Page 9 - Volume 9, Issue 3

THE “SOUND OF FREEDOM”: CHARACTERIZING JET NOISE FROM HIGH-PERFORMANCE MILITARY AIRCRAFT
Kent L. Gee, Tracianne B. Neilsen, and Alan T. Wall
Department of Physics and Astronomy Brigham Young University Provo, UT 84602
J. Micah Downing and Michael M. James
Blue Ridge Research and Consulting, LLC Asheville, NC 28801
“[T]urbofan engines that propel today’s high- performance tactical aircraft also produce “the sound of freedom” – noise levels sufficient to cause concern regarding personnel hearing loss on airfields and aircraft carriers and increased annoyance for communities near bases.”
Introduction
In 1932, the applied mathematician
Horace Lamb, a notable contributor
to the field of acoustics, addressed
the British Association for the
Advancement of Science. He reported-
ly quipped, “I am an old man now, and
when I die and go to heaven there are
two matters on which I hope for
enlightenment. One is quantum elec-
trodynamics, and the other is the tur-
bulent motion of fluids. And about the
former I am rather optimistic.” More
recently, Richard Feynman dubbed tur-
bulence “the most important unsolved
problem in classical physics.”1 Between
the difficulties in characterizing its
source and understanding the transi-
tion from mean fluid flow to wave
motion, the noise radiated from turbu-
lent jets is a topic that remains ill
understood. Numerous research stud-
ies, beginning with the seminal works of Sir James Lighthill in the 1950’s,2,3 have probed the origins or properties of jet noise. For example, a Google Scholar® search for publica- tions containing the exact phrase “jet noise,” yielded 678 results for the year 2012 alone.
Although greater reductions are still required, significant progress has been made to reduce commercial aircraft engine noise through the introduction of regulations4 and techno- logical advancements, including the development of high bypass flow ratio engines with chevrons. However, the low bypass turbofan engines that propel today’s high-perform- ance tactical aircraft also produce “the sound of freedom” – noise levels sufficient to cause concern regarding personnel hearing loss on airfields and aircraft carriers (see Figure 1 for typical maintainer positions) and increased annoyance for communities near bases. Figure 2, which shows compensa- tion, through 2005, to U.S. military veterans whose primary disability is hearing loss, indicates an alarming trend in hear- ing impairment. Though not all military hearing loss can be attributed to jet noise, noise reduction strategies designed to alter the turbulent flow, including various means of injecting fluid at the nozzle exit,5 advanced chevrons6 and corruga- tions,7,8 have been proactively explored for many years. In a
present Navy program, advanced
chevrons (see Figure 3) are being
installed on F/A-18E/F Super Hornet
9
nozzles and new corrugation designs
are being developed and tested. However, additional significant advances in reducing noise from high- performance aircraft require better understanding and quantification of the jet noise problem – both source and human impact. With this goal in mind, research offices (Office of Naval Research (ONR), Air Force Research Laboratory (AFRL), Strategic Research and Development Program, etc.) are sponsoring programs aimed at charac- terizing the physics of military jet noise generation and propagation. This article describes characteristics of supersonic jet noise as observed from recent analy- ses of extensive military jet aircraft measurements by the authors. Also
described are concurrent efforts by other investigators under a jet noise reduction10 program sponsored by ONR and NASA.
Supersonic Jet Noise: An Overview
The turbulence generated in supersonic, high-speed jet flows is responsible for the dominant noise associated with high-performance military engines. Supersonic jet noise can have multiple components, referred to as mixing noise, screech, and broadband shock-associated noise. Tam11 and Morris and Lilley12 provide reviews of these noise phenome- na for the interested reader. Because mixing noise in the aft region dominates the overall noise radiation, its characteris- tics are emphasized in this article.
The acoustic radiation associated with mixing noise from a jet is understood to originate both from unsteady fluctuations from small-scale eddies and from coherent
13-15
8 Acoustics Today, July 2013
This large-scale turbulence is comprised of varying lengths, amplitudes, and convection speeds, with an associated wavenumber spectrum. Some combinations of wavenumbers and axial velocities will result in a sonic disturbance and sub- sequent radiation to the far field. Other local pressure distur-
interaction between larger-scale turbulent features.