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Nov 1979

Volume 66, Issue S1, pp. S1-S89

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back to top Session KK. Noise VI: Noise Generation and Propagation
Contributed Papers
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Dispersion of sound in a combustion duct by fuel droplets and soot particles (A)

J. H. Miles and D. D. Raftopoulos

J. Acoust. Soc. Am. Volume 66, Issue S1, pp. S78-S78 (1979); (1 page)

Online Publication Date: 11 Aug 2005

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Dispersion and attenuation of acoustic plane wave disturbances propagating in a ducted combustion system caused by fuel droplet and soot emissions from a jet engine combustor are studied. The attenuation and dispersion are due to heat transfer and mass transfer and viscous drag forces between the emissions and the ambient gas. Theoretical calculations show sound propagation at speeds lower than the isentropic speed of sound at low frequencies. Experimental results are in good agreement with the theory.
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A time‐dependent difference theory for sound propagation in ducts with shear flow (A)

K. J. Baumeister

J. Acoust. Soc. Am. Volume 66, Issue S1, pp. S78-S78 (1979); (1 page)

Online Publication Date: 11 Aug 2005

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A time‐dependent numerical formulation is derived for sound propagation in a two‐dimensional straight soft wall duct with a sheared mean flow. The time‐dependent continuity and momentum equations are developed along with the soft wall boundary conditions. The appropriate governing equations and boundary conditions are solved simultaneously by an explicit iteration procedure. The analysis begins with a harmonic noise source radiating into a quiescent duct. This explicit method calculates stepwise in real time to obtain the transient as well as the “steady” state solution of the acoustic field. The von Neuman method is used to develop relationships which describe how sound frequency and grid spacing effect numerical stability. The soft wall boundary conditions require special treatment to keep the iteration technique stable. Example calculations are presented for sound propagation in hard and soft wall ducts. The time‐dependent analysis has been found to be superior to the steady finite difference and finite element techniques because of much shorter solution times and the elimination of matrix storage requirements.
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Flyover noise of a wide body aircraft (A)

S. K. Lanter, B. N. Shivashankara, F. G. Strout, and R. Ayyagari

J. Acoust. Soc. Am. Volume 66, Issue S1, pp. S78-S78 (1979); (1 page)

Online Publication Date: 11 Aug 2005

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Aircraft noise in the vicinity of airports is a well‐recognized problem. Rules and regulations are in effect that set limits on the noise levels of aircraft. Although the airplanes in the commercial fleet meet the appropriate regulations, there is always a constant effort within The Boeing Company to lower the noise levels. This requires a thorough understanding of the source mechanisms and propagation effects. The total noise of an airplane is composed of engine noise and airframe noise. The engine noise itself includes various components such as jet, core, fan, and turbine noise. There has been a lack of good quality flight noise data base, especially for wide body aircraft with high bypass ratio engines. To alleviate this, several test programs have been completed within The Boeing Company. The results of one of these programs, the test of a Boeing 747 airplane fitted with Pratt and Whitney JT9D engines, are presented in this paper. From the noise results, regions of jet noise dominance, fan noise dominance, and core noise dominance are identified. Methodology that will be used for more accurate breakdown of the noise components in regions where they are not dominant is outlined.
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Large scale model measurements of air frame noise using cross correlation techniques (A)

