure А.1. Lift coefficient for NACA S821

Figure А.1. Drag coefficient for NACA S821

Figure А. 2. Moment coefficient for NACA S821

Figure B.3. BEM performance results using LabView

Figure B.4. Block diagram of the BEM code

Figure С.1. The user interface for the progressive fatigue damage model

Figure С.2. Block diagram of the progressive damage model


Table 3.1. Calculated aerodynamic moment for respective blade cross section

Table 3.2. Fiber and matrix properties

Table 3.3. Composite Elastic Properties

Table 3.4. Strength properties for the composite materials

Table A.3. Blade properties

Table B.1. Parameters of the WP1.5MW machine

Table B.2. WP1.5MW Structural Blade Definition

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.

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 ofhaving short computation time and the model can be simulated without difficulty.

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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 ADAMS/SOLVER and ADAMS/View and it is a toolkit for analyzing wind­turbine 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 Riso 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 thesis 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

CHAPTER II: Aerodynamic Modeling

2.1 Methods for Calculating the Aerodynamic Forces

As mentioned earlier the aerodynamic forces used in this thesis 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 pre­bend 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 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.

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Figure 10.1: Schematic of blade elements; c, airfoil chord length; dr, radial length of element; r, radius; R, rotor radius; Ω, 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 very fast tool can be created [1]. The actuator disk theory assumes that the blade is replaced by a circular plane that changes the pressure, and creates a rotational force on the fluid, see figure 2.2.

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By the actuator disk theory the thrust can be calculated as the pressure drop over the disk.

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and the induced moment can be calculated as:

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where Ap is the pressure drop and A is the area of the disk i.e.

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assuming that the flow is incompressible and stationary Bernoulli's equation can be used to calculate p2 andp.3. This is done by calculating the state far upstream of the blade, and just before it (between 1 and 2) and calculating the state for far downstream of the blade andjust after it (between 4 and 3).


[1] The derivations shown in this chapter have been extracted from[9] and[10]


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Title: A Comparative Study of Static and Fatigue Behaviors for Various Composite Orthotropic Properties for a Wind Turbine Using a Coupled FEM-BEM Method