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-   -   Negative Absolute Pressure (https://www.cfd-online.com/Forums/fluent/43520-negative-absolute-pressure.html)

 mike January 29, 2007 05:47

Negative Absolute Pressure

Hi All!

I would like to simulate a system of tundish, slide-gate, submerged-entry nozzle and mould.

To whom, who do not know, what this is: This system is used in steel-industry to cast the molden steel into a mould, where the steel solidify. The submerged-entry nozzle (SEN) is mounted under the tundish. Steel runs from the tundish (bath level approx. 1m) thru the SEN in the mould. With the slide-gate you can decrease/increase the area, where the steel runs thru the SEN. This controls the mass flow thru the whole system.

My problem: I would like to calculate the mass-flow and the pressure-distribution in dependence of the position of the slide-gate. I used pressure-inlet (gauge-total-pressure 0) and pressure-outlet (gauge-pressure 0) as BC. Operating-pressure is 101300Pa (position at pressure-inlet). I activated gravity. The convergence of the setup is very well, the result for velocity is in a good range. But the pressure-distribution isn`t. I get a negative Abolute-Pressure, which is physically not possible. The Absolute pressure in Fluent (default-setting) is limited to 1Pa.

Have anybody an idea, what is wrong with my setup?

Mike

 shahid nadeem December 11, 2014 05:10

negative absolute total and static pressure

Hello

I am simulating the flow through the perforated gas distributor into the vaccum vessel.I am getting an absolute static pressure and absolute total pressure as negative value at the perforation outlet.Is it possible that we can get negative pressure value??..

 ghost82 December 11, 2014 05:50

Quote:
 Originally Posted by shahid nadeem (Post 523421) Hello I am simulating the flow through the perforated gas distributor into the vaccum vessel.I am getting an absolute static pressure and absolute total pressure as negative value at the perforation outlet.Is it possible that we can get negative pressure value??..

 shahid nadeem December 12, 2014 01:10

Dear Sir, Your Point is correct there cannot be negative absolute pressure.
But in my case the conditions are typical the Incompressible air is getting exit from atmospheric condition to vaccum condition of 1 pa. i am getting absolute static pressure as negative not the absolute total pressure. so i am still confuse.
The flow is getting exit through 10 mm holes there are 16000 holes.

 LuckyTran December 13, 2014 12:15

I think you are looking at gage pressures and not absolute pressures.

Absolute pressure (whether static or total) cannot be negative. Fluent should have thrown all sorts of errors and applied limiters as soon as it encountered a negative absolute pressure. The default limiter applies a lower limit of 1 Pa to the absolute pressure. There's no way for you to even get a negative absolute pressure unless you changed the limiter.

 yuanmengyuan1989 December 16, 2014 22:27

hello, i wonder where can we change the limiter of the Fluent?

Quote:
 Originally Posted by LuckyTran (Post 523783) I think you are looking at gage pressures and not absolute pressures. Absolute pressure (whether static or total) cannot be negative. Fluent should have thrown all sorts of errors and applied limiters as soon as it encountered a negative absolute pressure. The default limiter applies a lower limit of 1 Pa to the absolute pressure. There's no way for you to even get a negative absolute pressure unless you changed the limiter.

 LuckyTran December 17, 2014 12:35

Quote:
 Originally Posted by yuanmengyuan1989 (Post 524286) hello, i wonder where can we change the limiter of the Fluent?
In the GUI:
Solution => Solution Controls => Limits
There you can the limiters for minimum and maximum: pressure, temperature, and other turbulence quantities.

It is highly recommended that you leave these at their defaults and do not change these from their default values unless you are an expert. The defaults for pressure are min 1 Pa, and max 5e10 Pa.

 yuanmengyuan1989 December 17, 2014 21:32

thank you for your replay. i cannot agree with you more. i just want to see the limits of the pressure and i do not change it. As we see, the minmim value of the absoulte pressure in fluent is 1 Pa, why the absoulte pressre be negative? and the residual of each equation is so small during the iteration

Quote:
 Originally Posted by LuckyTran (Post 524379) In the GUI: Solution => Solution Controls => Limits There you can the limiters for minimum and maximum: pressure, temperature, and other turbulence quantities. It is highly recommended that you leave these at their defaults and do not change these from their default values unless you are an expert. The defaults for pressure are min 1 Pa, and max 5e10 Pa.

 pakk December 18, 2014 04:50

To be sure: when you say "i am getting absolute static pressure as negative", what exactly is negative in Fluent? The "static pressure" or the "absolute pressure"?

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