The 3D simulation always uses Cartesian coordinates. The more complicated 2D simulation is described as follows.
The 2D simulation defaults to automatic (which implies 2D axisymmetric for 2D meshes), but this can be changed by modifying the MICHELLE Solver (2D)/Solver Setup/Basic/Geometry/Geometry Type parameter in the simulation properties dialog box accordingly.
By choosing 2Dplanar, you indicate that the 2D geometry is to be treated as a plane in Cartesian coordinates. Since the structure must lie in the x-y plane, the solution is equivalent to the solution you would get if the structure extended to infinity in both directions in z. Structures lying in the y-z or z-x plane are not supported for 2D planar coordinates.
By choosing 2Daxisymmetric, you indicate that the 2D geometry is to be treated as a radial slice of a cylindrical structure. If the structure lies in the x-y plane, the x-axis is the cylindrical axis and the numerical solution is equivalent to that of a radially symmetric structure where your geometry defines half the structure cross-section (in other words, your geometry is the half of the cross-section in the positive y plane). If the structure lies in the z-x plane, the z-axis is the cylindrical axis and the numerical solution is equivalent to that of a radially symmetric structure where your geometry defines half the structure cross-section (in other words, your geometry is the half of the cross-section in the positive x plane). Structures lying in the y-z plane are not supported for 2D axisymmetric coordinates.
If you choose 2Daxisymmetric coordinates and your structure crosses the cylindrical axis, the geometry is invalid. If your structure is adjacent to the cylindrical axis, it is not necessary to explicitly define a boundary condition along that axis.
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