Kevin M. Leete

Preliminary Investigation of Multilobe Fighter Jet Noise Sources Using Acoustical Holography

An understanding of jet noise source mechanisms can facilitate targeted noise reduction efforts. This understanding has been enhanced with acoustic imaging technologies, such as near-field acoustical holography (NAH). In this study, multisource statistically optimized NAH (M-SONAH) was used to image the sound field near a tethered F-35 aircraft at multiple frequencies. A linear microphone array, placed along the ground, spanned the length of the jet exhaust plume. A multisource model of the sound field was included in the algorithm to incorporate the effects of the ground reflection on the measurement. Narrowband reconstructions elucidated fine details of the radiation patterns, such as multilobe radiation patterns (which may supersede “dual-lobe” patterns shown in previous studies), and broadband shock-associated noise. [Work supported by 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 near-field 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. J. Acoust. Soc. Am. 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 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. [Work supported by USAFRL through ORISE and the F-35 JPO. Distribution A - Approved for Public Release; Distribution is Unlimited. Cleared 06/28/2018; JSF18-643.]Multisource statistically-optimized near-field 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. J. Acoust. Soc. Am. 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 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 a...

Outdoor measurements of spherical acoustic shock decay

Prior anechoic measurements of a small acetylene-oxygen balloon explosion were used to study spherical weak-shock decay over short ranges [Muhlestein et al., J. Acoust. Soc. Am. 131, 2422-2430 (2012)]. Here, longer-range measurements conducted at the Bonneville Salt Flats with a larger balloon are described. Waveform and spectral characteristics and comparisons of the peak pressure decay with an analytical weak-shock model are presented. Weak shocks persist to at least 305 m, with an amplitude decay that is predicted reasonably well using the model. Deviations are discussed in the context of atmospheric effects and nonlinear ground reflections.

Mach stem formation in outdoor measurements of acoustic shocks.

Mach stem formation during outdoor acoustic shock propagation is investigated using spherical oxyacetylene balloons exploded above pavement. The location of the transition point from regular to irregular reflection and the path of the triple point are experimentally resolved using microphone arrays and a high-speed camera. The transition point falls between recent analytical work for weak irregular reflections and an empirical relationship derived from large explosions.

Dependence of High-performance Military Aircraft Noise on Frequency and Engine Power

To further understand the unique characteristics of military aircraft noise, the sound field in the vicinity of a tied-down F-35B was measured for various engine powers, from 25% to 150% Engine Thrust Request (ETR). Synchronous measurements along a linear ground array approximately parallel to the shear layer were used to image the entire field through multisource statistically optimized near-field acoustical holography (M-SONAH). The field in the direction of maximum radiation consists of multiple lobes in the spatiospectral domain, which are manifest as multiple local maxima in space for a fixed frequency or multiple peaks in the spectra at a fixed location. Multiple lobes are observed at non-afterburning and afterburning engine conditions. As frequency increases for a given engine power, lobes appear towards the sideline and shift aft until they disappear beyond the measurement aperture and new lobe(s) take their place. As engine power is increased at a fixed frequency, the forwardmost lobe increases in its relative contribution to the field, which is a major contributing factor to the forward shift in overall directivity with increasing engine power. Field reconstructions and data in the forward direction outside of the region of maximum radiation show the presence of broadband shock-associated noise (BBSAN) for 75% ETR and above. Reconstructions along the nozzle lipline of the jet indicate the BBSAN originates from approximately the same region as the sources for the main radiation direction

Full-spectrum near-field acoustical holography for fighter jet noise imaging

The development of acoustic imaging technologies over the previous decade has proven useful for increasing our understanding of the noise generation mechanisms inside the turbulent flows of full-scale tactical aircraft engines. In particular, advancements in near-field acoustical holography have allowed for jet noise source imaging from measurements made on tied-down aircraft over hard reflecting ground surfaces. These images have been limited in bandwidth due to the spatial resolution and extent of the holography arrays. However, improved aperture extension methods have allowed for representation of the lowest frequencies, and a new tool has been developed to produce accurate sound field images at above-Nyquist frequencies for broadband sources. These two technologies are implemented together to extend the imaging of fighter jet noise near-field toward the full spectrum required for an aircraft noise model.The development of acoustic imaging technologies over the previous decade has proven useful for increasing our understanding of the noise generation mechanisms inside the turbulent flows of full-scale tactical aircraft engines. In particular, advancements in near-field acoustical holography have allowed for jet noise source imaging from measurements made on tied-down aircraft over hard reflecting ground surfaces. These images have been limited in bandwidth due to the spatial resolution and extent of the holography arrays. However, improved aperture extension methods have allowed for representation of the lowest frequencies, and a new tool has been developed to produce accurate sound field images at above-Nyquist frequencies for broadband sources. These two technologies are implemented together to extend the imaging of fighter jet noise near-field toward the full spectrum required for an aircraft noise model.

Partial-field decomposition analysis of full-scale supersonic jet noise using optimized-location virtual references

Supersonic jet noise reduction efforts benefit from targeted source feature extraction and high-resolution acoustic imaging. Another useful tool for feature extraction is partial field decomposition of sources into independent contributors. Since such decomposition processes are nonunique, care must be taken in the physical interpretation of decomposed partially coherent aeroacoustic fields. The optimized-location virtual reference method (OLVR) is a partial field decomposition designed to extract physically meaningful source and field information through the strategic placement of virtual references within a reconstructed field. The OLVR method is applied here to obtain spatially distinct and ordered partial sources at multiple frequencies of a full-scale, high-performance supersonic jet engine operating at 100% engine power. Partial sources are shown to mimic behaviors of the total source distributions including monotonic growth and decay. Because of finite spatial coherence, multiple partial sources are used to reproduce far-field radiation away from the main lobe, and the number of required sources increases with increasing frequency. An analytical multiwavepacket model is fitted to the partial sources to demonstrate how OLVR partial fields can be leveraged to produce reduced-order models.

Comparison of inverse techniques for reproducing an extended, partially coherent sound field above a reflecting plane

The relative abilities of two advanced array processing techniques to reconstruct a partially correlated noise source similar to a high-speed jet are compared. M-SONAH is a modification of statistically optimized near-field acoustical holography that includes multiple types of wavefunctions [A. T. Wall et al., J. Acoust. Soc. Am., 137, 963–975 (2015)]. The hybrid method may be described as an advanced cross-beamforming algorithm with regularization [T. Padois et al., AIAA Paper 2013-2212]. A numerical, multiple wavepacket-based source distribution provides a reasonable model of the frequency-dependent directivity and coherence properties of a jet noise sound. An image source was included to account for a reflecting ground plane and the numerically modeled sound field was used an input to both the M-SONAH and hybrid methods. For M-SONAH, the image source was handled using an additional set of cylindrical wavefunctions and for the hybrid method, sources were assumed to be located both above and below the re...

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.]

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...