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Limits on a Dependent Variable

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Hi,

I have a relatively simple question that I couldn't find an answer for in the forums. I am using the General Form PDE mode to solve for a dependent variable, u. From the physics of the problem, I know that the value of u cannot exceed a certain value, say u_max. However, due to numerics, I obtain values larger than this maximum value which is unrealistic. Furthermore, those values cause roots of negative numbers etc that lead to further numerical instabilities in the solver. While I can use expressions such as (u_max+(u-u_max)*(u<u_max)) instead of u in the problematic expressions, it is not as straightforward as it sounds in the complex implementation. I am wondering if there is a feature in the COMSOL's solver where you can specify limits to a dependent variable. This has been a recurring issue that I faced several times in the past, and I would like to learn the more elegant solution if there is any.

Regards,
-Alper

1 Reply Last Post 11 mag 2011, 12:10 GMT-4
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)

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Posted: 1 decade ago 11 mag 2011, 12:10 GMT-4
Hi

you have an interesting remark there.

I know about the limits you can impose on variables in the optimisation module, but not for the general PDE solver.

I agree that this could be useful, from the external physicist / engineer point of view (we might know that for a given case i.e. complex or negative solutions are non-physical, COMSOL not , today),

Now I'm not sure it's that trivial from the math / computational point of view

--
Good luck
Ivar
Hi you have an interesting remark there. I know about the limits you can impose on variables in the optimisation module, but not for the general PDE solver. I agree that this could be useful, from the external physicist / engineer point of view (we might know that for a given case i.e. complex or negative solutions are non-physical, COMSOL not , today), Now I'm not sure it's that trivial from the math / computational point of view -- Good luck Ivar

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