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ABSTRACT: Jet noise research has seen increased use of autocorrelation analyses to glean physical insight about the source and its radiation properties. Length scales and other features have been identified in support of models incorporating large-scale (LSS) and fine-scale (FSS) turbulent structures. In this paper, the meaningful use of autocorrelation in jet noise analysis is further examined. A key finding is that the effect of the peak frequency on autocorrelation width needs to be removed prior to making conclusions about the relative LSS and FSS contributions. In addition, the Hilbert transform is applied to create an envelope of the autocorrelation function to more consistently define a characteristic time scale. These methods are first applied to the analytical LSS and FSS similarity spectra, previously developed by Tam et al. [AIAA 96-1716, 1996]. It is found that the envelope of the FSS similarity autocorrelation function is more similar to that of a delta function than the LSS envelope. These curves are used to more effectively quantify FSS and LSS features in noise spectra from the F-22A Raptor. [Work supported by ONR.].
The Journal of the Acoustical Society of America 05/2013; 133(5):3421. · 1.55 Impact Factor
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ABSTRACT: The two-source model for jet noise holds that turbulent mixing noise in jets is generated by uncorrelated, fine-scale (FSS) and partially correlated, large-scale (LSS) turbulent structures [Tam et al., J. Fluid Mech. 615, 253-292, (2008)]. The noise from an F-22A Raptor is modeled with an equivalent source consisting of two line arrays of monopole sources. These arrays, one correlated and one uncorrelated, with Rayleigh distributed amplitudes, account for both FSS and LSS sound propagation [Morgan, J. Acoust. Soc. Am. 129, 2442 (2011)]. The equivalent source parameters are selected based on Bayesian methods implemented with simulated annealing and fast Gibbs sampler algorithms. This method yields the best fit parameters, and the sensitivity of the solution is indicated by the generated posterior probability distributions. Analysis of the resulting equivalent sources shows that the directional, correlated line array has a greater effect on the near field sound, and the sensitivity of the array's parameters increases as the frequency increases. This equivalent source model can generate results up to 2500 Hz and accurately predict both near field and far field measurements. The analysis suggests that the shape of the source distribution changes as the frequency increases. [Work sponsored by the Office of Naval Research.].
The Journal of the Acoustical Society of America 05/2013; 133(5):3576. · 1.55 Impact Factor
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ABSTRACT: Acoustic intensity measurements of the F-22A Raptor are analyzed as part of ongoing efforts to characterize the noise radiation from military jet aircraft. Data were recorded from a rig of microphones and an attached tetrahedral intensity probe at various locations to the sideline and aft of the aircraft. Numerical analysis of the intensity at one-third octave band center frequencies along various measurement planes and at a 23 m radius arc reveals the magnitude and directionality of the vector acoustic intensity. Differences in the trends for low-frequency and high-frequency data are discussed and, via a simple ray tracing back toward the source, interpreted in terms of source location and extent. [Work supported by ONR.].
The Journal of the Acoustical Society of America 05/2013; 133(5):3421. · 1.55 Impact Factor
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ABSTRACT: The broadband, partially correlated noise radiated from supersonic jets has characteristics that scale with nozzle size and flow properties. In particular, the spectral content of jet noise and the variation with angle in many cases agree with empirically derived similarity spectra for large and fine-scale components of turbulent mixing noise [Tam et al., AIAA Paper 96-1716]. In previous studies, measurements made near the F-22 Raptor agreed remarkably well with the similarity spectra, with two exceptions. First, the high-frequency slopes seen in the data were shallower than the similarity spectra at many angles. Second, the data exhibit a double frequency peak, which is absent from the similarity spectra [Neilsen et al., J. Acoust. Soc. Am. 132, 1993 (2012)]. These observations are explored further by examining the spectral characteristics of noise from a different military jet and a laboratory-scale, unheated jet. In both cases, there is evidence that for supersonic cases the measured spectra are shallower than the similarity spectra due to nonlinear propagation effects. In addition, the military data support the observation that the double spectral peak is a feature of full-scale jet noise. Recommendations are made for applying the similarity spectra to predict spectral levels for full-scale jets. [Work supported by ONR.].
