# outflow BC's?

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 July 3, 2013, 02:03 outflow BC's? #1 Senior Member     Ehsan Join Date: Oct 2012 Location: Iran Posts: 2,209 Rep Power: 19 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? ScarFace likes this. __________________ Injustice Anywhere is a Threat for Justice Everywhere.Martin Luther King. To Be or Not To Be,Thats the Question! The Only Stupid Question Is the One that Goes Unasked.

 July 3, 2013, 04:18 #2 Senior Member     Mohsen KiaMansouri Join Date: Jan 2010 Location: CFD Lab Posts: 118 Rep Power: 8 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: Convective outlet boundary condition for Unsteady flows good luck __________________ “If you have an apple and I have an apple and we exchange these apples then you and I will still each have one apple. But if you have an idea and I have an idea and we exchange these ideas, then each of us will have two ideas.”

July 3, 2013, 08:49
#3
Senior Member

Ehsan
Join Date: Oct 2012
Location: Iran
Posts: 2,209
Rep Power: 19
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?
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__________________
Injustice Anywhere is a Threat for Justice Everywhere.Martin Luther King.
To Be or Not To Be,Thats the Question!
The Only Stupid Question Is the One that Goes Unasked.

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