Large-scale structure evolution in supersonic interacting shear layers

An experimental investigation is made of the evolution of the large-scale structures in a flow consisting of two planar turbulent shear layers formed by a Mach-3 planar jet bounded by a Mach-5 freestream. The flow is characterized by two planar shear layers at a convective Mach number of 0.28 that are independent in the near field but that interact farther downstream. Measurements were made using fast-response hot-wire probes and planar laser scattering from a condensed alcohol fog. The hot-wire data were used to calculate power spectra and cross correlations from which large-scale-structure length scales and orientation were inferred. The images reveal roller-like structures, but they do not appear as dominant or as coherent compared with low-Mach-number shear layers at a similar Reynolds number. The hot-wire data confirm the relatively unorganized and three-dimensional nature of the independent shear layers. In the far field, the visualizations reveal that the shear layers interact to form a more organized structure, similar to vortex shedding in incompressible turhulent wakes. These organized structures result in a distinct peak in the power spectrum and larger spanwise coherence lengths than for the independent shear layer.