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  • Documentation

    Why Do Geometry Operations Fail?

    This article covers the top reasons why imprint, internal and external flow volume extraction CAD Mode operations fail.

    Before we start, it is advised to visit the documentation page on CAD preparation and upload. In addition, please visit the following knowledge base article to find out how to find CAD faults.

    Common CAD Faults

    CAD faults, in general, will most likely cause geometry operations to fail. Therefore, you should make sure that before uploading the model to the platform, it does not have any CAD faults. The most common CAD faults are:

    • Small features/faces
    • Inexact edges
    • Overlapping faces/solids
    • Sliver faces
    • Knife edges/vertices
    • Discontinuous faces/edges
    • Bad faces
    • sheet/surface elements
    • Overlapping solids (interference)

    To check the CAD faults above, many CAD software have features to detect the common CAD errors.

    Following are some geometry operations you will find in SimScale CAD Mode.

    1. Imprint

    Always use the imprint operation if you have a model with multiple parts.

    • Imprint operation fails due to the interference between parts.
    example of an interference in a cad model to show one of the reasons why geometry operations fail
    Figure 1: Interference means an overlap between the solid parts.

    To resolve interference faults, first, you need to perform an interference check on the CAD model. Our CAD mode has this facility to detect and automatically fix interferences. For manual fixing, once you detect an overlap, you can subtract the body from the other one to remove the overlapping region. Alternatively, you can merge the two parts into one if this is suitable for your case. Check our Boolean operations integrated for this purpose.

    In conclusion, imprint operation will work on an interference-free CAD model.

    • Imprint operation fails if there is a small face or a sliver face on one of the contact faces. Clean the small and sliver faces in CAD Mode or on your CAD software, before uploading the model to the SimScale platform. In the following picture, you see a PCB board (green), covered by a resin layer (semi-transparent blue). The cooling is performed by a cooling pipe (grey). Imprint operation is necessary to capture the contacts between each part and execute a CHT analysis.
    cooling pipe has a small face at the contact
    Figure 2: Example of a small face on the contact interface

    2. Flow Volume Extraction

    To learn more about the flow volume extraction geometry operation, visit this Knowledge Base article.

    2.1 Internal Flow Volume

    If your model has openings and you would like to create a flow volume inside, use this geometry operation. This operation possibly fails because of the following CAD faults:

    • Interference between surfaces
    • Boundary faces are not parallel

    Boundary Faces

    The following picture is an example of the boundary faces not being parallel:

    example of non-parallel surfaces  in a cad model to show one of the reasons why geometry operations fail
    Figure 3: An example of parallel and not parallel boundary faces

    In this case, you will need to make sure that all the faces that are facing the environment outside of your model are parallel. The picture on the left shows that the two faces on the top are not parallel with the two faces on the side. The picture on the right shows an example of a parallel boundary face.

    • Boundary faces are not completely closing the openings of the cross-section

    The following example shows how the boundary faces at the opening should be created:

    example of an open boundary face in a cad model to show one of the reasons why geometry operations fail
    Figure 4: An example of closed and not closed boundary faces at the opening.

    You will also need to ensure that the boundary faces do not have any openings. This can be seen in the picture on the right where the top and side faces do not meet on the four corners. You can solve this by changing the four faces into one face.

    Contact Boundaries

    • An internal part has an edge-to-face or point-to-face contact with the boundaries of the fluid volume

    You see an edge-to-face contact example in the following picture:

    example of an open channel with an internal triangular component to show edge-to-face contact causing geometry operations to fail
    Figure 5: An example of an open channel, in edge-to-face contact with an internal component.

    Avoid any point-to-face or edge-to-face contact between the internal parts and the boundaries of the fluid volume. One way to avoid this issue is to trim the sharp edges and/or corners, in contact with the walls.

    • Two internal parts have an edge-to-edge contact: this happens when two parts share a single edge, and there are no face-to-face contacts. For example, in the CAD model below, there is an edge-to-edge contact between the laptop and the table:
    edge-to-edge contact causing geometry operation failure
    Figure 6: In an edge-to-edge contact, two parts share a single edge in common

    From a side view, the edge-to-edge contact is clear. One way to solve the problem is to generate a face-to-face contact by translating/extending one of the volumes:

    edge to edge contact flow volume extraction failure
    Figure 7: On the left, the edge-to-edge contact is highlighted on a side view. The image to the right highlights a possible fix to the problem, by translating the laptop and generating a face-to-face contact.

    Another possibility is to move the laptop to the right, preventing the edge-to-edge contact from occurring.

    • A boundary face is shared by multiple openings and cavities

    If multiple openings and cavities share the same boundary face, under some exceptional cases, internal flow volume extraction might fail. In this case, it is recommended to separate the boundary faces of the inlets.

    In the next example, there are two openings and seven cavities on the highlighted face. The flow volume will be generated inside the box, not in the cavities. Therefore, we had to separate (isolate) the boundary faces of the openings from the main face. To clarify, this operation needs to be done in CAD mode.

    internal flow volume extraction feature is being used on a box model with two openings and several cavities
    Figure 8: Isolating the boundary faces of the openings from the main face can prevent flow volume extraction failures in CAD mode

    2.2 External Flow Volume

    This operation possibly fails because of the following CAD faults:

    • Interference between surfaces
    • An internal part has an edge-to-face or a point-to-face contact with the boundaries of the fluid volume
    comparison of right and wrong way of generating an enclosure around a triangular component
    Figure 9: Ensure face-to-face contact or no contact while generating an enclosure

    Further Tips

    The following points are essential:

    • Firstly, avoid sheet/surface elements. In other words, the CAD model should only consist of solid parts.
    • Secondly, avoid overlapping solids (unless they represent cell zones).

    The following points are not essential, but we strongly recommend to perform a cost-effective simulation experience:

    • Model only the essential parts. In other words, remove unnecessary features or components, such as small nuts, bolts, pins, etc.
    • Fill leftover holes and gaps.
    this picture shows the comparison of an original pcb cad model and a cleaned one
    Figure 10: Common CAD cleaning measures on PCBs.


    If none of the above suggestions solved your problem, then please post the issue on our forum or contact us.

    Last updated: April 15th, 2022