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Old   September 21, 2015, 21:43
Default transient in the fluent
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Dear friends!

When I was doing analysis using ANSYS fluent doing transient analysis.I have some problems.
1.When I use fix time step ,if I set a big Time step size,the result will not converged in a time step and then change to the next time step .What's the influence? If I set a small time step size ,the result will converged in every time step size but it will spend a lot of time .
2.When I use adaptive time step ,the fluent will change the time step size .If the result is converged in this time step size ,it will change the time step size in next time step size, the next time step will not converged .How did this influence the results?

Thank you in advance!
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Old   September 22, 2015, 01:05
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In general, you get faster convergence with smaller time-steps because the solution changes less. You need to find a balance between time-step size and number of time-step which minimizes the overall computational time. However, generally people adjust the time-step so that convergence is achieved in <20 iterations or so (or <10).

With adaptive time stepping, you need to specify suitable minimum and maximum time-step sizes. You can also limit the maximum change in the time-step by adjusting the step change factor.

Even with adaptive time-stepping, it's still helpful to have the first few iterations performed with small time-steps so the option to include a number of fixed time steps can also help.
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