Adaptive Time Stepping: First Update Time

JuPa · January 7, 2015, 06:43

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?

ghorrocks · January 7, 2015, 06:54

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.

JuPa · January 7, 2015, 07:09

Thanks. I thought this was the case.

January 7, 2015, 06:43	Adaptive Time Stepping: First Update Time	#1
JuPa Senior Member Mr CFD Join Date: Jun 2012 Location: Britain Posts: 361 Rep Power: 15	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?

January 7, 2015, 06:54		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	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. JuPa likes this.

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January 7, 2015, 07:09		#3
JuPa Senior Member Mr CFD Join Date: Jun 2012 Location: Britain Posts: 361 Rep Power: 15	Thanks. I thought this was the case.