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Willshire, W. L., Jr.

Publications and source records attributed to Willshire, W. L., Jr..

11 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1986

The acoustic field of a point source in a uniform boundary layer over an impedance plane

The acoustic field of a point source in a boundary layer above an impedance plane is investigated anatytically using Obukhov quasi-potential functions, extending the normal-mode theory of Chunchuzov (1984) to account for the effects of finite ground-plane impedance and source height. The solution is found to be asymptotic to the surface-wave term studies by Wenzel (1974) in the limit of vanishing wind speed, suggesting that normal-mode theory can be used to model the effects of an atmospheric boundary layer on infrasonic sound radiation. Model predictions are derived for noise-generation data obtained by Willshire (1985) at the Medicine Bow wind-turbine facility. Long-range downwind propagation is found to behave as a cylindrical wave, with attention proportional to the wind speed, the boundary-layer displacement thickness, the real part of the ground admittance, and the square of the frequency.

Zorumski, W. E.↗
NASA NTRS1985

Long range downwind propagation of low-frequency sound

The propagation of low-frequency noise outdoors was studied using as the source a large (80-m diameter) 4-megawatt horizontal axis wind turbine. Acoustic measurements were made with low-frequency microphone systems placed on the ground at five downwind sites ranging from 300 m to 10,000 m (6.3 mile) away from the wind turbine. The wind turbine fundamental was 1 Hz and the wind speed was generally 12 - 15 m/s at the hub height (80 m). The harmonic levels, when plotted versus propagation distance, exhibit a 3 dB per doubling of distance divergence. Two plausible explanations identified for this cylindrical spreading behavior were propagation of the low frequency wind turbine noise via a surface wave and downwind refraction. Surface was amplitude predictions were found to be more than 20 dB smaller than the measured levels. Ray-tracing results were used to qualitatively explain measured trends. A normal mode approach was identified as a candidate method for low-frequency acoustic refraction prediction.

Willshire, W. L., Jr.↗
NASA NTRS1984

Noise and performance characteristics of a model scale X-wing rotor system in hover

Noise data for the hover condition were measured for a 8 m diameter X-wing model rotor mounted on an outdoor whirl tower. The noise was found to be a function of operating condition, in general increasing with performance. The X-wing spectra obtained from narrowband and 1/3-octave band analyses were dominated by the blade passage frequency harmonics. The circulation control slot blowing produced 1 to 10 kHz broadband noise which exhibited a jet velocity to the 5.4 dependency. Under the rotor plane loading noise was dominant, except at low values of blade pressure ratio where a sharp decrease in the blade passage frequency harmonic levels was observed. In the rotor plane, the contribution of the broadband noise sources was greater than under the rotor plane. The X-wing rotor system in hover exhibited flexibility in operating conditions to achieve a specific level of performance. The high collective, low blade passage ratio combinations which meet a given level of performance were associated with low overall sound pressure levels.

Willshire, W. L., Jr.↗
NASA NTRS1983

Rye Canyon X-ray noise test: One-third octave-band data

Acoustic data were obtained for the 25 ft. diameter X-wing rotor model during performance testing of the rotor system in hover. Data collected at the outdoor whirl tower test facility with a twelve microphone array were taken for approximately 150 test conditions comprised of various combinations of RPM, blade pressure ratio (BPR), and blade angle of attack (collective). The three test parameters had four values of RPM from 404 to 497, twelve values of BPR from 1.0 to 2.1, and six values of collective from 0.0 deg to 8.5 deg. Fifteen to twenty seconds of acoustic data were reduced to obtain an average 1/3 octave band spectrum for each microphone for each test condition. The complete, as measured, 1/3 octave band results for all the acoustic data are listed. Another part of the X-wing noise test was the acoustic calibration of the Rye Canyon whirl tower bowl. Corrections were computed which, when applied to as measured data, yield estimates of the free field X-wing noise. The free field estimates provide a more realistic measure of the rotor system noise levels. Trend analysis of the three test parameters on noise level were performed.

Willshire, W. L., Jr.↗
NASA NTRS1982

Investigation of effects of microphone position and orientation on near-ground noise measurements

Three ground level microphone mounting techniques (flush mounting, inverting the microphone over a plate, and lying the microphone on a plate) were compared over a frequency range from 315 Hz to 20 kHz and over a wide range of incidence angles in the Langley Jet-Noise Laboratory anechoic room. The flush mounted microphone, when compared with a free field microphone, exhibited approximate pressure doubling up to a frequency of 10 kHz. At frequencies less than 4 kHz, the inverted and lying microphones were in good agreement with the flush mounted microphone, and their 1/3-octave-band response spectra were fairly insensitive to the position of the microphone. At higher frequencies, the geometry of the microphone and its mounting became important. The inverted microphone 0.5 times the diameter above the plate gave the flattest response compared with the flush mounted microphone. Nondimensional analysis of the inverted and lying results showed that for Kappa H cos theta 0.7 the response of the microphones approached that of the flush mounted microphone.

