Abstract Summary
Prediction of aeroelastic stability for large horizontal axis wind turbines is a key step in the design process to ensure structural safety against catastrophic failures. Historically, flutter has not been a major issue to interrupt the safe operation of wind turbines, but the trend of going bigger with blade sizes might render flutter as a stability concern. Very large HAWTs with blade lengths of 143.4 m or 250 m are being considered for reducing LCOE; however, the slender nature of these very long blades might make them more susceptible to flutter. For horizontal axis wind turbines, classical flutter is an aeroelastic instability which occurs in the event of coupling between a flapwise mode and a torsional mode. At the onset of flutter, the vibration of the blade grows rapidly, and the inherent structural damping becomes insufficient to contain the growth of vibration which may lead to catastrophic failure of the structure. The edgewise instability only involves edgewise mode of vibration and the decrease in damping with increased RPM for this instability is slower compared to classical flutter instability. Classical Theodorsen’s unsteady aerodynamics theory has been widely used in obtaining forcing terms to analyze flutter and edgewise instability of horizontal axis wind turbines (HAWTs). In this study, three classical assumptions: 1) thin airfoil, (2) flat wake and (3) small angle of attack (AoA) assumption of Theodorsen’s theory have been examined, while also adding edgewise aerodynamic terms that are typically ignored. These classical assumptions along with addition of the edgewise aerodynamics have been revisited to examine their impact on prediction of both classical coupled-mode flutter and edgewise instability. The relaxation of the assumptions and addition of edgewise aerodynamic terms led to a new aeroelastic model with modified lift and moment equations. The new aeroelastic model is then applied to blades ranging in lengths from 20m to 246m to evaluate the impact of scale. For 3-bladed HAWTs, relaxation of assumptions had more impact on the classical coupled-mode flutter RPM than on edgewise instability RPM. Addition of edgewise aerodynamic terms affected the edgewise instability RPM more than the classic flutter RPM for all 3-bladed cases. For 2-bladed turbines, relaxing the thin airfoil assumption is the most prominent effect. In general, the assumptions have larger impact for the longer blade lengths. The following are the main contributions and findings of this study for flutter and edgewise instability analysis of HAWT blades: • Reformulation of Theodorsen’s unsteady aerodynamic theory by modifying the small angle of attack assumption, flat wake assumption and thin airfoil assumption. • Addition of edgewise aerodynamic terms into the lift and moment equation for flutter and edgewise instability RPM calculation. • Comparative analysis of 2-bladed and 3-bladed HAWT blades under the influence of the reformulated Theodorsen’s lift and moment equations that account for thick airfoils, large angle of attack, arbitrary wake, and edgewise aerodynamics. • Analysis of the effect of the aeroelastic model assumptions on a wide range of blade lengths from 20m to 246m.
