Use Free Tetrahedral () to create an unstructured tetrahedral mesh on a selection of domains, as shown in Figure 8-57. If no selection is specified, Free Tetrahedral creates a mesh on the remaining domains, boundaries, edges, and points. It inserts pyramids to match the tetrahedral mesh to any existing quad mesh on adjacent faces.

The Free Tetrahedral operation is available both for meshing sequences that generate mesh for a geometry and for meshes that define their own geometric model.

When you generate a mesh for a geometry, you can control the number, size, and distribution of elements by using one or several Size, Size Expression, Distribution, and Corner Refinement attributes.

For meshes that define their own geometric model, Free Tetrahedral () has a default Size subnode which, in practice, defines the size on the same selection as for the parent node. This subnode cannot be deleted or disabled. You can add more Size attributes to further control the element size.

To add a Free Tetrahedral node, select one or several domains in the Graphics, then choose one of the following:

Define the domains where you want to create an unstructured tetrahedral mesh. Choose the level of the geometry from the Geometric entity level list:

To scale the geometry during the meshing operation, change the x-scale, y-scale, and z-scale to positive real numbers, as done for the mesh in Figure 8-58. If any of the scale factors are not equal to one (1), the software scales the geometry in those directions before meshing. After meshing, it restores the geometry and mesh to fit the original size.

The scale factors make it possible to generate meshes that are anisotropic, and they are useful if the mesh generator creates many elements due to a thin geometry or if the mesh generation fails due to large aspect ratios in the geometry. Compare this to changing the Resolution of narrow region parameter in a mesh Size attribute to only affect the mesh size in narrow regions. Another alternative is using a Swept mesh operation for the thin parts of a geometry. The resulting hexahedral or prism elements usually have better element quality compared to the unstructured mesh after scaling the geometry. Consider using Remove Details to remove narrow face regions and thin domains, if not important to the simulation at hand. Yet another possibility is to simplify the geometry by using shell approximations in the physics, where appropriate.

Select the Smooth across removed control entities check box to smooth the transition in element size across removed Controlling the Mesh Size Using Mesh Control Entities. You can specify the number of smoothing iterations in the Number of iterations field. In the Maximum element depth to process field you can specify the maximum element depth for the mesh points to be smoothed.

From the Method list, choose the Delaunay tessellation method to use for creating a tetrahedral mesh:

In this section, you can control how much effort COMSOL Multiphysics puts into optimizing the minimum element quality and tuning the optimization for certain situations. Use the Mesh Statistics to see the full statistics of your mesh. All optimization is done on the volume elements while keeping the surface mesh fixed.

From the Optimization level list, choose one of the following levels:

As an example, refer to the tutorial Adjusting the Element Size for the Unstructured Mesh Generator and compare the amount of mesh elements plotted for qualskewness < 0.2 (see Figure 8-60). In this tutorial, the Maximum element growth rate is high which means that the element size can grow fast from the small elements on the boundaries into the domain. This results in fewer domain elements, but also more elements of less good quality. Increasing the Optimization level from Basic (image 2 below) to High (image 3 below) increases both the minimum and average quality. Compare this to lowering the Maximum element growth rate (image 4), which gives about the same minimum and average quality, but results in more elements in total.

There are additional settings under Accept lower element quality to that you can use if you can accept a lower mesh element quality. A Physics-Controlled Mesh will typically ensure that the mesh is good with respect to the physics at hand and is recommended. For user-controlled meshing sequences, wave and CFD problems are examples of computations that can be sensitive to too large and too small elements and might require changed settings.