A wall is a manifold surface composed of {line segments in 2D; triangular facets in 3D} termed facets. The surface is defined by a mesh. The surface properties of a wall can be specified independently for each facet. Walls can translate and rotate (defined by \(\{\mathbf{v},\mathbf{w}\}\), the generalized velocity and angular velocity, or spin of the wall about the reference point). Wall motion does not obey the equations of motion. Translational velocities can be applied to each vertex to model wall deformation (\(\{\mathbf{v}^{(k)}\}_{k=1,2,\ldots,n}\)—these values are specified directly). Wall deformation must preserve the manifold nature of the wall.

Walls may be created (using wall create) and facets may be added one at a time (using wall addfacet). The facets must be {vertex connected in 2D; edge connected in 3D} to previously existing facets if there are any. Alternatively, simple wall configurations can be generated with the wall generate command (e.g., boxes, planes, spheres, cylinders, cones, etc.). One may also import a wall from an .stl or .dxf file in 3D using the geometry logic (see the “Geometry” section).

The wall attribute and wall property commands are used to specify wall attributes and facet properties as discussed in “Model Components.” The entire attribute/property lists can be listed with the wall list attribute and wall list property commands along with the values of specific attributes/properties.