Study reveals intermittent buoyancy flux patterns in stratified turbulent flows across geophysical scales
Researchers used direct numerical simulations of stratified turbulent flows to characterize how buoyancy flux and mixing efficiency vary across a wide range of geophysical conditions. The study found strongly non-Gaussian, intermittent behavior in buoyancy flux—with kurtosis reaching approximately 100—indicating that vertical transport in stably stratified flows can be far more variable than classical models suggest. These findings have implications for understanding mixing in oceans and the atmosphere, where such intermittency affects energy transport and dissipation.
A new preprint posted to arXiv examines stratified turbulent flows through large-scale parametric direct numerical simulations (DNS) of the Boussinesq equations, varying the Froude number across geophysically relevant values (0.01 to 1) and exploring two Prandtl numbers (1 and 6), yielding buoyancy Reynolds numbers spanning roughly 0.06 to 2300. The study finds that buoyancy flux distributions exhibit strongly non-Gaussian tails with kurtosis reaching approximately 100, reflecting intense intermittency not only at small scales but also at large scales comparable to the mean flow. This large-scale intermittency is linked to long-time fluctuations in vertical velocity and temperature that locally generate turbulence, enhancing dissipation and vertical transport even under stable stratification. The domain-averaged buoyancy flux shows two distinct regimes: logarithmic growth with the buoyancy Reynolds number and an approach to a small offset as stratification intensifies. A simple energy model suggests that a defect between vertical kinetic and potential energy drives strong buoyancy flux events, triggering convective instabilities, two- and three-dimensional eddy formation, and rapid dissipation on a turnover timescale before the energetic cycle restarts in bursts. The skewness of buoyancy flux is shown to increase as a power-law with the buoyancy Reynolds number before saturating in the passive-scalar limit.
What's missing
The study is a preprint and has not yet undergone peer review. The simulations use idealized Boussinesq forcing and periodic boundary conditions, which may not fully capture the complexity of real geophysical flows such as ocean boundary layers or atmospheric boundary effects. The authors do not directly validate their DNS results against observational field data from oceans or the atmosphere, leaving the quantitative applicability of the scaling laws to real-world systems an open question. The range of Reynolds numbers achievable in DNS remains far below those of geophysical flows, which is a fundamental limitation of the approach.
What different sources said
- arXiv physicsCenter
Scaling laws and local enhancements of buoyancy flux in stratified turbulent flows
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