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Old   July 19, 2007, 07:51
Default Additional Variable Boundary Conditions!
  #1
Li
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Hi everyone!

In cfx, I created an additonal variable as a gas A and set it as transport equation. The main purpose of the simulation is to see the additional variable distribution in the room, but now I am confused with the boundary contition. The gas A is injected from the bottom of the room. At the beginning, I set it as an inlet boundary, but it seemed that I have to set the air velocity and there is no air to be injected in from the surface of the bottom in the room. Would you please tell me how I should set it in CFX? May I use wall boundary and then set boundary sources? Or set a subdomain and set a source with the equation of gas A? Actually I don't think adding source is a good idea according to my understanding of the sources.

Any comments will be appreciate, thank you very much!

Regards Li
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Old   July 19, 2007, 09:44
Default Re: Additional Variable Boundary Conditions!
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Robin
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Hi Li,

The boundary condition types (inlet, outlet, etc) refer to how the hydrodynamics are treated, not additional variables. If the AV originates at a wall, make it a wall and specify the additional variable value or flux. If it originates within a volume region, create a subdomain and introduce an AV source. If it originates from a point source (or a region smaller than your mesh resolution), create a point source.

Regards, Robin
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Old   July 19, 2007, 11:13
Default Re: Additional Variable Boundary Conditions!
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Li
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Hej Robin, thank you so much for your comments! I will try to set a wall and specify the AV flux first. Then I will come back to tell you if it works. Thanks a lot!
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Old   July 19, 2007, 11:31
Default Re: Additional Variable Boundary Conditions!
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Li
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Would you please tell me the difference between specifying AV 'flux in' and boundary source of AV equation sources in wall boundary condition? I read the help documents, but I still don't understand them well. Thank you!
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Old   July 19, 2007, 15:02
Default Re: Additional Variable Boundary Conditions!
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Robin
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Interesting question. I'm not sure if there is a difference.

Opaque, can you answer this?

-Robin
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Old   July 19, 2007, 15:34
Default Re: Additional Variable Boundary Conditions!
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opaque
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Dear Li,

For laminar flows, there is no difference between using a boundary source, or Flux in. However, for turbulent flows the value of the AV at the boundary is extracted from the "boundary layer/wall function" approximation when the Flux in option is used. That is not done for boundary source since a source is independent of the boundary layer behavior..

I think I did explain similar issue for heat transfer in a different thread.

Hope this helps,

Opaque

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Old   July 20, 2007, 02:53
Default Re: Additional Variable Boundary Conditions!
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Li
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Hej Opaque,

Thank you very much! It does help me a lot. I will search the thread you mentioned since I concern it too. At the same time, I am wondering if I could find this information you told me from the relative papers or CFD theory books or CFX help documents? Thanks a lot!

With kind regards Li

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Old   July 23, 2007, 10:27
Default Re: Additional Variable Boundary Conditions!
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opaque
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Dear Li,

You are welcome.

Search the CFX Solver Theory documentation for "Heat Flux in the Near-Wall Region".. That explains the heat transfer case, and the additional variable case is just similar to it.

Opaque

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Old   July 23, 2007, 10:50
Default Re: Additional Variable Boundary Conditions!
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Li
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Thanks a lot!

With kind regards Li
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