Limited Regions in 3D
In three dimensional problems, many figures to not fit readily into the extrusion model. In particular, there are frequently features that in reality exist only at very restricted positions in the extrusion dimension, and which create poor meshes when extruded throughout the domain.
FlexPDE implements the concept of LIMITED REGIONS to accommodate this situation.
A LIMITED REGION is defined as one that is considered to exist only in specified layers or surfaces of the domain, and is absent in all other layers and surfaces.
The LIMITED REGION will be constructed only in layers and surfaces specifically stated in the body of the REGION definition.
An example of this type of structure might be a transistor, where the junction structure of the device is present only in a very thin layer of the domain, while the substrate occupies the majority of the volume.
In earlier versions of FlexPDE, the shape of the junction structure was propagated and meshed throughout the extrusion dimension. Since version 4, the structure can be restricted, or LIMITED, to a single layer or a few layers.
For example, the following descriptor fragment defines a 3-unit cube with a 0.2-unit cubical structure in the center. The small structure is present in the layer 2 mesh only.
EXTRUSION Z=0, 1.4, 1.6, 3
START(0,0) LINE TO (3,0) TO (3,3) TO (3,0) TO CLOSE
LIMITED REGION 2
LAYER 2 K=9
LINE TO (1.6,1.4) TO (1.6,1.6) TO (1.4,1.4) TO CLOSE
See the User Guide section "Limited Regions" for a graphical example of this facility.