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Old   January 29, 2007, 05:47
Default Negative Absolute Pressure
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mike
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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?

Thanks in advance,

Mike
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Old   December 11, 2014, 05:10
Default negative absolute total and static pressure
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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??..
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Old   December 11, 2014, 05:50
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Quote:
Originally Posted by shahid nadeem View Post
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??..
EDIT: sorry, no cavitation, I read your fluid is gas..maybe your solution is not converged.
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Last edited by ghost82; December 11, 2014 at 10:11.
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Old   December 12, 2014, 01:10
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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.
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Old   December 13, 2014, 12:15
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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.
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Old   December 16, 2014, 22:27
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hello, i wonder where can we change the limiter of the Fluent?


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Originally Posted by LuckyTran View Post
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.
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Old   December 17, 2014, 12:35
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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.
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Old   December 17, 2014, 21:32
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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


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Originally Posted by LuckyTran View Post
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.
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Old   December 18, 2014, 04:50
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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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