HFSS15: Healing and Meshing

Potential problems with 3D Models

This section lists problems that can prevent a 3D model from being meshed successfully. Subsequent sections will describe how these problems can be detected and addressed.

ACIS errors

The underlying solid modeling technology used by ANSYS EM’s 3D products Maxwell, HFSS and Q3D is provided by the ACIS geometric modeler. You can create models directly in the drawing environment of these products using primitives, such as boxes, cylinders, etc. and operations on primitives, such as Boolean operations. In addition, you can import models produced by other CAD tools in a variety of formats such as STEP, IGES, etc. In ANSYS EM’s 3D products, all models have to be stored internally in ACIS’ native format, known as sat format. When you import models into ANSYS EM products, translators are invoked that convert the models to sat format. Often, models that were created in other CAD tools were created initially for other purposes than electromagnetic analysis, such as for mechanical design or just for display purposes. They may have imperfections that make them illegal to ACIS. Further, there can be compatibility issues between different versions and even flavors of modeling tools. All this can lead to errors in imported 3D models.

If you use ANSYS EM products to create geometry models, and thereby avoid model import and translation, you are unlikely to encounter such problems.

Mixed dimensionality

Even if a model is imported and translated without errors, there is a restriction to be aware of. ACIS can handle mixed-dimensionality models. One of the goals of ANSYS EM’s use of the ACIS modeling system is to create a valid volumetric mesh for simulation. Mixed-dimensionality models will not yield a valid volumetric mesh. Therefore, the ANSYS EM tools will not mesh objects with mixed dimensionality, so-called non-manifold objects. For instance, imagine a 3D object representing a curved metal plate with a small but finite thickness. If it reaches zero thickness somewhere while having non-zero thickness elsewhere, it has mixed dimensionality, 2D as well as 3D. You will get an error message saying that the object is non manifold. Of course, 2D and 3D objects can co-exist in a model, but any one object cannot be both 2D and 3D.

Intersecting objects

Another restriction is that ANSYS EM 3D tools don’t allow partial intersections (also known as partial overlaps) between 3D objects. Each element of the mesh has to belong unambiguously to one object. There is no problem if one object is enclosed completely inside a bigger object, but partial intersections lead to ambiguities. As long as there are partial object intersections, the mesh generator will not attempt to create a mesh. Instead, you will get an error message notifying you which objects are intersecting.

After you run a validation check, you can right-click on an intersection error message in the Message window, and select Go to reference from the shortcut menu. This selects the intersecting objects.

You must remove the intersections before you can proceed. You can do this by changing the shapes of objects slightly, or by subtracting one object from the other.

Caveat: if as a result of a subtraction the model has pairs of true surfaces that are coincident, that is, smooth curved surfaces that fit exactly one inside the other, you make it harder for the mesh generator to create a mesh. This is because ACIS will create segmentations on each of these surfaces, and these segmentations are not guaranteed to fit. Setting a small value for Surface Deviation under Mesh Operations >Assign>Surface Approximation increases your chance of success in such a case, but it is better to avoid such situations if you can.

Small features and misalignment

When there are no ACIS errors in the model, no non-manifold objects and no partial object intersections, the mesh generator can be invoked to create a valid mesh for the electromagnetic analysis. Even if the geometry is valid, mesh generation can still fail. Possible causes are the presence of very short edges, very small faces, long and thin sliver faces, and slight misalignments between faces that are supposed to be coincident.