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Continuity in a Beam Model

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Hey everyone,

I'm having a little trouble figuring out what seems like should be a very simple task in 4.0a. I've essentially drawn two lines and would like to connect them at the ends. I don't want them to be welded, so I've selected "Form Assembly" instead of "Form Union." In a mathematical sense, I'm trying to achieve boundary conditions such that the displacements are continuous at the connected ends but each line can rotate independently of the other.

I've attempted to do this by defining the connection point as a pin, but this doesn't achieve what I'd like because the connection shouldn't be constrained to zero displacement. I also tried to define an identity pair and then add a continuous boundary condition to the pair, but Comsol isn't allowing me to pick points for the identity pair, only domains. I'd like to simply pick the two end points on each line and couple them with an identity pair. The continuity condition should then constrain them to having the same displacement. Has anybody done something similar to this? Is it possible to define a point identity pair for one-dimensional geometries such as lines? Again, it seems like a very simple task, but I'm having trouble figuring out how to implement it.


4 Replies Last Post 27 juin 2011, 11:08 UTC−4
Nagi Elabbasi Facebook Reality Labs

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Posted: 1 decade ago 24 juin 2011, 14:44 UTC−4
One way to do that is to define an identity mapping at one of the two points that you want to join (you can define identity maps on points). Call it say id1, then add a prescribed displacement at the other point and set u=id1(u) and v=id1(v) and keep the rotation free.

Nagi Elabbasi
Veryst Engineering
One way to do that is to define an identity mapping at one of the two points that you want to join (you can define identity maps on points). Call it say id1, then add a prescribed displacement at the other point and set u=id1(u) and v=id1(v) and keep the rotation free. Nagi Elabbasi Veryst Engineering

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Posted: 1 decade ago 24 juin 2011, 15:43 UTC−4
I was thinking about doing it this way, but I'm not sure how to specify the displacement of the identity map in the prescribed displacement node. I followed your steps to create an identity map, then tried setting u=id1(u) and v=id1(v) at the second connection point, but get the error "unknown function id1." Am I specifying the variable name wrong? How do I access the "u" and "v" of id1? Is it id1.u and id1.v?

Thank you
I was thinking about doing it this way, but I'm not sure how to specify the displacement of the identity map in the prescribed displacement node. I followed your steps to create an identity map, then tried setting u=id1(u) and v=id1(v) at the second connection point, but get the error "unknown function id1." Am I specifying the variable name wrong? How do I access the "u" and "v" of id1? Is it id1.u and id1.v? Thank you

Nagi Elabbasi Facebook Reality Labs

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Posted: 1 decade ago 24 juin 2011, 16:06 UTC−4
The variable names are right. Make sure you name the identity mapping id1 and you select the proper point in the selection window.
The variable names are right. Make sure you name the identity mapping id1 and you select the proper point in the selection window.

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Posted: 1 decade ago 27 juin 2011, 11:08 UTC−4
Ah yes, it worked! My id mapping was named idmap1 instead of id1. Thank you.
Ah yes, it worked! My id mapping was named idmap1 instead of id1. Thank you.

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