The large scale structure and mixing characteristics of the flowfield surrounding a single, underexpanded, sonic, transverse jet injected into a Mach 1.6 crossflow were experimentally studied. The flowfield was investigated at three distinct sets of flow conditions corresponding to a jet-to-crossflow momentum flux ratio of 1.2, 1.7, and 2.2. Planar laser-induced fluorescence from acetone molecules and planar Mie scattering from condensed ethanol droplets were used to obtain spatially and temporally resolved flowfield visualizations in various side, end, and top view image planes. Statistical processing of the image ensembles produced mean and standard deviation images, two-dimensional spatial autocovariance fields representing the characteristics of the dominant turbulent structures, and probability density functions representing the instantaneous state of scalar mixing in the flowfield.; Time-averaged descriptions of the mixing characteristics of this transverse jet flowfield were shown to be inaccurate. Intermittent large scale structures of various sizes, shapes, and orientations strongly influence the distribution of the jet fluid and the crossflow fluid. Relatively high probabilities of unmixed fluid (whether jet or crossflow) persist in many of the mixing regions between the jet and crossflow. The most significant instantaneous mixing in this flowfield seems to occur in the center of wake region slightly below the jet centerline. A counter-rotating streamwise vortex pair in the jet cross section plays an important role in the scalar mixing processes, as it transports jet fluid down towards the wake and entrains crossflow fluid from below up into the jet. The streamwise vortex pair initially develops in an asymmetric and undulating manner, but gradually becomes more symmetric farther downstream of the Mach disk. A relatively large portion of the total jet fluid bypasses the Mach disk through the upstream edge of the barrel shock, thereby retaining a significant fraction of its momentum. This relatively high momentum jet fluid penetrates deeply into the crossflow, contributes to the formation of the largest turbulent structures, and affects the evolution of the streamwise vortex pair.
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