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.
- 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?
- 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?
- 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?
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.
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 http://forums.nrel.gov/t/resistant-moment-of-the-rotor-and-of-the-electric-generator/408/1 but I think that’s case when there’s no blade vibration.
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).