QUIZ

How much do you know about wind turbine aeroelasticity?

1. You have a blade design with negative aerodynamic damping of the first edgewise mode. Changing which paramter would be most effective to resolve this?
A: Edgewise stiffness 
B: Flapwise stiffness
C: Generator speed-torque relation
D: Structural pitch angle
2. A mode on a turbine consists of three blades flapping in the same direction: so all against the wind and then with the wind etc. If this frequency of flapping is 1.0 Hz in the rotating frame and the blades rotate at 12 RPM (0.2 Hz), which frequency or frequencies will you be able to measure in the tower?
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.
3. Another mode on a turbine consists of two blades vibrating in edgewise direction with the rotor rotation while the third blade vibrates against these two and vice versa. If the edgewise vibration frequency is 2.0 Hz and the turbine is still rotating at 12 RPM, which frequency or frequencies will you be able to measure in the tower?
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.

4. 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.
sites

Check your answers:

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.
sites
This is a clear illustration of so-called 'beats'.

We hope you have found this quiz useful and perhaps entertaining. If you want to learn more about wind turbine aeroelasticity, please check out our page on training.

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