SEARCH · Engineering Papers

Results for “spherical fitting”

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 325 records · Page 18

All sources NASA NTRS DOE OSTI

DOE OSTI2017

( Chandra Chandra) reveals heavy obscuration and circumnuclear star formation in Seyfert 2 galaxy NGC 4968

We present the Chandra imaging and spectral analysis of NGC 4968, a nearby (z = 0.00986) Seyfert 2 galaxy. We discover extended (∼1 kpc) X-ray emission in the soft band (0.5–2 keV) that is neither coincident with the narrow line region nor the extended radio emission. Based on spectral modeling, it is linked to on-going star formation (∼2.6–4 M{sub ⊙} yr{sup −1}). The soft emission at circumnuclear scales (inner ∼400 pc) originates from hot gas, with kT ∼ 0.7 keV, while the most extended thermal emission is cooler (kT ∼ 0.3 keV). We refine previous measurements of the extreme Fe Kα equivalent width in this source (EW=2.5{sub −1.0}{sup +2.6} keV), which suggests the central engine is completely embedded within Compton-thick levels of obscuration. Using physically motivated models fit to the Chandra spectrum, we derive a Compton-thick column density (N{sub H} > 1.25 × 10{sup 24} cm{sup −2}) and an intrinsic hard (2–10 keV) X-ray luminosity of ∼3–8 × 10{sup 42} erg s{sup −1} (depending on the presumed geometry of the obscurer), which is over two orders of magnitude larger than that observed. The large Fe Kα EW suggests a spherical covering geometry, which could be confirmed with X-ray measurements above 10 keV. NGC 4968 is similar to other active galaxies that exhibit extreme Fe Kα EWs (i.e., >2 keV) in that they also contain on-going star formation. This work supports the idea that gas associated with nuclear star formation may increase the covering factor of the enshrouding gas and play a role in obscuring active galactic nuclei.

79 ASTRONOMY AND ASTROPHYSICS↗

NASA NTRS2012

Comparison of PARASOL Observations with Polarized Reflectances Simulated Using Different Ice Habit Mixtures

Insufficient knowledge of the habit distribution and the degree of surface roughness of ice crystals within ice clouds is a source of uncertainty in the forward light scattering and radiative transfer simulations required in downstream applications involving these clouds. The widely used MODerate Resolution Imaging Spectroradiometer (MODIS) Collection 5 ice microphysical model assumes a mixture of various ice crystal shapes with smooth-facets except aggregates of columns for which a moderately rough condition is assumed. When compared with PARASOL (Polarization and Anisotropy of Reflectances for Atmospheric Sciences coupled with Observations from a Lidar) polarized reflection data, simulations of polarized reflectance using smooth particles show a poor fit to the measurements, whereas very rough-faceted particles provide an improved fit to the polarized reflectance. In this study a new microphysical model based on a mixture of 9 different ice crystal habits with severely roughened facets is developed. Simulated polarized reflectance using the new ice habit distribution is calculated using a vector adding-doubling radiative transfer model, and the simulations closely agree with the polarized reflectance observed by PARASOL. The new general habit mixture is also tested using a spherical albedo differences analysis, and surface roughening is found to improve the consistency of multi-angular observations. It is suggested that an ice model incorporating an ensemble of different habits with severely roughened surfaces would potentially be an adequate choice for global ice cloud retrievals.

Cole, Benjamin H.↗

DOE OSTI2018

First-principles study of the binding energy between nanostructures and its scaling with system size

The equilibrium van der Waals binding energy is an important factor in the design of materials and devices. However, it presents great computational challenges for materials built up from nanostructures. Here we investigate the binding-energy scaling behavior from first-principles calculations. We show that the equilibrium binding energy per atom between identical nanostructures can scale up or down with nanostructure size, but can be parametrized for large $\mathscr{N}$ with an analytical formula (in meV/atom), $E_b\mathscr{N} = a + b/\mathscr{N} + c/\mathscr{N}^2 + d/\mathscr{N}^3$ where $\mathscr{N}$ is the number of atoms in a nanostructure and $a, b, c$ and $d$ are fitting parameters, depending on the properties of a nanostructure. The formula is consistent with a finite large-size limit of binding energy per atom. We find that there are two competing factors in the determination of the binding energy: Nonadditivities of van der Waals coefficients and center-to-center distance between nanostructures. To decode the detail, the nonadditivity of the static multipole polarizability is investigated from an accurate spherical-shell model. Finally, we find that the higher-order multipole polarizability displays ultrastrong intrinsic nonadditivity, no matter if the dipole polarizability is additive or not.

