Similarity spectra analysis of high-performance jet aircraft noise

Abstract

Noise measured in the vicinity of an F-22A Raptor has been compared to similarity spectra found previously to represent mixing noise from large-scale and fine-scale turbulent structures in laboratory-scale jet plumes. Comparisons have been made for three engine conditions using ground-based sideline microphones, which covered a large angular aperture. Even though the nozzle geometry is complex and the jet is nonideally expanded, the similarity spectra do agree with large portions of the measured spectra. Toward the sideline, the fine-scale similarity spectrum is used, while the large-scale similarity spectrum provides a good fit to the area of maximum radiation. Combinations of the two similarity spectra are shown to match the data in between those regions. Surprisingly, a combination of the two is also shown to match the data at the farthest aft angle. However, at high frequencies the degree of congruity between the similarity and the measured spectra changes with engine condition and angle. At the higher engine conditions, there is a systematically shallower measured high-frequency slope, with the largest discrepancy occurring in the regions of maximum radiation.

Intensity analysis of the dominant frequencies of military jet aircraft noise

“…Recently, discrete frequency peaks were discovered, 4,11 from what appear to be two mutually-incoherent sources which exhibit distinct directionalities. 13 As seen in Fig.…”

“…The spectrum recorded at the ground-based microphone locations was matched to two similarity spectra curves, which correspond to radiation from large and fine-scale turbulence structures. 11,12 Currently, this generalized two-source representation is a prevalent model used in jet plume source characterization. Except at high frequencies, the similarity spectra curves follow the measured data well in the general frequency trend (from low frequency to several kHz).…”

Acoustical measurements during a static firing of the Space Launch System solid rocket motor

“…The general shapes of the spectra at 70° and 90° follow the LSS spectrum up to approximately 1 kHz, except for the low frequencies at 70°. This low frequency discrepancy has also been seen in high-power military aircraft noise in and near the direction of maximum sound radiation [9,10]. The measured rolloff above 1 kHz is likely due to atmospheric absorption, which Viswanathan pointed out was not explicitly included in the LSS/FSS fits, but is also complicated by the additional impact of nonlinear propagation.…”

“…In addition, it was proposed that the turbulent mixing noise at any radiation angle is a sum of LSS and FSS spectra. Agreement between the similarity spectra and measured jet noise spectra has been shown for laboratory-scale jets [6][7][8], and for high-power engines installed in military aircraft [9,10], with a few exceptions. Comparisons between the similarity and QM-1 spectra (shown in Figure 5) follow expected trends.…”

Sensitivity analysis of an equivalent source model for military jet aircraft noise

“…The two-source theory for jet noise supports this increase in the relative importance of the uncorrelated noise as frequency increases. 10,17 (Note that the absolute amplitude values are unimportant as the modeled levels are shifted in the end such that the maximum matches the maximum measured level.) Maps of the levels computed with the source distributions in Figure 6 are presented in Figure 7 - Figure 12 and illustrate the capabilities and limitations of the simple-source ESM.…”