Rob
Ansys Employee

If you're emptying a bottle (for example) you'd need to patch the pressure in that zone and run transient. For a "large" vessel you may be able to assume a constant inlet pressure somewhere. 

Typically, you will need suffcient space for the flow to establish, and any jets/plumes to form. You may also need to be very careful with initial conditions given the relatinship between pressure, temperature and density.