Unsteady pressure differential between inlet and outlet of the pipe for single phase
 

I am stuck with this simple but confusing problem. For example, I am simulating a single phase flow through a pipe. I am using water as the fluid. The flow is laminar and unsteady. The reason for using unsteady is because I want to observe the change in the pressure (pressure at inlet - pressure at outlet) with time. I have velocity inlet and pressure outlet boundary conditions. I have created a surface monitor using the inlet as surface and using Area-Weighted Average to record the static pressure Vs time. Since, the outlet pressure is zero, this static pressure should be equal to (pressure at outlet - pressure at inlet). The result obtained from Fluent are very confusing. At steady state, when the Pressure Vs time graph becomes flat, the pressure value agrees with the theoretical value. However, the pressure Vs time graph starts off at a very high pressure. I do not think this is the reality. Can you please help me with this.

I cannot find anything like this on internet. Can anyone please confirm my results!!! I have attached my Pressure Vs Time graph with this email.

Thank you very much.