#### 4.1` `Cubit Geometry Formats

##### 4.1.1` `Setting the Geometry Kernel

Set Geometry Engine {Acis|Facet}

##### 4.1.2` `Terms

Before describing the functionality in Cubit for viewing and modifying solid geometry, it is useful to give a precise definition of terms used to describe geometry in Cubit. In this manual, the terms topology and geometry are both used to describe parts of the geometric model. The definitions of these terms are:

Topology: the manner in which geometric entities are connected within a solid model; topological entities in Cubit include vertices, curves, surfaces, volumes and bodies.

Geometry: the definition of where a topological entity lies in space. For example, a curve may be represented by a straight line, a quadratic curve, or a b-spline. Thus, an element of topology (vertex, curve, etc.) can have one of several different geometric representations.

##### 4.1.3` `Topology

Within Cubit, the topological entities consist of vertices, curves, surfaces, volumes, and bodies. Each topological entity has a corresponding dimension, representing the number of free parameters required to define that piece of topology. Each topological entity is bounded by one or more topological entities of lower dimension. For example, a surface is bounded by one or more curves, each of which is bounded by one or two vertices.

##### 4.1.3.1` `Bodies and Volumes

A Cubit Body is defined as a collection of other pieces of topology, including curves, surfaces and volumes. The use of Body is not required, and is in fact deprecated in favor of using Volume. Bodies may still be used for grouping volumes, but it is suggested to use Groups instead.

Although a Body may contain groups of Surfaces or Volumes, for most practical purposes within the Cubit environment, a single Volume or Surface will belong to a single Body. For typical three-dimensional models, this means that there should be one Body for every Volume in the model, where the default Body ID is the same as the Volume ID. For this reason, in many instances the term Volume and Body are used interchangeably, although it is more consistent to always refer to Volumes and Volume IDs, and only use Bodies when absolutely necessary.

##### 4.1.3.2` `Non-Manifold Topology

In many applications, the geometry consists of an assembly of individual parts, which together represent a functioning component. These parts often have mating surfaces, and for typical analyses these surfaces should be joined into a single surface. This results in a mesh on that surface which is shared by the volume meshes on either side of the shared surface. This configuration of geometry is loosely referred to as non-manifold topology.

##### 4.1.4` `Bounding Box Calculations

Set Facet BBox [ON|Off]

Set Tight [[Bounding] [Box] [{Surface|Curve|Vertex} {on|off}]]

If curves are used, curve tesselation points will be included in the point list used to calculate the tight bounding box. This includes the vertices on the ends of the curves. One use for this is to find a more accurate tight bounding box, since curve tessellations are typically more fine than surface tessellations. However, in practice, it is recommended to just use surface tessellations. One special case is if the user sends in a list of curves as the criteria for the tight bounding box, the curve tessellations are always used, even if this parameter is false.

If vertices are used, vertex points will be included in the point list used to calculate the tight bounding box. In extremely large models, it could be advantageous to just use vertices. So the user would turn off both the surface and curve flags. One special case is if the user sends in a list of curves as the criteria for the tight bounding box, the curve tessellations are always used, even if the curve parameter is false and this parameter is true.

##### 4.1.5` `ACIS Geometry Kernel

ACIS is a proprietary format developed by http://www.spatial.com. Cubit incorporates the ACIS third party libraries directly within the program. The ACIS third party libraries are used extensively within Cubit to import, export and maintain the underlying geometric representations of the solid model for geometry decomposition and meshing. There are many ways to get geometry into the ACIS format. ACIS files can be exported directly from several commercial CAD packages, including SolidWorks, AutoCAD, and HP PE/SolidDesigner. Third party ACIS translators are also available for converting from native formats such as Parasolid, Catia, Pro/E, and many others. Cubit also uses the ACIS libraries for importing IGES and STEP format files.

Cubit is also able to import geometry from an Gambit file.

Importing and creating geometry using the ACIS geometric modeling kernel currently provides the widest set of capabilities within Cubit. All geometry creation and modification tools have been designed to work directly on the ACIS representation of the model.

##### 4.1.6` `Mesh-Based Geometry

In contrast to the ACIS format, Mesh-Based Geometry (MBG) is not a third party library and has been developed specifically for use with Cubit. Most of Cubit’ mesh generation tools require an underlying geometric representation. In many cases, only the finite element model is available. If this is the case, Cubit provides the capability to import the finite element mesh and build a complete boundary representation solid model from the mesh. The solid model can then be used to make further enhancement to the mesh. While the underlying ACIS geometry representation is typically non-uniform rational b-splines (NURBS), Mesh-Based Geometry uses a facetted representation. Mesh-Based Geometry can be generated by importing either an Exodus II format file or a facet file.

