17.7.6 What is the difference between the medial axis algorithm and the advancing front algorithm?

The medial axis algorithm and the advancing front algorithm are two meshing schemes that ABAQUS/CAE can use to generate a mesh when you are doing the following:

Meshing a surface with quadrilateral or quadrilateral-dominated elements using the free meshing technique.
Meshing a solid region with hexahedral or hexahedral-dominated elements using the swept meshing technique. (ABAQUS/CAE generates hexahedral and hexahedral-dominated meshes by sweeping the quadrilateral and quadrilateral-dominated elements generated by the two algorithms from the source side to the target side.)

The two algorithms are described as follows:

Medial axis

The medial axis algorithm first decomposes the region to be meshed into a group of simpler regions. The algorithm then uses structured meshing techniques to fill each simple region with elements. If the region being meshed is relatively simple and contains a large number of elements, the medial axis algorithm generates a mesh faster than the advancing front algorithm. Using the option to minimize the mesh transition may improve the mesh quality. The mesh transition option is available only for quadrilateral and hexahedral meshing.

Advancing front

The advancing front algorithm generates quadrilateral elements at the boundary of the region and continues to generate quadrilateral elements as it moves systematically to the interior of the region.

The elements generated by the advancing front algorithm will always follow the seeding exactly for quadrilateral-dominated and hexahedral-dominated meshes (except when you are creating a three-dimensional revolved mesh, and the profile being revolved touches the axis of revolution). For other meshes the elements generated by the advancing front algorithm will always follow the seeding more closely than those generated by the medial axis algorithm. When you are trying to mesh a surface, the advancing front algorithm supports meshing of virtual topology; the medial axis algorithm does not.

You may have to experiment with the two algorithms to obtain the optimal mesh. Figure 17–33 illustrates a simple shell region that was meshed with quadrilateral-dominated elements using the two meshing algorithms. In this example both algorithms generate an acceptable mesh.

Figure 17–33 Both algorithms generate acceptable meshes.

Because the elements produced by the advancing front algorithm follow your seeds, the resulting mesh may include some skew in the elements in narrow regions. Element skew is illustrated in Figure 17–34.

Figure 17–34 In some cases the advancing front algorithm generates elements with some skew.

In contrast, the advancing front algorithm may generate elements of a more uniform size with a more consistent aspect ratio, as shown in Figure 17–35. Uniform element size can play an important role in the analysis; for example, if you are creating a mesh for an ABAQUS/Explicit analysis, small elements in the mesh can unduly control the size of the time step. In addition, if it is important that the elements follow your seeds, the advancing front algorithm is preferable.

Figure 17–35 In some cases the advancing front algorithm produces a more uniform mesh.

In some cases, when you mesh multiple regions, ABAQUS/CAE generates a mesh with sheared elements at the interface between regions. Nodes in one region may be positioned differently than nodes in an adjacent region, which results in shear at the common boundary when ABAQUS/CAE merges the adjacent meshes. Figure 17–36 shows multiple swept regions and the resulting mesh generated by the medial axis algorithm.

Figure 17–36 Mesh shear is significant between adjacent regions using the medial axis algorithm.

The advancing front algorithm positions the nodes on the source side at the same location as your seeds; as a result, the mesh shear will be reduced. Figure 17–37 shows the same part meshed with the same seeding using the advancing front algorithm. However, as stated earlier, you may have to experiment with the two algorithms to obtain the optimal mesh.

Figure 17–37 Mesh shear is reduced between adjacent regions using the advancing front algorithm.

You use the Mesh Controls dialog box to choose the meshing algorithm. By default, ABAQUS/CAE does the following:

Chooses Medial axis if the element shape is Quad and the meshing technique is Free.
Chooses Medial axis if the element shape is Hex and the meshing technique is Sweep.
Chooses Advancing front if the element shape is Quad-dominated and the meshing technique is Free.
Chooses Advancing front if the element shape is Hex-dominated and the meshing technique is Sweep.

To display the Mesh Controls dialog box, select MeshControls from the main menu bar. For more information, see “Setting the mesh algorithm,” Section 17.15.6.