NREL 15MW Blade Mode Shape

Dear NREL team

Thanks for your endeavors on this in advance.

I am using the BModes to calculate the mode shape for the original NREL 15MW blades and tower at the beginning to validate my simulation procedure.

I have followed up the suggestions for the corresponding adjustments in the thread as per link NREL 5MW Blade Mode Shape - #5 by Paul.Feja

As there are still some discrepancies in blade natural frequencies compared to the results in the page 8 of report IEA Wind TCP Task 37, “Definition of the IEA Wind 15-Megawatt Offshore Reference Wind Turbine” published March 2020, I am wondering if you could give me some advices or comments on my settings as attached files for both blade and tower Bmodes files. (Tower file attached as well) Especially, the sections for Blade-tip or tower-top mass properties, could I extract them directly from the summery files as well? thanks

I would also like to check for few things here

  1. How could I include the blade flexibility into Bmodes to work out the blade natural frequencies?
  2. Does the tower Bmodes file include the information for RNA as a whole structure or only the information for tower itself?
  3. If I would like to generate the natural frequencies for whole system, how could I conduct it?

Thanks for your attention.

Best regards
Shuo-Ting Chiang
IEA-15-240-RWT_BModes_tower.docx (16.1 KB)
IEA-15-240-RWT_BModes_blade.docx (14.4 KB)
IEA-15-240-RWT_BModes_blade_sec_prop.txt (6.43 KB)

Dear Shuo-Ting Chiang,

Just a few comments:

  • Overall, it looks like you’ve specified these BModes input files correctly, except that you’ve specified a substructure mass and stiffness matrix for the tower while using hub_conn = 1, which means that the specified substructure mass and stiffness will not be used.
  • However, I’m not an expert on the IEA Wind 15-MW reference wind turbine and can’t confirm your tower-top mass, center of mass, and inertia values, or your substructure mass and stiffness values.
  • The tower-top mass, center of mass, and inertias specified in BModes for the tower should represent the properties of the complete RNA as a lumped rigid body. This mass is written to the ElastoDyn summary file (when the blades are modeled in ElastoDyn, not BeamDyn), but the center of mass and inertias are not. These, hoewever, can be obtained through a FAST / OpenFAST linearization analysis, e.g., as discussed in the following forum topic: How did you obtain the RNA properties you’ve set in this BModes input file?
  • The substructure mass and stiffness should be used if you are trying to derive the tower mode shapes for a system where the tower is connected to fixed-bottom offshore substructure (such as the monopile) or a floating offshore substructure (such as the UMaine semi). In this case, hub_conn should be set different from 1 and you should use the version of BModes referred to as BModes_JJ on this forum. Which system are you considering and how did you obtain the associated substructure mass and stiffness matrices?
  • Your BModes inputs look correct for the blade; the tip mass can be zero and hub_conn = 1 as you’ve specified them is OK. The natural frequencies and mode shapes of the blade are output from BModes after execution.
  • BModes can only model individual components like a blade or tower of a wind turbine. To calculate the full-system natural frequencies, including coupling between the blades and tower, the linearization functionality of OpenFAST can be used, as discussed many times on this forum.

Best regards,

Dear Jason,

Thanks for your kind and prompt reply.

These are really useful information for me to continue the further analysis.

I am using the monoplile for consideration but I haven’t considered the associated substructure mass and stiffness matrices for the tower modes analysis.

I am also wondering how I could get these information and what I think is to extract them from the subDyn summery files. do you have any advice on this?

I will follow up what you suggested for linearization analysis to evaluate the natural frequency for whole system and keep you posted after I have concrete results.

Thanks a lot again

Best regards
Shuo-Ting Chiang

Dear Shuo-Ting Ching,

Yes, you can derive the substructure mass and stiffness matrices needed as input to BModes from the SubDyn summary (specifically, the MBBt and KBBt matrices). This is documented in the online SubDyn manual on OpenFAST readthedocs: … e-coupling.

Best regards,