Reply To: The choice of boundary conditions

[Rob]

You can't use the same profile at both ends as they will (more or less) mean you have the same amount of fluid in the system. As you're deforming the domain you need more information, so, flow enters, potentially pools and then leaves. The inlet flow and outlet pressure may determine pooling but as both are transient you need to know those values, or potentially assign the motion. 

As CFD has moved from fixed density to compressible flow to deforming mesh the amount of input data has risen. In the "good old days" we just needed an inlet velocity and a decision on turbulence (yes or no)! That's no longer the case.