SEARCH · Search NASA

Results for “Sigma Models”

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 55 records · Page 3

All sources NASA NTRS DOE OSTI

NASA NTRS2013

Constraints on the Composition and Evolution of the Lunar Crust from Meteorite NWA 3163

The lunar meteorite NWA 3163 (paired with NWA 4881, 4483) is a ferroan, feldspathic granulitic breccia characterized by pigeonite, augite, olivine, maskelynite and accessory Tichromite, ilmenite and troilite. Bulk rock geochemical signatures indicate the lack of a KREEP- derived component (Eu/Eu* = 3.47), consistent with previously studied lunar granulites and anorthosites. Bulk rock chondrite-normalized signatures are however distinct from the anorthosites and granulites sampled by Apollo missions and are relatively REE-depleted. In-situ analyses of maskelynite reveal little variation in anorthite content (average An% is 96.9 +/- 1.6, 2 sigma). Olivine is relatively ferroan and exhibits very little variation in forsterite content with mean Fo% of 57.7 +/- 2.0 (2 sigma). The majority of pyroxene is low-Ca pigeonite (En57Fs33Wo10). Augite (En46Fs21Wo33) is less common, comprising approximately 10% of analyzed pyroxene. Two pyroxene thermometry on co-existing orthopyroxene and augite yield an equilibrium temperature of 1070C which is in reasonable agreement with temperatures of 1096C estimated from pigeonite compositions. Rb-Sr isotopic systematics of separated fractions yield an average measured Sr-87/Sr-87 of 0.699282+/-0.000007 (2 sigma). Sr model ages are calculated using a modern day Sr-87/Sr-86 Basaltic Achondrite Best Initial (BABI) value of 0.70475, from an initial BABI value Sr-87/Sr-86 of 0.69891 and a corresponding Rb-87/Sr-97 of 0.08716. The Sr model Thermomechanical analysis (TMA) age, which represents the time of separation of a melt from a source reservoir having chondritic evolution, is 4.56+/-0.1 Ga. A Sr model T(sub RD) age, which is a Rb depletion age and assumes no contribution from Rb in the sample in the calculation, yields 4.34+/-0.1 Ga (i.e. a minimum age). The Ar-Ar dating of paired meteorite NWA 4881 reveals an age of c. 2 Ga, likely representing the last thermal event this meteorite experienced. An older Ar-40/Ar-39 age of c. 3.5 Ga may record the thermal event which produced the granulitic texture. Additional chronological constraints will be provided by Sm-Nd systematics. Ferroan Anorthosites like NWA 3163 have been interpreted to represent direct lunar magma ocean (LMO) crystallization products. If this is the case, trace element concentrations in NWA 3163 primary mineral phases should be in equilibrium with residual LMO liquids present during crystallization of those phases. Results from petrogenetic modeling suggest that the NWA 3163 protolith did not form from crystallization of an initially LREE depleted LMO but rather require an initially chondritic LMO with early garnet crystallization. Furthermore, a two-stage crystallization model where plagioclase crystalized prior to pyroxene (93% vs. 99.5% of LMO crystallization) is implied.

McLeod, C. L.↗

NASA NTRS2003

Formation of Supermassive Black Holes in Galactic Bulges: A Rotating Collapse Model Consistent with the M(sub BH-sigma) Relation

