Modal stiffness and mass matrices extraction

[Spyros07]

Hello to the community!

I' m trying to extract the modal stiffness and mass matrices from a real eigenvalues analysis (SOL 103) for an aeroelastic simulation. I've tried to add DMAP alters for the Mhh and Khh matrices, nevertheless I haven't managed to extract them at all. Is there anyone familiar with this extraction? Any ideas would be very much helpful! Thanks in advance.

 

[jamesdean2]

Hi Spyros07,
Your best approach would be to skip the DMAP alters and use NASTRAN CMS (ie, superelement). You'll probably want a reduced model anyway for aeroelastic's. I'm pointing you in a different direction here, and it's not that you can't use DMAP, it just wouldn't be what I would call best practice. If you use the NASTRAN op4 format (ASCII) as your SE form, you can use the Python package 'pyYeti' to read the K and M matrices in and manipulate them into the proper format for your aeroelastic simulation. If you are trying to get the data into MatLab, I believe there is an op2 (binary) reader, I've seen use.

Here's a couple of resources:

https://www.youtube.com/watch?v=BEaYZ9Ev6mo

https://pyyeti.readthedocs.io/en/latest/index.html

 

[Spyros07]

Thank you very much for your prompt response jamesdean2! I tried your method, nevertheless i ran into two issues: 1) Matrices obtained are in the a-set, not in the h-set(modal) 2) Eigenfrequencies solution in MATLAB do not yield results similar to the NASTRAN ones. Has anyone had similar issues?

 

[jamesdean2]

Sorry, I think I misinterpreted your original post, that said, wrt item 1, typically if the aeroelastic analysis is not done internally within NASTRAN then the generalized stiffness/mass/damping matrices are taken from NASTRAN and processed externally. If you want NASTRAN to do that and if you haven't done this already, I recommend you look thru the nast/sssalter sub-directory of your NASTRAN installation, you will find examples of DMAP along with readme files that may be helpful to use along with the DMAP programmers guide. Wrt item 2, the typical problem-solving approach with an issue like this is to do a simple problem (ie. cantilever beam) with a known theoretical solution, from that result, you can identify what method is wrong and potentially the issue.