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-   -   absolute negative pressure???? (https://www.cfd-online.com/Forums/siemens/53529-absolute-negative-pressure.html)

 john October 27, 2003 02:35

absolute negative pressure????

Hi All,

I have simulated a flow in a closed channel involving water. But to my surprise I am getting -ve absolute pressure at many locations which is weird. Can anybody tell where things went wrong, if anybody faced similar problem before....

Thank you.

John

 Anton Lyaskin October 27, 2003 05:25

Re: absolute negative pressure????

May be in the real flow there should be cavitation?

 john October 27, 2003 06:20

Re: absolute negative pressure????

Thanks a lot Anton for your reply. But I did not understand the basis of your conclusion of cavitation. Do you mean to say if the cavitation model is not on the code has no other option than putting -ve presure values to satisfy continuity equation? Let me know if I am absolutely wrong or any other logic is there. Thanks again. - John

 Anton Lyaskin October 27, 2003 07:50

Re: absolute negative pressure????

The code dosn't care about ABSOLUTE pressure (in case of incompressible flow) - it deals with RELATIVE pressure. So from the point of view of mathematics your solution is correct - your pressure field does satisfy the continuity equation, because it's written for relative pressure, and there are no limitations for it. So you don't need to turn on cavitation model to satisfy the continuity.

But from the point of view of physics, absolute pressure in liquid can not be lower than some certain value (saturation pressure). Basicaly, all of your regions with negative absolute pressure will correspond to cavitation bubbles filled with saturated vapour (this is a bib-bib-big oversimplifaction).

 shahid nadeem December 11, 2014 09:06

Negative absolute pressure for air

Dear Friends,
I Would Like to share my experience regarding negative pressure.
i am solving a Perforated Pipe Distributor, the flow enters at atmospheric pressure and exits through the holes to the vaccum vessel maintained at 1 Pa.
The inlet area is 1500 mm and at the outlet there are 16600 number of holes of 10 mm diameter each.

After post processing i am getting a negative absolute pressure of -1358 Pa i am wondering how can there be a negative absolute pressure. WHether it is possible in vaccum conditions or is there any error in solving, if you have any idea please help me.

The Gas is Incompressiblel air at 333
Inlet: 33.33 Kg/s
Outlet 1 Pa

 Pauli December 11, 2014 13:08

Switch you gas to air - ideal gas.

 shahid nadeem December 12, 2014 01:18

Yes Sir even i think so but can you little explain me why you are thinking it should be compressible consideration because inlet mach number is 0.25, but i agree at the outlet there is a sudden expansion due to vaccum pressure of 1 pa.

Thank You

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