# LSS bending moment

Running fast for terrestrial wind turbine with rigid tower and rigid blades for steady wind speed 18m/s with no shear I found the LSSGagMya and LSSGagMza still are not zero.
Is there anyone who knows why the LSSGagMya and LSSGagMza are still varying harmonically with the time in this condition?
Does LSSGagMya define the total bending moment exerted from rotor to LSS about y direction? If no, what parameter defines such load in FAST?

Dear Mehdi,

I think those measurements are for virtual strain gages on the shaft and they rotate with the rotor. You should see a 1P gravity load on them.

Marshall

Hi,
Within adm file I set the controller aside and change the blade pitch angle manually. I set the blade pitch to 5 deg.
Also include following statement to measure the bending moment that exerted to LSS from rotor about y axis.
VARIABLE/31, FUNCTION=TY(3300,3030)
After running for 33 sec I got the following plot fo RotorMy:

Here also the plot of LSSGagMya:

As you see the amplitude of LSSGagMya is more than range of RotorMy where amazed me. I expected the amplitude of LSSGagMya be in range of RotorMy.
Note that the tower and blades are rigid and the wind speed is steady 18m/s with no shear. Also the generator torque is constant (=42kN).
Why these differences occurred? What is wrong?

Regards Mehdi,

Dear Mehdi,

I’m afraid that I have no idea what Rotor_MY is so I cannot address that issue.

I see that the LSSGagMya plot has a mean of 0, so I think that means you have no shaft tilt. For the shaft bending loads, you can easily check them by multiplying the mass of the rotor by the distance from the rotor CM to the shaft strain gage. You can get the total rotor mass and CM from ADAMS/View. The plots indicate that the value would be around 2000.

If you would explain what you mean by Rotor_MY, I may be able to help more.

Marshall

Thanks Marshal,

I myself defined the RotorMy_V by following statement to measure the bending moment that rotor exerts to LSS about y axis.
You can check the markers 3300 and 3030.
VARIABLE/31, FUNCTION=TY(3300,3030)

Hope you able to help more.

Regards,
Mehdi

Dear Mehdi,

*How many blades does this wind turbine have and is the hub modeled rigidly (without teeter)?
*Is the turbine upwind or downwind?
*Does the wind turbine have shaft tilt or yaw error?
*Does the wind turbine have precone or nonstraight blades?
*What have you defined ShftGagL to be in the FAST input file?

FAST input parameter ShftGagL determines the distance from the hub center (for a 2- or 3-bladed rotor with rigid hub) or teeter pin (for a 2-bladed rotor with teeter) to the virtual strain gage where the low-speed-shaft bending moments are output (via LSSGagMya, etc.). The virtual shaft strain gage is often placed at the main bearing. As Marshall said, you should see a 1P oscillation in LSSGagMya due to variation of the rotor weight in the rotating frame of the shaft. The 1P amplitude should equal the rotor weight times the moment arm. The moment arm would equal ShftGagL if the weight of the rotor is centered about the hub center/teeter pin (that is, if the rotor does not have precone, blade out-of-plane pre-bend, or a hub mass offset from the hub center).

I would also expect to see a 1P oscillation in your RotorMy_V if the weight of the rotor is not centered about the hub center/teeter pin, athough the amplitude would be smaller than LSSGagMya because the moment arm would be smaller (unless ShftGagL is set to zero). However, RotorMy_V is constant, which tells me that that the weight of the rotor is centered about the hub center/teeter pin and that the moment is aerodynamic related, likely due to a shaft tilt or yaw error, where the aerodynamic loads are not constant around the disk.

Best regards,

Dear Jason,
All the specifications of the turbine is for OC3-Hywind that you shared it in NREL web page but with a little change as bellow:
0 ShftTilt - Rotor shaft tilt angle (degrees)
0 Delta3 - Delta-3 angle for teetering rotors (degrees) [unused for 3 blades]
0 PreCone(1) - Blade 1 cone angle (degrees)
0 PreCone(2) - Blade 2 cone angle (degrees)
0 PreCone(3) - Blade 3 cone angle (degrees) [unused for 2 blades]
0 AzimB1Up - Azimuth value to use for I/O when blade 1 points up (degrees)
Turbine is upwind and the platform, blades and tower DOF’s have been disabled. The yaw motion of the nacelle has been locked as well.
ShftGagL=1.912.
As you see there is no tilt, cone,…,and it seems the rotor CM must be in hub CM. Also the wind is steady with speed 18m/s with no shear and y component.
I wonder where the constant bending moment comes from with magnitude of -260KNm. How such constant aerodynamic bending moment could be produced in such a condition?

Regards,
Mehdi,

Dear Mehdi,

Are you modeling the OC3-Hywind spar as fixed in space, or is the spar permitted to move under the influence of wind, waves, and gravity? Pitching of the spar would be equivalent to nonzero shaft tilt.

If the shaft is horizontal and there is no shear or yaw error, then I agree the aerodynamic loads should be balaced around the disk. If this is not the case, I think you should look a other outputs to better isolate the problem.

Best regards,

Dear Jason,
As I said before, I disabled all motions of the platform, so the floating wind turbine has been converted to terrestrial one.
I am looking for another factor that may cause this problem.

Regards,
Mehdi,

Dear Mehdi,

OK, I’m not sure I can offer further advice without more information. I think you should look a other outputs (e.g., blade root loads) to better isolate the problem.

Best regards,