SEARCH · Engineering Papers

Results for “Density estimation”

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.

SourceResearch connectionFrom yearThrough year

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

At least 937 records · Page 52

All sources NASA NTRS DOE OSTI

NASA NTRS1986

Ultrasonic characterization of structural ceramics

Ultrasonic velocity and attenuation measurements were used to characterize density and microstructure in monolithic silicon nitride and silicon carbide. Research samples of these structural ceramics exhibited a wide range of density and microstructural variations. It was shown that bulk density variations correlate with and can be estimated by velocity measurements. Variations in microstructural features such as grain size or shape and pore morphology had a minor effect on velocity. However, these features had a pronounced effect on ultrasonic attenuation. The ultrasonic results are supplemented by low-energy radiography and scanning laser acoustic microscopy.

Klima, S. J.↗

NASA NTRS1986

Nondestructive characterization of structural ceramics

Klima, S. J.↗

NASA NTRS1986

Mg II emission from shock waves in Mira variables

In order to investigate the atmospheric shock dynamics of Miras, a program to monitor the ultraviolet Mg II flux, the visual magnitude (using the IUE FES-magnitude) and whenever possible the hydrogen Balmer emission for several Miras was initiated. Results suggest that it should be possible to derive the density structure of the atmosphere and hence estimate quite accurately the kinetic energy input into the stellar atmosphere. The atmospheric shock models suggest that at phase equal 1.0 most of the energy which enters the region where the shock is located ends up in the wind. This is after the strongest Mg II emission is seen. From an observed velocity at infinity and an estimate of the energy input derived from the Mg II flux, it will be possible to determine the mass loss rate for these objects.

Brugel, Edward W.↗

NASA NTRS1987

Submm observations of IRAS galaxies

Twenty-six galaxies from the Infrared Astronomy Satellite (IRAS) point source catalog were observed at 250 and 1300 microns. The Far Infrared (FIR) spectra from 25 to 1300 microns are interpreted in terms of two dust components of about 16 and 53 K. Flux densities at 1300 microns are used to estimate the total gas mass M sub g. The warm dust luminosity L sup W is considered to be proportional to M sub g sup 0.9 in the mass range 3 x 10 to the 8th is equal to or less than M sub g/solar mass is equal to or less than 10 to the 12th power. The efficiency of star formation seems to be four times higher in barred and peculiar galaxies than in Sc types.

Chini, R.↗

NASA NTRS1987

AC impedance analysis of polypyrrole thin films

The AC impedance spectra of thin polypyrrole films were obtained at open circuit potentials from -0.4 to 0.4 V vs SCE. Two limiting cases are discussed for which simplified equivalent circuits are applicable. At very positive potentials, the predominantly nonfaradaic AC impedance of polypyrrole is very similar to that observed previously for finite porous metallic films. Modeling of the data with the appropriate equivalent circuit permits effective pore diameter and pore number densities of the oxidized film to be estimated. At potentials from -0.4 to -0.3 V, the polypyrrole film is essentially nonelectronically conductive and diffusion of polymer oxidized sites with their associated counterions can be assumed to be linear from the film/substrate electrode interface. The equivalent circuit for the polypyrrole film at these potentials is that previously described for metal oxide, lithium intercalation thin films. Using this model, counterion diffusion coefficients are determined for both semi-infinite and finite diffusion domains. In addition, the limiting low frequency resistance and capacitance of the polypyrrole thin fims was determined and compared to that obtained previously for thicker films of the polymer. The origin of the observed potential dependence of these low frequency circuit components is discussed.

Penner, Reginald M.↗

NASA NTRS1986

Probing thick vegetation canopies with a field microwave scatterometer

with a narrow-beam field microwave scatterometer having a variable slant range and a small slant-range resolution, investigators can estimate the distribution of backscattering sources in a medium as a function of slant range. From this, they can calculate the backscattering coefficient. With less confidence, they can estimate the volume extinction coefficient, the volume density of backscattering cross sections, the transmittance, and the backscattering coefficient of a semiinfinite medium with the same volumetric microwave properties. These parameters allow one to understand the interactions of microwaves with media such as vegetation canopies with their substrates.

Paris, J. F.↗

NASA NTRS1987

Observations of energetic ions on the nightside of Venus

Ions with energy greater than 40 eV have been measured on the nightside of Venus by the Pioneer Venus orbiter neutral mass spectrometer (ONMS). For periapsis altitudes beyond 1500 km, they are confined to solar zenith angles greater than 120 deg. The nightside energetic ion composition at 2000 km resembles the nightside thermal ion composition occurring at 1000 km with the dominant species being O(+). Postflight laboratory calibrations performed on the ONMS backup unit have been used to estimate the energetic O(+) ambient flux and density. The direction from which the energetic ions appear to be coming can be determined because the ONMS is mounted at an angle with respect to the spin axis of the spacecraft. Both tailward and antitailward components of the apparent ion flow are exhibited when projected into the ecliptic plane. The major component of the apparent ion flow is observed perpendicular to the ecliptic plane.

