Eddy convection in coaxial supersonic jets

We present experimental results on the morphology and evolution of large tu rbulent eddies in coaxial supersonic jets. The study encompassed Mach 1.5, axisymmetric, perfectly expanded jets composed of air ora mixture of helium and air. A double exposure planar laser-induced fluorescence (PLIF) system , with gaseous acetone as the tracer molecule. enabled visualization of the turbulent structure and of its evolution a short tints later. The convecti ve velocity of the eddies was extracted from the PLIF images by means of tw o-dimensional cross correlations. Eddies in the air jet propagate with a sp eed approximately 80% of the local centerline mean velocity, In the faster helium air jets eddies are measured to be supersonic with respect to the am bient air, a result corroborated by the visualization of Mach waves. In the helium-air jet addition of a Mach 0.82 secondary flow reduces the convecti ve velocity of the primary Eddies from 70 to 62% of the primary exit veloci ty. The speed of the secondary eddies is 44% of the secondary exit velocity . All turbulent motions in this coaxial helium air jet are intrinsically su bsonic, leading to substantial reduction of Mach waves and reduction in noi se. A refined empirical model for eddy convection in compressible jets is p roposed. The results of this study are relevant to mixing, combustion, and jet noise.