# Tower (and blades) mode shapes BMODES vs. FAST

Yinping Yang,

Mode shapes in real life change with operational parametes, such as rotor speed, pitch angle, and frequency of excitation. However, my experience has been that the accuracy of the mode shape (as long as they are close to correct) is not critical to the results of a loads analysis–the modal frequencies are far more important. So, we typically calculate the mode shapes only at one design point–typically the conditions at rated power. As wind turbines are getting larger and more flexible, the impact of the mode shapes will likely become more important.

I simply took your coefficients and scaled them so that they sum to unity. Again, as long as the mode shapes are close to correct and the system frequencies (and other inputs) are accurate, your simulation results should be good.

AirfoilPrep not only has a feature to extrapolate to the full range of AoA, but it also has features to (1) apply rotational augmentation (i.e., 3D corrections), (2) blend airfoil data (if needed), (3) calculate dynamic stall parameters, and (4) convert the airfoil data to AeroDyn format. So, AirfoilPrep is useful to use regardless of if the AoA range you have.

Best regards,

Hi,

I´m conducting a study of the tower´s mode shapes. In FAST v8, the files “NRELOffshrBsline5MW_ (name of substructure) _ElastoDyn_Tower.dat” needs the polynomials’ coefficients of tower´s fore-aft (FA) and side-to-side (SS) mode shapes (first and second orders) as input. I have noticed that these polynomials start at the origin (0;0) point and with end at (1;1) point. For the OC3 Tripod structure see figures 1 and 2.

I also noticed that the OC4Jacket structure has modes shapes very similar to those in the figures.

Analyzing the figures it is clear that the deformation at the tower’s base is zero, so I deduce that the Tripod substructure is rigid (i.e not flexible) in this analysis. Am I right?

I ´m studying a tripod-shaped structure, similar to OC3 Tripod structure and I evaluated the mode shpapes in ANSYS software. When I placed the tower’s base, on top of my substructure, (the piles are fixed, simulating the connection to the seabed), I noticed that the tower base has a non-zero deformation in first and second order mode shapes (as in Figure 3 for example). Thus, it is impossible to obtain a polynomial like the OC3Tripod ones, since the graphics never begin at (0;0) point.

Do you know if FAST v8 requires the polynomial to start at point (0;0) (i.e, zero deformation at the base)?

Regards,

Gabriel Maciel.

Dear Gabriel,

I have answered a similar question in my Apr 10, 2012 post in the following forum topic: http://forums.nrel.gov/t/turbine-soil-interaction-influence-on-the-mode-shapes/481/2.

Best regards,

Jason,

Thank you very much for your feddback.

Regards,

Gabriel Maciel.

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 (61.4 KB)

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: http://forums.nrel.gov/t/modes-of-bmodes/1782/1? 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,

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.

out26.SD.sum.txt (2.49 KB)
26mBModesout.txt (61.4 KB)
tower_dist._prop…txt (3.47 KB)

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 http://forums.nrel.gov/t/aerodyn-feedback/57/1), 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,

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 KB)

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 (61.4 KB)

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,

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,

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,

Dear Jason,

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

Sincerely,

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,

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,

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,

Dear Jason,

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

Sincerely,

Dear Dr.Jonkman,