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CAD Topology

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 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.


Most of the CAD models are described using the B-Rep method, which describes shapes using limits. A B-Rep model is composed 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 and the type of software you are using, the topology of your CAD model might be described differently. For a successful meshing and simulation setup, it is of great help to be aware of the topology of your CAD model in advance. That way you won’t waste time building the mesh only to realize later that it is either inaccurate or doesn’t support the simulation.

Topological Entities


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

Please note, that a CAD model can contain either one solid (= part) or multiple 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. Another way is to convert the whole assembly into a single part in the CAD software itself before meshing it, although make sure that this doesn’t interfere with the physics of the simulation. For a single solid mesh, this won’t be necessary.

A CAD model with a single solid is shown below. The tree on the top right shows that only a single solid is present.

CAD model with one solid part
Figure:1 The viewer showing a CAD model that only consists of one solid – a part

On the other hand SimScale can also deal with complete assemblies. Have a look at the image below. This time the tree shows multiple solid parts.

CAD model with many solid parts
Figure 2: The viewer showing a CAD model that consists out of several solids – an assembly


Sometimes a CAD model might only consist of faces and no solids at all. Such a model cannot be used to create a volume mesh.

CAD model with shells
Figure 3: The viewer showing a CAD model that only consists of faces, no solids

It looks the same as a solid model however, you should always refer the tree on the right to see the topological information. The “shell” indicator name in the above image shows that this model does not contain any solid information.


Mathematically speaking, an edge is a line segment between two points or formed due to the intersection of two or more surfaces. The edge characteristics of a CAD model are heavily influenced by the type of file format in which it is either created or imported into simulation software. Small features like holes or windings increase the count of edges and also give rise to sharp angles and small faces. Such features are generally not crucial from the simulation viewpoint and should be removed. This is also called CAD cleaning.


A vertex represents a point in space. The edges of the CAD geometry intersect into a vertex. These vertices are also referred to as nodes. Load boundary conditions in FEA can be assigned using these nodes. Hence a well defined topological information about the nodes will help to set up an accurate simulation.

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