CFX Boundary condition

rameezib · August 20, 2013, 02:29

Hello buddies,
I am trying to simulate a flow over an axial turbine whose rpm is low around 500 rpm. I know only the pressure drop across the turbine and volumetric flow rate. Fluid medium is ideal gas air. Using vol. flow rate,i could identify my inlet boundary condition as normal speed or mass flow inlet, but I'm very confused with the outlet boundary condition. Please help.

ghorrocks · August 20, 2013, 06:24

Then your outlet will be some form of pressure boundary. Read the CFX documentation on recommendations on boundary conditions for tips on exactly what sort pf pressure boundary, and the best practises guide for turbomachinery (also in the CFX documentation).

rameezib · August 20, 2013, 06:54

Yeah i have practised the tutorials, here what i wanted to ask is,
Can i specify pressure outlet as 0 Pa, keeping reference pressure as 0 Pa too
Or i should calculate the inlet pressure and then as i know pressure drop, subtract it from inlet pressure to specify the pressure outlet ?
Or should i simply keep the outlet open to atmosphere

ghorrocks · August 20, 2013, 07:09

I was not referring to the tutorials. I was referring to the CFX documentation which contains a lot of information about how to use the software effectively.

No, your outlet pressure is not going to be absolute zero pressure. If it is atmospheric pressure (1 bar absolute) then use 1 bar reference pressure and 0 bar pressure on the outlet.

rameezib · September 4, 2013, 01:02

Can we do vice versa? 0 bar as reference pressure and outlet as 1 bar because if i specify 1 bar as reference, it may effect my inlet condition. Thanks

ghorrocks · September 4, 2013, 02:57

You do not seem to understand. If your inlet is 2 bar absolute and the outlet is 1 bar absolute then use a reference pressure of 1 bar, an inlet of 1 bar and an outlet of 0 bar. All pressures are relative to the reference pressure. So if you change the reference pressure you change all pressures in the simulation.

August 20, 2013, 02:29	CFX Boundary condition	#1
rameezib New Member Rameez Join Date: May 2013 Posts: 19 Rep Power: 12	Hello buddies, I am trying to simulate a flow over an axial turbine whose rpm is low around 500 rpm. I know only the pressure drop across the turbine and volumetric flow rate. Fluid medium is ideal gas air. Using vol. flow rate,i could identify my inlet boundary condition as normal speed or mass flow inlet, but I'm very confused with the outlet boundary condition. Please help.

September 4, 2013, 01:02	Vice Versa	#5
rameezib New Member Rameez Join Date: May 2013 Posts: 19 Rep Power: 12	Can we do vice versa? 0 bar as reference pressure and outlet as 1 bar because if i specify 1 bar as reference, it may effect my inlet condition. Thanks

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August 20, 2013, 06:24		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,690 Rep Power: 143	Then your outlet will be some form of pressure boundary. Read the CFX documentation on recommendations on boundary conditions for tips on exactly what sort pf pressure boundary, and the best practises guide for turbomachinery (also in the CFX documentation).

August 20, 2013, 06:54		#3
rameezib New Member Rameez Join Date: May 2013 Posts: 19 Rep Power: 12	Yeah i have practised the tutorials, here what i wanted to ask is, Can i specify pressure outlet as 0 Pa, keeping reference pressure as 0 Pa too Or i should calculate the inlet pressure and then as i know pressure drop, subtract it from inlet pressure to specify the pressure outlet ? Or should i simply keep the outlet open to atmosphere

August 20, 2013, 07:09		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,690 Rep Power: 143	I was not referring to the tutorials. I was referring to the CFX documentation which contains a lot of information about how to use the software effectively. No, your outlet pressure is not going to be absolute zero pressure. If it is atmospheric pressure (1 bar absolute) then use 1 bar reference pressure and 0 bar pressure on the outlet.

September 4, 2013, 02:57		#6
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,690 Rep Power: 143	You do not seem to understand. If your inlet is 2 bar absolute and the outlet is 1 bar absolute then use a reference pressure of 1 bar, an inlet of 1 bar and an outlet of 0 bar. All pressures are relative to the reference pressure. So if you change the reference pressure you change all pressures in the simulation.