Several properties are associated with a grid including the following:

Table 5 Grid Properties

Dimension (required)

2D vs. 3D

Grid type (required)

Cartesian, curvilinear, extruded Cartesian, extruded curvilinear

Extrusion type (optional)

Type of extrusion used for K direction. Options include sigma stretch, Cartesian, curvilinear at corners, curvilinear at mid-sides. Must be defined for extruded grids.

Global coordinate system (optional)

Used to position the grid in 3D space.

Origin (optional)

The coordinates of the grid origin (I=0, J=0, K=0) in the global coordinate system

Orientation (required)

Right -hand or left hand rule

Dip (optional)

The angle of rotation about the global X axis (this is 0.0 for plan view 2D cases & 90 for vertically averaged 2D cases)

Bearing (optional)

The angle of rotation about the global Z axis

Computational Origin (optional)

The geometric corner of the grid that is the computational origin. By default this is the geometric origin (location 1).

U Direction (optional - only applies to 3D case)

The direction of the u axis which defines the position of the 3D grid triad on the geometric definition of the grid. This value defaults to either 1 or -1 depending on the computational origin.

NumI (required)

The number of cells in the I direction

NumJ (required)

The number of cells in the J direction

NumK (required - 3D Grids only)

The number of cells in the K direction (layers)

Cartesian Grids

For Cartesian grids the local coordinate location of each row, column, and layer (if 3D) boundary is specified. The first boundary in each direction is the local origin. Figure 7 shows a 2D Cartesian grid with the required grid cell boundary locations. The origin is always (0.0, 0.0) and therefore is not specified.

Figure 7 2D Cartesian grid with boundary locations

Curvilinear Grids

Curvilinear grids must have all coordinates defined for each grid corner. For a 2D grid, there are (NumI + 1) * (NumJ + 1) corners. A 3D grid has (NumI + 1) * (NumJ + 1) * (NumK + 1) corners. Figure 8 shows a 2D curvilinear grid.

Figure 8 2D curvilinear grid.

Extruded Grids

Some numerical models use grids that are 2D grids extruded in the K direction to form 3D grids. Both 2D curvilinear and 2D Cartesian grids can be extruded. The methods to extrude 2D grids include sigma-stretch, Cartesian, curvilinear at corners, and curvilinear at mid-sides. Sigma-stretch grids have top and bottom Kvalues (elevations) that may vary from column to column. Each layer of a sigma-stretch grid is a constant percentage of the K thickness. Cartesian extruded grids define constant K values for each layer in the grid (this is only used for 2D curvilinear grids because otherwise it would just be a 3D Cartesian grid). Curvilinear extrusion grids define the top and bottom K values for each layer and each corner or cell.

The CH3D hydrodynamic model supports 2D curvilinear grids extruded either by sigma-stretch or Cartesian. The groundwater model MODFLOW uses a 2D Cartesian grid extruded curvilinear at cell centers.