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 1,009 records · Page 56
All sourcesNASA NTRSDOE OSTI
NASA NTRS1978

Annoyance due to simulated blade-slap noise

The effects of several characteristics of blade slap noise on annoyance response were studied. These characteristics or parameters were the sound pressure level of the continuous noise used to simulate helicopter broadband noise, the ratio of impulse peak to broadband noise or crest factor, the number of pressure excursions comprising an impulse event, the rise and fall time of the individual impulses, and the repetition frequency of the impulses. Analyses were conducted to determine the correlation between subjective response and various physical measures for the range of parameters studied. A small but significant improvement in the predictive ability of PNL was provided by an A-weighted crest factor correlation. No significant improvement in predictive ability was provided by a rate correction.

Powell, C. A.↗
NASA NTRS1979

Earth radiation budget experiment and smart sensors

This paper presents the data analysis requirements for the Earth Radiation Budget Experiment and potential needs for a follow-on radiation budget system. The present requirements for determining the earth's radiation budget on scales from 250 by 250-km regions to global require two broadband measurements on each of three satellites. The instrument system is composed of wide- and medium-field-of-view radiometers and a narrow-field-of-view scanning radiometer. Modeled directional functions are required to interpret the data in terms of earth radiation fluxes. Meeting more stringent science requirements for a follow-on mission will require nine broadband channels with increased spatial and temporal sampling, resulting in six satellites and a fourfold increase in data transmission rates and ground-based data storage. Smart sensors can reduce the data and ground storage requirements by orders of magnitude with onboard processing, calibration, and attitude and ephemeris determination.

Young, G. R.↗
NASA NTRS1980

The cosmic X-ray background

The cosmic X-ray experiment carried out with the A2 Instrument on HEAO-1 made systematics-free measurements of the extra-galactic X-ray sky and yielded the broadband spectral characteristics for two extreme aspects of this radiation. For the apparently isotropic radiation of cosmological origin that dominates the extragalactic X-ray flux ( 3 keV), the spectrum over the energy band of maximum intensity is remarkably well described by a thermal model with a temperature of a half-billion degrees. At the other extreme, broadband observations of individual extragalactic X-ray sources with HEAO-1 are restricted to objects within the present epoch. While the non-thermal hard spectral components associated with unevolved X-ray emitting active galaxies could account for most of the gamma-ray background, the contribution of such sources to the X-ray background must be relatively small. In contrast, the 'deep-space' sources detected in soft X-rays with the HEAO-2 telescope probably represent a major portion of the extragalactic soft X-ray ( 3 keV) background.

Boldt, E. A.↗
NASA NTRS1979

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.↗
NASA NTRS1980

Noise due to tip vortex formation on lifting rotors

The high frequency broadband noise radiated by the local separated flow at rotor blade tips is analyzed. The aeroacoustic model is based on the convection of the tip region's separated flow pressure fluctuations past the trailing edge of the blade. The tip vortex formation and separation are modeled from available experimental studies. The separated flow pressure fluctuations are estimated using available separated flow experimental data and correlations. The trailing edge noise was approximated using a point dipole model. The point dipole strengths were evaluated by a method based on the trailing edge noise theory of Amiet. The point dipole approach compares favorably to stationary airfoil trailing edge noise analyses and experiments except near the plane of the airfoil. The rotor tip separation noise results are compared to other noise mechanism calculations and to experiments. The tip noise mechanism is shown to be quite important in the high frequency range. It is likely to be the major contributor to high frequency broadband noise for heavily loaded rotors with low turbulence inflow.

George, A. R.↗
NASA NTRS1980

Location and propagation of shock associated noise from supersonic jets

Shock associated noise from unheated supersonic jets is investigated through acoustic measurements in both the near and far fields. The peak Helmholtz number of broadband shock noise is found to be independent of nozzle pressure ratio when based on the length of the shock cells and the ambient speed of sound. This indicates that the acoustic wavelengths generated in the shock noise process are limited in size by the shock cell spacing. Excellent agreement between power spectral densities measured at various far-field angles is obtained at and above the peak shock noise frequency when source convection effects are included. Results show a directivity of broadband shock noise pointed in the upstream direction, with omnidirectionality being approached only at high pressure ratios. The relative importance of shock noise with respect to jet mixing noise is found to be maximum near the pressure ratio at which a Mach disc begins to form in the jet. Near-field measurements point to the downstream shock cells as the region where the dominant shock noise eminates from the jet.

Norum, T. D.↗
NASA NTRS1981

The cosmic X-ray background

The cosmic X-ray experiment performed with the A2 instrument on HEAO-1 was especially developed to make systematics-free measurements of the extragalactic X-ray sky and has yielded the broadband spectral characteristics for two extreme aspects of this radiation. For the apparently isotropic radiation of cosmological origin that dominates the extragalactic X-ray flux, the spectrum over the energy band of maximum intensity is remarkably well described by a thermal model with a temperature of a half-billion degrees. At the other extreme, broadband observations of individual extragalactic X-ray sources with HEAO-1 are restricted to objects within the present epoch. These X-ray sources include a large sample of active galaxies studied in some detail over a broad bandwidth for the first time. Details of the cosmic X-ray background at these two extremes are reviewed, and some models describing X-ray emission mechanisms are discussed.

