# Importance of change the Time Step Value for the Accuracy of a Transient CFD Simulation

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.

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.

• also if i have convergence on iterations for one time step ? Jun 24 '14 at 9:11
• if not why FLUENT put time variable steeping method as option on time step ? Jun 24 '14 at 9:30
• 1. No. Convergence in every time step only means that the algebraic error is small. This is independent of the approximation error.
– Jan
Jun 24 '14 at 10:28
• 2. Variable time stepping uses heuristics to estimate the error and to use larger time steps where the error is probably small anyways.
– Jan
Jun 24 '14 at 10:29
• Ok. I have boiling case , starting time the CFL=0 until boiling happen , so can I use different time step at each range ? Jun 24 '14 at 11:18