Blade root kinematics

Hello,

Is there anyway I can access the blade root kinematics for the case of a rigid rotor, i.e absolute translation and angular displacements,velocities and accelerations. Is this being evaluated internally somewhere, for.eg represented in the Q,QDT,Q2DT vectors?

Also, is it possible to change the inertia of these rigid blades on the fly?.. I plan to import the S-Func FAST block onto Simulink and I would like to the ability to change the value of the blade inertia at every time step.

I appreciate any help offered!

Thanks,

-Vishvas

Dear Vishvas,

The blade-root kinematics are not directly output from FAST. However, except for the rigid-body rotation and torsional compliance of the shaft, the nacelle, shaft, and hub are modeled rigidly in FAST. As such, you could derive the blade-root kinematics quite easily from knowledge of the nacelle kinematics and rotor rotation. FAST does have outputs associated with the nacelle kinematics and rotor rotation, so, this should be fairly straightforward. Alternatively, the blade-root kinematics are calculated internally to FAST without being output; you could customize the FAST source code so that these values are output.

There is no way to dynamically change the blade inertia in FAST at every step. You would have to customize the FAST source code to achieve this, but I’m not sure why you’d want to.

Best regards,

Jason,

Thank you very much for the information.

If its not too much of a pain, can you point me to some documentation that can help me locate where these internal calculations are done. I did spend a week trying to understand the source code, but apart from a few variables which were obvious in their meaning, I wasn’t able to fully understand other quantities/computations of interest.

I’m working on a very interesting problem for my M.S thesis, for which I do need the blade root kinematics and be able to change the inertia of the rigid blade dynamically. Instead of using modal methods to model structural dynamics, I plan to integrate a FEM code into FAST which will provide a more powerful capability to model virtually any kind of non-linear structural behavior. I’ve got most of the equations already derived and I’m in the process of implementing them now. (I can send you a copy of my work if I’m successful in my defense :slight_smile:)

Regards,
-Vishvas

Dear Vishvas,

Here are the key variables in the FAST source code are:
rQ(:slight_smile: = Position vector from the origin of the inertial frame to the apex of rotation. Note (:slight_smile: represents the 3 elements of the position vector
rS(K,J,:slight_smile: = Position vector from the origin of the inertial frame to node “J” on blade “K”. Note that the root is not explicitly solved for, but J=1 is the most in-board blade element. Alternative, the position vector to the blade root could be calculated via rQ(:slight_smile: + HubRad*j3(K,:), where j3(K,:slight_smile: is the unit radial vector directed from the apex of rotation along the pitch axis.
AngVelEH(:slight_smile: = Angular velocity of the hub
LinVelES(:slight_smile: = Translational velocity of the current node on the current blade (all nodes of all blades are not saved in a single array).
LinAccES(K,J,:slight_smile: = Translational acceleration of node “J” on blade “K”.

Note: The angular acceleration of the hub is not explicitly calculated, but you could calculate it similar to how AngAccEB is calculated. The correct equation would be:

AngAccEH = AngAccEHt
DO I = 1,NActvDOF
AngAccEH = AngAccEH + PAngVelEH (SrtPS(I),0,:)*QD2T(SrtPS(I))
ENDDO

We, too, are working on the introduction of a finite-element-based option in FAST. More information will follow.

Please post your thesis (or other relavent papers) here when complete.

Best regards,

Jason,

Thank you very much for these details. They are definitely helping me. I just have a couple more questions:

  1. Is the blade inertia used in the equations of motion? If yes, are the changes in inertia due to blade deformation taken into account? I ask this because I see that the Rotor Inertia(RotIner), though calculated and presented at the output is never used in the equations.
  2. Is there some document which details the equations used in FAST? I did read the document “Modeling of the UAE Wind Turbine for Refinement of FAST_AD” which provides a brief description of the equations used, but I was looking for something more detailed which I can use to correlate most, if not all the variables in the code to the physical turbine model. I hate to bug you with more questions about variable names.

Thanks!

-Vishvas

Dear Vishvas,

“Yes,” FAST uses blade inertia in the equations of motion. And “yes,” the inertia changes with blade deflection, which is why the fixed (undeflected) value of RotIner that is written to the FAST summary (*.fsm) file is not used by FAST’s equations of motion.

More inform on the FAST theory basis is available in the following forum topic: http://forums.nrel.gov/t/coupled-blade-modes-in-fast/314/1.

Best regards,

Jason,

Thanks for clarifying. I appreciate your help!

-Vishvas