Boldt, E.↗
NASA NTRS1981

Nonducted coherent VLF waves and associated triggered emissions observed on the ISEE-1 satellite

A description is given of new observations of nonducted coherent VLF waves from ground-based transmitters and associated VLF emissions in the magnetosphere. The data reported were acquired by the Stanford University VLF Wave Injection Experiment on the ISEE-1 satellite. The experiment has four main components, including a broadband (1-32 kHz) VLF receiver on ISEE-1 connected to a long electric antenna, a broadband (1-20 kHz) controllable VLF transmitter located at Siple Station in the Antarctic, various VLF navigation and communications transmitters, and ground stations in the Antarctic and Canada. The main goal of the experiment is to acquire understanding of interactions between coherent VLF waves and energetic particles in the magnetosphere, in particular the whistler mode instability through which both natural and stimulated VLF emissions are produced.

Bell, T. F.↗
NASA NTRS1980

Trailing edge noise from hovering rotors

A method has been developed to predict the high frequency broadband noise due to the interaction of convecting turbulent eddies with the trailing edges of a hovering rotor. The trailing edge noise from each blade was modeled as point dipole noise with spanwise loading corrections. This point dipole approximation was checked by applying the concept to a stationary airfoil in a moving medium with excellent results. In order to estimate the strength of the point dipole, the trailing edge noise theory of Amiet was used. The method was applied specifically to blade boundary layer turbulence and compared to incident atmospheric turbulence noise. The results indicate that the relative importance of these two mechanisms is related to the magnitudes of the intensity and of the length scales of the inflow and boundary layer turbulence. The results tend to fall below some available experimental data indicating that in those experiments other broadband noise sources were stronger than boundary layer-trailing edge noise. The approach which was developed is also applicable to other blade-turbulence interaction mechanisms such as local stall and tip noise.

Kim, Y. N.↗
NASA NTRS1981

Acoustic and turbulence measurements of a tone-excited jet with and without flight simulation

Acoustic as well as turbulence measurements were made of tone-excited jets to obtain an understanding of the broadband noise augmentation mechanism. Results for both heated and unheated jets with and without flight simulation are presented for a range of excitation frequencies and levels, as well as for the zero order and first order spinning modes. It is argued that although the phase-locked large-scale turbulence structure is at the root of the noise amplification process, the actual noise generation mechanism lies in the small-scale turbulence. Results show that (1) the phase velocity of the excited large scale instability waves is subsonic relative to the ambient fluid and (2) broadband jet noise is almost uniform at all frequencies. Moreover, when the large-scale and small-scale structures are plotted against forward velocity (for a given upstream excitation level of 141 dB), the change in large-scale turbulence is negligible with forward velocity, whereas the small-scale turbulence decreases for both excited and unexcited jets. Also, the corresponding difference in far-field noise does not appear to change significantly, indicating that the changes in small-scale turbulence are responsible for jet noise amplification.

Ahuja, K. K.↗
NASA NTRS1981

Crossflow effects on steady and fluctuating pressures on an ogive-cylinder cone-frustum model in supersonic separated flow

Wind-tunnel tests were conducted on an ogive-cylinder model with two axisymmetric protuberances having cone frustum angles of cone = 23 deg and 45 deg that were used to generate detached shock waves and the resulting separated flow areas downstream of the shock. The tests were conducted in a 9 by 7 foot supersonic wind tunnel at a free-stream Mach number of 2.0 and at Reynolds numbers of 1.5 x 1 million and 3.9 x 1 million, based on body diameter. The model had an afterbody fineness ratio of 8.3, and the ogive nose had a fineness ratio of 3.0. Two characteristics of the fluctuating pressures in surface vortex flows that result from the crossflow component, (velocity along the tunnel longitudinal axis free stream angle of attack), in combination with changes in the longitudinal pressure gradient were measured: (1) the broadband, rms-pressure coefficients and (2) the power spectral densities. Measurements are presented for various flow regions on the model such as the attached turbulent boundary layer, the detached frustum shock wave, and separated flow areas. The results indicate that the pressure fluctuations around or in the neighborhood of the foci of the vortex flows had broadband intensities and power spectral densities nearly identical to the levels previously measured in separated-flow regions at angles of attack of 0 deg.

Dods, J. B., Jr.↗
NASA NTRS1982

Sound measurements and observations of the MOD-OA wind turbine generator

Sound measurements are reported for a wind velocity of about 5 m/s and a power output of about 70 kW. Both broadband and narrowband data were obtained for a range of distances and azimuth angles from the machine. Both discrete frequency and broadband components were identified. Loading harmonics at multiples of the blade passage frequency and electrical generator harmonics at multiples of the shaft speed dominated the spectrum below 100 Hz. The 10,000 Hz peak is believed to be of mechanical origin in the nacelle and the other arises from blade aerodynamic sources. Aural detection distances of about 525 m upwind and 850 downwind were observed.

