Coherent Organization of Passive Scalar from a Point-Source in a Turbulent Boundary Layer

TL;DR

This study investigates how passive scalar plumes from a point source in a turbulent boundary layer meander and break up, highlighting the role of vortex structures in scalar transport and organization.

Contribution

It provides new insights into the spatial organization and dynamics of scalar plumes, emphasizing the influence of coherent vortices on plume meandering and breakup in turbulent boundary layers.

Findings

Coherent vortex motions dominate plume meandering and breakup.

Plume intermittency characterized by blob size, shape, and orientation.

Straining and breakup are primary modes of plume evolution.

Abstract

The spatial organization of a passive scalar plume originating from a point source in a turbulent boundary layer is studied to understand its meandering characteristics. We focus shortly downstream of the isokinetic injection ($1.5\le x/\delta \le 3$, $\delta$ being boundary layer thickness) where the scalar concentration is highly intermittent, the plume rapidly \textit{meanders}, and \textit{breaks-up} into concentrated scalar pockets due to the action of turbulent structures. Two injection locations were considered: the center of logarithmic-region and the wake-region of the boundary layer. Simultaneous quantitative acetone planar laser-induced fluorescence (Ac-PLIF) and particle-image velocimetry (PIV) were performed in a wind-tunnel, to measure scalar mixture fraction and velocity fields. Single- and multi-point statistics were compared to established works to validate the diagnostic novelties. Additionally, the spatial characteristics of plume intermittency were quantified using `blob' size, shape, orientation and mean concentration. It was observed that straining, break up and spatial reorganization were the primary plume-evolution modes in this region, with little small-scale homogenization. Further, the dominant role of coherent vortex motions in plume meandering and break-up was evident. Their action is found to be the primary mechanism by which the injected scalar is transported away from the wall in high concentrations (`large meander events (LMEs)'). Strong spatial correlation was observed in both instantaneous and conditional fields between the high concentration regions and individual vortex heads. This coherent transport was weaker for wake-injection, where the plume only interacts with outer vortex motions. A coherent-structure based mechanism is suggested to explain these transport mechanisms.