Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “broadbands”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 1,063 records · Page 59
All sourcesNASA NTRSDOE OSTI
NASA NTRS1976

Macroscopic study of time unsteady noise of an aircraft engine during static tests

Static tests of aircraft engines can exhibit greater than 10 dB random unsteadiness of tone noise levels because flow disturbances that prevail near test site facilities are ingested. Presumably such changes are related to installation and test site features. This paper presents some properties of unsteady noise observed at a NASA-Lewis facility during tests of a Lycoming YF-102 turbofan engine. Time and spatial variations in tone noise obtained from closely spaced far-field and inlet duct microphones are displayed. Long (0.5 sec) to extremely short (0.001 sec) intermittent tone bursts are observed. Unsteadiness of the tone, its harmonics, and the broadband noise show little similarity. In the far-field, identity of tone bursts is retained over a directivity angle of less than 10 deg. In the inlet duct, tone bursts appear to propagate axially but exhibit little circumferential similarity. They show only slight relationship to tone bursts observed in the far field. The results imply an intermittent generation of random mixtures of propagating duct modes.

Clark, B. J.↗
NASA NTRS1977

Acoustic performance of inlet multiple-pure-tone suppressors installed on NASA Quiet Engine 'C'

The purpose of the experimental program reported herein was to define the length of multiple pure tone (MPT) treatment required to reasonably suppress the MPT's produced by a supersonic tip speed fan and also determine what other suppression, broadband, and blade passing frequency (BPF), might be accomplished. The experimental results are presented in terms of both far-field and duct acoustic data. Front quadrant sound power level reduction in the far field is shown to agree with duct measurements over the range of treatment lengths. Detailed one-third octave and narrow band spectra at the maximum forward noise angle are presented. Some detailed analyses of one-third-octave band amplitudes are shown as a function of far-field angle. An approximate spinning mode duct propagation analysis is then introduced which predicts the acoustic suppression by the treatment on the multiple pure tones.

Bloomer, H. E.↗
NASA NTRS1977

Flight-Effects on Predicted Fan Fly-By Noise

The impact on PNLT (Perceived Noise Level, Tone corrected) and Fly-by EPNL (Effective Perceived Noise Level) when forward motion reduces the noise generated by the bypass fan of an aircraft engine was studied. Calculated noise spectra for a typical subsonic tip speed fan designed for blade passage frequency (BPF) tone cutoff were translated in frequency by systematically varying the BPF from 0.5 to 8 kHz. Two cases of predicted flight-effects on fan source noises were considered: reduced BPF tone level of 8 db and reduced broadband noise level of about 2 db in addition to reduced tone level. The maximum reduction in PNLT of the noise as emitted from the fan occurred when the BPF was at 4 kHz where the reductions were 7.4 and 10.0 db. The maximum reduction in EPNL of the noise as received during a 500-foot altitude fly-by occurred when the BPF was at 2.5 kHz where the reductions were 5.0 and 7.8 db.

Heidmann, M. F.↗
NASA NTRS1978

Compressor and fan wake characteristics

A triaxial probe and a rotating conventional probe, mounted on a traverse gear operated by two step motors were used to measure the mean velocities and turbulence quantities across a rotor wake at various radial locations and downstream stations. The data obtained was used in an analytical model developed to study how rotor flow and blade parameters and turbulence properties such as energy, velocity correlations, and length scale affect the rotor wake characteristics and its diffusion properties. The model, includes three dimensional attributes, can be used in predicting the discrete as well as broadband noise generated in a fan rotor, as well as in evaluating the aerodynamic losses, efficiency and optimum spacing between a rotor and stator in turbomachinery.

Reynolds, B.↗
NASA NTRS1978

Reduction of fan noise in an anechoic chamber by reducing chamber wall induced inlet flow disturbances

The difference between the flight and ground static noise of turbofan engines presents a significant problem in engine noise testing. The additional noise for static testing has been attributed to inlet flow disturbances or turbulence interacting with the fan rotor. In an attempt to determine a possible source of inflow disturbances entering fans tested in the Lewis Research Center anechoic chamber, the inflow field was studied using potential flow analysis. These potential flow calculations indicated that there was substantial flow over the wall directly behind the fan inlet that could produce significant inflow disturbances. Fan noise tests were run with various extensions added to the fan inlet to move the inlet away from this backwall and thereby reduce the inlet flow disturbances. Significant noise reductions were observed with increased inlet length. Over 5 db reduction of the blade passage tone sound power level was observed between the shortest and longest inlets at 90% fan speed and the first overtone was reduced 9 db. High frequency broadband noise was also reduced.

Dittmar, J. H.↗
NASA NTRS1976

Analytical modeling of under-the-wing externally blown flap powered-lift noise

The sound field produced by the interaction of a subsonic jet with a large-scale model of the under the wing externally blown flap in an approach attitude was analyzed. The analysis was performed to obtain a better understanding of the dominant noise sources and the mechanisms governing the peak sound pressure level frequencies of the broadband spectra. An analytical expression is derived which incorporates two available theories and experimental data; the expression predicts the sound field along a circular arc of approximately 120 deg measured from the upstream jet axis in the fly-over plane. The analysis compares favorably with test results obtained from two large-scale models, one using cold air from a conical nozzle and the other using hot gas from a TF-34 turbofan engine having a conical exhaust nozzle with a 12 lobe internal forced mixer. The frequency at which the peak sound pressure level occurs appears to be governed by a phenomenon which produces periodic formation and shedding of large-scale turbulence structures from the nozzle lip.

