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This means the overlap is 3 times larger on the left and right border of each LAM grid than at the top and bottom! This is done because the solver needs the grids to be 3 times larger in x- than in y-direction (minus 1 point).
Example
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to setup a 0.25° Yin-Yang grid
1. First calculate the size of the overlap when covering 8 times the grid resolution:
Grd_overlap = 8 * 0.25° = 2°
2. Then calculate the resulting total number (including 2 times the overlap) of core/free points in y-direction + 1 point:
Grd_nj = (90° + 2*2°) / 0.25° + 1 = 377
That's itall!
FYI, Grd_nj does not have to match the FFT criteria.
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