LARGE-SCALE STRUCTURE AND ENTRAINMENT IN THE SUPERSONIC MIXING LAYER

Experiments were conducted in a two-stream planar mixing layer at conv ective Mach numbers, M(c), of 0.28, 0.42, 0.50, 0.62 and 0.79. Planar laser Mie scattering (PLMS) from a condensed alcohol fog and planar la ser-induced fluorescence (PLIF) of nitric oxide were used for flow vis ualization in the side, plan and end views. The PLIF signals were also used to characterize the turbulent mixture fraction fluctuations. Vis ualizations using PLMS indicate a transition in the turbulent structur e from quasi-two-dimensionality at low convective Mach number, to more random three-dimensionality for M(c) greater than or equal to 0.62. A transition is also observed in the core and braid regions of the span wise rollers as the convective Mach number increases from 0.28 to 0.62 . A change in the entrainment mechanism with increasing compressibilit y is also indicated by signal intensity profiles and perspective views of the PLMS and PLIF images. These show that at M(c) = 0.28 the insta ntaneous mixture fraction field typically exhibits a gradient in the s treamwise direction, but is more uniform in the cross-stream direction . At M(c) = 0.62 and 0.79, however, the mixture fraction field is more streamwise uniform and with a gradient in the cross-stream direction. This change in the composition of the structures is indicative of dif ferent entrainment motions at the different compressibility conditions . The statistical results are consistent with the qualitative observat ions and suggest that compressibility acts to reduce the magnitude of the mixture fraction fluctuations, particularly on the high-speed edge of the layer.