Shepherd, K. P.↗
NASA NTRS1982

A climate index derived from satellite measured spectral infrared radiation

The vertical infrared radiative emitting structure (VIRES) climate index, based on radiative transfer theory and derived from the spectral radiances typically used to retrieve temperature profiles, is introduced. It is assumed that clouds and climate are closely related and a change in one will result in a change in the other. The index is a function of the cloud, temperature, and moisture distributions. It is more accurately retrieved from satellite data than is cloudiness per se. The VIRES index is based upon the shape and relative magnitude of the broadband weighting function of the infrared radiative transfer equation. The broadband weighting curves are retrieved from simulated satellite infrared sounder data (spectral radiances). The retrieval procedure is described and the error error sensitivities of the method investigated. Index measuring options and possible applications of the VIRES index are proposed.

Abel, M. D.↗
NASA NTRS1982

Technology thrusts for passive remote sensing. Sensing Technology Panel report

Technology thrusts for broadband spectrometry and gas filter radiometry development are identified. The development of 'smart' processing and cryogenics/cooling systems for all types of broadband spectrometers is advised. The use of large fixed gratings and the development of 1000-element arrays is recommended. Improvements suggested for interferometers are: the mitigation of background fluctuations; the development of multiaperature, multiband interferometers; and provision of in-flight alignment verification. Specifically recommended for gas filter radiometers are the development of gas filter cell technology, of liner and high dynamic range detectors, and of high uniform optical elements.

Source record↗
NASA NTRS1981

Broad band sound from wind turbine generators

Brief descriptions are given of the various types of large wind turbines and their sound characteristics. Candidate sources of broadband sound are identified and are rank ordered for a large upwind configuration wind turbine generator for which data are available. The rotor is noted to be the main source of broadband sound which arises from inflow turbulence and from the interactions of the turbulent boundary layer on the blade with its trailing edge. Sound is radiated about equally in all directions but the refraction effects of the wind produce an elongated contour pattern in the downwind direction.

Hubbard, H. H.↗
NASA NTRS1982

A coherent, solid state, 225 GHz receiver

A low noise, broadband, balanced mixer has been developed for use as a 225-GHz receiver. It utilizes a local oscillator (LO), whose frequency is nearly 1/4 the signal frequency, to mix the signal power to a low IF, which is then amplified. This same LO drives a similar mixer that is used to phase lock a 225-GHz transmitter. The use of a subharmonic mixer lowers LO cost, noise, and complexity, increases LO availability and reliability, and is compact and portable. Typical applications for this type of mixer include coherent receivers, passive imaging, multiple receiver systems, and broadband detection systems

Forsythe, R. E.↗
NASA NTRS1982

Visual and infrared observations of the distant Comets P/Stephan-Oterma /1980g/, Panther /1980u/, and Bowell /1980b/

The paper reports combined visual imagery and spectroscopy, near-infrared spectroscopy, and broadband infrared photometry of comets P/Stephan-Oterma (1980g), Bowell (1980b), and Panther (1980u) at intermediate heliocentric distances. The visual data indicate the existence of solid grains in extended halos around the nuclei of the three comets. Broadband near-infrared and thermal infrared measurements of Comet Panther suggest the presence of 2-4-micron-radius particles in the coma which most likely contain molecules incorporating the N-H bond, but which are more complex and less volatile than NH3. Such molecules can be produced in the grains by cosmic-ray reprocessing. Near infrared spectral features identical to those seen in comet Panther similary suggest the presence of a molecule incorporating the N-H bond in comet Bowell.

Jewitt, D. C.↗
NASA NTRS1983

An evaluation of circumferentially segmented duct liners

An evaluation of circumferentially segmented duct linings are performed by comparing attenuations of optimized segmented and uniform linings for a range of frequencies and source structures. Broadband suppression and the effects of changes to input modal structure are explored in these comparisons. Credence in the theoretical model was obtained by comparing predictions to results of a carefully controlled experiment performed on the Spinning Mode Synthesizer facility at the NASA Langley Research Center. Excellent agreement was obtained in these comparisons. Predicted results show that the optimized segmented liner is more effective than the optimized uniform liner for the higher order spinning mode sources with a hard-wall/soft-wall admittance variation representing an optimum configuration for the segmented liner. Predicted results also indicate that the segmented linear gives better broadband performance and that its suppression characteristics are not particularly sensitive to changes in input modal structure when compared to that of the uniform lining. It is shown that the greatest suppression benefit of segmented linings occur near mode cuton frequencies and that these benefits are obtained despite reductions in the total amount of acoustic treatment of fifty percent or more.

Watson, W. R.↗