Regarding the issue of the moment of inertia of RNA

[TianyuanWangi]

Description

Describe the bug here
Hi,
I noticed there might be an issue with the rotational inertia of the RNA in the sheet "Nacelle Mass Properties" of the file \Documentation\IEA-15-240-RWT_tabular.xlsx. The moment of inertia of the RNA about the TT coordinate system is:

378481086 271925264 252790348 848 19078409 170121

However, the moment of inertia about its own center of mass is:

499865526 342629218 343480100 691780 -12757978 -263827

May I ask whether this is caused by an incorrect calculation formula? Additionally, I noticed that some moments of inertia do not satisfy the parallel axis theorem and always have some discrepancies. What might be the reason for this?

Steps to reproduce issue

Please provide a minimum working example (MWE) if possible

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Current behavior

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Expected behavior

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Code versions

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• Python
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[gbarter]

Hello,

Thank you for catching this! You were correct that there was an error in calculating the final RNA center of mass (CoM). However, the error wasn't in the final value that was reported or an incorrect application of the parallel axis theorem, it was a "copy-paste" error in the spreadsheet only where the blades were listed as having the same CoM as the hub. The WISDEM code correctly accounts for the coning of the blade, such that the CoM of the blade set is further upwind of the hub CoM. The Excel spreadsheet in the develop branch has been updated with the correct CoM for the blades, which then makes the total RNA summation (via the parallel axis theorem) accurate for the CoM.

For the rotational inertia question, could you please explain a little bit more what error you see? What numbers would you expect compared to current values? There are many components and coordinate systems to keep track of, so it is entirely possible that a bug exists.

[TianyuanWangi]

Hello,

Thank you for catching this! You were correct that there was an error in calculating the final RNA center of mass (CoM). However, the error wasn't in the final value that was reported or an incorrect application of the parallel axis theorem, it was a "copy-paste" error in the spreadsheet only where the blades were listed as having the same CoM as the hub. The WISDEM code correctly accounts for the coning of the blade, such that the CoM of the blade set is further upwind of the hub CoM. The Excel spreadsheet in the develop branch has been updated with the correct CoM for the blades, which then makes the total RNA summation (via the parallel axis theorem) accurate for the CoM.

For the rotational inertia question, could you please explain a little bit more what error you see? What numbers would you expect compared to current values? There are many components and coordinate systems to keep track of, so it is entirely possible that a bug exists.

Hi gbarter
Thank you for your prompt reply and clarification. I believe that the moment of inertia of the RNA about its own COM cannot be greater than that about the TT. It can be derived using the parallel axis theorem to obtain the moment of inertia about its own COM:

1.0e+08 *

3.5844    0.0069   -0.1286
0.0069    2.0049   -0.0026

-0.1286 -0.0026 2.0135
targetInertia = targetInertia - mass*(dot(posVec,posVec)eye(3) - (posVecposVec'));

Here, posVec is the vector from RNA_TT to RNA_COM.

Kindly let me know if there are any issues.

Tianyuan

[gbarter]

Yes, you are correct again. There was indeed a +/- error in the shift from the summation in the tower-top coordinate system back to the CoM. An updated spreadsheet is once again available in the develop branch. Thank you for making improvements to the reference turbine material!

[TianyuanWangi]

Yes, you are correct again. There was indeed a +/- error in the shift from the summation in the tower-top coordinate system back to the CoM. An updated spreadsheet is once again available in the develop branch. Thank you for making improvements to the reference turbine material!

Hi gbarter,

It seems there may still be some issues with the results. The values I obtained are:

1.0e+08 *

    3.5844    0.0069   -0.1286
    0.0069    2.0049   -0.0026
   -0.1286   -0.0026    2.0135

Could you please check this again?

[gbarter]

I appreciate your rigor to get this right. When I did the calculation by hand, I also got slightly different numbers than what was posted last time, so I double checked the code, ran the calculations by hand and now things match (in the develop branch). Some comments:

• The RNA MoI summation is first done in the tower top coordinate system (not the CoM) and then then parallel axis theorem is reversed to determine the MoI at the CoM numbers.
• You can find the TT summation in the WISDEM code here and the postprocessing to get to the CoM MoI values here
• The code uses the tensor form of the parallel axis theorem given the complexities.
• I think you are using the MoI CoM numbers and then computing the tower top values. They should work out the same (I checked this by hand), but make sure you are handling all the terms correctly too, such as using the y^2+z^2 offsets for the MoIxx term.

[jennirinker]

@gbarter Do we need to update any of the HAWC2 input files with this change?

[gbarter]

@gbarter Do we need to update any of the HAWC2 input files with this change?

Pretty sure not. Neither HAWC2 or OpenFAST need the RNA mass properties as an input. The discussion here is about some of the errors found in my script to generate the summary RNA mass properties in the Documentation spreadsheet.