# CUBIT Geometry Formats

## Setting the Geometry Kernel

The geometry kernel can be switched between ACIS and Mesh-Based Geometry from the command line using the following command:

Set Geometry Engine {Acis|Facet}

The geometry engine will automatically be set when importing a model.

## 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.

## 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.

### 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.

### 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.

## Bounding Box Calculations

Bounding box calculations are used for many routines and subroutines in Cubit. These calculations are done using a faceted representation by default. To use the default modeling engine for more accurate (and longer) calculations change the Facet Bbox setting.

Set Facet BBox [ON|Off]

There are also various settings to control the accuracy of bounding box calculations based on point lists.

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

If surfaces are used, surface facet points will be included in the point list used to calculate the tight bounding box. This will include vertices and points on the curves. This is the default implementation.

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.