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NASA NTRS1992

Validation of finite element and boundary element methods for predicting structural vibration and radiated noise

Analytical and experimental validation of methods to predict structural vibration and radiated noise are presented. A rectangular box excited by a mechanical shaker was used as a vibrating structure. Combined finite element method (FEM) and boundary element method (BEM) models of the apparatus were used to predict the noise radiated from the box. The FEM was used to predict the vibration, and the surface vibration was used as input to the BEM to predict the sound intensity and sound power. Vibration predicted by the FEM model was validated by experimental modal analysis. Noise predicted by the BEM was validated by sound intensity measurements. Three types of results are presented for the total radiated sound power: (1) sound power predicted by the BEM modeling using vibration data measured on the surface of the box; (2) sound power predicted by the FEM/BEM model; and (3) sound power measured by a sound intensity scan. The sound power predicted from the BEM model using measured vibration data yields an excellent prediction of radiated noise. The sound power predicted by the combined FEM/BEM model also gives a good prediction of radiated noise except for a shift of the natural frequencies that are due to limitations in the FEM model.

Seybert, A. F.↗

NASA NTRS1979

Summary of noise reduction characteristics of typical general aviation materials

The paper presents the results of a large number of systematic tests to determine noise reduction characteristics of general aviation materials. Effects of material type (metallic and composite), thickness, panel stiffening, vibration damping materials, sound absorption materials and pressurization on noise reduction are included. Several promising methods for reducing cabin interior noise in light airplanes are discussed based on the results.

Roskam, J.↗

NASA NTRS1978

Sound production in a moving stream

Jet noise is modeled by particle-attached quadrupoles convected with the velocity of the actual fluid but positioned near a hypothetical instability-free vortex sheet. The strength of each quadrupole is Lighthill's stress tensor per unit mass. The work of Mani (1976) has shown that this type of model agrees well with experiment, and the present work establishes some of the equivalent sources needed for an exact analogy. The instability waves of the shear layer, as they grow into turbulence, are heard as sound that builds up as a precursor of the main turbulence-driven field. The circular compact jet is examined in some detail, and it is found that when the jet is very light it can provide a waveguide in which the effects of source activity persist for some time but eventually leak out as sound. This interaction greatly distorts the free field characteristics of the turbulent sources, so that Reynolds-stress-induced waves have an intensity that scales with the fourth power of jet velocity.

Dowling, A. P.↗

NASA NTRS1987

Acoustic effects on profile drag of a laminar flow airfoil

A two-dimensional laminar flow airfoil (NLF-0414) was subjected to high-intensity sound (pure tones and white noise) over a frequency range of 2 to 5 kHz, while immersed in a flow of 240 ft/sec (Rn of 3 million) in a quiet flow facility. Using a wake-rake, wake dynamic pressures were determined and the deficit in momentum was used to calculate a two dimensional drag coefficient. Significant increases in drag were observed when the airfoil was subjected to the high intensity sound at critical sound frequencies. However, the increased drag was not accompanied by movement of the transition location.

Shearin, John G.↗

NASA NTRS1956

Near Noise Field of a Jet-engine Exhaust II : Cross Correlation of Sound Pressures

Pressure cross correlations were obtained over a range of jet velocities both longitudinally and laterally for the overall sound pressure and for several frequency bands. The region of positive correlation was found to increase with distance downstream of the nozzle exit and was greater for lateral than for longitudinal correlations. In general, little change in the correlation curves was found as a function of jet velocity or frequency band width. Measurements made with a fixed and a movable microphone in a plate showed correlations similar to the free-field results. The results are interpreted in terms of pressure loads on surfaces.

LOADS, AERODYNAMIC↗

NASA NTRS1980

Avco Lycoming quiet clean general aviation turbofan engine

A fan module was developed using an existing turboshaft engine. The fan was designed using the latest in large engine noise control technology. A mixer was added to reduce the already low exhaust gas velocity. A nacelle incorporating sound treatment was provided for the test engine. A noise prediction model was used through the design process to evaluate the various design alternatives. Acoustic tests were then made to verify the prediction and identify the noise characteristics of the fan, core, jet, and sound treatment. Analysis of the recorded data yielded close agreement with the expected results. Core noise, as was expected, was the predominant source of noise for the quiet clean general aviation turbofan (QCGAT) engine. Flyover noise predictions were made which indicated that the Avco Lycoming QCGAT engine would meet the goals set for the QCGAT program.