Mckinzie, D. J., Jr.↗
NASA NTRS1978

Low frequency cabin noise reduction based on the intrinsic structural tuning concept: The theory and the experimental results, phase 2

Low frequency cabin noise and sonically induced stresses in an aircraft fuselage may be reduced by intrinsic tuning of the various structural members such as the skin, stringers, and frames and then applying damping treatments on these members. The concept is also useful in identifying the key structural resonance mechanisms controlling the fuselage response to broadband random excitation and in developing suitable damping treatments for reducing the structural response in various frequency ranges. The mathematical proof of the concept and the results of some laboratory and field tests on a group of skin-stringer panels are described. In the so-called stiffness-controlled region, the noise transmission may actually be controlled by stiffener resonances, depending upon the relationship between the natural frequencies of the skin bay and the stiffeners. Therefore, cabin noise in the stiffness-controlled region may be effectively reduced by applying damping treatments on the stiffeners.

Sengupta, G.↗
NASA NTRS1976

Status of photoelectrochemical production of hydrogen and electrical energy

The efficiency for conversion of electromagnetic energy to chemical and electrical energy utilizing semiconductor single crystals as photoanodes in electrochemical cells was investigated. Efficiencies as high as 20 percent were achieved for the conversion of 330 nm radiation to chemical energy in the form of hydrogen by the photoelectrolysis of water in a SrTiO3 based cell. The SrTiO3 photoanodes were shown to be stable in 9.5 M NaOH solutions for periods up to 48 hours. Efficiencies of 9 percent were measured for the conversion of broadband visible radiation to hydrogen using n-type GaAs crystals as photoanodes. Crystals of GaAs coated with 500 nm of gold, silver, or tin for surface passivation show no significant change in efficiency. By suppressing the production of hydrogen in a CdSe-based photogalvanic cell, an efficiency of 9 percent was obtained in conversion of 633 nm light to electrical energy. A CdS-based photogalvanic cell produced a conversion efficiency of 5 percent for 500 nm radiation.

Byvik, C. E.↗
NASA NTRS1978

Recent advances concerning an understanding of sound transmission through engine nozzles and jets

Experiments on the interaction between a turbulent jet and pure tone sound coming from inside the jet nozzle are reported. This is a model representing the sound transmission from sound sources in jet engines through the nozzle and the jet flow into the far field. It is shown that pure tone sound at low frequencies is considerably attenuated by the jet flow, whereas it is conserved at higher frequencies. On the other hand, broadband jet noise can be amplified considerably by a pure tone excitation. Both effects seem not to be interdependent. Knowledge on how they are created and on relevant parameter dependences allow new considerations for the development of sound attenuators.

Bechert, D.↗
NASA NTRS1978

Noise due to rotor-turbulence interaction

A procedure for calculating the noise due to turbulent inflow to a propeller or helicopter rotor in hover is summarized. The method is based on a calculation of noise produced by an airfoil moving in rectilinear motion through turbulence. At high frequency the predicted spectrum is broadband, while at low frequency the spectrum is peaked around multiples of blade passage frequency. The results of a parametric study of the variation of the noise with rotor tip speed, blade number, chord, turbulence scale, and directivity angle are given. A comparison of the theory with preliminary experimental measurements shows good agreement.

Amiet, R. K.↗
NASA NTRS1978

Helicopter external noise prediction and correlation with flight test

Mathematical analysis procedures for predicting the main and tail rotor rotational and broadband noise are presented. The aerodynamic and acoustical data from Operational Loads Survey (OLS) flight program are used for validating the analysis and noise prediction methodology. For the long method of rotational noise prediction, the spanwise, chordwise, and azimuthwise airloading is used. In the short method, the airloads are assumed to be concentrated at a single spanwise station and for higher harmonics an airloading harmonic exponent of 2.0 is assumed. For the same flight condition, the predictions from long and short methods of rotational noise prediction are compared with the flight test results. The short method correlates as well or better than the long method.

Gupta, B. P.↗
NASA NTRS1977

Ultraviolet observations of Mars and Saturn by the TD1A and OAO-2 satellites

Ultraviolet photometric and spectrophotometric observations of Mars and Saturn, obtained from two earth-orbiting satellites, are discussed. High-resolution data reveal no definite absorption features in the spectra of either planet. In order to reconcile the absence of a prominent absorption in the Mars data near 2150 A with preliminary Viking measurements of NO, this gas must be preferentially concentrated at high Martian altitudes. Broadband photometry from OAO-2 shows that atmospheric dust from the great dust storm of 1971-1972 on Mars reduced the ultraviolet geometric albedo by a factor of 3 at the height of the storm. Atmospheric energy deposition appears to be an important mechanism of the storm dynamics. The combined Saturn data indicate that NH3 is not the absorber that influences the UV spectrum of Saturn. The possibility that H2S is the absorber is investigated.

