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NASA NTRS2005

More on Magnetic Spectra from Correlated Crustal Sources on Mars

The spectral method for distinguishing crustal from core-source magnetic fields has been re-examined, modified and applied to both a comprehensive geomagnetic field model and an altitude normalized magnetic map of Mars. These observational spectra are fairly fitted by theoretical forms expected from certain elementary classes of magnetic sources. For Earth we found fields from a core of radius 3512 plus or minus 64 km, in accord with the 3480 km seismologic radius, and a crust represented by a shell of random dipolar sources at radius 6367 plus or minus 14 km, just beneath the 6371.0 km mean radius. For Mars we found only a field from a crust represented in same way, but 46 plus or minus 10 km below the planetary mean radius of 3389.5 km, and with sources about 9.6 plus or minus 3.2 times stronger than Earth's. It is remarkable that the same simple theoretical form should fairly fit crustal magnetic spectra for both worlds and return crustal-source depth estimates of plausible magnitude. Evidently, the idea of an ensemble of compact, quasi-independent, magnetized regions within these planetary crusts has some merit. Yet such estimates, at best a kind of average, depend upon both the observational spectrum fitted and the physical basis of the theoretical spectrum.

Voorhies, C. V.↗

NASA NTRS2015

A Chandrasekhar Mass Progenitor for the Type Ia Supernova Remnant 3C 397 from the Enhanced Abundances of Nickel and Manganese

Despite decades of intense efforts, many fundamental aspects of Type Ia supernovae (SNe Ia) remain elusive. One of the major open questions is whether the mass of an exploding white dwarf (WD) is close to the Chandrasekhar limit. Here, we report the detection of strong K-shell emission from stable Fe-peak elements in the Suzaku X-ray spectrum of the Type Ia supernova remnant (SNR) 3C 397. The high Ni/Fe and Mn/Fe mass ratios - (0.11-0.24 and 0.018-0.033, respectively) in the hot plasma component that dominates the K-shell emission lines indicate a degree of neutronization in the supernova ejecta that can only be achieved by electron capture in the dense cores of exploding WDs with a near-Chandrasekhar mass. This suggests a single-degenerate origin for 3C 397, since Chandrasekhar mass progenitors are expected naturally if the WD accretes mass slowly from a companion. Together with other results supporting the double-degenerate scenario, our work adds to the mounting evidence that both progenitor channels make a significant contribution to the SN Ia rate in star-forming galaxies.

G41.1-0.3); supernova remnants; nuclear reactions;↗

NASA NTRS1980

Convective penetrations and observable consequences of helium shell flashes in 3 and 6 solar-mass red supergiants

The results of 61 detailed numerical flash calculations are examined, with particular reference to 3 and 6 solar-mass stars with core masses ranging from 0.56 to 0.60 the solar mass and 0.94 to 0.95 the solar mass, respectively. First penetration of the convective envelope into pockets of carbon in the intershell region was found for the 15th flash for the 6 solar-mass case, with continuing deeper penetration for the flashes thereafter. However, no such type of envelope penetration could be found for the 3 solar-mass case.

Sackmann, I.-J.↗

NASA NTRS1994

The determination of the core mass at the helium flash in globular cluster stars

Evolutionary sequences for the red giant branch (RGB) phase of a representative globular cluster star have been computed in order to investigate the reliability of the current canonical values of the core mass M(sub c) at the helium flash. These computations were motivated by recent suggestions that the canonical values of M(sub c) may be systematically too small due to the numerical algorithms for shifting the hydrogen shell and advancing the chemical composition during the RGB phase. Our results show that these algorithms do not, in fact, introduce a significant error in the values of M(sub c). Moreover, we demonstrate that a procedure for advancing the chemical composition which is implicit only in the hydrogen abundance will systematically underestimate the amount of hydrogen fuel consumption between RGB models and therefore should not be used in RGB computations. Overall we estimate the uncertainty in the core masses of Sweigart & Gross (1978) due to numerical effects to be approximately equals 0.003 solar mass. From a consideration of the available canonical models we conclude that a change in the canonical values of M(sub c) by a few 10(exp -2) solar mass would require either a substantial change in the canonical input physics or some noncanonical effect such as rotation. Finally our models show that the use of short time steps can significantly increase the extent of the inner tail of the hydrogen shell. This effect may enhance the likelihood of hydrogen mixing following a helium shell flash in an asymptotic giant branch star.

