# Inlet pressure vs mass flow diference

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 March 18, 2011, 12:47 Inlet pressure vs mass flow diference #1 New Member   Miroslav Join Date: Jul 2010 Posts: 4 Rep Power: 7 Hello everybody, I am trying to make calculation of flow in turbocharger, I have one rotary domain there, but I have discover that when I set up mass flow BC on inlet and in res file I calculate pressure on inlet and than i set this pressure as a total or static (tryed both ) pressure on inlet I get dífferent values of mass flow in second case. Can you help me dicover what is going on ?

 March 20, 2011, 22:10 #2 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 10,931 Rep Power: 85 Did you average the pressure, or keep the pressure distribution over the inlet patch?

 March 21, 2011, 04:39 #3 New Member   Miroslav Join Date: Jul 2010 Posts: 4 Rep Power: 7 Hi I calculated avagare pressure. One possibilities is that I/O pressure to pressure is no so strongly defined than the mass flow on inlet and it gives to solver some freedom to get equilibrium. My model is qiute complicated, 4 domain , 1 static interfaces and 2 frozen rotors. What do you think about this assumption ? Thanks M.

March 21, 2011, 20:29
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Glenn Horrocks
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Quote:
 One possibilities is that I/O pressure to pressure is no so strongly defined than the mass flow on inlet and it gives to solver some freedom to get equilibrium.
I have no idea what this sentence means.

If you are applying a constant pressure across the boundary then that is an obvious difference to the mass flow inlet where pressure varies across the boundary.

 March 23, 2011, 04:55 #5 New Member   Miroslav Join Date: Jul 2010 Posts: 4 Rep Power: 7 Yeah Glenn, you are right that the comparison of pressure from BC (constant value) and the pressures from .res with BC mass -> P2 when P1 is changing across the boundary brings some mistake, but difference was much bigger. I was wondering if somebody has some experience with this efect. (Big difference between inlet pressure if it is set as BC or if it is calculated as a result of mass flow on inlet BC, I have to doublecheck that all cases had been converged enought, but i do not think there can be a problem ). We have made some measurement in real condition, and results from calculation was much more close to it , when I used mass-> P2 BCs.

March 24, 2011, 00:34
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J_A
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Quote:
 Originally Posted by Miroslav ...when I set up mass flow BC on inlet and in res file I calculate pressure on inlet and than i set this pressure as a total or static (tryed both ) pressure on inlet I get dífferent values of mass flow in second case. Can you help me dicover what is going on ?
Hi Miroslav, you are right!, if you set Total presure you are including the contribution of the flux, in concecuence the mass flow. But if you set the Static pressure as BC I/O, you will have "any" value as mass flow could be close the mass balance, so you need a initial condition in velocity calculated with the mass flow in your problem to have a better value en mass flow.
Hope to help

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