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Colocated multigrid
+---+---+---+---+---+---+---+---+---+
8 | x | x | x | x | x | x | x | x | x |
+---+---+---+---+---+---+---+---+---+
7 | 0 | | | | | | | | x |
+---+---+---+---+---+---+---+---+---+
6 | 0 | | | | | | | | x |
+---+---+---+---+---+---+---+---+---+
5 | 0 | | | | | | | | x |
+---+---+---+---+---+---+---+---+---+
4 | 0 | | | | | | | | x |
+---+---+---+---+---+---+---+---+---+
3 | 0 | | | | | | | | x |
+---+---+---+---+---+---+---+---+---+
2 | 0 | | | | | | | | x |
+---+---+---+---+---+---+---+---+---+
1 | 0 | | | | | | | | x |
+---+---+---+---+---+---+---+---+---+
0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | x |
+---+---+---+---+---+---+---+---+---+
0 1 2 3 4 5 6 7 8
- Pressure is defined on all cells:
- The 0 cells are stored but not used (we return p = 0 on these cells)
- The x cells are not stored (we implicitly return p = 0 out of bounds)
- Velocity is only defined in the inner cells, we always return 0 on the borders
The Coarse grid is as follows:
+-------+-------+-------+-------+-------+
| | | | | |
| x | x | x | x | x |
| | | | | |
+-------+-------+-------+-------+-------+
| | | | | |
| 0 | | | | x |
| | | | | |
+-------+-------+-------+-------+-------+
| | | | | |
| 0 | | | | x |
| | | | | |
+-------+-------+-------+-------+-------+
| | | | | |
| 0 | | | | x |
| | | | | |
+-------+-------+-------+-------+-------+
| | | | | |
| 0 | 0 | 0 | 0 | x |
| | | | | |
+-------+-------+-------+-------+-------+
So, the size of the total grid is (2^N+1)*(2^N+1), but we store only (2^N)*(2^N), and explicitly
set the pressure to 0 on the lower and left borders.