A jet exhausting over a plate at different distances away from the nozzle was investigated to simulate jets exhausting over airframe surfaces and the jet-ground interaction during take-off and landing operations. A supersonic rectangular nozzle of 2:1 aspect ratio and 1.5 design Mach number was tested with and without the plate. Far-field acoustics from the cold and heated jets at over-expanded, design, and under-expanded conditions were measured at the reflected, sideline, and shielded azimuthal directions. When the plate starts at the nozzle exit, the "scrubbing" and "scattering" noise from the surface-jet interaction was observed at the low-end frequencies of the reflected and shielded measurements and increased as the plate approached the jet. In the sideline, the plate attached to the nozzle exit diminished the broadband shock-associated noise while the shadowgraph results showed a connection to weakening of the shock-cell structures. In the cold jet, screech was mitigated with the plate attached at the nozzle exit. When the plate was moved away from the nozzle, screech tones were intensified at the under-expanded condition. Crackle levels were significantly intensified in the sideline within a range of plate positions. Noise levels in the shielded region were considerably lower due to the shielding effect.
When the turbulent structures in the shear layer of high-speed jets travel at supersonic convective speeds relative to the ambient speed of sound, they radiate Mach waves that become the dominant component of the overall perceived noise. This is consistent with the OASPL in the far field reaching a maximum in same direction as the Mach wave angle. When the speed of the supersonic jet exceeds a certain level, the steepening of the wave-front in the near field produces a noise feature called “crackle.” Both pressure wave steepening and crackle cannot be recognized in the spectrum of the pressure signal, but in the temporal waveform of the pressure. The statistics of the pressure signal and its time derivative, particularly skewness, have become standard measures of crackle in heated supersonic jets. Previous studies showed that it is possible to reduce far-field pressure skewness with the implementation of notched and chevron nozzles, and to mitigate Mach Wave radiation with secondary flow techniques. In this paper, we investigate the effect of chevrons on the pressure and dP/dt high-order statistics of a Md = 1.5 converging-diverging round conical nozzle, both in the near and far fields. Cold and heated jets, To = 300 K and 600 K, are tested at over, design, and under-expanded conditions. Far-field results of the heated jet showed that chevrons effectively reduce elevated levels of skewness and kurtosis of the pressure and dP/dt. These reductions are remarkable especially around the Mach Wave angle, the region in which high-order statistics tend to propagate. Near-field results corroborated the effectiveness of chevrons in the skewness reduction.
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