Symmetry Criteria for the Equality of Interior and Exterior Shape Factors With Exact Solutions

Abstract

Lienhard (2019, “Exterior Shape Factors From Interior Shape Factors,” ASME J. Heat Mass Transfer-Trans. ASME, 141(6), p. 061301) reported that the shape factor of the interior of a simply-connected region (Ω) is equal to that of its exterior (ℝ2\Ω) under the same boundary conditions. In that study, numerical examples supported the claim in particular cases; for example, it was shown that for certain boundary conditions on circles and squares, the conjecture holds. In this paper, we show that the conjecture is not generally true, unless some additional condition is met. We proceed by elucidating why the conjecture does in fact hold in all of the examples analyzed by Lienhard. We thus deduce a simple criterion which, when satisfied, ensures the equality of interior and exterior shape factors in general. Our criterion notably relies on a beautiful and little-known symmetry method due to Hersch which we introduce in a tutorial manner. In addition, we derive a new formula for the shape factor of objects meeting our N-fold symmetry criterion, encompassing exact solutions for regular polygons and more complex shapes.