How to Set Field Continuity Boundary Conditions in Comsol

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How can i set boundary condition on a part of a boundary

Posted Apr 18, 2016, 12:13 p.m. UTC

志远 陈

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I import a geometry into COMSOL 5.2 through livelink for SolidWorks. And it's been divided into several domains and boundaries automatically.
But I have encountered a problem. I want to set boundary conditions on just a small part of a boundary. For example, there's a geometry -- a cube. And i want to set temperature condition on a triangle area on a surface. What can i do to make this happen?

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Jeff Hiller COMSOL Employee

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Posted: 7 years ago Apr 19, 2016, 3:29 p.m. UTC

You will need to draw that triangle on the surface, either in the CAD package (SolidWorks, for you) before you import it into COMSOL, or in COMSOL. See www.comsol.com/community/forums/general/thread/15263/#p246712 regarding the latter.
Best,
Jeff

You will need to draw that triangle on the surface, either in the CAD package (SolidWorks, for you) before you import it into COMSOL, or in COMSOL. See https://www.comsol.com/community/forums/general/thread/15263/#p246712 regarding the latter. Best, Jeff

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Henrik Sönnerlind COMSOL Employee

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Posted: 7 years ago Apr 19, 2016, 9:16 p.m. UTC

Hi,

As long as the boundary condition is of Neumann type, another possibility is to write the load as a boolean expression based on the coordinates. Something like

17.4*(x>0.2)*(x<0.35)*(y<0.12)

This is of course only feasible for simple geometrical shapes of the loaded area. Also, the mesh must be fine enough, so that the discretization errors are small.

Actually the same idea can be used also for Dirichlet type boundary conditions, but with a little trick. If the degree of freedom is 'u', then the following expression for the value of a Dirichlet condition will prescribe u to 12 inside an area, and let u be free elsewhere:

if((x>0.2)*(x<0.35)*(y<0.12),12, u)

If anyone is curious about how this expression works, a Dirichlet condition is implemented as u - prescribed_value. But if the prescribed value happens to be 'u' itself, u - u will cancel during the symbolic evaluation and the constraint magically disappears.

Regards,
Henrik

Hi, As long as the boundary condition is of Neumann type, another possibility is to write the load as a boolean expression based on the coordinates. Something like 17.4*(x>0.2)*(x

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Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)

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Posted: 7 years ago Apr 20, 2016, 7:00 a.m. UTC

Hi Henrik

Thanks for the nice trick, then it should also apply to other cases, such as T for temperature (in K) etc
Worth a Blog presentation with several examples on different physics, for me :)

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Good luck
Ivar

Hi Henrik Thanks for the nice trick, then it should also apply to other cases, such as T for temperature (in K) etc Worth a Blog presentation with several examples on different physics, for me :) -- Good luck Ivar

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志远 陈

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Posted: 7 years ago Apr 22, 2016, 7:24 a.m. UTC

Thank you very much. I'll try that immediately

Thank you very much. I'll try that immediately

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Henrik Sönnerlind COMSOL Employee

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Posted: 6 years ago Jun 27, 2016, 1:23 p.m. UTC

Hi Ivar, The blog post that you suggested is now published! https://www.comsol.com/blogs/how-to-make-boundary-conditions-conditional-in-your-simulation/ Regards, Henrik

zirkleitaind.blogspot.com

Source: https://www.comsol.com/forum/thread/110073/how-can-i-set-boundary-condition-on-a-part-of-a-boundary

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