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number of iteration in fluent

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Old   December 22, 2018, 09:05
Default number of iteration in fluent
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Weiqiang Liu
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hello, all
I am doing a catalytic combustion of methane with fluent 17.0. The whole reaction set includes 340 gas phase reactions and 24 surface reactions.

the computation domain is two dimensional with 40000 cells. The case is steady state.

Now I am computing with 100000 iterations. the convergence standard is set as follows:
energy residual below 1e-6 and other residuals below 1e-3.

after 100000 iterations with 3 days of computing, all residual standards seemed reached the predefined value and some key parameters like temperature field, surface coverage of pt(s) are reasonable and continuous. however some other indexes like surface reaction rate of methane on the wall are discontinuous. I am wondering is the discontinuity due to having not converged.

can anybody give me some advice on how many iterations actually do I need on these similar cases according to your expertise.

thanks very much!

weiqiang
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Old   January 2, 2019, 09:41
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I don't think that your number of iterations is the problem. 100000 iterations should be more than enough for this calculation.

A lot of parameters like the combustion model, turbulence model, URFs, solver settings or the mesh can affect the convergence.
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Old   January 2, 2019, 11:09
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Quote:
Originally Posted by sufjanst View Post
I don't think that your number of iterations is the problem. 100000 iterations should be more than enough for this calculation.

A lot of parameters like the combustion model, turbulence model, URFs, solver settings or the mesh can affect the convergence.


now Iíve had 400000 iterations already. I patched a 2000K temperature to the whole computation domain. The maximum temperature after 400000 iterations is 1260k. I use species transport model with laminar finite rate.

Now the residuals are still oscillating. The default results for convergence are 1e-6 for energy and 1e-3 for others. I found itís not small enough. How do you usually judge convergence according to residuals regarding to chemical flow simulation.

Thanks
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