How do I set up atmospheric pressure for outlet?

swapnil313 · April 10, 2018, 20:45

I have two outlets at atmosphere exits.
How can I add pressure boundary conditions ?
I'm confused about the reference/static/total pressure settings.

ghorrocks · April 10, 2018, 23:23

What do you mean by "add pressure boundary conditions"?

Total pressure: https://en.wikipedia.org/wiki/Total_pressure
Static pressure: https://en.wikipedia.org/wiki/Static_pressure

Reference pressure is used by CFX to reduce numerical round-off errors. For instance if a 1 Pa pressure difference exists in a body at 1 bar absolute pressure:

* If the reference pressure is 0, then the pressure difference is 100001 [Pa] - 100000 [Pa] which is of course a 1 [Pa] difference, but if the numerics are only accurate to 4 significant figures you get 1.000e5 [Pa] - 1.000e5 [Pa] and the 1 [Pa] pressure difference has disappeared. A simulation based on this would have unexpected results as it cannot resolve the pressure variations. This effect is called round-off error.

* If the reference pressure is 1 [bar], then the pressure difference is 1 [Pa] - 0 [Pa] which is of course a 1 [Pa] difference. Now if we consider numerics which is only accurate to 4 significant figures you get 1.000e0 [Pa] - 0.000e0 [Pa] and the 1 [Pa] pressure difference is still resolved. A simulation based on this would can work as the 1 [Pa] pressure difference is accurately resolved, despite the limited accuracy of the numerics.

April 10, 2018, 20:45	How do I set up atmospheric pressure for outlet?	#1
swapnil313 New Member Swapnil Rane Join Date: Sep 2017 Posts: 23 Rep Power: 8	I have two outlets at atmosphere exits. How can I add pressure boundary conditions ? I'm confused about the reference/static/total pressure settings.

April 10, 2018, 23:23		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	What do you mean by "add pressure boundary conditions"? Total pressure: https://en.wikipedia.org/wiki/Total_pressure Static pressure: https://en.wikipedia.org/wiki/Static_pressure Reference pressure is used by CFX to reduce numerical round-off errors. For instance if a 1 Pa pressure difference exists in a body at 1 bar absolute pressure: * If the reference pressure is 0, then the pressure difference is 100001 [Pa] - 100000 [Pa] which is of course a 1 [Pa] difference, but if the numerics are only accurate to 4 significant figures you get 1.000e5 [Pa] - 1.000e5 [Pa] and the 1 [Pa] pressure difference has disappeared. A simulation based on this would have unexpected results as it cannot resolve the pressure variations. This effect is called round-off error. * If the reference pressure is 1 [bar], then the pressure difference is 1 [Pa] - 0 [Pa] which is of course a 1 [Pa] difference. Now if we consider numerics which is only accurate to 4 significant figures you get 1.000e0 [Pa] - 0.000e0 [Pa] and the 1 [Pa] pressure difference is still resolved. A simulation based on this would can work as the 1 [Pa] pressure difference is accurately resolved, despite the limited accuracy of the numerics. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

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