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Old   January 17, 2009, 09:11
Default Negative pressure
  #1
jksanil
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hi all, Today when i am solving my project, i had given the input for my model as 10bar and after solving i had drawn a contour at outlet of the model. to my suprise i am getting negative pressure. while the contour drawn at this region for mass flow is negative. what does that indicate

plz help me i am having submission on day after tomarrow

thanks you in advance
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Old   January 17, 2009, 14:38
Default Re: Negative pressure
  #2
John S.
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The pressure displayed is based on whatever you set your reference pressure as. For example, if you set your reference pressure 50 psi and the countour displays -10 psi, the absolute pressure on the countour would be 40 psi.
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Old   January 17, 2009, 15:44
Default Re: Negative pressure
  #3
jksanil
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MR. John thanks for your response...

In my case i had given my reference pressure as 1atm. and a negative pressure of 5.4e4 was shown. that indicates there is a pressure less than that of atm. is that correct??

ok as one responses to my posts i had informed that negative mass flow indicates a flow moving out of field. if it so how could mass can flow out with a pressure less than ref?

Coming to my case i need to analyse the back pressure of the system into which a mass flow rate of 0.5088Kg/sec is flowing. SO can u Plz give me the boundary conditions to be given for this case. if outlet condition i had given 0 relative pressure i am getting negative pressures. while for 5bar i am getting positive. if it is variable with outlet conditions???
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Old   January 18, 2009, 23:54
Default Re: Negative pressure
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HekLeR
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Yes, that is right. The absolute pressure is "101325 - 54000 = 46000" then, assuming Pa for all units. Negative mass flow means the flow is leaving the domain, so that is fine too.
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Old   January 19, 2009, 01:14
Default Re: Negative pressure
  #5
jksanil
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As u said dear HekLer I too acept that. As per my knowledge, Ref pressure is the pressure which is outside the considered flow regime. so fluid flowing from meshed fluid region to outside region (i.e. atmosphere) through outlet region. I mean to ask if pressure is negative then how the flow is going from so considered zone to outside?

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Old   January 23, 2009, 18:52
Default Re: Negative pressure
  #6
CycLone
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Reference pressure is just the pressure by which others are gauged, it has nothing to do with the external pressure. The external pressure is whatever you have defined at your boundary condition.

If your fluid is incompressible (i.e. you used a constant density fluid property), the solution will only depend on the relative pressures of your boundary conditions and will be independent of the reference pressure. In reality you wouldn't encounter a negative pressure; if the fluid is a gas the density would decrease and change the local flowfield, if it is a liquid, the liquid would cavitate when the pressure is low enough and there is sufficient mechanical energy in the fluid to cause cavitation. But these effects will only be included in your model if you set it up with the appropriate physics (i.e. use ideal gas instead of constant density or set up a multiphase analysis to model cavitation).

The reference pressure is only there to reduce round off error. If you are dealing with low speed flows, the relative pressure differences may be quite small relative to the overall pressure level. Usually you would set your reference pressure equal to one of your boundary pressures (and thus set that boundary to be zero relative to the reference pressure). Round off occurs because there are a limited number of significant digits, so when you add or subtract two large numbers that differ by a small amount, the smaller digits get truncated and thus the difference isn't captured.

-CycLone

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