5.3 Symmetries and Ghosts-Geometric Layer

The FEM framework can create ghosts not only of things that are on other processors, but also for various problem symmetries, like mirror reflection, and various types of periodicities. The interface for these ghosts is simple--you ask for the symmetries to be created, then you will get extra ghosts along each symmetry boundary. The symmetry ghosts are updated properly during any communication, even if the symmetry ghosts are ghosts of real local elements from the same chunk.

Figure 8: Illustrating symmetry ghost elements.

Figure 8 shows a chunk of a mesh for a rectangular domain with horizontal linear translational periodicity--that is, the domain repeats horizontally. Symmetry ghosts lie along the left and right sides; ordinary cross-processor parallel ghosts lie along the top edge where this chunk joins up with the rest of the domain; and the external boundary along the bottom of the chunk has no ghosts.

void FEM_Add_linear_periodicity( int nFaces,int nPer, const int *facesA,const int *facesB, int nNodes,const double *nodeLocs );
SUBROUTINE FEM_Add_linear_periodicity(nFaces,nPer,facesA,facesB, nNodes,nodeLocs)
INTEGER, INTENT(IN) :: nFaces, nPer, nNodes
INTEGER, INTENT(IN) :: facesA(nPer,nFaces), facesB(nPer,nFaces)
double precision, INTENT(IN) :: nodeLocs(3,nNodes)

Make facesA and facesB match up under linear translation. Each face of facesA must match up with exactly one face of facesB, but both the faces and the nodes within a face can be permuted in any order--the order is recovered by matching 3d locations in the nodeLocs array.

This call can be repeated, for example if the domain is periodic along several directions. This call can only be issued from init().

void FEM_Sym_coordinates(int elTypeOrMinusOne,double *locs);
SUBROUTINE FEM_Sym_coordinates(elTypeOrZero,locs)
INTEGER, INTENT(IN) :: elTypeOrZero
double precision, intent(inout) :: locs(3,<number of items>)

This call adjusts the 3d locations listed in locs so they respect the symmetries of their corresponding item. If elTypeOrZero is an element type, the locations are adjusted to match with the corresponding element; if elTypeOrZero is zero, the locations are adjusted to match up with the corresponding node.

This call is needed because symmetry ghost nodes and elements initially have their original locations, which must be adjusted to respect the symmetry boundaries. Thus this call is needed both for initial location data (e.g., from FEM_Get_node_data) as well as any communicated location data (e.g., from FEM_Update_ghost_field).

This call can only be issued from driver().