A Posteriori Finite-Element Output Bounds for the Incompressible Navier–Stokes Equations: Application to a Natural Convection Problem

We present a new Neumann subproblem a posteriori finite-element procedure for the efficient calculation of rigorous, constant-free, sharp lower and upper bounds for linear and nonlinear functional outputs of the incompressible Navier–Stokes equations. We first formulate the bound procedure; we derive and discuss a bound error expression; and we then demonstrate the capabilities of the method with numerical results obtained for natural convection problems. We also implement an optimal adaptive refinement strategy based on a local elemental decomposition of the bound gap.