Study of Passive Scalar Mixing in Stratified and Unstratified Turbulence Using Large-Eddy Simulations
Researchers used high-resolution simulations to study how passive scalars mix in stratified (layered density) versus unstratified turbulent flows. In the transverse direction, mixing behaves similarly in both cases but spreads slightly faster when stratified, while vertical mixing is severely suppressed by stratification. The findings have implications for modeling plume behavior and turbulent mixing in geophysical and industrial applications.
A new study employing large-eddy simulations examined passive scalar mixing in decaying homogeneous turbulence under stratified and unstratified conditions. The research modeled two mixing layers—one vertical and one transverse—to represent a plume much larger than the velocity length scale. Results show that transverse scalar mixing evolves similarly in both conditions, though stratification causes slightly faster spreading and higher scalar fluctuation intensity with more intermittent turbulent interfaces. Vertical mixing, however, is nearly eliminated by stratification, which constrains large-scale stirring. The stratified vertical layer initially grows until its width matches the vertical integral length of horizontal velocity, then stalls due to the Froude number constraint. The authors successfully modeled transverse scalar flux using one- or two-constant models depending on whether the mean profile is known, though the two-constant approach requires quasi-equilibrium conditions. The study used a Prandtl number of 0.7 and notes that higher Prandtl numbers may produce different results due to reverse buoyancy flux effects.
What's missing
The study acknowledges that results are specific to a Prandtl number of 0.7 and that higher Prandtl numbers may alter passive scalar mixing behavior through reverse buoyancy flux effects, suggesting this as an open question for future work. The applicability of findings to real-world geophysical or industrial plumes with different fluid properties and initial conditions is not discussed.
What different sources said
- arXiv physicsCenter
Evolution of passive scalar mixing layers in stratified and unstratified homogeneous turbulence
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