[lehoanganh07]

Dear Fluent user,

I am using mixture model for two phase flow: water-vapor in Nozzle. All boundary value is measure by experiment. But now I have problem with inlet boundary condition.
First simulation, I used pressure inlet BC. The results is very far different from experiment result ( Operation pressure is atmospheric pressure: 100130 Pa, static gauge pressure in let: 5510 pa, velocity: 10.8m/s,so I obtain total gauge pressure: 63770.51 Pa and vapor pressure: 1756 Pa). when I checked result, the pressure inlet rise to 160100 Pa in absolute and velocity reduce to 3 m/s (Note that Pressure outlet fixed 5700 Pa static absolute) and Cavity appare as supercavitation. In real situation, the length cavity just 1/4 Nozzle hole.
I am not really understand why have larger change in static pressure inlet. It may be cause the wrong result in cavity of bubble.
SO now, I change to velocity inlet BC and dont have result yet.

Could you help me to explant this error
Why this phenomenon appare (pressure change too larger)?
and which boundary condition: pressure inlet BC or velocity inlet BC is suitable?

Thank you very much.

[anuarun]

Hi friend,
Though this is an old thread, I am replying for other users' sake. I encountered the same problem.
The reason was that I gave absolute values instead of gauge at the boundaries and kept operating pressure as 1atm. Later I figured out that internally Ansys works with Gauge pressure only. But Vap pressure will be usually reported and used in absolute terms. Fluent will negate Operating pressure from the given absolute value to make it in Gauge terms.
The solution is either to keep operating pressure zero and to set all other pressures in terms of absolute quantities or give gauge pressure for inlet/outlet and keep operating pressure 1atm. In your case, since you mentioned gauge pressures, this doesno seem to be the problem.