Caldwell, J.↗
NASA NTRS1977

Calibration and performance of the Viking lander cameras

The paper discusses Viking lander cameras which have an angular resolution of 0.12 deg, and (for broadband imaging) a resolution of 0.04 deg, used for the acquisition of data in six spectral bands for color and near-infrared imaging. Attention is given to photogrammetric calibration techniques used in the determination of spatial and spectral brightness variations from image data. The effects of sampling on the achievable photogrammetric precision are described along with techniques for preflight spectral calibrations and the corrections required for degradation in the infrared response of the detectors. The effects of the known internal reflections on the qualitative images (such as the appearance of artifact clouds) are presented, noting their effects on skyline radiometry. The qualitative and quantitative effects of the signal quantization are briefly reviewed.

Patterson, W. R., III↗
NASA NTRS1977

Identification of water frost on Callisto

A description is presented of broadband (J, K, and L) and narrowband (3.0-3.8 micrometers) observations of Ganymede and Callisto which were made on the nights of November 7 and November 29, 1976 with a 28-inch infrared telescope. Based on the previous identification of water frost on the surface of Ganymede from shorter wavelength data, it is concluded that there is also water frost on the surface of Callisto, taking into account the presence of similar bands for both satellites in the spectral range from 3 to 4 micrometers.

Lebofsky, L. A.↗
NASA NTRS1978

Whistlers and VLF noises propagating just outside the plasmapause

Ground-based recordings have been made of broadband whistler mode signals propagating along the plasmasphere outer surface, i.e., beyond the region of steep plasmapause density gradients. In contrast to features found in other field line regions, the extension of signal frequencies into the 0.5-0.8 f(Heq) range is observed, (where f(Heq) is the equatorial electron gyrofrequency of the path). The effects are exhibited by the fine structure of the knee trace and other plasmapause irregularities, amplitude peaks and emissionlike behavior above the nose frequency, whistler mode echoing above 0.5 f(Heq), and narrowband noises and noise bursts propagating along the plasmapause outer surface.

Carpenter, D. L.↗
NASA NTRS1978

Laser amplification of incoherent radiation

The amplification of noise in a laser amplifier is treated theoretically. The model for the active medium and its description using density-matrix techniques, are taken from the theory of laser operation. The spectral behavior of the radiation in the nonlinear regime is studied and the formalism is written from the onset in the frequency domain. The statistics of the light are gradually modified by the nonlinear amplification process, and expressions are derived for the rate of change of fluctuations in intensity as a measure of statistical changes. In addition, the range of validity of Litvak's Gaussian-statistics approximation is discussed. In the homogeneous-broadening case, the evolution of initially broadband Gaussian radiation toward quasimonochromatic oscillations with laserlike statistics is explored in several numerical examples. The connections of this study with the time-domain work on self-pulsing in a ring-laser configuration, are established. Finally, spectral-narrowing and -rebroadening effects in Doppler-broadened media are discussed both analytically and with numerical examples. These examples show the distinct contribution of pulsations in the population ('Raman-type terms'), and saturation phenomena.

Menegozzi, L. N.↗
NASA NTRS1978

Collinear interaction of noise with a finite-amplitude tone

The interaction between a low-level noise band and a finite-amplitude tone is studied. Experiments are performed in the air and in a plane-progressive tube. Three cases are considered: where the tone is located at the low end of a three-octave noise band, where the tone is at the upper end of a narrower noise band, and where the tone is over an octave above a one-octave noise band. It is found that the noise portion of the spectrum becomes broadband and flat at short distances. Both these characteristics become extensive before the tone reaches the shock-formation stage. The source of the high-frequency noise energy is identified as the tone and its harmonics rather than the original noise band. Good agreement is found between these results and calculations based on the Earnshaw solution. It is noted that the study is directly applicable to jet aircraft noise.

Webster, D. A.↗
NASA NTRS1978

Evidence for a supermassive object in the nucleus of the galaxy M87 from SIT and CCD area photometry

Results are presented for broadband three-color (BVR) photometric observations of the peculiar elliptical radio galaxy M87, which were obtained with digital two-dimensional SIT and CCD detector systems on the Palomar 60-in. and 200-in. telescopes. The observations and the reduction procedures for the digital data are outlined, the luminosity profile of M87 is given, and a nuclear luminosity spike is found to be centered within 0.02 arcsec of the center of M87. Attempts are made to fit various theoretical models to the luminosity profile, and a satisfactory fit is obtained between the observed profile and a model involving a massive black hole in the galactic nucleus. A model-independent dynamical analysis of the central regions is performed which indicates that the nucleus of M87 contains a supermassive object of about 5 billion solar masses with a radius of no more than 100 pc and an M/L ratio of at least 60. The possible nature of this object is considered, and it is concluded that M87 is probably the most plausible candidate for a massive black hole in a galactic nucleus.

Young, P. J.↗