Transient Simulation for pipe flow

Sam

I used the uniform axial velocity at inlet & zero relative static pressure at outlet for my steady flow simulation (I'm using CFX5.7 now).

I now wish to simulate the transient pipe flow when the valve suddenly close at the inlet (valve is not included in the flow domain). Would it be possible to impose a step change of static pressure (rather than velocity) at the inlet as the boundary condition?

I want the inlet velocity to be part of the solution but learnt from user manual that static pressure at inlet is a very unreliable boundary condition. Any suggestion how to model this condition, please?

Thanks a lot, Sam.