Aerodynamic geometry of the wind turbine blade; Blade Element Momentum

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Convergence of the BEM calculation (HELICIEL)

The BEM is a discretization method where the blade is partitioned into elements. Therefore it is important to verify the sensibility of the number and size of elements on the final results. On the other hand if the element size suitable for convergence is too small we can always use a larger element size to save on computational power provided the difference in the results is not very large. We have considered the aerodynamic load as the parameter for convergence, Figure 2-10. It can be seen that the BEM converges above 20 blade elements. However the difference between 10 and 20 elements should be noted in order to verify which amount of blade elements is easier to model and also to verify if a lesser number of blades would affect harshly the final results. In fact the difference in the values of the parameters considered for the two is between 0.11% and 0.17%. Therefore for the sake of simplicity we will continue to use 10 blade elements.

Determination of the structure:

There are different blade cross sections in use presently. There are designs which include just an outer shell while others consider an internal stiffener we shall call from this point onwards as the spar. There are different types of spar. The blade sections are listed in the Figure 2-11.
The sections chosen have 8 geometric configurations with another 3 combinations depending on the layups. The sections having the word “Box” in their nomenclature signify that the blade surfaceshear web spar interface is reinforced with extra layups. Furthermore as part of an initial investigation the blades cross sections were modeled in ABAQUS FE software as prismatic cantilever beams with their proper layups and loaded with a uniform flap wise pressure load of 0.5 kPa. Furthermore since the specific width of each section is different hence the sectional weight was also considered in these simulations. To correlate the different types of materials used in the composite lade industry properties of Fiberglass/Epoxy, Carbon fiber/Epoxy and Kevlar/Epoxy composites were taken from the literature and applied to the simulations to ascertain which section and material showed the best performance.
The performance of the different sections is thus illustrated in terms of their deflection at the free end, maximum stress induced, weight and compliance. The results are illustrated in Figure 2-12.

Normal Wind conditions

NWP: The normal wind profile defines a vertical gradient of wind up to the atmospheric boundary layer. Furthermore the wind velocity increases with an increase in height.
NTM: the normal turbulence model predicts a temporal variation in the wind speed pour taking into account their fluctuations.

Extreme wind conditions

EWM: The extreme wind model models an extreme wind condition as that of a storm.
EOG: Extreme operating gust defines a short term increase in wind as that in case of a gust and then return of wind speed to normal.
EDC: The extreme direction change is the case where the wind speed changes suddenly and then
remains at the new directions.
ECG: In the extreme coherent gust case the wind speed rises sharply and stabilizes at this new speed.
ECD: The extreme coherent gust with direction change is a combination of sudden rise in wind speed and change in wind direction.
EWS: The extreme wind shear can be vertical or horizontal. These wind shears produce a sharp gradient along the radius of the rotor.
For the extreme wind model (EWM), the extreme operating gusts (EOG) and the extreme change in
direction (EDC), we define two cases. One with a recurrence of one year (EWM1, EOG1, EDC1,) and the other more severe with a recurrence of 50 years (EWM50, EOG50, EDC50). The case 6.1 of the standard IEC 61400-1 predicts a situation where the turbine would stop at an extreme wind case recurring every 50 years. Two situations are defined for this case: the first when the blade is vertical towards the top, the other where the blade is horizontal and the blade is subjected to a combined load of wind lift and its own weight. In service the loads applied and the bending moments can be evaluated. However for the extreme wind conditions the rotor is modeled as a flat plate with the entire wind load being applied as a drag force on this plate. The force is calculated in 2-22.

Equipment used for mechanical testing

In general the equipment used for the testing of material properties can be divided into 2 categories. The first are the machines used for loading the equipment and the second is the apparatus used for instrumentation. The equipment used for loading the specimens in tension and flexion are used depending upon their capacity, their capability to load specimens repeatedly and the degree of precision required for each test.
For instrumentation biaxial strain gauges with their data acquisition and treatment system, thermocouples, CCD camera and a Thermal camera are used for monitoring the tests and taking measurements. The equipment and their use are summarized in Table 3-7 –Table 3-9.


Table of contents :

