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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 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
NASA NTRS1989

Superclustering of quasi-stellar object absorption clouds

The clustering of C IV absorption line systems along the lines of sight to the 55 quasars in the survey of Sargent, Boksenberg, and Steidel is examined. Their original result that there is strong clustering of these systems at separations less than 1000 km/s is confirmed. The peculiar velocities of the clouds that combine with the one-dimensional nature of the sampling is shown to preclude any detailed comparison of the observed clustering strength on these scales with that found for nearby systems such as galaxies. The sample also contains significant clustering on larger scales, 1000-10,000 km/s. This probably reflects true spatial clustering of an amplitude exceeding that observed for any nearby systems, even rich clusters of galaxies. The signal is due mainly to high equivalent width absorbers and is dominated by a single very large 'absorption supercluster' in the quasar 0237-233. It seems likely that this structure was produced by processes other than gravitational instability.

Heisler, Julia↗
NASA NTRS1989

The polarizabilities of neon

The static dipole polarizability, second hyperpolarizability, quadrupole polarizability and dipole-dipole-quadrupole hyperpolarizability for the Ne atom are computed, using large Gaussian basis sets and coupled cluster wavefunctions. Correlation effects are substantial, ranging from 15 percent for the dipole polarizability to 40 percent for the second hyperpolarizability. It is confirmed by explicit calculation earlier hypotheses that g-type and higher angular momentum functions and core correlation effects make almost no contribution to these properties in Ne. Triple excitations, on the other hand, are very important, accounting for as much as 25 percent of the correlation contribution to the second hyperpolarizability. The best estimate of the second hyperpolarizability is 119 + or - 4 au, in good agreement with the recent calculations of Maroulis and Thakkar (1989) and the latest experimental result of Shelton (1989).

Taylor, Peter R.↗
NASA NTRS1990

Normal mode projections of GCM response to tropical forcing

The transient response of a general circulation model (GCM) is examined for the first 10 days of integration after a tropical heating source is switched on. Model output from 10 control and 10 experiment GCM runs are projected onto the normal modes of an adiabatic primitive equation model linearized about a basic state at rest to quantify the contributions from different modes to the 200 mb wind response. The response in the vicinity of the source is similar to that predicted by models linearized about uniform basic states, with important contributions from the internal Rossby and Kelvin modes in the tropics. The heating also excites internal gravity modes with contributions to the 200 mb winds about 40 percent of those of the internal Rossby modes through the 10-day integration time. The response in the external Rossby modes in the extratropics becomes statistically significant after five days.

Zhong, Weili↗
NASA NTRS1990

Flare heating and ionization of the low solar chromosphere. II - Observations of five solar flares

Two neutral Mg spectral lines formed in the temperature-minimum region and the low chromosphere, at 4571 and 5173 A, are used to quantify the changes in the atmospheric structure as a function of time during five solar flares. Eight proposed flare heating and ionization mechanisms and predictions of the effects of each on the temperature minimum region are discussed. Two Mg spectral observations made at the National Solar Observatory (Sacramento Peak), along with observations of hard and soft X-rays from the SMM and GOES satellites, are compared to the predictions of the eight proposed mechanisms. The initial effects in all five flares are consistent with backwarming by enhanced Balmer- and Paschen-continuum radiation originating in the upper chromosphere. Extended heating observed in two of the flares is most likely due to UV irradiation. In all cases heating by the dissipation of nonreversed electric currents, collisions with an electron or proton beam, irradiation by soft X-rays, and dissipation of Alfven waves are eliminated.

Metcalf, Thomas R.↗
NASA NTRS1991

Theoretical characterization of tetrahedral N4

Results are reported from ab initio theoretical calculations of the structure and energetics of tetrahedral N4. The theoretical basis and implementation of the computations are reviewed, and the results are presented in tables. It is found that a tetrahedral N4 molecule has energy 186 kcal/mole greater than that of two N2 molecules, with a 61-kcal/mole barrier between the two species. Brief consideration is given to the possible use of N4 (should a process for synthesizing it be developed) as a high-energy-density fuel for supersonic transport aircraft.

Lee, Timothy J.↗
NASA NTRS1991

The determination of accurate dipole polarizabilities alpha and gamma for the noble gases

Accurate static dipole polarizabilities alpha and gamma of the noble gases He through Xe were determined using wave functions of similar quality for each system. Good agreement with experimental data for the static polarizability gamma was obtained for Ne and Xe, but not for Ar and Kr. Calculations suggest that the experimental values for these latter ions are too low.

