|Groundwater modelling with MicroFEM • Lesson 8A: FemGrid grid generation|
In this lesson we will discuss the FemGrid generator and then build a simple unconfined model.
Originally, MicroFEM was developed for regional models. Such models often have irregular shapes, while spacing contrast (ratio of largest to smallest elements) is not extremely high. In those days MicroFEM came with a single grid generator: FemGrid. Later on, when MicroFEM was also used for civil engineering models (construction dewatering, dumping-grounds, groundwater remediation) that often need very small elements locally (sheetpile walls), FemMesh was developed as an alternative generator.
We will start by inspecting the example grid.
Files / New grid / Create new grid / FemGrid / [OK] / "Lesson 8" / [OK] / [Example input (F4)]
There are three tabs in the Tables: Fixed nodes, Segments and Regions. The tables contain the input for the FemGrid example grid. Check that the number of Fixed nodes = 8, Segments = 6 and Regions = 3.
[Fixed nodes (F8)] / [Nodes on segments (F8)]
There are 51 nodes in the map, in the shape of the MicroFEM logo.
Note: You can press (Esc) anytime you want to start again.
Click a cell in the Fixed nodes table and then use up- & down-arrow keys, while watching the map.
Do the same for the Segments table, and for the Regions table.
The Segments and Regions tables are wider than displayed on the screen.
Drag the bar between the Table and the Map to the left until all numbers in the tables are displayed.
Each Fixed node, Segment and Region has its own number. The order of the Fixed nodes in the table is irrelevant. This also applies to Segments and Regions. Each Region is defined by the numbers of its bordering segments.
The columns of the Region table show:
Similarly, each Segment is defined by the numbers of connected Fixed nodes.
Each Fixed node is defined by its (x, y) coordinates.
There is one IMPORTANT rule when defining your segments: A segment can be no longer than the boundary between TWO regions, or the outer boundary of ONE region.
Each region and each segment can have its own spacing, but it is recommended to use a spacing ratio of no more than TWO for adjacent regions. Also, choose a segment spacing between the spacings of the adjacent regions.
Check that these rules and recommendations apply to the example grid.
[F8] / [F8] / [F8] / [F8]
The grid consists of 108 nodes and 178 elements.
[Clear input data (F3)] / No
[Start with a hexagon (F5)] / [OK] / [F8] / [F8] / [F8] / [F8] / [F8] / [F8]
Now there is only one Region, bounded by only one Segment. The Segment is a closed line. It connects 6 nodes, but to define this segment we have to enter 7 numbers, because the first and last Fixed node is the same (Fixed node 2). There are 7 Fixed nodes. Only 6 are used to define the segment. Node 4 is a "free Fixed node". There can be as many free Fixed nodes as you like.
We will now create the grid for our new model. It is a triangular island bounded by the following coordinates: (0, 0), (8000, 3000) and (0, 6000). Somewhere near the centre of the island is a well. Around the well we want a finer spacing. The finer spaced region is also a triangle: (2000, 2000), (4000, 3000) and (2000, 4000). The well is located at (2620, 3025).
Enter the coordinates for all (7) Fixed nodes.
There are 2 segments: 1 = outer triangle, 2 = inner triangle.
There are 2 regions: 1 = bounded by the two triangles, 2 = within the inner triangle.
Spacing for inner triangle = 60 m and outer area = 120 m.
Choose your own spacing for the segments.
Create the grid.