# OUTFLOW BC explanation in fluent

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 February 16, 2013, 14:31 OUTFLOW BC explanation in fluent #1 New Member   Debanjan Deep Join Date: Mar 2009 Posts: 14 Rep Power: 10 I have a question regarding the mass flow rate during outflow boundary condition employed in FLuent. It requires a mass flow ratio in case of multiple outlets of Outflow BC. I am wondering why this mass flow rate ratio is required to put in fluent since I believe fully developed flow is sufficient bc condition to satisfy NS equation. Am I right? In case of multiple outflow outlet bc, it is mandatory to put those mass flow weight ratio to get a perfect simulation result as I have experienced before. Can anyone explain this from NS equation using some easy scheme like FDM? I am new in this field so any help would be highly appreciated! Thanks, Debanjan

 February 17, 2013, 10:28 #2 Senior Member   Lefteris Join Date: Oct 2011 Location: UK Posts: 203 Rep Power: 9 It has to do with the conservation of mass. Since there are no sources or sinks, what comes in must go out. Thus, the sum of the mass leaving the domain must be equal to the mass entering the domain, regardless of the number of the inlets and outlets. In the case of multiple outlets you determine the portion of the flow that "uses" the one or the other outlet. The fully developed flow you mentioned isn't a useful condition, actually it's not a BC at all. Best regards, Lefteris

February 18, 2013, 07:52
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andy
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Quote:
 Originally Posted by Aeronautics El. K. It has to do with the conservation of mass. Since there are no sources or sinks, what comes in must go out. Thus, the sum of the mass leaving the domain must be equal to the mass entering the domain, regardless of the number of the inlets and outlets. In the case of multiple outlets you determine the portion of the flow that "uses" the one or the other outlet. The fully developed flow you mentioned isn't a useful condition, actually it's not a BC at all. Best regards, Lefteris
A fully developed flow has zero normal gradient on the velocity components and a constant normal pressure gradient balancing the wall shear stress. It is a valid and sufficient boundary condition for outflow through a single exit but it is not a sufficient boundary condition for outflow through multiple exits. The OP was given reasons for this when he asked the same question a few posts below.

 February 18, 2013, 10:51 #4 Senior Member   Lefteris Join Date: Oct 2011 Location: UK Posts: 203 Rep Power: 9 You're absolutely right andy_ but as you said, for the specific problem they're not sufficient. Generally speaking though, these are the BC that should be used, you're right. __________________ Lefteris

 February 18, 2013, 12:01 #5 New Member   Debanjan Deep Join Date: Mar 2009 Posts: 14 Rep Power: 10 Well, my question here is why it is not sufficient for multiple outlets ? Can this be proved from NS equation point of view?

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