Irreversible mixing by unstable periodic orbits in buoyancy dominated stratified turbulence

Abstract

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 $\mathcal{E}=\chi /(\chi +\mathcal{D})$ ($\mathcal{D}$ 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 ${\overline{Re}}_B=\overline{\mathcal{D}}/(\nu N_B^2)$, and is bounded above by $1/6$, consistently with the classical model of Osborn (1980). There are qualitatively different physical properties between the unstable orbits that have lower irreversible mixing efficiency at low ${\overline{Re}}_B\sim O(1)$ and those with nearly optimal $\mathcal{E}\lesssim 1/6$ at intermediate ${\overline{Re}}_B\sim 10$. The weaker orbits, inevitably embedded in more strongly stratified flow, are characterised by straining or 'scouring' motions, while the more efficient orbits have clear overturning dynamics in more weakly stratified, and apparently shear-unstable flow.

References in corpus (6)

• The Significance of Simple Invariant Solutions in Turbulent Flows
• Recurrent flow analysis in spatiotemporally chaotic 2-dimensional Kolmogorov flow
• Layer formation in horizontally forced stratified turbulence: connecting exact coherent structures to linear instabilities
• Periodic motion representing isotropic turbulence
• The stability of stratified spatially periodic shear flows at low Péclet number
• Sustaining processes from recurrent flows in body-forced turbulence

Cited by in corpus (7)

• Asymptotic dynamics of high dynamic range stratified turbulence
• The linear instability of the stratified plane Couette flow
• Exploiting self-organized criticality in strongly stratified turbulence
• Data-Driven Stabilization of Periodic Orbits
• Stably stratified exact coherent structures in shear flow: the effect of Prandtl number
• Free-stream coherent structures in parallel compressible boundary-layer flows at subsonic Mach numbers
• Stabilising nonlinear travelling waves in pipe flow using time-delayed feedback