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TDS: Simple reaction - what's wrong here? (model attached)
Posted 12 ott 2017, 09:42 GMT-4 Chemical, Chemical Reaction Engineering Version 5.2 2 Replies
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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
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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
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Hey Jeff,
oh I see. Thanks a lot for your answer!
Best, Tobias