Sweigart, Allen V.↗

NASA NTRS1988

Dynamical effects on the core-mantle boundary from depth-dependent thermodynamical properties of the lower mantle

A common assumption in modeling dynamical processes in the lower mantle is that both the thermal expansivity and thermal conductivity are reasonably constant. Recent work from seismic equation of state leads to substantially higher values for the thermal conductivity and much lower thermal expansivity values in the deep mantle. The dynamical consequences of incorporating depth-dependent thermodynamic properties on the thermal-mechanical state of the lower mantle are examined with the spherical-shell mean-field equations. It is found that the thermal structure of the seismically resolved anomalous zone at the base of the mantle is strongly influenced by these variable properties and, in particular, that the convective distortion of the core-mantle boundary (CMB) is reduced with the decreasing thermal expansivity. Such a reduction of the dynamically induced topography from pure thermal convection would suggest that some other dynamical mechanism must be operating at the CMB.

Zhang, Shuxia↗

NASA NTRS1991

The energies and structure of X-ray K-absorption edges

Improved screening parameters are employed to compute the K threshold energies E(K) and photoelectric cross sections sigma(K) for the abundant atomic species of the interstellar gas. The E(K)-values for neutral C, N, and O differ from the values currently being employed in X-ray astronomy by several percent. The values are computed in an effective Coulomb field formulation, introducing an 'effective Coulomb' threshold, and convenient formulas are given for the extrapolation of the results to lower energies in the neighborhood of the real threshold energy. Within about 30-200 eV above threshold, the corrected cross sections display structure associated with direct secondary excitation of outer-shell electrons into the continuum. A unified description of this feature of the cross section is introduced, in terms of the secondary threshold energies and core relaxation factors. Both of these factors are computed with the improved screening parameters associated with the one-electron atomic states of the many-electron systems.

Gould, Robert J.↗

NASA NTRS2000

Residual Strength Analyses of Riveted Lap-Splice Joints

The objective of this paper was to analyze the crack-linkup behavior in riveted-stiffened lap-splice joint panels with small multiple-site damage (MSD) cracks at several adjacent rivet holes. Analyses are based on the STAGS (STructural Analysis of General Shells) code with the critical crack-tip-opening angle (CTOA) fracture criterion. To account for high constraint around a crack front, the "plane strain core" option in STAGS was used. The importance of modeling rivet flexibility with fastener elements that accurately model load transfer across the joint is discussed. Fastener holes are not modeled but rivet connectivity is accounted for by attaching rivets to the sheet on one side of the cracks that simulated both the rivet diameter and MSD cracks. Residual strength analyses made on 2024-T3 alloy (1.6-mm thick) riveted-lap-splice joints with a lead crack and various size MSD cracks were compared with test data from Boeing Airplane Company. Analyses were conducted for both restrained and unrestrained buckling conditions. Comparison of results from these analyses and results from lap-splice-joint test panels, which were partially restrained against buckling indicate that the test results were bounded by the failure loads predicted by the analyses with restrained and unrestrained conditions.

