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Defining a variable limited only for the boundary

Posted 28.04.2016, 06:46 GMT-4 Fluid & Heat, Computational Fluid Dynamics (CFD), Battery Design, Chemical Reaction Engineering, Modeling Tools & Definitions, Parameters, Variables, & Functions, Studies & Solvers 2 Replies

Swati Gupta
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Can we define a dependent variable for a physics which is only valid for particular boundary and not for the whole domain?
2 Replies Last Post 28.04.2016, 07:27 GMT-4
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 8 years ago 28.04.2016, 07:20 GMT-4
Hi

I'm not sure I catch you fully

But yes you can define your own equations and hence dependent variables for only one or a few Entities (Domains, Boundaries ...).

You have the math mode, or different options within the physics nodes ...

But what you also can do, is to define a variable (Definition Variables) on different Entities with different expressions (one per Entity though) so that the variable represent a valid results in the different Domains/Boundaries etc

--
Good luck
Ivar
Hi I'm not sure I catch you fully But yes you can define your own equations and hence dependent variables for only one or a few Entities (Domains, Boundaries ...). You have the math mode, or different options within the physics nodes ... But what you also can do, is to define a variable (Definition Variables) on different Entities with different expressions (one per Entity though) so that the variable represent a valid results in the different Domains/Boundaries etc -- Good luck Ivar
Swati Gupta
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Posted: 8 years ago 28.04.2016, 07:27 GMT-4
Thanks for the response!

Actually I am working on modelling of vanadium crossover in vfrb. I am using Nernst- Planck eqns and secondary current distribution to model membrane and tertiary current distribution to model electrolyte on the either side of the membrane. I am facing problem in modelling the boundaries of membrane-electrolyte interface. I am considering the interfacial area of finite thickness. And the thickness is assumed to be equivalent to the diffusion boundary layer that exist at the surface of a flow-by electrode . For simplicity membrane side thickness is assumed the same.
For this, in the nernst-planck eqn, concentration and migration terms are discretized as given in the attached image file. (Screenshot001.jpg)
Fluxes for both sides of electrolyte are- (Screenshot002.jpg)
I don't know how to incorporate the boundary fluxes equations. As of now I have manually typed those equations under in Variable under local definations. But the problem is how to calculate junction concentration (c_j) . How to equate those N_er and N_mr to find out (c_j)? I am using Nernst-Planck eqn & Secondary Current Distribution to model the membrane domain and tertiary current distribution & darcy law for both electrolyte domain.
I was thinking of defining variable c_j for the membrane-electrolyte boundary that's y I asked.But I don't think it's the proper way.
I am not getting at all how to model this fluxes.
Waiting for the response.
Thanks for the response! Actually I am working on modelling of vanadium crossover in vfrb. I am using Nernst- Planck eqns and secondary current distribution to model membrane and tertiary current distribution to model electrolyte on the either side of the membrane. I am facing problem in modelling the boundaries of membrane-electrolyte interface. I am considering the interfacial area of finite thickness. And the thickness is assumed to be equivalent to the diffusion boundary layer that exist at the surface of a flow-by electrode . For simplicity membrane side thickness is assumed the same. For this, in the nernst-planck eqn, concentration and migration terms are discretized as given in the attached image file. (Screenshot001.jpg) Fluxes for both sides of electrolyte are- (Screenshot002.jpg) I don't know how to incorporate the boundary fluxes equations. As of now I have manually typed those equations under in Variable under local definations. But the problem is how to calculate junction concentration (c_j) . How to equate those N_er and N_mr to find out (c_j)? I am using Nernst-Planck eqn & Secondary Current Distribution to model the membrane domain and tertiary current distribution & darcy law for both electrolyte domain. I was thinking of defining variable c_j for the membrane-electrolyte boundary that's y I asked.But I don't think it's the proper way. I am not getting at all how to model this fluxes. Waiting for the response.

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