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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 163 records · Page 9
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NASA NTRS1992

Theoretical study of the BeLi, BeNa, MgLi, MgNa, and AlBe molecules and their negative ions

The alkaline earth-alkali diatomics are found to have weak bonds, because the diffuse alkali valence s orbitals cannot form a bond of sufficient strength to pay the promotion energy of the alkaline-earth atoms. This leads to van der Waals bonding in the neutrals as well as the negative ions. In fact, the negative ions have larger binding energies than the neutrals as a result of the much larger polarizability of the negative ion. The binding energy of AlBe is significantly larger than the Be-alkali molecules, due to a covalent contribution to the bonding. The binding energy in AlBe(-) is considerably larger than AlBe; the binding energy of the X 3Sigma(-) state of AlBe(-) is computed to be 1.36 eV, as compared with 0.57 eV for the X 2Pi state of AlBe.

Bauschlicher, Charles W., Jr.↗
NASA NTRS1991

Resonant frequency calculations using a hybrid perturbation-Galerkin technique

A two-step hybrid perturbation Galerkin technique is applied to the problem of determining the resonant frequencies of one or several degrees of freedom nonlinear systems involving a parameter. In one step, the Lindstedt-Poincare method is used to determine perturbation solutions which are formally valid about one or more special values of the parameter (e.g., for large or small values of the parameter). In step two, a subset of the perturbation coordinate functions determined in step one is used in Galerkin type approximation. The technique is illustrated for several one degree of freedom systems, including the Duffing and van der Pol oscillators, as well as for the compound pendulum. For all of the examples considered, it is shown that the frequencies obtained by the hybrid technique using only a few terms from the perturbation solutions are significantly more accurate than the perturbation results on which they are based, and they compare very well with frequencies obtained by purely numerical methods.

Geer, James F.↗
NASA NTRS1992

A numerical method for systems of conservation laws of mixed type admitting hyperbolic flux splitting

The present treatment of elliptic regions via hyperbolic flux-splitting and high order methods proposes a flux splitting in which the corresponding Jacobians have real and positive/negative eigenvalues. While resembling the flux splitting used in hyperbolic systems, the present generalization of such splitting to elliptic regions allows the handling of mixed-type systems in a unified and heuristically stable fashion. The van der Waals fluid-dynamics equation is used. Convergence with good resolution to weak solutions for various Riemann problems are observed.

Shu, Chi-Wang↗
NASA NTRS1993

The multicomponent structure of bulges

The morphology of disk galaxies is usually described by two major components, the centrally concentrated spheroidal component, called the bulge, and an oblate disk. The ratio of there contribution to the total luminosity - the bulge-to-disk ratio - is one of the parameters characterizing the Hubble sequence. Following de Vaucouleurs (1948), for most galaxies the radial distribution of the outer spheroid is fairly well described by the r exp 1/4 law I(r)=I(sub 0) exp(-(alpha)r), whereas the radial luminosity distribution of the disk follows an exponential law: I(r)=I(sub 0) exp(-alpha(r exp 1/4)) (Freeman 1970), with r the radial distance from the center. I(sub 0) and alpha are characteristic constants for each individual galaxy. Parameters for the structural properties of these components give important constraints for models of the formation and evolution of galaxies. Therefore we have tried to decompose disk and bulge components from high S/N CCD observations of a sample of edge-on disk galaxies. A common procedure for the decomposition is to model one component in a region where it dominates and subtract it from the combined light distribution. This technique was successfully carried out e.g. by van der Kruit & Earl (1981, 1982) and Wakamatsu & Hamabe (1984, 1989). Here we present two more examples of bulge-dominated edge-on SO galaxies, namely ESO 506-G33 and NGC 7123, which show an additional small and concentrated central component besides disk and 'bulge'.

