Study on the influence of structural damping ratio on the aerodynamic stability of offshore wind turbine airfoils

As offshore wind turbines grow larger, nonlinear aerodynamic stability has become a critical challenge. For this challenge, this study designed the aeroelastic wind tunnel experimental system with pitch single-degree-of-freedom, simplifying the three-dimensional flexible blade into a two-dimensional rigid airfoil segment supported by elastic elements. Through wind tunnel tests for simultaneous measurement of pressure and vibration, the influencing patterns and mechanisms of the structural damping ratio on the aerodynamic stability of airfoils were thoroughly explored. The study discerned two types of torsional aeroelastic responses in airfoils: stable equilibrium state with high randomness and low amplitude, and stall flutter with high amplitude and near-sinusoidal oscillation, which exhibited clear stability boundaries and was limited to specific ranges of wind speeds. Increasing the structural damping ratio reduced the equilibrium angle of attack, stall flutter amplitude, and frequency, with these changes interacting with other influencing parameters like the installation angle of attack and incoming wind speed. The wind speed stability boundary of stall flutter was closely related to the installation angle of attack and damping ratio, yielding the bivariate fitting formulas for this boundary to support the design and safety assessment of large offshore turbine blades. Furthermore, the stability boundary of wind speed was positively correlated with the vibration frequency and negatively correlated with the amplitude, with the degree of correlation largely unaffected by the structural damping ratio. The leading-edge vortex's generation and shedding greatly affected the stall flutter characteristics. During the pitch-up phase, an increase in the structural damping ratio inhibited the leading-edge vortex's generation, thereby reducing the stall flutter amplitude. • Increasing structural damping ratio reduces stall flutter's amplitude and frequency nonlinearly, affected by factors like installation attack angle and wind speed. • Airfoil’s stall flutter is confined in a specific wind speed range, and the wind speed stability boundary of stall flutter is closely related to the installation attack angle and damping ratio. • The bivariate fitting formula for the wind speed stability boundary of stall flutter, concerning the installation attack angle and damping ratio, has been obtained. • Higher structural damping ratio inhibits leading-edge vortex formation, thereby reducing stall flutter amplitude.