Scalar measurements in compressible axisymmetric mixing layers

Abstract

An experimental study was conducted of mixing in supersonic axisymmetric shear layers using planar laser-induced fluorescence of seeded nitric oxide. Passive scalar images were obtained at convective Mach numbers (Mc) of 0.35, 0.82, and 1.3. Values of the fraction of mixed fluid (δm/δ1) were obtained at Mc=0.35 and 0.82, using a technique that employs fluorescence quenching to provide a direct measurement of fluid unmixedness. From the unmixedness measurement, an accurate value of the fraction of mixed fluid can be derived, even when the smallest mixing scales are not resolved. The visualizations reveal that at low Mc, the turbulent structures appear primarily as rollers, although the degree of axisymmetry of the rollers could not be determined. At high Mc, the structures were more irregular, and often appeared as the jagged, irregular structures that have been previously documented in planar layers. Concentration fluctuation statistics show that fluctuations are smaller at Mc=0.82 than at Mc=0.35, particularly on the high-speed edge of the layer, but those at Mc=1.3 are comparable to those at Mc=0.35. Using the quenching method, the fraction of mixed fluid, δm/δ 1, was found to be 0.45 and 0.48 at Mc=0.35 and 0.82, respectively. If the effect of the Reynolds number difference is taken into account, then the measured trend is even stronger. One can conclude then that the moderately compressible axisymmetric mixing layer has a slightly greater mixing efficiency than its incompressible counterpart.