Desde o início acreditou em mim e deu-me toda a confiança para ultrapassar as dificuldades que tive neste trabalho. O objetivo era conseguir uma redução efetiva da carga aerodinâmica no modelo computacional da pá.
Nomenclature
Glossary
Introduction
- Motivation
- Wind Energy Overview
- Historic Perspective
- Modern Wind Energy Context
- Objectives
- Thesis Outline
The current work aims to make an additional relevant contribution to the wind energy field and, above all, to understand how it is possible to respond – from a structural point of view – to the increasing size of the wind turbine blades. In the early 21st century, concerns arose about the competitiveness of wind energy and the decline in energy production costs occurred with the construction of larger structures.
Horizontal-Axis Wind Turbines
- Generic Overview
- Sources of Load on Blades
- Power and Torque Characteristics
- Blade Design and Properties
- Blade Section
- Blade Material Properties
- Airfoil Optimization
- Number of Blades
- Blade Twist Design
- Blade Thickness
- Tip-Speed Ratio
Such advances prevented the blades from having high RPM ratios1, although they eventually added some useless weight to the blade. In fact, high tip speed ratios are appreciated and desired because they can avoid the rotor from reaching excessive rotational speeds [7].
Aerodynamic Load Control
Active Load Control
- Variable Pitch Angle Blades
- Active Flow Control Techniques
The continuous increase in blade pitch angle until the flow separates from the blade surface, leading to a sudden loss of lift [26]. From an aerodynamic point of view, it is advantageous to introduce pitch control in the full blade length. However, during periods of extreme wind speeds, it is not possible to park the entire wing in the feather position, which can ultimately cause problems in maintaining the integrity of the structure.
Donald Lobitz, Dale Berg and Jose Zayas have published studies on the influence of these auxiliary systems, with the aim of achieving significant reductions in energy costs. Primarily, they theoretically concluded that ailerons positioned in the trailing edge are much more efficient than in the leading edge, supported by thin airfoil theory (see appendix A) and globally all devices that change effective camber are the most suitable to achieving an aerodynamic downforce. They focused their work in the study on microtabs, which are devices whose deflection is of the same order of magnitude as the boundary layer thickness.
Passive Load Control
- Stall Regulation
- Aerolastic Tailoring
- Bend-Twist Coupling
This type of device presents undoubted advantages compared to devices of conventional sizes, as they have a faster response and the associated aerodynamic load is greatly reduced. In addition, passive techniques do not respond to local variations, while the active loading approach can be independently adapted to each blade and is more beneficial to rotor performance [27]. The basic idea of aeroelastic tailoring is to take advantage of the twist of the vane and passively adapt it to the striking wind load.
Goeij [10] provided an elegant definition of aeroelastic tailoring in his work: 'the integration of directional stiffness into a structural design to control aerolastic deformation, both static and dynamic, in such a way that the aerodynamic and structural performance of that construction are affected. in an advantageous way.” The initial assumptions behind their work are that the blade deforms in response to wind input, causing it to both bend (pure bending) and rotate about the rotor axis. Turn to stall increases output close to nominal value and then begins a negative trend.
Aerodynamic Model
- Incompressible Potential Flow Fundamentals
- Boundary Conditions
- Vortex Flow
- Actuator Disk Concept
- Classical Blade Element Method Theory
- Numerical Models
- Panel Method
- BEM Iterative Solution
- Description of Aerodynamic Routine Program aero load.m
- Purpose and Objectives
- Input Variables
- Pressure distribution
- BEM Computation
- Aerodynamic Load Computation
- Program Routine
The circulation is therefore equal to the integration in a line segmented around a closed curveC (illustrated in Figure 4.22) in the flow, an arbitrary point of which has a velocityV. The thrust in the control volume of Figure 4.7 can be calculated from the integral momentum equation axis. In the fluxogram of Figure 4.11 it is possible to obtain a general overview of the set and sequence of routines commonly applied in each BEM code.
From the input variables, the developed algorithm runs an auxiliary software XFoil in the background. It also remains constant through all iterations, but must be set at the beginning of the simulation. The following fluxogram provides a rough understanding of the set of routines implemented in the code developed by the author.
Structural Model
- Linear Elasticity Foundations
- Finite Element Matrix Formulation
- Composite materials
- Description of Structural Mesh Generator WTB struct model.m
- Purposes and Objectives
- Input Variables
- Nodes Assembly
- Elements Assembly
- Load and Nodal Constraints Computation
If we consider an arbitrary three-dimensional nodal displacement vector, where[N] is the matrix containing shape functions. The constants of a lamina are given by. 5.28) where E11 is the longitudinal modulus, E22 is the transverse modulus, and Ef is the fiber modulus. Em is the matrix modulus, Vf and is the fiber volume fraction, νmis is the matrix Poisson's ratio, νf is the fiber Poisson's ratio, ν12 is the principal Poisson's ratio and G12 is the shear modulus.
The initial input needed to calculate the mesh is the number of divisions of leaf chord, span and webs. Nevertheless, the user has the freedom to choose the number of divisions into chords, spans and paths, and then from the set of calculated airfoil sections, all nodes are generated by linear interpolations of two adjacent sections. Once the sweep to the trailing edge is completed, the span coordinate is incremented and the same routine is applied, twice the number of user-defined span divisions.
