Rob
Ansys Employee

You don't set the inlet pressure, it's based on the local flow field. If your whole system is at elevated pressure you may be able to use fixed density. The operating pressure mimics the atmospheric pressure and boundaries are then set at gauge relative to the operating pressure. If the system is truely compressible, then zero operating pressure and setting the boundaries can be more stable. 

Note, the initial condition is just that. Once the solver has done some iterations it's no longer going to show the initial conditions in the results.