|Groundwater modelling with MicroFEM • Lesson 4B: Stopping criterion|
Step 2: Enter model data, boundary conditions and compute heads
We will here repeat some actions of Lessons 1B and 1C (steps 3-6)
[Input mode] / Select T1 / Open cell / 2000 / [F5]
Zoom in and make sure that the blinking current node is the centre node
Select Q1(Discharge) / Open cell / 1000 / [Enter]
[Walking mode] (or "Alt+1" on the keyboard).
Toolbar: [Ins] / [Del] / [F7]
[Input mode] / Select H1
Toolbar: [Fixed heads for marked nodes (F9)] / [Del]
Check that all boundary nodes are fixed (at 0.00 m)
Menu bar: Calculate / Go calculate
[Walking mode] / [F5]
Check the water balance. The model water balance shows an error: the boundary inflow is less than the well outflow.
Menu bar: Calculate / Options ... / set "Error of heads" = 1E–12 / [Calculate]
The "Error of heads" is a stopping criterion for the iterative procedure that computes all unknown heads. During one iteration a new value for all unknown (not fixed) heads is found, based on the heads of the previous iteration. In this way all heads are systematically improved (i.e. closer to the theoretical solution). The largest head improvement of all nodes is displayed on the screen at the end of each iteration. When this largest improvement is smaller than 1E–12, or 10–12, (the stopping criterion we used) the calculation process stops. To obtain the required accuracy takes about 450 iterations.
[Stop] / [OK] / [F5] / Check water balance / [F5]
[Zoom area] / Zoom in until the centre node is clearly shown / [Zoom area] (switch it off)
Check that the head in the well is –0.840 m. We can check this result with the analytical solution (Thiem equation, see Lesson 3: Step 13). With a well radius of 0.20 m and an approximate average distance to the boundary of 9000 m, the steady state drawdown in the well is computed as 0.8526 m.
Save this model for the next lesson.