Abstract
Wepresent experimentalresultson themorphology and evolution of large turbulenteddiesin coaxial supersonic jets.ThestudyencompassedMach1.5,axisymmetric,perfectlyexpandedjetscomposedofairoramixtureofhelium and air. A double-exposure planar laser-induced e uorescence (PLIF) system, with gaseous acetone as the tracer molecule, enabled visualization of the turbulent structure and of its evolution a short time later. The convective velocityoftheeddieswasextractedfromthePLIFimagesbymeansoftwo-dimensionalcrosscorrelations.Eddiesin theair jet propagatewith a speed approximately 80% ofthelocal centerline mean velocity. In thefaster helium-air jets eddies aremeasured to be supersonic with respect to the ambient air, a result corroborated by thevisualization of Mach waves. In the helium-air jet addition of a Mach 0.82 secondary e ow reduces the convective velocity of the primary eddies from 70 to 62% of the primary exit velocity. The speed of the secondary eddies is 44% of the secondary exit velocity. All turbulent motions in this coaxial helium-air jet are intrinsically subsonic, leading to substantial reduction of Mach waves and reduction in noise. A ree ned empirical model for eddy convection in compressible jets is proposed. The results of this study are relevant to mixing, combustion, and jet noise.
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