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

File formats

.stl

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.

1MB

naca4412_wing.stl

.ucd

The .ucd mesh file format can only store unstructured and structured meshes. A typical structure of a .ucd mesh file is shown below:

# <comment 1>
.
.
.
# <comment n>
<num_nodes> <num_cells> <num_ndata> <num_cdata> <num_mdata>
<node_id 1> <x> <y> <z>
<node_id 2> <x> <y> <z>
<node_id num_nodes> <x> <y> <z>
<cell_id 1> <mat_id> <cell_type> <cell_vert 1> ... <cell_vert n>
<cell_id 2> <mat_id> <cell_type> <cell_vert 1> ... <cell_vert n>
<cell_id num_cells> <mat_id> <cell_type> <cell_vert 1> ...<cell_vert n>

368KB

naca4412_wing.ucd

.wrl

The .wrl mesh file format can only store unstructured and structured meshes. A typical structure of a .wrl mesh file is shown below:

Shape {
    geometry IndexedFaceSet {
        coord Coordinate {
            point [
                <x> <y> <z>
                <x> <y> <z>
                <x> <y> <z>
                ...
            ]
        }
        coordIndex [
            <cell_vert 1> ... <cell_vert n>
            <cell_vert 1> ... <cell_vert n>
            <cell_vert 1> ... <cell_vert n>
            ...
        ]
    }
}

338KB

naca4412_wing.wrl