1. The work done by the aerodynamic forces on a wind turbine blade is calculated for a full cycle of the first edgewise mode. If this work done has a positive value, what does this say about the damping of the first edgewise mode?
A: Nothing, the work done during the mode shape cycle is not related to the damping of the mode
B: The mode is positively damped by the aerodynamic forces
C: The mode is negatively damped by the aerodynamic forces
2. You have a blade design with negative aerodynamic damping of the first edgewise mode. Changing which parameter would be most effective to resolve this?
5. You see a vibration resembling the graph on the right. What is going on in your turbine?
A. No problem, just some gust passing by.
B. An eigenmode without any damping is excited by an external force that is close to this frequency.
C. An eigenmode has negative damping, but due to the nonlinearities in the lift curve, it does not increase indefinitely.
D. An eigenmode with a lot of damping is excited several times by some external discrete force.
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A: Nothing, the work done during the mode shape cycle is not related to the damping of the mode
B: The mode is positively damped by the aerodynamic forces
C: The mode is negatively damped by the aerodynamic forces
The work done being positive means that energy is added to the motion, in other words the mode is negatively damped by the aerodynamic forces. In case the structural damping removes more energy than is added by these aerodynamic forces, the mode would still show some positive damping overall.
A: Edgewise stiffness
B: Flapwise stiffness
C: Generator torque-speed relation
D: Structural pitch angle
This angle defines the stiffest and most flexible directions. It varies along the blade. In plane motion has little to negative damping, out-of-plane motion usually has a lot of damping, so ensuring that there is out-of-plane motion in the edgewise mode can increase the damping of this mode.
A. 1.0 Hz
B. 0.8 Hz, 1.0 Hz and 1.2 Hz
C. 0.8 Hz and 1.2 Hz.
D. You will not be able to measure this vibration in the tower.
This mode shape is the same frequency in the rotating as in the non-rotating frame as the reaction force will be out-of-plane.
A. 2.0 Hz
B. 1.8 Hz, 2.0 Hz and 2.2 Hz
C. 1.8 Hz and 2.2 Hz.
D. You will not be able to measure this vibration in the tower.
This mode shape changes its frequency when looking in the rotating frame to + and - RPM when looking in the stand still frame. So an edgewise whirling mode in this case.
A. No problem, just some gust passing by.
B. An eigenmode without any damping is excited by an external force that is close to this frequency.
C. An eigenmode has negative damping, but due to the nonlinearities in the lift curve, it does not increase indefinitely.
D. An eigenmode with a lot of damping is excited several times by some external discrete force.
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This is a clear illustration of so-called 'beats'.
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