# Surfaces

A surface represents the geometry associated with a physical part. A surface is a two-dimensional geometric entity that may be used in automatic mesh generation.

The color of a surface is determined by the component collector to which it belongs.

A surface is comprised of one or more faces. Each face contains a mathematical surface and edges to trim the surface, if required. When a surface has several faces, all of the faces are maintained as a single surface entity. Operations performed on the surface affect all the faces that comprise the surface. In general, HyperMesh CFD automatically uses the appropriate number of and type of surface faces to represent the geometry.

The perimeter of a surface is defined by edges. There are four types of surface edges.

Surface edges are different from lines and are sometimes handled differently for certain operations.

The connectivity of surface edges constitutes the geometric topology.
Free Edges
A free edge is an edge that is owned by only one surface.
Free edges are colored red by default.
On a clean model consisting of surfaces, free edges appear only along the outer perimeter of the part and around any interior holes. Free edges that appear between two adjacent surfaces indicate the existence of a gap between the two surfaces. The automesher will leave a gap in the mesh wherever there is a gap between two surfaces.
Shared Edges
A shared edge is an edge that is owned, or shared, by two adjacent surfaces.
Shared edges are colored green by default.
When the edge between two surfaces is a shared edge, there is no gap or overlap between the two surfaces - they are geometrically continuous. The automesher always places seed nodes along the length a shared edge and will produce a continuous mesh without any gaps along that edge. The automesher will not construct any individual elements that cross over a shared edge.
Suppressed Edges
A suppressed edge is shared by two surfaces but it is ignored by the automesher.
Suppressed edges are colored blue by default.
Like a shared edge, a suppressed edge indicates geometric continuity between two surfaces but, unlike a shared edge, the automesher will mesh across a suppressed edge as if it were not even there. The automesher does not place seed nodes along the length of a suppressed edge and, consequently, individual elements will span across it. By suppressing undesirable edges you are effectively combining surfaces into larger logical meshable regions.
Non-Manifold Edges
A non-manifold edge is owned by three or more surfaces.
Non-manifold edges are colored yellow by default.
They typically occur at "T" intersections between surfaces or when two or more duplicate surfaces exist. The automesher always places seed nodes along their length and will produce a continuous mesh without any gaps along that edge. The automesher will not construct any individual elements that cross over a T-joint edge. These edges cannot be suppressed.