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At least 523 records · Page 29
All sourcesNASA NTRSDOE OSTI
NASA NTRS1979

Infrared analysis of polyethylene wear specimens using attenuated total reflection spectroscopy

Attenuated total reflection infrared spectroscopy was used to analyze ultrahigh molecular weight polyethylene wear test specimens. Three different specimens were analyzed. One specimen was gamma irradiated to a dose of 5.0 MRad, another to a dose of 2.5 MRad, and the final specimen was unirradiated. There was no conclusive evidence of chemical changes (i.e., unsaturation or oxidation) in the surface regions of any of the polyethylene samples. Therefore, it was concluded that the gamma irradiation sterilization procedure shoud not alter the boundary lubricating properties of the polyethylene.

Jones, W. R.↗
NASA NTRS1979

Attenuation of sound in a low Mach number nozzle flow

This paper examines the energy conversion mechanisms which govern the emission of low frequency sound from an axisymmetric jet pipe of arbitrary nozzle contraction ratio in the case of low Mach number nozzle flow. The incident acoustic energy which escapes from the nozzle is partitioned between two distinct disturbances in the exterior fluid. The first of these is the free-space radiation, whose directivity is equivalent to that produced by monopole and dipole sources. Second, essentially incompressible vortex waves are excited by the shedding of vorticity from the nozzle lip, and may be associated with the large-scale instabilities of the jet. Two linearized theoretical models are discussed. One of these is an exact linear theory in which the boundary of the jet is treated as an unstable vortex sheet. The second assumes that the finite width of the mean shear layer of the real jet cannot be neglected. The analytical results are shown to compare favourably with recent attenuation measurements.

Howe, M. S.↗
NASA NTRS1980

CTS attenuation and cross polarization measurements at 11.7 GHz

The long-term attenuation, cross-polarization, and rain-rate data monitored in Austin, Texas from the circularly polarized 11.7 GHz satellite beacon transmitter aboard the Communications Technology Satellite are analyzed. Data events are significantly more likely during April-September, than during October-March, except for ice deplorization which predominates during the winter months. A time of day dependence of the events is also noted. The 10 dB fade level is exceeded for .03% during the thunderstorm months. Isolation with the same probability is 23 dB.

Vogel, W. J.↗
NASA NTRS1980

Comparison of radar derived slant path rain attenuations with the COMSTAR beacon fades at 28.56 GHz for summer and winter periods

An experiment for testing and improving the accuracy of radar derived slant path attenuations of a 28.56-GHz COMSTAR beacon signal is described. The results of an additional data base consisting of five rain days during fall-winter periods of 1978-1979 are considered, over which 715 min of simultaneous radar and disdrometer data were obtained.

Goldhirsh, J.↗
NASA NTRS1981

A model of solar flux attenuation during eclipse passage and its effects on photoelectron emission from satellite surfaces

The basic theory of solar flux attenuation by the earth's atmosphere is reviewed and a model of the time-varying flux observed by a satellite during eclipse passage developed. The general model is applied to the specific problem of variations in photoelectron flux during penumbral passage and the effects of wavelength, solar activity, and atmospheric constituents on photoelectron emission investigated. Predictions of the photoelectron current expected from tungsten and aluminum surfaces are then successfully compared with actual observations from the ATS-5 and Injun 5 satellites confirming the validity of the model.

Garrett, H. B.↗
NASA NTRS1981

Noise transmission and attenuation for business aircraft

This paper describes analytical studies applicable for estimating the effects of noise transmission into light aircraft and commuter type aircraft. The propeller noise and turbulent boundary layer noise are considered. The analytical model described uses modal methods and incorporates flat stiffened panels for flat sided sidewalls and curved stiffened panels for cylindrical enclosures. The numerical results include noise attenuation with add-on treatments and the sensitivity of the transmitted noise to the discrete stiffening of the sidewall panels.

Vaicaitis, R.↗
NASA NTRS1980

Sound attenuation by liners in a blown flap environment

Sound propagation through a hot wall-jet flow over an absorbing wall is studied. The radiated sound field subject to the influence of flow convection and refraction is evaluated, and the nature of acoustic attenuation attributable to a sound absorbing liner is determined. Using a two-dimensional model, the noise field under the aircraft is also determined, and a slug-flow model is used to describe the influence of flow, density, and temperature on acoustic sources in jets. Results show significant changes in the radiated source due to the interference phenomenon, and a good absorber has the potential of changing the sound pressure range of variation to unity. A liner is also found to increase or decrease sound pressure, depending on the frequency.

Parthasarathy, S. P.↗
NASA NTRS1981

Analysis of UV protection requirements and testing of candidate attenuators for the Haloe optical instrument

Results of calculations are presented which simulate photolytic processes occurring in HALOE gas calibration cells exposed to extra-terrestrial solar ultraviolet photons. These calculations indicate that significant photolysis takes place in two of the sapphire-enclosed cells over the exposure periods of the proposed mission. A subsequent laboratory investigation is also described in which a high-voltage discharge hydrogen light source is used in conjunction with a vacuum ultraviolet spectrograph. The UV emission from this lamp was used to expose two candidate UV attenuators (ZnSe and coated Ge) to ascertain their suitability as UV filters while maintaining original infrared optical properties. Both materials were found to be effectively opaque to vacuum UV radiaton and suffered no adverse effects regarding their infrared transmissivity.

Nealy, J. E.↗
NASA NTRS1981

Stratospheric observations of the attenuated solar irradiance in the Schumann-Runge band absorption region of molecular oxygen

A spectrometer flown on the first Solar Absorption Balloon Experiment (SABE-1) observed the attenuated solar irradiance between 184 and 202 nm from an altitude near 40 km. These measurements provide a check on the absorption cross sections of molecular oxygen in the spectral region of the Schumann-Runge bands. Comparison of the measurements with calculations based on cross sections derived from laboratory data shows a general agreement although the irradiance measurements have large error bars near the centers of the absorption bands. The results imply that the 184-200 nm solar irradiance that penetrates to the stratosphere can be computed to an accuracy of + or - 30% or better by using presently available cross sections.

