Jet Noise Research Research Articles

Physics-based filtering of irrotational wavepackets from high-speed Schlieren

The eduction of acoustically efficient wavepacket structures from experimental measurements or high-fidelity numerical data aids in understanding different noise generation mechanisms in jets. This is often achieved by employing either signal processing techniques or modal decompositions. The former often require some user-defined thresholds that must be adjusted for each operating condition whereas the latter are more robust and can be applied to any jet flow-field irrespective of the operating conditions. The application of physics-based decompositions requires time-accurate spatio-temporally resolved data of the type obtained from well-validated numerical databases, which require substantial computational resources. There is an inherent advantage in extending decomposition-based approaches to experimental measurements, which can provide a substantially longer time series of data compared to numerical simulations. This investigation explores the application of Doak's Momentum Potential Theory to high-speed schlieren imaging, which is a widely used flow visualization tool in jet noise research. The procedure is applied to different test-cases, selected based on the availability of high-quality experimental measurements and numerical data. The coherent wavepackets obtained exhibit superior spatio-temporal coherence and radiative efficiency that capture the noise radiation mechanisms in the flow.

A history of jet noise research at the National Aeronautics and Space Administration.

This paper reviews jet noise research conducted at the National Aeronautics and Space Administration (NASA) from the early 1950s to the present day. Research conducted by NASA's predecessor, the National Advisory Committee for Aeronautics (NACA), and during the early years of NASA focused on turbojet noise, where a common approach for reducing jet noise was to limit the jet exit velocity to speeds that provided acceptable noise levels. Suppressors tested during this time resulted in thrust losses that were too severe to be implemented. With the introduction of turbofan engines in the 1960s, NASA shifted research to programs for both subsonic and supersonic aircraft applications with specific noise reduction goals. Subsonic research focused on increasing the bypass ratio of the engine to reduce the jet exit velocity of the core exhaust and adding mixers to the dual exhaust streams. Advances in computational methods improved aerodynamic designs and jet noise prediction tools. Supersonic applications proved to be more troublesome as programs aimed at large commercial transports required higher specific thrust engines. Changing the engine cycle to reduce jet noise was not compatible with mission range and speed requirements. Research for supersonic commercial aircraft remains an area of interest today at NASA.

A tale of two curves and their influence on rocket and supersonic jet noise research.

This letter describes how a landmark 1960s supersonic jet noise experiment influenced subsequent noise models. A discrepancy in other researchers' application of Potter and Jones's axial decomposition of the sound power generated from a laboratory-scale jet can be traced to an erroneous plot in the original report. Whereas most jet noise research indicates the dominant sound power is generated upstream of the supersonic core tip, propagation of this error in the ubiquitous NASA SP-8072 report has caused rocket noise modelers for five decades to disproportionately allocate sound power generation to the subsonic flow.

Connecting high-power jet noise characteristics with human annoyance

DoD has funded the development of advanced modeling tools for community noise exposures and for exploring noise reduction techniques. However, inadequate understanding of high-power jet noise sources and the effects of radiated noise on people are both limiting factors in mitigating community noise exposure. In the meantime, both ground personnel and community noise exposure lead to annoyance and disturbance, costly noise mitigation, and burdensome restrictions on operations. Therefore, DoD is exploring acoustical metrics that may better correlate jet noise exposure to human response. More broadly, DoD and the jet noise research community desire an increased understanding of jet noise source characterization to best focus on promising strategies for reducing jet noise impacts. This research project seeks to address these two needs by increasing the understanding of tactical jet noise sources and by correlating their physical noise features, such as noise source distribution, directivity, and shock content, with human response related to annoyance, disturbance, and the perception of crackle. This presentation will provide an overview of the project’s objectives. [Work Supported by AF SBIR Program.]DoD has funded the development of advanced modeling tools for community noise exposures and for exploring noise reduction techniques. However, inadequate understanding of high-power jet noise sources and the effects of radiated noise on people are both limiting factors in mitigating community noise exposure. In the meantime, both ground personnel and community noise exposure lead to annoyance and disturbance, costly noise mitigation, and burdensome restrictions on operations. Therefore, DoD is exploring acoustical metrics that may better correlate jet noise exposure to human response. More broadly, DoD and the jet noise research community desire an increased understanding of jet noise source characterization to best focus on promising strategies for reducing jet noise impacts. This research project seeks to address these two needs by increasing the understanding of tactical jet noise sources and by correlating their physical noise features, such as noise source distribution, directivity, and shock content...

