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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 541 records · Page 30

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NASA NTRS1975

Signal dispersion within a hippocampal neural network

A model network is described, representing two neural populations coupled so that one population is inhibited by activity it excites in the other. Parameters and operations within the model represent EPSPs, IPSPs, neural thresholds, conduction delays, background activity and spatial and temporal dispersion of signals passing from one population to the other. Simulations of single-shock and pulse-train driving of the network are presented for various parameter values. Neuronal events from 100 to 300 msec following stimulation are given special consideration in model calculations.

Horowitz, J. M.↗

NASA NTRS2010

Mechanisms of Diurnal Precipitation over the United States Great Plains: A Cloud-Resolving Model Simulation

The mechanisms of summertime diurnal precipitation in the US Great Plains were examined with the two-dimensional (2D) Goddard Cumulus Ensemble (GCE) cloud-resolving model (CRM). The model was constrained by the observed large-scale background state and surface flux derived from the Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) Program s Intensive Observing Period (IOP) data at the Southern Great Plains (SGP). The model, when continuously-forced by realistic surface flux and large-scale advection, simulates reasonably well the temporal evolution of the observed rainfall episodes, particularly for the strongly forced precipitation events. However, the model exhibits a deficiency for the weakly forced events driven by diurnal convection. Additional tests were run with the GCE model in order to discriminate between the mechanisms that determine daytime and nighttime convection. In these tests, the model was constrained with the same repeating diurnal variation in the large-scale advection and/or surface flux. The results indicate that it is primarily the surface heat and moisture flux that is responsible for the development of deep convection in the afternoon, whereas the large-scale upward motion and associated moisture advection play an important role in preconditioning nocturnal convection. In the nighttime, high clouds are continuously built up through their interaction and feedback with long-wave radiation, eventually initiating deep convection from the boundary layer. Without these upper-level destabilization processes, the model tends to produce only daytime convection in response to boundary layer heating. This study suggests that the correct simulation of the diurnal variation in precipitation requires that the free-atmospheric destabilization mechanisms resolved in the CRM simulation must be adequately parameterized in current general circulation models (GCMs) many of which are overly sensitive to the parameterized boundary layer heating.

Lee, M.-I.↗

NASA NTRS1974

The effect of repeated Compton scatterings on the diffuse X-ray background

Recent Monte Carlo computations, based on a speculative extrapolation of the electron spectrum, suggested that repeated scatterings contribute most of the extragalactic X-ray background around 3 MeV in a Compton model. We have redone this calculation analytically with a similar electron spectrum, but with very different results. The double-scatter contribution near 3 MeV does not exceed 1 percent and cannot improve the fit to data. The effect is largest near 1 keV but will probably not be observable.

Felten, J. E.↗

NASA NTRS1976

Noise propagation in urban and industrial areas

Noise propagation in streets and the discrepancies between theoretical analyses and field measurements are discussed. A cell-model is used to estimate the general background level of noise due to vehicular sources distributed over the urban area.

Davies, H. G.↗

NASA NTRS1981

Static tests of excess ground attenuation at Wallops Flight Center

An extensive experimental measurement program which evaluated the attenuation of sound for close to horizontal propagation over the ground was designed to replicate, under static conditions, results of the flight measurements carried out earlier by NASA at the same site (Wallops Flight Center). The program consisted of a total of 41 measurement runs of attenuation, in excess of spreading and air absorption losses, for one third octave bands over a frequency range of 50 to 4000 Hz. Each run consisted of measurements at 10 locations up to 675 m, from a source located at nominal elevations of 2.5, or 10 m over either a grassy surface or an adjacent asphalt concrete runway surface. The tests provided a total of over 8100 measurements of attenuation under conditions of low wind speed averaging about 1 m/s and, for most of the tests, a slightly positive temperature gradient, averaging about 0.3 C/m from 1.2 to 7 m. The results of the measurements are expected to provide useful experimental background for the further development of prediction models of near grazing incidence sound propagation losses.

Sutherland, L. C.↗

NASA NTRS1984

Fine-scale anisotropy of the cosmic microwave background in a universe dominated by cold dark matter

The fine-scale anisotropy of the cosmic microwave background radiation has been studied in cosmological models with a scale-invariant primordial adiabatic density fluctuation spectrum dominated by cold, weakly interacting particles. Normalization of the present fluctuation spectrum to the observed galaxy distribution results in excessive temperature anisotropy when compared to a recent upper limit on 4.5 arcmin unless the density parameter exceeds 0.4. When this result is combined with the requirement that the universe be at least 13 billion years old, it is found that if the cosmological constant is zero, then the density parameter is between roughly 0.4 and 1 and the Hubble constant is between roughly 60 km/s/Mpc and 50 km/s/Mpc.