Seshadri, B. R.↗

NASA NTRS1992

Theoretical investigation of gas-surface interactions

The investigation into the appearance of intruder states from the negative continuum when some of the two-electron integrals were omitted was completed. The work shows that, provided all integrals involving core contracted functions in an atomic general contraction are included, or that the core functions are radially localized, meaningful results are obtained and intruder states do not appear. In the area of program development, the Dirac-Hartree-Fock (DHF) program for closed-shell polyatomic molecules was extended to permit Kramers-restricted open-shell DHF calculations with one electron in an open shell or one hole in a closed shell, or state-averaged DHF calculations over several particle or hole doublet states. One application of the open-shell code was to the KO molecule. Another major area of program development is the transformation of integrals from the scalar basis in which they are generated to the 2-spinor basis employed in parts of the DHF program, and hence to supermatrix form. Particularly concerning the omission of small component integrals, and with increase in availability of disk space, it is now possible to consider transforming the integrals. The use of ordered integrals, either in the scalar basis or in the 2-spinor basis, would considerably speed up the construction of the Fock matrix, and even more so if supermatrices were constructed. A considerable amount of effort was spent on analyzing the integral ordering and tranformation for the DHF program. The work of assessing the reliability of the relativistic effective core potentials (RECPs) was continued with calculation of the group IV monoxides. The perturbation of the metal atom provided by oxygen is expected to be larger than that provided by hydrogen and thus provide a better test of the qualification of the RECPs. Calculations on some platinum hydrides were carried out at nonrelativistic (NR), perturbation theory (PT) and DHF levels. Reprints of four papers describing this work are included.

Dyall, Kenneth G.↗

NASA NTRS1992

The evolution of high-metallicity horizontal-branch stars and the origin of the ultraviolet light in elliptical galaxies

Evolutionary calculations of high-metallicity horizontal-branch stars show that for the relevant masses and helium abundances, post-HB evolution in the HR diagram does not proceed toward and along the AGB, but rather toward a 'slow blue phase' in the vicinity of the helium-burning main sequence, following the extinction of the hydrogen shell energy source. For solar and twice solar metallicity, the blue phase begins during the helium shell-burning phase (in agreement with the work of Brocato and Castellani and Tornambe); for 3 times solar metallicity, it begins earlier, during the helium core-burning phase. This behavior differs from what takes place at lower metallicities. The implications for high-metallicity old stellar populations in the Galactic bulge and for the integrated colors of elliptical galaxies are discussed.

Horch, E.↗

NASA NTRS1991

Interlaminar stress analysis at the skin-stiffener interface of a grid-stiffened composite panel

Results are presented of an analysis of the interlaminar stress components at the skin-wrap, skin-core, and wrap-core interfaces for an advanced-concept stiffened panel using a global/local finite element procedure. The procedure consists of a global model of two-dimensional shell elements that is used to design a grid-stiffened panel with blade-type stiffeners, a local model of three-dimensional solid elements that is used to compute interlaminar stress components, and a scheme devised to assign displacement boundary conditions for the local model that are derived from displacement and rotation data of a few nodes of the global model. The interlaminar stresses for this panel design were found to be well below typical tensile normal and shearing strengths of a graphite-epoxy material.

Wiggenraad, J. F. M.↗

NASA NTRS1994

Galactic stars applied to tests of the criterion for convection and semiconvection in an inhomogeneous star

The question of the effective criterion for convection and semiconvection to use in the inhomogeneous layers of models of massive stars is addressed here in terms of a choice between the Schwarzschild (temperature-gradient) criterion and the Ledoux (density-gradient) criterion. These two extremes give rise to and suppress, respectively, a fully convective zone (FCZ) in the layers immediately above the hydrogen-burning shell, if the star's mass is neither too low nor too high. Specifically, the existence of a large FCZ delays or prevents the star during core helium burning from becoming a red supergiant. The applicable range of initial stellar masses for a solar metallicity is approximately 13 to 30 solar mass. Nine different tests for the effective criterion for convection and semiconvection in Galactic supergiants, as well as a test using SN 1987A of the Large Magellanic Cloud, are performed, or reexamined, by using both old and new theoretical data, while three of the Galactic tests are repeated for association supergiants separately from cluster supergiants. Although eight of the 13 tests yield inconclusive results, five do support the Ledoux criterion, and three do so strongly.

