BCs for flexbodies

Hello,

Is there a way to include SPCs in your model when creating and MNF file for a flexbody? I am asking because when I tried creating an mnf file for a table-like structure, the 1st mode was completely off than when running a simple sol103. The points which were supposed to be constrained were moving freely in the modes (even though I had also included them in ASETS).

Is there another way that I can ensure the validity of the constraints when creating an mnf file?

Thanks,

Josef

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0 BThielem over 2 years ago

Hello Josef,

oh sorry, I have to step back somehow. I dont know good enough about mnf-file creation with SOL103 or in NASTRAN.

But nevertheless let me share my thoughts on this:

The frequencies of the modes of the mnf-file should not be compared to the frequencies of the modes of the same flexbody (or the full model in FEA) when its constrained.

In the mnf-file the fist six modes are rigid body modes (which have to be disabled, if they arent by default), then come the "free-free" modes, then come the static correction modes. Arent they ? I dont wonder that in the fist free-free modes (e. g. mode 7) the feet are moving freely. Seems correct to me. Why should the top part move if the feet arent constrained?

The use of RBE2 elements in mnf-file creation just means that the connection between the interface / master node and the slave nodes is defined as rigid. But the mnf-files interface / master nodes are still "open" for any sorts of connectors or forces.

If you now constrain the flexbody at the feet (with fixed joints to ground) and the model ist working properly, then I think the mnf-file is good. You cant compare the unconstrained mnf-File to the constrained FEA-model. An apple-to-apple-comparision is FEA full model constrained to mnf/reduced model constrained in the same way.

You wrote, in Adams the model is working properly. Is there the first mode at 1.5 Hz when its constrained with fixed joints (you didnt write)? If it is, then the mnf-file should be okay.

Best wishes

Bruno
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0 BThielem over 2 years ago

Hello Josef,

oh sorry, I have to step back somehow. I dont know good enough about mnf-file creation with SOL103 or in NASTRAN.

But nevertheless let me share my thoughts on this:

The frequencies of the modes of the mnf-file should not be compared to the frequencies of the modes of the same flexbody (or the full model in FEA) when its constrained.

In the mnf-file the fist six modes are rigid body modes (which have to be disabled, if they arent by default), then come the "free-free" modes, then come the static correction modes. Arent they ? I dont wonder that in the fist free-free modes (e. g. mode 7) the feet are moving freely. Seems correct to me. Why should the top part move if the feet arent constrained?

The use of RBE2 elements in mnf-file creation just means that the connection between the interface / master node and the slave nodes is defined as rigid. But the mnf-files interface / master nodes are still "open" for any sorts of connectors or forces.

If you now constrain the flexbody at the feet (with fixed joints to ground) and the model ist working properly, then I think the mnf-file is good. You cant compare the unconstrained mnf-File to the constrained FEA-model. An apple-to-apple-comparision is FEA full model constrained to mnf/reduced model constrained in the same way.

You wrote, in Adams the model is working properly. Is there the first mode at 1.5 Hz when its constrained with fixed joints (you didnt write)? If it is, then the mnf-file should be okay.

Best wishes

Bruno
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