# Reference Geometry

Reference geometry should be represented as flat node based (/XREF) or element based (/EREF) geometry.

In /EREF, the reference geometry parts of the airbag can be separate parts that are not connected.

In some cases, /XREF can also be used to with reference geometry that is not connected. In this case, each part referenced in the card should not connect to itself in the actual airbag model.

Reference geometry should be parallel to global XY plane.

Note: This is not a
requirement of FVM approach but instead simplifies defining the orthotropic
directions of airbag components.

For reference geometry that is a closed volume:

- the geometrical center of non-folded airbag should be at point X=0, Y=0, or Z=0
- normal directions of components in reference geometry should be the same as in folded airbag model
- intersections are not allowed in unfolded airbag reference geometry

For disconnected airbag reference geometry based on elements,
/EREF:

- each component should be in the Z=0 plane
- the components normal direction in the reference geometry should be the same as in folded airbag model

During design iterations, the reference geometry can be modified or morphed in order
to investigate the influence of minor airbag shape variations to airbag
characteristics.

Note: When using reference geometry, the time step for the
fabric is calculated based on the elements reference geometry size. Initial
material anisotropic directions are also calculated based on the reference
geometry.