Tower (and blades) mode shapes BMODES vs. FAST

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Gabriel.Maciel
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Re: Tower (and blades) mode shapes BMODES vs. FAST

Postby Gabriel.Maciel » Sun May 21, 2017 1:58 pm

Jason,

Thank you very much for your feddback.

Regards,

Gabriel Maciel.

Arsalan.Shah
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Re: Tower (and blades) mode shapes BMODES vs. FAST

Postby Arsalan.Shah » Sun Sep 23, 2018 3:00 am

Dear Jason,

Regarding the Mode shape poly fitting procedure via corresponding excel file presented by NWTC I came up with a question. I have run the BModes and obtained the first 20 mode shapes of the OC4 jacket OWT. Now as you recommended, I want to identify the prominent components of the coupled mode shapes in order to put them into Y column in the input sheet of the ModeShapePolyFitting work sheet. In addition, the FAST ElastoDyn_tower input file needs the first and second FA and SS mode shapes polynomial coefficients which normally are the first 4 mode shapes generated by BModes. But the f-a displacements relative to first 4 mode shapes are larger than the s-s displacements for each particular mode shape.
Does it mean that the first 4 mode shapes are FA modes ?! If not, would you please help me what to do in order to obtain first and second FA and SS mode shapes.

Best regards,

***My BModes' output is attached here***
26m - Copy.out.txt
(61.42 KiB) Downloaded 64 times

Jason.Jonkman
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Re: Tower (and blades) mode shapes BMODES vs. FAST

Postby Jason.Jonkman » Mon Sep 24, 2018 6:31 am

Dear Arsalan,

I took a brief look at your results and it is surprising to me that the fore-aft displacement seems to dominate over the side-to-side displacement until mode 8. Normally I'd expect more closely aligned pairs of fore-aft and side-to-side modes of similar frequency. I would question the accuracy of the eigensolution. Are you running the same BModes model you shared in the following post: viewtopic.php?f=4&t=2008? Have you checked the sensitivity of the eigensolution to tor_stff and axial_stff, which were set arbitrarily high? (Instead of using 5 orders of magnitude higher than flp_stff and edge_stff, perhaps try setting tor_stff and axial_stff 2-3 orders of magnitude higher.)

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

Arsalan.Shah
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Re: Tower (and blades) mode shapes BMODES vs. FAST

Postby Arsalan.Shah » Mon Sep 24, 2018 10:57 am

Dear Jason,

Yes I have run the model in the post you have mentioned but the apparent fixity length have been changed to 26 meter. Furthermore, I have decreased the order of tor_stff and axial_stff as you recommended but still f-a displacement dominate over the s-s displacement. I have attached my model coressponding files in the following.

Your sincerely,
out26.SD.sum.txt
(2.49 KiB) Downloaded 56 times

26mBModesout.txt
(61.42 KiB) Downloaded 56 times

tower_dist._prop..txt
(3.47 KiB) Downloaded 55 times

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Re: Tower (and blades) mode shapes BMODES vs. FAST

Postby Jason.Jonkman » Mon Sep 24, 2018 11:55 am

Dear Arsalan,

OK, I'm able to reproduce your results.

I noticed that the KBBt and MBBt matrices written by SubDyn are quite different between these models. It sounds like you've changed the apparent fixity length, but can you clarify what you changed? Are you expecting such large differences in KBBt and MBBt?

When I run BModes with the original KBBt and MBBt matrices (i.e. the original hydro_K and hydro_M matrices from BModes.txt from viewtopic.php?f=4&t=2008&start=15), the BModes output is much clearer, where I would conclude the following:
Mode 1 - freq=0.309 Hz; 1st tower side-to-side
Mode 2 - freq=0.312 Hz; 1st tower fore-aft
Mode 6 - freq=2.278 Hz; 2nd tower side-to-side
Mode 7 - freq=2.729 Hz; 2nd tower fore-aft

I would conclude the same mode # identification from 26mBModesout.txt (with different frequencies), but I'm not sure why the side-to-side deflection does not exceed the fore-aft deflection in mode 1 in this case.

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

Arsalan.Shah
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Re: Tower (and blades) mode shapes BMODES vs. FAST

Postby Arsalan.Shah » Mon Sep 24, 2018 2:09 pm

Dear Jason,

Firstly, I have to appreciate you for the time you have spent on checking my model. The only thing that I have changed in my last model comparing to the model you just ran( is the last four joints Z coordinate in my SubDyn input file as they have taken values -76 (i.e. -50 + (-26) ). As a result, the KBBT and MBBT matrices are changed considerably as the apparent fixity length have been increased from 12.46 to 26 m.

Best regards,
***BTW here is my SubDyn input file.

SDinp.rar
(3.97 KiB) Downloaded 52 times

Arsalan.Shah
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Re: Tower (and blades) mode shapes BMODES vs. FAST

Postby Arsalan.Shah » Wed Sep 26, 2018 10:01 am

Dear Jason,

I changed the the flp_iner and edge_iner values in tower section properties file from 0.0001 to 0.001 and it seems the problem has been solved. Now by looking at the BModes output the first four mode shapes can be considered as first and second S-S and F-A modes respectively. Would you please take a look at my result and let me know if it is correct or not ?

