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Convergence problems in multiphase flow!

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Old   December 11, 2007, 10:01
Default Convergence problems in multiphase flow!
  #1
Mikael
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Hello,

Im doing simulations for a helically coiled tube containing a liquid phase along with granules in the Euler- Euler multiphase solver. The problem is that I can't get it to converge. I started out by solving the flow without the volume fraction and got the residuals down to 10e-4 and then they flatened out. After that I added the volume fraction and got the residuals to decrease some without occilations. But after a while residuals starts to occillate very much. I have tried to lower the under relaxation but it didn't help. Somebody who knows what could be the problem?

Another thing is: is it necessary to use mass flow inlet, can't one use velocity inlet instead (the fluent manual says one should use mass-flow-inlet)?

//Mikael
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Old   December 11, 2007, 13:27
Default Re: Convergence problems in multiphase flow!
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bashu
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Are you running transient? If not you probably should.
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Old   December 11, 2007, 17:18
Default Re: Convergence problems in multiphase flow!
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Mikael
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I have tried solving transient and steady without any results. Same behaviour in both cases. I can add that I'm solving laminar because I got a Re-number of around 70.
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Old   December 12, 2007, 06:00
Default Re: Convergence problems in multiphase flow!
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rana
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it is difficult to converge this kind of problems in both steady and unsteady state i feel if your residuals do not diverge, then you can accept them as acceptable results.

before that, check for your under-relaxation parameters (do not decrease them too much even) and method of discretization

keep us posted..this is a genuine problem in multiphase flow
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Old   December 12, 2007, 10:39
Default Re: Convergence problems in multiphase flow!
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Emmery
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Hi,

you can check the main variables at some strategic points in the geometry. If their values are not evolving with iterations or has a periodic evolution (transient), then it can be considered as converge. Cheers
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Old   December 13, 2007, 11:01
Default Re: Convergence problems in multiphase flow!
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John Yang
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Multiphase flow problems in FLUENT is not well solved,so how about modify your mesh file or boundary conditions and try it again? Some problems are grid-sensitive.
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Old   December 13, 2007, 15:11
Default Re: Convergence problems in multiphase flow!
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Mikael
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Thank you for the help, I have now tried to change boundary conditions between velocity inlet and mass flow inlet without any result. Changed discretization also without results. However I havn't tried to change the grid so maybe I can try that out (my grid has very low skewness thou). So far I have gotten the massbalance to match between inlet and outlet to 89% accuracy, not that god...

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