# Global mass conservation in pipe flow

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 December 26, 2012, 03:29 Global mass conservation in pipe flow #1 New Member   Join Date: Jun 2010 Posts: 20 Rep Power: 7 Hi, I am simulating flow through a pipe. In my code, as is usual - I have a factor(ratio of mass in/mass out) multiplying the exit velocity for global mass conservation. At the inlet I have given a parabolic velocity profile and convective outflow bcs at the outlet. So now when the factor is calculated for the first time step, it becomes infinity(since mass out is zero at the exit boundary) and the whole thing blows up. How to resolve this? Thanks, Qrie

December 27, 2012, 04:58
#2
Senior Member

Rami Ben-Zvi
Join Date: Mar 2009
Posts: 147
Rep Power: 8
Quote:
 Originally Posted by qrie Hi, I am simulating flow through a pipe. In my code, as is usual - I have a factor(ratio of mass in/mass out) multiplying the exit velocity for global mass conservation. At the inlet I have given a parabolic velocity profile and convective outflow bcs at the outlet. So now when the factor is calculated for the first time step, it becomes infinity(since mass out is zero at the exit boundary) and the whole thing blows up. How to resolve this? Thanks, Qrie
Hi Qrie,

I don't think this trick is common, and it is actually unnecessary. If your code is properly written, the mass balance should be nearly perfect, especially for a pipe flow.

Good luck,
Rami

 December 27, 2012, 18:47 #3 Senior Member   Chris DeGroot Join Date: Nov 2011 Location: Canada Posts: 387 Rep Power: 6 Yep, correction is unnecessary. If you really want to do this, however, just set limits for the max/min factor that you want to apply so it never gets too big or small because this will be much worse than allowing a mass imbalance. In the early iterations it really doesn't matter if there is a mass imbalance because the code should sort it out on its own as it converges.

 Tags global mass, mass conservation, pipe

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