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Diffusion Problem employing a Triphasic material model
From a numerical perspective, the electric potential for triphasic materials is obtained from the solution of a quadratic equation whose coefficients depend on the solute concentrations and the fixed charge density. When some (or all) of these coefficients are zero (as occurs at the first time point in your model, since you set the initial concentrations and the fixed charge density in the triphasic domain to zero), the calculation of the electric potential evaluates the log of zero, thus producing NAN. Even if the values of the concentration are not exactly zero, starting with very small concentrations can produce negative concentration values (due to numerical roundoff errors) which also result in NAN when evaluating the logarithm. So it is best to use non-zero concentrations when performing triphasic analyses. This problem does not arise when solutes are neutral, since the electroneutrality condition is not used to evaluate the electric potential in that case.
You can overcome this problem in your analysis by changing the initial and boundary conditions as follows: Instead of using 0 initial concentration inside and prescribing 1 on the boundary, try using 1 for the initial concentration inside and 2 on the boundary. This should resolve the issue.
sorry, I've found the reason of the issue. The material properties were inconsistent! I was using the drag coeff value instead of the permeability. Furthermore, I was using the diffusivity values in m2/s instead of mm2/s.
It is working now!!!
Thanks a lot for the help and sorry for the basic mistake.
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