Mixing layer characteristics of coaxial supersonic jets

We present experimental results on mean ow development and mixing layer characteristics of single and dual-stream compressible air jets. The results are relevant to noise emission and mixing enhancement of high-speed turbulent jets. In the dual-stream jets, the primary ow was xed at Mach number 1.5 and the secondary stream was supplied at various subsonic Mach numbers and from a variety of nozzles. Coaxial and eccentric nozzle con gurations were investigated. In the coaxial arrangements, the secondary ow reduces the growth rate of the primary shear layer and elongates the primary potential core and the supersonic region of the jet. As a result, the entrainment rate of the coaxial jet is less than that of the single jet. The potential core length increases by as much as 68% when a thick secondary stream is supplied at Mach number 0.9. The eccentric con guration shows substantial improvement in mixing over the coaxial case and achieves an entrainment rate comparable to that of the single jet when the thickness of the secondary ow is relatively small. In the eccentric case, the maximum observed elongation of the primary potential core was 20% relative to the single jet case. An empirical model for predicting the primary and secondary potential core lengths of a coaxial jet is proposed.