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-   -   On the time step size. (https://www.cfd-online.com/Forums/fluent/36223-time-step-size.html)

mini March 31, 2005 22:49

On the time step size.
 
Hello everyone: During the unsteady computation, can the increased time step size affect the results and postprocessing? If I set the time step size 1e-05 initially and 1e-04 finally , are the results reasonable?

debs April 1, 2005 03:35

Re: On the time step size.
 
What is your numerical Scheme ? If the scheme is implicit the difference should not be so much . In an explicit scheme the difference can be considerable even for simple problems.


ap April 1, 2005 08:42

Re: On the time step size.
 
Right. Also consider the physical time scale of the system you're modeling. You should use a time step smaller than it.

Best regards, ap

khalid sultan April 3, 2005 07:46

Re: On the time step size.
 
Hi mini;

The time step - according to my knowledge - is dependent on: 1- the cell length. 2- the {|u+a|} term which express the maximum speed with which the information can travel. 3-and the CFL. If the cell has del(x) != del(y), you can take the smaller to compute the time step, although there are some other tricks that could be used.

Having this said, the time step should be calculated and not given. Of course if you give a little bit smaller time step than that computed, your solver will work. this is equvalent to decreasing the CFL, but you will increase; meanninglessly, your executaion time. The same arguement apply -to some extent- for a little bit increase.

Hoping it will help. cheers ;-)


ap April 3, 2005 10:27

Re: On the time step size.
 
This is true if you use an explicit method, while if you use a fully implicit one, the method is stable for each value of the time step.

However, this doesn't mean you can use a time step as big as you like. Obviously you still have the physical constraints on your dt to obtain an accurate solution.

ap

mini April 3, 2005 21:29

Re: On the time step size.
 
implicit scheme, thanks.

mini April 3, 2005 21:32

Re: On the time step size.
 
Yes. Thanks.


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