  The spatial organization of a passive scalar plume originating from a point
source in a 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 meanders, and
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 planar laser-induced fluorescence (PLIF) and particle-image
velocimetry (PIV) were performed in a wind-tunnel, using acetone vapors 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 the plume intermittency
were quantified using `blob' size, shape, orientation and mean concentration.
It was observed that straining and break up were the primary plume-evolution
modes in this region, with little small-scale homogenization. Further, the
dominant role of coherent vortex motions in meandering and break-up of the
plume was evident. Their action is found to be the primary mechanism by which
the scalar injected within the log-region is transported away from the wall
(`large meander events'). Strong spatial correlation was observed in both
instantaneous and conditional fields between the high scalar concentration
regions and the individual vortex heads. This coherent transport was weaker for
wake-injection case, where the plume only interacts with outer vortex motions.
A coherent-structure based mechanism is suggested to explain these transport
mechanisms.
