Abstract

In this paper we are continuing our work (Casas and Chrysafinos, SIAM J Numer Anal 50(5):2281---2306, 2012), concerning a priori error estimates for the velocity tracking of two-dimensional evolutionary Navier---Stokes flows. The controls are of distributed type, and subject to point-wise control constraints. The discretization scheme of the state and adjoint equations is based on a discontinuous time-stepping scheme (in time) combined with conforming finite elements (in space) for the velocity and pressure. Provided that the time and space discretization parameters, $$\tau $$? and $$h$$h respectively, satisfy $$\tau \le Ch^2$$?≤Ch2, error estimates of order $$\mathcal {O}(h^2)$$O(h2) and $$\mathcal {O}(h^{\frac{3}{2}-\frac{2}{p}})$$O(h32-2p) with $$p > 3$$p>3 depending on the regularity of the target and the initial velocity, are proved for the difference between the locally optimal controls and their discrete approximations, when the controls are discretized by the variational discretization approach and by using piecewise-linear functions in space respectively. Both results are based on new duality arguments for the evolutionary Navier---Stokes equations.

Highlights

  • In this paper we are continuing our work of [6] regarding the approximation of the following velocity tracking problem: (P)min J(u) u ∈ Uad where 1 J(u) = 2 γ + 2 T|yu(t, x) − yd(t, x)|2 dxdt Ω|yu (T, x) yΩ (x)|2 dx + λ 2

  • In [6] we presented space-time error estimates of order O(h), under suitable regularity assumptions on the data, when the controls are discretized by piecewise constants in space and time

  • Two parameters τ and h are associated to the numerical scheme and they were needed to satisfy the usual assumption τ ≤ Ch2 in order to prove that the discrete equation has a unique solution, and our estimate was optimal in L2(0, T ; H1(Ω)) norms for the state and adjoint

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Summary

Introduction

In this paper we are continuing our work of [6] regarding the approximation of the following velocity tracking problem:. Two parameters τ and h are associated to the numerical scheme (here τ and h, indicating the size of the grids in time and space) and they were needed to satisfy the usual assumption τ ≤ Ch2 in order to prove that the discrete equation has a unique solution, and our estimate was optimal in L2(0, T ; H1(Ω)) norms for the state and adjoint. We emphasize that the convective nature of the adjoint equation requires special attention We combine these estimates within the framework of [10, 8,7] (related to nonlinear elliptic pde control constrained problems), by exploring a localization argument and the second order condition.

Related results
Assumptions and preliminary results
Approximation of the state and adjoint-state equations
The discrete state equation
The discrete adjoint-state equation
Error estimates of the discrete optimal control problem

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