convergent-divergent nozzle in fluent
 

Hi, I'm trying to design a convergent-divergent nozzle in order to obtain the lowest pressure drop using the software Ansys Fluent in 2D mode. In every simulation I do I change a geometry value in order to study how that thing can influence pressure drops; then I consider the difference between inlet and outlet absolute pressure to calculate my pressure drops, but I notice that inlet absolute pressure changes in every simulation, why does this happen? In the boundary conditions I set velocity inlet with 0 gouge supersonic pressure, and outflow outlet, am I forgetting something?

Thank you for you attention.