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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.

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At least 1,279 records · Page 71
All sourcesNASA NTRSDOE OSTI
NASA NTRS1987

Application of the complex cepstrum to the location of acoustic sources near reflective surfaces

The complex cepstrum is used to correct bearing estimations of acoustic sources in the presence of a reflective surface. An automated liftering procedure is used which zeros out a block portion of the cepstrum including the echo information. The problem of the resulting distortion is alleviated by applying a coherence criterion to the recovered direct signals at each microphone. Thus to a large degree the interactive nature of cepstral processing is overcome for this application. For the test signals and geometries considered the cepstrum is shown to accurately correct for bearing errors in acoustic signals contaminated with reflections from nearby surfaces.

Fuller, C. R.↗
NASA NTRS1987

In-flight measurement of ice growth on an airfoil using an array of ultrasonic transducers

Results from three research flights to obtain in-flight ultrasonic pulse-echo measurements of airfoil ice thickness as a function of time using an array of eight ultrasonic transducers mounted flush with the leading edge of the airfoil are presented. The accuracy of the thickness measurements is found to be within 0.5 mm of mechanical and stereophotograph measurements of the ice accretion. The ultrasonic measurements demonstrate that the ice growth rate typically varies during the flight, with variations in the ice growth rate for dry ice growth being primarily due to fluctuations in the cloud liquid water content. Discrepancies between experimental results and results predicted by an analytic icing code underline the need for a better understanding of the physics of wet ice growth.

Hansman, R. John, Jr.↗
NASA NTRS1986

Primeval QSOs

The physical processes causing the formation of an increased number of QSOs at redshifts higher than 2 are discussed, along with the implications for the turn-on of active galactic nuclei (AGN) at the same time as QSOs. It is argued that optical and X-ray background data indicate that all but the brightest objects will decline in number at redshifts exceeding 2. Consideration is given to the possibility that accreting black holes are at the center of AGNs and formed within a few billion years of the turn-on of QSOs. A scenario is also explored wherein QSOs are remnant echoes of the epoch of galaxy formation and are thereby a suitable object with which to probe the parent population.

Cavaliere, A.↗
NASA NTRS1987

Rapid variability of the R Coronae Australis reflection nebula NGC 6729

Measurable changes in the surface brightness of the reflection nebula associated with R CrA occur over intervals as short as 24 hours. These and other more extreme variations are demonstrated with CCD images obtained over a 23-day period March 10 to April 2, 1984. During this time span R CrA brightened by 1.3 mag. The alterations in the appearance of the nebula NGC 6729 are apparently caused by the shadowing effects of clouds which are very close to the star, probably well within 1 Au. The spectrum of R CrA may itself vary slightly from night to night, and these changes are echoed by the surrounding nebula with an observable time delay. A photograph showing the complicated filamentary structure in NGC 6729 is reproduced and the relation between the reflection nebula and its illuminating star is discussed.

Graham, J. A.↗
NASA NTRS1986

ELF waves and ion resonances produced by an electron beam emitting rocket in the ionosphere

Results are reported from the ECHO-6 electron-beam-injection experiment, performed in the auroral-zone ionosphere on March 30, 1983 using a sounding rocket equipped with two electron guns and a free-flying plasma-diagnostics instrument package. The data are presented in extensive graphs and diagrams and characterized in detail. Large ELF wave variations, superposed on the strong beam-sector-directed quasi-dc component, are observed in the 100-eV beam-induced plasma when the beam is injected in a transverse spiral, but not when it is injected upward parallel to the magnetic-field line. ELF activity is found to be suppressed whenever the rocket passed through field lines with auroral activity, suggesting that the waves are produced by the interaction of the beam potentials, plasma currents, and return currents neutralizing the accelerator payload.

Winckler, J. R.↗
NASA NTRS1987

The area-time-integral technique to estimate convective rain volumes over areas applied to satellite data - A preliminary investigation

The use of the area-time-integral (ATI) technique, based only on satellite data, to estimate convective rain volume over a moving target is examined. The technique is based on the correlation between the radar echo area coverage integrated over the lifetime of the storm and the radar estimated rain volume. The processing of the GOES and radar data collected in 1981 is described. The radar and satellite parameters for six convective clusters from storm events occurring on June 12 and July 2, 1981 are analyzed and compared in terms of time steps and cluster lifetimes. Rain volume is calculated by first using the regression analysis to generate the regression equation used to obtain the ATI; the ATI versus rain volume relation is then employed to compute rain volume. The data reveal that the ATI technique using satellite data is applicable to the calculation of rain volume.

