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At least 199 records · Page 11

A study of production and stimulated emission of sound by vortex flows

The noise radiated by an elementary corotating vortex pair in a shear flow is calculated. It is shown that a small shear can substantially increase the noise while small reverse shears can annihilate vortex pairs and thus reduce the pair noise mechanism. The resonant excitation of an ensonified vortex pair and the broadband noise amplification of a six vortex cluster is calculated. The results are in qualitative agreement with recent experimental findings on jet broadband noise amplification.

Yates, J. E.↗

Supersonic rectangular jet impingement noise experiments

The discrete frequency sound produced by jets issuing from a convergent, rectangular nozzle of aspect ratio 4.24 was investigated. Experiments were performed both with the free jet and with the jet impinging on a hard ground surface. The impingement tones that dominate the impinging jet spectra show a definite staging behavior which appears to be biased toward the free jet screech frequency once the separation distance exceeds the region of substantial shock cell development. The frequency variation of the impingement tone stages fit the details of a feedback cycle if the disturbance convection velocity is chosen to be 20 percent higher than that necessary to satisfy the screech feedback loop. Phase locked optical records show a flapping mode of jet oscillation with tones at or near the screech frequency, with superimposed symmetric oscillations when a second dominant tone of unrelated frequency appears in the spectrum.

Norum, Thomas D.↗

Applicability of Fly Neighborly Noise Recommendations to UAM Quadrotors Undergoing Steady Maneuvers

This initial study examines whether the Helicopter Association International (HAI) Fly Neighborly operational recommendations that are based on single main rotor/tail rotor configurations will hold for non-conventional UAM rotorcraft with multiple rotors. The 6-occupant quadrotor concept vehicle designed under the NASA Revolutionary Vertical Lift Technology (RVLT) Project is studied. The tip speed is 550 ft/sec, with three blades per rotor (“550/3”). Predictions are made for three steady maneuvers: level turns, descending turns, and climbing turns. The RVLT Toolchain is exercised using CAMRAD II, pyaaron/AARON/ANOPP2 and a beta version of AMAT (ANOPP2 Mission Analysis Tool). AMAT provides functionality to acoustically model the curved flight paths associated with maneuvers. In addition to quadrotor trim and performance, this study includes analysis of azimuthal variations of the vertical blade loading and its derivative in the form of contour plots on a rotor plane and line plots at one radial location (r/R=0.765). Quadrotor noise trends are analyzed using maximum Overall Sound Pressure Level (OASPL) and Effective Perceived Noise Level (EPNL). The Fly Neighborly guideline that addresses descents (“Level turns are quieter than descending turns”) is predicted to hold for the RVLT Quadrotor as well.

UAM↗

A laboratory study of the subjective response to helicopter blade-slap noise

The test stimuli recorded during a recent field study consisted of 16 sounds, each presented at 4 peak noise levels. Two helicopters and a fixed-wing aircraft were used. The impulsive characteristics of one helicopter were varied by operating at different rotor speeds, whereas the other helicopter, the noise of which was dominated by the tail rotor, displayed little variation in blade-slap noise. Thirty-two subjects made noisiness judgments on a continuous, 11 point, numerical scale. Preliminary results indicate that proposed impulsiveness corrections provide no significant improvement in the noisiness predictive ability of Effective Perceived Noise Levels (EPNL). For equal EPNL, the two categories of helicopter stimuli, one of which was far more impulsive than the other, showed no difference in judged noisiness. Examination of the physical characteristics of the sounds presented in the laboratory highlighted the difficulty of reproducing acoustical signals with high-crest factors.

Shepherd, K. P.↗

Auralization of NASA N+2 Aircraft Concepts from System Noise Predictions

Auralization of aircraft flyover noise provides an auditory experience that complements integrated metrics obtained from system noise predictions. Recent efforts have focused on auralization methods development, specifically the process by which source noise information obtained from semi-empirical models, computational aeroacoustic analyses, and wind tunnel and flight test data, are used for simulated flyover noise at a receiver on the ground. The primary focus of this work, however, is to develop full vehicle auralizations in order to explore the distinguishing features of NASA's N+2 aircraft vis-à-vis current fleet reference vehicles for single-aisle and large twin-aisle classes. Some features can be seen in metric time histories associated with aircraft noise certification, e.g., tone-corrected perceived noise level used in the calculation of effective perceived noise level. Other features can be observed in sound quality metrics, e.g., loudness, sharpness, roughness, fluctuation strength and tone-to-noise ratio. A psychoacoustic annoyance model is employed to establish the relationship between sound quality metrics and noise certification metrics. Finally, the auralizations will serve as the basis for a separate psychoacoustic study aimed at assessing how well aircraft noise certification metrics predict human annoyance for these advanced vehicle concepts.

