Meshing in theory

To understand why we pay attention to good element quality, small aspect ratios and low distortion, let's take another look at the background. We are usually interested in displacements, strains and stresses. In three-dimensional space, this results in 15 unknowns, which require just as many equations. These are provided by continuum mechanics. But here we quickly reach our limits: The time required is enormous and analytical solutions are often not available. Anyone who has ever worked their way through pages of calculations involving the Airy stress function for a plate with a hole is all too familiar with these challenges.

The advantages are that a few settings automatically generate a mesh that can resolve both the global stiffness and the details that are important for contact calculations and stress evaluations. Another advantage over pure element size halving is that elements are only refined where necessary and not everywhere. The clear disadvantage is an increased number of nodes and elements. This means that we save engineering time in CAD preparation and a few steps in the iterative meshing process and buy this with more simulation time. This is where you, as the engineer, have to decide which path you want to take.

Of course, the following settings will never be applicable to all geometries, but I will give them to you as a good starting point (following illustration). Element size and Defeaturing are determined as before. You can see that the element size can be set slightly larger (previously 8mm). This is because we previously had to find a balance between small features and large areas. Now, however, the mesh can be coarser on the large surfaces, as we will cover the details with Curvature. Activate Curvature (or / and Proximity) and select a value close to the Defeaturing tolerance for the min. sizes (the mesh must not become smaller). Also select a normal angle that controls the fineness of the radii. 120° has proven to be a good starting value.

The script handles bolt meshing, for example, in a very elegant way. Select all bolt bodies, here via a string, and assign the Sweep/Axisymmetric meshing method, which results in a nice hexahedron mesh for this type of body. A local meshing size is then applied to the bolts so that bolt pretension and contacts in the support area are well mapped and no convergence difficulties arise.

Summary and recommendation

Meshing is often a process regulated by customer specifications or guidelines that have been developed over time. However, you should occasionally put your approach to the test and see whether it is better to throw old / outdated procedures overboard. Modern and powerful meshing algorithms, such as those implemented in Ansys, automatically generate meshes in the most complex structures. In order to be able to mesh as many structures as possible with these automatisms, only a few basic settings (adaptive sizing, defeaturing) are required, which you derive from your model. Special additional functions (Curvature and Proximity) allow you to resolve local areas in detail in addition to the global stiffness.

There’s more than one way to skin a cat and everyone meshes a little differently. The overall concept must be right! You can mesh with linear elements, only much finer, so that you get sufficient result quality. You can get so coarse that only the global stiffness is right, but you also have to accept that not every local detail is perfectly resolved. You can use different mesh densities to gain insights into model structure, stress evaluation and convergence behavior. Or you can use the longer calculating all-rounder mesh for as many tasks as possible. There are a variety of strategies and they all have their advantages and disadvantages. In the “Practice-Oriented Meshing in Ansys Mechanical” training course, you will learn suitable meshing procedures depending on the application and, if necessary, you can integrate these findings automatically using scripting. For this, I would like to recommend the “Automation within Ansys Mechanical using Python” training course.

As a final word, the following note: Of course you will find places in the meshes shown here that are not perfect and of course I can still rework them manually. But that was not my intention. I wanted to show that I/you/any user can automatically generate very good meshes in Ansys with a few solid basic settings, especially for complex and thin-walled structures. As an engineer, I can save myself a few hundred clicks and hours of CAD/meshing work with a clear conscience, accept a few less good elements and areas, but still achieve results of the desired quality. The art in FEM is, as in any numerical method, to minimize the error with reasonable effort, but be aware that you will never get it to zero.