OOF2: The Manual

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Skinny Elements (ones with a high aspect ratio) can cause poor finite element performance. Refining a Skeleton with the targets parameter of Refine set to AspectRatio will subdivide the long sides of quadrilateral Elements with high aspect ratios. (Triangular elements are excluded because refining a high aspect ratio triangle creates new elements that are just as bad as the original. To remove skinny triangles from a Skeleton, use the Rationalize Skeleton modifier.)

The aspect ratio of a quadrilateral element is computed by taking the ratio of the lengths its second longest and second shortest sides. The ratio is always greater than or equal to 1. This definition has the advantage of being easy to compute and only yielding large ratios for those quadrilaterals that will be improved by subdividing their long sides. (Quads whose long sides are adjacent are not refined. These quads can be removed by Rationalize. Some example of Elements that won't be refined are given in Figure 6.66.)

Figure 6.66. Elements not Refined

A collection of high aspect ratio elements that nevertheless won't be refined by CheckAspectRatio. (a) and (b) are triangles. (c) and (d) don't have a clear distinction between long and short edges, and (e) doesn't have its long edges on opposite sides of the element.

Figure 6.67 shows a Skeleton containing one skinny Element with its long sides marked (in bold) for refinement. Unlike CheckHomogeneity, which marks all the Segments of its target Elements, CheckAspectRatio marks only the long Segments. Figure 6.68 shows the result of refining the element.

Figure 6.67. An Element with a High Aspect Ratio

A Skeleton containing an element with a high aspect ratio.

Figure 6.68. Refining an Element with a High Aspect Ratio

The Skeleton produced by refining with modifier=Refine(targets=CheckAspectRatio(threshold=5.0), criterion=Unconditionally(), degree=Bisection(rule_set='conservative')).