hi, I've got a simple geometry: 2 pipes are connected on the same axis. the first one has ø32mm the second one has ø40mm. Massflow is about 1900L/min. (incompressible/ steady state / hydraulic oil) When my solution converged, I compute the drop pressure as the the total pressure difference between inlet (ø32) and outlet (ø40). drop pressure is ok with theoretical solution. BUT when I take a look at he static pressure distribution, the static pressure at the inlet is negative. (and positive at the outlet)

I cannot understand this phenomenon (and I don t trust this solution). Anyone could me explain that?

The main thing is that for incompressible flows, the pressure as an infinity of solutions. In Navier-Stokes, the only pressure term is grad(p), so there is an integration constant that is not defined. The pressure is defined only if you IMPOSE the pressure at one point (one boundary for example) of the flow.

If you do so, all the pressures will be defined relatively to this point.

So if you are working in incompressible, the pressure has no signification as absolute values, but only as relative values.

you're right. I checked, and I can ajust the pressure solution by setting the operating pressure (no need to recalculate) thanx