internal flow and external flow ?

Pathway0320

When I model the internal flow of the pipe, I set mass flow inlet and pressure outlet, and I can observe the internal flow in the pipe. If I want to know the velocity after 2mm of the pressure outlet, what should I do?

First, set second model. The pressure outlet of first model is set as mass flow inlet of second model and other boundaries as pressure outlet. (For mass conservation, so the two mass flow inlets are the same, right?) And how far should I define the boundary field? (The area of mass flow inlet is 0.08*2660)

Second, extend the pressure outlet of the first model. The model has another walls, and what types of this wall will be define? (symmetry, pressure outlet, or...)

Thanks a lot

sincerely