COMSOL Forums: restrict codomain of dependent variables, i,e. make them positive always?

Re: restrict codomain of dependent variables, i,e. make them positive always?

Thu, 04 Oct 2012 18:10:25 +0000

Dear Ivar,
thank you very much for the answer!
I was indeed not aware of that problem being a rather common problem, and searching specifically for negative *concentrations* did not come into my mind, so I am happy the appearance of them does apparently not necessarily mean that anything is fundamentally wrong with my problem :)
Do you know if the hint Tero gave in the thread you linked (defining a new variable that is set to a little bit over zero every step it is found to be negative if I understand it right), works in the newest version of COMSOL only, or in the old 3.5a one as well? (I do not know what kind of variable I should it define as, and how to tell COMSOL to solve for this instead of for my original dependent variable.. If you say it could work, I will try to figure out how to do it..)
All best,
Sabine

Re: restrict codomain of dependent variables, i,e. make them positive always?

Wed, 03 Oct 2012 20:46:30 +0000

Hi

You can stat by checking the KB (knowledge base) and te Forum, there are many discussions around how to cope with negative concentration, and often why they occure, and how to get around ;)

--
Good luck
Ivar

restrict codomain of dependent variables, i,e. make them positive always?

Wed, 03 Oct 2012 15:57:05 +0000

Hey everybody,

I have a model with ion concentrations as dependent variables amongst others. When solving my problem, COMSOL makes one of the concentrations negative! But it can only be posititve of course. So, how can I restrict the codomain of my dependent variable, i.e. tell COMSL that making the it negative is not allowed?

All best and thank you very much for your answers,
Sabine