The influence of rotation on natural frequencies of wind turbine blades with pre-bend

Abstract

The natural frequencies of wind turbine blades in the rotating state differ significantly from those in free vibration conditions. In this paper, an established model blade analysis with the multi-body method and modal parameter identification were used to investigate the influence of rotation on natural frequencies of wind turbine blades. For the blades with rated powers of 100 kW, 1.5 MW, and 2.3 MW, the frequency growth with the increase in rotating speed, as well as the influence of gravity and pre-bend on frequencies and intensity of vibrations, was investigated. Additionally, the Campbell diagrams were analyzed. The results reveal that the combined effect of centrifugal force and pre-bend can induce the vibration of flap-edge coupling, and the intensity of the coupled vibration enhances with the increase in rotating speed. Gravity has little effect on the natural frequencies of rotating blades, but it enhances the edgewise vibration. Blades with stronger flexibility have a greater dynamic stiffening effect mainly in the flapwise direction. As rotating speed increases, the first-order flapwise (1st flap) natural frequency increases significantly, while the growth rate of higher-order frequencies decreases successively. When the 100 kW, 1.5 MW, and 2.3 MW wind turbines reached their rated speeds, the 1st flap natural frequencies increased by 19.41%, 20%, and 15.91%, respectively. Additionally, in the Campbell diagram, the corresponding rotating speed at the intersection of the 1st flap frequency and 3P (3P is three times the rotation frequency of the wind rotor) increases when rotation is considered.