Torsional moment from tower

Dear developers
In particular Mr Jonkman
I have a question about the aerodynamic loading consideration on the [size=120]Tower[/size].
1- Does Fast (in the aerodyn or elastodyn module) compute the torsional loading in the tower?
The wind generates loads: Drag (Cd), Lift (Cl) and pitching moment (Cm).
I’ve seen Aerodyn’s option to enable or disable this function (USE_CM or NO_CM). But I believe this function is only for the blade. In the tower configurations (NRELOffshrBsline5MW_Onshore_AeroDyn_Tower.dat) it only has the drag coefficient (Cd).
I searched the input files, but I didn’t find such an option.
I also looked in the technical material (tutorial), but I didn’t find a reference, if any equation relates and gets the Cm from the Cd.

Thank you a lot for your support

Dear Carlos,

The tower aerodynamic load calculation in AeroDyn is very simple and assumes a cylindrical shape. So, there is no Cm computed on the tower for AeroDyn. That said, for operational cases, the aerodynamic loads on the rotor generally dominates over direct wind load on the tower, which is generally only impactful for parked/idling conditions when the rotor is not spinning, and the aerodynamic loads on the rotor can generate moments about the tower axis. Also, the drag force on the tower can also generate an aerodynamic moment on the tower because the drag force is applied in the deflected state, and so, the tower deflection can generate a moment arm for the tower drag force.

Best regards,

Dear Mr Jonkman,
Thank you a lot for your very fast response!!!

Dear OpenFAST team,

I investigate the torsional vibrations of a wind turbine tower and their damping behaviour, using an NREL 5 MW turbine. To do that, I start the OpenFAST simulations with an initial turbine yaw angle (e.g. 10 deg.) and record the curve of yaw angle (and some other properties like TwrBsMzt etc.) over time.

I have realized that the parameter YawDamp (under the NACELLE-YAW CONTROL section in ServoDyn input file) plays an important role on that. However, even if YawDamp = 0, the vibrations are damped over time, as seen in figure below. I have also tested to set the air density to a very low number (e.g. E-06 in order to eliminate the aerodynamic damping), but there are still some other factors damping the vibrations, that I cannot find out.

My question is: Are there any tower properties (input values) that we can manupulate which determines the tower structural damping of torsional vibrations ? Thanks in advance.

Best regards,

Dear @Salur.Basbug,

Is blade flexibility enabled? If so, the yaw motion could result in blade bending, which would in turn impact the yaw behavior.

Is the rotor spinning with tower flexibility enabled? If so, gyroscopics could also play a role, coupled with tower bending.

FYI: Instead of setting a very low air density, you can disable aerodynamics altogether in OpenFAST by setting CompAero = 0 in the OpenFAST primary (.fst) input file.

Best regards,

Thanks a lot for the tips, Jason!

Yes, the rotor spins and its rpm fluctuates (due to the yaw motion, I guess).
But now I removed the DOF’s (set = False) related to the blade, drive-train and tower flexibilities; hence there’s indeed no factor left damping the yaw angle fluctuations, acts like a pure mass-spring system.

It’s great, because now I can tune the YawDamp (versus measurment data) to mimic the tower structural torsion damping.

Best regards,

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