Tech Briefs

Automatic Free-Form All-Brick Elements in Meshes

Meshing is an important capability in the ADINA pre-processor that we are constantly working to improve. Not long ago, we presented the enhancements made to our automatic free-form hex dominant mesher and mentioned that an all brick mesh cannot be achieved for a general body. In this News, we describe an interesting case of an important class of problems where the ADINA mesher can generate an all-brick mesh.

When a geometric model does not have edges, it is possible to create a 100% hex mesh. Often, in biomedical engineering, models coming from Computer Tomography (CT) or Magnetic Resonance Imaging (MRI) are stored in the STL format (a list of triangles defining the surface of the model) and they typically do not have "hard" edges due to the way they are generated. When a model has no edges (typically, a body with a single face), there are no topological constraints on the hex mesher that prevent the generation of a mesh consisting of only brick elements.

Of course, an automatically generated free-form hex mesh is very desirable because for a given required accuracy in results, the number of elements required (and hence nodes used) is much less than when using a tet mesh.

The mesh shown below was created from an STL file describing a human torso (courtesy of AIM@SHAPE Shape Repository). The topology of the model is one body made up of a single body face (when loading the STL file into the ADINA interface, the ridge detection angle was set to 180 degrees). The mesh density follows the curvature of the model. A "boundary layer" is created all around the model in order to maximize the quality of the elements on the boundary. The mesh has about 177,000 elements and they are all bricks.

Figure 1  Mesh of human torso generated from STL file

Figure 2  Detail of above mesh showing the variation of mesh density

Figure 3  Partial cross-section showing the all-brick mesh inside

Being able to mesh these "edgeless" models with 100% bricks is powerful not only because the elements are all hexahedra but also because the meshes created have far fewer elements than tetrahedral meshes or even mixed meshes of the same density, as already mentioned above.

This methodology is also available as an option for meshing general ADINA-M bodies (the Parasolid based ADINA Modeler), i.e. bodies established in CAD programs. Note, however, there is then the need to use a fine enough mesh to match the topology of the CAD model. Hence, greater care needs to be exercised when using this capability with ADINA-M bodies.

This feature in ADINA of automatic free-form meshing of all-brick elements is clearly an important capability in many areas of three-dimensional analyses.

Stereolithography, STL format, CT, MRI, CAD import, brick mesh