2d or 2d axisymmetric? Former is a wedge, latter is a cone as far as Fluent is concerned.
The 2d options are generally a good option if both geometry and flow obey the symmetry constraint. Using a wing as an example, you'll get much the same result as for 3d for a long wing but with much lower cell count. It'll start to differ near the wing root or tip where 3d effects come into play.
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