Motivated by the observed correlation between black hole masses M(sub BH) and the velocity dispersion sigma of host galaxies, we develop a theoretical model of black hole formation in galactic bulges (this paper generalizes an earlier ApJ Letter). The model assumes an initial state specified by a uniform rotation rate OMEGA and a density distribution of the form rho = a(sup 2)(sub eff)per2piGR(sup 2)(so that a(sub eff)is an effective transport speed). The black hole mass is determined when the centrifugal radius of the collapse flow exceeds the capture radius of the central black hole (for Schwarzschild geometry). This model reproduces the observed correlation between the estimated black hole masses and the velocity dispersions of galactic bulges, i.e., M(sub BH) approximately equal to 10(sup 8) solar mass(sigma per 200 kilometers per second)(sup 4) where sigma = the square root of 2a(sub eff). To obtain this normalization, the rotation rate OMEGA approximately equal to 2 x 10(exp -15) rad per second. The model also defines a bulge mass scale M(sub B). If we identify the scale M(sub B) with the bulge mass, the model determines the ratio mu(sub B) of black hole mass to the host mass: mu(sub B) approximately equal to 0.0024(sigma per 200 kilometer per second), again in reasonable agreement with observed values. In this scenario, supermassive black holes form quickly (in approximately 10(exp 5) yr) and are born rapidly rotating (with a per M approximately 0.9). This paper also shown how these results depend on the assumed initial conditions; the most important quantity is the initial distribution of specific angular momentum in the precollapse state.

Adams, Fred C.↗

NASA NTRS1993

The influence of orography on the transport of atmospheric constituents

Given the fact that martian orography varies over a range of two scale heights (cf., about one scale height on Earth), it is thought that on Mars some aspects of the circulation should be affected to a large degree by the presence of orography. Initial results from two numerical models are presented that illustrate the effect of orography on cross-equatorial transport on Mars. On Earth, western boundary currents are found not only in the oceans as one would expect, but also in the troposphere, the most notable example being the East African Jet, which has an important effect on the monsoon circulation of the Eastern Indian Ocean. Two models were used for this investigation: a simple GCM (hereafter referred to as the SGCM); and a simple one-level barotropic model (BM). The SGCM solves the hydrostatic primitive equations on a sphere by representing model variables by spherical harmonics in the horizontal, with nonlinear terms being calculated in grid-point space. In the vertical, the model uses the sigma coordinate system in finite-difference form. The model represents the radiative processes in the atmosphere by a Newtonian relaxation to a specified zonal mean temperature state and surface drag by Rayleigh friction in the lowest model level.

Joshi, Manoj M.↗

NASA NTRS1996

BATSE Observations of the Large-Scale Isotropy of Gamma-Ray Bursts

We use dipole and quadrupole statistics to test the large-scale isotropy of the first 1005 gamma-ray bursts observed by the Burst and Transient Source Experiment (BATSE). In addition to the entire sample of 1005 gamma-ray bursts, many subsets are examined. We use a variety of dipole and quadrupole statistics to search for Galactic and other predicted anisotropies and for anisotropies in a coordinate-system independent manner. We find the gamma-ray burst locations to be consistent with isotropy, e.g., for the total sample the observed Galactic dipole moment (cos theta) differs from the value predicted for isotropy by 0.9 sigma and the observed Galactic quadrupole moment (sin(exp 2) b - 1/3) by 0.3 sigma. We estimate for various models the anisotropies that could have been detected. If one-half of the locations were within 86 deg of the Galactic center, or within 28 deg of the Galactic plane, the ensuing dipole or quadrupole moment would have typically been detected at the 99% confidence level. We compare the observations with the dipole and quadrupole moments of various Galactic models. Several Galactic gamma-ray bursts models have moments within 2 sigma of the observations; most of the Galactic models proposed to date are no longer in acceptable agreement with the data. Although a spherical dark matter halo distribution could be consistent with the data, the required core radius is larger than the core radius of the dark matter halo used to explain the Galaxy's rotation curve. Gamma-ray bursts are much more isotropic than any observed Galactic population, strongly favoring but not requiring an origin at cosmological distances.

Briggs, Michael S.↗

NASA NTRS1978

The uncertainty in ozone calculations by a stratospheric photochemistry model

At present, there is an apparent conflict between one-dimensional stratospheric photochemistry models used in predicting ozone depletion and average data for stratospheric ozone. This conflict is in three particulars - column density of O3 and ozone density in the regions around 30 km and 50 km altitude. A study of the sensitivity of one such model to the values of reaction rates, boundary conditions, solar intensities, photolysis cross sections, and O(1D) yield parameters reveals that even at the 2-sigma uncertainty limit due to these input parameters, the model does not overlap the data for O3 density at 30 km and 50 km. The data is outside the 1-sigma model uncertainty limit for O3 column density. The study also shows the relative contribution of the various parameters studied to the imprecision in these model results.

