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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.

In this study, ensembles of experimental data are presented and utilized to compare and validate two models used in the simulation of variable density, compressible turbulent mixing. Though models of this kind (Reynolds Averaged Navier Stokes and Large Eddy Simulations) have been validated extensively with more canonical flows in previous studies, the present approach offers novelty in the complexity of the geometry, the ensemble based validation, and the uniformity of the computational framework on which the models are tested. Moreover, all experimental and computational tasks were completed by the authors which has led to a tightly coupled experimental configuration with its digital twin. The experimental divergent shock tube facility and its data acquisition methods are described and replicated in simulation space. A 2D Euler model which neglects the turbulent mixing at the interface is optimized to experimental data using a Gaussian process. This model then serves as the basis for both the 2D RANS and 3D LES studies that make comparisons to the mixing layer data from the experiment. RANS is shown to produce good agreement with experimental data only at late flow development times. The LES ensembles generally show good agreement with experimental data, but display sensitivity to the characterization of initial conditions. Resolution dependent behavior is also observed for certain higher-order statistics of interest. Overall, the LES model successfully captures the effects of divergent geometry, compressibility, and combined non-linear instabilities inherent to the problem. The successful prediction of mixing width and its growth rate highlight the existence of three distinct regimes in the development of the instability, each with similarities to previously studied instabilities., Comment: 26 pages, 15 figures