ElastoDyn + BModes: Tower input file and mode shapes calculation

Wind & Water Computer-Aided Engineering Software Tools
21

Dear @Jason.Jonkman
When I use the binary TurbSim FF wind type, the following error occurs.

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22

Dear @Peng.Guo,

My guess is your first issue with steady wind is caused because your initial condition for blade pitch (BlPitch = 0) is far below the expected mean value conditioned on the mean wind speed of 20 m/s, which should be closer to 17.5 degrees based on Figure 3-1 from the IEA Wind 15-MW reference wind turbine specifications report: https://www.nrel.gov/docs/fy20osti/75698.pdf. Poor initial conditions of rotor speed and/or blade-pitch can cause convergence issues with ROSCO.

My guess is that you have a similar issue with your turbulence case.

Best regards,

23

Hi Jason et al,

Thanks a lot for all the time and effort you put into improving OpenFAST, and helping us. Your contribution to the wind turbine field can’t be appreciated enough.

Based on some measurements, I am trying to build a monopile offshore wind turbine model in OpenFAST. I have access to the support structure and wind turbine design data. The goal is to run time marching simulations based on a set of environmental conditions at hand and calculate the loads on the support structure and turbine. I have completed the aerodynamics and controller and am now setting up the support structure in OpenFAST. I have the support structure in SubDyn, and the first eigenfrequency and mode shape match the expected values. I have a question about Elastodyn_Tower file. I modified the mode shapes in the file based on the mode shapes I got from SubDyn. I know there are mathematical formulas for calculating the distributed tower properties. However, my question is, is there any way to calculate the tower distributed properties using the outputs of SubDyn?

Thanks for your help.
Rad

24

Dear @Rad.Haghi,

I’m not sure I fully understand your question as you would normally model part of the support structure (below the transition piece) in SubDyn and part of the support structure (above the transition piece) in ElastoDyn.

You could also model the support structure fully in SubDyn, in which case the platform reference point in ElastoDyn should be located at the tower top and tower in ElastoDyn should be effectively removed by disabling the tower-bending degrees of freedom and ensuring the flexible length is near zero.

Best regards,

25

Hi Jason,

Thanks for your reply.

My question was about Elastodyn_Tower.dat file. In that file, there are distributed tower properties and mode shapes. My question was if it is possible to have the distributed tower properties from SubDyn output.

Knowing I can model the support structure fully in SubDyn, I will go for that option. Is there any drawback to modeling an offshore wind turbine support structure using only SubDyn? If I model the support structure fully in SubDyn, what would have to the tower aerodynamics in Aerodyn?

Thanks for the help.
Rad

26

Dear @Rad.Haghi,

SubDyn doesn’t output the distributed tower properties and mode shapes needed by ElastoDyn directly, but it does output sectional properties (such as E and Ixx) that you could derive the distributed tower properties from. That said, a tower as modeled in SubDyn makes use of a circular cross section, so, you could calculate the distributed properties by hand quite easily.

The main disadvantages of modeling the full support structure (including the tower) in SubDyn are:

  1. SubDyn is linear whereas the tower in ElastoDyn considers geometric nonlinearities that may play a minor role in the response if the tower deflection is sizeable.
  2. There is no tower-aerodynamic coupling between SubDyn and AeroDyn implemented yet, so, if you model the tower in SubDyn, you must disable the tower influence/shadow and tower aerodynamic load models in AeroDyn.

Best regards,