Structuration type

Unstructured options refer to the parameters controlling the behavior of the unstructured meshes.

Unstructured mesher

• GiD can use three kinds of surface mesher, all of them based on advancing front technique.

- Rfast mesher generates the meshes faster because it works in the 2D space of the NURBS surface, and after the mesh is generated it map the nodes in the 3D space. It makes the mesher faster, but it has the drawback of providing with lower quality element in case the NURBS surface is highly distorted.
- Rsurf mesher follows an advancing front technique directly in the 3D space, so it is a little bit slower than RFast. However, it tends to provide with better quality meshes.
- MinElem mesher generates meshes with the minimum number of elements needed to represent the shape of the surface, according to the chordal error parameters defined in Chordal error. This kind of meshes are commonly used for visualization purposes, or to consider kinematic simulations, where the quality of the triangles (in terms of angle) is not so relevant.

Try only with the selected surface mesher:  option to force use only this mesher for surfaces, otherwise GiD can try another mesher internally when one of them fails to generate the mesh for some surface.

• GiD can use three kinds of unstructured volume meshers (all of them generate tetrahedra):

- Advancing front: This volume mesher is based on the advancing front technique. This mesher tends to give good quality tetrahedra, but it is not as fast as the other methods. It generates the meshes volume by volume (so all the surfaces of the model are previously meshed). It requires a very good quality of the contour surfaces meshes of the volume in order to be successful.
- Tetgen: The unstructured volume mesher is based on the Delaunay algorithm. This mesher is a faster than the Advancing front one, but it tends to give lower quality tetrahedra. This mesher is developed by the Numerical Mathematics and Scientific Computing research group of the Weierstrass Institute for Applied Analysis and Stochastics.
- OctTree: Octree-based tetrahedra mesher. This is a very robust and fast mesher. It generates the mesh of all the connected volumes together. As an octree-based mesher, it may refine too much the mesh in given regions, and it may present an alignment of the nodes of the mesh following the three main directions of the octree. If some part of the model has structured properties, this mesher is automatically discarded, and GiD uses some of the previous ones.

Try only with the selected volume mesher:  option to force use only this mesher for volumes, otherwise GiD can try another mesher internally when one of them fails to generate the mesh for some volume.

Skip entities automatically
These variables control whether a line or a point is skipped in the meshing process. Using this option, meshes can be generated with fewer elements than others because it is less dependent on the dimensions of geometrical surfaces. However, it is slower and can fail for distorted surface patches.
It has to be considered that an entity cannot be skipped considering some situations:

• if the entity has a higher entity higher than 2
• if the entity has structured meshing properties.
• if the entity has material, conditions or some group applied
• if the entity is shared by two supra-entities which have different information attached (in terms of conditions, materials or groups)

The parameters the user can define in this window are:

• Tangency limit angle: Angle (in degrees) between the normals of two adjacent surfaces/lines in their contact line/point above which the line/point won't be skipped.
• Take case on layers: If this variable is set, a line/point only will be skipped if it is in the same layer than the surfaces/lines it belongs to.

These parameters control if GiD skip or not an entity automatically. Apart from this, it can be assigned manually to some entity the information to be skipped or not: Skip or NoSkip Mesh Criteria (see Mesh criteria).

Structured options refer to the parameters controlling the behavior of the structured meshes.

• Allow automatic structured: If this preference is set, functions like Assign sizes by Chordal Error will define some surfaces as structured with highly distorted elements over them.
• Structured mesh in the inner part of volumes: If this option is set, the inner part of the volumes is meshed applying a tetrahedra pattern following an octree-based space decomposition. Then, the unstructured volume mesher is used to 'seam' this regular inner part with the contour elements. This option can make faster the mesh generation process depending on the characteristics of the model: the more massive is the model, the more fast the meshing process will be using this option.
• Symmetrical structured triangles: If this preference is set, structured triangle meshes will be topologically symmetrical. The triangles come directly from the splitting of a structured quadrilateral mesh into triangles (four triangles by each quadrilateral). Otherwise, each quadrilateral generates two triangles in a non-symmetrical way.
• Symmetrical structured tetrahedra: If this preference is set, structured tetrahedra meshes will be topologically symmetrical. The tetrahedra come directly from the splitting of a structured hexahedra mesh into tetrahedra (24 tetrahedra by each hexahedron). Otherwise, each hexahedron generates 6 tetrahedra in a non-symmetrical way.