W. R. Miller, W. C. Meecham, and W. F. Ahtye

J. Acoust. Soc. Am. Volume 66, Issue S1, pp. S78-S78 (1979); (1 page)

Online Publication Date: 11 Aug 2005

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Cross‐correlation techniques are used to measure the sound radiated by wing/flap airfoil configurations in the NASA‐Ames 40‐ by 80‐ft wind tunnel using a 5.8‐m‐high model, with three deployed flaps. The sound from flap corners exceeds other airframe noise by 10 dB and more; the noise from the leading, outbound corner of the leading flap seems to be the strongest. The sound is estimated using two formulas based on standard aeroacoustic theory and one method using the near‐, farfield crosscorrelation; this last is essentially independent of such theory—all three are in fair to good agreement with one another. The classic dipole angular distribution pattern for one dipole is compared with measurements; it is found that there is qualitative but not quantitative agreement. The dependence of intensity on U0 is roughly, though not exactly, U06, where U0 is the free stream speed. The turbulence length scales on the flap surface as determined by the characteristic time of the measured correlation function and the free stream speed, are from a few to many centimeters, on the order of the flap thicknesses. Time delays from the correlation between the far field signal and the surface source are determined from the correlation functions and are in good agreement with the flow‐refraction‐corrected results.
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Assessment at full scale of exhaust nozzle‐to‐wing size on STOL‐OTW acoustic characteristics (A)

U. von Glahn and D. Groesbeck

J. Acoust. Soc. Am. Volume 66, Issue S1, pp. S78-S79 (1979); (2 pages)

Online Publication Date: 11 Aug 2005

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On the basis of static aero/acoustic data obtained at model scale, the effect of exhaust nozzle size on flyover noise is evaluated for different STOL‐OTW nozzle configurations. Three types of nozzles are evaluated: a circular/deflector nozzle mounted above the wing, a slot/deflector nozzle mounted on the wing, and a slot nozzle mounted on the wing. The nozzle exhaust plane location, measured from the wing leading edge was varied from 10% to 46% of the wing chord (flaps retracted). Flap angles of 20° (takeoff) and 60° (approach) are included in the study. Initially, perceived noise levels (PNL) are calculated as a function of flyover distance at 152 m altitude. From these plots static EPNL values, defined as flyover relative noise levels, then are obtained as functions of nozzle size for equal aerodynamic performance (lift and thrust). On the basis of these calculations, the acoustic benefits attributable to nozzle size relative to a given wing chord size are assessed.
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Intermediate range explosion airblast propagation measurements (A)

J. W. Reed

J. Acoust. Soc. Am. Volume 66, Issue S1, pp. S79-S79 (1979); (1 page)

Online Publication Date: 11 Aug 2005

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Several hundred explosions of TNT, ranging in weight from 23 to 1134 kg, were fired at the NASA Space Center, Florida. Comprehensive meteorological measurements were made by rawinsonde balloons and on a nearby 150‐m tower, including winds, turbulence, temperatures, and humidity. A cruciform array of airblast gauges was operated, with gauges at 200 m, 500 m, 1 km, 2 km, and 5 km ranges from the explosions. Airblast results will be correlated against refractive atmospheric conditions, in hopes of establishing a functional relationship between overpressure decay with distance and the sound velocity gradient with height. Preliminary results of the analyses will be presented. [Work supported by DOD, DOE, and NASA.]
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Aircraft noise propagation and reception anomalies at a major airbase (A)

Philip Dickinson

J. Acoust. Soc. Am. Volume 66, Issue S1, pp. S79-S79 (1979); (1 page)

Online Publication Date: 11 Aug 2005

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When a U.S. Air Force Air Installation Compatible Use Zoning (AICUZ) study was presented to the local government authorities in the area surrounding one major U.S. airbase, there was some disbelief and much concern about the high noise exposures predicted. So the local government authorities commissioned their own land‐use compatibility study to be based on measured noise levels over an 18‐month period. The results of the study only exacerbated the worries—noise levels and exposures well in excess of those predicted and large numbers of people exposed to noise more than 20 dB in excess of that generally considered “intolerable.” Yet, complaints are so few as to be almost negligible, and the people seem remarkably healthy. Some of the reasons for the higher noise exposures relate to peculiar temperature effects in the high desert valley in which the base is situated. The low level of complaints is attributed to the scheduling and nature of the flights on the one hand and to the way of life of the local people on the other hand. It is suggested that the OSHA type regulations and EPA/US Air Force recommendations give a false impression of the compatibility of the environment in some circumstances.
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Acoustic wave generation by vortex shedding (A)

