For a transmission problem in a truncated two-dimensional cylinder located beneath the graph of a function u, the shape derivative of the Dirichlet energy (with respect to u) is shown to be well-defined and is computed. The main difficulties in this context arise from the weak regularity of the domain and the possible non-empty intersection of the graph of u and the transmission interface. The result is applied to establish the existence of a solution to a free boundary transmission problem for an electrostatic actuator.
Introducing the Dirichlet integrala classical result in shape optimization states that the shape derivative of J(O) is given by [11,20]. When the shape derivative is well-defined, it provides useful information on the Dirichlet energy itself and it is the basis for deriving first-order optimality conditions. However, the integral on the right-hand side of the shape derivative is only meaningful provided ϕ O has sufficient regularity (typically ϕ O ∈ H 2 (O)), which, in turn, requires sufficient regularity of the source term f and the open set O, see [11, Corollary 5.3.8] for instance. Source terms with low Sobolev regularity or depending on the admissible shape O in a non-smooth way are therefore excluded.Amongst the simplest situations featuring such a dependence is the differentiability with respect to O of the Dirichlet energy