an:05734615 Zbl 1200.35214 Fursikov, A.V.; Rannacher, R. Optimal Neumann control for the two-dimensional steady-state Navier-Stokes equations. EN Fursikov, Andrei V. (ed.) et al., New directions in mathematical fluid mechanics. The Alexander V. Kazhikhov memorial volume. Boston, MA: Birkh\"auser. Advances in Mathematical Fluid Mechanics, 193-221 (2010). ISBN 978-3-0346-0151-1/hbk 2010

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*35Q30 76D55 76D05 49J20 49K20 93C20 optimal control problem; stationary 2D Navier-Stokes system; generalized solution; minimization of drag; abstract Lagrange principle; Lagrange multiplier; optimality system The paper focuses on an optimal control problem for a 2D stationary Navier-Stokes system. The authors consider a domain $\Omega $ of $\Bbb R^2$ consisting of a rectangle without a subset bounded by a smooth curve $S $. The Navier-Stokes system $-\Delta v+v\cdot \nabla v+\nabla p=0$, $\nabla \cdot v=0$ is considered in $\Omega $. Dirichlet or Neumann boundary conditions are imposed on the different pieces of the boundary of this domain. The optimal control problem consists to minimize the functional $J=\int_{S}n\cdot \sigma \cdot e_{1}dx$ under the action of controls $u^{1}$ and $u^{2}$ imposed on subintervals of the horizontal parts of the boundary of the domain $\Omega $ through Neumann boundary conditions. Here $e_{1}$ is the first unit vector of $\Bbb R^2$ and $n\cdot \sigma =-pn+2{\cal D}(v)n$, where $n$ is the unit outer normal and $2{\cal D}(v)=(\partial _{j}v_{i}+\partial _{i}v_{j})_{i,j=1,2}$. The applied controls are supposed to satisfy $\Vert u^{1}\Vert ^{2}+\Vert u^{2}\Vert ^{2}\leq \gamma ^{2}$ in the $L^{2}$-norm of their respective domains, for some positive constant $\gamma $. The first main result of the paper proves the existence of a generalized solution of this optimal control problem. The authors start proving further properties of the generalized solution of the associated Stokes problem. They also prove the existence of a generalized solution of the Navier-Stokes problem assuming the existence of admissible collections $(v,p,u^{1},u^{2})$ for this Navier-Stokes problem and that the boundary data are small enough. This is done building a contraction operator associated to this Navier-Stokes problem. The proof of the existence of an optimal solution is obtained rewriting the original problem as a minimization problem for a continuous functional on a compact set. The second main result of the paper establishes the optimality system for the optimal solution. The authors here use the abstract Lagrange principle framework. The paper ends with the presentation of some briefly described numerical simulations. Alain Brillard (Riedisheim)