Wilson, C. A.↗

NASA NTRSDate not supplied

A Comparison of Aircraft Flyover Auralizations by the Aircraft Noise Simulation Working Group

The Aircraft Noise Simulation Working Group (ANSWr), comprised of NASA, DLR, and ONERA, recently completed an analysis campaign to compare aircraft noise simulation tools, establish guidelines for noise prediction, and launch activities to assess uncertainties associated with the simulation. The campaign included the analyses of two DLR conceptual aircraft, a reference tube-and-wing aircraft, and a low noise aircraft with engines mounted above the fuselage-wing-junction. While the total predicted noise for each of the concepts compared favorably among analyses at the peak level, significant differences were noted at the component level. This paper aims to further that effort by auralizing aircraft flyover noise associated with those predictions. Comparisons are made among the NASA, DLR/Empa, and ONERA generated sounds to determine how differences between the system noise prediction and auralization methods result in changes to the auralized sound.

Stephen A Rizzi↗

NASA NTRS1972

Aeroacoustic characteristics of jet flap type exhausts.

This work which is intended to shed more light on the acoustic and fluid dynamic characteristics of jet flap type exhaust flows, reports and discusses some typical experimental results obtained with cold model air jets in a reverberation chamber. Acoustic data on the total sound power output and the overall noise spectrum of various jet flap configurations are given and compared with data from conventional circular jets. In addition various flow data for the same jets resulting from hot wire measurements are reported and general conclusions are obtained regarding the expected noise source distribution. The results indicate the dependence of the acoustic and flow characteristics on the major nozzle parameters. For the jet flaps the total radiated sound power is found to follow a 3 to 6 power of the exhaust velocity while for the circular jets the 8 power law is confirmed. The noise produced in the secondary mixing region downstream of the flap trailing edge tends to exceed the noise issuing from the primary mixing region.

Gruschka, H. D.↗

NASA NTRS2009

An Excess Broadband Noise Observed with Overexpanded Jets

Results of an experiment on the characteristics of an excess noise occurring with convergent-divergent (C-D) nozzles in the overexpanded regime are presented in this paper. Data are obtained with five C-D nozzles and a convergent nozzle, all having the same exit diameter. The results clearly establish that the C-D nozzles are noisier in the low Mach number range of the overexpanded regime. This is evidenced from the directivity patterns as well as overall radiated sound power calculations. The excess noise is broadband in nature and is found to be more pronounced with nozzles having a larger half-angle of the divergent section. It appears to occur when a shock resides within the divergent section and results from random unsteady motion of the shock.

Zaman, K.B.M.Q.↗

NASA NTRS1980

Surface acoustical intensity measurements on a diesel engine

The use of surface intensity measurements as an alternative to the conventional selective wrapping technique of noise source identification and ranking on diesel engines was investigated. A six cylinder, in line turbocharged, 350 horsepower diesel engine was used. Sound power was measured under anechoic conditions for eight separate parts of the engine at steady state operating conditions using the conventional technique. Sound power measurements were repeated on five separate parts of the engine using the surface intensity at the same steady state operating conditions. The results were compared by plotting sound power level against frequency and noise source rankings for the two methods.

Mcgary, M. C.↗

NASA NTRS1989

UHB demonstrator interior noise control flight tests and analysis

The measurement and analysis of MD-UHB (McDonnell Douglas Ultra High Bypass) Demonstrator noise and vibration flight test data are described as they relate to passenger cabin noise. The analyses were done to investigate the interior noise characteristics of advanced turboprop aircraft with aft-mounted engines, and to study the effectiveness of selected noise control treatments in reducing passenger cabin noise. The UHB Demonstrator is an MD-80 test aircraft with the left JT8D engine replaced with a prototype UHB engine. For these tests, the UHB engine was a General Electric Unducted Fan, with either 8x8 or 10x8 counter-rotating propeller configurations. Interior noise level characteristics were studied for several altitudes and speeds, with emphasis on high altitude (35,000 ft), high speed (0.75 Mach) cruise conditions. The effectiveness of several noise control treatments was evaluated based on cabin noise measurements. The important airborne and structureborne transmission paths were identified for both tonal and broadband sources using the results of a sound intensity survey, exterior and interior noise and vibration data, and partial coherence analysis techniques. Estimates of the turbulent boundary layer pressure wavenumber-frequency spectrum were made, based on measured fuselage noise levels.

Simpson, M. A.↗

NASA NTRS1974

Digital servo control of random sound test excitation

A digital servocontrol system for random noise excitation of a test object in a reverberant acoustic chamber employs a plurality of sensors spaced in the sound field to produce signals in separate channels which are decorrelated and averaged. The average signal is divided into a plurality of adjacent frequency bands cyclically sampled by a time division multiplex system, converted into digital form, and compared to a predetermined spectrum value stored in digital form. The results of the comparisons are used to control a time-shared up-down counter to develop gain control signals for the respective frequency bands in the spectrum of random sound energy picked up by the microphones.

