Problem in defining domain reference pressure

dhiren012 · April 9, 2015, 07:47

I'm new to CFX. I'm trying to analyse the fluid flow in an externally pressurised gas journal bearing using CFX. I've presuure inlets as well as pressure oulets. Feed holes as inlet(10 bar absolute), clearance spaces open to atmospehere as oulets(1.5 bar absolute). I'm facing problems in defining the boundary conditions. What will be the domain reference pressure in this case.
Please help I'm not getting the concept of the domain reference pressure.

Opaque · April 9, 2015, 09:23

You can set the domain reference pressure to 0 [Pa], and use absolute pressure on the boundaries.

If there are concerns about round off issues with pressure, you can use the double precision option.

ghorrocks · April 9, 2015, 17:35

While a zero reference pressure means you don't need to understand what is going on, it does mean you are liable to see numerical round off errors.

Reference pressure is simply a pressure which all other pressures are referenced to. For wind flow around a building, where the absolute pressure varies from 99999Pa to 100001Pa - if you do not use a reference pressure you are then asking the numerics to resolve tiny pressure differences between 99999Pa and 100001Pa. If you use a reference pressure of 100000Pa, then the model uses pressures of -1Pa to +1Pa. So now the pressure difference is resolved in the first significant digit as opposed to the 5th significant digit. The reference pressure simulation will be much more resistant to numerical round off.

So in your case use a reference pressure of 1.5 bar and an inlet pressure of 8.5 bar with an outlet of 0bar. Simple.

dhiren012 · April 13, 2015, 07:47

Thank u opaque and ghorrocks.. But i'm facing another problem which was unnoticed before.. I've only one domain as fluid.. but in cfx-pre, there are 2 different fluid fluid interferences. why ??? (Note-the geometry has been sliced)
Pls help

ghorrocks · April 13, 2015, 18:05

They sound like the automatically generated interfaces. If they are wrong then delete them.

April 9, 2015, 07:47	Problem in defining domain reference pressure	#1
dhiren012 New Member Dhiren Join Date: Apr 2015 Posts: 2 Rep Power: 0	I'm new to CFX. I'm trying to analyse the fluid flow in an externally pressurised gas journal bearing using CFX. I've presuure inlets as well as pressure oulets. Feed holes as inlet(10 bar absolute), clearance spaces open to atmospehere as oulets(1.5 bar absolute). I'm facing problems in defining the boundary conditions. What will be the domain reference pressure in this case. Please help I'm not getting the concept of the domain reference pressure.

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April 9, 2015, 09:23		#2
Opaque Senior Member Join Date: Jun 2009 Posts: 1,804 Rep Power: 32	You can set the domain reference pressure to 0 [Pa], and use absolute pressure on the boundaries. If there are concerns about round off issues with pressure, you can use the double precision option.

April 9, 2015, 17:35		#3
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	While a zero reference pressure means you don't need to understand what is going on, it does mean you are liable to see numerical round off errors. Reference pressure is simply a pressure which all other pressures are referenced to. For wind flow around a building, where the absolute pressure varies from 99999Pa to 100001Pa - if you do not use a reference pressure you are then asking the numerics to resolve tiny pressure differences between 99999Pa and 100001Pa. If you use a reference pressure of 100000Pa, then the model uses pressures of -1Pa to +1Pa. So now the pressure difference is resolved in the first significant digit as opposed to the 5th significant digit. The reference pressure simulation will be much more resistant to numerical round off. So in your case use a reference pressure of 1.5 bar and an inlet pressure of 8.5 bar with an outlet of 0bar. Simple.

April 13, 2015, 07:47		#4
dhiren012 New Member Dhiren Join Date: Apr 2015 Posts: 2 Rep Power: 0	Thank u opaque and ghorrocks.. But i'm facing another problem which was unnoticed before.. I've only one domain as fluid.. but in cfx-pre, there are 2 different fluid fluid interferences. why ??? (Note-the geometry has been sliced) Pls help

April 13, 2015, 18:05		#5
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	They sound like the automatically generated interfaces. If they are wrong then delete them.