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Total pressure and mass flow boundary condition at inlet

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Old   May 6, 2013, 16:17
Default Total pressure and mass flow boundary condition at inlet
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Phillip
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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)
Attached Images
File Type: jpg diffuserRegion.jpg (16.1 KB, 54 views)
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Last edited by bscphil; May 7, 2013 at 04:22.
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Old   May 28, 2013, 00:44
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Tushar C.
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Quote:
Originally Posted by bscphil View Post
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??
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