Static aerodynamics

Hello everyone,

I am trying to understand the case when wind turbine works at static aerodynamic condition. I use 1.5 MW Baseline wind turbine to do the simulation in FAST. When I run FAST, I set wind speed and pitch angle to constant values. I disable YawDof, DrTrDof , TeetDof, TowerDof and CompNoise. Only the bladeDOF and CompAero are enabled in my simulation. However, I don’t quite understand the results from FAST.

  1. The rotor torque and rotor thrust at steady state is almost constant. However, it varies in a very small range ( which can be considered as constant of course). I wonder what’s causing the small variations in rotor torque and thrust? Shouldn’t they be constant at steady state?
  2. The blade deflection oscillates around a value at steady state. What’s causing the blade vibration? My understanding is that it’s caused by gravity, is that correct?
  3. From book “AeroDyn Theory”, I know that the AeroDyn takes blade deflection into account when analyzing aerodynamics by equation 2 in the book. Is this the way how FAST consider the coupling between blade structure and aerodynamics?

Dayuan Ju

Dear Dayuan,

Even with steady wind input without shear, the WindPACT 1.5-MW turbine has a shaft tilt that will induce asymmetry (skewed wake) in the aerodynamics, resulting in periodic instead of constant loads. A similar effect would be caused by a nacelle-yaw error (if there was one). Yes, gravity will have a role as well.

In general, the structural code will send the structural position/orientation/velocity of each aerodynamic analysis node (based on all structural motions, including rigid-body motions and beam deflections) to AeroDyn, which AeroDyn uses along with the local wind velocity to calculate the aerodynamic loads.

I hope that helps.

Best regards,

Dear Jason,

Thanks for your reply. That helps. From the simulation results, the rotor speed is constant while the blade deflection vibrates. But I think the blade elastic body motion (vibration) and rigid body motion (rotation) are coupled. So how could FAST get a constant rotor speed while the blade vibrates? I have read the forum but I think that’s case when there’s no blade vibration.

Dayuan Ju

Dear Dayuan,

In your first post, you mention that only the blade DOFs are enabled (along with CompAero, which enables aerodynamic loads); this is why the rotor speed is constant. To get a variation in rotor speed, you must enable the generator (GenDOF) or drivetrain (DrTrDOF) DOFs (or both).

Best regards,