[Denied]

Hello I am studying the results of a transient simulation of a flow of water though a duct that has expanding section and velocity at inlet increases in time according to a linear ramp profile.
Velocity at inlet is 0,1 m/s for 0,1 sec and then increases up to 1,5 m/s in the next 0,2 sec and then becomes constant.
I used a udf file for inlet velocity and I have monitored mass-weighted-avarage total pressure at inlet and used PISO.
i do not understand why the pressure monitor has the profile it has.When velocity increases at inlet it shows a step like increase and then decreases but not in a linear manner.Can someone explain this behaviour.
For post-processing what quantities can be interesting to focus on.