The Journal of the Acoustical Society of America 05/2013; 133(5):3420. · 1.55 Impact Factor
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ABSTRACT: Both axial and centrifugal fans are used to cool information technology (IT) equipment. These fans generate noise that can be annoying to their users, particularly the tonal noise that can be radiated. Work has focused on developing a method to attenuate the tonal noise associated with both of these types of fans. A compact system is used, whereby control sources are placed in close proximity to the fan. A genetic algorithm has been implemented to determine optimal source configurations. The attenuation associated with some configurations is found to be much more sensitive to error than others. For a given configuration, by using a relatively simple point source model it becomes possible to identify optimal near-field error sensor locations, which results in a compact noise control solution that provides significant global attenuation of the radiated tonal noise. This paper will review progress that has been made to apply this method to both axial and centrifugal fans. Experimental results confirm that it is feasible to achieve significant global control using this method.
The Journal of the Acoustical Society of America 05/2013; 133(5):3264. · 1.55 Impact Factor
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ABSTRACT: Near-field acoustical holography has been shown to be a useful tool for visualizing jet noise fields. It has been applied to a full-scale jet on an installed military aircraft with promising results, but the source characteristics in the extreme near field have been difficult to characterize because of the interference of acoustic reflections off the rigid reflecting plane beneath the jet. To provide accurate sound field reconstructions, a modified approach to statistically optimized near-field acoustical holography (SONAH) is implemented. In conventional SONAH, the sound field is represented by a matrix of elementary wave functions at all desired spatial locations. In this modified approach, advantage is taken of the property that arbitrary, user-defined functions can be selected for this matrix. Here, two sets of cylindrical wave functions, one centered on the jet centerline and one on the image source centerline, are used to obtain an accurate near-field reconstruction.
The Journal of the Acoustical Society of America 05/2013; 133(5):3296. · 1.55 Impact Factor
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ABSTRACT: As part of a sound system evaluation at Brigham Young University's football stadium and to assist in planning for future system design, measurements were made before and during games by an upper-level undergraduate acoustics class. The measurement experience provided significant training opportunities for the students. Teams of students used sound level meters to make recordings at numerous locations both inside and outside the stadium. These measurements were then correlated with data from stationary microphones placed near the field. From the data, the predicted slow, A-weighted equivalent levels in and around the stadium were calculated relative to an assumed 90 dBA on the sideline. Straightforward outdoor sound propagation prediction methods involving geometric spreading, atmospheric absorption, barriers, etc. were successfully used to validate the measured data within 1-2 decibels at many locations, including a location in the foothills to the southeast of the stadium at a distance of approximately 3 km. The students appreciated the hands-on experiences gained by participation in the measurements and analysis.
The Journal of the Acoustical Society of America 05/2013; 133(5):3319. · 1.55 Impact Factor
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ABSTRACT: 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.
The Journal of the Acoustical Society of America 04/2013; 133(4):2116-25. · 1.55 Impact Factor
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ABSTRACT: Noise measurements near the F-35A Joint Strike Fighter at military power are analyzed via spatial maps of overall and band pressure levels and skewness. Relative constancy of the pressure waveform skewness reveals that waveform asymmetry, characteristic of supersonic jets, is a source phenomenon originating farther upstream than the maximum overall level. Conversely, growth of the skewness of the time derivative with distance indicates that acoustic shocks largely form through the course of near-field propagation and are not generated explicitly by a source mechanism. These results potentially counter previous arguments that jet "crackle" is a source phenomenon.
The Journal of the Acoustical Society of America 02/2013; 133(2):EL88-93. · 1.55 Impact Factor
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ABSTRACT: The goal of active-learning techniques is to encourage the students to become involved with the material and take ownership for their learning, which fosters long-term knowledge and enjoyment of the subject. In this era of student-based learning outcomes, an active-learning approach is important because it focuses on what the students are doing to facilitate learning instead of what the instructor is trying to teach. To benefit most from class time, the students need to have the opportunity to actively engage with the material beforehand. If meaningful pre-class activities are required, it is easier to interact with the students during class. Some key methods for encouraging active learning during class include incorporating their pre-class experiences, conducting discussions, encouraging student participation, and evaluating student understanding with a response system, such as i-clickers. After the class time, students need apply what they have learned in answering additional questions on homework assignments and in hands-on laboratory experiences. Lessons learned after several years' worth of step-by-step efforts to approach these goals in an introductory acoustics class, which serves a wide range of majors as a general science elective, are presented.