Butler, D. M.↗

NASA NTRS2017

Fermi Gamma-Ray Pulsars: Understanding the High-Energy Emission from Dissipative Magnetospheres

Based on the Fermi observational data, we reveal meaningful constraints for the dependence of the macroscopic conductivity (sigma) of dissipative pulsar magnetosphere models on the corresponding spin-down rate, epsilon(sup dot). Our models are refinements of the FIDO (Force-free Inside, Dissipative Outside) models, which have dissipative regions that are restricted on the equatorial current sheet outside of the the light-cylinder. Taking into account the observed cutoff energies of all of the Fermi pulsars and assuming that (a) the corresponding gamma-ray pulsed emission is due to curvature radiation at the radiation-reaction-limit regime, and (b) this emission is produced at the equatorial current sheet near the light cylinder, we show that the Fermi data provide clear indications about the corresponding accelerating electric-field components. A direct comparison between the Fermi cutoff energies and the model ones reveals that if sigma increases with epsilon(sup dot) for high epsilon(sup dot)-values, while it saturates for low ones. This comparison indicates also that the corresponding gap width increases toward low epsilon(sup dot)-values. Assuming the Goldreich-Julian flux for the emitting particles, we calculate the total gamma-ray luminosity (L(sub gamma)). A comparison between the dependence of the Fermi L(sub gamma)-values and the model ones on epsilon(sup dot) indicates an increase of the emitting particle multiplicity with epsilon(sup dot). Our modeling, guided by the Fermi data alone, enhances our understanding of the physical mechanisms behind the high-energy emission in pulsar magnetospheres.

Kalapotharakos, Constantinos↗

NASA NTRS1982

The structure of 110 tilt boundaries in large area solar silicon

The models of Hornstra and their connection to the repeating group description of grain boundaries (7-10) are discussed. A model for the Sigma = 27 boundary containing a zig-zag arrangement of dislocations is constructed and it is shown that zig-zag models can account for the contrast features observed in high resolution transmission electron micrographs of second and third order twin boundaries in silicon. The boundaries discussed are symmetric with a 110 tilt axis and a (110) boundary plane in the median lattice (the median plane). The median lattice is identical in structure and halfway in orientation between the crystal lattices either side of the boundary.

Ast, D. G.↗

NASA NTRS1991

A numerical procedure for recovering true scattering coefficients from measurements with wide-beam antennas

A numerical procedure for estimating the true scattering coefficient, sigma(sup 0), from measurements made using wide-beam antennas. The use of wide-beam antennas results in an inaccurate estimate of sigma(sup 0) if the narrow-beam approximation is used in the retrieval process for sigma(sup 0). To reduce this error, a correction procedure was proposed that estimates the error resulting from the narrow-beam approximation and uses the error to obtain a more accurate estimate of sigma(sup 0). An exponential model was assumed to take into account the variation of sigma(sup 0) with incidence angles, and the model parameters are estimated from measured data. Based on the model and knowledge of the antenna pattern, the procedure calculates the error due to the narrow-beam approximation. The procedure is shown to provide a significant improvement in estimation of sigma(sup 0) obtained with wide-beam antennas. The proposed procedure is also shown insensitive to the assumed sigma(sup 0) model.

Wang, Qinglin↗

NASA NTRS1999

Surface Tension and Fingering of Miscible Interfaces

Experiments on miscible, buoyantly unstable reaction-diffusion fronts and non-reacting displacement fronts in Hele-Shaw cells show a fingering-type instability whose wavelengths (lambda*) are consistent with an interfacial tension (sigma) at the front caused by the change in chemical composition, even though the solutions are miscible in all proportions. In conjunction with the Saffman-Taylor model, the relation sigma = K/tau, where tau is the interface thickness and K approximately equal 4 +/- 2 x 10(exp -6) dyne, enables prediction of our measured values of lambda* as well as results from prior experiments on miscible interfaces. These results indicate that even for miscible fluids, surface tension is generally a more significant factor than diffusion in interfacial stability and flow characteristics.

