FAST error when reading mode shape inputs

Good Afternoon,

First off, thank you for making FAST available as a resource and managing this forum. I am currently trying to use FAST to run some analyses on a conceptual support structure that I am designing for the master thesis, and have thus far have been unable to run the required tests because when initializing ElastoDyn, the program seems to be trying to read the non-existent variable TwFAM1sh(1) – corresponding to the coefficient of the x^1 term of the mode shape fit polynomial. See attached pictures with error message “Invalid numerical input for file \8MW_Baseline\Vestas8MW_HybridBase_ElastoDyn_Tower.dat” occurred while trying to read TwFAM1Sh(1).

I used the ModeShapePolyFitting.xls spreadsheet provided, and for the improved method coefficients are only provided for the x^2-x^6 terms and it appears that the example files only include those coefficients in their mode shape inputs without issue. I also attempted to add an additional line of code to the input file with the x^1 coefficients from the direct method fit but that yielded the same error (“attempt2” image attached)

Just looking for any guidance as to how I can clear this error. Please let me know if I need to provide any additional information and thank you for your help.

M.C. Anderson
Vestas8MW_HybridBase_ElastoDyn_Tower.txt (3.62 KB)


Dear M.C. Anderson,

The error message referring to TwFAM1Sh(1) is incorrect; the message should refer to TwFAM1Sh(2), because–as you said–TwFAM1Sh(1) is never specified within the input file (and is always assumed to be zero).

Based on your tower input file, the error message is triggered because you have two blank lines between HtFract = 1.0 and the header for the Tower Fore-Aft Mode Shapes section of the input file.

I hope that helps.

Best regards,

Dear Jason,

My mode shape coefficients yielded by Normalized Improved Direct Method are as follows:
1st SS…a6…a5…a4…a3…a2
…-0.1911…1.1843…-0.0246…0.0109…1.0204
2nd SS
…-3.3430… 7.4950…-4.7084…3.2236…-1.6671
1st FA
…-0.1758…0.1454…0.0025…-0.0020…1.0299
2nd FA
…-1.9865…3.9697…-2.2354…2.1397…-0.8874
I have added them all to 1 and entered them into elastodyn file. Once I run the FAST V.8 an error is occured with the following description:
“Mode shape coefficients for tower fore-aft mode 1 must add to 1.0.
Mode shape coefficients for tower fore-aft mode 2 must add to 1.0.
Mode shape coefficients for tower side-to-side mode 1 must add to 1.0.
Mode shape coefficients for tower side-to-side mode 2 must add to 1.0.”
Do you know what is the problem ?

Best regards,

Dear Arsalan,

Your mode shape coefficients do not sum to one e.g. for the 1st SS tower mode, -0.1911 + 1.1843 + -0.0246 + 0.0109 + 1.0204 /= 1. From the related forum topic you’ve been posting in (http://forums.nrel.gov/t/tower-and-blades-mode-shapes-bmodes-vs-fast/121/1), my guess is you are copying the incorrect cells from the ModeShapePolyFitting.xls spreadsheet. My guess is your grabbing e.g. M9 - Q9, but these coefficients are not normalized. Instead, you should grab e.g. E6 - E10, which are normalized.

Best regards,