The Journal of the Acoustical Society of America 09/2012; 132(3):1923. · 1.55 Impact Factor
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ABSTRACT: The application of partial field decomposition (PFD) techniques based on a singular value decomposition to jet noise fields is useful for estimating the number of incoherent (equivalent) noise sources within a jet and for implementing near-field acoustical holography, but it does not generally provide physically meaningful partial fields (i.e. partial fields related to individual sources). Among several PFD methods that were designed to generate physically meaningful partial fields, the method developed by Kim et al. [JASA 115(4), 2004] finds the optimal locations of references in a sound field and places virtual references at those locations. In past investigations this method has been successfully applied to locate discrete numerical and physical sources and to generate partial fields related to each source. In this study, Kim's method is applied to a full-scale jet installed on a military aircraft in an attempt to obtain physically meaningful partial fields. The partial fields obtained using these optimally located virtual references are compared to the partial fields obtained from other PFD methods.
The Journal of the Acoustical Society of America 09/2012; 132(3):1891. · 1.55 Impact Factor
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ABSTRACT: Correlation and coherence analyses in the near field of military jet noise provide insight into source and radiation characteristics. Data were measured parallel to the exhaust centerline of an F-22A Raptor and spatial correlation and coherence values were calculated. Low spatial correlation at the sideline indicates radiation dominated by multiple incoherent sources. In contrast, the downstream region is characterized by high spatial correlation, suggesting radiation primarily from large-scale turbulent structures. Variations in spatial correlation in the axial direction can be related to the spectral dependence on measurement location, which supports the idea of a two-source jet-noise model. Coherence calculations, which decompose the correlation information into narrow frequency bands, further support this idea. [Work supported by the Office of Naval Research.].
The Journal of the Acoustical Society of America 09/2012; 132(3):1993. · 1.55 Impact Factor
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ABSTRACT: The extreme sound pressure levels radiated from rocket motors are such that nonlinear propagation effects can be significant. Here, free-field nonlinear propagation has been modeled for noise produced by a GEM-60 solid rocket motor. Measured waveforms were used as inputs into a numerical model based on the generalized Burgers equation. In both temporal and frequency domains the nonlinear predictions are significantly closer to the measured signals than free-field, linear predictions. In the temporal domain, shock coalescence and a transition from the weak-shock regime of propagation to the beginning of the old-age regime are clearly observed in both the nonlinear prediction and the measured data. These phenomena are completely missing in the linear prediction. In the frequency domain, continual transfer of energy upward in the spectrum reduces attenuation of high-frequency components when compared to predictions from the linear model. Various comparisons are made as a function of input distance for two different radiating angles from the rocket plume; these comparisons illustrate the importance of including nonlinear effects in rocket noise propagation modeling.
The Journal of the Acoustical Society of America 09/2012; 132(3):1992. · 1.55 Impact Factor
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ABSTRACT: Many far-field measurements of laboratory-scale jet noise have shown good agreement with the two similarity spectra developed to represent the contributions of fine-scale and large-scale turbulent structures [Tam et al., AIAA paper 96-1716, 1996]. Measurements near an F-22A Raptor provide a means to study how accurately the similarity spectra describe the noise from a full-scale, high-performance, jet engine. Comparisons have been made using ground-based microphones at 60° to 150° for three engine conditions: intermediate, military and afterburner, with more detailed analyses than described previously [Neilsen et al., J. Acoust. Soc. Am. 129, 2242 (2011) ]. The good agreement with Tam's predictions - the fine-scale spectrum at upstream and sideline angles and the large-scale spectrum in the maximum radiation direction - permits a quantitative analysis of the contributions of the two spectra at other angles. The radiated spectra and overall levels for all three engine conditions have been decomposed into contributions from the two spectra as functions of angle. Of particular interest is the appreciable contribution of fine-scale turbulence aft of the maximum radiation directions at military and afterburner conditions. [Work sponsored by the Office of Naval Research.].
The Journal of the Acoustical Society of America 09/2012; 132(3):1993. · 1.55 Impact Factor
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ABSTRACT: As part of investigations into the design of next-generation launch vehicles, near and far-field data were collected during horizontal static firings of reusable solid rocket motors. In addition to spectral analysis at individual microphone locations, the spatial variation of overall and one-third octave band pressure levels at sideline and polar arc arrays is considered. Analysis of the probability density functions reveals positively skewed pressure waveforms, but extreme skewness in the first-order estimate of the time derivative because of the presence of significant acoustic shocks.
