Choosing the apt time step size for a transient run

saisanthoshm88 · March 31, 2012, 13:52

I'm running a transient simulation and I see that some time steps are not converging with in the specified number of coefficient loops in CFX.

The number of coefficient loops specified is already 20 per time step, so I'd like to reduce the time step size now instead of increasing the number of coefficient loops per time step.

Is the time step size in any way related to the advection time in the .out file ?

If so, when I have the main domain and also some sub domains in the analysis so which advection time should be taken into consideration to decide the time step size?

Could some one please suggest me on this.

ghorrocks · March 31, 2012, 16:11

You should set the time step size by a sensitivity analysis. A good way to quickly converge on the correct time step is to use adaptive time steps, set to home in on 3-5 coeff loops per time step. This will quickly find the correct time step.

March 31, 2012, 13:52	Choosing the apt time step size for a transient run	#1
saisanthoshm88 Senior Member --------- Join Date: Oct 2010 Posts: 303 Rep Power: 17	I'm running a transient simulation and I see that some time steps are not converging with in the specified number of coefficient loops in CFX. The number of coefficient loops specified is already 20 per time step, so I'd like to reduce the time step size now instead of increasing the number of coefficient loops per time step. Is the time step size in any way related to the advection time in the .out file ? If so, when I have the main domain and also some sub domains in the analysis so which advection time should be taken into consideration to decide the time step size? Could some one please suggest me on this. __________________ Best regards, Santhosh.

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March 31, 2012, 16:11		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	You should set the time step size by a sensitivity analysis. A good way to quickly converge on the correct time step is to use adaptive time steps, set to home in on 3-5 coeff loops per time step. This will quickly find the correct time step.