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sphere in elastic medium

Posted Apr 26, 2013, 10:39 a.m. EDT Structural Mechanics Version 4.3a 5 Replies

Nick James
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Hi All.

I'm trying to impose some pressure on a spherical boundary (it is a very simple model) but I don't understand why Comsol is telling there is something wrong with it! I have added the elastic parameters and I was trying to add some pressure to the boundaries 6 and7.

Thank you for your help

nick
5 Replies Last Post Apr 26, 2013, 3:37 p.m. EDT
Jeff Hiller COMSOL Employee

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Posted: 1 decade ago Apr 26, 2013, 11:02 a.m. EDT
Your model is ill-posed because nothing is preventing the geometry from sliding in the z direction. You can prevent that by imposing a prescribed displacement of 0 in the z direction at point 5 for instance.
Note also that the file you attached is not applying a pressure but rather a vertical force to boundaries 6 and 7.

Jeff
Your model is ill-posed because nothing is preventing the geometry from sliding in the z direction. You can prevent that by imposing a prescribed displacement of 0 in the z direction at point 5 for instance. Note also that the file you attached is not applying a pressure but rather a vertical force to boundaries 6 and 7. Jeff
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago Apr 26, 2013, 11:04 a.m. EDT
Hi

you have not put any pressure but a vertical load, that means your part is accelerated vertically with no limits. You should ensure your model is restrained in all 6 doF (at least 3 for 2D axi), you have restricted 2 of3 one remains fully free.

if you rely only on forces, your system might run off by numerical integration on small errors, you need some "anti-windup" concept here to, as used in control theory, for that a simple "soft enough" spring (along Z motion here) well might do the job

--
Good luck
Ivar
Hi you have not put any pressure but a vertical load, that means your part is accelerated vertically with no limits. You should ensure your model is restrained in all 6 doF (at least 3 for 2D axi), you have restricted 2 of3 one remains fully free. if you rely only on forces, your system might run off by numerical integration on small errors, you need some "anti-windup" concept here to, as used in control theory, for that a simple "soft enough" spring (along Z motion here) well might do the job -- Good luck Ivar
Nick James
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Posted: 1 decade ago Apr 26, 2013, 11:09 a.m. EDT
Hi Jeff

Thank you. I agree about the pressure, sorry my mistake.
Concerning the boundary 5, I thouhgt it could be moving in the z direction like the 2 and 4 boundaries that are free. I'm not sure if i understand that!
Hi Jeff Thank you. I agree about the pressure, sorry my mistake. Concerning the boundary 5, I thouhgt it could be moving in the z direction like the 2 and 4 boundaries that are free. I'm not sure if i understand that!
Jeff Hiller COMSOL Employee

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Posted: 1 decade ago Apr 26, 2013, 12:04 p.m. EDT
Hi Nick,
I am talking about point 5, not boundary 5.
I suggested prescribing that point's vertical displacement to zero (rather than any other point's) because it will get you a nice symmetric solution.
Jeff
Hi Nick, I am talking about point 5, not boundary 5. I suggested prescribing that point's vertical displacement to zero (rather than any other point's) because it will get you a nice symmetric solution. Jeff
Nick James
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Posted: 1 decade ago Apr 26, 2013, 3:37 p.m. EDT
thanks it works perfectly!
thanks it works perfectly!

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