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The flowfields created by transverse injection of sonic gaseous jets through a circular nozzle into a supersonic crossflow have been experimentally investigated using planar Rayleigh/Mie scattering from silicon dioxide particles seeded into the crossflow stream. Helium and air were used as injectant gases allowing an examination of the effects of compressibility on the large-scale structural development and near-field mixing characteristics present within the flowfield. Instantaneous images from end and side view image planes show a highly three-dimensional interaction dominated by both large-and small-scale vortices. Analyses of these image ensembles provide jet spreading and penetration characteristics, standard deviation statistics, large-scale mixing information, and two-dimensional spatial correlation fields. Results indicate that injectant molecular weight variations do not strongly affect the jet's transverse penetration into the crossflow, although they lead to substantially different compressibility levels that dramatically influence the characteristics of the large-scale structures formed in the shear layer and the entrainment and mixing occurring between the injectant and crossflow fluids. The large-scale eddies tend to rapidly break-up in the low compressibility injection case while those in the high compressibility case remain coherent over a longer spatial range. Mixing layer fluctuations present in the low compressibility case intrude deeply into the jet fluid as compared to the high compressibility case, where these fluctuations are confined near the jet edge.

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