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. Setup:  Pressurebased, velocity formulation absolute, transient, gravity=9.81  VOF, implicit, open channel flow, implicit body force,  kepsilon, 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:) 
Hi,
What's about your residuals ? Did you achieve convergence ? 
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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 ;) 
You must obtain convergence before checking mass imbalance.
Try to iterate more until you obtain convergence 
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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...

I want to see the effect the step has on the flow pattern.

try to use steady state formulation

If it's an open channel flow, the top wall of your model should have a pressure BC of atmosphere

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