Stothers, Richard B.↗

NASA NTRS1994

Search for important weak interaction nuclei in presupernova evolution

A search is made for the most important electron captures and beta-decays after core silicon burning in massive stars. A nuclear statistical equilibrium code is used to compute isotopic abundances. Electron capture and beta-decay rates are estimated for the 150 most abundant isotopes in a simplified fashion which generally includes the strongest transitions. These estimates are made for nuclei in the fp-shell and use techniques similar to Fuller, Fowler, & Newman (1982a), and are compared to them. The general behavior of the rate of change of Y(sub e) is examined. These methods are then used to follow a typical stellar trajectory, seeking the most important weak interactions during the formation of the iron core. Ranked lists of nuclei are given, to prioritize more detailed studies on individual nuclei. Beta-decays are found to ben an important modification to the evolution below the rate of change of Y(sub e) approx. = 0.46 as the core approaches a state of dynamic equilibrium between electron captures and beta-decays.

Aufderheide, Maurice B.↗

NASA NTRS2003

The Effect of Surface Topography on the Nonlinear Dynamics of Rossby Waves

Boussinesq convection in rotating systems attracts a sustained attention of the fluid dynamics community, because it has intricate non-linear dynamics (Cross & Hohenberg 1993) and plays an important role in geophysical and astrophysical applications, such as the motion of the liquid outer core of Earth, the Red Spot in Jupiter, the giant cells in the Sun etc. (Alridge et al. 1990). A fundamental distinction between the real geo- and astrophysical problems and the idealized laboratory studies is that natural systems are inhomogeneous (Alridge et al. 1990). Heterogeneities modulate the flow and influence significantly the dynamics of convective patterns (Alridge et al. 1990; Hide 1971). The effect of modulations on pattern formation and transition to turbulence in Boussinesq convection is far from being completely understood (Cross & Hohenberg 1993; Aranson & Kramer 2002). It is generally accepted that in the liquid outer core of the Earth the transport of the angular momentum and internal heat occurs via thermal Rossby waves (Zhang et al. 2001; Kuang & Bloxham 1999). These waves been visualized in laboratory experiments in rotating liquid-filled spheres and concentric spherical shells (Zhang et al. 2001; Kuang & Bloxham 1999). The basic dynamical features of Rossby waves have been reproduced in a cylindrical annulus, a system much simpler than the spherical ones (Busse & Or 1986; Or & Busse 1987). For convection in a cylindrical annulus, the fluid motion is two-dimensional, and gravity is replaced by a centrifugal force, (Busse & Or 1986; Or & Busse 1987). Hide (1971) has suggested that the momentum and heat transport in the core might be influenced significantly by so-called bumps, which are heterogeneities on the mantle-core boundary. To model the effect of surface topography on the transport of momentum and energy in the liquid outer core of the Earth, Bell & Soward (1996), Herrmann & Busse (1998) and Westerburg & Busse (2001) have studied the nonlinear dynamics of thermal Rossby waves in a cylindrical annulus with azimuthally modulated height.

Abarzhi, S. I.↗

NASA NTRS2023

Ventilation Heat Exchanger / Flow Meter for xPLSS

The flow meter / heat exchanger (FMHX) in the ventilation loop of the exploration EMU cools the ventilation gas and measures the ventilation flow rate. The heat exchanger transfers heat from the ventilation gas to the thermal control loop via a miniature shell-and-tube heat exchanger. The flow meters calculate the flow rate of gas through the ventilation loop based on the pressure drop across the heat exchanger core. Creare has delivered four design validation test (DVT) heat exchangers and five DVT flow meters to NASA JSC to support development of the exploration portable life support system (xPLSS). This paper describes the design and performance of the DVT units.The heat exchangers are designed to cool the ventilation gas to a specified temperature with low pressure losses under the most challenging operating conditions. The measured performance of the DVT heat exchangers agrees well with design models and meets all performance requirements. The flow meters use a MEMS thermal flow sensor to produce a signal that is proportional to a small bypass flow around the heat exchanger core. They are designed to achieve high measurement accuracy across the full range of xPLSS operating conditions. We calibrated the flow meters in a special-purpose flow facility that simulates operation in the xPLSS ventilation loop. Calibration testing shows that DVT flow meters produce digital output for vent loop mass flow that meets NASA’s accuracy requirements across the range of xPLSS operating conditions. This paper reviews the design of the heat exchangers and flow meters and presents data from the final flow meter calibration testing, heat exchanger performance validation, and initial ground testing in NASA’s xPLSS.