Dettmar, Ralf-Juergen↗
NASA NTRS1993

A new observational and numerical study of tidal interactions in M81-M82-NGC3077 system

A nearby system of interacting galaxies M81-M82-NGC3077 triplet (D = 3.3 Mpc; Freeman & Madore 1988) has been studied using multi-wavelength observations and numerical simulations to obtain a comprehensive understanding on the dynamics and the consequences of tidal interactions in a group environment. The VLA 12-field Mosaic H I observations of 2 x 1.5 deg. region have revealed a vast array of H I filaments which suggests that the severity and extent of tidal disruptions far exceed the previous estimates. A tidal remnant of the former H I disk of M82 extending up to 30 kpc (in projection) is identified for the first time, and the pervasive effects of the tidal disruption are traced into the inner disk by optical and CO observations, including a kinematic trace of a large scale bar potential (Yun, Ho, & Lo 1992). The H I disk of M81 is traced out to 40 kpc in radius, and a large scale (l approx. 20 kpc) velocity anomaly ('High Velocity Trough'), which may be a remnant of a gaseous collision, is found within the disk of M81. The large H I bridge between M81 and NGC 3077 (van der Hulst 1979) is also found to extend approx. greater than 50 kpc further, bending around NGC 3077, toward M82. The total H I detected in this experiment, 5.6 x 10(exp 9) solar mass, represents the majority of the single-dish flux (Appleton, Davies, & Stephenson 1981) and suggests that the bulk of H I found in the region belongs to the three galaxies and the tidal filaments. The impact and details of the tidal interactions have been further examined through the use of numerical techniques. The 'restricted 3-body' approach was used to simulate the observed distribution of tidal H I streamers connecting the three galaxies, and the success of the simulation is further strengthened by the accurate predictions on the gas kinematics.

Yun, M. S.↗
NASA NTRS1993

Raman linewidths and rotationally inelastic collision rates in nitrogen

Using the N2-N2 interaction potential of van der Avoird et al. (1986) rotationally inelastic collision cross sections have been computed within the infinite order sudden (IOS) approximation, assuming the molecules are distinguishable. Methods for enforcing detailed balance and correcting for effects of inelasticity, which are ignored in the IOS approximation, are considered, including the energy corrected sudden (ECS) method. Suitably averaged cross sections are compared with experimental Raman Q-branch linewidths for temperatures from 295 to 1500 K and with recently measured room temperature state-to-state rates. Agreement is rather good, especially if ECS corrections are applied.

Green, Sheldon↗
NASA NTRS1992

Statistical crystallography of surface micelle spacing

The aggregation of the recently reported surface micelles of block polyelectrolytes is analyzed using techniques of statistical crystallography. A polygonal lattice (Voronoi mosaic) connects center-to-center points, yielding statistical agreement with crystallographic predictions; Aboav-Weaire's law and Lewis's law are verified. This protocol supplements the standard analysis of surface micelles leading to aggregation number determination and, when compared to numerical simulations, allows further insight into the random partitioning of surface films. In particular, agreement with Lewis's law has been linked to the geometric packing requirements of filling two-dimensional space which compete with (or balance) physical forces such as interfacial tension, electrostatic repulsion, and van der Waals attraction.

Noever, David A.↗
NASA NTRS1932

Speed and pressure recording in three-dimensional flow

Van der Megge Zijnen's spherical Pitot tube with its 5 test holes insures a simultaneous record of static pressure and magnitude and direction of velocity in three-dimensional flow. The report treats the method as well as the range of application of this Pitot in the light of modern knowledge on flow around spheres.