Doneaud, Andre A.↗
NASA NTRS1987

Tidal influences on vertical diffusion and diurnal variability of ozone in the mesosphere

Possible dynamical influences on the diurnal behavior of ozone are investigated. A time dependent one-dimensional photochemical model is developed for this purpose; all model calculations are made at 70 deg N during summer. It is shown that the vertical diffusion can vary as much as 1 order of magnitude within a day as a result of large changes in the zonal wind induced by atmospheric thermal tides. It is found that by introducing a dissipation time scale for turbulence produced by breaking gravity waves, the agreement with Poker Flat echo data is improved. Comparisons of results from photochemical model calculations, where the vertical diffusion is a function of height only, with those in which the vertical diffusion coefficient is changing in time show large differences in the diurnal behavior of ozone between 70 and 90 km. By including the dynamical effect, much better agreement with the Solar Mesosphere Explorers data is obtained. The results are, however, sensitive to the background zonally averaged wind. The influence of including time-varying vertical diffusion coefficient on the OH densities is also large, especially between 80 and 90 km. This suggests that dynamical effects are important in determining the diurnal behavior of the airglow emission from the Meinel bands.

Bjarnason, Gudmundur G.↗
NASA NTRS1987

The Newcastle meteor radar

A brief history and development of the Newcastle Meteor Radar system is given. Also described are its geographical coordinates and its method of operation. The initial objective when the project was commenced was to develop an entirely digital analyzer capable of recognizing meteor echo signals and recording as many of their parameters as possible. This objective was achieved.

Keay, Colin↗
NASA NTRS1988

An experimental and theoretical study of the ice accretion process during artificial and natural icing conditions

Real-time measurements of ice growth during artificial and natural icing conditions were conducted using an ultrasonic pulse-echo technique. This technique allows ice thickness to be measured with an accuracy of + or - 0.5 mm; in addition, the ultrasonic signal characteristics may be used to detect the presence of liquid on the ice surface and hence discern wet and dry ice growth behavior. Ice growth was measured on the stagnation line of a cylinder exposed to artificial icing conditions in the NASA Lewis Icing Research Tunnel (IRT), and similarly for a cylinder exposed in flight to natural icing conditions. Ice thickness was observed to increase approximately linearly with exposure time during the initial icing period. The ice accretion rate was found to vary with cloud temperature during wet ice growth, and liquid runback from the stagnation region was inferred. A steady-state energy balance model for the icing surface was used to compare heat transfer characteristics for IRT and natural icing conditions. Ultrasonic measurements of wet and dry ice growth observed in the IRT and in flight were compared with icing regimes predicted by a series of heat transfer coefficients. The heat transfer magnitude was generally inferred to be higher for the IRT than for the natural icing conditions encountered in flight. An apparent variation in the heat transfer magnitude was also observed for flights conducted through different natural icing-cloud formations.

Kirby, Mark S.↗
NASA NTRS1988

In-flight measurement of ice growth on an airfoil using an array of ultrasonic transducers

Results of preliminary tests to measure ice growth on an airfoil during flight icing conditions are presented. Ultrasonic pulse echo measurements of ice thickness are obtained from an array of eight ultrasonic transducers mounted flush with the leading edge of the airfoil. These thickness measurements are used to document the evolution of the ice shape during the encounter in the form of successive ice profiles. Results from 3 research flights are presented and discussed. The accuracy of the ultrasonic measurements is found to be within 0.5 mm of mechanical and stereo photograph measurements of the ice accretion.

Hansman, R. John, Jr.↗
NASA NTRS1988

Experimental measurements of heat transfer from an iced surface during artificial and natural cloud icing conditions

The heat transfer behavior of accreting ice surfaces in natural (flight test) and simulated (wind tunnel) cloud icing conditions were studied. Observations of wet and dry ice growth regimes as measured by ultrasonic pulse echo techniques were made. Observed wet and dry ice growth regimes at the stagnation point of a cylinder were compared with those predicted using a quasi steady state heat balance model. A series of heat transfer coefficients were employed by the model to infer the local heat transfer behavior of the actual ice surfaces. The heat transfer in the stagnation region was generally inferred to be higher in wind tunnel icing tests than in natural flight icing conditions.

Kirby, Mark S.↗
NASA NTRS1987

Objective method for analysis and tracking of convective cells as seen by radar

A special method has been developed for the study of cells that are embedded in convective rain systems. This method consists of a package of computer programs that use pattern recognition techniques on three-dimensional digital radar data to identify the rain cells, track them with time, and calculate their properties. The product of the computations is a comprehensive database of physically meaningful properties of rain cells, which can be used to infer the internal structure and the dynamics of convective rain systems. The cell-tracking method has been applied to the summer convective clouds of south Florida for the following purposes: (1) derivation of the relationship between the echo top height and the precipitation characteristics (e.g., area, water yield, rain intensity and duration of the rain cells); (2) study of the microphysical behaviorof cumulus clouds in relation to their cell properties; (3) evaluation of the effect of seeding on cumulus clouds on the cell scale; and (4) examination of cloud-to-ground lightning discharges in relation to convective cell intensity. The cell-tracking method is also currently being used in rain enhancement projects in Texas, Israel, and South Africa. The cell-tracking method, its products and their use in meteorological research are described in this paper.

