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Your problem is otherwise an elementary one but making diffusion coefficients concentration dependent may pose a problem. I have not been able to do that. Each domain can have its own diffusion coefficients, just add a Transport properties node below Transport of Diluted Species.

BR Lasse

Your problem is otherwise an elementary one but making diffusion coefficients concentration dependent may pose a problem. I have not been able to do that. Each domain can have its own diffusion coefficients, just add a Transport properties node below Transport of Diluted Species.

BR Lasse

Hello Sir, thanks for you reply. Well, when we have a continuos domain, make the difussion coefficient concentration dependent is not a big problem, but in this case the domain is not continuos, since the difussion coefficient is not the same along the domains, such that when comsol assembly the global matrix for instance stiffness matrix, some terms corresponds to domain1 and other correspond to domain2. I would like to know how to do that.

• I'd like to see how you make the concentration dependence of the diffusion coefficient.
• What do you mean a domain not being continuous?

Sorry for my delayed reply. With regard to your first question, in a previous simulation (a simple 1D problem and continuos domain, i mean, the difussion coefficient is the same nature) the module of "global definition", i define an "Analytic parameter", function name: D0, expression f(T,c) where T and c is the temperature a field variable respectively. Finally, in the module of Transport of diluted species, transport properties,Diffusion , we can define Dc as D0(c,T0[1/K]).

So how can I do this simultion but for 2 domains?, that is with 2 differente Dc

I tried this way: I defined an analytic function

1 - sqrt(K*c)

where K is a constant; this follows the Kohlrausch's law. But when inserting for the diffusion coefficient this expression (multiplied with D0) gives an error message due to the square root; the usual trick of writing c+eps did not work.

Attached a model where I used D = D0(1 - Kc) for the left domain and D = 0.1·D0(1 - Kc) for the right domain. D0 = 10^-5 cm²/s.

BR Lasse

Thank you so much for your example, from what i see, it is possible to raise this problem. Well, i am going to open another discussion, concerning this topic but with more complex situation. Thanks! Your comments will be welcome as well.

Hello Julio, I tried "Analytic parameter" as you metioned above but I couldn't plot the figure. Actually what I want to do is just to change constant diffusion coefficient D to diffusion dependent concentration, namely D/(1-c/c0), c is my variable in the module of Transport of diluted species. However when I set up this equation I got the error message as following: Failed to evaluate temporary symbolic derivative variable. - Variable: comp1.tds.D_cpxx@VDN$comp1.cp - Defined as: -((-Dp)/((1-comp1.cp)^2)) I am confused that where is the square from? It will be very kind to get any reply.

BR Liang

Hello Julio, I tried "Analytic parameter" as you metioned above but I couldn't plot the figure. Actually what I want to do is just to change constant diffusion coefficient D to diffusion dependent concentration, namely D/(1-c/c0), c is my variable in the module of Transport of diluted species. However when I set up this equation I got the error message as following: Failed to evaluate temporary symbolic derivative variable. - Variable: comp1.tds.D_cpxx@VDN$comp1.cp - Defined as: -((-Dp)/((1-comp1.cp)^2)) I am confused that where is the square from? It will be very kind to get any reply.

BR Liang

Hello Liang, it is really strange that you can not plot your analytic function, check carefully the module "Analytic parameter", and define well your arguments section, in this case i dont understand very well how you define the diffussion coefficient, i mean you wrote D=D/(1-c/co) or D=D0/(1-c/co), in the latter case your parameters must be "c,co and Do". Finally, in the module Transport of diluted species, subsection Transport Properties 1 you must type the name function in the section Dc. I hope you can comprehend my explanation. Best regards!

Hello Julio,

Thank you for your reply, It works now! However, what if c is a variable? I think I need to use other method, do you have any suggestion for this?

BRs Liang

Hello Julio,

Thank you for your reply, It works now! However, what if c is a variable? I think I need to use other method, do you have any suggestion for this?

BRs Liang

What do you mean?? c is the field variable, thus is variable.