spacesuit↗

NASA NTRS2023

Ventilation Heat Exchanger / Flow Meter for xPLSS

spacesuit↗

NASA NTRS2020

Design and Analysis of Buckling-Critical Large-Scale Sandwich Composite Cylindrical Test Articles

It has long been established in the literature that the buckling response of thin-shell structures can be very sensitive to the presence of small geometric and loading imperfections. The Shell Buckling Knockdown Factor Project (SBKF) was established by the NASA Engineering and Safety Center (NESC) to develop analysis-based shell buckling design recommendations for stiffened-metallic and composite launch-vehicle shell structures. Large-scale buckling tests were used to validate the modeling and analysis methods applied in developing these analysis-based recommendations. Herein, the test article design methodology for 8-ft-diameter, honeycomb-core sandwich composite cylinder validation tests is discussed and cylinder designs are presented. In this methodology, first, the sandwich composite design space was defined using several nondimensional parameters, and the desired test article design space was determined by examining the designs of launch-vehicle cylinder structures. Essentially all test article designs within certain design parameters were generated and then downselected based on simple closed-form failure calculations and the nondimensional design-space parameters. Four of these designs that spanned a significant portion of the design space of interest and had global buckling as the first predicted failure mode were selected and subjected to higher-fidelity finite element analyses (FEAs): shell-element-based analyses, axisymmetric-element-based analyses, and global-local analyses. The analysis flow discussed in this report supported the design objective. As the analysis flow progressed, designs were downselected so the fidelity of the analysis methods, and consequently their computational cost and accuracy, was increased. The selection of the FEA types created an analysis framework where particular methods complemented each other and reduced the uncertainty of the predicted test article responses. The analysis results are illustrated using several designs when the computationally expeditious closed-form analysis stage is discussed. Once this stage is complete, the higher-fidelity FEA types are illustrated using one selected detailed test article design. Both perfect and imperfect test article geometries were considered.

Buckling↗

NASA NTRS1999

Shape Optimization of Cylindrical Shell for Interior Noise

In this paper an analytic method is used to solve for the cross spectral density of the interior acoustic response of a cylinder with nonuniform thickness subjected to turbulent boundary layer excitation. The cylinder is of honeycomb core construction with the thickness of the core material expressed as a cosine series in the circumferential direction. The coefficients of this series are used as the design variable in the optimization study. The objective function is the space and frequency averaged acoustic response. Results confirm the presence of multiple local minima as previously reported and demonstrate the potential for modest noise reduction.

Robinson, Jay H.↗

NASA NTRS1976

The evolution of rotating stars. 1: Method and exploratory calculations for a 7 solar mass star

A method was developed which allows us to study the evolution of rotating stars beyond the main sequence stage. Four different cases of redistribution of angular momentum in an evolving star are considered. Evolutionary sequences for a 7 solar mass star, rotating according to these different cases, were computed from the ZAMS to the double shell source stage. Each sequence was begun with a (typical) equatorial velocity of 210 km/sec. On the main sequence, the effects of rotation are of minor importance. As the core contracts during later stages, important effects arise in all physically plausible cases. The outer regions of the cores approach critical velocities and develop unstable angular velocity distributions. The effects of these instabilities should significantly alter the subsequent evolution.

Endal, A. S.↗