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Posted: 10 years ago Mar 13, 2015, 4:13 a.m. EDT

To add information, I have tick the migration under electric field box in transport of diluted species module and the flux is difined as Ni=-D*grad(c)-zuF*c*grad(V)+c*u(velocity vector). Velocity vector is defined as zero here. Many Thanks!!!

Posted: 10 years ago Mar 13, 2015, 7:30 a.m. EDT

Hi This is merely a guess, but how dense is your mesh close to the boundaries? You may define zero flux BCs, but COMSOL calculates the flux based on your mesh, which is prone to smallest changes in the mesh. I have with the flux BC a lot and never been fully content. BR Lasse

Posted: 10 years ago Mar 13, 2015, 8:15 a.m. EDT

Dear Lasse, Thank you so much for your reply! My simulation geometry is just 2D 1 micrometer by 1 micrometer rectangular box. I use comsol predifined :Extremely fine size shown in the attachment. I just wonder whether the unsymmetric oppsite boundaries(one with +1V the other with -1V applied) is possible for zero flux since I see many models in gallery are zero flux for symmetric boundaries. Another question is does comsol first assume zero flux at BC then iterate? Thank you! Best, Leo [QUOTE] Hi This is merely a guess, but how dense is your mesh close to the boundaries? You may define zero flux BCs, but COMSOL calculates the flux based on your mesh, which is prone to smallest changes in the mesh. I have with the flux BC a lot and never been fully content. BR Lasse [/QUOTE]

Attachments:

• elemenentsize.png

Posted: 10 years ago Mar 14, 2015, 9:09 a.m. EDT

In principle zero flux condition is possible, the concentration profile is then set so that the concentration gradient opposes potential gradient, making flux zero: grad(c) + zF/(RT)*c*grad(U) = 0 --> grad(U) = - RT/(zF)*grad(ln(c)) I am not able to reply to your latter question, I do not know. Your mesh probably is very dense, but if your domain is that small, the minimum element size should perhaps further decreased in order to have better resolution at the boundaries. br Lasse

Posted: 10 years ago Mar 14, 2015, 10:12 a.m. EDT

Dear Lasse, Thank you so much! I have adoptted your suggestion of decreasing the minimal and maximal mesh sizes. I will report when I get the new resolution. Many thank! Best, Leo

Posted: 10 years ago Mar 17, 2015, 8:26 a.m. EDT

Dear Lasse, I have refined the mesh on the boundary, the flux do become smaller, but not within the error of comsol, I guess for my unsymmetric bc case, the flux might not be zero simultaneously... Thank you so much! Best, Leo

Posted: 10 years ago Mar 17, 2015, 8:51 a.m. EDT

OK. I still claim that your boundary conditions are quite OK :) Lasse

Posted: 10 years ago Mar 17, 2015, 10:41 a.m. EDT

Dear Lasse, My next trial will be putting dielectric layer outside the two opposing boudaries to make grad(V) less steep, hope this will help get normal zero flux bc. I will update if it can work~~ Many thanks! Best, Leo

Posted: 10 years ago Mar 24, 2015, 11:33 a.m. EDT

Dear Lasse, I have tried several ways but all failed. I tried to use pointwise constrain -user defined-constraint expression/constrain force expression to make boundary flux to be zero but also failed. I am so curious why pointwise constrain fail to give what it constains to? Thanks a lot! Best Regards, Leo