77 NANOSCIENCE AND NANOTECHNOLOGY↗

NASA NTRS1995

Photochemical modeling of Titan's atmosphere

We have developed a new photochemical model of Titan's atmosphere which includes all the important compounds and reactions in spherical geometry from the surface to 1240 km. Compared to the previous model of Yung et al. (1984), the most significant recent change in the reactions used is the updated methane dissociation scheme (Mordaunt et al. 1993). Moreover, the transfer of the solar radiation in the atmosphere and the photolysis rates have been calculated by using a Monte Carlo code. Finally, the eddy diffusion coefficient profile is adjusted in order to fit the mean vertical distribution of HCN retrieved from millimeter groundbased observations of Tanguy et al. (1990); using new values for the boundary flux of atomic nitrogen (Strobel et al. 1992). We have run the model in both steady-state and diurnal modes, with 62 species involved in 249 reactions. There is little difference between diurnal and steady-state results. Overall our results are in a closer agreement with the abundances inferred from the Voyager infrared measurements at the equator than the Yung et al. results. We find that the catalytic scheme for H recombination invoked by Yung et al. only slightly improves the model results and we conclude that this scheme is not essential to fit observations.

Toublanc, D.↗

NASA NTRS2004

A Geomagnetic Estimate of Mean Paleointensity

To test a statistical hypothesis about Earth's magnetic field against paleomagnetism, the present field is used to estimate time averaged paleointensity. The estimate used the modern magnetic multipole spectrum R(n), which gives the mean square induction represented by spherical harmonics of degree n averaged over the sphere of radius a = 6371.2 km. The hypothesis asserts that low degree multi-pole powers of the coresource field are distributed as chi-squared with 2n+1 degrees of freedom and expectation values, where c is the 3480 km radius of the Earth's core. (This is compatible with a usually mainly geocentric axial dipolar field). Amplitude K is estimated by fitting theoretical to observational spectra through degree 12. The resulting calibrated expectation spectrum is summed through degree 12 to estimate expected square intensity F(exp 2). The sum also estimates F(exp 2) averaged over geologic time, in so far as the present magnetic spectrum is a fair sample of that generated in the past by core geodynamic processes. Additional information is included in the original extended abstract.

Voorhies, Coerte V.↗

NASA NTRS1990

An evolving relativistic jet model for the BL Lacertae object Markarian 421

Coordinated observations of the nearby BL Lacertae object Mrk 421 obtained during May 1980, January 1984 and March 1984, are described. These observations give a time-frozen picture of the continuous spectrum of Mrk 421 at X-ray, ultraviolet, optical, and radio wavelengths. The observed spectra have been fitted to an inhomogeneous relativistic jet model described by Hutter and Mufson. The studies show that it is difficult to describe the data by a single jet whose properties vary as a function of time in a self-consistent manner unless the flat spectrum, hard X-ray tail present in January 1984 is due to an additional short-lived component of emission. These jet models are compared with homogeneous, spherically symmetric, synchrotron self-Compton models for this source.