Abib, Mohammed↗

NASA NTRS1980

Microwave remote sensing of snowpacks

The interaction mechanisms responsible for the microwave backscattering and emission behavior of snow were investigated, and models were developed relating the backscattering coefficient (sigma) and apparent temperature (T) to the physical parameters of the snowpack. The microwave responses to snow wetness, snow water equivalent, snow surface roughness, and to diurnal variations were investigated. Snow wetness was shown to have an increasing effect with increasing frequency and angle of incidence for both active and passive cases. Increasing snow wetness was observed to decrease the magnitude sigma and increase T. Snow water equivalent was also observed to exhibit a significant influence sigma and T. Snow surface configuration (roughness) was observed to be significant only for wet snow surface conditions. Diurnal variations were as large as 15 dB for sigma at 35 GHz and 120 K for T at 37 GHz. Simple models for sigma and T of a snowpack scene were developed in terms of the most significant ground-truth parameters. The coefficients for these models were then evaluated; the fits to the sigma and T measurements were generally good. Finally, areas of needed additional observations were outlined and experiments were specified to further the understanding of the microwave-snowpack interaction mechanisms.

Stiles, W. H.↗

NASA NTRS1992

The asymmetric distribution of satellite galaxy velocities

Companion galaxies tend to have redshifts that exceed that of their associated primary. We show that the latest sample of satellite galaxies (from Zaritsky et al.) corroborates this assertion. We examine whether selection biases and contamination by galaxies that are not physical satellites can create the observed velocity asymmetry. As shown by Valtonen and Byrd (1986) for previous samples, simple models of the selection process reproduce the sense of the asymmetry, but not its magnitude unless the samples are dominated by interlopers. We expand on this model by using numerical simulations that include a simple model of galaxy clustering, and demonstrate that clustering can increase the magnitude of the expected asymmetry for a particular level of contamination. Depending on input assumptions, these models imply 1 sigma lower limits on the contamination level in the Zaritsky et al. (1992) sample of between 8 and 20 percent. Using the results from the analysis of that sample and the models, we conclude that, for the type of satellite galaxy sample discussed, here, a moderate level of contamination (about 10 percent) is sufficient to account for the observed asymmetry.

Zaritsky, Dennis↗

NASA NTRS1994

The Local Supercluster as a test of cosmological models

The Local Supercluster kinematic properties (the Local Group infall toward the Virgo Cluster and galaxy density distribution about the Virgo Cluster) in various cosmological models are examined utilizing large-scale N-body (PM) simulations 500(exp 3) cells, 250(exp 3) particles, and box size of 400 h(exp -1) Mpc) and are compared to observations. Five models are investigated: (1) the standard, Cosmic Background Explorer Satellite (COBE)-normalized cold dark matter (CDM) model with omega = 1, h = 0.5, and sigma(sub 8) = 1.05; (2) the standard Hot Dark Matter (HDM) model with omega = 1, h = 0.75, and sigma(sub 8) = 1; (3) the tilted CDM model with omega = 1, h = 0.5, n = 0.7, and sigma(sub 8) = 0.5; (4) a CDM + lambda model with omega = 0.3, lambda = 0.7, h = 2/3, and sigma(sub 8) = 2/3; (5) the PBI model with omega = 0.2, h = 0.8, x = 0.1, m = -0.5, and sigma(sub 8) = 0.9. Comparison of the five models with the presently available observational measurements v(sub LG) = 85 - 305 km/s (with mean of 250 km/s), delta(n(sub g))/(n(sub g)-bar) = 1.40 + or - 0.35) suggests that an open universe with omega approximately 0.5 (with or without lambda) and sigma(sub 8) approximately 0.8 is preferred, with omega = 0.3-1.0 (with or without lambda) and sigma(sub 8) = 0.7-1.0 being the acceptable range. At variance with some previous claims based on either direct N-body or spherical nonlinear approaches, we find that a flat model with sigma(sub 8) approximately 0.7-1.0 seems to be reasonably consistent with observations. However, if one favors the low limit of v(sub LG) = 85 km/s, then an omega approximately 0.2-0.3 universe seems to provide a better fit, and flat (omega = 1) models are ruled out at approximately 95% confidence level. On the other hand, if the high limit of v(sub LG) = 350 km/s is closer to the truth, then it appears that omega approximately 0.7-0.8 is more consistent. This test is insensitive to the shape of the power spectrum, but rather sensitive to the normalization of the perturbation amplitude on the relevant scale (e.g., sigma(sub 8)) and omega. We find that neither linear nor nonlinear relations (with spherical symmetry) are good approximations for the relation between radial peculiar velocity and density perturbation, i.e., nonspherical effects and gravitational tidal field are important. The derived omega using either of the two relations is underestimated. In some cases, this error is as large as a factor of 2-4.