Rizzi, Stephen A.↗

Near noise field characteristics of Nike rocket motors for application to space vehicle payload acoustic qualification

Results of a series of noise measurements that were made under controlled conditions during the static firing of two Nike solid propellant rocket motors are presented. The usefulness of these motors as sources for general spacecraft noise testing was assessed, and the noise expected in the cargo bay of the orbiter was reproduced. Brief descriptions of the Nike motor, the general procedures utilized for the noise tests, and representative noise data including overall sound pressure levels, one third octave band spectra, and octave band spectra were reviewed. Data are presented on two motors of different ages in order to show the similarity between noise measurements made on motors having different loading dates. The measured noise from these tests is then compared to that estimated for the space shuttle orbiter cargo bay.

Hilton, D. A.↗

Toward a Psychoacoustic Annoyance Model for Urban Air Mobility Vehicle Noise

A psychoacoustic test was performed to obtain annoyance responses to noise from a quadrotor Urban Air Mobility (UAM) vehicle to aid in the development of a model of annoyance to UAM vehicle noise. Previous analysis of that test concluded that a psychoacoustic annoyance (PA) model, including the effects of loudness, sharpness, fluctuation strength and roughness, correlated well with the collected annoyance responses. This motivated the assessment of other PA models available in the literature, as well as the development of a new PA model based on the previously collected annoyance responses to UAM noise. To build the PA model for UAM noise, annoyance ratings to individual sounds and annoyance comparisons between pairs of sounds are first combined into a latent annoyance scale that has a correlation coefficient of 0.98 with the raw responses and that is based on just-noticeable-differences (JNDs) in annoyance. Examples for two sounds that differ in annoyance by 1 JND show when there is 75% agreement about which sound is more annoying. This latent annoyance (JND) scale also gives insight into the interplay of various perceptual components, such as overall loudness, temporal effects and spectral effects. The proposed PA model for UAM noise is fit to the latent annoyance scale, includes the sound quality effects mentioned above, as well as a term for tonality, and offers a step forward in the prediction of annoyance to UAM vehicle noise.

Urban Air Mobility↗

Large Eddy Simulation in the Computation of Jet Noise

Noise can be predicted by solving Full (time-dependent) Compressible Navier-Stokes Equation (FCNSE) with computational domain. The fluctuating near field of the jet produces propagating pressure waves that produce far-field sound. The fluctuating flow field as a function of time is needed in order to calculate sound from first principles. Noise can be predicted by solving the full, time-dependent, compressible Navier-Stokes equations with the computational domain extended to far field - but this is not feasible as indicated above. At high Reynolds number of technological interest turbulence has large range of scales. Direct numerical simulations (DNS) can not capture the small scales of turbulence. The large scales are more efficient than the small scales in radiating sound. The emphasize is thus on calculating sound radiated by large scales.

Mankbadi, R. R.↗

Numerical Solutions to the Third CAA Workshop Benchmark Problems

This paper presents numerical solutions to the problems of propagation of sound waves through a transonic nozzle, shock-sound interactions, and automobile noise involving feedback of the third NASA Computational Aeroacoustics (CAA) Workshop on benchmark problems. The numerical algorithm is based on a dual time scheme for temporal discretization and a third-order finite volume scheme for spatial discretization. The aims of this study are to apply a dual time stepping scheme to treat aeroacoustic problems of sound propagation and to validate our CAA solver with the benchmark problems for developing a numerical tool for noise analysis and control.

Loh, Roy H.↗

Jet noise suppression by porous plug nozzles

Jet noise suppression data presented earlier by Maestrello for porous plug nozzles have been supplemented by the testing of a family of nozzles having an equivalent throat diameter of 11.77 cm. Two circular reference nozzles and eight plug nozzles having radius ratios of either 0.53 or 0.80 were tested at total pressure ratios of 1.60 to 4.00. Data were taken both with and without a forward motion of coannular flow jet, and some tests were made with a heated jet. Jet thrust was measured. The data were analyzed to show the effects of suppressor geometry on nozzle propulsive efficiency and jet noise. Jet noise reductions in overall sound pressure level of up to 15 dB were obtained with a 3.5 percent reduction in thrust, but reductions of 10 dB were obtained at thrust reductions on the order of 0.3 percent. The noise reductions for subsonic pressure ratios were much less than those for supersonic pressure ratios.

