Oscillation in local axial wind component and AOA

Wind & Water Rotor Aerodynamics
1

Hi

In my FAST simulations I got some unexpected oscillations in Axial local wind component (experienced by blade element) and AOA under following situation:

  • Downwind three bladed wind turbine Test03 of FAST (with blade, tower degrees of freedom are true. Even if I make them False still getting the oscillations)
  • Used full field turbulence wind generated with TurbSim i.e., the wind file 42m_12m/s.wind
  • Tower velocity deficit zero.

I get an oscillating response with the frequency of the rotor revolution in axial wind component and AOA (I didn’t check for forces). I want to know what causes these oscillations?

Thanks in advance, Best regards

Ramzan

2

Dear Ramzan,

An oscillation in local wind speed and AoA could be caused by the following effects:

  • Nonzero shaft tilt and/or tower deflection.
  • Wind shear (vertical or horizontal).
  • Yaw error.

Test03 in the FAST CertTest uses a shaft tilt of zero, but it does include wind shear and yaw error.

Best regards,

3

Dear Jason,

Thanks for the feedback. I am selecting 3 blades and getting an oscillating response with the frequency of the rotor revolution. I think, in this case oscillating response should be faster than rotor revolution, even if it is coming from the effects notified by you. Could you kindly comment/guide in this context.

Best regards.

4

Dear Ramzan,

In the rotating frame (e.g., the blade root), you will see loads that are harmonics of the rotor speed, 1P, 2P, 3P, etc. But in the fixed frame (e.g., rotor speed, rotor torque, or tower), these loads are translated into harmonics of 3-times the rotor speed, 0P, 3P, 6P, etc., unless the rotor loads are imbalanced (for a 3-bladed rotor).

Best regards,

5

Dear Jason,
Thanks a lot. Best regards,
Ramzan