Geometry abstraction, polygon geometry, and convergent geometry > Polygon geometry and abstraction

Understanding the geometry abstraction process

There are two different ways you can abstract the polygon geometry in your FEM file to optimize it for meshing:

You can use the software's automatic abstraction capabilities during either 2D or 3D meshing.
You can choose Home tab→Polygon Geometry group→Auto Heal Geometry to manually abstract your model.

Whether you choose to perform the abstraction during meshing or by using Auto Heal Geometry, the abstraction process is the same. In both cases, the software searches your model for geometric features that are so small that they can prevent the software from being able to mesh or solve your model. During the abstraction process, the software eliminates:

Short edges.
Sliver faces.
Highly pinched regions of the geometry.

Seams inserted into periodic geometry

For models that contain periodic (360°) geometry, such as cylinders or cones, the software automatically inserts a single edge or seam during the automatic abstraction process. This seam insertion:

Occurs automatically during the 2D or 3D meshing process.
Facilitates the generation of the mesh.
Is not controlled by the minimum element length tolerance.
Creates two new non-periodic faces from a single periodic face. The face ID associated with the original periodic face is not preserved.

Understanding the Minimum Element Length model tolerance

The key difference between the different ways to perform the abstraction is in how you define the minimum element length tolerance value. The software uses the minimum element length tolerance to determine which features to eliminate during the abstraction.

In the 2D and 3D meshing dialog boxes, you define the minimum element length as a percentage of the overall element size.
In the Auto Heal Geometry dialog box, you define the minimum element length as an absolute measurement.

In general, the abstraction process is designed to abstract features that are smaller than 10% of your overall element size. Removing features below that size helps ensure that your model will mesh with elements that have an aspect ratio greater than 10:1, which is required by many solvers. However, you should always use caution not to set the Minimum Element Length tolerance too high. In general, you should use caution when using a Minimum Element Length tolerance that is greater than 20% of the overall element size you intend to use to mesh the geometry. The software enforces the following limits on the Minimum Element Length value:

In the 2D Mesh dialog box: The Minimum Element Length value must be less than 40% of the overall element size. If the CAD Curvature Abstraction check box is cleared.The Minimum Element Length value must be less than 70% of the overall element size If the CAD Curvature Abstraction check box is selected.
In the 3D Tetrahedral Mesh dialog box, the Minimum Element Length value must be less than 20% of the overall element size.

Abstraction process limitations

The abstraction process is limited to abstracting away small features. The abstraction process does not:

Suppress holes.
Transform radius corners of fillets into 90° angles.
Turn sheet bodies into solid bodies.

Removing short edges

The software abstracts any edges that are shorter than the specified Minimum Element Length tolerance. This prevents the software from creating an element with a very short edge on that portion of the geometry.

Removing sliver faces

The software abstracts any sliver faces whose width (W) is smaller than the specified Minimum Element Length tolerance.

The following graphic shows an example of a sliver face on polygon geometry.

When the software meshes the geometry, the software abstracts away the sliver face. Notice how the software doesn't include this face in the mesh.

Eliminating pinched regions

The software also abstracts away any highly pinched regions of the geometry. A pinched region is a very narrow region of a surface whose width is smaller than the specified Minimum Element Length tolerance.

In the case of a pinched region, the software evaluates the extent of the pinched region, isolates the pinched region, and then tries to merge it with the adjacent geometry. The following graphic shows an example of a pinched region.

When the software meshes the geometry, the pinched region is absorbed into the adjacent geometry.

Learn more

Geometry abstraction and polygon geometry

Polygon geometry

Polygon geometry versus lightweight facet bodies

Understanding CAD issues reported during FEM creation

How edges are represented in a solid model

Modeling tolerance values

Comparing geometry idealization and geometry abstraction

Displaying suppressed edges in polygon geometry

Checking CAE model consistency

Naming of polygon bodies, meshes, and properties

Tools for identifying and repairing polygon body problems

Synchronizing polygon body names and attributes to the CAD geometry

Controlling polygon body tessellation

Polygon body update process

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