High-performance Military Aircraft Research Articles

Sound quality analysis of far-field noise from a high-performance military aircraft

Noise from high-performance military aircraft can pose challenges to community relations near airfields. Accurately predicting and quantifying community impacts is important for efforts to minimize such impacts and reduce annoyance. In this study, sound recordings measured 305 m from a tethered F-35 aircraft operating at various engine conditions are analyzed using sound quality metrics. The calculated metrics are inputs for a model of perceived annoyance used to estimate the relative contributions of loudness and other sound quality features to annoyance. These results can help inform future efforts at noise reduction by identifying potentially relevant sound quality components of the jet noise as well as helping inform discussions of noise policy on military bases. [Work supported by a USAFRL SBIR and the F-35 JPO. Distribution A: Approved for public release; distribution unlimited. Cleared 07/10/2017; JSF17-714.]

Spectral decomposition of turbulent mixing and broadband shock-associated noise from a high-performance military aircraft

Sound from high-performance military aircraft originates primarily from the turbulent mixing noise, but at smaller inlet angles, broadband shock-associated noise (BBSAN) is present. The similarity spectra of the two components of turbulent mixing noise developed by Tam et al. [AIAA Paper 96–1716 (1996)] represent noise associated with fine and large-scale turbulent structures and provide reasonable fits for ideally expanded, supersonic jet noise. For non-ideally expanded jet flow, BBSAN contributions to the spectral shape need to be included in spectral decompositions in the sideline and forward directions. A model proposed by Tam et al. [J. Sound Vib. 140, 55–71 (1990)] and later simplified by Kuo et al. [AIAA Paper 2011–1032 (2011)] provides a spectral function that models the BBSAN spectral shape. The ability of the BBSAN and similarity spectra shapes to account for the measured spectra is evaluated for ground-based microphones that covered a spatial aperture from 35 to 152 degrees. Spectral decompositions at low and high engine powers are compared. Using turbulent mixing noise similarity spectra decomposition in conjunction with BBSAN empirical fits, a better equivalent source model can be developed. [Work supported by the Office of Naval Research and the F-35 JPO. Distribution A: Approved for public release; distribution unlimited.]

Characterization of broadband shock-associated noise from high-performance military aircraft

For nonideally expanded jets, broadband shock-associated noise (BBSAN) is a feature in the sideline and forward directions. While BBSAN has been studied fairly extensively for laboratory-scale jets, its presence and characteristics in full-scale, tactical aircraft noise need to be evaluated. Noise measurements on a tied-down F-35 provide the opportunity to characterize full-scale BBSAN using a linear ground array that spanned a large angular aperture: 35–152 degrees relative to the front of the aircraft. The main questions are whether the full-scale BBSAN shares the same characteristics as those observed in laboratory-scale BBSAN and if current models capture the features of full-scale BBSAN. The variation in the spectral shape, peak frequency, and peak level of full-scale BBSAN across angle for different engine powers is explored and compared to prior laboratory studies. Comparisons are also made with models for BBSAN based on stochastic theory ([Tam et al., J. Sound Vib. 140, 55–71 (1990)]) and the simplified model used in Kuo et al. [AIAA Paper 2011–1032 (2011)] for lab-scale BBSAN. Frequency-dependent convective speed estimates obtained from the current BBSAN models are compared to estimates based on directivity. [Work supported by the Office of Naval Research and the F-35 JPO.]

Numerical validation of using multisource statistically optimized near-field acoustical holography in the vicinity of a high performance military aircraft

Multisource statistically optimized nearfield acoustical holography (M-SONAH) is an advanced holography technique [Wall et al., J. Acoust. Soc. Am. 137, 963–975 (2015)] that has been used to reconstruct the acoustic field from measurements taken in the vicinity of a high-performance military aircraft [Wall et al. 139, 1938 (2016)]. The implementation of M-SONAH for tactical jet noise relies on creating an equivalent wave model using two cylindrical sources, one along the jet centerline and one below the ground as an image source, to represent the field surrounding an aircraft tethered to a reflecting ground run up pad. In this study, the spatial and frequency limitations of using the M-SONAH method to describe the field of a tethered F-35 is explored by using the same measurement geometry as at a recent test, but substituting the sound field obtained from a numerical source for the measurement data. The M-SONAH reconstructions are then compared to numerical benchmarks. A spatial region and frequency bandwidth where bias errors are low are identified and provide validation for the use of this method in tactical jet noise source and field reconstructions. [Work supported by USAFRL through ORISE and the F-35 JPO. Distribution A: Approved for public release; distribution unlimited. Cleared 07/10/2017; JSF17-714.]

