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 immortality July 3, 2013 02:03

outflow BC's?

what set of BC's we can put in output flow boundaries?
One that i have test is fixedValue of pressure.
It makes the pressure of neighbor internalField values of pressure goes higher than around cells in compressible cases.
Another may be fixedValue velocity,but i saw some crash in this selection.
Can we set both velocity and pressure there from CFD view?
totalPressure is unstable on outflow.
What are the experiences my friends has ever had on this topic?

 cfdonline2mohsen July 3, 2013 04:18

Dear Ehsan
the b.c. for presurre at the outlet is like:
Code:

```outlet     {         type            fixedValue;         value          uniform 0;     }```
because for many cases usually the flow faces the atmosphere air at the outlet.

the type of b.c. for velocity depends on your outlet boundary. if your outlet boundary is far enough so that you don't have any reverse flow then the zeroGradient is a suitable choice
Code:

```outlet     {         type            zeroGradient;     }```
but if you have reverse flow at the outlet then convective outlet b.c. should be used. take a look at the following post:

http://www.cfd-online.com/Forums/ope...tml#post385820

good luck

 immortality July 3, 2013 08:49

1 Attachment(s)
Hi Mohsen
I have a problem in compressible flows outlet BC on p.
p in neighbour cells of the patch goes very high.
could you have a look into it?
its a simple tube that only part of right wall is open.I need p on BC be 800000 but it seems it can't decrease below a limit(I'm not sure about the exact value but maybe around 1500000)
why its so?its numerical or physical?mean in real world pressure at outlet can't be decrease more from a specific pressure? :confused: :)

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