Abstract
Vibration characteristics of two-blade and three-blade wind turbines are investigated. The system is modeled by a coupled system of the flexible tower (consisting of a mass and a spring) with two degrees of freedom and the blades with a single degree of freedom, and these blades are subjected to wind pressure which varies depending on the height from the ground. The vibrations of the two-blade and three-blade wind turbines are theoretically analyzed to determine the natural frequency diagrams, frequency responses, stationary time histories and their FFT results. It is found that several response peaks appear at the lower range of the rotational speed ω because of both the wind pressure and the parametric excitation terms. The vibrations at these peaks include multiple frequencies consisting of higher harmonics. The response curves near the highest peak exhibit soft spring types due to the nonlinearities of the restoring moments of the blades, and they predominantly include 2ω and 3ω and their higher harmonics in two-blade and three-blade wind turbines, respectively. The number of blades influences the frequencies of vibrations. In the numerical simulation, "localization phenomena" in the blades, which vibrate at different amplitudes, are observed.
