We have experimentally studied the effect of microjets on the flow field of a Mach 0.9 round jet. Planar and three-dimensional velocity field measurements using particle image velocimetry show a significant reduction in the near-field turbulent intensities with the activation of microjets. The axial and normal turbulence intensities are reduced by about 15% and 20%, respectively, and an even larger effect is found on the peak values of the turbulent shear stress with a reduction of up to 40%. The required mass flow rate of the microjets was about 1% of the primary jet mass flux. It appears that the microjets influence the mean velocity profiles such that the peak normalized vorticity in the shear layer is significantly reduced, thus inducing an overall stabilizing effect. Therefore, we seem to have exploited the fact that an alteration in the instability characteristics of the initial shear-layer can influence the whole jet exhaust including its noise field. We have found a reduction of about 2 dB in the near-field overall sound pressure level in the lateral direction with the use of microjets. This observation is qualitatively consistent with the measured reduced turbulence intensities.

This paper describes the results of a study examining the flow and acoustic characteristics of an axisymmetric supersonic jet issuing from a sonic and a Mach 1.5 converging-diverging (C-D) nozzle and impinging on a ground plane. Emphasis is placed on the Mach 1.5 nozzle with the sonic nozzle used mainly for comparison. A large-diameter circular plate was attached at the nozzle exit to measure the forces generated on the plate owing to jet impingement. The experimental results described in this paper include lift loss, particle image velocimetry (PIV) and acoustic measurements. Suckdown forces as high as 60% of the primary jet thrust were measured when the ground plane was very close to the jet exit. The PIV measurements were used to explain the increase in suckdown forces due to high entrainment velocities. The self-sustained oscillatory frequencies of the impinging jet were predicted using a feedback loop that uses the measured convection velocities of the large-scale coherent vortical structures in the jet shear layer. Nearfield acoustic measurements indicate that the presence of the ground plane increases the overall sound pressure levels (OASPL) by approximately 8 dB relative to a corresponding free jet. For moderately underexpanded jets, the influence of the shock cells on the important flow features was found to be negligible except for close proximity of the ground plane.

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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The unsteady velocity field generated by an underexpanded jet has been investigated using stereoscopic particle image velocimetry (PIV). A 4:1 aspect ratio converging–diverging rectangular nozzle designed to operate at a fully expanded condition of $M=1.44$ was used. The nozzle was operated at off-design conditions to generate imperfectly expanded jets with intense screech tones. Phase-locked PIV measurements show the spatial and temporal evolution of the three-dimensional jet with high fidelity. In addition to the globally averaged mean and turbulence velocity field data, the phase-averaged data for the velocity and vorticity fields were also obtained. The turbulence quantities were resolved into contributions from the periodic and random motions. The deformation of the periodic spanwise structures results in the formation of strong streamwise vortices that appear to govern the mixing of the jet. It is shown that the presence of coherent vorticity of significant strength, in addition to the shock cell strength, is largely responsible for determining the screech intensity.

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