[cosine]

Dear all,

I am modeling the concentration of a solute in a multi-domain simulation using an additional variable (transport equation).
At both sides of the interface boundary i define no flux boundary condition and source flux and transfer coefficient. In the one domain i have negative transfer coefficient (sink) and at the other positive (source) is that correct?
Hoever, in the results i receive some negative values which are not reliable.
I use refined mesh as well as smaller time steps, but nothing.
Do you have any idea? How can i bound the additional variable?
I will attach later the CCL file.