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-   -   negative static pressure !!!! (http://www.cfd-online.com/Forums/fluent/28699-negative-static-pressure.html)

 Barbara July 9, 2001 13:34

negative static pressure !!!!

HI!!!! I'm working on sails, so my velocity is about 7 m/s. If I set Fluent with 'velocity inlet' all the boundary condition I find Static Pressure negative in the field.....I think it is a Fluent problem,because if I change one of the boundary condition in 'pressure outlet' I find a reasonable stetic pressure. Any ideas??

 Jonas Larsson July 9, 2001 13:54

Re: negative static pressure !!!!

Perhaps you have confused absolute static pressure and gauge static pressure? Fluent computes and displays all pressures as relative pressure from an "operating pressure", which you can set. Negative gauge pressure just means that your pressure is below the operating pressure.

 Barbara July 9, 2001 15:32

Re: negative static pressure !!!!

When I ask Fluent to display: 'Contours --> Pressure --> static pressure ' what does It plot?? Real static pressure or Gauge static pressure? I set every kind of pressure as 101325 Pa ( Operating, gauge ...). What's wrong??

 Jonas Larsson July 9, 2001 16:45

Re: negative static pressure !!!!

It plots gauge pressure. The real static pressure is gauge pressure + operating pressure.

 Barbara July 11, 2001 02:37

Re: negative static pressure !!!!

Thank you for the ideas, it's good for an half.I speak to Fluent tecnical support...it plots the real pressure but as a difference from the value of pressure in the point you define the operating pressure.So, the difference is not between pressure in the point and Operating pressure!!! is between pressure in the point and pressure in the point you choose to define the operating pressure.You can't know the value of pressure in the field !!!! Fluent uses difference of pressure when works with incompressible flow and is not able to give you back the absolute value of pressure. That's all!! Bye Barbara

 John C. Chien July 11, 2001 04:44

Re: negative static pressure !!!!

(1). For incompressible flows, since the density is always constant, the solution is floating by a constant value. That is, the pressure level will not affect the velocity field. (2). You have to be careful, becuase this is not the case when the density is not constant, say low mach number case, where the pressure will affect the density directly. ( though the effect is small at low Mach number)

 new anne August 11, 2012 13:37

Hi dear friends
I have this problem in my studies about cavitation.static pressure values are negative along the spillway .it is not correct.i don't know what the problem is...

 Mario June 5, 2013 15:01

Hi,

I have the same problem. I'm trying to find the pressure drop in a high flow check valve. after the check I have negative pressure. I use incompressible flow.

 oj.bulmer June 7, 2013 10:44

It is fairly normal to have negative static pressure after the check valve, since the pressure energy gets converted into kinetic energy. After some distance pressure recovers and you will see increased pressure, but they still may remain negative. The negative value you see is the difference of actual pressure and operation pressure.

OJ

 Mario June 11, 2013 12:36

But the absolute pressure should be limited to the lower limit. If the minimum pressure limit is 0, how can the absolute pressure can be negative.

 oj.bulmer June 11, 2013 13:27

Typically, whenever static pressure refers to the gauge static pressure, ie above the operating/atmospheric pressure. Whenever one refers to absolute pressure, they mention it as absolute.

When gauge static pressure is negative, it means it is below operating pressure, but it is positive.

OJ

 Mario June 11, 2013 13:52

In my case, it's the absolute pressure that is negative. That why I have an interrogation. Is fluent consider absolute pressure as gage pressure?

MT

 oj.bulmer June 11, 2013 13:59

It is very strange. Are you sure it is absolute pressure?

OJ

 jonygajar June 21, 2013 07:21

In my case also, absolute pressure is negative in whole the domain. In the range of -1e+9 which is not possible and with every iteration is goes up. Anyone know why this happens?

 Gimlas June 28, 2013 19:04

I've got another question related with this topic. I've made a simulation of air flow in a vertical pipe which is 81 m high. I've defined the velocity inlet 4,464 m/s and pressure outlet 0 Pa. Fluent calculated correctly the dynamic pressure which is 12.2 Pa. But I've got e serious problem with the static pressure. According to the calculations the static pressure at the inlet is equal to 175 Pa whereas using Bernoulli's equations it should be equal at least 973.4 Pa (omitting head losses due to the friction etc.). I've made different approaches: different meshes (element number, mesh type etc.), constant density vs incompressible gas equation, 1st and 2nd order equations, different solver setting. The outcome is always the same. Where is my mistake?

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