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Documentation

CAD Preparation and Upload

Preparing, uploading, and adapting your CAD model for simulation is the first step in setting up a simulation. Some tips for CAD preparation are outlined below:

CAD Upload/Import

To upload a CAD file on to the SimScale workbench:

  1. Open an existing project or create a new one.
  2. To open the upload dialog, click the “+” button next to Geometries in the simulation setup tree.
  3. Drag and drop your CAD file or click to open the file selection dialog.
CAD import dialog box in simscale
Figure 1: SimScale CAD upload dialog. Either upload files directly from your machine or import via Onshape.

Alternatively, choose the import via Onshape to directly pull a design from your Onshape library.

Supported CAD Formats

SimScale supports the following CAD formats:

  • Parasolid (.x_t, .x_b)
  • SolidWorks (.sldprt, .sldasm)
  • Autodesk Inventor (.iam, *.ipt)
  • Rhino (.3dm)
  • CATIA (.CATPart, .CATProduct)
  • ACIS (.sat, .sab)
  • STEP (.stp, .step)
  • IGES (.igs, .iges)
  • STL (.stl)

This list is sorted in the preferred order of formats. Generally, it is recommended to upload the model in the native format of the tool it was created with. (e.g. as .sldprt if modeled in Solidworks). SimScale natively uses the Parasolid CAD kernel. In order to have all CAD manipulation functionality available within SimScale, the CAD model needs to be successfully translatable into the Parasolid format.

CAD Upload Options

CAD optimization options
Figure 2: CAD model upload optimization options recommended for simulations

While uploading the CAD model it undergoes optimization steps automatically to best suit the simulation. These steps are explained in detail below. Although not advisable, the user can turn these off if needed.

Facet-split on import

Facet models contain geometry that is not described by parametric functions but by a surface mesh consisting usually of triangles. Often the whole faceted geometry is stored in one big surface part and can not be easily accessed to define e.g. boundary conditions on separate faces. In order to do that you can split the faceted parts of the models by a surface angle. For all angles that are higher than the given value, the algorithm tries to separate the geometry by introducing separate faces. Some formats contain only faceted data, for e.g., *.stl files; others like Parasolid, Step, or Rhino can contain mixed parametric and faceted parts. For parametric geometry, this setting doesn’t take any effect.

Automatic Sewing

Automatic sewing tries to connect parts of the model that are stored separately but touch exactly. In case a closed shell can be achieved, it additionally creates a solid body bound by the original faces. As most simulations are carried out on three-dimensional domains and require solid regions as input it is recommended to use this option on import.

Improve data on import

This option tries to improve the topology (e.g. edges, vertices) and geometry of the model by adjusting tolerances, simplifying entities, etc. As this option should improve CAD operations and data handling for all downstream applications it is recommended to use it on import. For very complex models it can take a considerable amount of time though, therefore you can also opt-out and reconsider in case you face issues in geometry handling or meshing.

Optimize for LBM / PWC

This option allows you to import an *.stl file that is optimized for the Incompressible LBM and Wind Comfort analysis types. It leaves out complex import steps like sewing and cleanup that are not required by the LBM solver and therefore also allows to import big and complex models fast.

Assembly Upload

To upload an assembly file from a particular CAD tool please collect all the related part and sub-assembly files along with the assembly file and create a *.zip file. Then upload this *.zip file using the upload dialog. Find a detailed guide on assembly upload here. Currently, SimScale supports the assembly upload feature for the following formats:

CAD ToolNative Part formatNative Assembly format
CATIA.catpart.catproduct
Fusion 360 / Inventor.ipt.iam
SolidWorks.sldprt.sldasm
Table 1: CAD Tools and their respective part and assembly formats
Cad model in viewer
Figure 3: Once uploaded and converted successfully, the model will be shown in the viewer and listed in the Geometries section of the navigation tree.

Note

In case round parts of your geometry appear to have corners, don’t mind: We automatically simplify your geometry for displaying purposes to make sure that you can fluently interact with the model. Internally, especially for the meshing process, SimScale uses the fully featured geometry.

