Investigation of multi-lobed fighter jet noise sources using acoustical holography and partial field decomposition methods

Abstract

Full-scale tactical aircraft noise exhibits multiple radiation lobes not seen in laboratoryscale jets. These lobes have different radiation directions yet appear to have similar, overlapping source regions. Near-field acoustical holography (NAH) source reconstructions, in conjunction with partial field decomposition (PFD) methods that produce physically meaningful partial fields, are used in the current work to investigate the nature of these radiation patterns. First, it is shown that the two main radiation lobes are highly incoherent, suggesting independent partial sources. Second, these lobes are isolated as mutually orthogonal partial fields. In this representation, the lobes seem to be generated by independent yet spatially coincident extended partial sources. Source comparisons are made between non-afterburner and afterburner engine powers to investigate whether afterburner combustion produces any sources that are fundamentally different from those of nonafterburner operations. The current results show no qualitative changes occur due to the addition of the afterburner thrust aside from minor variations in source distribution, level, and the nature of the overlap between the multiple lobes.

Source characterization of full-scale jet noise using acoustic intensity

“…At afterburner and military engine conditions, data was taken at a sampling rate of 48 and 96 kHz, respectively. A significantly better understanding of the turbulent mixing noise in the geometric near field of a high-power military jet aircraft has resulted from the previous analysis of the data, using holography 35 and partial coherence decomposition 36 , correlation and coherence analysis 37 , multiple beamforming methods 38 , similarity spectra analysis 39 and equivalent source modeling 40 . Attached to the top of the 90 microphone array was a custom three-dimensional intensity probe [see Fig.…”

“…source mechanisms 36 . In addition, an intensity-based equivalent source model using two analytical, mutually incoherent steered line array sources has produced features consistent with these measurements, with greater success than the use of a single steered line array 31 .…”

“…This effect is most pronounced at military engine conditions, where the estimated source region covering z = 6-7.5 m at 200 Hz shifts upstream to z = 3.5-5.5 at 250 Hz, the next third-octave band. As noted above, the nature of this transition region and the possibility of two discrete source mechanisms are currently being explored 36,39,48 .…”

Partial-field decomposition analysis of full-scale supersonic jet noise using optimized-location virtual references

“…For example, the target of a previous investigation was to separate the incoherent contributions of two jet radiation lobes, so the two VRs were simply located in the far field near the center of each lobe. 24 In the current work, contributions to the field from isolated source components (based on their spatial coherence in the near field) are desired. The OLVR procedure used here relies on the subprocesses of back propagation toward the source through the use of NAH, a PFD method based on a singular value decomposition (SVD-based PFD), 27 the MUSIC algorithm, 19,23 and the Gauss elimination technique (Cholesky decomposition) that is integral to a second PFD algorithm called partial coherence decomposition (PCD).…”

“…In order to obtain physically meaningful spatial distributions of PFs along the length of a jet axis, and account for partially coherent sources, the VRs selected here are optimized to have high MUSIC powers and low mutual coherence and to be spatially ordered. It should be noted that a different VR selection process with a different goal (i.e., the isolation of two partially independent radiation features) was implemented in a previous study, 24 emphasizing the fact that PFD is a nonunique process and the desired information dictates the method.…”