Rice, Julia E.↗
NASA NTRS1991

The energy separation between the classical and nonclassical isomers of protonated acetylene - An extensive study in one- and n-particle space saturation

The energy separation between the classical and nonclassical forms of protonated acetylene has been reinvestigated in light of the recent experimentally deduced lower bound to this value of 6.0 kcal/mol. The objective of the present study is to use state-of-the-art ab initio quantum mechanical methods to establish this energy difference to within chemical accuracy (i.e., about 1 kcal/mol). The one-particle basis sets include up to g-type functions and the electron correlation methods include single and double excitation coupled-cluster (CCSD), the CCSD(T) extension, multireference configuration interaction, and the averaged coupled-pair functional methods. A correction for zero-point vibrational energies has also been included, yielding a best estimate for the energy difference between the classical and nonclassical forms of 3.7 + or - 1.3 kcal/mol.

Lindh, Roland↗
NASA NTRS1991

Coronal dynamics of a quiescent active region

Post-SMM repair Flat Crystal Spectrometer (FCS) observations of coronal X-ray line broadening in a nonflaring active region are discussed. The resulting map of FCS line profiles across the central X-ray bright portion of a large active region shows a clear variation in line width for the Mg XI resonance line at 9.17 A, but no evidnece of line shifts. Line ratios yield T(e) of about 3 x 10 to the 6th K within 10 percent for most of the map. The brightest, broadest profiles are consistent with emission from the tops of loops overlying the magnetic neutral line. For the spatially averaged Mg XI resonance line profile, the 'excess velocity' v(ex) corresponding to the excess width above the thermal value equals the thermal velocity for Mg ions, 45 km/s; for some individual bright pixels v(ex) is 60 km/s or more. The observed coronal line broadening is interpreted as a signature of the mechanisms responsible for coronal heating in active regions.

Saba, Julia L. R.↗
NASA NTRS1991

A theoretical study of Na(H2O)n(+) (n = 1-4)

The successive H2O binding energies of Na(H2O)n(+) are in excellent agreement with experiment and are rather insensitive to electron correlation since the bonding is predominantly electrostatic. A point-charge model shows that changes in the successive binding energies are due primarily to ligand-ligand repulsion. Vibrational frequencies and IR intensities are determined for Na(H2O)n(+) (n = 1-4) at the self-consistent-field and second-order Moller-Plesset levels of theory to facilitate experimental study of these ions.

Bauschlicher, Charles W., Jr.↗
NASA NTRS1991

Macromolecular recognition: Structural aspects of the origin of the genetic system

Theoretical simulation of prebiotic chemical processes is an invaluable tool for probing the phenomenon of evolution of life. Using computational and modeling techniques and guided by analogies from present day systems we, seek to understand the emergence of genetic apparatus, enzymatic catalysis and protein synthesis under prebiotic conditions. In one possible scenario, the RNA enzymatic reaction plays a key role in the emergence of the self-replicating and offers a clue to the onset of enzymatic catalysis prior to the existence of the protein biosynthetic machinery. Our ultimate goal is to propose a simple RNA segment which contains the specificity and catalytic activity of the contemporary RNA enzyme and which could emerge in a primordial chemical environment. To understand the mechanism of ribozyme catalyzed reactions, ab initio and semi-empirical (ZINDO) programs were used to investigate the reaction path of transphosphorylation. A special emphasis was placed on the possible catalytic and structural roles played by the coordinated magnesium cation. Both the inline and adjacent mechanisms of transphosphorylation have been studied. Another important aspect of this reaction is the identity of the functional groups which are essential for the acid base catalysis. The structural characteristics of the target helices, particularly a possible role of G center dot T pair, is under examination by molecular dynamics (MD) simulation technique. Modeling of the ancestral aminoacyl-tRNA synthetases (aRS) may provide important clues to the emergence of the genetic code and the protein synthetic machinery. Assuming that the catalytic function evolved before the elements of specific recognition of a particular amino acid, we are exploring the minimal structural requirements for the catalysis of tRNA aminoacylation. The molecular modeling system SYBYL was used for this study based on the high resolution crystallographic structures of the present day tyrosyl-adenylate:tyrRS and tRNA(Gln): ATP:glnRS complexes. The trinucleotide CCA of the 3'-end tRNA is placed into the active site pocket of tyrRS, based on the scheme of interaction between tRNA(Gln) and glnRS, and upon the stereochemistry of the tyrRS:tRNA:Tyr-AMP transition state. This provides a model of the non-specific recognition of a tRNA's 3'-end by an aRS, which might be similar to that of the ancestral aRS's. In the next step, modeling of the rest of the acceptor stem of tRNA (Tyr) with tyrRS is carried out.

