# Total pressure and mass flow boundary condition at inlet

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May 6, 2013, 16:17
Total pressure and mass flow boundary condition at inlet
#1
New Member

Phillip
Join Date: Mar 2012
Location: Germany
Posts: 27
Rep Power: 6
Dear Foamer's,

i've some problems with my boundary conditions for a internal flow through a diffuser and the outflow in atmosphere.

For my case i must set the inlet total pressure + the inlet mass flow rate, because the diffuser is after an rotor/stator stage, which drops the total pressure to e.g. 101325 Pa + 680 Pa. The mass flow rate through the stage is constant e.g. 11.75 kg/s. The flow should be expanding to atmospheric static pressure e.g. 101325 Pa. Ok, this are the main restrictions...

My first attempt was for a incompressible RANS simulation with "simpleFoam":
• inlet:
• U: fixedValue = 96.8m/s (plug profile)
or U: flowRateInletVelocity = 11.75kg/s (plug profile)
• walls:
• U: fixedValue = 0m/s
• atmosphere:
• p: fixedValue = 101325 Pa
This attempt calculates the right mass flow rate, but the total pressure was wrong (101325 Pa + ~5500 Pa).

My second attempt was:
• inlet:
• p: totalPressure = 101325 Pa + 680 Pa
• walls:
• U: fixedValue = 0m/s
• atmosphere:
• p: fixedValue = 101325 Pa
This attempt calculates the right total pressure at inlet, but the wrong mass flow rate (~5.5 kg/s).

My third attempt was:
• inlet:
• p: totalPressure = 101325 Pa + 680 Pa
• U: flowRateInletVelocity = 11.75kg/s
• walls:
• U: fixedValue = 0m/s
• atmosphere:
• p: fixedValue = 101325 Pa
This attempt crashes after some iteration.

My fourth attempt was:
• inlet:
• p: totalPressure = 101325 Pa + 680 Pa
• U: pressureInletVelocity
• walls:
• U: fixedValue = 0m/s
• atmosphere:
• p: fixedValue = 101325 Pa
This calculates very high mass flow rates.

Summary: i could not find out which boundary conditions are exactly right to set total pressure and mass flow rate (or velocity) at the inlet.

I hope somebody with some experience in a similar problem can help me, thanks and sorry for my poor english.

(the attached picture is a overview of the region)
Attached Images
 diffuserRegion.jpg (16.1 KB, 100 views)

Last edited by bscphil; May 7, 2013 at 04:22.

May 28, 2013, 00:44
#2
Senior Member

Tushar Chourushi
Join Date: Jul 2009
Location: IIT-Indore, India
Posts: 319
Blog Entries: 1
Rep Power: 10
Quote:
 Originally Posted by bscphil Dear Foamer's, i've some problems with my boundary conditions for a internal flow through a diffuser and the outflow in atmosphere. For my case i must set the inlet total pressure + the inlet mass flow rate, because the diffuser is after an rotor/stator stage, which drops the total pressure to e.g. 101325 Pa + 680 Pa. The mass flow rate through the stage is constant e.g. 11.75 kg/s. The flow should be expanding to atmospheric static pressure e.g. 101325 Pa. Ok, this are the main restrictions... My first attempt was for a incompressible RANS simulation with "simpleFoam": inlet:p: zeroGradient U: fixedValue = 96.8m/s (plug profile) or U: flowRateInletVelocity = 11.75kg/s (plug profile) walls:p: zeroGradient U: fixedValue = 0m/s atmosphere:p: fixedValue = 101325 Pa U: zeroGradient This attempt calculates the right mass flow rate, but the total pressure was wrong (101325 Pa + ~5500 Pa). My second attempt was: inlet:p: totalPressure = 101325 Pa + 680 Pa U: zeroGradient walls:p: zeroGradient U: fixedValue = 0m/s atmosphere:p: fixedValue = 101325 Pa U: zeroGradient This attempt calculates the right total pressure at inlet, but the wrong mass flow rate (~5.5 kg/s). My third attempt was: inlet:p: totalPressure = 101325 Pa + 680 Pa U: flowRateInletVelocity = 11.75kg/s walls:p: zeroGradient U: fixedValue = 0m/s atmosphere:p: fixedValue = 101325 Pa U: zeroGradient This attempt crashes after some iteration. My fourth attempt was: inlet:p: totalPressure = 101325 Pa + 680 Pa U: pressureInletVelocity walls:p: zeroGradient U: fixedValue = 0m/s atmosphere:p: fixedValue = 101325 Pa U: zeroGradient This calculates very high mass flow rates. Summary: i could not find out which boundary conditions are exactly right to set total pressure and mass flow rate (or velocity) at the inlet. I hope somebody with some experience in a similar problem can help me, thanks and sorry for my poor english. (the attached picture is a overview of the region)

Good one... first attempt seems to be the best practice in case of diffusing problems, but don't know how come your Total pressure is varying a lot.. Are you able to reach convergence??

 April 19, 2015, 07:56 change flow rate to static pressure #3 New Member   Ali hassan Join Date: Apr 2015 Posts: 2 Rep Power: 0 Hello everyone, I am doing validation for previous article about CNG mixer, my problem is using outlet static pressure instead of out flow because they ANSYS fluent don't allow to use out flow if I use inlet pressure, so, is there any one can help me to get static pressure from mass flow rate? Thanks

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