Discussion Closed This discussion was created more than 6 months ago and has been closed. To start a new discussion with a link back to this one, click here.

TDS: Simple reaction - what's wrong here? (model attached)

Please login with a confirmed email address before reporting spam

Dear Colleagues,

using transport of diluted species, there are two species with concentration C1 and C2 confined in a square. Initial value for C1=1 and C2=0. Reaction for C1: k(C2-C1) Reaction for C2: -k(C2-C1), with k=.01 [1/s].

All boundaries are set to No-Flux.

In a stationary study shouldn't this result in both concentrations at .5? The solution calculated by COMSOL is c1=c2 but far away from .5. The following warning occurs:

"There was an error message from the linear solver. The relative error (0.43) is greater than the relative tolerance."

I saw other posts that talk about this warning but it did not seem to apply.

Any hints are highly appreciated!

Best, Tobias



2 Replies Last Post Oct 12, 2017, 11:14 a.m. EDT
Jeff Hiller COMSOL Employee

Please login with a confirmed email address before reporting spam

Posted: 6 years ago Oct 12, 2017, 10:40 a.m. EDT
Updated: 6 years ago Oct 12, 2017, 11:12 a.m. EDT

Hello Tobias,

This is an example of a problem with more than one solution. In other words your problem is underconstrained.

Just check out your equations and boundary conditions; you'll see that as long as c1 and c2 are spatially constant and equal, they will satisfy your equations and boundary conditions - not just if they are equal to 0.5. COMSOL cannot pick among the infinite number of solutions that exist the one that you expected without some guidance.

You can make the solution unique by, for example, imposing the point constraint c1-.5 in one corner. Then the solution is unique and COMSOL finds it with no problem.

Jeff

-------------------
Jeff Hiller
Hello Tobias, This is an example of a problem with more than one solution. In other words your problem is underconstrained. Just check out your equations and boundary conditions; you'll see that as long as c1 and c2 are spatially constant and equal, they will satisfy your equations and boundary conditions - not just if they are equal to 0.5. COMSOL cannot pick among the infinite number of solutions that exist the one that you expected without some guidance. You can make the solution unique by, for example, imposing the point constraint c1-.5 in one corner. Then the solution is unique and COMSOL finds it with no problem. Jeff

Please login with a confirmed email address before reporting spam

Posted: 6 years ago Oct 12, 2017, 11:14 a.m. EDT

Hey Jeff,

oh I see. Thanks a lot for your answer!

Best, Tobias

Hey Jeff, oh I see. Thanks a lot for your answer! Best, Tobias

Note that while COMSOL employees may participate in the discussion forum, COMSOL® software users who are on-subscription should submit their questions via the Support Center for a more comprehensive response from the Technical Support team.