Kent L. Gee

The role of nonlinear effects in the propagation of noise from high-power jet aircraft

To address the question of the role of nonlinear effects in the propagation of noise radiated by high-power jet aircraft, extensive measurements were made of the F-22A Raptor during static engine run-ups. Data were acquired at low-, intermediate-, and high-thrust engine settings with microphones located 23-305 m from the aircraft along several angles. Comparisons between the results of a generalized-Burgers-equation-based nonlinear propagation model and the measurements yield favorable agreement, whereas application of a linear propagation model results in spectral predictions that are much too low at high frequencies. The results and analysis show that significant nonlinear propagation effects occur for even intermediate-thrust engine conditions and at angles well away from the peak radiation angle. This suggests that these effects are likely to be common in the propagation of noise radiated by high-power aircraft.

On the Perception of Crackle in High-Amplitude Jet Noise

Crackle is a phenomenon sometimes found in supersonic jet noise and can comprise an annoying and dominant part of the overall perceived noise. In the past, crackle has been commonly quantified by the skewness of the tune waveform. In this investigation, a simulated waveform with a virtually identical probability density function and power spectrum as an actual F/A-18E afterburner recording has been created by nonlinearly transforming a statistically Gaussian waveform. Although the afterburner waveform crackles noticeably, playback of the non-Gaussian simulated waveform yields no perception of crackle at all, despite its relatively high skewness

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.

Measurement and Prediction of Noise Propagation from a High-Power Jet Aircraft

Static engine run-up noise measurements have been made on the F-22 Raptor at low and high power settings. At afterburner, the propagation measurements reveal significant evidence of nonlinearity in that there is much greater high-frequency energy than is predicted by linear theory. The measurements have been compared against the results of a nonlinear numerical model based on the generalized Mendousse-Burgers equation. Although the model simplifies the propagation environment in that it neglects ground effects and atmospheric variability, agreement between the measured and nonlinearly predicted spectra is quite favorable. This comparison demonstrates that nonlinear effects can play a significant role in the propagation of high-amplitude noise and that prediction of these effects is possible with this type of numerical model.

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.

Bicoherence analysis of model-scale jet noise

Bicoherence analysis has been used to characterize nonlinear effects in the propagation of noise from a model-scale, Mach-2.0, unheated jet. Nonlinear propagation effects are predominantly limited to regions near the peak directivity angle for this jet source and propagation range. The analysis also examines the practice of identifying nonlinear propagation by comparing spectra measured at two different distances and assuming far-field, linear propagation between them. This spectral comparison method can lead to erroneous conclusions regarding the role of nonlinearity when the observations are made in the geometric near field of an extended, directional radiator, such as a jet.

Acoustical Nonlinearities in Aircraft Flyover Data

Numerous analyses techniques have been proposed as means of characterizing acoustical nonlinearities in high-thrust engine noise. These include probability distributions for the pressure and the time derivative of the pressure (i.e., the gradient), the skewness and kurtosis coefficients of the pressure and its time derivative, and Howell-Morfey nonlinear indicators. In this paper, a number of these analyses techniques are applied to acoustic data recorded during a series of military jet flyovers. The analysis examines these different measures as a function of microphone height above the ground. This analysis provides strong indications that microphone should be mounted well above the ground to properly measure nonlinearities in high-thrust engine noise.

Identification of Nonlinear and Near-field Effects in Jet Noise Using Nonlinearity Indicators

In the collection and analysis of high-amplitude jet noise data for nonlinear acoustic propagation, both model-scale and full-scale measurements have limitations. Model-scale measurements performed in anechoic facilities are usually limited by transducer and data acquisition system bandwidths and maximum propagation distance. The accuracy of fullscale measurements performed outdoors is reduced by ground reflections and atmospheric effects. This paper describes the use of two nonlinearity indicators as complementary to ordinary spectral analysis of jet noise propagation data. The first indicator is based on an ensemble-averaged version of the generalized Burgers equation. The second indicator is the bicoherence, which is a normalized version of the bispectral density. These indicators are applied to Mach-0.85 and Mach-2.0 unheated jet noise data collected at the National Center for Physical Acoustics. Specifically, the indicators are used to separate geometric near-field effects from nonlinear propagation effects for the Mach-2.0 data, which cannot be done conclusively using comparisons of power spectral densities alone.

Preliminary Analysis of Nonlinearity in F/A-18E/F Noise Propagation

*† ‡ § Analyses of recent F/A-18E/F military power and afterburner measurements suggest that the noise propagation is nonlinear in the far field. Spectral broadening takes place as the radiated noise propagates from 18 to 150 m, the limits of the measurement range. This broadening phenomenon cannot be readily explained in terms of linear propagation effects. Calculation of a nonlinearity indicator derived by Howell and Morfey supports the assertion that nonlinear propagation effects are present. Furthermore, skewness and kurtosis calculations indicate that the noise data distributions are non-Gaussian over the propagation range at these engine settings. Finally, the measured spectra have been compared against predictions obtained with existing nonlinear spectral evolution methods. Even though the prediction results compare poorly with the actual measurements, this is attributable to limitations of the spectral evolution methods themselves. I. Introduction HIS paper contains a preliminary analysis of F/A-18E/F Super Hornet static engine run-up noise measurements made at NAVAIR Lakehurst, NJ on 15 April 2003. Results of the analysis show evidence of nonlinear acoustic propagation effects. These effects are typified by a spectral broadening in which energy is transferred from midfrequencies to the ends of the spectrum. The evolution of a finite-amplitude noise spectrum may be explained in terms of two time-domain phenomena. Waveform steeping is responsible for the transfer of energy from mid to high frequencies, whereas shock coalescence and a corresponding increase in time scale accounts for a relative increase of energy at low frequencies 1 . In this paper, the measurement setup and apparatus as well as analysis procedure are first described. Next, the probability density function (PDF) and related statistical quantities are calculated. The measured spectra are compared with linear predictions, which take into account spherical spreading and atmospheric absorption, as well as nonlinear predictions from two existing jet noise prediction schemes 2-4 . The assertion that the disparity between linear predictions and measurements are caused by nonlinear propagation is supported by calculation of a quantity derived by Howell and Morfey as a useful nonlinearity indicator 3 .

Application of theoretical modeling to multichannel active control of cooling fan noise

Multichannel active control has been applied to the global reduction of tonal noise from a cooling fan. In order to achieve consistent far-field attenuation of multiple harmonics of the blade passage frequency (BPF) of the fan, an analytical model has been applied to the control system in order to determine appropriate transducer configurations. The results of the modeling show that the additional global reduction possible by locating acoustically compact secondary sources coplanar with a compact primary source rapidly lessens as the number of symmetrically placed sources is increased beyond three. Furthermore, the model suggests that there are locations in the extreme near field of the sources that can be considered ideal for the minimization of far-field radiated power. Experiments carried out show that a four-channel control system is more effective than a two-channel system at achieving far-field attenuations, especially at the higher harmonics of the BPF for the fan tested. In addition, greater far-field mean-square pressure attenuations are achieved with the error microphones located along the calculated ideal regions than for nonideal placement.