Research on Vibration Suppression of a Wind Mill

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As the development of wind mil generation, vibrations and sounds became a serious problem. However, most of the research on wind mills are those related to the generation of the electric power, such as, the shape of blade, number of blades, etc. Recently, the size of the blade becomes large. Some wind mills has a long blade of almost 50 meters. Therefore, very unique vibration problems which are not observed in the ordinary rotating machinery occur. For example, the wind forces at the top position and the bottom position of the blade are different and this difference makes a periodic excitation force. When a blade passes near the pole, a pressure impulse works. Such excitation forces may cause various kinds of vibrations of the blades. We clarified the followings.(1) Due to the periodic excitation force, a harmonic resonance occurs. (2) Since the restoring forces at the top position and the bottom position are different, this system becomes a periodically-excited system. (3) The coexistence of the parametric excitation due to the gravity and the periodic excitation due to wind force causes various kinds of superharmonic resonances. (4) As the stiffness of the blade decreases, the effect of parametric excitation increases. (5) Unstable zone appears when the stiffness of the blades becomes smaller than a certain value due to the periodic excitation.From this research, it became clear that wind turbines has a potential to cause vibrations even if they are made without manufacturing errors.