Cen, Renyue↗

NASA NTRS2013

NF3: UV Absorption Spectrum Temperature Dependence and the Atmospheric and Climate Forcing Implications

Nitrogen trifluoride (NF3) is an atmospherically persistent greenhouse gas that is primarily removed by UV photolysis and reaction with O((sup 1)D) atoms. In this work, the NF3 gas-phase UV absorption spectrum, sigma(delta,T), was measured at 16 wavelengths between 184.95 and 250 nm at temperatures between 212 and 296 K. A significant spectrum temperature dependence was observed in the wavelength region most relevant to atmospheric photolysis (200-220 nm) with a decrease in sigma(210 nm,T) of approximately 45 percent between 296 and 212 K. Atmospheric photolysis rates and global annually averaged lifetimes of NF3 were calculated using the Goddard Space Flight Center 2-D model and the sigma(delta,T) parameterization developed in this work. Including the UV absorption spectrum temperature dependence increased the stratospheric photolysis lifetime from 610 to 762 years and the total global lifetime from 484 to 585 years; the NF3 global warming potentials on the 20-, 100-, and 500-year time horizons increased less than 0.3, 1.1, and 6.5 percent to 13,300, 17,700, and 19,700, respectively.

nitrogen trifluoride↗

NASA NTRS1985

Creep-fatigue behavior of NiCoCrAlY coated PWA 1480 superalloy single crystals

Single crystal specimens of a Ni base superalloy, PWA 1480, with a low pressure plasma sprayed NiCoCrAlY coating were tested in various 0.1 Hz fatigue and creep fatigue cycles both at 1015 and 1050 C. Creep fatigue tests of the cp, pc, and cc types were conducted with various constant total strain ranges employing creep dwells at various constant stresses. Considerable cyclic softening occurred as was evidenced particularly by rapidly increasing creep rates in the creep fatigue tests. The cycle time in the creep fatigue tests typically decreased by more than 80 percent at 0.5 N sub f. Though cyclic life did correlate with delta epsilon sub in a better correlation existed with sub f for both the fatigue and creep fatigue tests, and poor correlations were observed with either sigma sub max or the average cycle time. A model containing both delta sigma and delta sigma (sub in), N sub f = alpha delta sigma (sub in) beta delta sigma gamma, with best fit values of sigma for each cycle type, but the same values of beta and gamam, was found to provide good correlations. Life lines were not greatly different among the cycle types, differing only by a factor of about three. The cp cycle life line was lowest for both test temperatures, however among the other three cycle types there was no consistent ranking. For all test types failure occurred predominately by multiple internal cracking originating at pores. The strong correlation of life with delta sigma may reflect a significant crack growth period in the life of the specimens.