Rein, Robert↗
NASA NTRS1991

Macromolecular recognition: Structural aspects of the origin of the genetic system

Theoretical simulation of prebiotic chemical processes is an invaluable tool for probing the phenomenon of the evolution of life. Using computational and modeling techniques and guided by analogies from present day systems, we seek to understand the emergence of the genetic apparatus, enzymatic catalysis and protein synthesis under prebiotic conditions. Modeling of the ancestral aminoacyl-tRNA-synthetases (aRS) may provide important clues to the emergence of the genetic code and the protein synthetic machinery. The minimal structural requirements for the catalysis of tRNA aminoacylation are being explored. A formation of an aminoacyl adenylate was studied in the framework of ab initio molecular orbital theory. The role of individual residues in the vicinity of the TyrRS active site was examined, and the effect of all possible amino acids substitutions near the active site was examined. A formation of aminoacyl tRNA was studied by the molecular modeling system SYBYL with the high resolution crystallographic structures of the present day tRNA, aRS's complexes. The ultimate goal is to propose a simple RNA segment that is small enough to be build in the primordial chemical environment but maintains the specificity and catalytic activity of the contemporary RNA enzyme. To understand the mechanism of ribozyme catalyzed reactions, ab initio and semi-empirical (ZINDO) programs were used to investigate the reaction path of transphosphorylation. A special emphasis was placed on the possible catalytic and structural roles played by the coordinated magnesium cation. Both the inline and adjacent mechanisms of transphosphorylation were studied. The structural characteristics of the target helices, particularly a possible role for the G-T pair, is also studied by a molecular dynamics (MD) simulation technique.

Rein, Robert↗
NASA NTRS1989

The determination of accurate dipole polarizabilities alpha and gamma for the noble gases

The static dipole polarizabilities alpha and gamma for the noble gases helium through xenon were determined using large flexible one-particle basis sets in conjunction with high-level treatments of electron correlation. The electron correlation methods include single and double excitation coupled-cluster theory (CCSD), an extension of CCSD that includes a perturbational estimate of connected triple excitations, CCSD(T), and second order perturbation theory (MP2). The computed alpha and gamma values are estimated to be accurate to within a few percent. Agreement with experimental data for the static hyperpolarizability gamma is good for neon and xenon, but for argon and krypton the differences are larger than the combined theoretical and experimental uncertainties. Based on our calculations, we suggest that the experimental value of gamma for argon is too low; adjusting this value would bring the experimental value of gamma for krypton into better agreement with our computed result. The MP2 values for the polarizabilities of neon, argon, krypton and zenon are in reasonabe agreement with the CCSD and CCSD(T) values, suggesting that this less expensive method may be useful in studies of polarizabilities for larger systems.

Rice, Julia E.↗
NASA NTRS1992

Relationships between toughness and microstructure of reaction bonded Si3N4

Fracture toughnesses of nominally identical batches of reaction bonded silicon nitride (RBSN) differed significantly (about 2.0 and about 2.7 MPa sq rt m). Detailed fractographic and microstructural characterizations investigated underlying factors. Subtile differences between high and low toughness RBSN and between constituent Si powders have been revealed through SEM/FEG, TEM, BET, Hg-porosimetry, and XRD. The results illustrate the need for behavioral models to guide microstructural design and to interpret properties of brittle materials with intermediate levels of porosity.