Mufson, S. L.↗

NASA NTRS2013

Fast, Large-Area, Wide-Bandgap UV Photodetector for Cherenkov Light Detection

Due to limited resources available for power and space for payloads, miniaturizing and integrating instrumentation is a high priority for addressing the challenges of manned and unmanned deep space missions to high Earth orbit (HEO), near Earth objects (NEOs), Lunar and Martian orbits and surfaces, and outer planetary systems, as well as improvements to high-altitude aircraft safety. New, robust, and compact detectors allow future instrumentation packages more options in satisfying specific mission goals. A solid-state ultraviolet (UV) detector was developed with a theoretical fast response time and large detection area intended for application to Cherenkov detectors. The detector is based on the wide-bandgap semiconductor zinc oxide (ZnO), which in a bridge circuit can detect small, fast pulses of UV light like those required for Cherenkov detectors. The goal is to replace the role of photomultiplier tubes in Cherenkov detectors with these solid-state devices, saving on size, weight, and required power. For improving detection geometry, a spherical detector to measure high atomic number and energy (HZE) ions from any direction has been patented as part of a larger space radiation detector system. The detector will require the development of solid-state UV photodetectors fast enough (2 ns response time or better) to detect the shockwave of Cherenkov light emitted as the ions pass through a quartz, sapphire, or acrylic ball. The detector must be small enough to fit in the detector system structure, but have an active area large enough to capture enough Cherenkov light from the sphere. The detector is fabricated on bulk single-crystal undoped ZnO. Inter - digitated finger electrodes and contact pads are patterned via photolithography, and formed by sputtered metal of silver, platinum, or other high-conductivity metal.

Wrbanek, John D.↗

DOE OSTI2017

A novel electron mirror pulse compressor

An electron mirror-based pulse compressor design has been developed for improving the temporal resolution of dynamic/ultrafast transmission electron microscopes and ultrafast electron diffraction cameras. The improvement will enable electron microscopes and diffraction cameras to better resolve the dynamics of reactions in the areas of solid state physics, chemistry, and biology. The design utilizes a combination of mirror optics and a magnetic beam separator, which exploits the symmetry inherent in reversing the electron trajectory in the mirror in order to compress the pulse. This system can also simultaneously correct the spherical and chromatic aberration of the objective lens for improved spatial resolution. For pulsed experiments with a practical bunch charge, the correction of the chromatic aberration coefficient counters the spread in the electron energies induced by the space charge of the pulse to make possible the probing of the sample with high spatial resolution. The pulse compressor can accommodate pulses with a range of electron densities and energy spreads. Additionally, it is designed to fit into both ultrafast electron diffraction cameras and dynamic/ultrafast transmission electron microscopes. Consequently, this instrument is suitable for enhancing the study of the structure, composition, and bonding states of new materials at ultrafast time scales.

47 OTHER INSTRUMENTATION↗

NASA NTRS1973

Mars gravity derived from the long-period motion of Mariner 9

The spherical harmonic coefficients in the Mars gravity field are determined using a two-stage filtering process. In the first stage, a weighted least-squares orbit determination processor is applied to Mariner 9 radio data to estimate the mean Kepler elements of each orbit. A finite difference method is then used to generate orbital element rates. In the second stage, the gravity coefficients are determined using another weighted least-squares processor which fits the long period Lagrange perturbation equations to the estimated rates. The resulting gravity potential and associated equipotential surface is presented and error sources discussed. Gravity field results are applied to various physical properties of Mars, such as moments of inertia.-

Ferrari, A. J.↗

NASA NTRS2018

Blunt-Body Paradox and Improved Application of Transient-Growth Framework

The “Reshotko-Tumin transition criterion" based on optimal transient growth successfully correlates laboratory measurements of roughness induced transition over blunt body configurations. Even though transient growth has not been conclusively linked to the measured onset of transition, the above correlation denotes the only available physics-based model for subcritical transition in blunt body flows, since the latter do not support any modal instabilities at typical experimental conditions. Unlike other established models based on empirical curve fits that are valid for a specific subclass of datasets, the optimal-growth-based transition criterion appears to provide a reasonable correlation with measurements in various wind tunnel and ballistic range facilities and for a broad range of surface temperature ratios. This paper is focused on optimal growth calculations that improve upon significant shortcomings of the computations underlying the Reshotko-Tumin correlation. The improved framework is applied to leeward transition over a spherical section forebody that was tested in the Mach 6 Adjustable Contour Expansion wind tunnel at Texas A&M University. The computed results highlight the significance of nonparallel basic state evolution, curvature terms, and an optimization procedure that varies both inflow and outflow locations of the transient growth interval. More important, the results indicate that the modified correlation is very close to its original form, and hence, that the accuracy of the transient-growth-based transition criterion is not compromised by using a more thorough theoretical framework. Yet the results also show that the optimal energy gain up to the predicted transition onset location can be rather small, highlighting the need to further investigate the optimal growth criterion for additional experimental configurations and to also uncover the in-depth physics underlying blunt body transition.

