In-plane self-excitation of two-bladed horizontal axis wind turbine rotors due to blade elasticity and gravity

The two-bladed rotor is one of the promising concepts to emerge from the study of super large wind turbines. However, the rotor is prone to generating larger vibrations compared with conventional three-bladed rotors. In particular, in-plane vibration is hard to avoid because its damping is small. Furthermore, blades are becoming more flexible as wind turbines are getting larger. Inplane self-excitation of a 10-MW wind turbine with a two-bladed rotor was studied in this article through aero-elastic simulations. This study shows that even if the blade deformations are almost the same, large rotor in-plane self-excitation does not occur in a three-bladed rotor; however, it does occur in a two-bladed rotor. The self-excitation was shown to be caused by a combination of blade in-plane elasticity and gravity. Furthermore, the mechanism was theoretically demonstrated through simplified models that showed a mass and a spring.