Best regards,

newBMout.txt
(61.42 KiB) Downloaded 55 times

Jason.Jonkman
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Re: Tower (and blades) mode shapes BMODES vs. FAST

Postby Jason.Jonkman » Wed Sep 26, 2018 4:59 pm

Dear Arsalan,

Very interesting! The natural frequencies calculated by BModes did not change by much, but increasing flp_iner and edge_iner a little definitely makes the S-S and F-A mode shapes more readily identifiable. I'm not sure why this sensitivity exists in BeamDyn (likely a numerical problem), but I'll have to remember this sensitivity in the future.

I would interpret the modes as follows (same ordering as before):
Mode 1 - freq=0.286 Hz; 1st tower side-to-side
Mode 2 - freq=0.288 Hz; 1st tower fore-aft
Mode 6 - freq=2.033 Hz; 2nd tower side-to-side
Mode 7 - freq=2.336 Hz; 2nd tower fore-aft

Modes 3-5 show different coupling of the tower to the jacket, but they are not 2nd tower modes.

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

Arsalan.Shah
Posts: 53
Joined: Thu Nov 16, 2017 11:27 am
Organization: IAU-Science and Research branch
Location: IR

Re: Tower (and blades) mode shapes BMODES vs. FAST

Postby Arsalan.Shah » Thu Sep 27, 2018 3:59 am

Dear Jason,

I really appreciate you for your attention. But would you please interpret that why you have chosen modes 6 and 7 instead modes 3 and 4 ? Is there any consideration except domination of displacements in order to decide which ones have to be selected ?

Best regards,

Jason.Jonkman
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Re: Tower (and blades) mode shapes BMODES vs. FAST

Postby Jason.Jonkman » Thu Sep 27, 2018 6:48 am

Dear Arsalan,

If you look at modes 3 and 4 in the ModeShapePolyFitting.xls spreadsheet, you'll notice that they are simply repeats of modes 1 and 2 (but couple to the substructure differently) i.e. the curvature of the beam does not change sign. When you look at modes 6 and 7, you clearly see that they are second modes i.e. the curvature of the beam changes sign once along the beam. In ElastoDyn, the actual tower deformation will be formed by some linear combination of the modes, so, you'll want to provide the first two mode shapes to have a solid basis for deformation.

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

Arsalan.Shah
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Joined: Thu Nov 16, 2017 11:27 am
Organization: IAU-Science and Research branch
Location: IR

Re: Tower (and blades) mode shapes BMODES vs. FAST

Postby Arsalan.Shah » Thu Sep 27, 2018 7:17 am

Dear Jason,

It is completely clear for me now. I really appreciate you.

Sincerely,

Arsalan.Shah
Posts: 53
Joined: Thu Nov 16, 2017 11:27 am
Organization: IAU-Science and Research branch
Location: IR

Re: Tower (and blades) mode shapes BMODES vs. FAST

Postby Arsalan.Shah » Sat Sep 29, 2018 7:49 am

Dear jason,

In regard to poly fitting procedure, according to FAST guideline and as you mentioned earlier we must sum a2-a6 coefficients to one preceding to move them into Elastodyn_tower file as the structure mode shapes. But given that the picture I have attached here, which row have to be considered ? Does the projection method's row (the highlighted one in the picture) is the target ?

Best regard,

polytest.jpg
polytest.jpg (656.74 KiB) Viewed 2721 times

Jason.Jonkman
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Re: Tower (and blades) mode shapes BMODES vs. FAST

Postby Jason.Jonkman » Sat Sep 29, 2018 8:15 pm

Dear Arsalan,

As described in the "ReadMe" worksheet, I would normally recommend that you use the Normalized Improved Direct Method when you are deriving the modes for ElastoDyn from BModes. Make sure that you specify the slope at the bottom of the beam (as taken from the BModes output) in addition to the x and y data.

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

Arsalan.Shah
Posts: 53
Joined: Thu Nov 16, 2017 11:27 am
Organization: IAU-Science and Research branch
Location: IR

Re: Tower (and blades) mode shapes BMODES vs. FAST

Postby Arsalan.Shah » Sat Sep 29, 2018 9:30 pm

Dear Jason,

In the picture attached to my last post the improved normalized direct method has yielded quite large amounts. Is it correct yet ?

Best regards,

Arsalan.Shah
Posts: 53
Joined: Thu Nov 16, 2017 11:27 am
Organization: IAU-Science and Research branch
Location: IR

Re: Tower (and blades) mode shapes BMODES vs. FAST

Postby Arsalan.Shah » Sat Sep 29, 2018 11:03 pm

Dear Jason,

The problem is solved by applying the correct slope at the bottom. I really appreciate you.

Sincerely,


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