Jet Noise

Philip J. Morris* and K. Viswanathanf Department of Aerospace Engineering, The Pennsylvania State University, University Park, PA 16802, USA ^ The Boeing Company, Seattle, WA 98124, USA

Abstract This chapter provides an overview of the present un­derstanding of jet noise from both an experimental and analyti­cal viewpoint. First, a general review of experimental observations is provided. Only single axisymmetric jets are considered. Then a historical review of theoretical contributions to jet noise under­standing and prediction is provided. The emphasis is on both the assumptions and shortcomings of the approaches, in addition to their successes. The present understanding of jet noise generation mechanisms and noise predictions is then presented. It is shown that there remain two competing explanations of many observed phenomena. The ability of the different approaches to predict jet noise is assessed. Both subsonic and supersonic jets are considered. Finally, recent prediction methods and experimental observations are described.

1 Introduction

The advent of the jet engine as the preferred propulsion system, first for mili­tary aircraft and then for commercial aircraft, highlighted the problem of jet noise. The extremely high noise levels of the small military jets of the Sec­ond World War needed to be reduced significantly before larger jet-powered aircraft could be introduced into civilian service. In the early 1950’s letters began appearing in the Times of London complaining about “the screaming of jet fighters” at seaside towns in England on the weekends and a Presi­dential Commission identified noise as the “principal nuisance factor,” for people who live near airports (see Bolt (1953)). Initially, research was ex­perimental. In England, jet noise studies were being conducted at Cranfield by Westley and Lilley (1952) and in Southampton by Powell (1953a). In the United States, von Gierke (1953) at Wright-Patterson Air Force Base and Hubbard and Lassiter (1953) at Langley Field were also conducting jet and propeller noise measurements. Powell (1954) provided a survey of jet

R. Camussi (Ed.), Noise Sources in Turbulent Shear Flows: Fundamentals and Applications, CISM International Centre for Mechanical Sciences, DOI 10.1007/978-3-7091-1458-2_3,

© CISM, Udine 2013

noise experiments conducted to that time. The seminal theoretical contri­butions were made by Lighthill (1952), Lighthill (1954). In addition, the introduction of his Acoustic Analogy provided a framework for the correla­tion of experimental data. For example, the theory resulted in scaling laws such as the “eighth power law” for the radiated power as well as the high and low frequency dependencies of the spectrum. A review of this theory and its subsequent extensions is given in Section 3.1.

These notes are not intended to be a comprehensive review of jet noise research over the last fifty years. There have been several excellent reviews during this period. These include (among others); Lighthill (1963), Ribner (1964), Goldstein (1976), Ffowcs Williams (1977), Lilley (1995), Goldstein (1995) and Tam (1995a). A good reference for an overview of aircraft noise is given by Smith (1989). Here we have emphasized key theoretical and experimental studies and the latest developments. These notes represent our opinions, not always unanimous, and we apologize in advance for any omissions.

2 Experimentally Observed Characteristics of Jet Noise

There are three principal jet noise components. These are the turbulent mixing noise, broadband shock-associated noise, and screech tones. The latter two components are present only for supersonic jets and only when the nozzle is operated at off-design conditions. The relative importance of the three components is strongly dependent on the noise radiation direc­tion. Turbulent mixing noise is dominant in the aft direction, while the shock noise is dominant in the forward direction for round nozzles. Most commercial jet engines have fixed nozzle geometry, with the jet Mach num­ber being subsonic during takeoff. During climb and at cruise, the ambient pressure is much lower than at sea level and the nozzle is often operated at supersonic conditions, generating shock noise. Several experimental studies since the seventies have investigated the characteristics of the three noise components. First the salient experimental results are presented.

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