Krisam, F↗
NASA NTRS1993

The roles of fluid motion and other transport phenomena in the morphology of materials

Two crystallization problems were studied: the growth of protein crystals, in particular the influence of colloidal forces and convection, and the influence of interface resistance on the growth of dendritic crystals. The protein study involved both experimental and theoretical work; the work of dendrites was entirely theoretical. In the study of protein crystallization, experiments were carried out where crystals were grown in the presence and absence of natural convection. No evidence was found that convection retards crystal growth. The theoretical study focused on the influence of colloidal forces (electrostatic and London-van der Waals) on the interaction between a protein molecule and a flat crystal surface. It was shown that the interaction is extremely sensitive to colloidal forces and that electrostatic interactions play a strong role in deciding whether or not a molecule will find a favorable site for adsorption. In the study of dendritic growth, the role of an interfacial resistance on the selection processes was examined. Using a computational scheme, it was found that the selected velocity is strongly dependent on the magnitude of the interfacial resistance to heat transfer. This is a possible explanation for discrepancies between the theoretical and experimental results on succinonitrile.

Saville, D. A.↗
NASA NTRS1993

Nested Krylov methods and preserving the orthogonality

Recently the GMRESR inner-outer iteraction scheme for the solution of linear systems of equations was proposed by Van der Vorst and Vuik. Similar methods have been proposed by Axelsson and Vassilevski and Saad (FGMRES). The outer iteration is GCR, which minimizes the residual over a given set of direction vectors. The inner iteration is GMRES, which at each step computes a new direction vector by approximately solving the residual equation. However, the optimality of the approximation over the space of outer search directions is ignored in the inner GMRES iteration. This leads to suboptimal corrections to the solution in the outer iteration, as components of the outer iteration directions may reenter in the inner iteration process. Therefore we propose to preserve the orthogonality relations of GCR in the inner GMRES iteration. This gives optimal corrections; however, it involves working with a singular, non-symmetric operator. We will discuss some important properties, and we will show by experiments that, in terms of matrix vector products, this modification (almost) always leads to better convergence. However, because we do more orthogonalizations, it does not always give an improved performance in CPU-time. Furthermore, we will discuss efficient implementations as well as the truncation possibilities of the outer GCR process. The experimental results indicate that for such methods it is advantageous to preserve the orthogonality in the inner iteration. Of course we can also use iteration schemes other than GMRES as the inner method; methods with short recurrences like GICGSTAB are of interest.

Desturler, Eric↗
NASA NTRS1994

The resonance lines of MG2 as diagnostics of the upper solar chromosphere

The resonance lines of singly ionized magnesium, the MgII h&k lines at about 280 nm, are two of the small number of lines in the solar spectrum that are optically thick in the chromospheric part of the solar atmosphere. Potentially these lines contain information on the initial temperature rise that occurs at the top of the photosphere. Unfortunately, few good observations of the lines exist due to their wavelength near 280 nm the ultraviolet. However, a fair number of observations (on the order of 200) are available from the data base of the UltraViolet Polarimeter and Spectrometer (UVSP) instrument that flew on board of NASA's Solar Maximum Mission (SMM) satellite. In addition, this data base contains a number of spectra that include the Mg I resonance line at (lambda)285.2nm, just longward of the h&k lines. The neutral magnesium line is not as strong as its ionic counterparts and samples slightly lower parts of the atmosphere. Its width is a sensitive diagnostic of the ionization balance between neutral and singly ionized magnesium, which determines the opacity scale (and formation height) of other diagnostically important MgI lines like the 457.1 nm intercombination line, the magnesium b lines and the infrared MgI emission lines near 12 microns. Analysis of the observed line profiles shows that it is necessary to include the effects of partial frequency redistribution (PRD) in the formation of the line as in the case of the h&k lines. This implies that the core of the line is very sensitive to the way scattering is treated in the modeling of the line, and in turn this allows us to separate the uncertain effects in the atomic data (viz. the Van der Waals broadening) from the uncertainties in the underlying atmospheric model. The main objective of this research was to compare observed spectra of the magnesium resonance lines against theoretical line profiles calculated from recent models of the solar atmosphere by Fontenla et al., hereafter called FAL. These models extend earlier solar models by Avrett et al. and provide yet a better match between observed and calculated spectra, in particular of the hydrogen Lyman line, by including the effects of ambipolar diffusion of hydrogen and helium in the mass and energy transport equations for the chromosphere-corona transition region. Although the transition region models can be built self-consistently (without additional observational input) once the proper boundary conditions are given at the top of the chromosphere, there are uncertainties in the models precisely because these boundary conditions are not well known. With the analysis of UVSP magnesium observations we strive to constrain these uncertainties.

