negative pressure

mAx

hi all, I have a problem. I have a nozzle, which is supposed to reduce a load pressure. This reduced pressure is acting behind a piston and move it in closed position. I modelised it in 2d axisymmetric. The load pressure is about 220 bar, after a steady calculation, I checked the force on the piston, and it seems that the piston is opening, which is not possible. I saw that the pressure after the nozzle is negative (absolute pressure). It seems that the high pressure loss generates cavitation. So behind the piston, I assumed that Fluent integrates this negative pressure on the rear piston. Anyone has any idea for preventing this effect? Should I take a very high operating pressure for reducing this effect? thank you for your help. mAx