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Continuity convergence problem

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Old   December 13, 2010, 16:54
Default Continuity convergence problem
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Ankur gupta
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Hello,

I am using a 2-phase, euler model to model laminar mixing flow across static mixers.
I am using a uniform tet mesh. Using 3-D,double precision

My continuity does not seem to converge. Residual is around 1.0e-03.

I have tried -:
1) increasing mesh density
2) increasing downstream length
3) changing the under-relaxation factor, it helps only a little.

When I use two similar fluids (of same viscosity) the convergence is still better, around 2.0e-05. But when the fluids are of extremely different viscosity (ratio 1:1000), the convergence is 1.0e-03

Can somebody please help me. I really need help with this

Thanks
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Old   December 13, 2010, 19:36
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2e-5 seems reasonable, although often the best way to determine convergence is to look at the flow variables (mass flow, for instance) and see the total imbalance in the system.

If the mass in is approximately equal to the mass out, the pressures are behaving as you expect, etc., you might consider the solution converged, whether or not it reaches some arbitrarily low value for residuals.

Also, how are you refining the mesh? If you're refining in Fluent and you have a number of highly skewed cells, Fluent won't really decrease the skewness. It's better to return to design modeler or gambit and simply improve the mesh that way.
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Old   December 13, 2010, 20:23
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hello,

thanks for your reply..

2.0e-5 is coming when the viscosities of the fluids are same..

and 1.0e-3 is coming when the viscosities are very different..(1000 times)

i am refining skewness from fluent..

but do you think creating a dense mesh near the high gradient region will help ??
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