Modelling in PreComp & BModes the 5MW NREL blade

Hi everyone,

I’m trying to model the 5MW NREL baseline WT blades in PreComp, the problem is that I can not find the lay-out materials properties nor the webs configuration. I’ve tried to contact Sandia Lab as this thread says (http://forums.nrel.gov/t/structural-design-of-the-wind-turbine-blade/271/7) but I had no answer. I need to model the wind turbine because I need the coupled structural properties of the blade (I don’t know if I’m using the right words here but I’m refering to the terms that are not in the diagonal of the stiffness matrix) these can be obtained with PreComp, they are the “S_XXX” parameters in the output file. I would be very glad if someone can put some light in this question.

My second issue is with the BModes preprocessor (using the output PreComp file), I want to know why are 20 modes set up as default in the inputs files of the 5MW tower if FAST just uses the 2 first modes. I also would like to know if there is a fast way to see the 1st and 2nd modes for the blades reading the output file. And to finish, should the eigenfrequencies that we obtain running BModes be the same as the ones in the 5MW definition pdf? Or are they different because the blades are “alone” during the BModes running?

Thanks for all the support that this forum gives, before posting I’ve tried to search for older posts but had no success.

Dear Oriol,

You can find the Sandia report by Resor referred to in the aforementioned forum topic here: prod.sandia.gov/techlib/access-c … 132569.pdf.

It is simply hard-coded in BModes to output the first 20 eigenmodes. While BModes doesn’t directly output the polynomial coefficients needed by FAST, we’ve supplied the “ModeShapePolyFitting.xls” spreadsheet in the FAST archive to aid in the effort of deriving the polynomial coefficients.

The natural frequncies predicted by BModes should be close, but not identical to the full-system frequenices. I say “close, but not identical” because the full-system frequencies will be impacted by the coupling to other degrees-of-freedom (DOFs) in the system–beyond just the isolated beam DOFs calculated by BModes.

Best regards,

Thank you very much for the pdf, it really helped me to build the 5MW blade turbine, now I’m trying to adjust the number of layers of each material that each Layup configuration has, in order to match at the maximum the properties of the 5MW NREL, when I get a good configuration I will share it here, then maybe people could improve it even more.

Dear Jason,

I’ve modelled a blade that more or less meet the 5MW NREL baseline wind turbine blade data, now while working on BModes I have a few doubts about the “Blade-tip properties”. I think in the particular case of the 5MW NREL wind turbine there is no tip mass because there is no tip brake in the model, then all this section should be set to 0.0 value? My other question is about the hub_conn flag, I know while simulating with FAST the whole wind turbine we have a cantilevered beam, but for the blade alone simulation is it better to do it in mode 2 (free-free)?

Thanks

Dear Oriol,

For the NREL 5-MW rotor blade I would not consider any point mass/inertia at the tip.

If you are trying to simulate in BModes a rotor blade attached to the hub, I would treat the root in a cantilevered condition. While BModes cannot model the entire turbine and the rest of the turbine is not rigid, it is more realistic to treat the root in a cantilevered condition than as “free”.

Best regards,

Dear Jason,

last question: should the rotational speed in the input file be set to the nominal rotational speed of the rotor (12.1 rpm)?

thanks

Dear Oriol,

Mode shapes in real life change with operational parameters, such as rotor speed, pitch angle, and applied load. However, my experience has been that the accuracy of the mode shape (as long as they are close to correct) is not too 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 (at the rated speed and nominal pitch angle). As wind turbines are getting larger and more flexible, the impact of the mode shapes will likely become more important. You could always do a sensitivity study to see if changes in the mode shape with different operational conditions have an influence on the results.

Best regards,