Response discrepancies when using FAST and another software

Hello

I am conducting analysis on wind turbine dynamics using Oasys GSA software. I modelled my tower as an offshore monopile NREL 5MW wind turbine (with blades and rotor simply lumped at the top) and I used FAST8 outputs for rotor thrust (LSShftFxa) as an external force acting at the hub level (blade DOFs were FALSE in input files). When I compared results, the amplitudes in my model were several times higher and the steady state response was reached much later than in FAST. When I changed the structural damping from 1% to 15% in my model the responses matched almost perfectly. My question is whether that is attributable to aerodynamic damping, since my model does not involve anything related to blade rotation/motion. If it is the case, is there anyway way to obtain aerodynamic damping ratios from FAST?

Thank you !

Dear Nikita,

Your analysis approach sounds similar to the one discussed in the following forum topic: http://forums.nrel.gov/t/fast-tower-model/968/1, where my comments likely apply (particularly, my Aug 06, 2014 post).

Regarding aerodynamic damping, a FAST linearization analysis (currently available only in FAST v7) can produce a linear state-space model that includes aerodynamic damping. Alternatively, I’ve seen publications (not NREL’s) that discuss how to estimate aerodynamic damping for a given rotor.

Best regards,

Dear Jason

Thank you very much for the information and the link. Indeed the problem is related to the RNA assembly - when I removed the RNA mass and modified my tower to have the same stiffness and first natural frequency as before, there was a very good agreement between FAST and my model results. Nevertheless, is there a method to “filter out” loads related to RNA mass and inertia from FAST load outputs? Ideally I would want to keep the RNA mass at the top of my model (I have an option to add inertia to it).

Thank you !

Dear Nikita,

Eliminating the gravity and inertial loads from the FAST output to calculate the applied aerodynamic loads is discussed in the following forum topic: http://forums.nrel.gov/t/the-effect-of-tilt-angle/769/1.

Best regards,

Dear Jason

Thank you very much for the information provided - it was very insightful. I have eliminated RNA inertial loads by disabling all DOFs (and setting ShftTilt to 0, since Blade DOFs are always off) and obtained rotor thrust which I applied to my model (I have not yet applied other loads and external moments as I presume they have little effect on response in X direction, please correct me if I am wrong). When comparing to FAST results of the tower top displacement (all DOFs were on except Blades, Teeter, Drivetrain and Gen Dofs, no wave action) I got a good agreement for amplitudes but the frequencies were slightly different.

For my model I get the first natural frequency as 0.2912 Hz. Based on the posts from this thread - http://forums.nrel.gov/t/using-aggregate-mass-in-adams-to-check-nrel-cs-monopile-bmi/696/1 still be applicable to it?

Another issue is with fore-aft (My) mudline moments - for some reasons my amplitudes during transient response are much larger even though ttds are more or less the same - could that come from omission of RNA’s inertias again?

Lastly, I do understand that platform DOFs are related to the base in case of a monopile OWT, but is it important to always have all 6 platform DOFs on (how exactly do heave, sway and surge relate to it)?

Thank you!

Dear Nikita,

Your approach to modeling the tapered tower sounds reasonable. I would guess the difference in frequencies are related to the neglecting of the rotational inertia of the RNA (especially the pitch/roll inertia of the rotor), and perhaps the neglecting of the blade flexibility, in your software. The lumped mass and inertia of the rigid RNA from Jun 07, 2013 post in the forum topic you referenced are directly applicable.

I’m not sure why the fore-aft moments are much larger in your model, but I would guess that’s related to a lack of damping (e.g. aerodynamic), which you likely don’t capture by applying independently derived wind-thrust loads to your model.

The rationale for why all 6 platform DOFs should be enabled in FAST’s ElastoDyn module when used together with SubDyn is explained in Section 5.4 of the SubDyn User’s Guide and Theory Manual: wind.nrel.gov/nwtc/docs/SubDyn_Manual.pdf. Enabling heave is important for transferring the turbine weight to (and subsequent settling of) the substructure; likewise, enabling surge and sway are important for transferring the tower-base shear forces to (and subsequent transverse deflection of) the substructure.

Best regards,