Rosenfeld, Daniel↗
NASA NTRS1987

The radar-glory theory for icy moons with implications for radar mapping

The anomalous radar echoing properties of three ice-clad moons of Jupiter appear to be due to glory-like backscattering from buried craters. The enormous glare from these sources would impair geologic studies based on standard methods of radar mapping. It is not known whether similar or different problems will arise in the radar study of other icy surfaces in the outer solar system, or of the unseen surface of Titan. In any event, the results from the moons of Jupiter illustrate the role of exploratory measurements and the importance of possible bistatic radar-mapping techniques based on the use of separated transmitters and receivers.

Eshleman, Von R.↗
NASA NTRS1988

Analysis of algorithms for the retrieval of rain-rate profiles from a spaceborne dual-wavelength radar

The ability to retrieve rain-rate profiles from a dual-wavelength spaceborne radar system operating at 13.6 and 35 GHz is analyzed. The fundamental problem of extracting either the attenuation and/or the reflectivity from the backscatter echo, which contains both contributions, is addressed. Three algorithms, the backscatter, the attenuation coefficient, and the dual-wavelength methods, are examined. These algorithms are tested using four rain-rate profiles derived from radar measurements. In particular, measured (true) values are compared with calculated (retrieved) rain rates applying the algorithms with superimposed uncertainties assuming a suggested spaceborne dual-wavelength radar system. Error values of rain rates are determined where these values reflect failure of the assumptions utilized in the derivation of the algorithms, rain backscatter noise, and instrument noise. It is concluded that no single technique gives rise to a panacea in the making of accurate rain measurements and that difficulties exist with each method.

Goldhirsh, Julius↗
NASA NTRS1988

VHF radar remote sensing of atmospheric parameters over Taiwan during the time period of Typhoon Wayne

The VHF radar located at the subtropical and low latitude observing site of Taiwan has been used to observe atmospheric parameters from the troposphere to the middle atmosphere during the time passage of Typhoon Wayne in August 1986. Time-dependent wind velocities with three-dimensional profiles have been plotted by processing the recorded data of the backscattered echo power and the Doppler spectral width of the signal returns from the VHF radar. Propagation characteristics of the gravity waves excited by the enhanced convective motions of Typhoon Wayne have been detected from the Fourier power spectrum analysis of the three-dimensional wind velocities from multiple beams of the VHF radar and their cross-correlation analysis. The density perturbations caused by the propagation of the gravity waves can be calculated from the VHF radar observations. The VHF radar can also be used to detect time-dependent vertical profiles of the liquid water content. The present study shows that the equivalent rainfall rate calculated from the VHF radar is in good agreement with the ground-based measurement of rainfall rate at the same area.

Hung, R. J.↗
NASA NTRS1988

Radiometric calibration analysis of SIR-B imagery

The second flight of the NASA Spaceborne Imaging Radar (SIR-B) collected nearly 8 hours of digital synthetic-aperture radar (SAR) data of the earth's surface. This paper analyzes the performance of the SAR to determine the accuracy of the relative radiometric calibration of these data. Procedures are presented for deriving accuracy estimates as well as limitations under which these estimates are valid. The impact on calibration of an RF breakdown in the primary antenna feed system cable is evaluated. An analysis of the platform stability based on the SAR echo data is also presented in conjunction with its resultant effect on the calibration accuracy. Finally, numerical error bounds are derived with guidelines on their utilization.

Wall, Stephen D.↗
NASA NTRS1988

Ultrasonic techniques for aircraft ice accretion measurement

Ultrasonic pulse-echo measurements of ice growth on cylinders and airfoils exposed to both artificial (icing wind tunnel) and natural (flight) icing conditions are presented. An accuracy of + or - 0.5 mm is achieved with the present method. The ultrasonic signal characteristics associated with each of the two types of icing regimes identified, wet and dry ice growth, are discussed. Heat transfer coefficients are found to be higher in the wind tunnel environment than in flight. Results for ice growth on airfoils have also been obtained using an array of ultrasonic transducers. Icing profiles obtained during flight are compared with mechanical and stereo image measurements.

Hansman, R. John, Jr.↗