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(ASK) negative pressure detected on my outlet.

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Old   March 21, 2011, 04:27
Default (ASK) negative pressure detected on my outlet.
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hi friends
i'm modeling butterfly valve plus pipe in FLUENT 6.2, after i had my simulation then i display my pressure contour, i got my pressure was negative on the outlet, why that happen?

regards,
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Old   March 21, 2011, 04:40
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Hi,
note that you've used gauge pressures.
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Old   March 21, 2011, 06:43
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i dont have any data about gage pressure, one that i know for sure is just open loop with the asumption that static outlet pressure is 1 Atm. how bout that??
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Old   March 21, 2011, 10:50
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ok, your outlet absolute pressure is 1atm. ensure that you set operating pressure correctly. your absolute pressures are evaluated relative to operating one. and double check your boundary conditions.
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Old   March 21, 2011, 22:13
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i set the operating condition in 1Atm, is it correct?
my boundary condition is velocity inlet and pressure outlet
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Old   March 22, 2011, 03:32
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then you should set gauge pressure in B.C. panel at the outlet. if you have static absolute pressure of 1 atm at the outlet set 0 pa there.
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Old   March 22, 2011, 05:09
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that's what i did, my friend amir but the pressure still negative, why?
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Old   March 22, 2011, 08:22
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if you set outlet gauge pressure to 0 pa, the pressure in all cells should be positive unless for supersonic flows or when your case doesn't converge yet. check whether the pressure at the outlet is zero or not and also check residuals.
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Old   March 22, 2011, 08:38
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i have my case converged obviously. yeah i have checked it and the outlet pressure is zero. any idea what was going wrong? my reynold number is 449.500
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Old   March 22, 2011, 08:51
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so, it may happen just after a valve because of separation zones and after that pressure increases to zero at the outlet.
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Old   March 22, 2011, 11:58
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is it normal?does it make a sense?
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Old   March 22, 2011, 13:30
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yes,
after separation zones it usually occurs.
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Old   March 22, 2011, 21:30
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yes dear my friend amir i know it usually occurs..cavitation and cavitation is always obtained in the zone where the pressure is below the atmosphere but could it be possible reach negative value???
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Old   March 23, 2011, 03:17
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your gauge pressure in those zones is negative not absolute one. (e.g. if you get -1000 pa, it means that your absolute pressure there is 100325 pa) it just shows that pressure there is lower that the value at the outlet.
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Old   March 23, 2011, 05:07
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so how about if the pressure is positive? for example my inlet pressure is 550.673 Pa, it means that the absolute is 101.325 + 550.673 Pa, right??
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Old   March 23, 2011, 08:46
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Quote:
Originally Posted by sincity View Post
so how about if the pressure is positive? for example my inlet pressure is 550.673 Pa, it means that the absolute is 101.325 + 550.673 Pa, right??
yes, of course.
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Old   March 23, 2011, 10:07
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what an interseting conversation with you, thanks amir may Allah bless you, are u moslem?
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Old   March 23, 2011, 10:15
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you're welcome.
yes I am.
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Old   March 23, 2011, 11:56
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me too.
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