Willshire, W. L., Jr.↗
NASA NTRS1981

Ground effects on aircraft noise for a wide-body commercial airplane

Ground effects on noise from a high-bypass ratio engined aircraft are investigated. A 747 is flown at altitudes from 30 to 960 m past a 20-microphone array, and ground effects are calculated from noise measurements as a function of elevation angle and slant range. Results are given in terms of EPNL units and are compared with previous results of a turbojet-powered T-38 and SAE-recommended empirical lateral attenuation prediction procedures. Theoretical predictions are made based on a method developed by Pao et al. (1978) and are compared with both 747 and T-38 results, demonstrating good agreement although a consistent underprediction of peak measured attenuation is observed. Measured ground effects are found largest at 200 Hz, and no definite shielding differences are apparent for a directivity angle of 122.5 deg. Less lateral attenuation of effective perceived noise is measured for both aircraft and is attributed to the dominance of low-frequency noise in the source spectrum.

Willshire, W. L., Jr.↗
NASA NTRS1981

SEL and EPNL noise duration coefficients for the 747 and T-38 aircraft

Duration coefficient were calculated for Boeing 747 and T-38 airplanes for sound exposure level (SEL) and effective perceived noise level (EPNL) scales. The measured SEL suration coefficients were 8.4 for the Boeing 747 and 5.5 for the T-38 result was in good agreement with a previous result for a similar F-5 airplane. In EPNL, the duration coefficients were 7.2 for the Boeing 747 and 5.7 for the T-38. The difference in the results between the two airplanes is believed to be due to their different engine noise source spectra. The difference in the Boeing 747 results in the two different metric was due to the different frequency weighting of A-weighted sound pressure level, used in SEL, and tone-corrected perceived noise level, used in EPNL, when applied to the 747 spectra.

Willshire, W. L., Jr.↗
NASA NTRS1981

Lateral Attenuation of High-By-Pass Ratio Engined Aircraft Noise

A flight experiment was conducted to investigate the lateral attenuation of high by pass ratio engined airplanes. A B-747 was flown at low altitudes over the ends of two microphone arrays. One array covering a lateral distance of 1600 m consisted of 14 microphones positioned over grass. The second array covered a lateral distance of 1200 m and consisted of 6 microphones positioned over a concrete runway. Sixteen runs were flown at altitudes ranging from 30 to 960 m. The acoustic information recorded in the field was reduced to one third octave band spectral time histories and synchronized with tracking and weather information. Lateral attenuation as a function of elevation angle was calculated in overall, A-weighted, tone-corrected perceived noise level, and effective perceived noise level units. The B-747 results are compared with similar results for a turbojet-powered T-38 airplane and the SAE recommended lateral attenuation prediction procedure. Less lateral attenuation was measured for the B-747 than for the T-38. The B-747 lateral attenuation values also fell below the SAE curve.

Willshire, W. L., Jr.↗
NASA NTRS1980

Assessment of ground effects on the propagation of aircraft noise: The T-38A flight experiment

A flight experiment was conducted to investigate air to ground propagation of sound at gazing angles of incidence. A turbojet powered airplane was flown at altitudes ranging from 10 to 160 m over a 20-microphone array positioned over grass and concrete. The dependence of ground effects on frequency, incidence angle, and slant range was determined using two analysis methods. In one method, a microphone close to the flight path is compared to down range microphones. In the other method, comparisons are made between two microphones which were equidistant from the flight path but positioned over the two surfaces. In both methods, source directivity angle was the criterion by which portions of the microphone signals were compared. The ground effects were largest in the frequency range of 200 to 400 Hz and were found to be dependent on incidence angle and slant range. Ground effects measured for angles of incidence greater than 10 deg to 15 deg were near zero. Measured attenuation increased with increasing slant range for slant ranges less than 750 m. Theoretical predictions were found to be in good agreement with the major details of the measured results.

Willshire, W. L., Jr.↗
NASA NTRS1979

Ground effects on aircraft noise

A flight experiment was conducted to investigate air-to-ground propagation of sound near grazing incidence. A turbojet-powered aircraft was flown at low altitudes over the ends of two microphone arrays. An eight-microphone array was positioned along a 1850 m concrete runway. The second array consisted of 12 microphones positioned parallel to the runway over grass. Twenty-eight flights were flown at altitudes ranging from 10 m to 160 m. The acoustic data recorded in the field reduced to one-third-octave band spectra and time correlated with the flight and weather information. A small portion of the data was further reduced to values of ground attenuation as a function of frequency and incidence angle by two different methods. In both methods, the acoustic signals compared originated from identical sources. Attenuation results obtained by using the two methods were in general agreement. The measured ground attenuation was largest in the frequency range of 200 to 400 Hz. A strong dependence was found between ground attenuation and incidence angle with little attenuation measured for angles of incidence greater than 10 to 15 degrees.

Willshire, W. L., Jr.↗