Study of Passive Scalar Mixing in Stratified and Unstratified Turbulence Using Large-Eddy Simulations
Researchers used high-resolution large-eddy simulations to study how passive scalars mix in stably stratified versus unstratified homogeneous turbulence, finding that stratification nearly eliminates vertical mixing while only modestly enhancing transverse spreading. The work models a large-scale plume scenario using two scalar mixing layers oriented in the vertical and transverse directions, with simulations run at a Prandtl number of 0.7. The findings have implications for understanding pollutant dispersion, oceanic mixing, and atmospheric transport in stably stratified environments.
A new preprint on arXiv presents high-resolution large-eddy simulations (LES) of decaying stratified and unstratified homogeneous turbulence to investigate passive scalar mixing, a problem relevant to atmospheric and oceanic flows. The study employs two scalar mixing layers — one vertical and one transverse — as a simplified model for a large-scale plume embedded in a turbulent, stably stratified medium. In the transverse direction, stratification leads to slightly faster scalar spreading and higher scalar fluctuation intensity, along with a more intermittent turbulent/non-turbulent interface compared to the unstratified case. In the vertical direction, however, stratification almost entirely suppresses mixing by inhibiting large-scale stirring; the scalar layer grows only until its width scales with the vertical integral length of the horizontal velocity, which is itself constrained to keep the vertical Froude number near unity, after which spreading effectively ceases. For modeling the stratified scalar flux in the transverse direction, the authors find that a one-constant model performs well when the mean scalar profile is known, while a two-constant model is adequate only when the scalar is in quasi-equilibrium with the velocity field. The study notes that results were obtained at a Prandtl number of 0.7, and the authors anticipate that higher Prandtl numbers — which would alter the reverse buoyancy flux — could meaningfully change passive scalar mixing behavior.
What's missing
The study is a preprint and has not yet undergone formal peer review. Results are limited to a Prandtl number of 0.7; the authors themselves flag that higher Prandtl number regimes (relevant to, e.g., salinity in the ocean) remain unexamined. The simulations assume decaying turbulence with no mean shear or rotation, which may limit applicability to real geophysical flows.
What different sources said
- arXiv physicsCenter
Evolution of passive scalar mixing layers in stratified and unstratified homogeneous turbulence
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