Bauer, A. B.↗

Lobed Mixer Design for Noise Suppression Acoustic and Aerodynamic Test Data Analysis

A comprehensive database for the acoustic and aerodynamic characteristics of several model-scale lobe mixers of bypass ratio 5 to 6 has been created for mixed jet speeds up to 1080 ft/s at typical take-off (TO) conditions of small-to-medium turbofan engines. The flight effect was simulated for Mach numbers up to 0.3. The static thrust performance and plume data were also obtained at typical TO and cruise conditions. The tests were done at NASA Lewis anechoic dome and ASK's FluiDyne Laboratories. The effect of several lobe mixer and nozzle parameters, such as, lobe scalloping, lobe count, lobe penetration and nozzle length was examined in terms of flyover noise at constant altitude. Sound in the nozzle reference frame was analyzed to understand the source characteristics. Several new concepts, mechanisms and methods are reported for such lobed mixers, such as, "boomerang" scallops, "tongue" mixer, detection of "excess" internal noise sources, and extrapolation of flyover noise data from one flight speed to different flight speeds. Noise reduction of as much as 3 EPNdB was found with a deeply scalloped mixer compared to annular nozzle at net thrust levels of 9500 lb for a 29 in. diameter nozzle after optimizing the nozzle length.

Mengle, Vinod G.↗

International Congress on Recent Developments in Air- and Structure-Borne Sound and Vibration, 2nd, Auburn Univ., AL, Mar. 4-6, 1992, Proceedings. Vols. 1-3

The papers presented in this volume cover a variety of topics, including sound intensity, structural intensity, modal analysis and synthesis, statistical energy analysis and energy methods, passive and active damping, and boundary element methods. Attention is also given to diagnostics and condition monitoring, material characterization and nondestructive evaluation, active noise and vibration control, sound radiation and scattering, and finite element analysis.

Crocker, Malcolm J.↗

New aspects of subsonic aerodynamic noise theory

A theory of aerodynamic noise is presented which differs from Lighthill's theory primarily in the way in which convection of the noise sources is treated. The sound directivity pattern obtained from the present theory agrees better with jet-noise directivity data than does that obtained from Lighthill's theory. The results imply that the shear-noise contribution to jet noise is smaller than previously expected.

Goldstein, M. E.↗

Generation of Higher Order Modes in a Rectangular Duct

Advanced noise control methodologies to reduce sound emission from aircraft engines take advantage of the modal structure of the noise in the duct. This noise is caused by the interaction of rotor wakes with downstream obstructions such as exit guide vanes. Mode synthesis has been accomplished in circular ducts and current active noise control work has made use of this capability to cancel fan noise. The goal of the current effort is to examine the fundamental process of higher order mode propagation through an acoustically treated, curved duct. The duct cross-section is rectangular to permit greater flexibility in representation of a range of duct curvatures. The work presented is the development of a feedforward control system to generate a user-specified modal pattern in the duct. The multiple-error, filtered-x LMS algorithm is used to determine the magnitude and phase of signal input to the loudspeakers to produce a desired modal pattern at a set of error microphones. Implementation issues, including loudspeaker placement and error microphone placement, are discussed. Preliminary results from a 9-3/8 inch by 21 inch duct, using 12 loudspeakers and 24 microphones, are presented. These results demonstrate the ability of the control system to generate a user-specified mode while suppressing undesired modes.

Gerhold, Carl H.↗

Database of Inlet and Exhaust Noise Shielding for Wedge-Shaped Airframe

An experiment to measure the noise shielding of the blended wing body design concept was developed using a simplified wedge-shaped airframe. The experimental study was conducted in the Langley Anechoic Noise Research Facility. A wideband, omnidirective sound source in a simulated engine nacelle was held at locations representative of a range of engine locations above the wing. The sound field around the model was measured with the airframe and source in place and with source alone, using an-array of microphones on a rotating hoop that is also translated along an axis parallel to the airframe axis. The insertion loss was determined from the difference between the two resulting contours. Although no attempt was made to simulate the noise characteristics of a particular engine, the broadband noise source radiated sound over a range of scaled frequencies encompassing 1 and 2 times the blade passage frequency representative of a large, high-bypass-ratio turbofan engine. The measured data show that significant shielding of the inlet-radiated noise is obtained in the area beneath and upstream of the model. The data show the sensitivity of insertion loss to engine location.

Gerhold, Carl H.↗

Predicting Jet Noise for Full-Scale Low-Boom Aircraft

Supersonic vehicle research addresses the development of tools, technologies, and knowledge to help eliminate technical barriers to practical commercial supersonic flight. While the major focus of this research has been on reducing the ground signature at supersonic cruise, airplanes must also satisfy noise constraints during takeoff and landing at subsonic speeds, and jet noise is the main sound source. We use computational aeroacoustic sim- ulation tools to assess new designs at these lower speeds. We are further scrutinizing both modeling and simulation practices and the use of hybrid Reynolds-averaged Navier Stokes/Large- Eddy Simulations methods and developing new methods to predict jet noise on a full flight configuration.

Stich, Gerrit-Daniel↗