Using rotor furl to model nacelle tilting

Wind & Water Structural Analysis
1

Dear all,

I am trying to model tilting of the nacelle with respect to the tower head. This was possible in older versions of FAST, I guess. Now, nacelle tilting has been replaced by the more general rotor furl DoF, as stated in the user manual. However, to my understanding activating rotor furl will allow the drive train (rotor blades + hub) to move with respect to the nacelle frame of reference, but not the nacelle to tilt with respect to the tower head frame of reference. Is that correct? If so, is there any other way of simulating nacelle tilting?

Best regards,
Wolfgang

2

Dear Wolfgang,

That’s right. FAST’s rotor-furl DOF allows the rotor and drivetrain (hub, shafts, gearbox, and generator) to move relative to the “nacelle”. By properly locating the rotor-furl hinge, and lumping the nacelle mass into the hub, should be able reasonably model the dynamics of a tilting nacelle.

Best regards,

3

Dear Jason,

thank you for your reply.

I had a look at the furl input file and understand now what you mean with mass lumping. However, I am not sure what happens to masses and inertias that I specify for furling (in addition to rotor and blades). Will they be added to the overall masses and inertias already specified in the nacelle, i.e. do I have to substract them from
NacMass and NacYIner in order to get the overall mass balance right?

Thanks for your help.

Regards,
Wolfgang

4

Dear Wolfgang,

The masses and inertias specified in FAST furling input file are added to other masses and inertias specified.

Best regards,

5

Dear all,

help with the following questions would be very much appreciated:

  1. When using rotor furling to simulate a nacelle tilting dof masses and inertias other than those of the rotor have to be properly lumped. The eigen-inertia of the hub aroung the tilt axis is nowhere specified. From this I take that it has to be lumped into the rotor furl inertia term, whereas the hub mass (being part of the rotor) is properly taken into account by FAST automatically. Is that the case?
  2. In general, since for the hub only mass and inertia around the rotor axis are specified in the .fst file I presume that its inertia around a vertical (yaw) axis is neglected unless this contribution is lumped into the yaw inertia of the nacelle. Correct?

Thank you for your advice.

Best regards,
Wolfgang

6

Dear Wolfgang,

Your understanding is correct. But just to clarify, the specified hub mass will lead to a hub inertia about the rotor-furl axis in FAST equal to HubMass*(distance from (and normal to) the rotor-furl axis to the hub center of mass)^2. The term that is missing is the hub inertia about the hub center of mass.

The same principle applies to the hub inertia about the nacelle-yaw axis.

Best regards,

7

Dear Jason,

thank your for your answer. That was very helpful indeed.
The responsiveness in this forum is really amazing, which makes working with FAST a real pleasure. Big compliment to NREL!

Best regards,
Wolfgang