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mixture model, Problem with negative volume fractions
Posted May 13, 2011, 11:39 a.m. EDT 2 Replies
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i try to model the settling of sand grains in a fluid due to gravity (suspension of sand and oil). I use the mixture model laminar flow.
The 2d-domain contains 3 noslip-walls and 1 slip-wall (upper wall). All walls are with "no dispersed phase flux" condition. The slip-velocity follows "Hadamard-Rybczinski" and the mixture viscosity with "Krieger-type".
The intial velocities are zero and the pressure ist constrained to zero at the upper corner-points.
The initial volume fraction of the dispersed phase is 0.1 all over the domain and the maximum volume fraction is 0.62.
The solution has to be time-dependent an the solver MUMPS is used. The time discretization comes from "generalized alpha".
The model is attached.
The calculation starts without problems, but after a while negative volume fractions appear. Later on, volume fractions greater as the specified maximum appear.
Now the time steps decrease rapidly and the solver stops without convergence.
How can we avoid these senseless volume fractions, maybe they are the cause of my problem?
How can we tune up the solver to get faster results?
Thanks for your help!
with best wishes
Uwe
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Hello Uwe Glaubach
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I have the exact same problem. Anybody could find a solution?
Thanks,
Marco
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Currently i'm working on mixer modeling using rotating machinery and mixture model. To put constraint on volume fraction (for example average volume fraction is 0.1 during mixing) you can use weak constraint on certain point. First you add component coupling average, than take all domain. Then make a variable, for example "phid_constr", put a value "aveop1(phid)-0.1". Then on weak constraint, use user defined, then put on constraint expression "phid_constr" and on constraint force expression "test(phid_constr". This will add auxiliarry dependent variable that have to be solved.
Regards,
Aditya