Abstract Summary
The current trend in offshore wind energy is to design and install systems with larger swept areas that yield unprecedented efficiency standards. Nevertheless, large blades are needed to achieve these objectives. As a result of aerodynamic and structural tailoring, long and slender blades are produced that are also progressively susceptible to various dynamic instability phenomena during operational conditions. One of these phenomena is the bending-torsion flutter that may lead either to structural failures and system or system breakdowns. Over the last years the Author has been involved in the examination of blade flutter under the influence of stochastic disturbances, e.g., turbulence and aeroelastic load errors. The relevance of this research stems from the potential risk of unstable motion at an angular frequency close to the rated angular speed of operational blade motion. A reduced-order Markov model has been proposed and used to examine the effects of the various perturbations. One of the features that the model accounts for, is the coupling between angular speed and rotationally sampled wind turbulence. Mean-square stability has been predominantly considered; results have shown that various perturbations may negatively impact the instability threshold. In this study the model is employed to investigate moment stability beyond mean squares, observing that the instability may involve nonlinear state propagation, parametric perturbation, and vibration amplitude dependency. Third-order instability will be investigated and compared against previous numerical results. The NREL 5MW reference wind turbine blade is used as a benchmark example.
