Spatial dependence of levels and spectral characteristics of the near-field noise spectra from the afterburning F-22A Raptor and their transition toward far-field behavior are described. It is shown that the measured spectra in the vicinity of the aircraft show relatively good agreement with overall shape of large and fine-scale similarity spectra, with two exceptions. First, the measured spectral shapes have shallower slopes at high frequencies than the similarity spectra at most downstream locations. The variation in high-frequency slope with downstream distance is quantified and, in the vicinity of the maximum radiation direction, approaches the 1/f~2 limit associated with shock formation. This measured slope agrees with some previous laboratory and full-scale measurements of supersonic jet noise. Second, the spectra at downstream distances corresponding to the region of maximum radiation exhibit a double peak, a characteristic not predicted by the similarity spectra nor seen in laboratory-scale measurements. In addition, the maximum in the peak-frequency region does not vary continuously with downstream distance, but rather exhibits discrete frequency jumps, with relative contributions of the different peaks varying as a function of downstream distance. These observations have implications in finding ties between the noise from high performance, full-scale engines and laboratory-scale experiments and computational modeling efforts. Furthermore, they indicate the limitations in applying the present similarity spectra models to full-scale engine noise.
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