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Adaptive Time Stepping: First Update Time

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Old   January 7, 2015, 06:43
Question Adaptive Time Stepping: First Update Time
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I'm running a transient simulation from t = 0s until t = 1200 s (20 minutes).

I don't care about the first 5 or 10 seconds as the results are usually garbage - in this period the simulation is just starting up and I want to overcome the initial transient.

The adaptive time stepping scheme tries to find an ideal time step at all times. To avoid this I've set the first update time to 10 seconds, using an unusually large time step to quickly overcome the initial transient. After 10 seconds the adaptive time stepping bit kicks in converging within 5 - 10 coefficient loops (it's multiphase).

What are your thoughts on my strategy to avoid solving the initial transient?
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Old   January 7, 2015, 06:54
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As long as the poor resolution of the initial transient does not affect the later fluid flow then it sounds fine. What you describe is very common in compressible flow simulations as acoustic waves bounce around everywhere when the simulation starts up. If you don't care about this just use some big timesteps to get through it quickly.
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Old   January 7, 2015, 07:09
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Thanks. I thought this was the case.
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