CAD preparation

CAD Preparation

The work for setting up a successful simulation does not start with the mesh creation. It starts with preparing your CAD model for the simulation. Often the time spent to prepare a CAD model for a simulation is well invested, since it can save you a headache later in the process. Find below some advice on how to prepare your CAD model for a successful simulation with SimScale.

General hints

  • Start simple and iterate: In order to save time, a proven strategy for the simulation setup is to create it for a very simple version of your problem in order to see whether or not the simulation approach is viable before investing plenty of time into the setup of a complex simulation and not getting results in the end.

  • Before you start the simulation, think the complete workflow through in order to identify potential problems. If you have a rough picture of how the mesh shall look like and how you will apply boundary conditions, one often sees potential in preparing the CAD model for it

  • Try to get a clear understanding of what you are trying to analyze. This helps to decide whether certain effects can be neglected or not. For example if you think that the inertia effects in a structural mechanics simulation are not significant, why not starting with a static analysis? This will save you plenty of computing time and might lead to good results. If not, you can than switch to a more sophisticated analysis like a dynamic one.

Format and Importing

Choose a format

SimScale supports the generic CAD formats

  • STEP

  • IGES

  • BREP

as well as the triangulated surface format

  • STL

If you have the choice, STEP is the preferred one since it’s superior in terms of the conversion process to IGES. The STL format can only be used for fluid mechanics analysis since only certain meshing algorithms on SimScale support it.

Export options

All CAD systems come with their own STEP and IGES export methods which may differ in the details. One important aspect when exporting to the STEP format is to make sure that the exported CAD model contains the information about the solids that might be present in your model. Once uploaded to SimScale you can counter-check whether or not your CAD model contains solids. The event log shows the amount of relevant entites such as solids, face and shells. If there is no solid present, it seems that your export was not successful and you should check if there are any other options to export your model.

The event log of the geometry upload

The event log of the geometry upload informs you about the entities of your model

Model dimension

The dimension of your model will be very important for the simulation setup. Whether your model is scaled in m or in mm will have a huge impact on your results. So you should check, whether your CAD system export and the SimScale import did preserve the dimensions. You can do that via the Bounding box diagonal length in the Geometry Event Log after uploading your CAD model (see the figure above). For example if you chose to use SI units, SimScale expects the CAD model to be scaled in m. If that is not consistent with your model, you can use the scaling operations within SimScale to get the correct unit.

Complexity reduction

Very often you can simplify your CAD model to get more accurate simulation results in shorter time. Find some of these situations below.


Often, your CAD model does contain many detailed features because of manufacturing constraints or the installation. Good examples are small holes and windings. These detailed features might be relevant for the final manufacturing but often they do not affect analysis results even though they increase the meshing and computing time significantly. A good approach to this is think upfront, which parts of your model are relevant to the analysis and which ones you can leave out to make your simulation more efficient.

Remove small entities

Especially for mesh generation, very small entities can be a problem. If you have very small faces with sharp angles the surface meshing might fail. Therefore check your CAD model for very small entities and remove them before trying to create a mesh for it. Find below a model, where faces had to be merged before a surface mesh could be generated.

A bench-vice model

The bench vice model with the problematic region marked in red

Detail of the problematic region

Detail: Very small face with very sharp angles. Causes the surface mesher to fail

Detail of the cleaned region

Detail after CAD cleaning: Problematic face was removed, meshing works fine.

Use symmetry

If you problem is symmetric, you can significantly reduce computing time by just uploading one part of the whole model and use symmetry boundary conditions.

Divide and Conquer

Very often it is not the best approach to analyze your complete CAD assembly in one single analysis. Often it is more helpful to divide your problem in small chunks and analyze them one by one.

Simulation specifics

Often the boundary conditions or other simulation specific entities have requirements to the CAD-model. Find below some hints:

Entities for boundary conditions

The SimScale platform will keep all entities of your CAD model (faces, edges, nodes) during the mesh creation process. So if you want to apply a boundary condition later on a specific face or edge, you have to make sure that this face exists in the CAD model

Faces in the CAD model prepared for the analysis

Prepared CAD model such that one can directly start assigning boundary conditions

Fluid volume extraction

To run a fluid mechanics simulation, you have to provide the actual fluid volume. For internal fluid flow analyses this means that you have to extract the region in your CAD model as a single part and upload this part to SimScale. Often this means that you have to invert your CAD assembly to identify where the fluid actually flows. Some CAD systems do have dedicated tools for such operations.

Extraction of Fluid volume for fluid mechanics simulation

The actual CAD model on the left and a streamline visualization of the fluid volume on the right

Trouble shooting

Find some hints to deal with occuring problems on SimScale:

  • “Problem in surface meshing”: Check your CAD model for very small features or intersecting regions.