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results depends on time step

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Old   November 5, 2015, 09:19
Exclamation results depends on time step
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Mieszko Młody
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Dear Foamers,
I encountered a worrying problem , the results depend on the time step,
even if the Cu number is much smaller than 1. It happens in sonicFoam and reactingFoam.
e.g. I had significantly different results changing time step from 1e-5 to 2e-5.
(Cu number ~ 0.15 and 0.3 respectively).

Any thoughts, comments ?

thx
ZMM
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Old   November 5, 2015, 10:31
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Half your time step again and check if you see any convergence. There is nothing particularily worrying about the time step dependence of your simulation. Co<1 is a guideline for stability, not accuracy.
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Old   November 5, 2015, 10:31
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It's quite possible that you still have numerical errors with these time step sizes. To rule this out, successively decrease the time step size until the results don't change anymore.
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Old   November 5, 2015, 11:12
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Yes,
that is true,
but I observed as well, that calculations with smaller time steps gets unstable.
e.g.
having some reasonably changes in the results with decreasing time step, there is a min time step below which the calculations gets numerically unstable...
which seems to be counterintuitive, because there should be a minimal time step below which there is no further changes in the results.

thanks for your reply
ZMM
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Old   November 6, 2015, 10:17
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Have you tried refining your mesh? If your mesh is too coarse you may have to refine it until, again, you don't get any different results. Then you may get stable and accurate solutions for larger timesteps.
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