From BModes output to FAST (Blade file)

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Alvaro.Olcoz
Posts: 22
Joined: Thu Nov 15, 2018 5:46 pm
Organization: Universidad Pública de Navarra
Location: Navarra

From BModes output to FAST (Blade file)

Postby Alvaro.Olcoz » Wed Mar 20, 2019 10:32 pm

Dear Jason,
I am currently trying to reproduce in BModes the mode shapes available in FAST v8 CertTest of the SWRT machine in order to learn and be able to define my own baldes' in the future. As you stated in other posts, I have selected the blade sectional properties in order to obtain the uncoupled mode shapes (only for the first modes).
My concern (surely trivial sorry, but I have no background in modal analysis yet) is that in the output file ,for the span locations = 1 (end node) I expected to get the value of 1; in the flap disp channel for flapwise modes and in the lag dip for edwise modes (matching with the polynomic function value evaluated at x=1, which is one).
Then, after reading the BModes user guide I have found that the mode shapes given by BModes are mass-normalized. I do not understand this well, because if a needed to multipy a scalar (mass value) to the displacements obtained, and I wanted to get an unitary displacement for the last node in all the flap and edge modes, the actual displacement value in the output file at the last node would need to be equal (since the mass of the blade remains the same) for all the modes. There is certainly something that I misunderstand.
All in all, I get somehow stuck in the process between the BModes output file and the polynomial factors required by the FAST blade properties file ( ModeShapePolyFitting.xlsx may be needed).
Thak you very much in advance.
Best regards,
Alvaro Olcoz
PS: Attached you can find the BModes output file
Attachments
BModes_output.txt
(7.07 KiB) Downloaded 11 times

Jason.Jonkman
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Re: From BModes output to FAST (Blade file)

Postby Jason.Jonkman » Thu Mar 21, 2019 6:46 am

Dear Alvaro,

An eigenvector can be normalized in any number of ways. I'm sure you find the definition of mass normalization in the literature. If you want to normalize such that the tip displacement is unity, you can divide all elements of the eigenvector by the current value at the tip. Regardless, the ModeShapePolyFitting.xlsx spreadsheet does not care how the eigenvector specified in the x and y columns of the Input sheet is normalized--the tip displacement does not need to be unity. Simply copy the data from the span_loc and flap disp columns into the x and y columns of the Input sheet, and the spreadsheet will do the rest.

Please note that at the time the SWRT FAST model was made, the BModes preprocessor was unavailable, so, the Modes preprocessor was used in place of BModes to generate the polynomial coefficients of the blade for the SWRT FAST model.

Best regards,
Jason Jonkman, Ph.D.
Senior Engineer | National Wind Technology Center (NWTC)

National Renewable Energy Laboratory (NREL)
15013 Denver West Parkway | Golden, CO 80401
+1 (303) 384 – 7026 | Fax: +1 (303) 384 – 6901
nwtc.nrel.gov

Alvaro.Olcoz
Posts: 22
Joined: Thu Nov 15, 2018 5:46 pm
Organization: Universidad Pública de Navarra
Location: Navarra

Re: From BModes output to FAST (Blade file)

Postby Alvaro.Olcoz » Fri Mar 22, 2019 1:26 pm

Dear Jason,
Thank you very much for your rapid help one more time!
Best regards,
Alvaro


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