|Groundwater modelling with MicroFEM • Lesson 1B: Create and run a simple model|
In lesson 1A we looked at step 1: Creating an extremely simple grid (mesh, network) of only six equilateral elements
Step 2: The MicroFEM user interface
Now that we have a model, the GUI (Graphical User Interface) looks quite different. The Map, the Status bar and the Table (on the right hand side) now all show information.
Map: Shows the model grid, just as in "Grid generating mode".
Move with the cursor from node to node. Check the information on the Status bar.
Click on each node.
Clicking on a node makes that node the "current node". The current node is blinking (alternating white and black).
Table: All (five) cells in the table contain zero values. Click on any cell to make it the "active cell".
Toolbar: There are 4 (zoom and pan) + 4 (mode) buttons on the right hand side.
Read the hints.
Try the four zoom and pan buttons. Note the scale on the Status bar.
Click on the Mode buttons: Input mode (mode 2), Drawing mode (mode 3), Alter-grid mode (mode 4), and back to Walking mode (mode 1).
Note that there is a separate Toolbar for each mode.
Step 3: Model data input
Select Input mode.
All data of the model are still zero (default value). The Table shows the model data of the "current node". You can modify model data in "Input mode" only.
We start by entering transmissivities of 2000 m2/d for all nodes.
Table: Make the "trans. [m2/d]" cell (3rd from the top) the active cell.
Click on the blue cell (the cell with the zero). The cell opens for input (white cell, blue text).
Enter 2000, and then press F5 on the keyboard (or [all nodes (F5)] on the Toolbar.
We will now enter a well discharge of 1000 m3/day for the centre node only.
Map: click on the node in the centre of the grid. This node will become blue (blinking blue and black)
On the Status bar the position of the node is given by "x=0.00 y=0.00"
Table: Make the "discharge [m3/d]" cell (bottom of the list) the active cell.
By using the Enter key you assign the entered value to the current node only.
Step 4: Mark all boundary nodes
"Marking nodes" is an important procedure in MicroFEM. It is the way to select a particular part (subset) of the nodes. You mark nodes when you want to do something with that group of nodes (i.e. that particular part of the model) only. In this case we want to do something with all boundary nodes (in this case all nodes, except the centre node).
Select the Walking mode.
Click the [Walking mode] button (the first of the lower right 4 buttons), or "Alt+1" on the keyboard.
The Toolbar for the Walking mode is displayed. Read the hints of all buttons of this toolbar. Note that the keyboard equivalents from left to right are: Ins, Del, F2, F3, .... , F12
Menu bar: Commands
You will see that the available commands correspond one-to-one with the toolbar buttons.
Toolbar: [Delete marks (Del)] / [Mark model boundary (F7)]
Marked nodes are connected by yellow lines.
The "current node" is on the left side of the Scale (1:50,000 or something similar). The cursor node (the node closest to the cursor) is on the right. A yellow square is added when the node is marked.
Step 5: Specify fixed head boundary conditions
All boundary nodes are marked. The centre node is not marked. By default a model boundary is a no-flow boundary in MicroFEM. By default the top of the model is also a no-flow boundary. The base of the model is always a no-flow boundary.
When we do not change the boundary condition, there is no groundwater boundary flow possible. When a well discharges groundwater in such a model, there is no steady-state solution possible.
We will now set the model boundary (all boundary nodes) to a "fixed head" boundary condition.
Select Input mode by clicking the [Input mode] button (the second of the lower right four buttons).
The Toolbar for the Entering mode is displayed.
Read the hints of all buttons of this toolbar.
Note that the keyboard equivalents from left to right are: Ins, Del, F2, F3, F4, two buttons without a keyboard equivalent, F5, .... , F12.
Table: Make the "head [m]" cell the active cell (you do not have to open this cell).
Toolbar: [Fixed heads for marked nodes (f9)]
Nothing seems to happen, but when you click on each node, and look at the table, you may see the results of our last action. All the boundary nodes show the same information in the Table: Transmissivity = 2000 m2/d, Head = 0.00 m and fixed, Discharge = 0 m3/d. Only the centre node is different: Transmissivity = 2000 m2/d, Head = 0.00 m (no "fixed" text), Discharge = 1000 m3/d.
Step 6: Compute model heads
Menu bar: Calculate / Go calculate
Check that the head of the centre node has changed to –0.144 m.
You may move (drag) the bar between the Table and the Map a little to the left.
In the next lesson it will be explained why this result must be equal to: (√3)/12 = 0.1443376.
Step 7: Save the results
Menu bar: Files / Save all
If you forget to save your files, MicroFEM will remind you when you close the program by clicking the upper right cross button.