Investigations on the effects of structural damping on vortex-induced vibration response of an airfoil at a high angle of attack via the aero-damping map

Abstract

Large-scale modern wind turbines at standstill are prone to vortex-induced vibrations. In this study, we propose the use of the aero-damping map to investigate the complex vibration responses of the wind turbine airfoil at 90° of attack angle with different levels of structural dampings. The vibration amplitude and response frequency in the lock-in condition and soft lock-in conditions agree well with the contour line on which the sum of aerodynamic damping and structural damping is equal to zero. The mechanism of frequency soft lock-in is explored from the aspect of energy transfer that when the equilibrium state cannot be maintained at the natural frequency due to high structural damping, the system locks to a frequency between the natural frequency and vortex shedding frequency of the stationary airfoil to achieve lower aerodynamic damping and more energy absorption from the air. The transient response of the beat vibration is also investigated with the aero-damping map combined with the dynamic mode decomposition method. It is found that the lock-in mode and von Kármán mode coexist in the unsteady flow field during beat vibration. The competition between the two modes causes the system to be in an intermittent state of alternating frequency lock-in stage with lower aerodynamic damping and unlock-in stage with higher aerodynamic damping, hence resulting in the amplitude amplification and attenuation alternately.