C. M. Bowline and W. Y. Abbott

J. Acoust. Soc. Am. Volume 66, Issue S1, pp. S79-S79 (1979); (1 page)

Online Publication Date: 11 Aug 2005

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It has been theorized that thrust perturbations observed in solid propellant rocket motors are caused in part by vortex shedding off flow obstructions (inhibitors), and the acoustic instability resulting from vortex flow between pairs of inhibitors. Acoustic measurements were made during wind tunnel tests developed to simulate flow between inhibitors. Results concerning geometric relationships and flow velocities are presented in this paper. In general, the flow between sets of baffles in a wind tunnel can create extremely large noises at predictable frequencies, given particular geometries and flow velocities.
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Determination of two‐stroke engine exhaust noise by the method of characteristics (A)

Adrian D. Jones

J. Acoust. Soc. Am. Volume 66, Issue S1, pp. S79-S79 (1979); (1 page)

Online Publication Date: 11 Aug 2005

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A unique computational technique was developed for the method of characteristics solution of a one‐dimensional flow in a duct as applied to the wave action in an engine exhaust system. By using the method it was possible to compute the detailed unsteady flow in both straight pipe and tuned expansion chamber exhaust systems as matched to the flow from the cylinder of a small two‐stroke engine. The radiated exhaust noise was then determined by assuming monopole radiation from the tailpipe outlet. Very good agreement with experiment on an operating engine has been achieved in the calculation of both the third‐octave radiated noise and the associated pressure cycles at several locations in the different exhaust systems. Of particular interest is the significance of nonlinear behavior which results in wave steepening and shock wave formation. The calculation method developed differs from those of others, principally that of Blair and that of Karnopp, Dwyer, and Margolis. The method computes the precise paths on the x−t plane of a finite number of C+, C−, and P characteristics, thereby obtaining high accuracy in determining the tailpipe outlet velocity and hence radiated noise.
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Application of partial coherence methods for identification of interior noise sources in aircraft (A)

James T. Howlett

J. Acoust. Soc. Am. Volume 66, Issue S1, pp. S79-S79 (1979); (1 page)

Online Publication Date: 11 Aug 2005

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Effective interior noise control procedures require identification of the noise sources and the noise transmission paths. Recent developments in computational procedures have led to increased interest in partial coherence analyses for source/path determination. The theoretical aspects of this method have been explored by Bendat and also by Dodds and Robson; however, only a few reports of applications to practical problems are available. Previous authors reported only partial success in identifying sources. Therefore, improvements of the approach are felt to be needed. This paper describes the latest results of an ongoing effort to develop partial coherence techniques for interior noise source/path determination in aircraft. The paper includes a summary of the computational techniques as developed by Bendat [J. Sound Vib. 49(3), 293–308 (1976)] and illustrates the application to a two‐input, single‐output system with coherence between the inputs. Solution of this system by previously used methods illustrates the occurrence of erroneous results (including negative psd) in the course of the calculation. New results are presented to suggest possible sources of the negative psd, and to suggest a procedure for avoiding this problem. The augmentation of the calculations on a digital computer interfaced with a two‐channel real‐time analyzer is also discussed. Future plans in the development of the partial coherence method includes application to experimental source identification in laboratory tests using simulated aircraft noise sources and fuselage transmission paths.
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The influence of solid bodies on low mach number vortex sound (A)

F. Obermeier

J. Acoust. Soc. Am. Volume 66, Issue S1, pp. S80-S80 (1979); (1 page)

Online Publication Date: 11 Aug 2005

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See Also: Erratum

Show Abstract
The mechanisms of sound generation by unsteady, subsonic flows in the presence of solid boundaries are investigated. For that purpose an alternative integral representation for the radiated pressure field is applied which is different from the generally used integral representation introduced by Lighthill and Curle. The main advantage of our method consists in a linear dependence of its integrand on the time derivative of the vorticity fluctuations in the hydrodynamic near field, while the ordinary Green's function has to be substituted by a “vector Green's function.” This vector Green's function can be chosen for the flow fields appropriate in such a way that surface integrals do not appear. In particular the paper is concerned with two‐dimensional flow and sound fields caused by a pair of spinning vortices and superimposed stationary potential flows along a finite plate or around a cylinder. Analytical solutions are determined by applying the method of matched asymptotic expansions.
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