Measurement methods for high-performance jet aircraft noise inside a hardened aircraft shelter

High-performance military aircraft regularly operate inside hardened aircraft shelters (HAS). The F-35A aircraft must be certified as safe to operate inside a HAS before it can be deployed and used in such structures worldwide. Acoustic levels at maintainer locations allow for noise dose estimates and regulation of personnel mission support in HASs to mitigate risks of hearing damage. Acoustic levels impinging on the airframe are compared against engineering design limits in order to prevent a reduction in operational lifespan of the aircraft due to acoustic fatigue. The Air Force Research Laboratory, the Royal Netherlands Air Force, and the Dutch national laboratories NLR and TNO collaborated on a set of acoustic measurements for an F-35A operating inside a HAS at Leeuwarden airbase in the Netherlands. The methods, analysis, and qualitative findings of the acoustic measurements are presented here. [Work supported by RNLAF and by the F-35 JPO. Distribution A: Approved for public release; distribution unlimited. Cleared 01/24/2017; JSF17-035.]

Spatiotemporal analysis of high-performance military aircraft noise during ground run-up

Recent measurements of high-performance military aircraft noise have revealed that full-scale jet noise has features and structures that are still only partly understood, such as the presence of multiple acoustic radiation lobes in the aft direction at certain frequencies. Spatiotemporal analyses of a ground-based microphone array measurement of the noise from a tethered F-35 at various engine conditions are used to investigate these features of the sound field. The ground array covered an angular aperture of 35–152 degrees relative to the front of the aircraft. The large angular aperture allows for a detailed investigation of the correlation and coherence at frequencies exhibiting multi-lobe behavior. This spatiotemporal analysis yields further evidence of the characteristics of multi-lobe behavior in high-performance, full-scale jet noise. [Work supported by an Office of Naval Research grant, a USAFRL SBIR, and the F-35 JPO. Distribution A: Approved for public release; distribution unlimited. Cleared 07/10/2017; JSF17-714.]

Self-similarity of level-based wavepacket modeling of high-performance military aircraft noise

Construction of the equivalent acoustic source model for high-performance military aircraft noise presented in this paper begins with a decomposition of sound levels, from ground-based microphones 11.7 m from a high-performance military aircraft, into portions matching the similarity spectra associated with large-scale and fine-scale turbulent mixing noise (LSS and FSS). [Tam et al. AIAA Paper 96-1716 (1996)]. A Fourier transform of the spatial distribution of the decomposed levels yields frequency-dependent wavenumber spectra [Morris, Int. J. Aeroacoust. 8, 301-316 (2009)]. Each LSS-educed wavenumber spectrum corresponds to a wavepacket, consisting of a spatially varying amplitude distribution with a constant axial phase relationship. This wavepacket model produces a directional, coherent sound field. However, the asymmetry in the LSS-educed wavenumber spectra may indicate that a nonuniform phase relationship is a more physical choice that could be correlated to the axial variation in convective speed. The FSS component is modeled with a line of incoherent monopoles whose amplitude distribution is related to the FSS-educed wavenumber spectrum. While this level-based model is limited, the addition of the FSS-related source yields a better match to the noise environment of a high-performance military aircraft than is found using a single coherent wavepacket. [Work supported by ONR.]

Azimuthal coherence of the sound field in the vicinity of a high performance military aircraft

Mixing noise from a jet engine originates from an extended spatial region downstream of the nozzle and is partially correlated both spatially and temporally. Previously, the coherence properties in the downstream (axial) direction of the sound field of a tethered military aircraft were investigated, resulting in the identification of different spatial regions based on coherence length [B. M. Harker et al., AIAA J. 54, 1551-1566 (2016)]. In this study, a vertical array of microphones to the side of the jet plume is used to obtain the azimuthal coherence of the sound field. Although multipath interference effects and a limited angular aperture make coherence length calculation impossible, information about upper and lower bounds can be extracted. The measured azimuthal coherence as a function of downstream distance and frequency is then compared to that predicted by sound field reconstructions using multisource, statistically optimized near-field acoustical holography (M-SONAH) [A. T. Wall et al., J. Acoust. Soc. Am., 139, 1938-1950 (2016)]. This comparison helps to benchmark the performance of a reduced-order M-SONAH algorithm that employs only axisymmetric cylindrical basis functions to represent the direct and image sound fields. [Work supported by USAFRL through ORISE (2016).]

Near-field array measurements of F-35 aircraft noise

Noise source measurement and modeling efforts are being conducted for current and future generations of high-performance military aircraft. A near-field microphone array was used in a noise measurement of F-35A and F-35B aircraft at Edwards Air Force Base in 2013 in order to extract source models. The two aircraft measured were tethered to the ground on a concrete run-up pad. The array consisted of 76 sensors, spanned 32 m, and was placed on the ground approximately 7 m from the jet plume. The engines operated over a full range of power settings from idle through full afterburner. In preparation for the execution of source modeling efforts, these near-field data are explored and compared to far-field data published previously [James et al., AIAA Paper 2015-2375]. [Work supported by USAFRL through F-35 JPO.]

