Free Domain#

Description#

Meshes are generated from “domains” which consist of a closed set of edges, and an arbitrary number of closed internal boundaries. Free meshes are meant to be auto-meshed with a particular automatic meshing algorithm (unfortunately the method cannot be changed for an existing domain).

The outer domain boundary is first selected in a counter-clockwise sense in order to make a closed domain. Any number of continuous edges can be selected. After the outer boundary has been defined, inner “holes” can be created by selecting a closed boundary of edges in a clockwise sense.

  • Delaunay Triangles The Delaunay mesher is an all triangle mesher with the ability to have gentile gradients in the element size, and control over the element geometry. This is the preferred method for non-critical locations where high gradients in element size exist. Some control over the meshing is available with the following parameters:

    • min angle: An angle measure defining what is a “skinny element.” It’s a loose definition, but generally zones of skinny elements will be subdivided in order to get triangles with minimum angle greater than this number.

    • propagation: A loose measure of element size propagation through the domain. The algorithm looks at neighboring element sizes to determine if there is excessive change in the size. Smaller (down to 1) yields a finer mesh.

  • Paving is an automatic meshing method that generates all-quad elements and requires the outer domain boundary to have an even number of sides per element. This method performs optimally when the element sizes are relatively uniform. Some control over paving is provided through the following parameters, although they offer limited control:

    • seam angle: The angle below which interior angles of the boundary are seamed. Sometimes changing this can let the algorithm pass where it had trouble (rec. angle between 15 and 55).

    • size factor: A multiplier controlling the proximity check of edges which are about to meet. Interior paving boundaries which are “close” will be seamed. factors from 0.0 to 0.3 are pretty good.

    If difficulties occur in paved meshing, or heavily distorted elements are created, the best thing to do is alter edge node spacings and re-try. Some effort is generally required to get a nice mesh with this method.

Dialog Options#

The free domain pop-up dialog has some parameters which need to be specified:

  • Elset name: The name of an elset which will be applied to all the elements created for the domain (change using cmd-M when the domain is selected (yellow)).

  • s3d, c2d, cax, etc: element geometry to use. Note axisymmetric has its own “space.” All types can be extended to 3D, but if the domain is outside the \(z=0\) plane, s3d should be selected.

  • Linear - Quadratic: Element interpolation.

  • Normal - Reduced: Element integration order.

  • Delaunay Triangles: The Delaunay refinement method for triangles.

  • Paving: All quad meshing.

../../_images/FreeDomain.svg

Example 1#

The following is an example of Delaunay triangulation of a multiply connected geometry defined in 3D. Note there are several domains, allowing the node spacing and meshing parameters to be defined locally.

../../_images/delaunay1a.fig.svg

Example 2#

The following is an example of paved meshing, with the click sequences labeled. One can see the location where advancing meshes collided, and seaming was performed.

../../_images/pave4.fig.svg

Paving works for 3D meshes as well, as seen below. Note the geometry flaw was meshed (because the proximity check was reached with the different advancing fronts in 3D. The resulting “surface” is a coincidence of the seaming. Some of the difficulties in the current Paving algorithm are due to a generalized implementation for paving 3D surfaces.

../../_images/box.fig.svg