Adaptive Time Stepping: First Update Time

JuPa · January 7, 2015, 05:43

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, 05:54

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, 06:09

Thanks. I thought this was the case.

January 7, 2015, 05:43	Adaptive Time Stepping: First Update Time	#1
JuPa Senior Member Mr CFD Join Date: Jun 2012 Location: Britain Posts: 361 Rep Power: 14	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, 05:54		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	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.

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Extrusion with OpenFoam problem No. Iterations 0	Lord Kelvin	OpenFOAM Running, Solving & CFD	8	March 28, 2016 11:08
Unstabil Simulation with chtMultiRegionFoam	mbay101	OpenFOAM Running, Solving & CFD	13	December 28, 2013 13:12
number of iterations per time step	chouki	FLUENT	1	August 13, 2013 00:11
same geometry,structured and unstructured mesh,different behaviour.	sharonyue	OpenFOAM Running, Solving & CFD	13	January 2, 2013 22:40
Orifice Plate with a fully developed flow - Problems with convergence	jonmec	OpenFOAM Running, Solving & CFD	3	July 28, 2011 05:24

January 7, 2015, 06:09		#3
JuPa Senior Member Mr CFD Join Date: Jun 2012 Location: Britain Posts: 361 Rep Power: 14	Thanks. I thought this was the case.