Wavepacket source modeling of high-performance military aircraft jet noise from cross-beamforming analysis

Wavepacket models provide a convenient representation of jet noise source phenomena because of the extended, partially correlated nature of the turbulent mixing noise. When treated as an equivalent source model, they are useful to estimate features of both the radiated noise as well as the source characteristics to assist in jet noise reduction efforts. In this study, advanced cross-beamforming techniques are applied to measurements in the vicinity of a high-performance military aircraft. These results are then decomposed into an azimuthally-averaged multi-wavepacket representation of the data, which can then be treated as an equivalent source. Estimates of the field levels and coherence properties using the equivalent source are compared with measurements, and results from the multi-wavepacket model show good agreement with benchmark measurements over a range of frequencies that contribute significantly to the overall radiation. The capabilities and limitations of the model to estimate field properties are quantified with respect to benchmark measurements in the mid and far fields as a function of frequency. Results indicate that the multi-wavepacket representation is an improvement over single-wavepacket models, which do not incorporate spatiotemporal features of the radiation. [Work supported by ONR and USAFRL through ORISE.]

Quadspectral nonlinearity analysis of military jet aircraft noise waveforms

Understanding the impact of jet noise can be improved by quantifying the nonlinearity in a signal with a single-microphone measurement. Based on the quadspectral Morfey-Howell indicator, a nonlinearity gain factor called ν N has been derived from an ensemble-averaged, frequency-domain version of the generalized Burgers equation [Miller et al., AIP Conf. Proc. 1685, 090003 (2015)]. This gain factor gives a quantitative expression for the change in sound pressure level spectrum over distance. Past results show that ν N accurately characterizes nonlinear evolution of waves in simulation and model-scale jet data. Here, noise waveforms from a high-performance military jet aircraft are characterized using the ν N indicator; results are compared with those from other indicators that have been used previously (e.g., derivative skewness, time-waveform steepening factor, etc.). Far field results show that the nonlinear gains at high frequencies (>1 kHz) tend to balance the absorption losses, thus establishing the c...

Derivative skewness values of shock-containing noise waveforms

Acoustic shocks present in military aircraft noise are often referred to as crackle, a component of jet noise that can significantly alter perception and annoyance. Quantifying the shock content within a waveform is important for gauging possible effects that these shocks may have. In the past, the derivative skewness has been used to quantify the steepening of waveforms throughout propagation, representing average behavior throughout the waveform. Though past use has been mostly qualitative, recent work has given physical meaning to derivative skewness values of sinusoids and provided recommendations on sampling rate and thresholds that can indicate significant shocks within the waveform. Using these recommendations, the derivative skewness of high-performance military aircraft noise is estimated for various locations and engine conditions. Derivative skewness values are compared with shock counting schemes based on various criteria. The comparisons show a strong relationship between high derivative skewness values and the strength and number of shocks within a waveform.

Analytical intensity calculated from a wavepacket model and comparison to intensity measurements near a high-performance military aircraft

To create an equivalent source description of jet noise, a wavepacket model is used to determine the acoustic vector intensity away from the source. This paper represents an initial investigation into using the measured vector acoustic intensity to define wavepacket parameters. The complex pressure of a line source is defined according to an analytical hyperbolic tangent wavepacket model and Rayleigh integration is used to find the pressure, particle velocity, and time-averaged intensity at observer locations. The parameters that define the shape of the wavepacket are initially based on prior pressure level-based optimization carried out for ground-based microphones. The resulting calculated acoustic intensity vectors are compared to vector intensity measurements previously taken near a tethered high-performance military aircraft at military and afterburner engine conditions. The wavepacket parameters are then varied to provide an optimal fit using the intensity, rather than pressure level, as the cost function, and the differences between the two models are described. [Work supported by the Office of Naval Research.]

Spatiotemporal correlation of high-performance military aircraft jet noise

Correlation analyses of pressure measurements on a ground-based array of microphones of noise from a tethered high-performance military aircraft provide insights into the sound field variation with position and engine conditions which are fundamental in the continued development of more complete jet noise models. Time-scaled, single-point (auto)correlation functions confirm that to the side of the nozzle exit, the temporal correlation envelope decays very rapidly, whereas the envelope decays more slowly in the maximum radiation region and farther downstream. Two-point space-time (cross) correlation functions confirm that noise from a single engine operating at intermediate power is more similar to that from heated, laboratory-scale jets, whereas additional features seen at military power and afterburner are unique, including a feature which is likely related to the dual directivity lobe observed in the far field of military aircraft. A complementary coherence analysis provides estimates of spatial coherence lengths as a function of frequency and location. Field coherence lengths are utilized in analyzing coherence lengths of equivalent source distributions obtained from applying DAMAS-C to the ground-based array data. The cumulative results of these investigations provide a full-scale military jet noise benchmark that should be considered when evaluating laboratory-scale jet studies and simulations of jet noise.

