Faulty Elements Generated by FancyExtruderGenerator

[miaoyinb]

Bug Description

The current algorithm used in FancyExtruderGenerator may generate faulty 3D elements during extrusion. To be specify, the generated 3D elements (i.e., PRISM6, PRISM18, HEX8, and HEX27) do not follow the node indexing convention. As a result, these 3D elements have negative volume and lead to negative Jacobian.

Steps to Reproduce

This issue usually happens in two cases:

1. If a 2D element (mesh) can be extruded in (x y z) (e.g., (0 0 1)) direction without any issue, simply use (-x, -y, -z) (e.g., (0 0 -1)) direction to extrude it will lead to a faulty 3D element (mesh).
2. For a 2D mesh in XY-plane, if we use TransformGenerator to rotate it (0 180 0) to achieve an effective "mirroring" operation, and then stitch it to another original 2D mesh in XY-plane. The result 2D mesh cannot be extruded correctly in any direction because it contains two types of 2D elements: one can be extruded in (x y z) direction, the other can be extruded in (-x -y -z) direction.

Impact

FancyExtruderGenerator may generate faulty elements that cannot be used in FEM simulation without warning/error messages.

[miaoyinb]

@GiudGiud @eshemon FYI

[miaoyinb]

@GiudGiud
I have a way to fix these faulty elements on the fly. Will push it soon and we can discuss if that is the best approach. Thanks.