MappedGrid does not override get_cell_map

[Ady0333]

Describe the bug

MappedGrid computes and stores a composed geometric map

geo_map = phys_map ∘ model_map

in its constructor, but it does not override get_cell_map.

Because of this, calling get_cell_map on a MappedGrid falls back to the default implementation for Grid, which reconstructs the map using linear interpolation of the deformed node coordinates.

function get_cell_map(trian::Grid)
  cell_to_coords = get_cell_coordinates(trian)
  cell_to_shapefuns = get_cell_shapefuns(trian)
  lazy_map(linear_combination, cell_to_coords, cell_to_shapefuns)
end

This reconstruction ignores the stored geo_map.

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Impact

For nonlinear or curved phys_map, the reconstructed map becomes a piecewise linear approximation of the geometry instead of the intended composed map. This can lead to incorrect Jacobians and integration results during FE assembly.

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Affected file

src/Geometry/MappedDiscreteModels.jl

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Possible fix

Return the stored map from MappedGrid:

get_cell_map(grid::MappedGrid) = grid.geo_map

[Ady0333]

Please assign me this issue.