Optimal mixing in two-dimensional stratified plane Poiseuille flow at finite P\'eclet and Richardson numbers.

We consider the nonlinear optimisation of irreversible mixing induced by an initial finite amplitude perturbation of a statically stable density-stratified fluid. A constant pressure gradient is imposed in a plane two-dimensional channel. We consider flows with a finite P\'eclet number $Pe=500$ and Prandtl number $Pr=1$, and a range of bulk Richardson numbers $Ri_b \in [0,1]$. We use the constrained variational direct-adjoint-looping (DAL) method to solve two optimization problems, extending the optimal mixing results of Foures et al. (2014) to stratified flows, where the mixing of the scalar density has an energetic cost, and thus has an observable dynamic effect. We identify initial perturbations of fixed finite kinetic energy which maximise the time-averaged kinetic energy developed by the perturbations over a finite time interval, and initial perturbations that minimise the value of a `mix-norm', as defined by Thiffeault (2012) and shown by Foures et al. (2014) to be a computationally efficient and robust proxy for identifying perturbations that minimise the variance of a scalar distribution at a target time. We demonstrate, for all bulk Richardson numbers considered, that the time-averaged-kinetic-energy-maximising perturbations are significantly suboptimal at mixing compared to the mix-norm-minimising perturbations. By considering the time evolution of the kinetic energy and potential energy reservoirs, we find that mix-norm-minimising optimal perturbations lead to a flow which, through Taylor dispersion, very effectively converts perturbation kinetic energy into `available potential energy', which in turn leads rapidly and irreversibly to thorough and efficient mixing, with little energy returned to the kinetic energy reservoirs.