Many of the same operations that can be done with traditional CAD geometry can also be done with mesh-based geometry. While all mesh generation operations are available, only some of the geometry operations can be used. For example, the following can be done with geometric entities that are mesh-based:

Some operations that are not yet available with mesh-based geometry include:

##### 4.1.6.1` `Creating Mesh-Based Geometry Models

Mesh based geometry models can be created in one of two ways

While both of these methods create geometry suitable for meshing, there are some significant differences:

Exodus II files

Exodus II contains a mesh representation that may include 3D elements, 2D elements, 1D elements and even 0D elements. It may also contain deformation information as well as boundary condition information. The import mesh geometry command is designed to decipher this information and create a complete solid model, using the mesh faces as the basis for the surface representations. Exodus II is most often used when a solid model that has previously been meshed requires modification or remeshing. Importing an Exodus II file will generate both geometry and mesh entities, assigning appropriate ownership of the mesh entities to their geometry owners. Deleting the mesh and remeshing, refining or smoothing are common operations performed with an Exodus II model.

Facet files

The facet file formats supported by Cubit are most often generated from processes such as medical imaging, geotechnical data, graphics facets, or any process that might generate discrete data. Importing a facet file will generate a surface representation only defined by triangles. If the triangles in the facet file form a complete closed volume, then a volume suitable for meshing may be generated. In cases where the volume may not completely close or may not be of sufficient quality, a limited set of tools has been provided. In addition to the standard meshing tools provided in Cubit, it is also possible to use the triangle facets themselves as the basis for an FEA mesh.

##### 4.1.6.2` `Improving Mesh-Based Geometry Models for Meshing

In many cases, the triangulated representations that are provided from typical imaging processes are not of sufficient quality to use as geometry representations for mesh generation. As a result, Cubit provides a limited number of tools to assist in cleaning up or repairing triangulated representations.

##### 4.1.6.2.1` `Using tolerance on STL files

Stereolithography (STL) files, in particular, can be problematic. The import mechanism for STL provides a tolerance option to merge near-coincident vertices.

##### 4.1.6.2.2` `Using the stitch option on AVS and facet files

The stitch option on the import facets|avs command provides a way to join triangles that otherwise share near-coincident vertices and edges. This is useful for combining facet-based surfaces to generate a water-tight model.

##### 4.1.6.2.3` `Using the improve option on facet files.

The improve option on the import facets command will collapse short edges on the boundary of the triangulation. This option improves the quality of the boundary triangles.

##### 4.1.6.2.4` `Smoothing faceted surfaces.

Smooth <surface_list> Facets [Iterations <value>] [Free] [Swap]

The free option can be used to ignore the tangent plane projection. Used too much, the free option can collapse the model to a point. One of two iterations of this option may be enough to clean up the triangles enough to be used for a finite element mesh.

The swap option can be used to perform local edge swap operations on the triangulation. The quality of each triangle is assessed and edges are swapped if the minimum quality of the triangles will improve.

##### 4.1.6.2.5` `Creating a thin offset volume

Offset surfaces may be generated from an existing facet-based surface. This would be used in cases where a thin membrane-like volume might be required where only a single surface of triangles is provided. This command may be accomplished by using the standard create body offset command

The result of this command is a single body with an inside and outside surface separated by a small distance which is generally suitable for tet meshing. This command is currently only useful for small offsets where self-intersections of the resulting surface would be minimal. It is most useful for bodies that may be initially composed of a single water-tight surface.

##### 4.1.6.2.6` `Creating volumes from surfaces

Create Body Surface <surface_id_range>

##### 4.1.6.3` `Meshing Mesh-Based Models

Mesh <entity_list> From Facets

Import Facets <filename> Make_elements

Creating triangular finite elements in this manner can greatly speed up the mesh generation process, however it is limited to non-manifold topology. If the triangular elements are to be used for tetrahedral meshing (i.e. all edges of the triangulation should be connected to no more than two triangles)

##### 4.1.6.4` `Exporting Mesh-Based Geometry

Mesh-Based geometry models and their mesh may be exported by one of the following methods:

Exodus II

Exporting to an Exodus II file saves the finite element mesh along with any boundary conditions placed on the model. It will not save the individual facets that comprise the mesh-based geometry surface representation. Importing an Exodus II file saved in this manner will regenerate the surfaces only to the resolution of the saved mesh.

Facet files

Export Facets ’filename’ <entity_list> [Overwrite]

Export STL [ASCII|Binary] ’filename’ <entity_list> [Overwrite]