Help for setting varying timesteps

darookie · September 10, 2009, 04:46

The simple solution is always hard to get. After having searched in the forum for a good solution to set varying timesteps with an CEL expression without sucess, here is my way to obtain:
first 300 iterations -> 0.7s
next 300 iterations -> 0.01s

CEL Expression:
0.7[s]-0.69*step(citern-301)[s]

I hope I can help somebody who has the same task.
andrej

ghorrocks · September 10, 2009, 06:32

You could also have done it with a 1D interpolation function. Same end effect, but possibly easier to understand.

darookie · September 11, 2009, 05:08

I wanted to change the time in a jump function. But is a linear change better in regard to convergence?

ghorrocks · September 11, 2009, 07:32

You can define step functions in the 1D interpolation function. The following series of points define a step function good enough for most people:
(0,0),
(0.999999999999,0),
(1,1),
(2,1)

Yes, a sudden change in time step size can cause convergence issues. It will cause accuracy issues as well. Ramping the time step change will help both problems.

September 10, 2009, 04:46	Help for setting varying timesteps	#1
darookie New Member Andrej Hoyningen Join Date: Jul 2009 Posts: 22 Rep Power: 16	The simple solution is always hard to get. After having searched in the forum for a good solution to set varying timesteps with an CEL expression without sucess, here is my way to obtain: first 300 iterations -> 0.7s next 300 iterations -> 0.01s CEL Expression: 0.7[s]-0.69*step(citern-301)[s] I hope I can help somebody who has the same task. andrej

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September 10, 2009, 06:32		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	You could also have done it with a 1D interpolation function. Same end effect, but possibly easier to understand.

September 11, 2009, 05:08		#3
darookie New Member Andrej Hoyningen Join Date: Jul 2009 Posts: 22 Rep Power: 16	I wanted to change the time in a jump function. But is a linear change better in regard to convergence?

September 11, 2009, 07:32		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	You can define step functions in the 1D interpolation function. The following series of points define a step function good enough for most people: (0,0), (0.999999999999,0), (1,1), (2,1) Yes, a sudden change in time step size can cause convergence issues. It will cause accuracy issues as well. Ramping the time step change will help both problems.