Frederick, J. E.↗
NASA NTRS1983

Radar modeling of space diversity associated with slant path rain attenuation at variable path angles, frequencies, and drop size distributions

Single and joint terminal slant path attenuation statistics at frequencies of 28.56 and 19.04 GHz were derived from radar data. Statistics were independently obtained for path angles of 20, 45, and 90 deg, in order to examine how path angle influences both single terminal and joint probability distributions. A prediction technique is demonstrated to work well for calculating both single and joint terminal distributions at other path angles. Diversity gains and autocorrelation function dependence on site spacing were determined employing the radar modeling results.

Goldhirsh, J.↗
NASA NTRS1984

Measuring and Modeling of the Dielectric Properties and Attenuation of Vegetation

The dielectric properties of vegetation material--primarily agricultural plants--as a function of moisture content and microwave frequency was measured and used to develop dielectric mixing models for the vegetation-water mixture. A model for the loss factor of a vegetation canopy was also developed. During the first phase of this investigation, three waveguide transmission systems covering from 1 to 2-GHz, from 3.5 to 6.5-GHz, and from 7.5 to 8.5-GHz bands were constructed and calibrated. By measuring the magnitude and phase of the field transmission coefficient of a given sample, it was possible to calculate the real and imaginary parts of the complex dielectric constant of the sample. Measurements were made for numerous samples of leaves and stalks of wheat and corn, and for wheat heads. Also, dielectric measurements were made of the liquid included in the vegetation material after extraction by mechanical means. The propagation loss is more than an order of magnitude greater than had previously been assumed. Various types of dielectric mixing models were investigated in terms of the available data, and a propagation model was developed and evaluated against direct canopy attenuation measurements. The canopy measurements were made by transmitting a signal from a radar antenna mounted atop a truck-mounted boom, and using a small antenna mounted on a rail beneath the canopy to receive it.

Ulaby, F. T.↗
NASA NTRS1984

Measuring and Modeling of the Dielectric Properties and Attenuation of Vegetation

The dielectric properties and attenuation of vegetation was measured and modeled. The purpose was to: (1) measure the microwave dielectric properties of vegetation material as a function of moisture content and microwave frequency; (2) develop dielectric mixing models for the vegetation-water mixture; (3) develop a model for the loss factor of a vegetation canopy; (4) relate the results of (1) and (2) to (3); and (5) test the model in (3) against direct canopy transmission measurements.

F Ulaby↗
NASA NTRS1983

Rain cell size statistics as a function of rain rate for attenuation modeling

Rain cell size statistics as a function of rain rate have been deduced by employing a radar data base of rain reflectivity data acquired over a three-year period at Wallops Island, VA. These cell statistics have important applications in slant path rain attenuation modeling and remote sensing of the earth's surface from space at frequencies above 10 GHz.

Goldhirsh, J.↗
NASA NTRS1983

Yearly variations of rain-rate statistics at Wallops Island and their impact on modeled slant path attenuation distributions

Rain gauge measurements at Wallops Island, VA over a five-year period have been reduced and cumulative rain-date distributions have been determined for yearly, successively combined years, and overall average cases. Yearly variations of the predicted slant path attenuation statistics for two International Radio Consultative Committee (CCIR) models (referred to as modified method I and method II) and the global model at 28.56 GHz were calculated using the yearly measured rain rates at Wallops Island. This frequency was selected as it coincided with the beacon frequency of te COMSTAR geostationary satellite which was also simultaneously monitored at Wallops Island for a period of three years enabling a comparison with the measured rain-fade statistics. The year to year variations of the predicted fades as well as the percent deviations relative to the five-year average case were examined.

Goldhirsh, J.↗
NASA NTRS1984

Slant path rain attenuation and path diversity statistics obtained through radar modeling of rain structure

Single and joint terminal slant path attenuation statistics at frequencies of 28.56 and 19.04 GHz have been derived, employing a radar data base obtained over a three-year period at Wallops Island, VA. Statistics were independently obtained for path elevation angles of 20, 45, and 90 deg for purposes of examining how elevation angles influences both single-terminal and joint probability distributions. Both diversity gains and autocorrelation function dependence on site spacing and elevation angles were determined employing the radar modeling results. Comparisons with other investigators are presented. An independent path elevation angle prediction technique was developed and demonstrated to fit well with the radar-derived single and joint terminal radar-derived cumulative fade distributions at various elevation angles.

Goldhirsh, J.↗
NASA NTRS1983

The attenuation of sound in a randomly lined duct

The kinetic theory of wave propagation in random media is applied to assess the relative merits of the peripherally segmented, axially segmented and 'checkerboard' configured acoustic duct treatments proposed by Mani (1976). It is argued that, in a first approximation, a multisegmented liner involving many discrete and distinct sections may be modelled as a liner whose impedance varies randomly with position. This results in a relatively simple analytical description of the acoustic properties of the liner in terms of a system of linear, first order ordinary differential equations, whose solution gives the distribution with axial distance of the acoustic modal intensities. The theory is expected to be valid provided that the variation in the impedance from segment to segment is small, so that the conclusions obtained here are of a preliminary nature, and must be regarded as giving a general indication of features which should be incorporated into the design of efficient acoustic liners. The principal such conclusion is that a checkerboard liner offers the best means of increasing the attenuation over and above that of a uniformly lined duct. On the other hand, only minimal gains are to be derived from the use of strictly peripherally segmented liners.

Howe, M. S.↗