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Old   May 14, 2009, 05:51
Question Pressure boundary conditions
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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?

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Old   May 14, 2009, 18:26
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Hi,

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

Glenn Horrocks
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Old   May 15, 2009, 00:35
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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.
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Old   May 15, 2009, 04:00
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Quote:
Originally Posted by ghorrocks View Post
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 .

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Old   May 15, 2009, 04:02
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Quote:
Originally Posted by suihenry View Post
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?
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Old   May 15, 2009, 04:34
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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.
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Old   May 20, 2009, 04:12
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Thanks, the key point is the outlet condition. I got it.
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