Avrett, Eugene H.↗
NASA NTRS1994

Electrical characterization of 6H crystalline silicon carbide

Crystalline silicon carbide (SiC) substrates and epilayers, undoped as well as n- and p-doped, have been electrically characterized by performing Hall effect and resistivity measurements (van der Pauw) over the temperature range of approximately 85 K to 650 K (200 K to 500 K for p-type sample). By fitting the measured temperature dependent carrier concentration data to the single activation energy theoretical model: (1) the activation energy for the nitrogen donor ranged from 0.078 eV to 0.101 eV for a doping concentration range of 10(exp 17) cm(exp -3) to 10(exp 18) cm(exp -3) and (2) the activation energy for the aluminum acceptor was 0.252 eV for a doping concentration of 4.6 x 10(exp 18) cm(exp -3). By fitting the measured temperature dependent carrier concentration data to the double activation energy level theoretical model for the nitrogen donor: (1) the activation energy for the hexagonal site was 0.056 eV and 0.093 eV corresponding to doping concentrations of 3.33 x 10 (exp 17) cm(exp -3) and 1.6 x 10(exp 18) cm(exp -3) and (2) the activation energy for the cubic site was 0.113 and 0.126 eV corresponding to doping concentrations of 4.2 x 10(exp 17) cm(exp -3) and 5.4 x 10(exp 18) cm(exp -3).

Lempner, Stephen E.↗
NASA NTRS1929

The Problem of Space Travel: The Rocket Motor

This is an English translation of Noordung's 'Das Problem der Befahrung des Weltraums'. It is a part of the NASA History Series. The book provides Noordung's insight as to how a space station might be constructed. Keep in mind that the author passed away in 1929. It contains ideas that were criticized for faults and those that were acclaimed for their potentially successful ingenuity.

Hermann Noordung↗
NASA NTRS1994

Stellar populations in edge-on galaxies from deep CCD surface photometry, 1: NGC 5907

We present extremely deep charge coupled device (CCD) surface photometry of the edge-on Sc galaxy NGC 5907. Our data reach reliably to a surface brightness of R=27 mag/sq arcsec, some two magnitudes fainter than any previous work. We obtained this improvement using a 2048X2048 CCD with a wide (approximately 24 min) field, which made it possible to sky subtract directly from the galaxy frame, and by taking many dark sky flatfields. Our analysis of these data, using a full 2D model fitting procedure with a detailed error model, confirm the thin disk parameters of van der Kruit & Searle (1981). In particular, we confirm that the galaxy's disk has a radial cutoff and a constant scale height with radius. We find evidence for a stellar warp in this system, which has the same orientation as the H I warp. Our deep data also confirm that NGC 5907 has no thick disk. This suggests that theories of thick disk formation from star formation in the early stages of disk collapse, or by secular heating mechanisms, are unlikely to be correct, because they would predict that every galaxy would have a thick disk. Thick disk formation from the accretion of satellite galaxies is more likely.

Morrison, Heather L.↗
NASA NTRS1995

Propagation of nuclear burning fronts on accreting neutron stars: X-ray bursts and sub-hertz noise

