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Old   November 21, 2011, 14:45
Default Mass Flow Problem
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Good evening

I am creating mass in my model, and I get very high velocities in the air phase. Anybody that have an idea where the mistake/s could be?

Geometry data: Length of model = 10m, width=1m,
Inlet area and outlet area = 1.25m^2, height step =0,25m.

The geometry is set to be 70% filled with water.

Set-up:

- Pressure-based, velocity formulation absolute, transient, gravity=-9.81
- VOF, implicit, open channel flow, implicit body force,
- k-epsilon, relizable, standard wall functions
- Primary phase = air (constant density) and secondary phase = water
- Inlet boundary: Multiphase, free surface level -2.14,bottom level -3.015.
Air, mass flow rate = 0.045 kg/s
Water, mass flow rate = 87,5 kg/s
- Outlet boundary: Outflow, flow rate weighting = 1
- Reference values: v=0.1m/s
- Solution method: PISO

Any help would be appreciated
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Old   November 22, 2011, 12:51
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Hi,
What's about your residuals ? Did you achieve convergence ?
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Old   November 22, 2011, 15:07
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Quote:
Originally Posted by laurentb View Post
Hi,
What's about your residuals ? Did you achieve convergence ?
After 800 iterations it has still not converged. The residual for the continuity is 10^-2.

Under relaxation factors was set up:
Pressure = 0.3
Density = 1
Body Forces = 1
Momentum = 0.5
Volume Fraction = 0.5
Turbulent Kinetic Energy = 0.8
Turbulent Viscosity =1

Thank you for trying to help me
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Old   November 23, 2011, 04:08
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You must obtain convergence before checking mass imbalance.
Try to iterate more until you obtain convergence
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Old   November 23, 2011, 04:44
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Quote:
Originally Posted by laurentb View Post
You must obtain convergence before checking mass imbalance.
Try to iterate more until you obtain convergence
Thanks. I will do some more iteration to try to get convergence, but do you think the under relaxation factors seems reasonable? Is it corect to set this up as a transient problem, or should it have been steady state?
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Old   November 23, 2011, 04:57
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The choice between steady and transient depends on what you want to do, what's the aim of your work ? don't matter about under relaxation factors before you are not sure of what you want to do exactly...
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Old   November 23, 2011, 08:48
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I want to see the effect the step has on the flow pattern.
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Old   November 23, 2011, 09:21
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try to use steady state formulation
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Old   November 24, 2011, 02:13
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If it's an open channel flow, the top wall of your model should have a pressure BC of atmosphere
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Old   November 24, 2011, 04:54
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If it's an open channel flow, the top wall of your model should have a pressure BC of atmosphere
Thank you. That sounds like a good advise
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