HydroGeoSphere/Selecting Mesh Components

In order to assign boundary conditions, material properties etc. we need to be able to choose subsets of the grid. The method of choice must be flexible and easy to use as well as being able to handle complex input requirements.

Grid components

The following is a list of grid components, ranked in order of increasing complexity:

1. nodes -- used to assign initial heads and first-type boundary conditions
2. segments -- used to represent wells, tile drains or observation wells
3. faces (triangles or rectangles) -- used to represent fractures or high-conductivity planes (as 2-D triangular or rectangular elements) and to assign second- and third-type boundary conditions to these as well as 3-D prism or block elements.
4. elements (blocks, prisms or tetrahedra) -- sometimes used to assign hydraulic conductivities or distribution coefficients
5. zones -- generally used to assign material properties such as hydraulic conductivity. Elements are grouped into zones by assigning them the same ID number.

We will assign to all members of a grid component an attribute called ‘chosen’, which can be toggled on or off by the user. If the attribute is chosen for certain members of a component, then subsequent instructions issued by the user will affect those members only. For example, the following section of a hypothetical prefix.grok file would initially turn off all chosen nodes (i.e. instruction clear chosen nodes which requires no further input), then turn on only those nodes satisfying the requirement that they are within 1.e-5 distance units of the plane defined by the equation x = 0.0 (i.e. instruction choose nodes x plane followed by two lines of input):

clear chosen nodes
choose nodes x plane
0.0                   X coordinate of plane
1.e-5                 distance criteria

Once these nodes were chosen, we could set the property of interest by issuing another instruction like:

specified head
1
0.0 10.0

In this case we are assigning a constant head of 10.0 to all chosen nodes at time 0.0, which will apply for the duration of the simulation. Note that the instruction specified head acts on nodes by definition. It is up to the user to be aware of which components each group of instruction acts on.

The effect of issuing two such instructions in succession is cumulative. For example, the following input would choose nodes which were within 1.e-5 distance units of the planes at x = 0.0 and x = 10.0:

clear chosen nodes
choose nodes x plane
0.0                   X coordinate of plane
1.e-5                 distance criteria
choose nodes x plane
10.0                  X coordinate of plane
1.e-5                 distance criteria

The following sections introduce all the instructions which are available for choosing subsets of the various grid components.