[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)