output parameters for blade tip motions in FAST

Hi everybody,

I am pretty new in wind turbine community, and I have a question about the blade tip motions.
I am modeling a rigid wind turbine structure in FAST. I made the tower and the blades rigid by disabling the degrees of freedom related to the tower and blades mode shapes.
As the blades are rigid, and I have put the initial blade tip deflections in the primary input file equal to zero, I expected to have zero blade tip deflections. But, the code outputs show a time varying tip deflection.
I would be grateful if anybody let me know what has caused this nonzero tip deflection.

Appreciation in advance
Best Regards

Dear Salehyar,

Disabling the blade and tower modal degrees of freedom (DOFs) and providing zero-valued initial conditions should produce zero-valued tip deflections for all time. Are you sure you’ve disabled FlapDOF1, FlapDOF2, and EdgeDOF for the blade and TwFADOF1, TwFADOF2, TwSSDOF1, and TwSSDOF2 for the tower? Are you sure you’ve selected the correct output channels?

Best regards,

Dear Jason,

Thank you for your reply to my post. These are the related lines from the primary input file:
false FlapDOF1 - First flapwise blade mode DOF (flag)

false FlapDOF2 - Second flapwise blade mode DOF (flag)

false EdgeDOF - First edgewise blade mode DOF (flag)
False TwFADOF1 - First fore-aft tower bending-mode DOF (flag)

False TwFADOF2 - Second fore-aft tower bending-mode DOF (flag)

False TwSSDOF1 - First side-to-side tower bending-mode DOF (flag)

False TwSSDOF2 - Second side-to-side tower bending-mode DOF (flag)
0.0 OoPDefl - Initial out-of-plane blade-tip displacement (meters)

0.0 IPDefl - Initial in-plane blade-tip deflection (meters)

and the following is the requested output:

And the results are time varying deflections.

Best Regards

Dear Salehyar,

All of those settings and outputs look correct for FAST to predict no deflection.

You may see very small nonzero values (say, 0.000001 meters or smaller) due to the numerics of how the FAST calculations are performed. Are the deflections you are seeing small like this or sizeable? Very small values do not indicate a problem with the simulation and should not be of concern.

Best regards,

Dear Jason,

The deflections are small, so they are not of concern.
I have two more questions regarding the blades outputs, and your response would be appreciated.
Question 1: In what conditions the rotor speed, given to the code in initial conditions of the primary input file,is fixed? and in what conditions it change with time? how can we see the time history of the rotor speed?
Question 2: I made the blades and the tower rigid by disabling the related DOFs, the precone angle of the blades is set to zero, and the rotor has a rotational speed.
The output parameters for blades local span motions show that the acceleration in x and y directions is very small,(can be considered zero), but the acceleration in z direction is sizable. How can the blades have acceleration in local z direction(blade coordinate system)?

Many thanks for your cooperation.
Best Regards

Dear Salehyar,

If you turn the GenDOF off then the rotor speed will stay constant throughout the simulation. You will have to turn on the GenDOF and pick a suitable generator model to allow the rotor speed to change.

I have not looked at the local span motions output by FAST and I may be wrong but I would think the acceleration in the z direction would be the centripetal acceleration. You could probably do a quick check, knowing the blade radial location and rotor speed, to see if the output makes sense.

Hope this helps.

Best Regards,


Dear Subin,

Many thanks.

Best regards

Subin answered the questions perfectly. Thanks, Subin!

Best regards,