We propose a hybridizable discontinuous Galerkin (HDG) method to approximate the solution of a distributed optimal control problem governed by an elliptic convection diffusion PDE. We derive optimal a priori error estimates for the state, adjoint state, their fluxes, and the optimal control. We present 2D and 3D numerical experiments to illustrate our theoretical results. ). X. Zheng thanks Missouri University of Science and Technology for hosting him as a visiting scholar; some of this work was completed during his research visit. 1 arXiv:1801.00082v1 [math.NA] 30 Dec 2017Optimal control problems for convection diffusion equations arise in applications [21] and are also an important step towards optimal control problems for fluid flows. Therefore, researchers have developed many different numerical methods for this type of problem including approaches based on finite differences [3], standard finite element discretizations [14][15][16], stabilized finite elements [2,19], the symmetric stabilization method [4], the SUPG method [13,17], the edge-stabilization method [5,28], mixed finite elements [16,29,31], and discontinuous Galerkin (DG) methods [17,20,26,27,30,32,33].DG methods are well suited for problems with convection, but they often have a higher computational cost compared to other methods. Hybridizable discontinuous Galerkin (HDG) methods keep the advantages of DG methods, but have a lower number of globally coupled unknowns. HDG methods were introduced in [9], and now have been applied to many different problems [6,8,[10][11][12][22][23][24][25].HDG methods have recently been successfully applied to two PDE optimal control problems. Zhu and Celiker [34] obtained optimal convergence rates for an HDG method for a distributed optimal control problem governed by the Poisson equation. The authors have also studied an HDG method for a difficult Dirichlet optimal boundary control problem for the Poisson equation in [18]. We proved an optimal superlinear convergence rate for the control in polygonal domains. Despite the large amount of work on this problem, a superlinear convergence result of this type had only been previously obtained for one other numerical method on a special class of meshes [1].Due to these recent results and the favorable properties of HDG methods, we continue to investigate HDG for optimal control problems for PDEs in this work. Specifically, we consider the above distributed control problem for the elliptic convection diffusion equation, and apply an HDG method with polynomials of degree k to approximate all the variables of the optimality system (4), i.e., the state y, dual state z, the numerical traces, and the fluxes q = −∇y and p = −∇z. We describe the HDG method and its implementation in Section 2. In Section 3, we obtain the error estimatesWe present 2D and 3D numerical results in Section 4 and then briefly discuss future work.
HDG scheme for the optimal control problemWe begin by setting notation. Throughout the paper we adopt the standard notation W m,p (Ω) for Sobolev spaces on Ω...