Experimental results on the flow structure, mean-flow characteristics, and mixing of dual-stream compressible axisymmetric and high-aspect ratio rectangular (2D) jets are presented. The research is relevant to noise emission, thermal signature, and combustion in high-speed turbulent jets. The primary flow is set at Mach number 1.5 and the secondary stream is supplied at four subsonic Mach numbers from nozzles of variable area and shape. Pitot probe surveys are conducted to obtain velocity profiles, from which the lengths of the potential core and the supersonic region of the jet are determined, in addition to measures of the jet spread and mixing. Flow structure is visualized using schlieren photography and instantaneous planar laser-induced fluorescence (PLIF), and measurements of scalar mixing are obtained from time-averaged PLIF. Images from a double-exposure version of PLIF provide views on the morphology and evolution of large-scale turbulent structures in the mixing layers of the jet, from which the eddy convective velocity are measured. Addition of a secondary annular flow, via a convergent nozzle, to an axisymmetric jet reduces the convective velocity of the eddies in the primary shear layer. The reduction of primary eddy convective velocity is consistent with elimination of Mach waves in the nearfield of the jet when the convective Mach number of eddies in both primary and secondary shear layers are subsonic. The secondary flow reduces the growth rate of the jet and stretches the primary potential core and supersonic region of the jet with increasing secondary flow thickness and/or Mach number. On the other hand, the jet grows at a faster rate and the potential-core elongations are smaller when the secondary flow is supplied in an asymmetric arrangement about the primary jet. While the secondary flow issued from a converging geometry nozzle stabilizes the jet, flow from a convergent-divergent nozzle operated at off-design conditions exhibits instability and destabilizes the adjacent flow. Jet mixing is enhanced and the potential core length is halved. In the 2D jet, the same instability and mixing enhancement are achieved with the secondary flow applied on one side or both sides of the primary jet.
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