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BCs for flexbodies

Josef K.
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
  • 0
    Hello Josef,
    perhaps I dont exactly get your question, but can it be that the mnf-file is okay and has interface nodes on the locations of the BCs. And that you only forgot to define the BCs in Adams? If you didnt constraint the interface nodes in Adams in the same way than in FEA (eg NASTRAN) than there are no constraints...
    With my best wishes Bruno
  • 0
    Hello Bruno,
     
    Reading my own post now I see that I did not state my question properly. Let me clarify.
     
    My model is a table-like structure, with its "feet" bolted on the ground. This should mean that the nodes at the bottom of the feet should be completely still at all times.
     
    When buidling a simple FEA model (with SPCs on the feet) and running a SOL103 the first mode is present at 1.5 Hz. It is a bending mode with the top wiggling about and the feet fixed to the ground.
     
    When solving a SOL103 to generate the MNF file, instead of adding the SPCs (since they can't be used in the MNF creation process) I used RBE2 elements for the nodes which were previously used in the SPCs and added the master nodes to an ASET in the model. To the best of my knowledge, this is how you are supposed to build the MNF files.
     
    When viewing the modes of the MNF file, the first one appeared at about 2.9 Hz. What seemed completely off to me though is the fact that in this mode the top part of the table was still whie the feet were moving freely (still a bending mode). I also made sure I included the correct nodes in the ASET.
     
    What I am asking is wether I have made a mistake in the way I tried to constrain the flexible body. Is there a different way to make sure that the feet are constrained when building the MNF file, so that its modal properties are correct?
     
    As far as the Adams model is concerend, I did constrain them there (fixed joints to the ground) and the model is working properly. But the first mode of the MNF files should be at 1.5 Hz and not at 2.9 Hz.
     
    Thanks,
     
    Josef
     
     
  • 0
    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 Thinking? 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
  • 0
    Hello bruno,
     
    Per the last part of your answer, as far as I know, the modes of an mnf file do not change when it is constrained i.e., since the 1st mode is at 2.9 Hz (in Adams), even though I used fixed joints, it would still remain the same. I have no way of making it change throught Adams.
     
    As far as the MNF file creation is concerned, my problem perhaphs is in the understandning of how the mnf file creation works. When running simple eigenmodes analysis (sol103) constraining the body for it to be in its "final state" leads to its true modes (since in real life, it wouldn't be free-free). So the same should hold true in the mnf file generation. Thatsa why I asked if there was some mistake in the way I constrained the body for the mnf file creation.
     
    I might be mistake from the beginning though. Can you build an MNF file for a flexbody which will not be free-free? Or regardless of where I place constraints, the MNF file will always containt its free-free modes?
     
    Kind regards,
     
    Josef
  • 0
    Hello Josef,
    ​
    "the modes of an mnf file do not change when it is constrained"
    Yes. This is because the mnf-file (in likely most cases) is an open, free-free representation of the flexible body. Like your table being unconstrained.
    ​
    "I have no way of making it change throught Adams."
    Yes. The mnf file as the representaion of the flexbody remains the same(, regardless how it is constrained).
    ​
    "When running simple eigenmodes analysis (sol103) constraining the body for it to be in its "final state" leads to its true modes (since in real life, it wouldn't be free-free)."
    Yes. Now it is constrained AND you did the eigenmodes analysis. This combination shows real life behaviour.
    ​
    "So the same should hold true in the mnf file generation."
    No. Generally it is recommended (and e. g. in Adams/ViewFlex the default setting) that any interface node should have all 6 dofs in ASET. Then you have the big advantages that you can later apply any kind of joint or force on the interface node AND the modal base of the flexbody is good. The disadvantages are having more constraint-modes and so having minimal performance reduction.
    ​
    "Thatsa why I asked if there was some mistake in the way I constrained the body for the mnf file creation. [...] Can you build an MNF file for a flexbody which will not be free-free?"
    If you know well about your assembly and you have to reduce the number of constraint modes, then sure you can constrain the body in the mnf file generation process. Then you are "directly" closer at its "final state". E. g. in Adams/ViewFlex you can lock DOFs for any Interface Node in mnf-file generation.
    But did you really do this? I suppose no. Again: The attachment method RBE2 only defines the connection between Master Node and Slave Nodes (as rigid). But the RBE2 in no way are Joints which connect the interface node to the surrounding.
    Last but not least the mnf-file creation should not be able to "use" an Interface Node on which all 6 dofs are locked, because then no constraint mode can be built. In my view at least one dof has to be open for the Int Node in the mnf file gen process.
    ​
    -->
    If you want the Flexbody to be in its "final state", then you have to attach the Joints in Adams (if you didnt lock the respective dofs during the mnf file generation (and I suppose you didnt)). The modes of the mnf-file then do not change, because its still the same. But the modes of the Linear Modes Analysis in Adams do change and (only) this is the "true final state modes".
    ​
    Wave
     
    Bruno​
  • 0
    Thanks! This was very helpful. One final question
     
    "(if you didnt lock the respective dofs during the mnf file generation (and I suppose you didnt))"
     
    how do I do this? I mean how do I lock the dofs in Adams/Vieflex?
     
    Thanks,
     
    Josef
  • 0
    Have a body (with constraints) in Adams View
    Launch ViewFlex from the Ribbon (Bodies tab, Flexible Bodies container, ViewFlex button)
    Check Advanced Settings
    Select Attachments (Mesh will be generated)
    Select Find Attachments
    (Attachment Method for each Row can be chosen from RBE2 or RBE3)
    In the column Rel.DOF you now can define the dofs. But as I see now, you have to define the dofs, which later will definitely not be constrained and therefore will not have to be taken into consideration when building the constraint-modes (the default 0 indicates that there will be 6 constraint-modes)
     
    The F1-help says for Rel.DOF:
    Degrees of freedom (DOF) to be released in the modal analysis. Any released DOF do not allow constraints with other bodies. The required input is a series of integers, from 1 to 6, indicating that the corresponding DOF must be released:
    ■0 = No DOF released
    ■1 = X-axis translation
    ■2 = Y-axis translation
    ■3 = Z-axis translation
    ■4 = X-axis rotation
    ■5 = Y-axis rotation
    ■6 = Z-axis rotation
    For example, if you want to release x translation, z translation, and y rotation, enter 135. To release all rotational DOF, enter 456.
     
    So I have to admit that the wording is a bit opposite to what I said before. You cant "lock" the dofs in Adams/ViewFlex. Instead you have to release the dofs which definitely are of no interest for you. Perhaps you have to try a little bit to get a better understanding.
     
    Bruno
  • 0
    Thank you! I'll look further into it!
     
    Kind regards,
     
    Josef
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