I have a transient simulation for a case from 0 to 3 sec , actually i interest the solutions on time range from 2 to 3 sec , as my velocity and mesh size , my time step should be 0.0000000001 sec , my question is can I make my time step is 0.0001 sec for time range 0 to 1.9 and the i use 0.0000000001 for time range from 1.9 to 4 sec ( the range i interested ) ? BTW I use ANSYS FLUENT as a commercial CFD software.
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Certainly not. Since the approximation error is simply accumulating during the time integration.
You can never compensate the errors you made earlier.
Thus, in terms of accuracy, this is not a good idea.
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$\begingroup$ also if i have convergence on iterations for one time step ? $\endgroup$ Jun 24, 2014 at 9:11
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$\begingroup$ if not why FLUENT put time variable steeping method as option on time step ? $\endgroup$ Jun 24, 2014 at 9:30
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$\begingroup$ 1. No. Convergence in every time step only means that the algebraic error is small. This is independent of the approximation error. $\endgroup$– JanJun 24, 2014 at 10:28
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$\begingroup$ 2. Variable time stepping uses heuristics to estimate the error and to use larger time steps where the error is probably small anyways. $\endgroup$– JanJun 24, 2014 at 10:29
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$\begingroup$ Ok. I have boiling case , starting time the CFL=0 until boiling happen , so can I use different time step at each range ? $\endgroup$ Jun 24, 2014 at 11:18