Nakich, R. B.↗

NASA NTRS1990

Wind turbine acoustics

Available information on the physical characteristics of the noise generated by wind turbines is summarized, with example sound pressure time histories, narrow- and broadband frequency spectra, and noise radiation patterns. Reviewed are noise measurement standards, analysis technology, and a method of characterizing wind turbine noise. Prediction methods are given for both low-frequency rotational harmonics and broadband noise components. Also included are atmospheric propagation data showing the effects of distance and refraction by wind shear. Human perception thresholds, based on laboratory and field tests, are given. Building vibration analysis methods are summarized. The bibliography of this report lists technical publications on all aspects of wind turbine acoustics.

Hubbard, Harvey H.↗

NASA NTRS1987

A parametric study of mean loading effects on airfoil gust interaction noise

The influence of airfoil mean loading parameters on the sound generated by interaction with short wavelength vorticity and entropy gusts is examined. It is found that there are two mean flow parameters of importance, the effective leading edge incidence angle and the airfoil lift coefficient. The former controls the total sound power level, while the latter has an important effect on the directivity pattern. For vorticity gusts, positive gust angles result in noise levels which increase with leading edge loading (LEL), while for negative gust angles an increase in LEL may actually reduce the noise level. For low levels of LEL, entropy gusts generate less sound than vorticity gusts, while for larger values of LEL, entropy gusts are at least as important as vorticity gusts.

Kerschen, E. J.↗

NASA NTRS2008

Preliminary Analysis of Acoustic Measurements from the NASA-Gulfstream Airframe Noise Flight Test

The NASA-Gulfstream joint Airframe Noise Flight Test program was conducted at the NASA Wallops Flight Facility during October, 2006. The primary objective of the AFN flight test was to acquire baseline airframe noise data on a regional jet class of transport in order to determine noise source strengths and distributions for model validation. To accomplish this task, two measuring systems were used: a ground-based microphone array and individual microphones. Acoustic data for a Gulfstream G550 aircraft were acquired over the course of ten days. Over twenty-four test conditions were flown. The test matrix was designed to provide an acoustic characterization of both the full aircraft and individual airframe components and included cruise to landing configurations. Noise sources were isolated by selectively deploying individual components (flaps, main landing gear, nose gear, spoilers, etc.) and altering the airspeed, glide path, and engine settings. The AFN flight test program confirmed that the airframe is a major contributor to the noise from regional jets during landing operations. Sound pressure levels from the individual microphones on the ground revealed the flap system to be the dominant airframe noise source for the G550 aircraft. The corresponding array beamform maps showed that most of the radiated sound from the flaps originates from the side edges. Using velocity to the sixth power and Strouhal scaling of the sound pressure spectra obtained at different speeds failed to collapse the data into a single spectrum. The best data collapse was obtained when the frequencies were left unscaled.

Khorrami, Mehdi R.↗

NASA NTRS1972

A study of the variable impedance surface concept as a means for reducing noise from jet interaction with deployed lift-augmenting flaps

The feasibility of quieting the externally-blown-flap (EBF) noise sources which are due to interaction of jet exhaust flow with deployed flaps was demonstrated on a 1/15-scale 3-flap EBF model. Sound field characteristics were measured and noise reduction fundamentals were reviewed in terms of source models. Test of the 1/15-scale model showed broadband noise reductions of up to 20 dB resulting from combination of variable impedance flap treatment and mesh grids placed in the jet flow upstream of the flaps. Steady-state lift, drag, and pitching moment were measured with and without noise reduction treatment.

Hayden, R. E.↗

NASA NTRS1976

Sound generated in the vicinity of the trailing edge of an upper surface blown flap

Far-field noise data indicated that for practical upper surface blown flap configurations, the noise radiated below the flap is dominated by the noise generated in the vicinity of the trailing edge. Sound field by turbulent mixing in the trailing edge wake is investigated experimentally and theoretically. Hot-wire measurements were made downstream of the trailing edge to determine the gross turbulence mixing characteristics of the flow. This information is used as input to a theoretical analysis of the sound field. Favorable agreement is found between predicted and measured far-field noise directivities at various frequencies and noise power spectra at various directions.

Tam, C. K. W.↗

NASA NTRS1978

A new technique for reducing test section noise in supersonic wind tunnels

A new technique is presented for obtaining a low-noise test environment in a high-speed wind tunnel. This technique utilizes the fact that the primary noise source for high supersonic/hypersonic wind tunnels is radiated noise from the turbulent, tunnel-wall boundary layer. Because of the high directionality of sound in supersonic flows this test section noise originates far upstream on the walls of the nozzle at the 'acoustic origin'. It is shown that tailoring the nozzle contour to reduce the acoustic origin Mach number significantly reduces the noise level in the upstream half of the nozzle test rhombus. Experimental noise measurements are presented from a conventional, Mach 5 nozzle and are compared with measurements from a rapid-expansion, Mach 5 nozzle.

Anders, J. B.↗