Autocorrelation analysis of military jet aircraft noise

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 Contributions of John M. Seiner towards Aeroacoustic Facilities and Experimental Techniques

John M. “Jack” Seiner leaves behind a legacy of contributions to the science of Aeroacoustics. This memorial article concentrates on his contributions to the facilities and instrumentation he conceived, developed, and utilized to forward our understanding of jet noise, supersonic flows, and related acoustic issues. From his earliest days at NASA Langley Research Center in 1974, Jack worked to understand the physics of the problems he studied and strove to correctly model the physics of the problem at hand correctly in the laboratory. For jet noise research, this drove the requirement that the facility had to provide heated air to the model nozzle. Jack believed in the future of computational fluid dynamics and supported the development of codes to calculate shock containing jets and later advancements. When a facility capability or experimental technique did not exist to answer his questions, he worked to secure funding and develop these capabilities and techniques to demonstrate their value to aeroacoustic research. He was a pioneer in the study of twin-plume resonance and using propane fueled burners to achieve realistic jet plume conditions in the laboratory.

Does a Model-Scale Nozzle Emit the Same Jet Noise as a Jet Engine?

A careful experimental study has been carried out to verify whether a model-scale nozzle produces the same jet noise as a jet engine, with the twin goals of 1) validating the practice of carrying out jet noise research at model scale for full-scale applications, and 2) aiding the identification of jet noise in measured total spectra from jet engines, which have contributions from several noise sources. A special test was carried out to measure pure jet noise from a low-bypass ratio jet engine at all the angles. Aeroacoustic measurements were made from model-scale nozzles at the Boeing Low-Speed Aeroacoustic Facility at the same operating conditions as the jet engine. Many issues that are pertinent for the comparison of scaled model data with engine data have been investigated thoroughly. These include the requirements of the instrumentation system for model and engine tests, suitable methodology for the calculation of the atmospheric attenuation coefficients, propagation effects, data repeatability, scaling of jet noise spectra, effects of the disparate Reynolds numbers and mixing perimeters between model nozzles and engines, etc. The data indicate that the weather corrections are accurate even at very high frequencies of interest in model tests. A new phenomenon of nonlinear propagation effects is identified for subsonic heated jets when the convective Mach number exceeds unity. The convective Mach number is shown to be a better indicator of nonlinear processes than high sound amplitudes,as has been believed in the past As demonstrated, with the proper scaling factors, both narrowband and one-third-octave jet spectra can be collapsed. The concept of increasing the shear perimeter for achieving better mixing and hence reducing noise has no merit, even at supercritical pressure ratios. Excellent agreement between scaled model and engine spectra is demonstrated at all angles and frequencies for a variety of power levels; thus, it is possible to acquire high-quality spectra at high frequencies, provided proper care is taken. Finally, the major conclusion from the preceding results is that jet noise research carried out at model scale is applicable to jet engines.

Subsonic jet aeroacoustics: associating experiment, modelling and simulation

An overview of jet noise research is presented wherein the principal movements in the field are traced since its beginnings. Particular attention is paid to the evolution of our understanding of what we call a “source mechanism” in free shear flows; to the theoretical, experimental and numerical studies which have nurtured this understanding; and to the currently unresolved conceptual difficulties which render analysis of experimental and numerical data so difficult. As it is clear that accelerated progress in this field of research can be made possible by a more effective synergy between the theoretical, experimental and numerical disciplines—one which draws in particular on the impressive recent progress in experimental and numerical techniques—we endeavour to elucidate the various “source” characteristics identified by these different means of study; the points on which the studies agree or disagree, and the significance of such accord or discord; and, the new analysis possibilities which can now be realised by effectively associating experiment, modelling and simulation.

