Oasys LS-DYNA Environment 8.1 VOLUME 3 ... - Oasys Software

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Oasys LS-DYNA Environment: User Guide (Version 8.1) folded bags, an initial timestep as low as 0.1 microsec may be necessary, rising to the LS-DYNA default by 5 millisec. REFERENCE GEOMETRY By default, the undeformed state of the fabric elements is defined by the initial geometry. However, this is not always wanted. By using the airbag reference geometry feature, it is possible to define the flat, unfolded mesh geometry separatety from the initial geometry. LS-DYNA then compares the current geometry of an element with the reference geometry, instead of the undeformed geometry, to calculate the strains and stresses in an element. Stresses are zero until an element size reaches that of its equivalent in the reference geometry. There are two main reasons for using this feature: ! Folding the mesh distorts the elements around the fold lines so that the initial geometry is inaccurate. This can lead to an incorrect shape when the bag is inflated unless the reference geometry is used. ! A “shrunken” unfolded bag may be used to avoid initial contact with the occupant or to achieve a reasonable initial volume. The airbag will inflate to the correct size if it is defined using the reference geometry. The coordinates of the airbag nodes in the folded or reduced state is included under *NODES as usual. The coordinates of the same nodes in the flat, full-size state are given under *AIRBAG_REFERENCE_GEOMETRY. For reduced size airbags, use *AIRBAG_REFERENCE_GEOMETRY_RDT. This sets the timestep based on the reference, not the reduced state. If the fabric is defined as orthotropic, the angles are relative to the reference geometry: this simplifies the definition for folded airbags. JETTING In a real airbag, the pressure on the portions of fabric directly in front of the inflator will be higher than the pressure on other parts of the bag, because the fabric in front of the inflator feels momentum forces from the gas entering the bag. However, the internal pressure in an LS-DYNA control volume is constant throughout the volume at any given time. LS-DYNA can simulate the effect of the momentum forces from the gas that rushes out of the inflator. It does this by applying extra forces to nodes that are in the inflator’s line-of-sight. The user defines the inflator position and direction, and a “cone angle” over which the forces are to be applied. The jetting forces are calculated automatically from the mass inflow and gas Page 16.4
Oasys LS-DYNA Environment: User Guide (Version 8.1) parameters and applied over the cone angle using a Gaussian distribution, with the highest forces applied to nodes along the centreline of the cone. Virtual origin (Focus) Hole diameter Pressure is applied to surfaces that are in the line of sight to the virtual origin Airbag Gaussian profile Cone center line Note that the jetting forces are only an approximation and do not address most of the gas dynamics issues. The airbag types to use with the jetting model are *AIRBAG_WANG_NEFSKE_JETTING and *AIRBAG_WANG_NEFSKE_MULTIPLE_JETTING. The only difference between them is that the pressure distribution over the cone angle is user-defined by a load curve in the MULTIPLE_JETTING type, instead of being automatically defined as a Gaussian curve. *AIRBAG_WANG_NEFSKE_JETTING $ SID 1[ SIDTYP ][Rbody ID][ V scale][ P scale][ V init ][mass dmp][ SPSF ] 1 1 0 1.0 1.0 0.000E+00 5.000E+01 0.7 $ Cv ][ Cp ][ Temp ][LC temp ][LC mflow][tank vol] 8.5e8 1.1e9 800 1 $ C23 ][ LCC23 ][ A23 ][ LCA23 ][ CP23 ][ LCCP23 ][ AP3 ][ LCAP23 ] 1.0 1500. $P ambint][rho amb ][ G conv ] 1.009E-01 1.200E-12 1.000E+00 $ IOC ][ IOA ][ IVOL ][ IRO ][ IT ][ LCBF ] $ jet focal point x, y and z ][ jet vector head x, y and z ][cone ang][ beta ] 0.000E+00 0.000E+00-7.000E+00 0.000E+00-0.300E+00 0.300E+00 1.000E+00 $ secondary jet focal x y z ][part set][cutoff a] 2 3.000E+01 *SET_PART_LIST 1 1 *SET_PART_LIST 2 1 $ ====================== LOAD CURVES ====================== *DEFINE_CURVE 1 $ massflow into airbag $ Time (s) mass flow rate (kg/s) 0.00000E+00 0.00000E+00 0.010000 1.8e-3 0.040000 0.0 0.10000 0.0 Page 16.5
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Oasys LS-DYNA Environment 8.1 VOLUME 3 ... - Oasys Software

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