## Docs

To run a simulation on a CAD model, the model needs to fulfill somewhat more restrictive criteria than one might be used to. The reason is since a mesh needs to be generated it is not enough that the CAD model just looks correct. This involves some cleaning aspects, discussed under CAD preparation but also some topological aspects. Here you can find some hints on them.

For a correct and efficient setup of your simulation, you should make sure that your CAD model has the right topology for the simulation you want to run. In general a CAD model consists of different types of topological entities such as solids, faces, edges or vertices. You should be aware of this topology since it will have an impact on how the mesh is generated for it.

## General¶

Most of the CAD models are described using the B-Rep method, which allows to describe shapes using its limits. In B-Rep a model is composed out of two parts:

• Topology: Describes how elements are bounded and connected
• Geometry: Describes the shape of each element

Depending on how you built your CAD model as well as depending on the type of CAD software you are using, the topology of your CAD model might be desribed differently. For a successful meshing and simulation setup it is of great help to be aware of the topology of your CAD model. That way you will not try to build a volume mesh for a surface model, which in general does not work.

## Topological entities¶

### Solids¶

To run a simulation based on a volume mesh, your CAD model needs to contain solids. The mesh algorithm will detect them automatically and will be able to create volume meshes for it.

Please note, that a CAD model can contain only one solid (= part) or multile solids (= assembly). This is relevant for example if you plan to run a structural simulation. If you are dealing with an assembly, each solid will be meshed as a single mesh, such that you need to connect these solids later via contact constraints to get a valid simulation. For a single mesh, this will not be necessary.

The figure below shows a CAD model that only consists out of a single solid.

The viewer showing a CAD model that only consists out of one solid - a part

You can see it either in the scene tree on the right of the viewer or in the event log after you upload the CAD model. The solid count shows 1.

The event log after uploading a solid model that contains a single solid

On the other hand SimScale can also deal with complete assemblies. Have a look at the image below.

The viewer showing a CAD model that consists out of several solids - an assembly

The scene tree on the right in the viewer shows multiple solids and their associated faces. The event log this time shows a number of 10 solids in this CAD model.

The event log after uploading a solid model that contains multiple solids

### Faces¶

A CAD model might also only consist out of faces and no solids at all. This means that you can not create a volume mesh for such a CAD model. The image below shows such a model.

The viewer showing a CAD model that only consists of faces, no solids

It looks the same as a solid model, however the tree on the right shows the “shell” indicator name that shows, that this model does not contain any solid information. The event log shown in the image below indicates it even clearer. The solid count is zero.

The event log after uploading a shell model - a CAD model, only consisting out of faces