Local blade whirl and global rotor whirl interaction

Investigation of stall induced vibrations has shown that the actual value of the edgewise blade frequency relative to the coupled, global rotor tilt-yaw frequencies is important for development of the vibrations. They develop more easily if the blade frequency is close to one of the rotor tilt-yaw frequencies referred to common coordinates.

This observation indicates that an important coupling exists between the blade edgewise modes and the global rotor modes. The presented work aims at providing insight into the physics of this coupling by use of simpel models, which take into account the effects believed to be most important in the considered state of operation.

We assume that the dominating inertia force on the blades originates from the edgewise vibration. The resulting inertia force at the hub, which turns out to be a force rotating in the rotor plane with the edgewise frequency, is then considered as the forcing on the supporting structure, i.e. the main shaft, the nacelle frame and the tower.

A four degrees of freedom model of the rotor and the supporting structure is used to illustrate the global rotor tilt-yaw mode shapes during operation. A mathematical model for the work performed by the inertia force from the blade vibration on the hub movement shows, how energy might by exchanged between the structural elements.

In addition, full aeroelastic calculations are used to support and extend the conclusions from the simple models, when the complex, real wind turbine structure is considered. The in-plane inertia force describes an elliptical path in the rotor plane, and we denote the corresponding blade deformation local blade whirl.

Likewise, the hub moves along an elliptical orbit, when the rotor tilt-yaw modes are excited, and we denote this movement global rotor whirl. Using these terms our concern is the interaction between local blade whirl and global rotor whirl.