Week - 8 Mass Scaling
OBJECTIVE
- Use the mass scaling to reduce the simulation time.
- Ensure mass scaling doesn’t cross 8% and the stability is intact. Use parameters DT2MS and TSSFAC to achieve this.
- After explicit simulation trials, check its effect on the implicit solution.
Note: The times shown in this exercise is the preliminary estimated time shown by LS dyna solver. This is affected by different background processes. Also the actual time will be less than the estimated time in most cases but will follow the trend in similar computational conditions.
PROCEDURE
Mass scaling refers to a technique where a non-physical mass is added to the simulation in order to increase the timestep size and thereby less simulation time. Since the run time is a function of density which in turn is a function of mass, the faster solution can be achieved if there is more mass.
For this generally 5 to 8 % of additional mass can be considered effective without altering the solution as a whole.
In LS-Dyna this can be done using Control_Timestep keyword by altering the DT2MS and TSSFAC values.
- DT2MS
This is the parameter which is used to define the new time step of the simulation and thus this affects the additional mass provided for the simulation.
Ideally the values of DT2MS < 0 to ensure that only negative values of dt are adjusted whereas positive values are left as it is. This yields us a resultant positive mass.
- TSSFAC
This parameter doesn’t affect the mass scaling but it affects the timestep size and thus simulation speed. It is the timestep scale factor that is multiplied with DT2MS to get the new timestep
Time step, dt = DT2MS x TSSFAC.
EXPLICIT ANALYSIS
Without mass scaling i.e. DT2MS = 0
From the image we can see that estimated simulation time without the mass scaling is 34 hr and 44 min (i.e. 125075 sec). So upon doing mass scaling we expect to gradually reduce the simulation time without adding more than 8 % of the mass.
Ideally in order to verify stability of the system, values like stress, strain or displacement should remain unchanged. But since we are not going to check the results of the simulation, Stability criteria can be verified by ensuring that the time step dt is not changed drastically and this can be done using TSSFAC card.
With mass scaling TSSFAC = 0.9 and varying DT2MS
- DT2MS = -3.5e5
- DT2MS = -5e5
- DT2MS = -7e5
- DT2MS = -8.5e5
- DT2MS = -1.0e-4
- DT2MS = -1.03e-4
- DT2MS = -1.05e-4
- DT2MS = -1.028e-4
Below table shows the result for different cases of explicit analysis with TSSFAC = 0.9
|
Sr. No. |
DT2MS |
Mass added |
% Mass added |
Total time (hrs and min) |
Total Time (sec) |
|
1 |
-0.0000350 |
0.0 |
0.0000 |
31 hr 18 min |
112692 |
|
2 |
-0.0000500 |
375.0 |
0.0013 |
28 hr 37 min |
103030 |
|
3 |
-0.0000700 |
40670.0 |
0.1429 |
24 hr 2 min |
86579 |
|
4 |
-0.0000850 |
451600.0 |
1.5870 |
19 hr 12 min |
69162 |
|
5 |
-0.0001000 |
1827000.0 |
6.4230 |
16 hr 29 min |
59393 |
|
6 |
-0.0001030 |
2295000.0 |
8.0660 |
14 hr 22 min |
51779 |
|
7 |
-0.0001050 |
3058000.0 |
10.7400 |
14 hr 16 min |
51370 |
|
8 |
-0.0001028 |
2260000.0 |
7.9440 |
14 hr 24 min |
51880 |
With mass scaling TSSFAC = 0.8 and varying DT2MS
- DT2MS = -3.5e5
- DT2MS = -5e5
- DT2MS = -7e5
- DT2MS = -8.5e5
- DT2MS = -1.0e-4
- DT2MS = -1.03e-4
- DT2MS = -1.05e-4
- DT2MS = -1.028e-4
Below table shows the result for different cases of explicit analysis with TSSFAC = 0.8
|
Sr. No. |
DT2MS |
Mass added |
% Mass added |
Total time (hrs and min) |
Total Time (sec) |
|
1 |
-0.0000350 |
0.0 |
0.0000 |
34 hr 26 min |
123992 |
|
2 |
-0.0000500 |
375.0 |
0.0013 |
33 hr 42 min |
121363 |
|
3 |
-0.0000700 |
40670.0 |
0.1429 |
23 hr 32 min |
84740 |
|
4 |
-0.0000850 |
451600.0 |
1.5870 |
19 hr 23 min |
69786 |
|
5 |
-0.0001000 |
1827000.0 |
6.4230 |
15 hr 54 min |
57272 |
|
6 |
-0.0001030 |
2295000.0 |
8.0660 |
16 hr 10 min |
58252 |
|
7 |
-0.0001050 |
3058000.0 |
10.7400 |
15 hr 41 min |
56493 |
|
8 |
-0.0001028 |
2260000.0 |
7.9440 |
15 hr 28 min |
55712 |
Below graph shows the brief comparison for the DT2MS vs Running Time for TSSFAC = 0.9 and 0.8
IMPLICIT ANALYSIS
For implicit analysis, additional keywords like Control_Inmplicit_General, Control_Implcit_Auto, Control_Implccit_Solver and Contol_Implcit_Solution are to be created.
Now from the above simulation we are aware what percentage of mass is added at different DT2MS values. So 3 different DT2MS values are tested to understand the effect of mass scaling on implicit analysis.
- DT2MS = 0
- DT2MS = -3.5e-5
- DT2MS = -1e-4
From above 3 images it is clear that mass scaling doesn’t affect the simulation time in case of implicit analysis. The simulation time and timestep in implicit is mainly controlled using Control_Implcit_Auto card using parameters like ITEOPT, ITEWIN, DTMIN and DTMAX but defining the iteration and timestep limits. For this exercise timestep is kept high as we are focused on identifying the trend and not the result itself. Otherwise smaller timestep is to be defined so as to consider the effects of non-linearity in the model.
CONCLUSION
From the above activity, we get to understand the functionality of mass scaling used in simulation to reduce the simulation time. Following things can be derived from the activity.
- Time step can be controlled using the mass of the simulation
- Mass scaling is only useful for the explicit simulation. Also in order to maintain the credibility of the simulation, time step should not be altered drastically and both these can be achieved using DT2MS and TSSFAC parameters.
- Mass scaling is dependent only on DT2MS and not TSSFAC.
- Implicit analysis is not affected by mass scaling.