Encyclopaedia Index



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Causes a dashed grid to be drawn on the plot, starting from the axis tick marks. The grid is removed by another GRID command.

See also HELP on : GRID DEFINE


(also see GRID command, below)

The lattice of points, lines, surfaces, and volumes which are imagined to subdivide the domain in which the fluid flows.



GRID settings....are effected in GROUPs 2 (for time), 3 (for the x-direction), 4 (for the y-direction) and 5 (for the z-direction), by the setting of values of LSTEP, TLAST, TFRAC, NX, XULAST, XFRAC, NY, YVLAST, YFRAC, NZ, ZWLAST and ZFRAC.

The commands GRDPWR and SUBGRD are useful in this regard. The entries on GRDPWR, SUBGRD, GROUP n, LSTEP, TLAST, etc provide further information. Regions with different grid distributions can be defined: see entries on NREGT, IREGT, GRDPWR, RSET etc.

Body-fitted coordinate grids are set in group 6; see the entry BODY-F for further information, also GSET and other entries in Group 6.

The grid-generation menu provides an interactive environment for creating Cartesian, cylindrical-polar or body-fitted grids. See the MENU entry for further details.


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[Grid] activates the grid sub-menu to set a new grid element to be drawn.


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G[rid] CE[ntre] <plane> <number> <subregion> <options>.... plots a grid at a cell centre, rather than at a cell face.

This may be useful when it is necessary to plot contours or vectors directly onto a grid, rather than in the space between two grids, as happens with the normal GRID command.

All parameters are the same as for the GRID command.

See also : GRIDS

Grid check for BFCs

(see GRDCHK)


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G[rid] C[lear]....deletes all grid elements, whether switched on or off.



The command GRID DEFINE enables you to obtain more closely-spaced grids allowing specify of the required intervals.

For logarithmically-scaled axes, the GRID command will draw a ruled grid at the powers of ten. To rule a finer grid based on the subsidiary calibrations, the GRID DEFINE command should be used. As usual, this will request grid spacings in the x-and y-directions. In a direction plotted logarithmically, a number must be entered for the grid spacing, although it will not be used.

In a direction plotted linearly, the number will be used for the grid spacing (as is always the case for linear plots).


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G[rid] D[elete]....deletes the last grid element, whether switched off or on.


Grid dimension, setting of

(see GSET(D,...))

Grid distortion

PHOENICS is most commonly used with a cartesian or cylindrical-polar grid geometry; however, facilities also exist for employing grids which are 'stretched' or 'bent' so as to conform with shapes of special interest to you. It is the purpose of Group 6 to accommodate the corresponding grid-defining data inputs.

Grid distributions,setting of

(see RSET)

Grid extent in Y-direction,specifying the

(see YVLAST)


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G[rid] H[atch] <plane< <number> <subregion> <options>....

plots a hatched grid or subregion of grid. A hatched grid consists of vertical and/or horizontal lines with a user-specified spacing, and is often used to improve the appearance of a plot where too many grid lines might obscure the picture.

The program will then prompt for hatch direction, which may be any or all of X, Y and Z, and a hatch spacing for each direction, which represents a fraction of the grid size in that direction.

NOTE that once set, these parameters will be retained for all subsequent hatched grids, unless reset by the HATCH command.

Grid lines,multiplying number of

(REFINE command; also see TR218)

Grid lines,setting high curvature of

(see UUP, VUP, WUP logicals, Gr 6)

Grid location,specification of

(ORIGIN command; also see TR218)

Grid manipulation with PINTO

(see TR218)

Grid mesh,matching of

(see GSET(M,...))

Grid meshes,copying of

(see GSET(C,...))

Grid No

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[Grid No] is the sequence number of the GRID element in the stack. Typing in any valid number (>the total number of grid element) will pop up the corresponding grid element and make it become the current element.

Only the attributes of the current GRID element can be modified.


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G[rid] OF[f] <element range>....switches off the specified grid element(s), which will not appear in subsequent plots until switched on again.

See also : GRID ON


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G[rid] ON <element range>....switches on the specified grid element(s), which will appear in subsequent plots.

See also : GRID OFF

Grid origin,displacement of

(see ZWADD)


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G[rid] OU[t] <plane> <number> <subregion> <options>.... plots an outline of the specified grid, i.e. one with no interior detail.

See also : GRIDS

Grid planes,transferring of

(see GSET(T,...))

Grid points,setting internal

(see GSET(B,...))

