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Nodes doesn't match degrees of freedom
Posted 19 ott 2021, 09:52 GMT-4 Structural & Acoustics, Geometry, Mesh Version 5.6 2 Replies
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Dear all,
I am doing and eigenfrequency study for a circular plate. I want to do a modal anaylisis in Matlab with my stiffness and mass matrices. I can export my matrices but the dimensions doesn't match my nodes/ degrees of freedom. I have a triangular shell mesh with 27 nodes and 40 elements which of 12 boundary elements. In theory each nodes should have 6 dof's 3 translation and 3 rotation. The solution of the eigenfrequency analysis says that 558 number of degrees of freedom are solved. This suggest that there are 93 nodes (558/6) but in my mesh there are only 27. How is this possible? If I export my x0 vector and reduced K and M matrices the dimensions give 465 dofs. This suggest that the Rz dof in shell elements has no contribution to the analysis and can be ignored. This still leads to 93 nodes (465/5).
I don't understand why this happens. With this I cannot continue with my modal analysis create an actuator matrix to control the first 20 modes.
Can anybody help me with this problem?
Best regards,
Sander
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You are probably only counting the corner nodes. If a quadratic discretization is used, there are also 3 midside nodes in each element.
You are correct about one rotational DOF being eliminated. Effectively there are only 5 DOF per node, as long as there are no foldlines, T-joints etc.
-------------------Henrik Sönnerlind
COMSOL
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Hi Henrik,
Thanks this helps a lot!
Regards,
Sander