Week - 8 Mass Scaling

MASS SCALING

OBJECTIVES

                In this challenge, we have to Use mass scaling to reduce the runtime of a model and ensure the stability of mass scaling. The hard limit on mass scaling is 8% and stability should be completely intact. This means that two simulations, one with mass scaling and one without should behave the same way. The submission should contain the trials for mass scaling and the data generated henceforth. Both the DT and TSSFAC parameters should be optimized. 

            Then the result is compared with a Histogram chart and learn how to reduce the stimulation time in Ls-Dyna.

MASS SCALING

Mass-scaling refers to a technique whereby nonphysical mass is added to a structure to achieve a larger explicit timestep.

Anytime you add nonphysical mass to increase the timestep in a dynamic analysis, you affect the results (think of F=m*a). Sometimes the effect is insignificant and in those cases adding nonphysical mass is justifiable. Examples of such cases may include the addition of mass to just a few small elements in a noncritical area or quasi-static simulations where the velocity is low and the kinetic energy is very small relative to the peak internal energy. In the end, it's up to the judgment of analysts to gauge the effect of mass scaling. You may have to reduce or eliminate mass scaling in a second run to gauge the sensitivity of the results to the amount of mass added.

One can selectively employ mass scaling by artificially increasing the material density of the parts you want to mass-scale. This manual form of mass scaling is done independently of the automatic mass scaling invoked with DT2MS in *CONTROL_TIMESTEP.

PROCEDURE

Import the given .k file into Ls-prepost.

First, the stimulation is run without any changes in the model in the Ls-Dyna solver and the stimulation time is noted as 37hr 50min i.e 136221s.

Then we have to include the control Time step card then change the TSSFAC and DT2MS value.

TRAIL 1:

In the first trial we have to keep the value of DT2MS and TSSFAC value as default where 0.9 and -3.5E-4 respectively, the simulation run time will be 51hr 56min.

TRAIL 2:

In this trial we have to keep the value of DT2MS and TSSFAC value 0.9 and -5.5E-4 respectively, the simulation run time will be 42hr 32min.

TRAIL 3:

In this trial we have to keep the value of DT2MS and TSSFAC value 0.9 and -7.5E-4 respectively, the simulation run time will be 28hr 57min.

TRAIL 4:

In this trial we have to keep the value of DT2MS and TSSFAC value 0.9 and -9.5E-4 respectively, the simulation run time will be 21hr 58min.

TRAIL 5:

In this trial, we have to keep the value of DT2MS and TSSFAC value 0.9 and -1.05E-3 respectively, the simulation run time will be 30hr 47min.

TRAIL 6:

In this trial, we have to keep the value of DT2MS and TSSFAC value 0.9 and -1.03E-3 respectively, the simulation run time will be 21hr 48min.

TRAIL 7:

In this trial we have to keep the value of DT2MS and TSSFAC value 0.9 and -1.029E-3 respectively, the simulation run time will be 26hr 0min.

TRAIL 8:

In this trial we have to keep the value of DT2MS and TSSFAC value 0.9 and -1.028E-3 respectively, the simulation run time will be 21hr 37min.

The above table and histogram represent the difference in the percentage of mass addition concerning DT2MS value, decreasing the value of DT2MS reduce the time step.   

Now we keep the DT2MS value constant and change the TSSFAC value

TRAIL 9:

In this trial we have to keep the value of DT2MS and TSSFAC value 0.8 and -1.028E-3 respectively, the simulation run time will be 23hr 44min.

TRAIL 10:

In this trial we have to keep the value of DT2MS and TSSFAC value 0.7 and -1.028E-3 respectively, the simulation run time will be 28hr 23min.

TRAIL 11:

In this trial we have to keep the value of DT2MS and TSSFAC value 0.6 and -1.028E-3 respectively, the simulation run time will be 38hr 40min.

The above table and histogram represent it is clearly denoted that the mass addition percentage and added mass value doesn’t change concerning TSSFAC but reduce the TSSFAC value increases the Timestep of the stimulation.   

IMPLICIT ANALYSIS

We have to do implicit analysis and compare with explicit analysis for the same model, where the control cards, material, curve are already defined in the model we have to shift the analysis by impliocit_auto and implicit_general cards.

In the implicit method, the run time was reduced to 25min 24s (15245s).

CONCLUSION

In this challenge, we learn how mass scaling affects the run time of the stimulation while comparing explicit and implicit analysis.

We have to change the DT2MS value in the implicit problem but it doesn’t affect the run time of the stimulation because we are assigning the stimulation time and time step in the implicit_auto card.

Mass scaling is only used in the explicit analysis where we have higher strain rates over 10units/s like car crash simulations, implicit analysis is used when the strain rate is minimum.