CAD preparation is the first step of setting up a simulation. Some tips for CAD preparation are outlined below:
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 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.
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. Please refer to the article CAD Scaling[Insert Link] for more information.
Very often CAD can be simplified to get more accurate simulation results in shorter time. A few cases are listed below
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 and they increase the meshing and computing time significantly. Therefore, such features should be removed during the CAD preparation.
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. These features need to be removed before meshing. An example is demonstrated below, where faces had to be merged before a surface mesh could be generated.
The bench vice model with the problematic region marked in red
Detail: Very small face with very sharp angles. Causes the surface mesher to fail
Detail after CAD cleaning: 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.
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
Often the boundary conditions or other simulation specific entities have requirements to the CAD-model. A few examples are explained below:
Entities for boundary conditions
The Simscale platform keeps all entities of the CAD model (faces, edges, nodes) during the meshing process. Therefore, to apply a boundary condition on a specific face or edge, it needs to exist as a separate entity in the CAD model.
Prepared CAD model such that one can directly start assigning boundary conditions
Fluid 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 Fluid Volume Extraction feature, which can be found under the geometry operations menu. More information can be found here Fluid Volume Extraction[Insert Link].
The actual CAD model on the left and a streamline visualization of the fluid volume on the right
CAD specifics for Conjugate Heat Transfer simulations
For Conjugate Heat Transfer (CHT) simulations, interfaces are required for simulating the heat transfer between two regions. These interfaces are generally created automatically during the meshing process. To enable the mesher to create the interfaces automatically, CAD should have face cutouts at the mating surfaces. This is illustrated in the case below:
Sample CAD for CHT application
Figure shows a sample CAD for a CHT application, containing three regions, one fluid region and two solid regions.
Ideally, the CAD should appear as shown in the figure below. Here, the surfaces that are in contact with other parts, are precisely cut into separate surfaces.
CAD with face cutouts
This can be done in the CAD preparation stage using the respective CAD tool or it can also be performed on the Simscale platform using the Imprint feature. More information can be found here Imprinting[Insert Link].
Find some hints to deal with occurring problems on SimScale:
“Problem in surface meshing”: Check your CAD model for very small features or intersecting regions.
Last updated: December 24th, 2018
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