Defining a Curved Surface Mesh Operation

A curved surface mesh operation is applied to all objects in the model. To illustrate, a model that has two cylindrical objects is used. For curved surface mesh operation, the two important controls for the Manual settings are Normal Deviation and Surface Deviation. Curved surface operations with respect to these Manual Setting controls are described below.

Normal Deviation Angle

By default, Normal Deviation is 22.5 degrees in HFSS. When specifying curved surface mesh operation, consider the sizes and shapes of all curved objects to obtain the desired faceted triangulation model for the mesh.

The following illustrations show a model that has two cylindrical objects.

In HFSS 3D Layout, the 90 degree sweeping curves are faceted as 4 segments since Normal Deviation is 22.5 degrees by default. If you specify the surface normal angle as 5 degrees at the model level, there are 18 segments on both cylindrical curved surfaces as illustrated in the following figures. Such a level of granularity is useful for large curved surfaces, but for small curved surfaces, it creates an extremely dense mesh. Therefore, use caution to assign a curved surface mesh operation with small Normal Deviation unless all the curved objects in the model have equivalent curvatures and sizes.

Maximum Surface Deviation Length

An appropriate value of Surface Deviation supplements the Normal Deviation setting. When you set Surface Deviation in addition to the Normal Deviation setting, it can further improve the geometry representation. By specifying Surface Deviation you can control the difference between the faceted face and the real curved face. Surface Deviation is defined in user unit. Consider the same model with two cylindrical objects. In this mode, if you specify 30 degrees as the Normal Deviation, there are 3 segments on both cylindrical surfaces as expected.

Since the radius of the large cylindrical surface is 6mm, set 0.06mm as the Surface Deviation in the curved surface mesh operation. This ensures we have the desired faceting geometric model.

To summarize, the Surface Normal Angle of 30 degrees provides one facetation of the geometry, and the Surface Deviation of 0.06 mm controls the difference from the real curvatures. Since the small faceted cylindrical surface already has the difference smaller than 0.06 mm, the surface is maintained as is. However, since the large cylindrical surface has a gap greater than 0.06mm, the facetation is made finer to adhere to the specified length.

As a rule of thumb, large surface normal angle can control the small curved objects while small Surface Deviation can control the large curved objects in the model-level mesh operation. Surface Normal Angle is effective for skewed curved surfaces.

Aspect Ratio

Aspect Ratio is the ratio of the longest length of an element to its shortest length. Aspect ratio controls the faceting triangle ratio. It is needed only when you want a highly uniform mesh on the curved surfaces or when the curved object is either thin or has a narrow gap. By default, Classic mesher uses 20.0 as the Aspect Ratio whereas TAU mesher uses 8.0 as the Aspect Ratio.

When there is a narrow gap between two curved surfaces, a small value of aspect ratio ensures the gap to be uniform. The figures below illustrate a geometric model with the aspect ratio of 20 and 2. Smaller aspect ratio ensures better representation of the curvature for skewed curved surfaces, especially in thin curved layers.

Use caution when assigning a small aspect ratio, since it will produce a dense mesh at many other curved surfaces which may not require the denseness.