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Negative Concentration in Domain

Posted Mar 5, 2015, 4:10 p.m. EST General, Electrochemistry Version 4.4 4 Replies

Kumar Krishnamoorthy
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I've been trying to simulate the transfer of ions in a nano-channel using a Nernst-Placnk-Poisson-Boltzmann system, and I'm getting Negative Concentrations in the domain. Can someone help me with identifying possible mistakes in my model ?

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4 Replies Last Post Mar 9, 2015, 2:20 p.m. EDT
Lasse Murtomäki
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Posted: 9 years ago Mar 6, 2015, 3:18 a.m. EST
Hi

The problem of negative concentrations has appeared on this forum repeatedly, and there is no easy way out. I have faced the same problem with convective diffusion. I almost got rid of it with better meshing in the channels. I used swept mesh across the channel with at least 10 elements and geometric distribution so that at the channel boundary the mesh was at densest. Convective diffusion is a notorious problem.

br
Lasse
Hi The problem of negative concentrations has appeared on this forum repeatedly, and there is no easy way out. I have faced the same problem with convective diffusion. I almost got rid of it with better meshing in the channels. I used swept mesh across the channel with at least 10 elements and geometric distribution so that at the channel boundary the mesh was at densest. Convective diffusion is a notorious problem. br Lasse
Sri Puranam
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Posted: 9 years ago Mar 6, 2015, 12:08 p.m. EST
Hi Kumar,

What are your solver settings? Do you use a fully coupled solver ? You can use a segregated solver and use the "lower limit" option and force the lower limit to be zero.

If you problem is very stiff, you will have some convergence issues when you do this. You will have to mesh the "negative concentration region" very densely. Also I would suggest that you ramp up any source terms in your problem.

Let me know if it helps.

Thanks!
Sri.
Hi Kumar, What are your solver settings? Do you use a fully coupled solver ? You can use a segregated solver and use the "lower limit" option and force the lower limit to be zero. If you problem is very stiff, you will have some convergence issues when you do this. You will have to mesh the "negative concentration region" very densely. Also I would suggest that you ramp up any source terms in your problem. Let me know if it helps. Thanks! Sri.
Walter Frei COMSOL Employee

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Posted: 9 years ago Mar 6, 2015, 2:03 p.m. EST
You will also find this knowledgebase article addresses this question.:
www.comsol.com/support/knowledgebase/952/

Best,
You will also find this knowledgebase article addresses this question.: http://www.comsol.com/support/knowledgebase/952/ Best,
Kumar Krishnamoorthy
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Posted: 9 years ago Mar 9, 2015, 2:20 p.m. EDT
Thank you all for your comments ! I found that having a finer mesh size helped with the issue.
Thank you all for your comments ! I found that having a finer mesh size helped with the issue.

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