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Imposing a constraint

Posted Oct 11, 2011, 9:22 a.m. EDT Version 4.0a, Version 4.2 5 Replies

Lakshmi Selvakumaran
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Hi,

I am trying to impose a constraint that the average of the x-stress tensor be zero over the whole domain, but I couldnt. Can anyone suggest a way to impose this constraint on the system?

Thanks.

5 Replies Last Post Oct 18, 2011, 7:30 a.m. EDT
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago Oct 11, 2011, 10:33 a.m. EDT
Hi

try to add a weak constraint on the variable and impose it to zero, rather by creating an new variable and make it piloted by the result to change your driving BC

PS it works smoother in the later versions ;)

--
Good luck
Ivar
Hi try to add a weak constraint on the variable and impose it to zero, rather by creating an new variable and make it piloted by the result to change your driving BC PS it works smoother in the later versions ;) -- Good luck Ivar
Lakshmi Selvakumaran
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Posted: 1 decade ago Oct 11, 2011, 1:02 p.m. EDT
Hi Ivar,

Thank you very much for your reply. i did try to impose it as a week constraint but instead of the constraint being imposed, the Lagrange multiplier became zero. Is there a way to strictly enforce this condition?

Thanks.

Best
Lakshmi.
Hi Ivar, Thank you very much for your reply. i did try to impose it as a week constraint but instead of the constraint being imposed, the Lagrange multiplier became zero. Is there a way to strictly enforce this condition? Thanks. Best Lakshmi.
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago Oct 11, 2011, 4:22 p.m. EDT
Hi

yes by defult it optimises to "0" so ou must move all items to the left side of the equation, if you want to impose "u = 3" you should write "u - 3" in the field entry, when COMSOl imposes "=0" you will end up with "u=3"

Check the doc, and turn on the "equation view" in the preferences, and see how COMSOL handles weak constraints

--
Good luck
Ivar
Hi yes by defult it optimises to "0" so ou must move all items to the left side of the equation, if you want to impose "u = 3" you should write "u - 3" in the field entry, when COMSOl imposes "=0" you will end up with "u=3" Check the doc, and turn on the "equation view" in the preferences, and see how COMSOL handles weak constraints -- Good luck Ivar
Lakshmi Selvakumaran
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Posted: 1 decade ago Oct 17, 2011, 10:23 a.m. EDT
Hi Ivar,

Thanks for your reply. I upgraded to COMSOL 4.2 now. I understand that COMSOL optimises the constraint to zero. But when I try to impose this constraint, a non symmetric matrix is formed and then it returns with a singular matrix error. I have attached the file. Please have a look and let me know what you think.

Thanks for your time.

Best
Lakshmi.
Hi Ivar, Thanks for your reply. I upgraded to COMSOL 4.2 now. I understand that COMSOL optimises the constraint to zero. But when I try to impose this constraint, a non symmetric matrix is formed and then it returns with a singular matrix error. I have attached the file. Please have a look and let me know what you think. Thanks for your time. Best Lakshmi.

Attachments:

Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago Oct 18, 2011, 7:30 a.m. EDT
Hi

try to avoid point and edge constraints in 3D, you are making singular points, and the solver will anyhow have difficulties to finds its way, and your results will at least be locally very wrong

Cut your surface with a cylinder and apply loads on a boundary, and use pressure, then you are sure you are perpendicular to the surface. If you have large deformations, consider to turn the option on, it takes longer to solve and needs often more RAM, be aware

--
Good luck
Ivar
Hi try to avoid point and edge constraints in 3D, you are making singular points, and the solver will anyhow have difficulties to finds its way, and your results will at least be locally very wrong Cut your surface with a cylinder and apply loads on a boundary, and use pressure, then you are sure you are perpendicular to the surface. If you have large deformations, consider to turn the option on, it takes longer to solve and needs often more RAM, be aware -- Good luck Ivar

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