Vectorial variational problems in $L^\infty$ constrained by the Navier-Stokes equations

Abstract: We study a minimisation problem in $L^p$ and $L^\infty$ for certain cost functionals, where the class of admissible mappings is constrained by the Navier-Stokes equations. Problems of this type are motivated by variational data assimilation for atmospheric flows arising in weather forecasting. Herein we establish the existence of PDE-constrained minimisers for all $p$, and also that $L^p$ minimisers converge to $L^\infty$ minimisers as $p\to\infty$. We further show that $L^p$ minimisers solve an Euler-Lagrange system. Finally, all special $L^\infty$ minimisers constructed via approximation by $L^p$ minimisers are shown to solve a divergence PDE system involving measure coefficients, which is a divergence-form counterpart of the corresponding non-divergence Aronsson-Euler system.