Hi, I am using Comsol 5.3a. At the moment using Darcy as well as transport of dilute species modulus by coupling these two equations . I am doing parametric study from small range of diffusion coefficient from 10^-9 to 10^-15, such that investigating concentration of solute across my geometry. However, each time I decrease Diffusion to 10^-14 to 10^-15 range, I get negative concentration next to narrower part of channel. I even used small meshes with distribution of fixed numebr of 100, still getting negative concentration in some parts of domain. Is there anyone who khows how I should solve this problem?
Many thanks, Azin