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-   -   NEGATIVE PRESSURE??? (http://www.cfd-online.com/Forums/fluent/40089-negative-pressure.html)

REZA March 11, 2006 20:45

NEGATIVE PRESSURE???
 
HI I am modeling a cylinder inside the wind tunnel, but I am getting negative pressure up and down the cylinder and down stream of the flow? would you please help me to find the problem? thanks REZA

Ahmed March 11, 2006 23:00

Re: NEGATIVE PRESSURE???
 
What is your operating pressure?

REZA March 11, 2006 23:08

Re: NEGATIVE PRESSURE???
 
101375 Pa and the aditional 50 Pa is because of 10 m/s air velocity.

Ahmed March 11, 2006 23:14

Re: NEGATIVE PRESSURE???
 
You have two options: 1- Either repeat the analysis and setting the operating pressure to Zero 2- Or add the value of the operating pressure (101325) to your results You have to understand that the pressure solved for using the Navier Stokes equations is the "Absolute Pressure" not the gauge pressure as you did.

REZA March 12, 2006 02:24

Re: NEGATIVE PRESSURE???
 
Thanks a lot Ahmed but I tried both method, and still the problem has not been solved. air has a velocity of 10m/s. what should I exactly put for operating condition & pressure outlet? I really appreciate your help. REZA

mAx March 12, 2006 09:15

Re: NEGATIVE PRESSURE???
 
hi, wich pressure is negative? absolute or static?

Ahmed March 12, 2006 11:04

Re: NEGATIVE PRESSURE???
 
if you set the operating pressure to zero then you have to use absolute values when setting your boundary conditions, and do not forget the solution limits, the minimum is 1 Pa by default, it is almost impossible that you get a solution with negative values as you say, you must be doing something utterly wrong, use the Send Email button and write a detailed email with your boundary conditions or send me a case and data files

REZA March 12, 2006 16:07

Re: NEGATIVE PRESSURE???
 
Hi mAx Both pressures are negative!!!!

mAx March 13, 2006 02:24

Re: NEGATIVE PRESSURE???
 
local negative absolute pressure can show apparition of cavitation. But it occures only with high drop pressure

kharicha March 13, 2006 05:55

Re: NEGATIVE PRESSURE???
 
Is your flow compressible ?

if no, then the value of the pressure is not important, only the gradient of pressure acts physically.....looks in the NS equations....the value of the pressure is nowhere. use a reference pressure equal to 0 or one. Then you get a maximum and a minimum value of pressure, and what is physically important it is the difference between them.

If yes, then the density, temperature field can change due to change of pressure. The value can be also important for example if you deal with cavitation, phase change, boiling etc...

Last thing, be carefull! your static pressure distribution (and your maximum and minimum value) depends strongly on the drag (shape and viscosity)...a good meshing is again required.

zhang March 13, 2006 06:56

Re: NEGATIVE PRESSURE???
 
I am also have the problem too

Ahmed March 13, 2006 16:17

Re: NEGATIVE PRESSURE???
 
mAx wrote: "negative absolute pressure" This must be a new addition to science, take your results to the Nobel committe

john March 14, 2006 02:27

Re: NEGATIVE PRESSURE???
 
What Max said is right. He never meant that this is possible in physical world. But solver put -ve value for absolute pressure if there is cavitation. This is that way to check presence of cavitation. It is in other words 'an error'.

It is not necessary to show your such a arrogant presence, without knowing the fact.

mAx March 14, 2006 02:57

Re: NEGATIVE PRESSURE???
 
Thanks John. Ahmed you should read between the lines. I particulary underlined the use of the term ABSOLUTE pressure, because it is naturally impossible to have negative absolute pressure. But in Fluent if you have a such domain, it can show presence of cavitation. If I don't know the solution of a posted topic, I won't post a bullshit. Please read following topic, I posted in October 2005: http://www.cfd-online.com/Forum/flue...cgi?read=32697 Over there I mentionned, that negative absolute pressure isn't physically possible. So please, read and re-read the topic before you post bullshit

pitamber.methia September 24, 2013 05:12

negative pressure
 
Hi
I have the same problem
in pressure results it is showing the negative absolute pressure.
how it can be possible i am unable to understand. Can someone help


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