Kasprzak, W. T.↗

NASA NTRS1987

A method of long-term radar shower data analysis

Complex photographic and radar meteor observations have been carried out since 1957. Using the available observational data, the density of incident flux of meteoroids was estimated over a wide mass range of 0.001 to 100 g. To avoid the influence of apparatus selectivity a special technique was applied. The application of this technique to the radar shower data analysis is discussed in detail.

Simek, M.↗

NASA NTRS1987

Incident meteoroid flux density

Complex photographic and radar meteor observations were carried out. Using the available observational data, the density of incident flux of meteoroids was estimated over a wide mass range of 0.001 to 100 g. To avoid the influence of apparatus selectivity a special technique was applied. The main characteristics of this technique are given and discussed.

Badadjanov, P. B.↗

NASA NTRS1990

The interaction of an artificial electron beam with the earth's upper atmosphere - Effects on spacecraft charging and the near-plasma evironment

The ionization created in the earth's upper atmosphere by an electron beam emitted from a low altitude spacecraft is described by two coupled nonlinear differential electron transport equations for the up-going and down-going differential energy flux. The equations are solved numerically, yielding estimates of the differential electron energy flux density at the spacecraft location. At altitudes below 200-250 km and for beam energies around 1 keV, it is shown that secondary electrons supply a significant contribution to the return current to the spacecraft and thereby reduce the spacecraft potential. The numerical results are in good agreement with observations from the Charge 2 sounding rocket experiment.

Neubert, T.↗

NASA NTRS1991

Inflationary axion cosmology

If Peccei-Quinn (PQ) symmetry is broken after inflation, the initial axion angle is a random variable on cosmological scales; based on this fact, estimates of the relic-axion mass density give too large a value if the axion mass is less than about 10 to the -6th eV. This bound can be evaded if the universe underwent inflation after PQ-symmetry breaking and if the observable universe happens to be a region where the initial axion angle was atypically small. Consideration of fluctuations induced during inflation severely constrains the latter alternative is shown.

Turner, Michael S.↗

NASA NTRS1991

Infrared polarization images of star-forming regions. I - The ubiquity of bipolar structure

The inefficiency of the stellar formation process leads rather generally to high residual dust densities, and so to the existence of infrared reflection nebulosity (IRN), in regions of star formation. Polarization images of several star-forming regions with mass outflows (GSS 30, S255, GL 5180, GL 2591, GGD 27, and NGC 7538) presented here: (1) establish the universality of bipolarity and of shell or cavity structure in the IRN consistent with that of CO outflow; (2) identify the source of the mass outflow in each case; (3) show that the opening angle near this central source is large; and (4) demonstrate several instances of multiple shells, probably arising from episodic mass loss. Astrometry of 2.2-micron sources with arcsecond accuracy identifies the illuminating source of each IRN uniquely with a compact H II region or a bright IR source. The polarization images provide strong evidence for large-scale dust toroids around each of these sources. The density and mass of these disks are estimated from the extinction through the disk.

Tamura, M.↗

NASA NTRS1991

Infrared spectroscopy of a Cygnus A - Implications for the obscured active nucleus

Near-infrared spectroscopic observations of the central regions of the luminous radio galaxy Cygnus A are presented and interpreted in terms of an obscured quasar nucleus. Strong emission is detected in the molecular hydrogen lines 1-0 S(1) and 1-0 S(3), the strengths of which are accounted for through heating by the nuclear hard X-ray source. The large equivalent widths of these molecular hydrogen lines and the near-infrared narrow hydrogen recombination lines suggest that the observed nuclear continuum is strongly attenuated at 2 microns. The observed upper limit to the flux of broad Pa-alpha implies an extinction to the putative broad line region AV(BL) of at least 24 mag, and the observed continuum intensity of the nuclear point source at 2.2 microns gives an extinction of 43 +/-9 mag toward the optical-infrared continuum. These estimates are consistent with the gas column density inferred from the low-energy X-ray cutoff. Strong forbidden Si VI 1.962-micron line emission from Cygnus A is also reported.