Freestream conditions↗

NASA NTRS1994

A family of models for spherical stellar systems

We describe a one-parameter family of models of stable sperical stellar systems in which the phase-space distribution function depends only on energy. The models have similar density profiles in their outer parts (rho propotional to r(exp -4)) and central power-law density cusps, rho proportional to r(exp 3-eta), 0 less than eta less than or = 3. The family contains the Jaffe (1983) and Hernquist (1990) models as special cases. We evaluate the surface brightness profile, the line-of-sight velocity dispersion profile, and the distribution function, and discuss analogs of King's core-fitting formula for determining mass-to-light ratio. We also generalize the models to a two-parameter family, in which the galaxy contains a central black hole; the second parameter is the mass of the black hole. Our models can be used to estimate the detectability of central black holes and the velocity-dispersion profiles of galaxies that contain central cusps, with or without a central black hole.

Tremaine, Scott↗

NASA NTRS1979

The magnetic field of Mercury

Data from Mariner 10 observations of Mercury indicate that there exists an intrinsic magnetic field of the planet, sufficiently strong at present to deflect the solar wind flow around the planet and to form a detached bow shock wave in the super Alfvenic solar wind. Four methods used to analyze the magnetic field data and derive quantitative values for the description of the planetary field include (1) comparison of bow shock and magnetopause relative positions at Mercury to those at Earth; (2) direct spherical harmonic analysis of the data; (3) modeling of the magnetosphere by an image dipole and infinite 2-D current sheet in addition to the planetary field; and (4) scaling of a mathematical model for the terrestrial magnetosphere. The results obtained yield dipole moments ranging from 2.4 to 5.1x10 to the 22d power, with the lower values associated cw cm with certain models using partial quadrupole and octupole terms to improve the least squares fitting of models to observations.

Ness, N. F.↗

NASA NTRS2017

Chandra Reveals Heavy Obscuration and Circumnuclear Star Formation in Seyfert 2 Galaxy NGC 4968

We present the Chandra imaging and spectral analysis of NGC 4968, a nearby (z = 0.00986) Seyfert 2 galaxy. We discover extended (approx. 1 kpc) X-ray emission in the soft band (0.5-2 keV) that is neither coincident with the narrow line region nor the extended radio emission. Based on spectral modeling, it is linked to on-going star formation [approx. 2.6-4 Mass compared to Earth yr(exp.− 1)]. The soft emission at circumnuclear scales (inner approx. 400 pc) originates from hot gas, with kT approx. 0.7 keV, while the most extended thermal emission is cooler (kT approx. 0.3 keV). We refine previous measurements of the extreme Fe K alpha equivalent width in this source (EW 2.5 + 2.6/-1.0 keV), which suggests the central engine is completely embedded within Compton-thick levels of obscuration. Using physically motivated models fit to the Chandra spectrum, we derive a Compton-thick column density [N(sub H) is greater than 1.25× 10(exp 24) cm(exp.− 2)] and an intrinsic hard (2-10 keV) X-ray luminosity of approx. 3-8× 10(exp. 42) erg s(exp. − 1) (depending on the presumed geometry of the obscurer), which is over two orders of magnitude larger than that observed. The large Fe K Alpha EW suggests a spherical covering geometry, which could be confirmed with X-ray measurements above 10 keV. NGC 4968 is similar to other active galaxies that exhibit extreme Fe K Alpha EWs (i.e., greater than 2 keV) in that they also contain on-going star formation. This work supports the idea that gas associated with nuclear star formation may increase the covering factor of the enshrouding gas and play a role in obscuring active galactic nuclei.

LaMassa, Stephanie M.↗

DOE OSTI2022

Development of an x-ray radiography platform to study laser-direct-drive energy coupling at the National Ignition Facility

A platform has been developed to study laser-direct-drive energy coupling at the National Ignition Facility (NIF) using a plastic sphere target irradiated in a polar-direct-drive geometry to launch a spherically converging shock wave. To diagnose this system evolution, eight NIF laser beams are directed onto a curved Cu foil to generate Heα line emission at a photon energy of 8.4 keV. These x rays are collected by a 100-ps gated x-ray imager in the opposing port to produce temporally gated radiographs. The platform is capable of acquiring images during and after the laser drive launches the shock wave. A backlighter profile is fit to the radiographs, and the resulting transmission images are Abel inverted to infer radial density profiles of the shock front and to track its temporal evolution. Here, the measurements provide experimental shock trajectories and radial density profiles that are compared to 2D radiation-hydrodynamic simulations using cross-beam energy transfer and nonlocal heat-transport models.

