Dynamics of Stratified Turbulence and Mixing

Stratified turbulence occurs when fluid density varies under gravity, producing buoyancy forces that inhibit vertical motions and foster anisotropic flow structures. The competition between inertial forces, viscous dissipation and buoyancy is characterised by the Reynolds number and Froude number, leading to distinct regimes: weakly stratified turbulence, where buoyancy effects are nascent; intermediately stratified turbulence, marked by non-equilibrium anisotropy; and strongly stratified turbulence, dominated by layered, patchy motions and internal gravity waves. Within these regimes, diapycnal mixing—irreversible exchange across density surfaces—governs the transport of heat, carbon and nutrients in oceans and atmosphere. Progress in measurement, simulation and theory has elucidated energy pathways, mixing efficiencies and the role of coherent vortices in stirring density interfaces. Such insights underpin more accurate climate models, pollutant dispersion forecasts and industrial mixing processes by quantifying how turbulence and stratification together regulate global and local exchange mechanisms.

Research from Nature Portfolio

No recent Nature Portfolio content available.

Dynamics of Stratified Turbulence and Mixing publication trend

The graph below shows the total number of articles in dynamics of stratified turbulence and mixing across all publications each year (not limited to Nature Index journals).

Technical terms

Stratified turbulence: Turbulent flow in which stable density gradients under gravity produce buoyancy forces that suppress vertical motions.

Froude number: Dimensionless ratio of inertial to buoyancy forces, Fr = U/(N L), where U is characteristic velocity, N is buoyancy frequency and L is length scale.

Reynolds number: Dimensionless ratio of inertial to viscous forces, Re = U L/ν, where ν is kinematic viscosity.

Buoyancy frequency: Also known as Brunt–Väisälä frequency; natural oscillation frequency of fluid parcels in stable stratification, N = √[(–g/ρ)(dρ/dz)].

Holmboe waves: Interfacial instabilities in stratified shear layers, characterised by counter-propagating vortical motions at density interfaces.

Mixing efficiency: Fraction of turbulent kinetic energy irreversibly converted into potential energy change by mixing across density surfaces.

Diapycnal mixing: Vertical transport of mass, heat or chemicals across surfaces of constant density in a stratified fluid.

References

1. New insights into experimental stratified flows obtained through physics-informed neural networks. Journal of Fluid Mechanics (2024).
2. Mixing in forced stratified turbulence and its dependence on large-scale forcing. Journal of Fluid Mechanics (2020).
3. The evolution of coherent vortical structures in increasingly turbulent stratified shear layers. Journal of Fluid Mechanics (2022).