Phased-array measurements of full-scale military jet noise

Abstract

Beamforming techniques for aeroacoustics applications have undergone significant advances over the past decade to account for difficulties that arise when traditional methods are applied to distributed sources such as those found in jet noise. Nevertheless, successful source reconstructions depend on array geometry and the assumed source model. The application of phased-array algorithms to ground array measurements of a fullscale tactical jet engine at military and afterburner engine conditions yield different source reconstructions. A deconvolution approach for the mapping of acoustic sources (DAMAS) is utilized to remove array effects seen in conventional beamforming and allows for improved interpretation of results. However, the distributed nature of the jet noise source, as well as large correlation lengths at low frequencies, can result in inaccurate source locations and/or amplitudes for both conventional beamforming and DAMAS. Results using DAMAS-C, an extension of DAMAS, indicate the degree of source correlation within the military aircraft noise. Source reconstructions on the jet centerline for different one-third octave band frequencies confirm the greater source correlation at low frequencies. These preliminary results represent the first implementation of DAMAS-C on full-scale jet noise data. Nomenclature = Point-spread functions matrix = Total beamform response vector = Beamwidth of the array CSM = Cross spectral matrix = Ambient sound speed = Scaling factor to limit residual component variability = Steering element = Steering vector = Frequency ′ = Cross spectral elements = Conjugate matrix transpose = Wavenumber = Measurement location index = Number of measurement locations = Scan point index = Number of scan points ( ) = Residual component at each iteration 2 0 = Distance from measurement to reference location = Distance from measurement to scan location = Iteration number = Matrix transpose = Vector of monopole source strengths Δ = Spacing between scan points 0 = Cross-source amplitude from and 0 ( ) = Source strength at scan point and iteration = Beamformed response for scan point

Cylindrical acoustical holography applied to full-scale jet noise

“…5 from about z ¼ 10 to 27 m. In a preliminary study by Tam and Parrish (2014), combustion noise was suggested as a possible mechanism for the generation of the second lobe. However, combustion noise would likely be localized at the nozzle exit, which is not seen in the present work or in recent phased-array analyses of the same data set (Harker et al, 2014). Hence, further work is needed to investigate the cause of the two lobes, which are also present in other high-power military aircraft noise data sets (Norum et al, 2004;McInerny et al, 2007;Schlinker et al, 2007;Gee et al, 2005Gee et al, , 2008Greska and Krothapali, 2008).…”

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.…”

“…3 and assuming a peak source location near 5-6 m downstream. In the time domain, time of arrival analysis can estimate the apparent direction of signal propagation, or the overall intensity vector direction, not unlike beamforming 38 or phase speed characterizations 37 . In the frequency domain, this time of arrival difference presents itself as differences in the complex pressures themselves, resulting in a nonzero imaginary part of the cross spectrum between microphone pairs.…”

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

“…Turbulent flow and combustion data are not yet available to corroborate or refute these claims about jet acoustic source mechanisms, but several acoustic imaging techniques have shown F-22A source distributions centered on regions well downstream of the nozzle. 7,16,17 In addition, the supersonic engine tested by Schlinker et al 1 did not exhibit an increase in noise from the nozzle exit region in the direction of maximum radiation with the addition of fuel.…”

“…F-22A source reconstructions 7,17 have also been obtained through advanced beamforming 21 and near-field acoustical holography (NAH) 22,23 methods. Both reconstructions suggest that two or more overlapping source regions generate radiation lobes that point in different directions, resulting in multiple spatial/spectral peaks in the field.…”