Ward, Martin J.↗

NASA NTRS1992

Fluid flow in a jet and the Ca XIX line profiles observed during solar flares

The present study considers two types of fluid models as methods for reproducing the blueshifts and line broadening observed in soft X-ray lines during the rise of solar flares. The fluid models representing the laminar flow of material in a jet and through a pipe were used to derive the velocity at each location in the flow. The optimal values for the free parameters in the jet model were within the ranges expected for solar flare conditions. The jet was much smaller than typical flare volumes estimated from spatial images, but the resulting densities were consistent with the upper limit of densities derived using density-sensitive line ratios. This supports previous reports of small filling factors and is interpreted as evidence for a chromospheric origin for the flow. Stability analysis indicates that, although the laminar jet can reproduce the observations, instabilities may cause the flow to become turbulent, and that this laminar treatment is not strictly valid.

Bornmann, P. L.↗

NASA NTRS1992

Low-mass X-ray binary evolution and the origin of millisecond pulsars

The evolution of low-mass X-ray binaries (LMXBs) is considered. It is shown that X-ray irradiation of the companion stars causes these systems to undergo episodes of rapid mass transfer followed by detached phases. The systems are visible as bright X-ray binaries only for a short part of each cycle, so that their space density must be considerably larger than previously estimated. This removes the difficulty in regarding LMXBs as the progenitors of low-mass binary pulsars. The low-accretion-rate phase of the cycle with the soft X-ray transients is identified. It is shown that 3 hr is likely to be the minimum orbital period for LMXBs with main-sequence companions and it is suggested that the evolutionary endpoint for many LMXBs may be systems which are the sites of gamma-ray bursts.

Frank, Juhan↗

NASA NTRS1992

An improved Venus gravity field from Doppler tracking of the Pioneer Venus Orbiter and Magellan spacecraft

A total of 365,000 Pioneer Venus Orbiter (PVO) S-band Doppler data points were fit in arcs ranging from one to four revolutions in length by estimating the spacecraft position and velocity, atmospheric density, and a solar pressure parameter at each arc epoch. The resultant converged PVO orbits were used to produce 194 information arrays for the Venus spherical harmonic coefficients complete to degree and order 21. A similar procedure was used to produce a second set of 72 information arrays for a 21 x 21 gravitational field from a limited, but global, sampling of Magellan S- and X-band data from cycles 1 and 2. The PVO and Magellan information arrays were combined and solved to obtain the 21 x 21 spherical harmonic expansion designated JPL-Venus Gravity Model 6A (VGM6A). The VGM6A harmonic field field produced significant improvements in the Doppler residual statistics obtained from field validation fits to both PVO and Magellan tracking data in comparison with alternative representations of Venus gravity.

Mcnamee, J. B.↗

NASA NTRS1992

Helium escape from the Earth's atmosphere - The charge exchange mechanism revisited

We have studied the escape of neutral helium from the terrestrial atmosphere through exothermic charge exchange reactions between He(+) ions and the major atmospheric constituents N2, O2 and O. Elastic collisions with the neutral background particles were treated quantitatively using a recently developed kinetic theory approach. An interhemispheric plasma transport model was employed to provide a global distribution of He(+) ions as a function of altitude, latitude and local solar time and for different levels of solar ionization. Combining these ion densities with neutral densities from an MSIS model and best estimates for the reaction rate coefficients of the charge exchange reactions, we computed the global distribution of the neutral He escape flux. The escape rates show large diurnal and latitudinal variations, while the global average does not vary by more than a factor of three over a solar cycle. We find that this escape mechanism is potentially important for the overall balance of helium in the Earth's atmosphere. However, more accurate values for the reaction rate coefficients of the charge exchange reactions are required to make a definitive assessment of its importance.

Lie-Svendsen, O.↗

NASA NTRS1994

Comments on the statistical analysis of excess variance in the COBE differential microwave radiometer maps

Cosmic anisotrophy produces an excess variance sq sigma(sub sky) in the Delta maps produced by the Differential Microwave Radiometer (DMR) on cosmic background explorer (COBE) that is over and above the instrument noise. After smoothing to an effective resolution of 10 deg, this excess sigma(sub sky)(10 deg), provides an estimate for the amplitude of the primordial density perturbation power spectrum with a cosmic uncertainty of only 12%. We employ detailed Monte Carlo techniques to express the amplitude derived from this statistic in terms of the universal root mean square (rms) quadrupole amplitude, (Q sq/RMS)(exp 0.5). The effects of monopole and dipole subtraction and the non-Gaussian shape of the DMR beam cause the derived (Q sq/RMS)(exp 0.5) to be 5%-10% larger than would be derived using simplified analytic approximations. We also investigate the properties of two other map statistics: the actual quadrupole and the Boughn-Cottingham statistic. Both the sigma(sub sky)(10 deg) statistic and the Boughn-Cottingham statistic are consistent with the (Q sq/RMS)(exp 0.5) = 17 +/- 5 micro K reported by Smoot et al. (1992) and Wright et al. (1992).

Wright, E. L.↗