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Old   August 26, 2008, 15:06
Default Openings and compressible flow
  #1
underGroundMan
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Hi

I have set up a case in which I have opening & total pressure at the inlet, and opening & static pressure at the outlet. I am running compressible simulation. Is it ok to use openings at both ends with compressible simulation?

If I do not use opening on either end, I get a message from solver stating "A wall has been placed the outlet/inlet". Under what circumstances one can ignore this message. Thank you.

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Old   August 26, 2008, 16:13
Default Re: Openings and compressible flow
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John S.
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CFX will build an artifical free slip wall at a boundary if it's massflow calculation says flow wants to go opposite than what the boundary allows. I usually ignore this warning if it's showing <2% or so of the total boundary area is walled off. If it's building large walls at your boundary you probably need to redefine your boundary conditions. One other thing you cuold try is to set the expert parameter "build artificial wall = f" to tell CFX not to build the free slip surfaces.
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Old   August 26, 2008, 16:23
Default Re: Openings and compressible flow
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CycLone
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Telling it not to build artificial walls is effectively making it an opening with nothing defined for the flow going in the opposite direction.

You should specify a boundary condition that makes sense (i.e. is a good engineering approximation) at that location. If flow can go in either direction or you don't know which direction it might go in, then an opening is OK. But keep in mind that if the flow is reversed or separated near the opening you will not have the right behavior, because you are excluding what should be happening further upstream or downstream of the boundary condition.

-CycLone
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Old   August 27, 2008, 13:14
Default Re: Openings and compressible flow
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F S
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Hi

Try moving the inlet/outlet boundries a little away from their present position. This type of message can occour due to small regions of recirculating flow. See CFX documentation on defining inlet/outlet boundries. If the wall is being placed at around 2-3% of face then you can ignore the message.

Good luck
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Old   August 28, 2008, 08:10
Default Re: Openings and compressible flow
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underGroundMan
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Thanks for the suggestion guys.

Regards

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