Tracianne B. Neilsen

Similarity spectra analysis of high-performance jet aircraft noise

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.

Extraction of acoustic normal mode depth functions using vertical line array data

A method for extracting the normal modes of acoustic propagation in the shallow ocean from sound recorded on a vertical line array (VLA) of hydrophones as a source travels nearby is presented. The mode extraction is accomplished by performing a singular value decomposition (SVD) of individual frequency components of the signals temporally averaged, spatial cross-spectral density matrix. The SVD produces a matrix containing a mutually orthogonal set of basis functions, which are proportional to the depth-dependent normal modes, and a diagonal matrix containing the singular values, which are proportional to the modal source excitations and mode eigenvalues. The conditions under which the method is expected to work are found to be (1) sufficient depth sampling of the propagating modes by the VLA receivers; (2) sufficient source-VLA range sampling, and (3) sufficient range interval traversed by the source. The mode extraction method is applied to data from the Area Characterization Test II, conducted in September 1993 in the Hudson Canyon Area off the New Jersey coast. Modes are successfully extracted from cw tones recorded while (1) the source traveled along a range-independent track with constant bathymetry and (2) the source traveled up-slope with gradual changes in bathymetry. In addition, modes are successfully extracted at multiple frequencies from ambient noise.

Skewness and shock formation in laboratory-scale supersonic jet data

Spatial properties of noise statistics near unheated, laboratory-scale supersonic jets yield insights into source characteristics and near-field shock formation. Primary findings are (1) waveforms with positive pressure skewness radiate from the source with a directivity upstream of maximum overall level and (2) skewness of the time derivative of the pressure waveforms increases significantly with range, indicating formation of shocks during propagation. These results corroborate findings of a previous study involving full-scale engine data. Further, a comparison of ideally and over-expanded laboratory data show that while derivative skewness maps are similar, waveform skewness maps are substantially different for the two cases.

An iterative implementation of rotated coordinates for inverse problems

A generalized inversion method is presented that uses a rotated coordinates technique [Collins and Fishman, J. Acoust. Soc. Am. 98, 1637-1644 (1995)] in simulated annealing to invert for both the location of an acoustic source and parameters that describe the ocean seabed. The rotated coordinates technique not only aids in the inversion process but also indicates the coupling of the source and environmental parameters and the relative sensitivities of the cost function to changes in the various parameters. The information obtained from the rotated coordinates provides insights into how the inversion problem can be effectively decoupled. An iterative process consisting of multiple simulated annealing runs that each use a different set of rotated coordinates is demonstrated. This multistep algorithm is called systematic decoupling using rotated coordinates and is especially helpful when inverting for a large number of unknown parameters. The cost function minimized in the inversion algorithm is model-data cross-hydrophone spectra summed coherently over frequency and receiver pairs. The results of applying this inversion method to simulated data are presented in this paper.

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

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 laboratory-scale 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 acros...

Localization of multiple acoustic sources in the shallow ocean

An iterative, rotated coordinates inversion technique is applied in conjunction with spatial narrowband filters in matrix form to localize multiple acoustic sources in the shallow ocean. When the iterative, rotated coordinates inversion technique is applied to broadband horizontal line array data designed to simulate a group of moving ships, the bearing of the loudest source is quickly identified. Both passband and stop band matrix filters are constructed for an angular sector centered on the identified bearing. Data filtered with the passband filters are used in the inversion to obtain estimates of the remaining source parameters, range, depth, course angle and speed, and prominent environmental parameters. Additional inversions applied to data modified by the stop band filters yield the bearing of the next loudest source. The procedure is repeated until all detectable sources have been localized. Results of applying this strategy to several simulated cases are presented.

Military jet noise source imaging using multisource statistically optimized near-field acoustical holography

The identification of acoustic sources is critical to targeted noise reduction efforts for jets on high-performance tactical aircraft. This paper describes the imaging of acoustic sources from a tactical jet using near-field acoustical holography techniques. The measurement consists of a series of scans over the hologram with a dense microphone array. Partial field decomposition methods are performed to generate coherent holograms. Numerical extrapolation of data beyond the measurement aperture mitigates artifacts near the aperture edges. A multisource equivalent wave model is used that includes the effects of the ground reflection on the measurement. Multisource statistically optimized near-field acoustical holography (M-SONAH) is used to reconstruct apparent source distributions between 20 and 1250 Hz at four engine powers. It is shown that M-SONAH produces accurate field reconstructions for both inward and outward propagation in the region spanned by the physical hologram measurement. Reconstructions across the set of engine powers and frequencies suggests that directivity depends mainly on estimated source location; sources farther downstream radiate at a higher angle relative to the inlet axis. At some frequencies and engine powers, reconstructed fields exhibit multiple radiation lobes originating from overlapped source regions, which is a phenomenon relatively recently reported for full-scale jets.

Spectral Characterization in the Near and Mid-field of Military Jet Aircraft Noise

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 !/! ! 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.

Source characterization of full-scale jet noise using acoustic intensity

Vector acoustic intensity provides both the direction and magnitude of energy flow at the probe location and is, hence, more informative than acoustic pressure measurements. However, this important quantity has seen little application previously in aeroacoustics. In the present work, an intensity probe, consisting of four microphones, captured the radiated field to the sideline and aft of a tethered, full-scale military jet aircraft as one engine was operated at multiple engine conditions. Data from each probe location provide a frequency-dependent map of the sound flow near the aircraft. The vector acoustic intensity is estimated using a recently developed processing technique that extends the upper frequency limit of the traditional cross-spectrum-based calculations. The dominant intensity vectors are traced back to the jet centerline as a method of approximating the extent and location of the source region as a function of frequency. As expected for jet mixing noise sources, the resulting source region estimates contract and move upstream with increasing frequency. A comparison of estimated source regions and intensity directionalities between military and afterburner engine conditions reveals important distinctions in the sound fields.

Cylindrical acoustical holography applied to full-scale jet noise

Near-field acoustical holography methods are used to predict sound radiation from an engine installed on a high-performance military fighter aircraft. Cylindrical holography techniques are an efficient approach to measure the large and complex sound fields produced by full-scale jets. It is shown that a ground-based, one-dimensional array of microphones can be used in conjunction with a cylindrical wave function field representation to provide a holographic reconstruction of the radiated sound field at low frequencies. In the current work, partial field decomposition methods and numerical extrapolation of data beyond the boundaries of the hologram aperture are required prior to holographic projection. Predicted jet noise source distributions and directionality are shown for four frequencies between 63 and 250 Hz. It is shown that the source distribution narrows and moves upstream, and that radiation directionality shifts toward the forward direction, with increasing frequency. A double-lobe feature of full-scale jet radiation is also demonstrated.