Hello,I want to create in my model a negative mass flow rate (production term),how do I define the region of cells where this mass outflow is applied ? For the moment I have only one continuum region which is water. Thanks if you can hepl me.

In your mesh, select the boundary element faces which will form the location of mass injection/extraction. Then, define the appropriate boundary condition. (ie: often, one can achieve a mass extraction by defining the BC as a mass flow inlet, but pointing the vector directions out of the boundary). If you have a solution already (which did not have this new BC location tagged in the mesh, you can "write-bc" the boundary conditions, and write interpolation file... then read in new mesh and "read-bc" and interpolate exisitng solution... with new BC location now newly defined.