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

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

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!

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!

Interesting question. I'm not sure if there is a difference.

Opaque, can you answer this?

-Robin

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

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

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

Thanks a lot!

With kind regards Li