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-   -   negative static pressure (http://www.cfd-online.com/Forums/fluent/43113-negative-static-pressure.html)

Selo melo December 12, 2006 18:25

negative static pressure
 
I receive negative static pressure values in fluent while solving a pressure regulator problem. I change the boundary types, conditions and there is still a negative value. How can i overcome the problem?

Besides, i cant enter any pressure value in velocity inlet boundary condition. How does fluent calculate the pressure , what is its reference point, operating pressure?

One more and last... At the same time i wanna enter pressure and flowrate at an inlet. Will i use mass flow inlet B.C.? If so to assign a 4 atm pressure at inlet will i enter 4 atm initial gauge pressure and 0 atm operating pressure?


Jason December 13, 2006 10:11

Re: negative static pressure
 
A common reason for negative static pressure is having a pressure in the operation conditions (the static pressure you're looking at is always gauge pressure, but if you set your operating pressure to 0, then gauge pressure = absolute pressure).

Another common reason is BCs, and you're all over the place on your BCs... Velocity BC should not be used for compressible flow and mass flow BC should not be used for incompressible flow. See the fluent manual, or search the forum for details (this comes up a lot, and it's been explained better than I could possibly do so, so rather than confusing you, I recommend searching the forum).

If you're modeling compressible flow, use the mass flow inlet. I typically set the operating conditions to 0 for compressible flow. This makes the BCs a little more straight forward. Now, in the mass flow BC, all pressures are in absolute, and the initial/supersonic gauge pressure is the absolute static pressure.

If you're modeling incompressible flow, then you know the density and area, you can calculate the velocity from the mass flow rate. Therefore use the velocity BC.

I hope this clarifies things some.

Good luck, Jason

Selo melo December 13, 2006 13:27

Re: negative static pressure
 
Thanks.

What about the location in operating pressure... Do i need to define a location other than (0,0,0) ?

Jason December 14, 2006 09:42

Re: negative static pressure
 
Checkout ch 8.15 in the users manual. The reference pressure location is a way of keeping the pressure from floating when you have incompressible flow without any pressure boundaries. For your case, you can leave it alone.

Good luck, Jason

wayne December 18, 2006 06:44

Re: negative static pressure
 
Hi jason as i know the pressure in fluent is not a "really" one it is P'=P-ru0*g*r(g&r are vectors) so when a fluid is at rest is P'=0 so there would be negative Static pressure when the really pressure P is smaller then ru0*g*r. is that right?

Jason December 18, 2006 10:30

Re: negative static pressure
 
P' is the hydrostatic pressure while P is the static pressure. If gravity is off, this term would fall out, but if gravity is on and if you define a density in the operating conditions, then this would be the density used in the rho*g*r term. This could be negative if P is less than rho*g*r, but that could only happen if P was a gauge pressure. Absolute pressure can't be negative (if the numerics run away, that's another story, but physically it can't happen).

What I was describing was the use of gauge vs. absolute pressure.

Pgauge = Pabsolute - Preference

If gravity is on and you define a reference point, then the absolute pressure would be adjusted so that the gauge pressure at the reference location is 0 (only for incompressible fluids without a pressure inlet BC). So if gravity were on and there was no (or very little) fluid motion, you'd be right, then anything "above" this location would have a negative static pressure, and anything "below" this location has a positive pressure.

Jason


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