A priori error analysis for HDG methods using extensions from subdomains to achieve boundary conformity

We present the first a priori error analysis of a technique that allows us to numerically solve steady-state diffusion problems defined on curved domains Ω by using finite element methods defined in polyhedral subdomains D_h ⊂ Ω. For a wide variety of hybridizable discontinuous Galerkin and mixed methods, we prove that the order of convergence in the L²-norm of the approximate flux and scalar unknowns is optimal as long as the distance between the boundary of the original domain Γ and that of the computational domain Γh is of order h. We also prove that the L²-norm of a projection of the error of the scalar variable superconverges with a full additional order when the distance between Γ and Γ_h is of order h^5/4 but with only half an additional order when such a distance is of order h. Finally, we present numerical experiments confirming the theoretical results and showing that even when the distance between Γ and Γ_h is of order h, the above-mentioned projection of the error of the scalar variable can still superconverge with a full additional order.