An experimental investigation of high-compressibility mixing layers

An investigation of the effects of high compressibility conditions on the two-stream, planar turbulent mixing layer is performed in a unique shock tunnel driven, supersonic mixing layer facility. Compressibility levels, previously unattainable in traditional blowdown wind tunnels, are reached to examine their effect on the growth and development of large-scale structures which dominate the entrainment behavior of mixing layers. Visualizations of the shear layer are achieved by schlieren imaging and planar laser induced fluorescence (PLIF) of two, seeded tracer species, acetone and nitric oxide. Side, plan, and end view visualizations of three compressibility conditions (M_c = 0.85, 1.71, and 2.64) provide information on large-scale structure character and dimensionality.; The first focus of this work is an accurate measurement of the shear layer growth rate at compressibility conditions greater than M_c = 1. Ensemble averaged schlieren images provide a measure of the visual growth rate and are captured for a wide compressibility range (M_c = 0.85 to 2.84). Second, spatially resolved, non-intrusive, laser-based imaging techniques are employed to probe the underlying three-dimensional structure of highly compressible shear layers that is masked by line-of-sight integrated imaging. A final emphasis is on the extension of a PLIF technique, cold chemistry imaging of mixed fluid, to low pressure mixing layer environments.; Consistent with the trends seen in previous mixing layer experiments and computations, all the imaging techniques reveal that the high compressibility shear layer is dominated by three-dimensional streamwise oriented structures with limited transverse dimension. The growth rate of these structures tends to asymptote to 18% of its incompressible value above M_c = 1.5. PLIF imaging results uncover three-dimensional shock structures which are caused by slow scalar structures convecting in a supersonic flow, but do not confirm the existence of shocklets in highly compressible turbulence. Visualizations of the mean and instantaneous scalar field at M_c = 2.64 suggest the applicability of gradient transport mixing models over structure based techniques at high compressibility. Also, the mixedness of the shear layer at this higher compressibility condition is seen to slowly increase with compressibility and Reynolds number when compared to prior cold chemistry results.