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Difference in result due to time step

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Old   August 23, 2010, 03:32
Default Difference in result due to time step
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prateek jain
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Hi all,

In high resolution scheme steady state, when I changing the time step results or different.
Why it is happening ?
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Old   August 23, 2010, 07:37
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Are you asking when you change the time step size in a steady state simulation why are the results different?
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Old   August 23, 2010, 23:50
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hi,

Yes change in result due to change in time step
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Old   August 24, 2010, 06:31
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It should not make a difference. If the time step gets small enough the Reynolds Averaging gets unreliable, but as long as you are larger than a certain amount it should not make any difference.
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Old   August 24, 2010, 06:47
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Thanks.
How I can adjust/ decide the time step.
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Old   August 24, 2010, 18:50
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Do the tutorials, read the documentation. It is all discussed there.
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Old   August 25, 2010, 08:04
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CCTech
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Hi

As said above

Your results should not change with change in the timestep for steady state. Please check if your simulations are well converged before comparing two results.

1 Residuals at least 10-3
2 Imbalances less than 1%
3 Monitored point is stable with respect to iterations

Thanks
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Old   August 25, 2010, 23:18
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prateek jain
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Hi,

Thanks,
All three points are achived.

But results are different.

What should I do?
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Old   August 26, 2010, 06:44
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Usually this is because your time step is too small. Your time step needs to be big enough to "average" out the turbulent fluctuations into a mean flow. To do this, your time step needs to be larger than the eddy turnover time of the largest eddies.

So, in short, increase the time step size until your result converges.
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Old   August 26, 2010, 23:26
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Hi, Thanks

Can you give me one example for that.
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Old   August 27, 2010, 06:44
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This is a general principle learnt from doing CFD for decades. I can't give you a specific example for it. Just trust me. Or even better, fiddle with the time step and see what happens - then you will prove it to yourself.
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Old   August 30, 2010, 00:03
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Thanks I will try it.
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