Problem with a boundary condition in Comsol

[ivanguru]

hi, I'm a chemical engineering student with a little problem with Comsol multiphysics;
in practice, i have to solve a problem of diffusion in a solid sphere.
after drawing the domain, i have to set a boundary condition on sphere's surfaces.
this condition, for my problem, is FLUX=Kc(Cb-C) and Cb (bulk concentration) is

proportional to an integration coupling variable, that i have previously fixed
In particular, Cb is equal to costant * mass_released, where mass_released is equal

to volume integral of C0 (initial concentration) minus C

After setting all variables and running simulation, it gives me an error referred to

evaluation of variable Cb
I think the problem is that it's difficult to use in a boundary condition a variable that

changes during simulation (in fact it is time dependent)..


I hope someone can help me, have a nice day!

 

[Aaron Bex]

What you could try is to combine the two equations to form a single equation and use that as the boundary condition. This would mean combining the equation for Cb with the equation for FLUX, then using the resulting equation for the boundary condition. This should solve your problem.

 

[oswaler]

Dear student,

Thank you for reaching out with your problem in Comsol multiphysics. It seems like you have identified the issue correctly - using a time-dependent variable in a boundary condition can be challenging. My recommendation would be to try using a time-dependent function instead of a variable in your boundary condition. This way, the function will automatically update with time and you won't run into any evaluation errors.

You can also try breaking down your problem into smaller, simpler steps to troubleshoot and identify the issue. Check if the function or variable is being properly evaluated at each time step, and if there are any other factors that may be affecting the boundary condition.

Additionally, I would suggest consulting the Comsol support team for further assistance. They may be able to provide specific guidance or troubleshooting tips for your particular problem.

I wish you the best of luck in solving this issue and hope you have a successful simulation. Have a great day!