Encyclopaedia Index

### BFC

------- PIL logical; default=F; group 6 ----

BFC....must be set T, when body-fitted coordinates are required, after the settings of NX, NY and NZ, so that the default grid can be initialized over NX*NY*NZ cells.

If NX, NY and NZ are all less than the dimensions of the XFRAC, YFRAC and ZFRAC arrays, and a Cartesian or polar grid has been specified, the default BFC grid is set to be the same as the existing grid.

In polar cases, ie if CARTES is F, the default BFC grid is of polar type, (ie the same as a polar grid, except that grid lines of constant IY are not arcs but straight lines).

If no grid has been specified, or the first condition above is not met, the default grid is set to a cartesian box of extent XULAST, YVLAST and ZWLAST, in which the grid is uniformly spaced in each direction.

See BODY-F for further information.

### BFC, cell-corner coordinates

(see SETBFC logical, Group 6)

### BFC,corner-coordinate access in PIL

The XC, YC and ZC arrays can be accessed on the right- and left- hand sides of expressions. For example:
 XC(1,4,5)=6 -assigns a value to an element of the X- coordinate array; YC(NX,NY,NZ) -prints out a value from the Y-coordinate array; XX=1+ZC(1,3,5) -assigns a value to XX from an expression involving the ZC array.

The argument indices in the coordinate arrays must be either constants or simple variables. For example, the following is ILLEGAL: ZC(1,NY+1,1)=2

### BFC geometry, initiating restart of

(see RSTGEO logical, Group 6)

### BFC geometry, saving of

(see SAVGEO logical, Group 6)

### BFC grid check

(see GRDCHK command, Group 6)

### BFC

grid coordinates, generation of (see MAGIC command, Group 6)

### BFC

grid files in AUTOPLOT

PHOENICS BFC grid files. In this case plots may be made of any grid plane, or set of planes, viewed along any of the cartesian axes, or from an arbitrary 3-D view point.

### BFC grid information, naming file for

(see NAMXYZ character, Group 6)

### BFC grid Satellite arrays

(see XC, YC, ZC real arrays, Group 6)

### BFC grid, reading corner coordinates of

BFC grid-specification commands (see GSET, Group 6)

### BFC velocities

For BFCs, there are two methods of calculating the velocity vector, according to two averaging procedures.

By default, or if you use the command SET VECTOR AVERAGE ON, the resolutes at all six faces of each cell are used to calculate the vector.

If, however, you enter SET VECTOR AVERAGE OFF, only the High, North and East values of the resolutes are used. For BFC calculations, this method gives plots more consistent with the way in which the momentum equations are formulated in Earth. In BFC cases, where there are large velocity changes across a cell, the use of SET VECTOR AVERAGE ON may lead to erroneous vector plots.

### BFC x-cyclic boundaries, switching of

(see XCYIZ command, Group 6)

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