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mass flow rate at different cross section !!

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Old   September 9, 2003, 00:32
Default mass flow rate at different cross section !!
  #1
Nyquist
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When we get the mass flow rate from different cross section in a flow field, they are different. Are they not mass conservation? How will you explain it?
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Old   September 9, 2003, 06:10
Default Re: mass flow rate at different cross section !!
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Thomas
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must be same, when you do not have any sources/sinks. how did you obtain the mass flow rate?
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Old   September 9, 2003, 23:10
Default Re: mass flow rate at different cross section !!
  #3
cjtune
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There is no rule that says mass flow rate must be the same through any cross-section in the interior of the domain. The sum of mass flow rates must only be zero when taken from ALL domain BOUNDARIES.
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Old   September 10, 2003, 01:20
Default Re: mass flow rate at different cross section !!
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Thomas
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you are absolutely right - it depends on the geometry! when you have a straight pipe with one inlet and one outlet the mass flow rate is equal in every cross section. I just considered an too easy problem.
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Old   September 10, 2003, 01:46
Default Re: mass flow rate at different cross section !!
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cjtune
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err...to be more precise, it depends on the flow field... imagine taken cross-sections parallel to and perpendicular to the streamlines.
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Old   September 10, 2003, 12:38
Default Re: mass flow rate at different cross section !!
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Robin
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Draw a control volume around any part of your simulation. Now if you take the sum of the mass flows through that control volume and add any sources/sinks, they must equal zero.

If the two planes you created cut through the entire flow (ie. there are only walls between them and no other inlet/outlets/passages) and there are no sources/sinks, the mass flow rates should be equal. If they are not, either your solution is not converged or your post-processor is introducing interpolation errors (if it uses ip mass flows, the results should be exact).

Although mass flows should equal across all boundary conditions, this is not sufficient a conservation criteria as the solver may be enforcing mass flow conservation at the boundary to accelerate convergence in the computational domain.

Regards, Robin
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Old   September 11, 2003, 15:38
Default Re: mass flow rate at different cross section !!
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Joern Beilke
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Even a pipe flow does not have equal mass flow in any section at one time-step or iteration if the flow is compressible and transient. You have to look for the time averaged values.

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Old   September 15, 2003, 12:13
Default Re: mass flow rate at different cross section !!
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Robin
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If the flow is compressible and transient, you will have to account for the accumulation of mass within the control volume, but mass is still conserved.

-Robin
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