The mesh file
The APM Preprocessor supports mesh files in .stl format. The .stl file is a commonly used mesh file format which represents bodies with triangular elements. If a body is represented only with triangular elements its mesh is said to be unstructured. Conversely, if a body is represented only with quadrilateral elements its mesh is said to be structured. The images below show an unstructured mesh of a fixed-wing UAV, a structured mesh of a NACA0015 wing, and a hybrid (mixed) mesh of a fixed-wing UAV.

Unstructured mesh of a fixed-wing UAV

Structured mesh of a NACA0015 wing

The .stl mesh file format can only store unstructured meshes. However, the APM Preprocessor supports a non-standard .stl mesh file format which can store structured meshes. A typical structure of a .stl mesh file is shown below:
solid "name"
facet normal nx ny nz
outer loop
vertex x y z
vertex x y z
vertex x y z
endloop
endfacet
facet normal nx ny nz
outer loop
vertex x y z
vertex x y z
vertex x y z
endloop
endfacet
...
...
...
endsolid
Each mesh element is defined by the
facet
keyword. The normal
keyword and the nx
, ny
, nz
variables specify the unit normal vector of the element.The vertex
keyword and the x
, y
, z
variables specify a vertex of the element. Since each element has 3 vertices the vertex
keyword and the x
, y
, z
variables are repeated three times.For structured meshes, a fourth vertex is added to each element.
A .stl file with four vertices per element is non-standard and is specific to the APM Preprocessor.
Last modified 4mo ago