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seemingly converged solution changes after restart

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Old   September 3, 2013, 10:03
Default seemingly converged solution changes after restart
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athan
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Hello:

I was wondering if anyone encountered the following, rather wierd, problem and if anyone could offer help/suggestions. I am using FLUENT.

This is a low pressure CVD problem of fiber coating with surface reactions. The solution is steady state. The 3D equation being solved are momentum, energy (with radiation) and a few species concentrations. For a few cases what I find is that when I restart a seemingly converged solution without changing anything, the steady state solution converges to a different state. Before the restart, the seemingly converged showed all traits of convergence: the weight gains had plateaued out, the residuals were small (<1e-4) and plateaued and so on. Also if I allow the seemingly converged solution to run much longer without interruption it would remain in the same (seemingly) converged state.

Clearly interrupting and restarting is doing something; as if waking the solution out of a metastable state. Has anyone encountered such a problem? What I also find is that for these cases when I restart, the residuals spike quite a bit although I am not changing anything in the restart operation. This too is surprising.

Thanks

Athan
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Old   September 3, 2013, 10:57
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Jon
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Is it restarting from scratch or does it 'blip'?

It could be something as simple as the precision it is written out in. If you are solving in double and writing out in single precision that might be your problem.

Good luck.
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