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Boundary constraints: How to tie nodes in one DOF only?
Posted 23.06.2020, 06:02 GMT-4 Parameters, Variables, & Functions, Structural Mechanics, Equation-Based Modeling Version 5.5 5 Replies
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(Comsol 5.5, Structural mechanics, 3D model)
I have a part which contains an initially flat boundary surface (top surface in sketch). Upon application of boundary conditions that do not act directly on that surface, the surface moves and deforms.
How can I constrain the movement in a way such that all nodes on that top surface move the same amount in Z, but freely in X and Y? Almost like a Roller, that can move "globally" up and down. In other words: The nodes on the surface are allowed to move absolutely in Z, but not relatively to each other.
I tried both the roller and periodic boundary conditions, but then, the surface does not move at all in Z.
I assume I will have to use Weak Constraints, but what Constraint Expression can I use? Thank you!
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The Prescibed Displacement node allows you to...prescribe a displacement in one axis but not the others.
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Thanks, Dave. I do not want to prescribe the displacement of the upper surface, though, but have it determined by the other boundary conditions.
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I think you are looking for one of the special variants of Normal Direction Condition in the Symmetry node, probably Free displacement.
See the attached screenshot.
-------------------Henrik Sönnerlind
COMSOL
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As an old-timer, I was going to propose using a nonlocal integration coupling to pick up the value of the vertical displacement at one of the top corners and then imposing it as the vertical displacement of the entire top via a prescribed displacement. The attached file does that.
Jeff
-------------------Jeff Hiller
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Thanks so much, Henrik and Jeff! It looks like Henrik's solution does exactly what I want. I have no idea how I missed that option, after having spent all these hours playing around with the boundaries. (For simplicity, I haven't checked your solution, Jeff.)