Representation of wind turbine blade responses in power production load cases by linear mode shapes

Abstract

The wind turbine industry is designing large MW size turbines with very long blades, which exhibit large deflections during their operational life. These large deflections decrease the accuracy of linear models such as linear finite element and modal-based models, in which the structure is represented by linear mode shapes. The aim of this study is to investigate the competence of the mode shapes to represent the large blade responses in normal operation load cases. For this purpose, blade deflections are projected onto the linear modal space, swept by mode shape vectors. The projection shows the contribution of each mode and the projection error. The blade deflections are calculated by a nonlinear aero-servo-elastic solver for power production fatigue load cases with normal turbulence. The mode shapes are calculated at the steady-state deflected blade position computed at different wind speeds. Three reference turbine blades are used in the study to evaluate the effects of various blade design parameters such as length, stiffness, mass, and prebend. The results show that although the linear mode shapes can represent the flapwise and edgewise deflections accurately, axial and torsional deflections cannot be captured with good accuracy. The geometric nonlinear effects are more apparent in the latter directions.The results indicate that the blade deflections occur beyond the linear assumptions. KEYWORDSgeometric nonlinearity, structural dynamics, wind energy, wind turbine aeroelasticity, wind turbine loads INTRODUCTIONThe wind turbine design process requires a wide range of different simulations including load and stability analysis and design optimization.Although, some analyses need models with only one uncoupled turbine property such as aerodynamic and structure, others need coupled models which have couplings between the structure, the aerodynamics, and the controller. Coupled turbine analyses generally use low fidelity models because of the high computational cost and difficulties in the coupling process. For example, turbine load analyses are generally performed by the blade element momentum (BEM) 1 method for the aerodynamic part and beam solvers or reduced order models for the structural part. The structural model can be geometrically linear or nonlinear depending on the solver capabilities and available computation resources. The focus of this study is to assess the capabilities of using linear mode shapes in reduced order structural models in stability, aeroelastic, and load analysis of modern turbines.Linear modes constitute a simple and fundamental modeling alternative for most of the linear dynamic problems. Small size, cost effective, and reliable reduced order models 2-6 can be constructed by the mode shapes to calculate the structural response. It is also the key part of the stability analysis. Therefore, reduced order models are also preferred in wind turbine load and stability analysis. 7,8 Although, these models work very well for the structures with small deflections, their accuracy decreases as...

Effect of tunneling cracks on structural property degradation of wind turbine blades

“…During the design process of wind turbines, analyzes that couple the structural, aerodynamic and control response of the different components are typically used to define design loads. These coupled analyzes normally use low fidelity models to reduce computational cost and facilitate the coupling pro-5 cess [1]. For rotor blades, for example, beam models based on 2D cross-section analysis [2,3] are typically used, with which the blade cross-section stiffness properties are estimated efficiently.…”