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Time Step

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i having problem in time step.

i checked the minimum time step in the model that comes around 1.934e-03. 

when i run the model in radioss with out mass scaling it takes the DT= 0.1448E-02.

but usually the minimum time step * 0.9 will be taken in radioss as initial time step, when we calculate that 0.001934 * 0.9 = 0.0017406

what is reason behind this

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thanks ivan 


i tried with time step DT/NODA/CST 0.144E-02 i am having mass error and energy error as follow 



RESTART FILES: 2D_SHELL_0001_[0001-0001].rst WRITTEN
 NC=  605000 T= 8.7604E+02 DT= 1.4481E-03 ERR=-60.0% DM/M= 3.2230E-02
 ELAPSED TIME=       1579.40 s  REMAINING TIME=     304910.89 s
 NC=  605500 T= 8.7676E+02 DT= 1.4481E-03 ERR=-60.0% DM/M= 3.2230E-02
 ELAPSED TIME=       1580.85 s  REMAINING TIME=     304938.64 s
 NC=  606000 T= 8.7749E+02 DT= 1.4481E-03 ERR=-60.2% DM/M= 3.2230E-02
 ELAPSED TIME=       1582.20 s  REMAINING TIME=     304945.54 s
 NC=  606500 T= 8.7821E+02 DT= 1.4481E-03 ERR=-60.3% DM/M= 3.2230E-02
 ELAPSED TIME=       1583.49 s  REMAINING TIME=     304941.06 s
 NC=  607000 T= 8.7894E+02 DT= 1.4481E-03 ERR=-60.4% DM/M= 3.2230E-02
 ELAPSED TIME=       1584.80 s  REMAINING TIME=     304941.00 s
 NC=  607500 T= 8.7966E+02 DT= 1.4481E-03 ERR=-60.5% DM/M= 3.2230E-02
 ELAPSED TIME=       1586.16 s  REMAINING TIME=     304950.37 s
 NC=  608000 T= 8.8038E+02 DT= 1.4481E-03 ERR=-60.6% DM/M= 3.2230E-02
 ELAPSED TIME=       1587.61 s  REMAINING TIME=     304976.63 s
 NC=  608500 T= 8.8111E+02 DT= 1.4481E-03 ERR=-60.7% DM/M= 3.2230E-02
 ELAPSED TIME=       1588.93 s  REMAINING TIME=     304978.07 s
 NC=  609000 T= 8.8183E+02 DT= 1.4481E-03 ERR=-60.8% DM/M= 3.2230E-02
 ELAPSED TIME=       1590.30 s  REMAINING TIME=     304988.53 s

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Large contact energies relative to total energy can cause large negative Energy Errors because contact energy is not part of the Energy Error equation. If the simulation has friction and a lot of sliding contact, then the large contact energy and resulting energy error can be considered acceptable.


Your model without friction had reasonable energy error.




Step loading functions can cause divergence in implicit or dynamic effects in explicit.

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The input deck is attached.


Note the AMS settings used (0.67 scale factor and 2e-3 timestep) are not ideal. First, run with DT/NODA mass scaling until you get reasonable negative energy without friction (15%) and mass error (2%) throughout the simulation. Then set up the AMS run trying to hit the energy error % of DT/NODA run (as well as stress and displacements). 


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