Vittorio, N.↗

NASA NTRS1986

High-resolution optical and ultraviolet absorption-line studies of interstellar gas

Recent progress in the characterization of the interstellar medium (ISM) by means of optical and UV spectral data is summarized. The gas is studied by focusing on background stars whose spectra can be accurately modeled to provide the light source for the absorption-line scans. The capabilities of earth- and space-based instruments which have been and are used for the surveys are delineated. The distributions of diffuse gas densities and characteristics of the cold, warm and hot gas in the Galaxy are described in terms of the elemental abundances, kinetics and distributions of the gas. Particular note is taken of gas in the solar neighborhood and around SNR, and of absorption-line data of cosmological significance.

Cowie, Lennox L.↗

NASA NTRS1986

IRAS observations of the zodiacal background

IRAS sky survey measurements are used to constrain a model for the interplanetary dust emission. Efforts to date, although hampered somewhat by calibration uncertainties, indicate that the cloud is inclined to the ecliptic by approximately 1.7 deg, with its line of ascending nodes at 70 deg ecliptic longitude. The data is well fit by a dust density distribution presently described in cloud symmetry plane coordinates, where the dust has a gray emissivity and an equilibrium temperature at 1 AU of 280 K.

Good, J. C.↗

NASA NTRS1989

Chemical consequences of the initial diffusional growth of cloud droplets - A clean marine case

A simple microphysical cloud parcel model and a simple representation of the background marine aerosol are used to predict the concentrations and compositions of droplets of various sizes near cloud base. The aerosol consists of an externally-mixed ammonium bisulfate accumulation mode and a sea-salt coarse particle mode. The difference in diffusional growth rates between the small and large droplets as well as the differences in composition between the two aerosol modes result in substantial differences in solute concentration and composition with size of droplets in the parcel. The chemistry of individual droplets is not, in general, representative of the bulk (volume-weighted mean) cloud water sample. These differences, calculated to occur early in the parcel's lifetime, should have important consequences for chemical reactions such as aqueous phase sulfate production.

Twohy, C. H.↗

NASA NTRS1990

Studies of compact objects with Einstein - Review and prospects

X-ray images and spectra of a wide range of systems containing compact objects were obtained with the Einstein X-ray Observatory. Accreting white dwarfs, neutron stars and black holes were observed in binary systems in the Galaxy, and new constraints were derived for their formation, nature and evolution. Massive black holes were studied in active galactic nuclei, and X-ray spectra (and evolution) of AGN have led to a new model for the diffuse X-ray background.

Grindlay, Jonathan E.↗

NASA NTRS1991

The Interstellar Gas Experiment (IGE)

The Interstellar Gas Experiment (IGE) exposed thin metallic foils in order to collect neutral particles from the interstellar gas. These particles were entrapped in the foils along with precipitating magnetospheric and ambient atmospheric particles. Seven of these foils collected particles arriving from seven different directions as seen from the spacecraft for the entire duration of the Long Duration Exposure Facility (LDEF) mission. The authors' mass spectroscopy analysis of the noble gas component of these interstellar particles detected isotopes of helium and neon. These preliminary measurements suggest that the various isotopes are occurring in approximately the expected amounts and that their distribution in direction of arrival is close to what models predict. The analysis to subtract the background fluxes of magnetospheric and atmospheric particles is still in progress. The hope of this experiment is to investigate the noble gas isotopic ratios of this interstellar sample of matter which originated outside the solar system.

Lind, Don↗

NASA NTRS1991

Pressurization and expulsion of cryogenic liquids: Generic requirements for a low gravity experiment

Requirements are presented for an experiment designed to obtain data for the pressurization and expulsion of a cryogenic supply tank in a low gravity environment. These requirements are of a generic nature and applicable to any cryogenic fluid of interest, condensible or non-condensible pressurants, and various low gravity test platforms such as the Space Shuttle or a free-flyer. Background information, the thermophysical process, preliminary analytical modeling, and experimental requirements are discussed. Key parameters, measurements, hardware requirements, procedures, a test matrix, and data analysis are outlined.

