For a case extruding patches left and right:
sourcePatches (left right);
if the optional zoneNames entry is also specified in extrudeMeshDict
zoneNames (leftCells rightCells);
the cells extruded from the left patch are added to the leftCells cellZone and
the cells extruded from the right patch are added to the rightCells cellZone.
Alternatively if a single zone name is specified, e.g.
zoneNames (extrudedCells);
then cells extruded from the left and right patches are added to the
extrudedCells cellZone.
If the number of patches to extrude is large it might be more convenient for
the cells extruded from each patch to be added to a cellZone named the same as
each patch, this option is selected by setting zoneNames to patchNames:
zoneNames (patchNames);
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