MBC3 of Linearized FAST Model


thank you again for this great platform and your support.

I’m using Fast to create linearized models for control design.
After the linearization i always apply the multiblade coordinate transformation to transform rotating values (blade root bending moments) into a fix frame with
cosine, sine and zero components. Today i faced a problem which i couldn’t solve myself.

When i apply a wind step of +0.1m/s to the linearized and MBC3 transformed model i expect a step change in the 0-component (blade root bending moment flap)
and no changes in the cosine and sine component. Since there is no tilt, no wind shear or any other asymmetry.

What i get are the following responses…

From left to right are the 0-component, COS-component and SIN-component of the blade root bending moments in flap direction.
Operation point is 14 m/s with 12.6deg pitch angle.

Question: What causes these drifting signals in the COS- and SIN-component?
The linear increasing signals are not physically.

What i have done so far is:

  • increasing azimut steps > no influence
  • decreasing both the displacement and velocity tolerances in the linear_input file > again no influence

I appreciate every idea to solve this issue. Thanks

as i’ve seen now, the drifting effect in COS- and Sin-component is less once the tower is completely stiff (no tower DOF enabled).

yellow w/ stiff tower
magenta w/ flexible tower

A magnification of the SIN-component signal shows the drift is still present but weaker.



Further investigations show a drift in the time series of a FAST simulation as well.

The blade root bending moment drifts away with time - while the wind speed and rotor speed are absolutely constant.

Does anyone know what the root cause is?

Dear Ingo,

It looks like you’ve shown that the drift in the COS and SIN components of the blade-root flapwise bending moment in the MBC3-transformed linearized model is the result of tower deflection.

When the tower-bending DOFs are disabled, you see a very minor drift in the COS and SIN components of the blade-root flapwise bending moment in the MBC3-transformed linearized model. Perhaps this is a result of a very small change in rotor speed as a result of the step change in wind? Or has the generator and drivetrain DOFs been disabled in your model?

For the time-domain results, are you plotting the blade-root flapwise bending moment time series directly or have you applied the MBC3 transform to the time series? If you have not applied any transforms, have you confirmed that the blade-pitch angle and nacelle-yaw angle are fixed? What DOFs are enabled in the model and does the solution make more sense if you disable DOFs?

Best regards,