Refine Elements

Increasing the density of mesh elements can be accomplished by selecting a set of elements and selecting the Refine Elements command from the Mesh menu. This brings up the Refine Elements dialog.

Elements to Refine

The top portion of the dialog is used to specify which elements in the mesh are to be refined. If the Refine all 3D mesh elements option is selected, all elements in the mesh are refined regardless of which elements are selected. If the Refine selected 3D mesh elements option is selected, only the selected elements of the mesh are refined.

Even if the Refine selected 3D mesh elements option is selected, a few elements that were not selected must also be altered. This is due to the fact that the elements that were selected for refinement are refined, disjoint faces are created between the selected elements and the non-selected elements directly adjacent to the selected elements. To eliminate these disjoint faces, some transition elements are identified and refined. Transition elements are defined as any non selected element that shares at least one node with an element that is selected for refinement.

Refinement Method

There are three methods of refinement that can be used. The difference among the three methods is the shape of the resulting mesh elements. Each of the three methods is described below.

Vertical Column Refinement

Vertical column refinement is used to split hexahedra and wedges in the X and Y directions only, as shown below:

Vertical column refinement of (a) hexahedra and (b) wedges.

Vertical column refinement was designed to be used with meshes created by extruding a 2D mesh through several layers. Meshes created in this manner are composed strictly of hexahedra and wedges and can be made by following the mesh extrusion procedure.

Depending upon the type and orientation of the elements in a 3D mesh, vertical column refinement may not be possible. When the Refine Elements options is selected from the Mesh menu, the entire mesh is checked to see if it can be refined using vertical column refinement. If vertical column refinement is not possible, the Vertical column refinement option is dimmed. In order for vertical column refinement to be possible, the following conditions must be met.

If the entire mesh is to be refined, all elements in the mesh must be either hexahedra or wedges.
If only a selected portion of the mesh is to be refined, all selected elements must be either hexahedra or wedges.
All wedges to be refined must be oriented in space such that their top and bottom faces correspond to the triangular faces of the wedge.
Both the top and bottom faces of each element to be refined must be on the boundary or adjacent to other elements that are also to be refined.
All transition elements (i.e. elements not intended to be refined but share at least one node with an element that was selected for refinement) must also satisfy conditions 2 and 3 above.

All Elements To Tetrahedra Refinement

All element types to tets refinement is used to convert any of the four basic element types to tetrahedra. This option is especially useful since some finite element solvers require meshes to be composed strictly of tetrahedra.

The Coarse refinement and Fine refinement options are used to specify the degree of refinement to be applied. If the Fine refinement option is selected, each tetrahedron is divided into eight smaller tetrahedra, each pyramid is divided into 16 smaller tetrahedra, each wedge is divided into 24 smaller tetrahedra, and each hexahedron is divided into 48 smaller tetrahedra. As with vertical column refinement, it is possible to refine either the entire mesh or selected portions of a mesh using the Fine refinement method.

All elements to tetrahedra fine method of refinement of (a) hexahedra, (b) wedges, (c) pyramids, and (d) tetrahedra.

If the Coarse refinement option is selected, each pyramid is divided into two smaller tetrahedra, each wedge is divided into either three, or eight tetrahedra, and each hexahedra is divided into five, six, or twelve tetrahedra as shown below. Tetrahedra are not refined. Unlike the Fine refinement method, it is not possible to refine only a selected portion of a mesh when using the coarse method. The entire mesh gets refined.

All elements to tetrahedra coarse method of refinement of (a) hexahedra, (b) wedges, and (c) pyramids.

Retain Element Types Refinement

Retain element types refinement is used to convert any of the four basic element types to smaller elements of the same type. For example, each hexahedra is divided into eight smaller hexahedra as shown below. Pyramids are the exception since they are divided into five smaller pyramids and four tetrahedra. It is possible to divide a pyramid into four smaller pyramids, but the resulting pyramids are of poor quality.

Like vertical column refinement, it is possible to refine only selected portions of a mesh when using Retain element types refinement. However, it is not always possible to retain element types in the transition elements. If the original mesh is composed of strictly tetrahedra, any selected region of the mesh can be refined without introducing elements other than tetrahedra. However, if the mesh contains any element type other than tetrahedra, pyramids and wedges will be introduced into the transition region.

Retain element types refinement of (a) hexahedra, (b) wedges, (c) pyramids, and (d) tetrahedra.