Lightfoot, Annamarie↗
NASA NTRS1992

Low-frequency predictability of the Dynamical Extended-Range Forecast Experiment

The objective of the study was to analyze data from the Dynamical Extended-Range Forecast Experiment conducted from January 1986 to March 1987, and to evaluate differences between analysis and forecasts with emphasis on the tropical 30-50-day oscillation. The diagnostic toll used is the projection of analysis and forecast data onto the normal modes of a primitive equation model. Examination of zonal-wind anomalies in the tropics shows that the forecast model predicts propagation of intraseasonal variations more accurately for slow propagation rates. The forecast amplitude is generally weaker than the analyzed amplitude. Analyzed kinetic energy and error fields exhibit similar horizontal scales for internal and external modes. External Rossby-mode components maximize in the extratropics while Rossby internal modes exhibit patterns that extend over the entire globe.

Nogues-Peagle, Julia↗
NASA NTRS1992

FONO - A difficult case for theory and experiment

High levels of ab initio theory are used to investigate the equilibrium structures, vibrational spectra, and relative energetics of FNO2, cis-FONO, and trans-FONO isomers. FNO2 is determined to be 36.9 +/- 2.5 kcal/mol (0 K) more stable than cis-FONO, which is more stable than trans-FONO by 2.5 +/- 1.0 kcal/mol (0 K). The molecular structure of cis-FONO is shown to have typical F-O and central O-N single bond distances, indicating that cis-FONO has a structure analogous to HONO. The computed vibrational spectrum of cis-FONO is shown to be consistent with IR matrix isolation experiments, including isotopic shifts. The experimentally deduced structure of FONO is shown to be incorrect because one of the vibrational bands included in the normal coordinate analysis is either a combination band or an overtone.

Lee, Timothy J.↗
NASA NTRS1993

Monolithic Continuous-Flow Bioreactors

Monolithic ceramic matrices containing many small flow passages useful as continuous-flow bioreactors. Ceramic matrix containing passages made by extruding and firing suitable ceramic. Pores in matrix provide attachment medium for film of cells and allow free movement of solution. Material one not toxic to micro-organisms grown in reactor. In reactor, liquid nutrients flow over, and liquid reaction products flow from, cell culture immobilized in one set of channels while oxygen flows to, and gaseous reaction products flow from, culture in adjacent set of passages. Cells live on inner surfaces containing flowing nutrient and in pores of walls of passages. Ready access to nutrients and oxygen in channels. They generate continuous high yield characteristic of immobilized cells, without large expenditure of energy otherwise incurred if necessary to pump nutrient solution through dense biomass as in bioreactors of other types.

Stephanopoulos, Gregory↗
NASA NTRS1992

Connected triple excitations in coupled-cluster calculations of hyperpolarizabilities: Neon

We have calculated the second hyperpolarizability gamma of neon using the CCSD(T) method. The accuracy of the CCSD(T) approach has been established by explicit comparison with the single, double and triple excitation coupled-cluster (CCSDT) method using extended basis sets that are known to be adequate for the description of gamma. Our best estimate for gamma(sub 0) of 110 +/- 3 a.u. is in good agreement with other recent theoretical values and with Shelton's recent experimental estimate of 108 +/- 2 a.u. Comparison of the MP2 and CCSD(T) hyperpolarizability values indicates that MP2 gives a very good description of the electron correlation contribution to gamma(sub 0). We have combined MP2 frequency-dependent corrections with the CCSD(T) gamma(sub 0) to yield values of gamma(-2 omega;omega,omega,0) and gamma(exp K)(-omega;omega,0,0).

Rice, Julia E.↗
NASA NTRS1992

On-line implementation of nonlinear parameter estimation for the Space Shuttle main engine

We investigate the performance of a nonlinear estimation scheme applied to the estimation of several parameters in a performance model of the Space Shuttle Main Engine. The nonlinear estimator is based upon the extended Kalman filter which has been augmented to provide estimates of several key performance variables. The estimated parameters are directly related to the efficiency of both the low pressure and high pressure fuel turbopumps. Decreases in the parameter estimates may be interpreted as degradations in turbine and/or pump efficiencies which can be useful measures for an online health monitoring algorithm. This paper extends previous work which has focused on off-line parameter estimation by investigating the filter's on-line potential from a computational standpoint. ln addition, we examine the robustness of the algorithm to unmodeled dynamics. The filter uses a reduced-order model of the engine that includes only fuel-side dynamics. The on-line results produced during this study are comparable to off-line results generated previously. The results show that the parameter estimates are sensitive to dynamics not included in the filter model. Off-line results using an extended Kalman filter with a full order engine model to address the robustness problems of the reduced-order model are also presented.

Buckland, Julia H.↗