Contributions of Computational Aeroacoustics to Jet Noise Research and Prediction

A brief review of recent progress in the field of computational aeroacoustics (CAA) is proposed. This paper is complementary to the previous reviews of Tam [(1995a) “Computational aeroacoustics: issues and methods”, AIAA J. 33(10), 1788–1796], Lele [(1997) “Computational Aeroacoustics: a review”, AIAA Paper 97–0018, 35th Aerospace Sciences Meeting and Exhibit, Reno, Nevada] and Glegg [(1999) “Recent advances aeroacoustics: the influence of computational fluid dynamics”, 6th International Congress on Sound and Vibration, Copenhagen, Danemark, 5–8 July, 43–58] on advances in CAA. After a short introduction concerning the current motivations of jet noise studies, connections between computational fluid dynamics (CFD) and CAA using hybrid approaches are discussed in the first part. The most spectacular advances are probably provided by the direct computation of jet noise, and some recent results are shown in the second part.

Designing Clean Jet-Noise Facilities and Making Accurate Jet-Noise Measurements

The main objective of this paper is to provide guidelines for designing and calibrating a high quality, static, jet-noise research facility and making high-quality jet noise measurements. Particular emphasis is placed on methodology for determining if internal noise is dominant in the jet noise spectrum. A section of this document is devoted to clarifying the terminology associated with microphone frequency response corrections and providing a step-wise description of other corrections that must be applied to the measured raw spectra before the jet noise data can be considered accurate and ready for use for extrapolation to full-scale jet engine noise.

Jet Noise: Since 1952

Jet noise research was initiated by Sir James Lighthill in 1952. Since that time, the development of jet noise theory has followed a very tortuous path. This is, perhaps, not surprising for the understanding of jet noise is inherently tied to the understanding of turbulence in jet flows. Even now, our understanding of turbulence is still tenuous. In the fifties, turbulence was regarded as consisting of a random assortment of small eddies. As a result, the primary focus of jet noise research was to quantify the noise from fine-scale turbulence. This line of work persisted into the eighties. The discovery of large turbulence structures in free shear flows in the early seventies led some investigators to begin questioning the validity of the then established theories. Some went further to suggest that, for high-speed jets, it was the large turbulence structures/instability waves of the flow that were responsible for the dominant part of jet mixing noise. Development of a quantitative theory of noise from large turbulence structures/instability waves took place during the next 15 years. Precision instrumentation and facilities for jet noise measurements became available in the mid-eighties. This permitted a large bank of high-quality narrow band jet noise data to be gathered over the subsequent years. Recent analysis of these data has provided irrefutable evidence that jet noise, in fact, is made up of two basic components; one from the large turbulence structures/instability waves, the other from the fine-scale turbulence. This is true even for subsonic jets. In this paper, some of the crucial research results of the past 44 years, that form the basis of our present understanding of jet noise generation and propagation, are discussed.

Jet noise: The early history.

The invention of the jet engine and its development during World War II as the propulsive system for fighter aircraft led aircraft designers in the late 1940s to investigate its use as the power plant for civil air transports. The aim was improved speed, economics, and efficiency over that of slower propellor‐driven aircraft. However two major problems existed, (a) its high fuel consumption, and (b) its noise at full take‐off power. Innovative University jet noise research in the UK in the late 1940s and early 1950s quickly led to strong industry–university collaboration. This resulted in the development of the corrugated or lobed nozzle for jet noise reduction and later to the introduction of the bypass engine achieving significant reductions in both specific fuel consumption and jet noise reduction. The experiments by R. Westley and G. M. Lilley at Cranfield and by Professor E.J. Richards and A. Powell at Southampton University, Professor Sir James Lighthill’s theoretical studies at Manchester University, and the skill of F.B. Greatrex and his team at Rolls‐Royce all contributed to this successful revolution in civil aircraft design. The paper discusses the rapid development of this early jet noise research and the corresponding complementary work in the USA.

