Standalone Run of FVM Airbag

FVM airbag should be run standalone to verify correct deployment behavior and airbag characteristics. The FV merging parameters and time step control can be modified to reduce simulation time while still providing realistic results.

Typical FV parameters are entered in the Engine file as (can be also added in /MONVOL/FVMBAG1, /MONVOL/FVMBAG2 cards as of 2019.1):
/DT/FVMBAG/1
1.5 1.0e-05
/FVMBAG/MODIF
666000001
1  0.010  0.001  1

Variation of time step scaling coefficient and global merging parameter may be necessary to provide proper time step and number of FV at the end of the simulation.

Number of finite volumes and FV time step can be plotted in HyperGraph.


Figure 1.

The number of finite volumes should reduce smoothly to 2-10% of initial number of finite elements. The merging strategy should also provide a time step higher than the minimal time step required in crash simulations.

The merging of FV should checked in Engine output file:
    2  0.6688E-03  0.2863E-03 FVBAG  666000001   0.0%   2.739      0.1415E-14   0.000     -0.8103E-19   0.000
 ** MONITORED VOLUME ID: ********TIME STEP:  0.2878E-03 FINITE VOLUME:   21940
 
 ** MONITORED VOLUME ID: ******** - FINITE VOLUME MESH UPDATE **
    NUMBER OF FINITE VOLUMES :    30605
    SUM VOLUME FINITE VOLUMES :   155386.459        (VOLUME AIRBA  155386.459    )
    SUM AREA SURFACE POLYGONS :   227440.887        (AREA AIRBAG   227440.887    )

A reasonable number of finite volume are needed and finite volume merging should not cause the number to drop immediately to one finite volume.

The finite volume time step in the /DT/FVMBAG card should be less than the initial finite volume time step output in Starter output. Merging parameter Cgmerg in /FVMBAG/MODIF should not be very high to provide smooth FV merging history.

The sum of finite volume volumes should be equal to the actual airbag volume as:
** MONITORED VOLUME ID:        1 - FINITE VOLUME MESH UPDATE **
NUMBER OF FINITE VOLUMES :     1565
SUM VOLUME FINITE VOLUMES :   298121.912        (VOLUME AIRBA 298121.912)
SUM AREA SURFACE POLYGONS :   230093.158        (AREA AIRBAG   230093.158)
Every time finite volumes are merged, this message appears in the Engine output.
44  0.1320      0.2865E-02 FVBAG          1   1.0%   7.222      0.1104       0.000      0.4265E-01   0.000
** MONVOL ID        1 FINITE VOLUME MESH UPDATE - LOOPING - NUMBER OF FINITE VOLUMES :     1564
** MONVOL ID        1 FINITE VOLUME MESH UPDATE - LOOPING - NUMBER OF FINITE VOLUMES :     1563
** MONITORED VOLUME ID:        1TIME STEP:  0.4293E-02 FINITE VOLUME:     396

The gas motion inside the airbag should be reviewed by contour plotting the animation output of the temperature and velocity of the gas. The motion should clearly develop from the inflator into the airbag volume and account for the airbag topology (internal walls, baffles, vent holes, etc.).

Airbag gas parameters should be compared to uniform pressure airbag run. Significant differences should be inspected and explained.


Figure 2. Comparison of airbag output for AIRBAG1 and FVMBAG1 simulations


Figure 3. Comparison of venting parameters for AIRBAG1 and FVMBAG1 simulations

The vent area and vent outflow should be plotted for each vent and controlled and compared to uniform pressure run.