Grid regions

You use the REGEXT command to set default dimensions of grid regions See Encylopaedia entry for 'REGEXT'.

Grid specification with PINTO

(see TR218)

Grid systems in PINTO

(see TR218)

Grid-generation menu

The grid-generation and grid-handling procedures of PHOENICS underwent major re-development during 1991-2. The main new features were:-


Mouse-driven grid generation in GridMenu

The VIEW facility for body-fitted coordinates has been transformed into a mouse-driven grid-generation facility, which allows the user to 'drop' points, and construct lines and frames using a mouse (or the arrow keys when a mouse is not available). Grids can be matched to frames without leaving the new system, and the 'grid check' facility (previously available only through the GRDCHK command) can be used to inspect the quality of the mesh. ('Grid check' uses a colour code to indicate the orthogonality of the grid at each point.)

Settings effected in the VIEW environment are transferred automatically to the GridMenu session, and recorded as PIL commands. The MENSAV facilities will also record and replay VIEW sessions.

Grid-specification commands for BFCs

(see GSET, Group 6)


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GRI[D] D[EFINE] [a b]

Causes a dashed grid to be drawn on the plot, using a as the x-interval and b as the y-interval. If a & b are unspecified, the program will prompt for them. The grid is removed by another GRID command. For a logarithmic scale, the grid is drawn at the subsidiary axis marks. See also HELP on : GRID


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G[rids] <plane> <number> <subregion> <options>....plots the grid for a given plane and number. If a subregion is specified, only that part of the grid will be drawn.

A subregion consists of the extent of the grid in the other two coordinate directions, and either or both may be omitted if the total extent in that direction is to be used. For example, GRID X 1 Y 5 M Z 1 4 will plot the X 1 grid over the region Y cells 5 to the maximum, Z cells 1 to 4.

Options available are COLOUR <number> to plot the grid in a specified colour, and DASH <number> to plot the grid in a specified line-style. For example, GRID X 1 Y 5 7 DASH 1.



PHOENICS can employ grids of three distinct kinds, namely:

A cartesian grid is composed of cells formed by the intersection of three sets of mutually perpendicular parallel planes, on any one of which either x, y or z is a constant, these quantities being the distances in the three coordinate directions. The spacings between the planes can be arbitrary functions of those distances.

A cylindrical-polar grid, by contrast, consists of cells formed by the intersection of:

In cylindrical coordinates, the location y=0 need not correspond to the axis. It is displaced from the axis by the radial extent RINNER (the default value of which is zero). Thus, for flow in an annulus, y=0 corresponds to the inner surface of the annulus.

Once again, the intervals of x, y and z may be arbitrarily chosen.

A curvilinear grid is best imagined by supposing that a regular cartesian grid is embedded in a jelly-like medium, which is then squeezed, stretched, bent and twisted in an arbitrary way. All the cells which were originally in contact with one another remain so; but their shapes may have changed considerably.

Curvilinear grids are so often used for flow simulations in which it is desired that the grid should conform to the curved surface of some body, that they are often called body-fitted coordinates, or BFC's for short. It is indeed under these headings (or rather BFC and BODY-F) that they appear elsewhere in this encyclopaedia.

Both cartesian and polar grids are strictly orthogonal; however, mild departures from orthogonality can be accommodated by the use of 'stretching factors', which are particularly useful when it is desired to represent adequately some thin region of varying thickness, such as a jet or a boundary layer. The "porosity" feature of PHOENICS is used to effect the stretching.

There is one mode of stretching which is far from mild, but which PHOENICS can still handle very well; this is a time- varying stretching or compression of the grid in the low-high (ie z) direction.

One example is the cyclically varying motion of a grid which just fills the space between the top of a piston and the top of the combustion chamber of a reciprocating internal- combustion engine. In this case, the planes of which the intervening distances vary with time are those normal to the z-direction.

(See PHENC entry: ZMOVE)


(see Centred grids)

Grids,curvilinear,definition of

(see Curvilinear grids)

Grids,displaying of

(see Displaying grids)

Grids, one-dimensional in PINTO

(see One-dimensional grids)

Grids, orthogonal, generation of

(see Orthogonal grids)

Grids, plane-by-plane modification of

(see VIEW)

Grids, ruled, in AUTOPLOT

(see Ruled grids)

Grids, specifying a new grid with PINTO

(see TR218)

Grids, three-dimensional in PINTO

(see Three-dimensional grids)

Grids, two-dimensional in PINTO

(see Two-dimensional grids)