E. J. Richards and jet noise research at Southampton 1951–56.

Elfyn Richards became the first Professor of Aeronautical Engineering at University College, Southampton, in September 1950 and he immediately began planning for jet noise research. The author joined him in January 1951, remaining until August 1956. Some events of that period are reviewed. With Richards’ boundless energy and infectious enthusiasm, his indispensible help with more than a touch of impatience—kindly toward the author, less so for administrative barriers—progress was remarkably rapid, though the experimental apparati were in most respects very modest by later standards. The eighth power law for turbulent jet noise power (and the awkward lack of proportionality of peak frequency with velocity), the screech of choked jets and supersonic edgetones were all discovered in the first year. Feedback theories for the latter two, pulse jet noise, and other investigations such as shock wave interaction and means of reducing screech were soon to follow. Acoustically inducted fatigue (AIF) of aircraft structures caused by jet noise was predicted and soon after was found to occur in practice. Richards played some role in all of these topics, but his innumerable ideas and widening noise interests could not possibly all be addressed in that period.

Aerodynamic noise — a review of the contributions to jet noise research at the College of Aeronautics, Cranfield 1949-1961 (together with some recent conclusions)

SummaryIt is a pleasure and indeed an honour to be invited to contribute this paper to the 70th Birthday Tribute to Professor A. D. Young, in recognition of his longstanding friendship and as a colleague of the author for many years.It was Professor A. D. Young, at the College of Aeronautics in 1949, who gave the necessary guidance and encouragement to the author in starting research into aerodynamics noise, which was possibly one of the first attempts made in making a quantitative assessment of the noise from a turbulent air jet.The paper discusses the early experimental work and its conclusions, and how it led to attempts to reduce jet noise through the use of vortex generators and the corrugated nozzle. The comparison between theory and experiment is commented on with reference to the early work on acoustic sources and the questions this comparison raised. A final section is included which attempts to close the gap in our understanding of jet noise generation.

Analysis and correction of ground reflection effects in measured narrow-band sound spectra using cepstral techniques

Ground reflections generate undesirable effects on acoustic measurements such as those conducted outdoors for jet noise research, aircraft certification, and motor vehicle regulation. Typically, microphones have been placed a few feet above the ground leading to cancellations and reinforcements at low frequencies due to interference of the direct signal and from the ground. Theoretical treatment of ground reflections is difficult, and the problem is generally controlled by modifying experiments to include ground or high pole microphones which can be subject to operational problems. Analytical methods applicable to narrow-band sound spectra have been developed for signal processing and can be used to correct ground reflection effects. This paper shows how cepstral techniques developed in speech processing and used to find the pitch period of a voiced speech segment can be adapted to identifying the echo delay time and to correcting for ground reflection effects. Sample results are presented using both simulated and measured sound pressure level narrow-band spectra. The technique can readily be adapted to existing fast-Fourier-transform-type spectrum measurement instrumentation to provide field measurements of echo time delays.

A New Anechoic Facility for Supersonic Hot Jet Noise Research at Lockheed-Georgia

Much confusion about jet noise has come about as a direct result of inadequate facilities and insufficient knowledge and control of test conditions, in many cases giving rise to completely erroneous conclusions. The facility described here has been carefully designed, accounting for the shortcomings of other facilities and being guided by the stringent demands of ongoing jet noise research at Lockheed. The design goal was the capability of testing model jets up to 2000°F at pressure ratios as high as 8, in a free-field environment anechoic at all frequencies above 200 Hz. A comprehensive series of flow visualization and temperature mapping experiments in a one-sixth scale-model anechoic room was conducted. The results dictated the design of the exhaust collector/muffler to provide entrainment and room cooling air in the quantities demanded by the jet operating conditions. In order to optimize the choice of material and anechoic wedge design to achieve the 200-Hz requirement, a special impedance tube was used extensively in performance evaluations. Some features of the facility are (1) an exhaust collector providing air in quantities dictated by the particular jet operating condition with no special forced-air injection or fan installation and (2) a “cherry-picker” crane used to gain access to instrumentation, etc., for maintenance, calibration, and setup, thus eliminating the need for access platforms, etc. The crane is stowed by remote control under an anechoic cover during all test operations. The superior performance of the room is described by intensity/distance plots obtained from a fixed loudspeaker source and moving microphone installation.