FREE-FREE Modes

Hello

I have a model that I am checking for any errors. I ran a free-free modal analysis. Instead of getting 6 rigid body modes (zero), I am getting the first 3 modes as 0 while, the other 3 (rotational) are not perfectly zero. It has a value of 0.2

Any recommendation on how to debug and know what is going on?

0 Ken Walker over 5 years ago
This could be a grounding problem caused by unintended internal constraints.

Typical grounding causes are:

Ill-conditioned MPC’s

Incorrectly connected spring elements

Coordinate system errors (definition and/or references)

PS entry on GRID card (Permanent single-point constraint)

Improperly defined DMIG matrices (Superelement errors)

To identify potential grounding problems, add GROUNDCHECK(SET=(G,N))=YES to your Nastran model (goes in CASE CONTROL). It has additional options; read the QRG for details. Look in f06 for PASS/FAIL under "RESULTS OF RIGID BODY CHECKS".
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0 shaheer.iqbal over 5 years ago
Hello,

Thank you for the response, So I debugged my model and found the following fix.

I was using glued contact within my model. I deleted the glued contact and put a RBE2 contact to attach the bodies and it fixed the error.

Any suggestion why would a glued contact not give perfect rigid body modes?
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0 Ken Walker over 5 years ago
Short answer: you will get rigid modes any where 2 parts aren't successfully glued. There can be multiple causes. The most common include:

Contact detection distance (ERROR) too small,

Missing elements in contact bodies (BSURF),

Incorrect IGLUE flag (need 2 or 4 for moment carrying glue with shells/bars).

Incorrect contact options (COPTS/COPTM)

To visualize contact status, include BOUTPUT=YES as a Case Control Result Request to get contact status for Subcase 0. Be sure to save results as HDF5 or OP2 to post-process.
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