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Old   November 10, 2011, 07:26
Default biphasic model
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Hi, this is my 2 thread, I hope you can help me again.
I'm working on an anaerobic digester. I have to make a simulation with a biphasic model where are entering the 87% of sludge and the 13% of air from the same inlets. the inlets are 2 and they are on the bottom of the digester, the outlets are on the top and are 2 as well. I tried to make 2 simulation, one with VOF and another with multiphase segregated flow but both of them are diverging. Maybe I put the wrong boundary conditions, i tried to put the inlets as flow-split or velocity inlets, for the outlet flow-split or pressure outlets. I try to follow the tutorial of the Star manual setting the same models and parameters, obviously linked with my problem, but no convergence. If someone has got a suggestion please don't be shy Thanks
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Old   November 10, 2011, 09:43
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I havent much experience with multiphase problems but i would suggest that you should read the section Modeling Multiphase Flow in the user guide. Especially about Phase Interactions and Eulerian Multiphase Model.

Maybe anyone else can give you more detailed help.

Have fun
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Old   November 11, 2011, 14:51
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The most important question is: How do you judge convergence or divergence? A lot people are only looking on residuals and forget that residuals in Star-CCM+ are normalized.

For the "multiphase segregated flow" simulation (I think, it is an Eulerian multiphase, right?), consider that it tends to diverge when running turbulent. When you're running turbulent while your flow actually is laminar, switch of the turbulence model.
When it's turbulent, reduce under-relaxation factors to very low values, reduce time step size etc.
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Old   December 17, 2011, 15:22
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Another tip when running turbulent segregated multiphase is to first run until convergence using the laminar model and then turn on turbulence.
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