[sanjiiv]

hello,
what is mean by operating pressure in fluent.
Is is it is absolute pressure.
where it will act inside the domain or like gauge pressure acting on the boundaries.

or not, please tell me what is the difference between operating and gauge pressure setting .whaat is its relation in problem setting how to handle it in solving a problem.

thank you,

[LuckyTran]

In Fluent, all pressure inputs and plots and such are gauge pressures.

P_absolute = P_operating + P_gauge

The default operating pressure is 101325 Pa. So if you put a pressure inlet as 0 pressure (0 gauge pressure), then the absolute pressure at the inlet is 101325 Pa.

The operating pressure setting applies to your entire simulation and also affects all cells in the domain. Hence when you make a pressure contour plot, you're actually looking at the gauge pressure and not the absolute pressure.

For cases with body forces there's also a hydrostatic component which is a little more complicated.

[sanjiiv]

If that means, then in analyses of compressible flow over CD nozzle problem,
we will set the two variant pressure at the pressure inlet, one is gauge total pressure as 101325 pa and other is supersonic/initial gauge pressure as 99348 pa.why we are setting like this, what is the logic behind it...

[LuckyTran]

They are two completely unrelated things.

There are only two cases where the supersonic / initial gauge pressure value is used. The name of the box reflects these two uses.

1) When your inlet is supersonic, the flow properties at the inlet do not depend on anything occurring downstream (in the domain). Hence when the inlet is supersonic, you must completely specify all properties at the inlet. For the supersonic case, you must specify both the stagnation and static pressure (both in gauge pressures, because everything in Fluent is gauge pressures). If your inlet is not supersonic, then the supersonic gauge pressure input is never needed and never used. You can put in any number and it will be ignored.

2) During the initialization, if you choose compute from and select the inlet, it will use the static pressure you have specified in the supersonic / initial gauge pressure. A lot of people choose to do their initialization this way, although I personally never do it.

You Fluent user's guide has all this information neatly written. In general, if you run into these types of problems and get confused, just click the help button.

[sanjiiv]

Then what was the purpose of setting gauge total pressure as atm condition 101325pa ,which you mention it as stagnation pressure .
But in the CD nozzle problem the inlet is basically subsonic ..what was that ?..