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Mismatch inlet and outlet flow rates

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Old   December 18, 2017, 04:36
Default Mismatch inlet and outlet flow rates
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Hello all,

I am currently stuck in a problem with Fluent (18.2) modelisation.

I have a simple pipe in which some water is flowing. I used SST model because I want to study transitional regimes. It is a steady/pressure-based/absolute velocity formulation case. No temperature/energy effects are taken here. Operating conditions: pressure=1 atm.

The solver method is coupled with Courant number of 10 (mesh ~5E-4)

I defined a mass flow inlet:
- Reference frame Absolute
- Mass flow rate 0.001 Kg/s
- Supersonic/Initial Gauge Pressure 0 Pa
- Direction specification method: Normal to Boundary
- Turbulence: Intermittency 1, Intensity 5%, Viscosity ratio 10

A pressure outlet:
- Backflow reference frame: absolute
- Gauge pressure 101325 Pa
- Pressure profile multiplier
- Backflow direction specification method: normal to boundary
- Backflow pressure specification: static pressure
- Turbulence: Intermittency 1, Intensity 5%, Viscosity ratio 10

No slip condition walls

A symmetry (taken into account the hald of pipe)

I would say that the calculation converges well and it reach residuals @ 1E-6 smoothly.

When I check the flow rate however I have a correct inlet mass flow @ 0.001 Kg/s but an unexpected outlet flow rate @ 0.00125 Kg/s ( greater velocities at the outlet)

Have you any ideas of why these two values of mass flow mismatch ? I have no hits of it!

Last edited by kiodos; December 18, 2017 at 05:52.
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Old   December 18, 2017, 15:20
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Not sure if this would fix it or not, but for your pressure outlet you said that the gauge pressure is 101325 Pa. Should it be 0 Pa? Other than that I've got no clue.
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Old   December 19, 2017, 09:05
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Yep already set 0 Pa at the outlet because working conditions are 1 atm...
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