Difference in result due to time step

prateek · August 23, 2010, 03:32

Hi all,

In high resolution scheme steady state, when I changing the time step results or different.
Why it is happening ?

ghorrocks · August 23, 2010, 07:37

Are you asking when you change the time step size in a steady state simulation why are the results different?

prateek · August 23, 2010, 23:50

hi,

Yes change in result due to change in time step

ghorrocks · August 24, 2010, 06:31

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.

prateek · August 24, 2010, 06:47

Thanks.
How I can adjust/ decide the time step.

ghorrocks · August 24, 2010, 18:50

Do the tutorials, read the documentation. It is all discussed there.

CCTech_Pune · August 25, 2010, 08:04

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

prateek · August 25, 2010, 23:18

Hi,

Thanks,
All three points are achived.

But results are different.

What should I do?

ghorrocks · August 26, 2010, 06:44

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.

prateek · August 26, 2010, 23:26

Hi, Thanks

Can you give me one example for that.

ghorrocks · August 27, 2010, 06:44

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.

prateek · August 30, 2010, 00:03

Thanks I will try it.

August 23, 2010, 03:32	Difference in result due to time step	#1
prateek 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		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,692 Rep Power: 143	Are you asking when you change the time step size in a steady state simulation why are the results different?

August 23, 2010, 23:50		#3
prateek New Member prateek jain Join Date: Aug 2010 Posts: 8 Rep Power: 15	hi, Yes change in result due to change in time step

August 24, 2010, 06:31		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,692 Rep Power: 143	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.

August 24, 2010, 06:47		#5
prateek New Member prateek jain Join Date: Aug 2010 Posts: 8 Rep Power: 15	Thanks. How I can adjust/ decide the time step.

August 24, 2010, 18:50		#6
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,692 Rep Power: 143	Do the tutorials, read the documentation. It is all discussed there.

August 25, 2010, 08:04		#7
CCTech_Pune New Member CCTech Join Date: Jul 2010 Posts: 20 Rep Power: 15	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

August 25, 2010, 23:18		#8
prateek 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?

August 26, 2010, 06:44		#9
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,692 Rep Power: 143	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.

August 26, 2010, 23:26		#10
prateek New Member prateek jain Join Date: Aug 2010 Posts: 8 Rep Power: 15	Hi, Thanks Can you give me one example for that.

August 27, 2010, 06:44		#11
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,692 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.

August 30, 2010, 00:03		#12
prateek New Member prateek jain Join Date: Aug 2010 Posts: 8 Rep Power: 15	Thanks I will try it.