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An experimental investigation has been carried out on a supersonic jet of air issuing from an M = 1.44 converging-diverging rectangular nozzle of aspect ratio 4. Particle image velocimetry measurements of the flow field along with near-field acoustic measurements were made. The effect of injection of a small amount of water (∼5% of the mass flow rate of the jet) into the shear layer of the jet, on the unsteady flow structure and sound generation were examined. The presence of water droplets in the jet modified the turbulence structure significantly, resulting in axial and normal r.m.s. velocity reductions of about 10% and 30%, respectively, as compared to that of a normal jet. An even larger effect is found on the peak values of the turbulent shear stress with a reduction of up to 40%. The near-field noise levels (OASPL) were found to reduce by about 2-6 dB depending on the location of the injection and the water mass flow rate. Far-field acoustic measurements carried out on a heated M = 0.9 (jet exit velocity = 525 m s −1) jet show significant (6 dB) reductions in the OASPL with moderate amounts of water injection (17% of the mass flow rate of the jet) suggesting that the technique is viable at realistic engine operating conditions. 132 A. Krothapalli and others confined to a definite wedge sector. They emanate from the region within the first few diameters downstream of the nozzle exit. These are generated by small-scale disturbances (or eddies) that are being convected at supersonic speeds so that they emit Mach waves in the direction defined by a disturbance convection velocity and the atmospheric speed of sound (Phillips 1960; Ffowcs Williams 1965; Ffowcs Williams & Maidanik 1965). Although these waves lie in an important range of the spectrum (1 ∼ 4 kHz in the case of full scale engines), they may not have enough intensity at far distances to contribute significantly to the far-field noise. These waves are eliminated by surrounding the jet with a gas stream that has a higher speed of sound, thus resulting in subsonic convection velocities of the small disturbances, as demonstrated by Oertel & Patz (1981) and more recently by Papamoschou (1997). The second field is highly directional, peaking at smaller angles relative to the jet axis (or larger angles to the inlet axis). This noise field is generated from large-scale instabilities reaching peak amplitude in the region somewhat upstream of the end of the potential core. These sources are associated with the unsteady flow on a scale that is comparable with the local shear layer width (Bishop, Ffowcs Williams & Smith 1971). The spectral intensity of this sound field generally contains two distinct peaks (Laufer, Schlinker & Kaplan 1976). One is associated with highly directional Mach waves characterized by high positive pressure peaks in the far-field microphone signal (Ffowcs Williams, Simson & Virchis 1975; Laufer et al. 1976). These Mach waves are of significant strength as compared to those seen very close to the jet exit as discussed above. It is found th...

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Boundary-layer transition at different free-stream turbulence levels has been investigated using the particle-image velocimetry technique. The measurements show organized positive and negative fluctuations of the streamwise fluctuating velocity component, which resemble the forward and backward jet-like structures reported in the direct numerical simulation of bypass transition. These fluctuations are associated with unsteady streaky structures. Large inclined high shear-layer regions are also observed and the organized negative fluctuations are found to appear consistently with these inclined shear layers, along with highly inflectional instantaneous streamwise velocity profiles. These inflectional velocity profiles are similar to those in the ribbon-induced boundary-layer transition. An oscillating-inclined shear layer appears to be the turbulent spot-precursor. The measurements also enabled to compare the actual turbulent spot in bypass transition with the simulated one. A proper orthogonal decomposition analysis of the fluctuating velocity field is carried out. The dominant flow structures of the organized positive and negative fluctuations are captured by the first few eigenfunction modes carrying most of the fluctuating energy. The similarity in the dominant eigenfunctions at different Reynolds numbers suggests that the flow prevails its structural identity even in intermittent flows. This analysis also indicates the possibility of the existence of a spatio-temporal symmetry associated with a travelling wave in the flow.

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