The problem of flow-induced stability of wind turbine blades has been recognized by the literature for quite some time [e.g. (Hansen 2002)]. Traditionally, the issue of flutter for a standard-size wind turbine blade has been avoided either because the rotor’s operational angular speeds are low, or the blades are appropriately parked during high-wind events. Nevertheless, wind-turbine blades are becoming longer, slenderer and more sensitive to wind-induced vibration, fluid-structure interaction and flow-induced instabilities such as coupled flutter. Modeling simplifications and uncertain aerodynamics loads, for example related to wind tunnel errors, are the root cause of perturbations that may alter the deterministic flutter threshold. Stochastic analysis is required to address this issue. This abstract and the full paper describe the derivation of a theoretically-based, numerically-implemented model for the evaluation of stochastic flutter of wind turbine blades. Numerical results analyze the flutter of the reference NREL (National Renewable Research Laboratory) 5-MW wind turbine blade.In contrast with previous work on the estimation of flutter probability of wind turbine blades (Li and Caracoglia 2019), this study proposes a new, theoretically-based and numerically-implemented model to identify the conditions for stochastic stability of wind turbine blades. The model is based on implementation of stochastic calculus methods. Dynamic stability in a non-deterministic context depends on its definition, such as almost-sure stability. In the context of long-span bridge aerodynamics this problem has been examined in the recent past through implementation of the theory of (largest) Moment Lyapunov Exponent (MLE) for dynamical systems, which may be used to test the statistical moment stability. The same approach is utilized in this study to formulate and solve a suitable reduced-order model of the blade dynamics. The model is inspired by an analogous model used for rotorcraft stability under inflow turbulence perturbations (Fujimori et al. 1979; Lin et al. 1979). The dynamic equations of motion are based on a continuous formulation of the loads and response along the blade’s longitudinal axis, exploiting the model by Hodges and Dowell (1974). The inflow velocity is defined under the condition of still-air wind, i.e. blades are assumed to rotate at no wind and zero initial twist. The reference inflow velocity depends on the blades’ angular speed of rotation and proportional to the relative (radial) position of the blade cross section with respect to the center of the rotor on the rotor plane. This hypothesis is acceptable since inflow velocity is mainly controlled by the rotation of the blades and attached flow conditions are necessary to trigger flutter.References:Fujimori, Y., Lin, Y. K., and Ariaratnam, S. T. (1979). "Rotor blade stability in turbulent flows-part II." AIAA J., 17(7), 673-678.Hansen, M. H. (2002). "Aeroelastic instability problems for wind turbines." Wind Energy, 10, 551-577.Hodges, D. H., and Dowell, E. H. (1974). "Nonlinear equations of motion for the elastic bending and torsion of twisted nuniniform rotor blades." Techincal Note D-7818, National Aeronautics and Space Administration (NASA).Li, S., and Caracoglia, L. (2019). "Surrogate Model Monte Carlo simulation for stochastic flutter analysis of wind turbine blades." J. Wind. Eng. Ind. Aerod., 188, 43-60.Lin, Y. K., Fujimori, Y., and Ariaratnam, S. T. (1979). "Rotor blade stability in turbulent flows-part I." AIAA J., 17(6), 545-552.

Stochastic Dynamics of Rotating Wind Turbine Blades Influenced by Turbulence and Aeroelastic Uncertainties: Recent Developments

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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