Chapter 1 State of the art 
1.1 Advances in Blade technology
1.2 Methods of fabrication
1.3 Resistance of composites to damage
1.4 Resistance to delamination and de-bonding
1.4.1 Analytical approach
1.4.2 Finite Element approach using VCCT.
1.4.3 Numerical approach using cohesive zone models.
1.5 Fatigue behavior of Materials.
1.6 Delamination fatigue.
1.7 Wind turbine form design
1.8 Fabrication of Blades
1.9 Full-scale testing of Wind Turbine Blades
1.10 Finite element modeling of Wind Turbine Blades
1.11 Conclusion
Chapter 2 Design methodology for Wind turbine Blades 
2.1 Design Parameters
2.1.1 Swept area and Power generated
2.1.2 Coefficient of performance
2.2 Calculation of the shape of the wind turbine blade
2.2.1 Aerodynamic geometry of the wind turbine blade; Blade Element Momentum
2.2.2 Optimum design of the blade
2.3 Power generated by the rotor
2.4 Convergence of the BEM calculation (HELICIEL)
2.5 Determination of the structure:
2.6 Normal Wind conditions
2.7 Extreme wind conditions
2.8 Determination of the blade cross section
2.8.1 Method of maximum stress
2.8.2 Method of maximum deflection
2.9 Finite element modeling of the blade
2.10 Materials used for the modeling
2.11 Modeling of the material
2.12 Determination of the initial layup
2.13 Convergence of Mesh
2.14 Modeling of the structural response of the blade using finite elements
2.15 Inertial loading due to blade’s weight
2.16 Aerodynamic loading due to incident wind
2.17 Centrifugal load due to rotation of the blade
2.18 Behavior subject to global loading: Aerodynamic, gravitational and centrifugal
2.19 Modifications to the initial blade design
2.20 Conclusions and Discussion
Chapter 3 Experimental characterization of material properties 
3.1 Fabrication of composite panels
3.2 Materials
3.3 Equipment used for mechanical testing
3.4 Determination of fiber content
3.5 Quasi static material properties
3.5.1 Tensile tests
3.5.2 Flexural tests
3.6 Discussion on results
3.7 Summary of test results
Chapter 4 Fracture Resistance in Monolithic Interfaces 
4.1 Mode I opening mode: Experimental setup
4.2 Analytical calculation of Strain energy release rate for Mode I (GI)
4.2.1 MBT 1 (Modified Beam Theory 1)
4.2.2 MBT 2 (Modified Beam Theory 2)
4.2.3 CBBM (Compliance Based Beam Method).
4.2.4 CC (Compliance Calibration Method).
4.2.5 Calculation using J-integral
4.2.6 Virtual crack closure technique (VCCT)
4.3 Results of Mode I DCB tests
4.4 Mode II shearing mode: Experimental setup
4.5 Analytical calculation of Strain energy release rate for Mode II (GII)
4.5.1 Beam theory (BT)
4.5.2 Shearing Height theory (SH)
4.5.3 Compliance calibration (CC)
4.5.4 Virtual Crack Closure Technique (VCCT)
4.6 Conclusions and Discussion
Chapter 5 Fracture Resistance in Bonded Interfaces and Sandwich Structures
5.1 Mode I opening mode: Experimental setup
5.2 Analytical calculation of Strain energy release rate Mode I (GI)
5.2.1 MBT 1 (Modified Beam Theory 1)
5.2.2 MBT 2 (Modified Beam Theory 2)
5.2.3 CC (Compliance Calibration Method)
5.2.4 Virtual crack closure technique (VCCT)
5.3 Results of Mode I DCB tests
5.4 Mode II shearing mode bonded beams: Experimental setup
5.5 Analytical calculation of Strain energy release rate Mode II (GII): Bonded Beams
5.5.1 Beam theory (BT)
5.5.2 Compliance calibration (CC)
5.6 Results of Mode II ENF tests for bonded beams
5.7 Mode II shearing mode of sandwich beams: Experimental setup
5.8 Analytical calculation of Strain energy release rate Mode II (GII): SandwichmBeams
5.9 Mode II SLB results for Sandwich beams
5.10 Observations on Results
Chapter 6 Experimental study of behavior of materials under cycli loading
6.1 Composite material
6.1.1 Test procedure
6.1.2 Measurements
6.1.3 Self heating and degradation of stiffness
6.1.4 Effect of loading rate
6.1.5 Endurance limit using self heating
6.1.6 Rate of damage progression
6.2 Fatigue tests on UD specimens
6.2.1 Degradation of stiffness
6.2.2 Failure Modes
6.3 Discussion
6.4 Self heating tests on NORPOL
6.5 Conclusions and Discussion
Chapter 7 Finite element modeling of the wind turbine blade 
7.1 Characterization of materials
7.1.1 Modeling
7.1.2 Discussion
7.2 Characterization of interfaces
7.2.1 Model setups for DCB tests
7.2.2 Discussion on Results
7.2.3 Model setups for ENF tests
7.3 Modeling of single lap joints
7.3.1 Modeling strategy
7.3.2 Discussion on Results
7.4 Modeling of the complete blade
7.4.1 Modeling strategy
7.4.2 Tier 1: Complete blade model
7.4.3 Tier 2: 3D Shell blade section
7.4.4 Tier 3: 3D Solid blade section
7.4.5 Tier 4: Sub model of the Tier 3 solid model
7.4.6 Tier 5: Sub model of Tier 4 including cohesive zones
7.5 Conclusions and Discussion
Conclusions and Perspectives


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