Spatiotemporal-Correlation Analysis of Jet Noise from a High-Performance Military Aircraft

Abstract

Correlation analyses of ground-based acoustic-pressure measurements of noise from a tethered F-22A provide insights into the sound-field characteristics with position and engine condition. Time-scaled single-point (auto) correlation functions show that, to the side of the nozzle exit, the temporal-correlation envelope decays rapidly, whereas the envelope decays more slowly in the maximum radiation region and farther downstream. This type of spatial variation has been previously attributed to a transition from fine-to large-scale mixing noise in laboratoryscale jets. Two-point space-time (cross) correlation functions demonstrate that noise from a single engine operating at intermediate power is similar to that from a heated, convectively subsonic laboratory-scale jet, whereas additional features are seen at afterburner, relative to supersonic laboratory jets. A complementary coherence analysis provides estimates of coherence lengths as a function of frequency and location. Acoustic coherence lengths across the ground microphone array are used to analyze one-dimensional, equivalent-source-coherence lengths obtained from the DAMAS-C beamforming algorithm. The source coherence reaches its maximum downstream of the maximum source level, suggesting that uncorrelated sources meaningfully contribute to the dominant source region. In addition to revealing further the nature of the sound field near an advanced tactical engine, the characteristics seen should be useful as a phenomenological comparison point for those trying to model military-scale results both experimentally and numerically.

Dependence of High-performance Military Aircraft Noise on Frequency and Engine Power

“…Harker et al [16] show OTO band spectra as well as correlation analyses for data collected from a ground-based linear array similar to the one in this study, though from a different aircraft with a rectangular nozzle. Fig.…”

“…Qualitatively, it appears that the total field is a superposition of these lobular structures, which implies some incoherence between the lobes. This has been seen by Harker et al [16] for a similar aircraft and is currently being investigated in Ref. [23].…”

“…Previous observations of a "dual-peak" or multi lobe phenomena in Refs. [7,11,15,16] appear to be subsets of the more complete picture described herein.…”

“…Additionally, correlation analysis in Fig. 8 of Harker et al [16] show that the signals recorded in the region of maximum OASPL at afterburner have significant correlation with two different time delays, which correspond to two sets of waves with different phase speeds propagating across the array, resulting in different far-field directivities. A similar correlation analysis performed on the same dataset of this paper is shown by Fig.…”

“…Multilobe radiation is a feature of military aircraft jet noise that is seen in several datasets [7,[10][11][12][13][14][15][16][17], though no conclusive explanation of its cause has been given. These lobes are described as either multiple peaks in the spectra at a given measurement location or as multiple local maxima in space across the array when plotting a single frequency.…”