[KiruBiru]

Hi all,

Using ANSYS CFX. Need to fine pressure drops across the system (see the attachment).
The fluid is air, ref. pressure for the domain 1 atm., no heat transfer involved, k-e model used, convergence is OK (RMS 1e-5 reached, imbalances are OK, the monitored values were stable on the Solver graphs).

Issues:
- Why max pressure at the outlet is smaller than the area averaged at the outlet? Is something wrong with my formula, then?
- Why are all the pressure values negative? I could understand in case of two values substracted (delta) but not for the actual values.
- I would normally expect pressure 0 Pa at all inlets (relative pressure is set to 0 Pa after all) but this is not the case.
- The combination of Total Pressure at the inlet and Normal Speed at the outlet is claimed as robust in the ANSYS guidelines but the outcome is strange (see above).
Some time ago, I did a CFD on a similar situation and assigned Static Pressure = 0 Pa to the inlets and the same normal speed of 6,8 m/s to the outlet. Came to much more predictable results than this “stable” combination.
What am I missing?

[ghorrocks]

Why max pressure at the outlet is smaller than the area averaged at the outlet?

The numbers you show have the max value larger than the average, as you would expect.

Why are all the pressure values negative?

The pressure is relative to the reference pressure. You defined the inlet at zero pressure, so the pressure will decrease from there to the outlet where it will be more negative.

I would normally expect pressure 0 Pa at all inlets

You set the total pressure, not the static pressure. Also read the CFX Theory documentation on exactly how the boundary conditions are applied for some background.

The combination of Total Pressure at the inlet and Normal Speed at the outlet is claimed as robust in the ANSYS guidelines but the outcome is strange

Your results look as expected to me.

[KiruBiru]

Thanks ghorrocks. I think I am onto it.