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August 23, 2010, 03:32 |
Difference in result due to time step
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#1 |
New Member
prateek jain
Join Date: Aug 2010
Posts: 8
Rep Power: 15 |
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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August 23, 2010, 07:37 |
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#2 |
Super Moderator
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,701
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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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August 23, 2010, 23:50 |
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#3 |
New Member
prateek jain
Join Date: Aug 2010
Posts: 8
Rep Power: 15 |
hi,
Yes change in result due to change in time step |
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August 24, 2010, 06:31 |
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#4 |
Super Moderator
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
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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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August 24, 2010, 06:47 |
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#5 |
New Member
prateek jain
Join Date: Aug 2010
Posts: 8
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Thanks.
How I can adjust/ decide the time step. |
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August 24, 2010, 18:50 |
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#6 |
Super Moderator
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
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Do the tutorials, read the documentation. It is all discussed there.
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August 25, 2010, 08:04 |
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#7 |
New Member
CCTech
Join Date: Jul 2010
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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 CCTech Pune |
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August 25, 2010, 23:18 |
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#8 |
New Member
prateek jain
Join Date: Aug 2010
Posts: 8
Rep Power: 15 |
Hi,
Thanks, All three points are achived. But results are different. What should I do? |
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August 26, 2010, 06:44 |
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#9 |
Super Moderator
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
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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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August 26, 2010, 23:26 |
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#10 |
New Member
prateek jain
Join Date: Aug 2010
Posts: 8
Rep Power: 15 |
Hi, Thanks
Can you give me one example for that. |
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August 27, 2010, 06:44 |
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#11 |
Super Moderator
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,701
Rep Power: 143 |
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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August 30, 2010, 00:03 |
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#12 |
New Member
prateek jain
Join Date: Aug 2010
Posts: 8
Rep Power: 15 |
Thanks I will try it.
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