M3 3127
Speaker
Qi Zhou, Department of Civil Engineering, University of Calgary.
Title
Layers and interfaces in stably stratified waters: A DNS perspective
Abstract
Flows in stably stratified natural waters often develop self-organized layers and sharp density interfaces that profoundly influence mixing and transport. Using direct numerical simulations (DNS), we investigate the dynamics and instability mechanisms that lead to the emergence of such layered structures. In a stratified wake, we show that turbulence evolves toward a locally critical state characterized by spontaneous shear layers and clustering of high-dissipation regions around a critical Richardson number. Despite substantial decay in bulk turbulence intensity, this self-organized regime persists and supports two-way energy exchange, including kinetic energy backscatter, which is reminiscent of self-organized criticality observed in other complex systems. We further examine how layering affects mixing efficiency by tracking the irreversible flux coefficient, which stabilizes between 0.45 and 0.49 as the flow transitions into the layered state. This plateau corresponds to a characteristic Ozmidov-to-Thorpe length-scale ratio and a universal flux–gradient relationship consistent with the constant-power mixing scenario proposed in the literature. Finally, analysis of the stratified Taylor–Green vortex reveals the zigzag instability as a potential mechanism driving vertical scale selection, with its fastest-growing mode scaling inversely with the Froude number. Together, these findings demonstrate how stratified turbulence self-organizes into persistent layered structures that regulate the energetics and mixing behaviour of natural aquatic systems.