Engineering topics

Shepherd, Kevin P.

Publications and source records attributed to Shepherd, Kevin P..

At least 37 records · Page 2

NASA NTRS1992

Building vibrations induced by noise from rotorcraft and propeller aircraft flyovers

Noise and building vibrations were measured for a series of helicopter and propeller-driven aircraft flyovers at WFF during May 1978. The building response data are compared with similar data acquired earlier at sites near Dulles and Kennedy Airports for operation of commercial jet transports, including the Concorde supersonic transport. Results show that noise-induced vibration levels in windows and walls are directly proportional to sound pressure level and that for a given noise level, the acceleration levels induced by a helicopter or a propeller-driven aircraft flyover cannot be distinguished from the acceleration levels induced by a commercial jet transport flyover. Noise-induced building acceleration levels were found to be lower than those levels which might be expected to cause structural damage and were also lower than some acceleration levels induced by such common domestic events as closing windows and doors.

Shepherd, Kevin P.↗

NASA NTRS1992

Overview of NASA human response to sonic boom program

For some routes the ability to fly at supersonic speeds over land as well as over water would greatly enhance the time benefit to the passenger. It would also increase the productivity and economic viability of the aircraft. There are no reliable guidelines which can be used to determine a sonic boom exposure which would be acceptable for overland supersonic flight. In addition to the peak pressure of the sonic boom, the detailed shape of the signature will also influence the perception, and therefore the community response, to sonic boom exposures. Initially, the program aims to develop the capability to predict human response to individual sonic booms. This will enable a quantitative assessment of the benefit of 'low boom' aircraft configurations and will also serve to guide the design of the aircraft and its operating conditions. This capability will form the foundation of studies to determine the relationship between sonic boom exposure and community response. Only then will it be possible to assess the feasibility of acceptable overland supersonic flight.

Shepherd, Kevin P.↗

NASA NTRS1992

Sonic boom acceptability studies

The determination of the magnitude of sonic boom exposure which would be acceptable to the general population requires, as a starting point, a method to assess and compare individual sonic booms. There is no consensus within the scientific and regulatory communities regarding an appropriate sonic boom assessment metric. Loudness, being a fundamental and well-understood attribute of human hearing was chosen as a means of comparing sonic booms of differing shapes and amplitudes. The figure illustrates the basic steps which yield a calculated value of loudness. Based upon the aircraft configuration and its operating conditions, the sonic boom pressure signature which reaches the ground is calculated. This pressure-time history is transformed to the frequency domain and converted into a one-third octave band spectrum. The essence of the loudness method is to account for the frequency response and integration characteristics of the auditory system. The result of the calculation procedure is a numerical description (perceived level, dB) which represents the loudness of the sonic boom waveform.

Shepherd, Kevin P.↗

NASA NTRS1992

Assessment and design of low boom configurations for supersonic transport aircraft

A review is presented of presently utilized sonic boom prediction and minimization techniques. The three-pronged approach - acceptability studies, atmospheric propagation studies, and configuration design and operation, to the sonic boom problem as an element of the High Speed Research Program are discussed. Experimental and theoretical results of concepts designed to validate present minimization methods are given.

Darden, Christine M.↗

NASA NTRS1991

A loudness calculation procedure applied to shaped sonic booms

Described here is a procedure that can be used to calculate the loudness of sonic booms. The procedure is applied to a wide range of sonic booms, both classical N-waves and a variety of other shapes of booms. The loudness of N-waves is controlled by overpressure and the associated rise time. The loudness of shaped booms is highly dependent on the characteristics of the initial shock. A comparison of the calculated loudness values indicates that shaped booms may have significantly reduced loudness relative to N-waves having the same peak overpressure. This result implies that a supersonic transport designed to yield minimized sonic booms may be substantially more acceptable than an unconstrained design.

Shepherd, Kevin P.↗

NASA NTRS1991

Technical bases for High Speed Civil Transport environmental acceptability

Uncertainties concerning atmospheric pollution, airport-community noise, and sonic booms, are the concerns of NASA's High-Speed Research Program. Current analytical predictions are of an operational regime centered on Mach 2.4/20-km altitude where the goal of 5-gm equivalent NO2 emissions/kg fuel can result in no more than 1-percent column ozone depletion. Jet-noise suppressors using a mixer-ejector device are seen as capable of furnishing substantial noise reduction. Low-boom aerodynamic configurations are under scrutiny to maximize aircraft economic performance through overland supersonic flight.

Wesoky, Howard L.↗

NASA NTRS1991

A New Simulator for Assessing Subjective Effects of Sonic Booms

A man rated and computer driven sonic boom simulator which has been constructed and placed in operational use at NASA-Langley is described. The simulator is used to study human subjective reactions to sonic booms and has the capability of producing a wide range of signatures under controlled conditions. Results are presented to illustrate the capability of the simulator to generate user specified N-wave and shaped booms having rise times as low as 0.5 milliseconds and peak overpressures up to 191 Pa (4 psf). The validity of the simulator as a lab research tool for studying human subjective response to sonic booms was demonstrated by successful completion of a preliminary test designed to compare loudness of N-wave sonic booms with results obtained by other investigators. Excellent agreement of the preliminary test data with existing data was observed. This provided confidence in the experimental methodology and established the simulator as a viable tool for performing detailed evaluations of sonic boom loudness and acceptability within the lab. environment.

