boundary conditions for the mass-sink in a square domain
 

Dear friends:
We consider a 2D square computation domain with a mass-sink(Q kg/s) located at the center position.
I want to solve the flow fields and pressure field in the domain with the SIMPLE algorithm. But I am confused by how to set the boundary conditions on the 4 edges of the computational domain.
Could you please give me some suggestions.
Regards.

Of course, if the problem is divergence-free you cannot set not-permeable walls but you need to allow mass to flow and balance the mass-sink

Thank you very much. But how to decide velocity at the boundaries.

this is an issue that depends on the flow problem you wnat to simulate ...you could concentrate the flow mass only in a part of the boundary, on more parts or all along the boundaries.
What should correspond physically the 4 edges of the domain?