Total rotor aerodynamic load

this is my first post, I hope to do things correctly.
I am running the IEA-15-240-RWT-Monopile openfast scripts.
I have modified the FST script by flagging only Elastodyn (CompElast) and Subdyn (CompSub).
I modified the initial conditions in the elastodyn script by setting 0 to every iI.C.
I run it for 300 sec and monitored the Fore-Aft displacement of the tower (TTDspFA)
I was actually expecting a null value since I thought I did not have any external forces (no wind or sea). Nevertheless, I obtained a quite important oscillation of the tower with negative mean value (is negative upwind?). I understood that this effect is due to the rotor whose speed is small but not zero (as set in the i.c.).
I have then activated the Aerodyn and Inflow modules (but not Servodyn). In this case, the oscillation starts similarly to the previous case but quickly reaches a positive mean value following the increase in the speed of the rotor.
Figures are uploaded.

My questions are:

  1. Why the rotor is not 0 for the whole duration of the analysis in the first case?
  2. It seems that the thrust induced by the spinning of the blades induces a quasi-static displacement of the tower which is greater than the oscillations produced by the wind. Is it correct this interpretation?
  3. What does the output of the total rotor aerodynamic load (e.g. “RtAeroFxg”) consists of? Is it the resultant of the forces and moments of the blades subjected to the wind load or does it also include the aerodynamic effect of the rotation of the blades due to their rotation?

Thanks in advance

Dear @Alessandro.Tombari,

Here are my answers to your questions:

  1. Disabling all but the ElastoDyn and SubDyn modules does not mean that you eliminate all loads. The load from gravity is still applied, which, due to the overhanging weight of the rotor, will tilt the tower upwind, as you are seeing. This motion generates small inertia forces, which according to your description causes the rotor to spin by some small amount (although I don’t see that from your plots; the rotor speed looks zero for the case w/o wind in your plot).

  2. No. The negative (upwind) displacement is caused by gravity. The positive (downwind) displacement is caused by the aerodynamic load, when AeroDyn and InflowWind are enabled.

  3. Presumably you are referring to AeroDyn output RtAeroFxh (not g). This the total aerodynamic load applied to the rotor (integrated across all blades) and project along the x axis of the hub (along the shaft).

Best regards,

Thank you Jason for your detailed answers. I have a couple of doubts if you can help me.

Thank you, it makes sense now. I didn’t realize the gravity load can cause an initial deflection of 0.2m.
I did not show the rotor speed for the case without wind by itself but it is an harmonic rotation with max magnitude of about 0.002 rpm.
So the large oscillation of the tower that can be seen in the previous plot (w/o wind) is just due to the dynamic effect (like an impulse) due to the upward tilting. I did not realize the damping could be so low.

I understood that RtAeroFxh is related to the hub coordinate while RtAeroFxg is the same quantity but related to the global coordinate system.

I carried out a further analysis with wind but disabling the generator of (GenDOF = False and CompServo = 0) in order to have 0 rotor speed.
RotSpeed is 0 in the output but RtSpeed is non-zero but small.
The tower is now oscillating around the permanent deformation due to the upward tilt and damping out with the time.
Therefore, it seems that the effect of the wind (10m/s) is not so relevant to the tower displacement while the rotation of the blades generates an important thrust.
Sorry if they are trivial observations but just to understand if I am using the software correctly.
Thank you

Dear @Alessandro.Tombari,

Yes, that all makes sense.

I forgot that rotor aerodynamic loads in global (g) coordinates where added to AeroDyn recently :blush:.

With GenDOF = False, a small RtSpeed is the result of drivetrain torsion.

Best regards,

Dear @Jason.Jonkman ,
thank you very much for your kind replies.
I have just a final concern.
Is it realistic to have a lateral deflection of about 0.2-0.4m due to the gravity (overturning moment due to the hub mass)?
I think the ratio lateral deflection/tower height is much larger than the one obtained for the 5MW tower.
Thank you

Dear @Alessandro.Tombari,

Yes, this gravity-induced tower deflection is expected for this large 15-MW turbine with large overhanging weight of the rotor-nacelle assembly.

Best regards,