Irreversible mixing by unstable periodic orbits in buoyancy dominated stratified turbulence

We consider turbulence driven by a large-scale horizontal shear in Kolmogorov flow (i.e. with sinusoidal body forcing) and a background linear stable stratification with buoyancy frequency N-B(2) imposed in the third, vertical direction in a fluid with kinematic viscosity nu. This flow is known to be organised into layers by nonlinear unstable steady states, which incline the background shear in the vertical and can be demonstrated to be the finite-amplitude saturation of a sequence of instabilities, originally from the laminar state. Here, we investigate the next order of motions in this system, i.e. the time-dependent mechanisms by which the density field is irreversibly mixed. This investigation is achieved using 'recurrent flow analysis'. We identify (unstable) periodic orbits, which are embedded in the turbulent attractor, and use these orbits as proxies for the chaotic flow. We find that the time average of an appropriate measure of the 'mixing efficiency' of the flow E = chi/(chi + D) (where D is the volume-averaged kinetic energy dissipation rate and chi is the volume-averaged density variance dissipation rate) varies non-monotonically with the time-averaged buoyancy Reynolds numbers (Re-B) over bar = (D) over bar /D(nu N-B(2)), and is bounded above by 1/6, consistently with the classical model of Osborn (J. Phys. Oceanogr., vol. 10 (1), 1980, pp. 83-89). There are qualitatively different physical properties between the unstable orbits that have lower irreversible mixing efficiency at low (Re-B) over bar similar to O(1) and those with nearly optimal E