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My time remaining keep increasing

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Hi guys,  i have a problem with my model when i ran the simulation. I would like to do the Danner crash test using radioss solver. But, when i ran my simulation, the time remaining indicator kept increasing. So,  i would to know whether have i done wrong? Can someone please explain this situation to me? Please.

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Hi,

 

in explicit method the timestep is affected by characteristic element length, stiffness and density. Please go through the timestep chapter in Free eBook: Crash Analysis With RADIOSS – A Study Guide. Also recommended is 2017 Radioss user guide starting from page 30.

 

Timestep_Crash_Analysis_with_RADIOSS.pdf940.97 kB · 13 downloads

 

Review what is causing very low time step in the engine out file:

•For an element, check the related material (especially its Young modulus and density in case of an elastic-plastic material; and its viscosity in case of a visco-elastic material). There must not be an error in the units system that this data is given in. 

Check the size of the element, since elemental length is proportional to time step. If some elements are distorting badly during simulation it will lead to very low time step and the run will terminate. In this case use small strain formulation (in the property card).

•For a node, check the characteristics of connected elements. If the node is on the master side or the slave side of an interface, this interface must be verified.

•For an interface, the gap of the interface must be verified if some failure happens on the master or the slave side of the interface. Check the interfaces defined in the model for any penetrations.

 

https://altair-2.wistia.com/medias/osd349gz4k

 

To reduce solving time use mass scaling (DT/NODA) or advanced mass scaling (AMS). Any targeted model for AMS application should run first in /DT/NODA/CST with a reasonable energy error (ERROR < +2%) and acceptable added mass (MAS.ER < 0.02) along its simulation time. A model unable to run with /DT/NODA/CST will not run with AMS either. Output the number of AMS iterations per cycle via /DT/AMS/Iflag - Iflag =2 may help to monitor convergence quality at no extra CPU cost.

Maximum allowed iterations before the divergence stops is 1000. 75 to 100 iteration per cycle is a sign of a poor convergence 50 still may provide some speedup

30 iterations or less is considered a good convergence.There should be only a few cycles where INTER is controlling the timestep. 

 

Use Multi-Domain Technique if your model has different subdomains characterized by different mesh sizes and consequently very different minimal time step. 

 

Additionally, you can use single precision (Radioss panel options: -sp) to speed up computation. 

Run time remaining can increase during the run even if the timestep does not drop. This happens when available CPU power is used during the run for other tasks (use Windows task manager to check how much CPU is utilized).

 

Negative Energy Error represents energy dissipated from the system and this can be from many sources including plastic deformation and Hourglass energy. 

If you have included coefficient of friction then the energy error is due to coefficient of friction which is justified. If this amount of energy error is there when no coefficient of friction is defined, then it may be a case of penetration which is problematic. The combined energy error (contact + hourglass) should be less than 15% of the total energy. 

 

Energy error.pdf

 

The following contact settings are recommended for type 7:

Istf=4

Igap=2

Fscale_Gap=0.8

INACTI=6

Gap_min=1

Fric = 0.1

Iform=2

 

You can try increasing the Gapmin in your TYPE7 interface which will allow the contact to work sooner and prevent deep penetration. It is also possible to use  /DT/INTER/DEL in the engine file to release slave nodes from the interface when time step drops below the value given in this option /DT/INTER/DEL- however, if there are many nodes released during the run the results can be questionable.

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