Pressure boundary conditions

windhair · May 14, 2009, 05:51

Hello, all

I want to model a simple flow inside pipe, what I want to specify is the pressure at inlet. But there are some problem I do not really understand.

The pressure is 3 atm
When I set ref.pressure to 1 atm, and relative static pressure to 2 atm, I can get the results, but when I modify the reference pressure to 3 atm, and relative statc pressure to 0 atm, the solve stops in the first iteration, and no result is generated, why?

Thanks all

ghorrocks · May 14, 2009, 18:26

Hi,

What type of flow is it? Compressible? Transonic? You will need to describe what you are doing more fully.

Glenn Horrocks

suihenry · May 15, 2009, 00:35

In CFX, the ref. pressure is the environment reference pressure. because usually the fluid flow is in the nature condition, so ref. pressure is 1 atm.. As explained, if a fluid flows in a special pressure condition like fluid flow in a vacuum environment, the ref.pressure should be set as 0 atm. Or if you simulate a air flows in a very high altitude (the atmosphere is supposed as 0.5 atm), the ref. pressure should be set as 0.5 atm.
So if you set the ref. pressure as 3 atm, the outlet should be set as -2 atm to make the outlet is in a atmosphere environment. But usually we don't set like this.

windhair · May 15, 2009, 04:00

Quote:

Originally Posted by ghorrocks

Hi,

What type of flow is it? Compressible? Transonic? You will need to describe what you are doing more fully.

Glenn Horrocks

, the pipe is one part of the large system, what I want to simulate is only the pipe part, we adjust the inlet pressure as the testing parameter. It is around 3atm. The flow is mixed gas, subsonic .

Thanks

windhair · May 15, 2009, 04:02

Quote:

Originally Posted by suihenry

In CFX, the ref. pressure is the environment reference pressure. because usually the fluid flow is in the nature condition, so ref. pressure is 1 atm.. As explained, if a fluid flows in a special pressure condition like fluid flow in a vacuum environment, the ref.pressure should be set as 0 atm. Or if you simulate a air flows in a very high altitude (the atmosphere is supposed as 0.5 atm), the ref. pressure should be set as 0.5 atm.
So if you set the ref. pressure as 3 atm, the outlet should be set as -2 atm to make the outlet is in a atmosphere environment. But usually we don't set like this.

Thank you for the answer. If the opening is not to the atmosphere, should I specify the outlet pressure accordingly?

suihenry · May 15, 2009, 04:34

if you set the outlet as opening type, you should always set the outlet pressure, the value can be various.
if the inlet pressure is 3 atm and the outlet pressure is P(any value) in your case, it's better to set inlet pressure as 2 atm, the ref.pressure as 1 atm, the outlet as (P-1) atm.
if the outlet is not to atmosphere, you can define the value of inlet and outlet yourself. for e.g. if the inlet pressure is 3 atm, the outlet pressure is 1.5 atm, the environment pressure is 0.5 atm in your case, it's better to set inlet pressure as 2.5 atm, the ref.pressure as 0.5 atm, the outlet as 1 atm.

windhair · May 20, 2009, 04:12

Thanks, the key point is the outlet condition. I got it.

May 14, 2009, 05:51	Pressure boundary conditions	#1
windhair New Member Join Date: Mar 2009 Posts: 23 Rep Power: 17	Hello, all I want to model a simple flow inside pipe, what I want to specify is the pressure at inlet. But there are some problem I do not really understand. The pressure is 3 atm When I set ref.pressure to 1 atm, and relative static pressure to 2 atm, I can get the results, but when I modify the reference pressure to 3 atm, and relative statc pressure to 0 atm, the solve stops in the first iteration, and no result is generated, why? Thanks all

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May 14, 2009, 18:26		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,700 Rep Power: 143	Hi, What type of flow is it? Compressible? Transonic? You will need to describe what you are doing more fully. Glenn Horrocks

May 15, 2009, 00:35		#3
suihenry New Member henry wang Join Date: May 2009 Posts: 13 Rep Power: 16	In CFX, the ref. pressure is the environment reference pressure. because usually the fluid flow is in the nature condition, so ref. pressure is 1 atm.. As explained, if a fluid flows in a special pressure condition like fluid flow in a vacuum environment, the ref.pressure should be set as 0 atm. Or if you simulate a air flows in a very high altitude (the atmosphere is supposed as 0.5 atm), the ref. pressure should be set as 0.5 atm. So if you set the ref. pressure as 3 atm, the outlet should be set as -2 atm to make the outlet is in a atmosphere environment. But usually we don't set like this.

May 15, 2009, 04:34		#6
suihenry New Member henry wang Join Date: May 2009 Posts: 13 Rep Power: 16	if you set the outlet as opening type, you should always set the outlet pressure, the value can be various. if the inlet pressure is 3 atm and the outlet pressure is P(any value) in your case, it's better to set inlet pressure as 2 atm, the ref.pressure as 1 atm, the outlet as (P-1) atm. if the outlet is not to atmosphere, you can define the value of inlet and outlet yourself. for e.g. if the inlet pressure is 3 atm, the outlet pressure is 1.5 atm, the environment pressure is 0.5 atm in your case, it's better to set inlet pressure as 2.5 atm, the ref.pressure as 0.5 atm, the outlet as 1 atm.

May 20, 2009, 04:12		#7
windhair New Member Join Date: Mar 2009 Posts: 23 Rep Power: 17	Thanks, the key point is the outlet condition. I got it.