Fluid-Structure Interaction
Fluid Structure Interaction Methods
The tight coupled model is the most common model in FSI solvers because it presents good accuracy in nonlinear problems. The aerodynamic load is mapped to the structure, the structural displacement is transferred to the fluid solver, and a new aerodynamic must be merged into a deformed structural mesh of the previous iteration [ 44 ]. The latter model is known as loose coupling, and the aerodynamic and structural models are solved independently until convergence, and information is exchanged immediately thereafter.
Obviously, this method loses accuracy compared to the others, because the information until convergence is completely lost.
Loose Coupling FSI Schemes
Under steady-state axial flow conditions, the blade has a constant aerodynamic load and the equation of motion continues with Equation 5.23. The structural mesh is initially undeformed and the CFD tool calculates the aerodynamic load for the undeformed structure. Then the output of the CFD solver is coupled to the FEM tool, which will apply the aerodynamic load to the deformed mesh and give a new output against an updated deformed mesh, which will be reintroduced into the CFD solver .
In the first iteration, {Faero} is calculated with the BEM code and the displacement field is calculated relative to the undeformed mesh. The CFD solver, on the other hand, accepts the deformed mesh and performs an aerodynamic load calculation based on the deformed mesh and returns it to the CSM solver. This time, the total aerodynamic load in the current iteration is given by the CFD solution and the difference between the current and previous BEM solutions.
Simplified Coupling Procedure
The iterative process is repeated until the convergence of {Faero} and displacement vector {q} is verified. The updated Faero is then introduced into M AT LABR and the iterative process is performed sequentially until Faero converges. Theoretically, the calculation times should not be exaggerated, since non-linear effects are not taken into account during the simulations.
The generic coupling sequence is represented in Figure 6.5 and it shows the framework for all numerical models developed during this work.
Parametric Study
- Wind Turbine NREL 5 MW Data
- Baseline Blade Parameters
- Baseline Results
- Structural Performance
- Aerodynamic Performance
- Fibres Orientation
- Thickness Distribution
- Shear Webs Location
- Material Reinforcement
- Parametric Study Summary
The deformation of the shear webs in Figure 7.3(b) follows the same pattern as the outer surface in Figure 7.3(a). With respect to the twist in Figure 7.4(c), it begins a sharp curve up to 10m, which is slightly softened by the presence of shear webs from then on. The sudden change in curvature reported earlier is the main source of stress, as shown in Figure 7.5, since the maximum is obtained at the same spatial position illustrated by Figure 7.4(c).
The results of CP, shown in figure 7.6(a), are calculated by applying equation (7.3) in the area covered by BEM, which does not include the root section (about 10m). a) Drift coefficient distribution (b) Lift coefficient distribution. Based on similar research [5], two simple thickness distributions were implemented in the structural model, as illustrated in figure 7.12. Carbon/epoxy composite is applied in two of the inner layers, as shown in figure 7.21.
Enhanced Blade Design
- Design properties
- Coupled Analysis
- Structural Performance
- Aerodynamic Performance
- Static Analysis including Inertial Loads
Finally, the reinforcement done in the previous chapter proved to be successful and was integrated into the laminate stack, as shown in Figure 8.1(a) labeled material 2. The twist distribution in Figure 8.4 is also lower. when the solution has converged, but that difference. The stress distribution in Figure 8.5 shows that the reinforcement provided after the root area was not sufficient to alleviate the high stresses in that area, although the maximum value in Table 8.1 is clearly lower than the estimated yield strength of composite in the previous period. chapter.
This improved design does not significantly affect the aerodynamic parameters, however deviations are visible in both Figures 8.6 and 8.7, especially in the second half of the span where the twist is more noticeable. In the first case, this is a direct consequence of the reduction of the incident angle. Similar to Fig. 8.5, the stress graph in Fig. 8.11 shows a congested area near the root, not only due to the mesh insertion area, but also where areas of high suction are found.
Conclusions
Achievements
Knowledge of wind turbines requires many different engineering sciences such as aerodynamics, composite materials, structural mechanics and many others. From the set of parameters, it was possible to evaluate those that were more important for the structural behavior of the blade. From the evaluated parameters, an improved design of the studied blade was calculated, which met the desired goals.
Future Work
Bibliography
Appendix A
Thin Airfoil Theory
Lift is defined as the component of the pressure force normal to the freestream direction, and drag is defined as the component of the pressure force along the freestream direction. About a general point on the x-axis whose coordinates are given by (a,0), the pitching moment per unit extension is given in dimensional and nondimensional forms respectively by. While the pitching moment can be determined for any point in space, it is usual to calculate the pitching moment M and the pitching moment coefficient Cm around the quarter chord, i.e.
The center of pressure is defined as the point about which the pitching moment is zero. As the flow conditions change (for example, angle of incidence α changes), the center of pressure will change. The aerodynamic center is defined as the point where the pitching moment (or the pitching moment coefficient) is independent of α.
Appendix B
Matlab APDL Code Generator