Miner, R. V.↗

NASA NTRS1988

Creep-fatigue behavior of NiCoCrAlY coated PWA 1480 superalloy single crystals

Miner, R. V.↗

NASA NTRS2011

A Debris Disk Case Study: 49 Ceti with Herschel

Gas-poor debris disks represent a fundamentally different class of circumstellar disk than gas-rich protoplanetary disks. Their gas probably originates from the same source as the dust, i.e. planetesimal destruction, but the low gas densities make it difficult to detect. So far, Herschel has detected far-IR gas emission from one debris disk, Beta Pictoris. Here I discuss a well-known debris disk system in the GASPS survey, 49 Ceti. It serves as a case study for modeling low-density gas in optically thin disks. The dust disk appears to be spatially resolved at 70 um. Most interestingly, there appears to be a hint of ClI 158 urn emission at the roughly 2 sigma level. Preliminary modeling suggests that reconciling the sub-mm CO emission from this system with the weak or non-existent far-IR atomic lines may require an unusual chemical composition in the gas of this disk.

Roberge, Aki↗

NASA NTRS1990

Microwave Conductivity of Laser Ablated YBa2Cu3O7-delta Superconducting Films and Its Relation to Microstrip Transmission Line Performance

We report on the values of the microwave conductivity in the normal (sigma(subN)) and superconducting (sigma*=sigma(sub1)-j sigma(sub2)) states of two laser ablated YBa2CU3O7(sigma) thin films at 35 GHz, in the temperature range from 20 to 300 K. The films 0.7 and 0.4 micrometers) were deposited on LaA10(sub3) by laser ablation. The conductivity was obtained from the microwave power transmitted through the films and assuming a two-fluid model. Values of sigma(subN) approximately 2.3 X 10(exp5) S/m at room temperature for both films, and of sigma(sub1) approximately 6.3 X 10(exp5) and 4.6 X 10(exp5) S/m at temperatures around 80 K were obtained for the 0.7 and 0.4 micrometer films respectively. For sigma(sub2) values of 4.9 X 10(exp6) and 5.4 X 10(exp6) S/m were obtained for the 0.7 and 0.4 micrometer films at 80 K. The expected conductor losses and Q-factor of a superconducting ring resonator were calculated using these conductivity values. The theoretical values were then compared with the experimental results obtained for a resonator fabricated from one of these films.

Bhasin, K. B.↗

NASA NTRS1993

SAR terrain classifier and mapper of biophysical attributes

In preparation for the launch of SIR-C/X-SAR and design studies for future orbital SAR, a program has made considerable progress in the development of an SAR terrain classifier and algorithms for quantification of biophysical attributes. The goal of this program is to produce a generalized software package for terrain classification and estimation of biophysical attributes and to make this package available to the larger scientific community. The basic elements of the SAR (Synthetic Aperture Radar) terrain classifier are outlined. An SAR image is calibrated with respect to known system and processor gains and external targets (if available). A Level 1 classifier operates on the data to differentiate: urban features, surfaces and tall and short vegetation. Level 2 classifiers further subdivide these classes on the basis of structure. Finally, biophysical and geophysical inversions are applied to each class to estimate attributes of interest. The process used to develop the classifiers and inversions is shown. Radar scattering models developed from theory and from empirical data obtained by truck-mounted polarimeters and the JPL AirSAR are validated. The validated models are used in sensitivity studies to understand the roles of various scattering sources (i.e., surface trunk, branches, etc.) in determining net backscatter. Model simulations of sigma (sup o) as functions of the wave parameters (lambda, polarization and angle of incidence) and the geophysical and biophysical attributes are used to develop robust classifiers. The classifiers are validated using available AirSAR data sets. Specific estimators are developed for each class on the basis of the scattering models and empirical data sets. The candidate algorithms are tested with the AirSAR data sets. The attributes of interest include: total above ground biomass, woody biomass, soil moisture and soil roughness.

Ulaby, Fawwaz T.↗