Study Identifies Two Distinct Pathways to Turbulent Mixing in Strongly Stratified Flows
Researchers used linear theory and numerical simulations to identify two mechanisms by which horizontal shear instabilities drive turbulence and mixing in strongly stratified fluids with no initial vertical shear. The first pathway involves vertical shear emerging directly from the primary instability, while the second involves a longer cascade through columnar vortices and secondary instabilities. The findings have implications for understanding mixing processes in geophysical and astrophysical flows where strong density stratification is present.
A new study published on arXiv investigates how turbulence and diapycnal mixing (mixing across density layers) can occur in strongly stratified flows that lack initial vertical shear. Using a combination of linear stability analysis and direct numerical simulations, the researchers demonstrate that horizontal shear instabilities can trigger turbulence even when traditional vertical shear-based mechanisms are absent. The study identifies two distinct pathways: one where vertical shear emerges rapidly from vertically-modulated eigenmodes and becomes unstable to small-scale Kelvin-Helmholtz instabilities, and another where a vertically-invariant eigenmode first creates long-lived columnar vortices that subsequently become unstable to three-dimensional hyperbolic instabilities. Importantly, the researchers find that both pathways inevitably lead to the emergence of vertical shear and small-scale mixing instabilities at sufficiently high buoyancy Reynolds numbers, though they excite different vertical scales and produce different mixing efficiencies.
What's missing
The study does not discuss potential applications or observational validation of these theoretical findings in natural systems such as oceans, atmospheres, or stellar interiors, though such applications are implied by the focus on geophysically-relevant parameters.
What different sources said
- arXiv physicsCenter
Two pathways to diapycnal mixing in strongly stratified flows with no initial vertical shear
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