Leatherwood, Jack D.↗

NASA NTRS1991

Sound attenuation apparatus

An apparatus is disclosed for reducing acoustic transmission from mechanical or acoustic sources by means of a double wall partition, within which an acoustic pressure field is generated by at least one secondary acoustic source. The secondary acoustic source is advantageously placed within the partition, around its edges, or it may be an integral part of a wall of the partition.

Shepherd, Kevin P.↗

NASA NTRS1991

Aeroacoustics of large wind turbines

This paper reviews published information on aerodynamically generated noise from large horizontal axis wind turbines operated for electric power generation. Methods are presented for predicting both the discrete frequency rotational noise components and the broadband noise components, and results are compared with measurements. Refraction effects that result in the formation of high-frequency shadow zones in the upwind direction and channeling effects for the low frequencies in the downwind direction are illustrated. Special topics such as distributed source effects in prediction and the role of building dynamics in perception are also included.

Hubbard, Harvey H.↗

NASA NTRS1991

Active sound attenuation across a double wall structure

The possibility of achieving significant local and global sound attenuation across a flat double wall is demonstrated. It is also shown that sound can be prevented from entering the interior of a cabinlike environment. The approach used is unlike established active noise control techniques.

Grosveld, Ferdinand W.↗

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 NTRS1990

Loudness of shaped sonic booms

A loudness model is adopted to study the feasibility of designing and operating a supersonic transport to produce minimized sonic booms. The loudness contours in this technique extend to a lower frequency (1 Hz) and thus are appropriate for sonic booms that contain significant low frequency energy. Input to the loudness calculation procedure is the power spectral density of the pressure-time signature. Calculations of loudness, for both indoor and outdoor conditions, demonstrate that shaped sonic booms are potentially more acceptable than N-waves possessing the same peak overpressure.

Shepherd, Kevin P.↗

NASA NTRS1989

Noise radiation characteristics of the Westinghouse WWG-0600 (600kW) wind turbine generator

Acoustic data are presented from five different WWG-0600 machines for the wind speed range 6.7 to 13.4 m/s, for a power output range of 51 to 600 kW and for upwind, downwind and crosswind locations. Both broadband and narrowband data are presented and are compared with calculations and with similar data from other machines. Predicted broadband spectra are in good agreement with measurements at high power and underestimate them at low power. Discrete frequency rotational noise components are present in all measurements and are believed due to terrain induced wind gradients. Predictions are in general agreement with measurements upwind and downwind but underestimate them in the crosswind direction.

Shepherd, Kevin P.↗

NASA NTRS1989

Comparison of measured and calculated sound pressure levels around a large horizontal axis wind turbine generator

Results are reported from a large number of simultaneous acoustic measurements around a large horizontal axis downwind configuration wind turbine generator. In addition, comparisons are made between measurements and calculations of both the discrete frequency rotational harmonics and the broad band noise components. Sound pressure time histories and noise radiation patterns as well as narrow band and broadband noise spectra are presented for a range of operating conditions. The data are useful for purposes of environmental impact assessment.

Shepherd, Kevin P.↗

NASA NTRS1989

Environmental noise characteristics of the MOD5-B (3.2 MW) wind turbine generator

Both narrow band and broad band acoustic data were obtained for the MOD5-B wind turbine for a range of wind speeds from 5.8 to 14.3 m/s; for a range of power outputs from 300 to 3100 kW; and for various azimuth angles and distances. Comparisons are made with those of other large machines and with predictions by available methods. The highest levels occur at the lower frequencies and generally decrease as the frequency increases. Low frequency rotational noise components were more intense than expected for an upwind machine and are believed to result from localized wind gradients across the rotor disk due to upwind terrain features. Predicted broad band spectra follow the general trends of the data but tend to underestimate the levels in the frequency range where the turbulent boundary layer-trailing edge interaction noise is expected to be significant.

Shepherd, Kevin P.↗

NASA NTRS1989

Low frequency acoustic emissions from large horizontal axis wind turbines

Available test data and theoretical predictions of LF noise from large wind turbines of the type to be used for energy generation are briefly summarized. The main LF noise sources are identified as tower-wake/blade interactions and rotor-plane inflow gradients. Sound-pressure time histories, measured and calculated narrow-band and rotational noise spectra, and noise radiation patterns for the WTS-4 and WWG-0600 wind turbines are presented graphically.

Hubbard, Harvey H.↗

NASA NTRS1989

Aircraft noise induced building vibration and effects on human response

The acoustic loads resulting from aircraft noise and sonic booms which can induce vibration in the structure and result in radiation of noise into its interior, rattling of items in contact with the structure, and the perception of the inhabitants that the structure is vibrating, are investigated. In particular, the response of buildings, particularly residential structures, to aircraft noise and the resulting effects on human response have been the subjects of considerable research at the NASA Langley Research Center. These studies are reviewed with particular emphasis on the response of houses to aircraft overflight noise and any increase in noise annoyance caused by the perception of vibration and rattling.

Powell, Clemans A.↗

NASA NTRS1988

Wind turbine acoustics research bibliography with selected annotation

Citations of documents are included, which represent the state-of-the-art of technology in each of the following acoustics subject areas: Prediction of Wind Turbine Noise; Acoustic Measurements for Wind Tunnels; Effect of Wind Turbine Noise on Building Structures, People and Communities; Atmospheric Propagation; and Measurement Technology Including Wind Screens. Documents are listed in chronological order in each section of the paper, with key documents and associated annotation listed first. The sources are given along with acquisition numbers, when available, to expedite the acquisition of copies of the documents.

Hubbard, Harvey H.↗