Chapters:
1: Introduction
2: Simple example
3: Invocation
4: Finer Control
5: X-Y Plots
6: Contour Plots
7: Image Plots
8: Examples
9: Gri Commands
10: Programming
11: Environment
12: Emacs Mode
13: History
14: Installation
15: Gri Bugs
16: Test Suite
17: Gri in Press
18: Acknowledgments
19: License
Indices:
Concepts
Commands
Variables

## 8.11: Finite Element Model mesh

Finite Element Models (used in fluid mechanics) employ non-rectangular meshes, and plotting these meshes requires a few intermediate steps. Consider the common case of triangular elements. Suppose two data files exist describing the mesh, the first, ``model.nodes`' say, consists of a description of the x-y coordinates of the nodes (vertices) of the triangles. The second, ``model.elements`' say, consists of a description of which triplet of nodes defines each triangle in the mesh. Here, from a sample application, is a node file called ``model.nodes`':

    1	1	1
    2	2	1
    3	1	2
    4	3	1.5
    5	2	2
    6	1.5	3

Here is the corresponding file of the elements, called ``model.elements`'

    1	1	2	3
    2	2	5	3
    3	2	4	5
    4	3	5	6

In each of these files, the first column is a reference number. Thus, ``model.elements`' indicates that the first triangle is defined by the nodes numbered ``1`', ``2`' and ``3`' as defined in ``model.nodes`'. More specifically, the triangle is defined by vertices at (x,y) locations (1,1), (2,1), and (1,2).

A Gri program, named ``FEM.gri`', to draw the nodes is the following.

    set missing value -99.99
    # Create data using perl-script ...
    system FEM.pl model.nodes model.elements > tmp
    # ... then plot it ...
    open tmp
    read columns x y
    close
    draw curve
    # ... and, finally, clean up the temporary file
    system rm tmp

The work of interpreting the data files is done by the perlscript that follows, named ``FEM.pl`'

    #!/usr/bin/perl -w
    $missing = -99.99;              # missing value
    $node_file = $ARGV[0];
    $element_file = $ARGV[1];
    open (NODE, $node_file)
        or die "Cannot open '$node_file' file";
    open (ELEM, $element_file)
        or die "Cannot open '$element_file' file";

    # Read in node information, creating arrays
    # named $node_x[] and $node_y[]. Check that
    # the first column (the index) makes sense.
    $max_node = 1;
    while(<NODE>) {
        ($index, $node_x[$max_node], $node_y[$max_node])
            = split;
        die "Node mismatch at index=$index"
            if ($index != $max_node);
        $max_node++;
    }

    # Read in triangle elements, into arrays
    # $a[], $b[], and $c[].  Check that the
    # first column (the index) makes sense.
    $max_elem = 1;
    while(<ELEM>) {
        ($index, $a[$max_elem], $b[$max_elem], $c[$max_elem])
            = split;
        die "Element mismatch at index=$index"
            if ($index != $max_elem);
        $max_elem++;
    }

    # Print out triangles suitable for plotting in gri.
    for ($i = 1; $i < $max_elem; $i++) {
        print $node_x[$a[$i]], " ", $node_y[$a[$i]], "\n";
        print $node_x[$b[$i]], " ", $node_y[$b[$i]], "\n";
        print $node_x[$c[$i]], " ", $node_y[$c[$i]], "\n";
        # Repeat first, to close the triangle.
        print $node_x[$a[$i]], " ", $node_y[$a[$i]], "\n";
        print $missing, " ", $missing, "\n";
    }

The resultant image is below.