Re: rigid body mass matrices

[Steven Doyle]

PyNastran is not calculating any of that. It’s simply pulling data that is there.

I’m not sure how Nastran handles the rigid body mass matrices. I’d assume it was just based on the g-set and follow the method described in the basic dynamics guide.

Femap allows you to request different sets of rigid body mass matrices, so I’d guess there are different names for different sets.

I’m sure other people here have a better understanding than I do.

On Thu, Aug 1, 2024 at 12:55 AM Cédric Dumoulin <cedu...@gmail.com> wrote:

Hi Steve, 

I would like to import the different rigid body mass matrices (G, N, ...) computed in Nastran using pyNastran. I'm currently working using Simcenter 2306 distribution of Nastran (more specifically, I would like to ensure that I import the one obtained from the G set reduction). 

In the example here, I only have a superelement as DMIG in the model which has been generated with both fixed (BSET) and few free (CSET) boundary conditions (which explains why I have such a difference in the rigid body masses with or without spc...)

I then run a SOL103, with the node which where in the BSET fixed. 
The mass matrices reduced with the G and N sets in f06 file are correct. But the matrix I get in pyNastran (RBMASSS) corresponds to the matrices reduced with F and A sets. See only_bset_fixed folder in minimum working example provided). 

What surprising me, it that, when I fix all the nodes, the rigid body matrix from F and A sets are obviously fully populated of '0', but then, pyNastran is giving me the correct matrix (corresponding to either G, N, or N+auto).  see all_fixed folder in minimum working example provided. 

Could you tell me how I can extract a specific rigid body matrix using pyNastran? 

Thank you in advance for your help, 

Best regards, 

Cédric 

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[Cédric Dumoulin]

Hi Steve, 

Thank you for you answer! I did not expect that PyNastran was calculating anything there, I'm was just trying to get the different matrices calculated by Nastran. I understand from you answer that those matrices are simply not present in the OP2, otherwise they would have been extracted! I'll simply parse them from the f06 then :) 

Best regards, 

Cédric