The Journal of the Acoustical Society of America 09/2012; 132(3):1992. · 1.55 Impact Factor
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ABSTRACT: Multi-microphone arrays embedded in a rigid spherical housing have been used to estimate field quantities such as vector intensity. However, the measured pressure waveforms are modified by the scattering of the incident pressure wave on the sphere. The frequency response function and the corresponding impulse response function for microphones in a sphere can be created using Mie scattering theory. This permits analysis of the scattering effect on pressure measurements for shock-containing broadband waveforms, such as those produced by solid rocket motors. The abrupt pressure rises associated with shocks are seen to be either overestimated or underestimated depending on the angle of the measuring microphone. These shocks are the most affected portion of the waveform due to their high frequency content. In addition, deconvolution of measured rocket signals using the predicted impulse responses of the microphones in the array provides an apparently consistent estimate of the free-field signal at the probe center.
The Journal of the Acoustical Society of America 09/2012; 132(3):1995. · 1.55 Impact Factor
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ABSTRACT: Development of the next-generation space flight vehicles has prompted a renewed focus on rocket sound source characterization and near-field propagation modeling. Improved measurements of the noise near the rocket plume are critical for direct determination of the noise environment. They are also crucial in providing inputs to empirical models and in validating computational aeroacoustics models. NASA's SP 8072 acoustic load prediction model is a widely used method for predicting liftoff acoustics. SP-8072 implements two Distributed Source Methods (DSM-1 and DSM-2), which predict the loading as the sum of the radiated field from each source distributed along the plume. The prediction model depends largely on empirical curve fits computed from historical data to determine the source power and frequency content at distances along the plume. Preliminary results from measurements of a static horizontal firing of Alliant Techsystems Orion 50S XLG performed in Promontory, UT are analyzed with respect to the historical data that drives the SP-8072 prediction model.
The Journal of the Acoustical Society of America 09/2012; 132(3):1991. · 1.55 Impact Factor
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ABSTRACT: An effort to characterize the aeroacoustic source regions and noise environment around launch vehicles has resulted in study of the hardware and processing methods used to calculate acoustic intensity. Because of the extremely harsh measurement environment and other source region characteristics, these investigations have included selection, calibration, and arrangement of microphones and examination of the required pressure and particle velocity estimates. The results of analytical, laboratory, and field experiments are described as a summary of lessons learned during the on-going effort.
The Journal of the Acoustical Society of America 09/2012; 132(3):1985. · 1.55 Impact Factor
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ABSTRACT: Multireference partial field decomposition (PFD) can be used to generate coherent holograms for scan-based near-field acoustical holography measurements. PFD is successful when the reference array completely senses all independent subsources, but meeting this requirement is not straightforward when the number of subsources and their locations are ambiguous (such as in aeroacoustic sources). A figure of merit based on spatial coherence lengths, called references per coherence length (RPLc), is a useful metric to guide inter-reference spacing in the array design so that the source is spanned. Coherence length is defined as the axial distance over which the ordinary coherence drops from unity to some desired value. Numerical experiments involving an extended, partially correlated source show that sufficiency of the reference array for different source conditions may be simply expressed in terms of RPLc. For sources of varying spatial coherence and over a large range of frequencies, one reference per coherence length is equivalent to sensing all independent subsources.
The Journal of the Acoustical Society of America 09/2012; 132(3):2075. · 1.55 Impact Factor
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ABSTRACT: Multireference partial field decomposition (PFD) can be used to generate coherent holograms for near-field acoustical holography measurements. PFD is most successful when the reference array completely senses all independent subsources, but meeting this requirement is not straightforward when the number of subsources and their locations are ambiguous (such as in aeroacoustic sources). A figure of merit based on spatial coherence lengths, called references per coherence length (RPL(C)), is a useful metric to guide inter-reference spacing in the array design. For numerical, extended, arbitrarily coherent sources one reference per coherence length results in a sufficient reference array.
The Journal of the Acoustical Society of America 09/2012; 132(3):EL215-21. · 1.55 Impact Factor
