So for example could you supply a flow rate at an inlet and have an outlet which extracted the same mass flow that was entering at that inlet? Yes this is possible. So taking an example, where you have two internally linked boundaries, where one is an inlet into the domain and one an outlet from the domain. There will obviously be other inlets and/or outlets present, but this is just linking two of them together. If you set a flow rate on the Inlet, you can then set the mass flow leaving the outlet boundary using an expression. An example would be: Inlet = 5 m/s Outlet = Sum(MassFlow, GetBoundary(“@inlet”), “Simple”) Here you are setting the outlet mass flow to be equal to the Inlet mass flow for that particular boundary pairing There are other useful expressions in the AIM Manual