# Negative Absolute Pressure

 Register Blogs Members List Search Today's Posts Mark Forums Read

 January 29, 2007, 05:47 Negative Absolute Pressure #1 mike Guest   Posts: n/a 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

 December 11, 2014, 05:10 negative absolute total and static pressure #2 Member   shahid Join Date: Oct 2013 Posts: 74 Rep Power: 3 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??..

December 11, 2014, 05:50
#3
Senior Member

Daniele
Join Date: Oct 2010
Location: Italy
Posts: 908
Rep Power: 15
Quote:
 Originally Posted by shahid nadeem 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??..
__________________

Last edited by ghost82; December 11, 2014 at 10:11.

 December 12, 2014, 01:10 #4 Member   shahid Join Date: Oct 2013 Posts: 74 Rep Power: 3 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.

 December 13, 2014, 12:15 #5 Senior Member   Lucky Tran Join Date: Apr 2011 Location: Orlando, FL USA Posts: 605 Rep Power: 12 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.

December 16, 2014, 22:27
#6
New Member

Join Date: Jul 2012
Posts: 25
Rep Power: 5
hello, i wonder where can we change the limiter of the Fluent?

Quote:
 Originally Posted by LuckyTran 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.

December 17, 2014, 12:35
#7
Senior Member

Lucky Tran
Join Date: Apr 2011
Location: Orlando, FL USA
Posts: 605
Rep Power: 12
Quote:
 Originally Posted by yuanmengyuan1989 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.

December 17, 2014, 21:32
#8
New Member

Join Date: Jul 2012
Posts: 25
Rep Power: 5
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 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.

 December 18, 2014, 04:50 #9 Senior Member   Join Date: Nov 2013 Posts: 677 Rep Power: 9 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"?

 Thread Tools Display Modes Linear Mode

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is OffTrackbacks are On Pingbacks are On Refbacks are On Forum Rules

 Similar Threads Thread Thread Starter Forum Replies Last Post abminternet OpenFOAM 4 July 4, 2011 11:45 mengyue1 FLUENT 0 November 26, 2009 07:40 flar.t CFX 1 December 19, 2006 00:20 Selo melo FLUENT 5 December 18, 2006 10:30 Tania FLUENT 8 March 30, 2004 18:41

All times are GMT -4. The time now is 22:38.