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Limits on a Dependent Variable
Posted May 11, 2011, 11:44 a.m. EDT 1 Reply
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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
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 May 11, 2011, 12:10 p.m. EDT
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Posted:
1 decade ago
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
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