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Old   February 26, 2008, 12:17
Default outflow BC
  #1
selim
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Hi, In Fluent there is outflow BC. I couldnt find the equivalance of this in CFX? Any help?

Thanks, ---Selim
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Old   February 26, 2008, 12:50
Default Re: outflow BC
  #2
Fusion
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I don't know how Fluent works, whoever in CFX there are 2 type of outlet BC: "outlet", which simply allows the fluid to exit from your domain; and "opening", which allows the flow to re-enter into your domain also. The latter is preferrable since the former, having particular pressure distributions at the surface, imposes a "wall" condition to the cells in which the flow are sucted into your domain, thus creating a partly obstructed outlet surface (unphysical behaviour).
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Old   February 26, 2008, 17:36
Default Re: outflow BC
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Glenn Horrocks
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Hi,

I would not say in general that an opening is preferable to an outlet. If a boundary could have flow in both directions then an opening is required. However, if flow is only out of the boundary (or if a small region of backflow is present but it is not important to the results) then an outlet is preferable as they tend to be more numerically stable and converge faster.

Regards, Glenn Horrocks
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Old   February 26, 2008, 19:27
Default Re: outflow BC
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Rogerio Fernandes Brito
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It depends on the situation. I use openning to not use wall, because i wanna simulate a small region, īcause i donīt have a fast personal computer, neither a distributed simulation!

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Old   February 27, 2008, 04:29
Default Re: outflow BC
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Subha
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Hai Glenn,

When we have a new problem, how can we conclude that the boundary has a flow, only in one direction or both?

I simulated a flow in which I used a '0 Pa' static pressure outlet. After some steps I had a warning saying that there are reverse flows in my domain. Then I had to change it Opening. Is this the only method to know whether this boundary has reverse flows or not?

Regards,

Subha.
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Old   February 27, 2008, 17:50
Default Re: outflow BC
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Glenn Horrocks
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Hi,

You can always look in CFX-Post to see the reverse flows.

As I said previously, the reverse flows and artificial walls at outlets are not necessarily bad, providing they are not significantly influencing results. In fact the additional numerical stability they have is very useful.

Glenn Horrocks
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Old   March 1, 2008, 11:02
Default Re: outflow BC
  #7
Hekler
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This is an extrapolated condition in fluent.

The equivalent in CFx is a supersonic outlet. This is only available for compressible flows. For incompressibl just set a pressure outlet and switch it to a supersonic outlet in the command editor.
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