Supported CAD Operations

After upload, some additional preparation might be required. The following CAD manipulation options are currently available from within the SimScale workbench:

  • Scaling
    In case the uploaded model does not match the original real dimensions of the modeled object, the Scaling operation can be used to adapt the model size accordingly. The dimension of the model is very important for the simulation setup. Scaling the model in meters(m) or millimeters(mm) will have a large impact on the results.
  • Imprinting
    Only required for Conjugate Heat Transfer (CHT) simulations. Here interfaces are defined for simulating the heat transfer between two regions. It is required that these interfaces are defined by two congruent faces on both contacting regions. In case this requirement is not met by the assigned CAD model, the platform will recommend an automatic Imprinting operation.
  • Flow volume extraction
    A fluid mechanics simulation is performed on the actual fluid volume. For an internal flow problem, this means that the fluid volume needs to be extracted from the CAD geometry. This can be done in the CAD preparation stage using the respective CAD tool. It can can also be performed on the SimScale platform using the Flow Volume Extraction feature, which can be found under the geometry operations menu. More information can be found here: Fluid Volume Extraction
  • Surface splitting
    This operation only applies to *.stl models. Often those kind of models consist only out of a single face, making it necessary to split the model based on a user-defined feature angle for later assignment.

All currently available CAD operations can be found in the context menu of each geometry listed in the geometries section.

cad operations context menu
Figure 4: Right-click on the CAD model to open the context menu and select “Add geometry operation” in order to open the list of all available CAD operations that can be applied to the model. Depending on the file format used, not all CAD operations might be available.

CAD Preparation

Depending on the complexity and quality of your CAD model, some preparation and clean up work might be required. Most of this clean up work should be done before importing the model into SimScale. The following general guidelines might help get you to a first successful iteration of your simulation.

General Hints

  • Start simple and iterate
    A proven strategy for the simulation setup is to create it for a very simple version of the problem in order to see whether or not the simulation approach is viable.
  • Think the complete workflow through
    Having an idea about the mesh and the application of boundary conditions can help with the CAD preparation.
  • Have a clear understanding of the problem
    This helps to decide whether certain effects can be neglected or not.

Diving deeper, keep in mind the following requirements for a successful and accurate simulation:

Model dimension

The dimension of the model is very important for the simulation. If there are any discrepancies between the model units and the SimScale units this can lead to unrealistic geometrical dimensions. In this case, it is crucial to perform a scaling operation.

Figure 5: In case of uploading an *.stl file, the model dimension can be selected.

The default dimension used within the workbench in Meter. The user can change it to other units from the dropdown as shown above although this option is only available for .stl files that do not contain unit information in the file.

Complexity reduction

Very often the CAD model can be simplified to get more accurate simulation results in a shorter time. A few cases for potential optimization potential are listed below.

Defeaturing

CAD models often contain many detailed features because of manufacturing constraints or the installation. Good examples are small holes or windings. These detailed features might be relevant for the final manufacturing but they do not affect simulation results, but only increase the meshing and computing time significantly. Therefore, such features should be removed during CAD preparation before import into SimScale.

Remove Small Entities

Small entities can be a problem when it comes to meshing. If there are very small faces with sharp angles the surface meshing might fail. An example is demonstrated below, where faces had to be merged before a surface mesh could be generated. Such small entities should also be removed during CAD preparation before import into SimScale.

example of small features that should be cleaned up before import
Figure 6: Left: Very small face with very sharp angles. Causes the surface mesher to fail. Right: Problematic face was removed, meshing works fine.

Use of Symmetry

If the problem is symmetric, computation time can be significantly reduced by using symmetry boundary conditions and performing simulations on just a part of the whole CAD model. In such a case only one instance of the symmetric parts of the model should be imported.

Divide and Conquer

Analyzing smaller parts of the entire CAD one by one, rather than analyzing the entire CAD in a single simulation, can help reduce the complexity of the problem and speed up the simulation.

Simulation specific preparation

Often the boundary conditions or other simulation solver specifics define additional requirements on the CAD-model. A few examples are explained below:

Entities for Boundary Conditions

To apply a boundary condition on a specific face or edge, it needs to exist as a separate entity in the CAD model. Therefore, to e.g. assign a load condition only to a certain area, this area needs to exist as an entity in the model.

example geometry with split surfaces
Figure 7: Prepared CAD model such that one can directly start assigning boundary conditions. In this case the front face of the bumper has been split into multiple parts.

Cad Faults

In case your model could not successfully be translated into the SimScale-specific internal format, all entities that failed translation will be exposed after upload. In order to progress further, try fixing all faulty parts and upload the model once more.

cad faults interface
Figure 8: Faulty topological entities will be listed in the scene tree. Click on a fault to visualize it in the viewer. To progress further, all faults need to be resolved first and another instance of the cleaned geometry needs to be uploaded.

CAD Topology

In general, a CAD model consists of different types of topological entities such as solids, faces, edges and vertices. It’s important to be aware of this topology since it will have an impact on mesh generation and simulation setup. Find an introduction to CAD topology here.

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