47 OTHER INSTRUMENTATION↗

DOE OSTI2016

Re-acceleration model for radio relics with spectral curvature

Most of the observed features of radio gischt relics, such as spectral steepening across the relic width and a power-law-like integrated spectrum, can be adequately explained by a diffusive shock acceleration (DSA) model in which relativistic electrons are (re-)accelerated at shock waves induced in the intracluster medium. However, the steep spectral curvature in the integrated spectrum above ∼2 GHz detected in some radio relics, such as the Sausage relic in cluster CIZA J2242.8+5301, may not be interpreted by the simple radiative cooling of postshock electrons. In order to understand such steepening, we consider here a model in which a spherical shock sweeps through and then exits out of a finite-size cloud with fossil relativistic electrons. The ensuing integrated radio spectrum is expected to steepen much more than predicted for aging postshock electrons, since the re-acceleration stops after the cloud-crossing time. Using DSA simulations that are intended to reproduce radio observations of the Sausage relic, we show that both the integrated radio spectrum and the surface brightness profile can be fitted reasonably well, if a shock of speed u{sub s} ∼ 2.5–2.8 × 10{sup 3} km s{sup −1} and a sonic Mach number M{sub s} ∼ 2.7–3.0 traverses a fossil cloud for ∼45 Myr, and the postshock electrons cool further for another ∼10 Myr. This attempt illustrates that steep curved spectra of some radio gischt relics could be modeled by adjusting the shape of the fossil electron spectrum and adopting the specific configuration of the fossil cloud.

79 ASTRONOMY AND ASTROPHYSICS↗

NASA NTRS1973

Mars gravity derived from long-period motion of Mariner 9

The spherical harmonic coefficients in the gravity field of Mars are determined using a two stage filtering process. In the first stage, a weighted least squares orbit determination processor is applied to Mariner 9 Doppler tracking data to estimate Kepler elements and rates. In the second stage, the Kepler element rates are used as input to another weighted least squares processor that estimates gravity coefficients using the Lagrange perturbation equations. The novelty in this method is the fitting of element rates rather than the elements themselves. This procedure decouples the results from the effects of internal spacecraft dynamics.

Ferrari, A. J.↗

NASA NTRS1975

The Bulk Lunar Electrical Conductivity

The electrical conductivity structure was studied of a spherically layered moon consistent with the very low frequency magnetic data collected on the lunar surface and by Explorer 35. In order to obtain good agreement with the lunar surface magnetometer observations, the inclusion of a void cavity behind the moon requires a conductivity at shallow depths higher than that of models having the solar wind impinging on all sides. By varying only the source parameters, a conductivity model can be found that yields a good fit to both the tangential response upstream and the radial response downstream. This model also satisfies the dark side tangential response in the frequency range above 0.006 Hz, but the few data points presently available below this range do not seem to agree with the theory.

Leavy, Donald Lucien↗

NASA NTRS1992

Time-dependent mapping of the magnetic field at the core-mantle boundary

We consider the problem of constructing a time-dependent map of the magnetic field at the core-mantle boundary. We use almost all the available data from the last 300 years to produce two maps, one for the period 1690-1840 and the other for 1840-1990. We represent the spatial dependency of the field using spherical harmonics, the time dependency using a cubic B-spline basis, and seek the smoothest solutions compatible with the observations. We argue that, for observations from permanent magnetic observatories, the most efficient strategy is to use the first differences of annual means; for satellite data, the most efficient strategy is simply to limit the number of data used so as to minimize any tendency to map the crustal field into the core field. The resulting model fits the observatory data better than any previous model. The resulting time-dependent field map exhibits much of the same structure in the field and its secular variation identified in earlier studies.

Bloxham, Jeremy↗