Week - 8 Mass Scaling
Aim: 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%
Procedure:
Implicit Analysis
1. Open the .k file in LS-Dyna.
2. Check the unit sytem used. Go to Model - Keyword Manger - Control - Units.
Kg - m - sec unit system is used.
3. Delete the existing Timestep card.
4. Check the simulation time. Go to Model - Keyword Manger - Control - Termination.
Simulation Runtime = 600 sec.
5. Save the Model and Run the simulation. Kill the run, ctrl + c and type quit.
Estimated total clock time is 40867 sec (11 hrs 21 mins).
6. Run the Simulation using previous time step card.
Estimated total clock time = 39627 sec (11 hrs)
Mass Percentage increase = 0 %
We have to use mass scaling to reduce the runtime of a model and ensure the stability of mass scaling.
To do this we have do to trial and error experiment with DT and TSSFAC parameters.
DTINIT: Initial time step size
TSSFAC: Scale factor for computed time step
DT2MS: Time step size for mass scaled solutions
Negative DT2MS: Mass is added to only those elements whose timestep would otherwise be less than TSSFAC x DTINIT
Positive DT2MS: Mass is added or taken away from elements so that the timestep of every element is the same.
I will be using Negative DT2MS method.
Trail 1:
TSSFAC: 0.8 and DT2MS: -2.500e-05
Estimated total clock time = 43189 sec (11 hrs 59 mins)
Mass Percentage increase = 0 %
Trail 2:
TSSFAC: 0.8 and DT2MS: -2.500e-03
Estimated total clock time = 845 sec (14 mins)
Mass Percentage increase = 5.6423E+04 %
Trail 3:
TSSFAC: 0.8 and DT2MS: -2.500e-04
Estimated total clock time = 8728 sec (2 hrs 25 mins)
Mass Percentage increase = 4.7600E+02 %
Trail 4:
TSSFAC: 0.8 and DT2MS: -1.020e-04
Estimated total clock time = 10695 sec (2 hrs 58 mins)
Mass Percentage increase = 7.47 %
Trail 5:
TSSFAC: 0.8 and DT2MS: -1.0289e-04
Estimated total clock time = 15904 sec (4 hrs 25 mins)
Mass Percentage increase = 7.999 %
Trail 6:
TSSFAC: 0.8 and DT2MS: -1.02899e-04
Estimated total clock time = 14578 sec (4 hrs 2 mins)
Mass Percentage increase = 8.005 %
| Trail | TSSFAC | DT2MS | Mass Percentage Increase | Estimated Total Clock Time |
| 1 | 0.8 | -2.500e-05 | 0 % | 43189 sec (11 hrs 59 mins) |
| 2 | 0.8 | -2.500e-03 | 56423 % | 845 sec (14 mins) |
| 3 | 0.8 | -2.500e-04 | 476 % | 8728 sec (2 hrs 25 mins) |
| 4 | 0.8 | -1.020e-04 | 7.47 % | 10695 sec (2 hrs 58 mins) |
| 5 | 0.8 | -1.0289e-04 | 7.999 % | 15904 sec (4 hrs 25 mins) |
| 6 | 0.8 | -1.02899e-04 | 8.005 % | 14578 sec (4 hrs 2 mins) |
Trial 5 is best suitable as per the hard limit of mass scaling (i.e 8%).
Histogram:
Note: all the DT2MS value are in negative. Only for representation it is shown in positive.
Implicit Analysis
Trail 1
1. Using Trial 5 .k file (TSSFAC: 0.8 and DT2MS: -1.0289e-04). Create CONTROL_IMPLICIT_GENERAL, CONTROL_IMPLICIT_SOLUTION, CONTROL_IMPLICIT_SOLVER & CONTROL_IMPLICIT_AUTO.
Go to Model - Keyword - Control.
2. Save the model and run the simulation.
Total clock time = 55 sec
Mass Percentage increase = 0 %
Trail 2
TSSFAC: 0.9 and DT2MS: -3.500e-05 
Total clock time = 55 sec
Mass Percentage increase = 0 %
Comparison of Explicit run time Vs Implicit run time:
| Trail | D2TMS | TSSFAC | Implicit Runtime (Sec) | Explicit Runtime (Sec) |
| 1 | -1.0289e-04 | 0.8 | 55 | 15904 |
| 2 | -3.500e-05 | 0.9 | 55 | 39628 |
- In Implicit method there is no added mass and mass scaling ratio and the computational time is very less 55 seconds.
- The Factor DT2MS and TSSFAC don't affect Implicit Analysis as the Implicit analysis doesn't use a Physical Time Factor.
CONCLUSION:
- The objective of mass scaling is achieved for the given model by varying the values of DT2MS and TSSFAC using the trial and error method.
- By considering the hard limit on mass scaling being 8%, the optimized value of DT2MS = -1.0289E-4 and TSSFAC= 0.8 with the lowest possible runtime required to complete the simulation is 15904 sec (4 hrs 25 mins).
- The values of DT2MS and TSSFAC do not affect the runtime required to complete the simulation for implicit analysis.
- The Scale factor Don't affect the Stability of the Simulation it Affects the DT2MS To perform abrupt changes.