We identify a new regime of time dependent helium burning for high accretion rate neutron stars and suggest that this burning is the origin of the low-level luminosity variations (on timescales of 10-10(exp 4) s, designated the 'very low-frequency noise'(VLFN) by van der Klis and collaborators) always detected in the brightest accreting X-ray sources. Only two nuclear burning regimes were previously recognized. At accretion rates in excess of the Eddington limit (dot-M approximately greater than (1-3) x 10(exp -8) solar mass/yr), the accreted matter fuses steadily. At very low dot-M, the star's entire surface is rapidly (approximately less than 10 s) burned by a fast propagating convective burning front at regular intervals, giving quasi-periodic Type I X-ray bursts. We show that for the observationally interesting range of 5 x 10(exp -10) solar mass/yr approximately less than dot-M approximately less than 10(exp -8) solar mass/yr, parts of the stellar surface burn slowly. At these accretion rates, a local thermonuclear instability starts a fire which propagates horizontally at v approximately 300 cm/s. The fire propagates around the flammable surface in roughly the same time it takes to accrete enough fuel for the next instability (approximately 10(exp 3)-10(exp 4), so that only a few fires are burning at once, giving rise to large luminosity flares. Nuclear burning is always time dependent for sub-Eddington local accretion rates: a local patch undergoes a recurrent cycle, accumulation fuel for hours until it becomes thermally unstable or is 'ignited' by a nearby burning region. The global pattern of burning and the resulting luminosity are thus very dependent on how fast nuclear fires spread around the star. The nuclear burning luminosity is not uniform over the stellar surface and so may provide a handle on measuring, or constraining, the spin periods of these neutron stars.

Bildsten, Lars↗
NASA NTRS1994

A survey of nebulae around Galactic Wolf-Rayet stars in the southern sky, 1

Images are presented from the first half of a survey of all Galactic Wolf-Rayet stars in the catalog of van der Hucht et al. (1981) residing in the southern skies. Previous surveys used only existing broad-band photographic plates. Encouraged by successes using CCD imaging with interference filters of the LMC and northern Galaxy (Miller & Chu 1993), we have expanded the survey to the southern hemisphere. In the first half of our southern survey, H alpha and (O III) narrow-band CCD images of fields centered on known Wolf-Rayet stars have indicated the existence of six new ring nebulae as well as revealing previously unobserved morphological features in the known ring nebulae. An example of this is an almost perfect ring of (O III) emission residing interior to the previously observed H alpha filaments of the Wolf-Rayet ring nebulae RCW 104. Our surveys to date indicate that 21% of all Wolf-Rayet stars have ring nebulae, with WN-type Wolf-Rayet stars having a greater likelihood for an associated ring.

Marston, A. P.↗
NASA NTRS1994

A model of sonoluminescence

A bubble of air, trapped at the center of a spherical container of water on the surface of which spherical sound waves are maintained by transducers, may emit light, a phenomenon known as sonoluminescence. The surface of the bubble expands and contracts in obedience to the Rayleigh-Lamb equation, which requires knowledge of the gas pressure on the surface of the bubble. In many investigations of bubble pulsations, it is assumed that the air in the bubble moves adiabatically. To understand sonoluminescence, however, it is necessary to allow for the possibility that shocks are generated within the bubble. We couple the Rayleigh-Lamb equation governing the bubble radius to Euler's equations governing the motion of air in the bubble, and solve the two equations simultaneously. The air is modelled by a van der Waals gas. Results are presented for a number of slightly different conditions of excitation, but in which the response of the system is widely different.

Wu, C. C.↗
NASA NTRS1996

Restructuring of Dust Aggregates in the Solar Nebula

We discuss the results of a recent effort to analyze the mechanical stability of dust aggregates with a detailed model of the physical properties of a contact between grains. This model contains both elastic repulsion forces and attractive van der Waals/dipole/metallic forces along with a description of the energy dissipation due to rolling, sliding, and breaking of contacts. We find that (1) aggregates formed from single sized grains via Particle-Cluster-Aggregation remain fluffy, (2) collisions with other aggregates and with large grains may lead to compaction (3) the velocities of small grains and aggregates in the early solar nebula are too small to produce marked compaction as long as the aggregates are small, and (4) internal restructuring of aggregates is a potentially large sink of energy which could enable the sticking of large bodies even at collision velocities of the order of several hundred cm/s.

Dominik, C.↗