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What is the physical reason for this constraint? Will the species be removed from the solution if the concentration becomes too high? Will it diffuse away from highly concentrated regions much quicker than the normal diffusion? How does the concentration increase? Maybe the source of increased concentration simply delivers no more of the species if the concentration becomes too high. All of these physical effects can be implemented in COMSOL I guess.

Hello,
There exists the Pointwise Constraint, which you could define in the domain where you want to limit the dependent variable. But it should be compatible with the PDE equation, and perhaps does not work as a threshold.
Also you can do a non-perfect approach, similar to changing the diffusion coefficient to:
if(step1(thres-u),c,c*10),
where 'thres' is the threshold you want to apply to 'u' (dependent vble.), 'step1' is a suitable smooth step function (to be tuned) and c is the normal value of your diffusion coefficient.
With this silly trick you are telling COMSOL: if u is below the threshold, all is normal, but if it's over it, c must raise, what implies that laplacian must lower. Of course, you should tune this expression for you particular PDE, also the number 10 and the function step1.
Perhaps some of those ideas can work for you.
Jesus.

Good day:

I am trying to find the most effective method of implementing a constraint on a dependent variable.

I am solving a classical diffusion equation (using Equation based modeling)of concentration 'c' and I do not want my variable 'c' to exceed some value 'cmax' on the entire domain I am modeling on.

What is the most effective way that COMSOL allows this to be done?

Thank you for any help given in advance.

Hi Philip,

I am in a same dilemma as you. Have you solved this problem? I'll appreciate it if you instruct me to deal with it. I am looking forward to your reply.

Yuhong

Hi

I am also facing a similar problem. I have defined continuity equation with the help of PDE but there has to be an upper limit on the density term. I used pointwise constraint but that is making the density constant having the maximum value.

Thanks