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Flow exiting a convergent-divergent nozzle operated at severely overexpanded conditions, such that a shock wave is formed inside the nozzle, exhibits a strong instability that causes mixing enhancement in the flow itself and can destabilize an adjacent flow. The latter property enables mixing enhancement of an arbitrary jet via axial injection of a secondary gas flow. A systematic investigation of this phenomenon reveals that the instability is associated with shock-induced nozzle flow separation. In this paper we review key results that demonstrate the potential of this mixing technique and provide insight into the relevant physical processes. In addition, we establish criteria for the occurrence and location of the separation shock, and construct correlations between mixing enhancement and shock strength. [ABSTRACT FROM AUTHOR]

An experimental study of simultaneous multipoint measurements in the flowfield and acoustic field of a Mach 1.75 cold-air jet is presented. A series of four optical-deflectometer probes measured turbulent fluctuations in or near the jet flow, and eight microphones recorded the far-field pressure in the direction of peak emission. The correlation methodology involves the coherence between the delay-and-sum beamformer outputs of the optical-deflectometer probes and the microphones. This procedure yields results with greater fidelity and higher coherence levels than obtained with individual optical-deflectometer-to-microphone correlations. With the optical-deflectometer probes in the jet shear layer, there is a significant correlation, on the order of 0.1, between the turbulent fluctuations and farfield noise. As the optical-deflectometer probe moves transversely away from the jet, its correlations with the microphone beamformer first drops and then increases. This trend signifies the transition from hydrodynamic to acoustic pressure fields. In the vicinity of the nozzle exit, the peak coherence between the beamformed optical-deflectometer and microphone signals coincides with the physical location of the optical-deflectometer probe. However, as the shear layer thickens downstream, the peak coherence generally lags the probe location, which is a probable result of acoustic refraction by the mean flow. [ABSTRACT FROM AUTHOR]

We present a computational study, validated by mean-flow experiments, of a dual-stream nozzle simulating the exit conditions of a supersonic turbofan engine with noise-suppressing fan flow deflectors. The study is conducted for eight nozzle configurations and two operating conditions: a cold condition at which mean velocity surveys were conducted and against which the computational code was validated and a hot condition that corresponds to the takeoff engine cycle and at which acoustic data were collected. The code predictions successfully replicate the mean velocity fields and the inflectional layers of the experimental flows. The code is then extended to the conditions of the actual engine cycle. The computations reveal a similar velocity profile for the hot and cold conditions when the axial distance is normalized by the potential core length. For both conditions, the vane deflectors reduce the turbulent kinetic energy k on the underside of the jet. An overall noise source strength is modeled as the axial integral of k7/2. A significant correlation is found between the reduction in the noise source strength and the reduction in the peak level of the overall sound pressure level. [ABSTRACT FROM AUTHOR]