Inclusion of a ground-reflecting plane in wavepacket modeling of military jet noise

Equivalent source models for high-speed jet noise are intended to represent the acoustic properties of the turbulent mixing noise. A wavepacket ansatz previously applied to obtain equivalent sources from anechoic laboratory measurements is modified to include both a direct and image complex pressure representation of the source to model the presence of a ground-reflecting plane. This is important for modeling the sound field at maintainer and flight deck personnel positions because the interference effects caused by ground-reflected propagation paths significantly influence the sound levels and vary with frequency and location. The ability of the direct-plus-image wavepacket model to yield the interference patterns observed across large planes of data (2 m tall by 23 m long) measured near a high-performance military aircraft is evaluated. In particular, the variation in the nulls of the sound pressure level across the planes as a function of frequency predicted by this wavepacket model are compared to the measurements between 4 and 25 m from the engine nozzle exit. Comparisons with previous work based on a Rayleigh-distributed source amplitudes are also provided. [Work supported by the Office of Naval Research.]

Estimating aircraft noise levels and spectra from aircraft flyover and ground operations

The noise from military aircraft operations can adversely affect people within the civilian community near an airbase and within the military community living and operating within the confines of the airbase. The aircraft noise levels and spectra should be estimated to determine the building noise attenuation treatments to achieve the desired interior noise levels. This presentation describes a method to estimate the noise levels and spectra from both ground operations and flyover operations of high performance military jet aircraft for the community near the airbase and the airbase proper. Sound levels are predicted over a typical airbase region using an equivalent source model for a fighter jet aircraft and atmospheric propagation modeling. Overall level contours are generated over the entire area, and spectral levels are predicted at desired points corresponding to noise sensitive building locations. [Work supported by USAFRL through ORISE.]

Acoustic intensity near a high-powered military jet aircraft.

The spatial variation in vector acoustic intensity has been calculated between 100 and 3000 Hz near a high-performance military aircraft. With one engine of a tethered F-22A Raptor operating at military power, a tetrahedral intensity probe was moved to 27 locations in the geometric near and mid-fields to obtain the frequency-dependent intensity vector field. The angles of the maximum intensity region rotate from aft to sideline with increasing frequency, becoming less directional above 800 Hz. Between 100 and 400 Hz, which are principal radiation frequencies, the ray-traced dominant source region rapidly contracts and moves upstream, approaching nearly constant behavior by 1000 Hz.

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.

Control strategy for aircraft vapor compression system operation

Future, high-performance military aircraft will likely use vapor compression systems to manage the temperatures of multiple loads which are time-varying, dissimilar, and have high-turndown ratios. Experiments were performed using a scaled vapor compression system over a hypothetical flight profile to compare the temperature control performance and energy consumption of a superheat and capacity control strategy (Method A) against those of an alternate cycle-based strategy (Method B). Method B modulated the compressor speed to control the saturated suction temperature (SST) and modulated the expansion valves to directly control the evaporator heat load outlet temperature. The vapor compression system used two evaporators with dynamic and dissimilar heat loads. Method B offered better temperature control performance for the profile examined. The coefficient of performance (COP) of Method B was slightly greater than that of Method A. However if the overcooling produced by Method A was penalized, the useful COP of Method B would be meaningfully higher. Lastly, Method B did not require back pressure control valves. The use of fewer components is important for aircraft weight and reliability concerns.

Spectral variations near a high-performance military aircraft

Spectral characteristics of jet noise depend upon location relative to the nozzle axis. Studies of the spectral variation in the far field led to a two-source model of jet noise, in which fine-scale turbulent structures are primarily responsible for noise radiation to the nozzle sideline and large-scale turbulent structures produce the broad, dominant radiation lobe farther aft. Detailed noise measurements near an F-22A Raptor shed additional insights into this variation. An initial study [Neilsen et al., J. Acoust. Soc. Am. 133, 2116–2125] was performed with ground-based microphones in the mid-field. The similarity spectra associated with the large and fine-scale turbulent structures [Tam et al., AIAA paper 96–1716 (1996)] provide a reasonable representation of measured spectra at many locations. However, there are additional features that need further investigation. This paper explores the presence of a double peak in the spectra in the maximum radiation direction and a significant change in spectral shape at the farthest aft angles using data from large measurement planes (2 m × 23 m) located 4–6 jet nozzle diameters from the shear layer. The spatial variation of the spectra provides additional insight into ties between the similarity spectra and full-scale jet noise. [Work supported by ONR.]