Discussion Forum

I am working on a project where I have to model the drug concentrations through a porous media from a solid body designed in solidworks. I am relatively new to COMSOL so I was modeling a simplistic diffusion system (no flux, 0 initial concentration on the domain of interest). There are two domains from the imported solidworks file, one loaded with drug and another for diffusion throughout.

The drug loaded concentration layer has a diffusion coefficient through that porous media, and porosity of that loaded layer. This is in direct contact with the bone with 0 initial concentration, diffusion coefficients through the bone, and porosity of bone. The drug is removed according to its half life. There is a thin diffusion layer on the interface surface between the two domains.

When I run the time study and take the volumetric averages/minimums, the surface values are negative. I have tried following the advice in the support knowledge base 952 (https://www.comsol.com/support/knowledgebase/952) but I don't believe I want inflow in my system, using the machine epsilon didn't change the results, and I wasn't sure how to use a step function, and using the lower limit on a segregated solver in the settings. I do need the minimums on the drug concentration, so can't use the zeros given by this method. I am currently running an extremely fine mesh, but it is taking a very long time and I would like to know if there are better ways around this.