Learning Forum

[king79]

Hi All,

I am having some difficulty with interval outputs using NODOUT. My simulation time is 0.125 seconds and I requested output results for every 1.25e-3 seconds and the termination time is 0.125. However, Ls Prepost does not report the results at exactly 1.25e-3 time interval. It only provides an approximate interval of 1.22e-3 seconds. Is there a fix for this kind of problem?

Thanks

[Reno Genest]

Hello @king79
The output interval should be the same or very close to what you specify in *DATABASE_NODOUT. You may see 1.25e-3 or 1.249999e-3 in LSPP. What *DATABASE_NODOUT command do you use?

Reno.

[Imtiaz Gandikota]

I suggest writing ascii output (*DATABASE_NODOUT, binary=1) and check the interval. In LSPP, instead of looking at the plot, save the plot to a file and compare the interval with ls-dyna nodout ascii file. You can also use interpolate option to specify more points while saving curves to a file using LSPP.

[king79]

I have implemented your suggestions but LSPP results from the NODOUT using ascii output does not produce the requested time intervals. it seems the only way out is to interpolate which is strange.
Here is a breakdown of my input:
*Control_Implicit_Geneneral: DT0= 1.25e-3 (all other options are default)
*Control_Termination: Endtim =0.125 (all other options are default)
*Database_D3Plot: DT=1.25e-3 (all other options are default)
*Database_Nodout: DT=1.25e-3, binary = 1 (all other options are default)

[Reno Genest]

Are you using constant time step? Or automatic time step? You specify the initial time step DT0=1.25e-3, but if you use automatic time step, the following time steps may be larger depending on convergence. Make sure you set IAUTO=0 on *CONTROL_IMPLICIT_AUTO to use constant time steps.

Reno.

[king79]

I am using constant time-step and IAUTO=0 per default value.

[king79]

Do I need to select *Control_implicit_auto in order to activate constant time-step?

[Reno Genest]

The default should be IAUTO=0 when the keyword *CONTROL_IMPLICIT_AUTO is not included in the input file. But, you can try to include the keyword just to make sure.

Reno.

[Reno Genest]

You can also follow the solution progression in the messag file or the d3hsp file and make sure the time step the LS-DYNA solver takes is constant.

Reno.

[king79]

Ls Dyna is solving it at a constant time internal for majority of the simulation; however, it is using a different time-step from what I requested.

[Reno Genest]

If you run the following model, does it use a constant timestep? You can set DT0=0.1 and run:
On my machine, the above model runs with a constant timestep.

Reno.

[king79]

I was able to re-run my original model with the time step that Ls Dyna was suggesting and obtained a constant time interval in the results which was consistent with the constant-time step. It appears ls dyna uses a constant time-step that would guarantee convergence. I will try to run your example model later to see for I obtain similar results. Thank you.