Vandresar, Neil T.↗

NASA NTRS1991

Background issues for on-line aircraft documentation

Currently, almost all aircraft documentation in commercial aircraft cockpits is presented via hardcopy manuals. Several recent projects are aimed at eliminating all paper documentation in cockpits using electronic libraries. Electronic libraries encompass diverse information bases including aircraft system documentation, operations and procedures, checklists, maintenance logs, minimum equipment lists, maps and charts, and flight management information. These electronic libraries are envisioned to be embedded in the avionics so as to provide real time monitoring and display of information. Background issues are examined (motivation, information retrieval models, and preliminary designs) for the on-line presentation of aircraft systems documentation including operations, procedures, and checklists.

Russell, C. Ray↗

NASA NTRS1992

He I absorption by Lyman-alpha clouds and low-redshift Lyman-alpha clouds

Sources and sinks of radiation are used to model the evolution of the metagalactic ionizing background radiation field. It is shown that, if the comoving number density of quasars approached constancy at high redshift, as suggested by some recent observations, the observed ionizing background might possibly have been produced by the observed quasars alone. Consideration is also given to the UV radiation from decaying neutrons proposed by Sciama (1990), as an extra possible source. If is found that, if the background spectrum were as soft as in the Sciama scenario, the observed Ly-alpha clouds would contain detectable amounts of neutral helium, producing an 'He I forest' almost as dense as the hydrogen forest, due to the 584-A transition 1s-squared-1s2p. It is argued that observations of the Ly-alpha forest contradict the Sciama hypothesis.

Miralda-Escude, Jordi↗

NASA NTRS1992

A hydrodynamic treatment of the cold dark matter cosmological scenario

The evolution of structure in a postrecombination Friedmann-Robertson-Walker universe containing both gaseous baryons and cold dark matter (CDM) is studied by means of an Eulerian code coupled with a standard particle-mesh code. Ionization state and radiative opacity are calculated in detail, and the hydrodynamic simulations make it possible to compute properties of gas distribution on scales larger than three cell sizes. The model yields a soft X-ray background consistent with the latest cosmic nucleosynthesis values, and can accurately reproduce the galaxy-galaxy two-point correlation. The rate of galaxy formation peaks at a relatively late epoch. With regard to mass function, the smallest objects are stabilized against collapse by thermal energy: the mass-weighted mass spectrum peaks in the vicinity of m(b) = 10 exp 9.2 solar masses with a reasonable fit to the Schecter luminosity function if the baryon mass to blue light ratio is approximately 4. Overall, the simulations provide strong support for the CMD scenario. Of particular interest is that, while the baryons are not biased on scales greater than 1/h Mpc, the galaxies are, and that the 'galaxies' have a correlation function of the required slope and the correct amplitude.

Cen, Renyue↗

NASA NTRS1993

Estimating effective data density in a satellite retrieval or an objective analysis

An attempt is made to formulate consistent objective definitions of the concept of 'effective data density' applicable both in the context of satellite soundings and more generally in objective data analysis. The definitions based upon various forms of Backus-Gilbert 'spread' functions are found to be seriously misleading in satellite soundings where the model resolution function (expressing the sensitivity of retrieval or analysis to changes in the background error) features sidelobes. Instead, estimates derived by smoothing the trace components of the model resolution function are proposed. The new estimates are found to be more reliable and informative in simulated satellite retrieval problems and, for the special case of uniformly spaced perfect observations, agree exactly with their actual density. The new estimates integrate to the 'degrees of freedom for signal', a diagnostic that is invariant to changes of units or coordinates used.

Purser, R. J.↗

NASA NTRS1992

HEAO-1 analysis of Low Energy Detectors (LED)

The activities at Penn State University are described. During the period Oct. 1990 to Dec. 1991 work on HEAO-1 analysis of the Low Energy Detectors (LED) concentrated on using the improved detector spectral simulation model and fitting diffuse x-ray background spectral data. Spectral fitting results, x-ray point sources, and diffuse x-ray sources are described.

Nousek, John A.↗

NASA NTRS1993

Impact analysis of composite aircraft structures

The impact analysis of composite aircraft structures is discussed. Topics discussed include: background remarks on aircraft crashworthiness; comments on modeling strategies for crashworthiness simulation; initial study of simulation of progressive failure of an aircraft component constructed of composite material; and research direction in composite characterization for impact analysis.

Pifko, Allan B.↗