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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 595 records · Page 33
All sourcesNASA NTRSDOE OSTI
NASA NTRS1988

Accurate experimental and theoretical comparisons between superconductor-insulator-superconductor mixers showing weak and strong quantum effects

A systematic study of the gain and noise in superconductor-insulator-superconductor mixers employing Ta based, Nb based, and Pb-alloy based tunnel junctions was made. These junctions displayed both weak and strong quantum effects at a signal frequency of 33 GHz. The effects of energy gap sharpness and subgap current were investigated and are quantitatively related to mixer performance. Detailed comparisons are made of the mixing results with the predictions of a three-port model approximation to the Tucker theory. Mixer performance was measured with a novel test apparatus which is accurate enough to allow for the first quantitative tests of theoretical noise predictions. It is found that the three-port model of the Tucker theory underestimates the mixer noise temperature by a factor of about 2 for all of the mixers. In addition, predicted values of available mixer gain are in reasonable agreement with experiment when quantum effects are weak. However, as quantum effects become strong, the predicted available gain diverges to infinity, which is in sharp contrast to the experimental results. Predictions of coupled gain do not always show such divergences.

Mcgrath, W. R.↗
NASA NTRS1989

Neutron starquakes and the nature of gamma-ray bursts

The possibility that gamma-ray bursts originate from quakes deep in the solid crust of a neutron star is investigated. Seismic waves are radiated if shear stress is relieved by brittle fracture. However they cannot propagate directly to the surface but are temporarily trapped below a reflecting layer. The shaking of the stellar surface couples the seismic waves to Alfven waves which propagate out into the magnetosphere. The crust-magnetosphere transmission coefficient strongly increases with wave frequency and magnetic field strength. Alfven wave luminosities sufficient to power galactic gamma-ray bursts are possible if magnetic fields greater than 100 billion G cover at least part of the stellar surface. As the Alfven waves propagate out into the low density magnetosphere, they become increasingly charge starved, thereby accelerating particles to relativistic energies.

Madau, P.↗
NASA NTRS1990

Corrosion fatigue crack propagation in metals

This review assesses fracture mechanics data and mechanistic models for corrosion fatigue crack propagation in structural alloys exposed to ambient temperature gases and electrolytes. Extensive stress intensity-crack growth rate data exist for ferrous, aluminum and nickel based alloys in a variety of environments. Interactive variables (viz., stress intensity range, mean stress, alloy composition and microstructure, loading frequency, temperature, gas pressure and electrode potential) strongly affect crack growth kinetics and complicate fatigue control. Mechanistic models to predict crack growth rates were formulated by coupling crack tip mechanics with occluded crack chemistry, and from both the hydrogen embrittlement and anodic dissolution/film rupture perspectives. Research is required to better define: (1) environmental effects near threshold and on crack closure; (2) damage tolerant life prediction codes and the validity of similitude; (3) the behavior of microcrack; (4) probes and improved models of crack tip damage; and (5) the cracking performance of advanced alloys and composites.

Gangloff, Richard P.↗
NASA NTRS1994

Heterogeneous Interactions of ClONO2 and HCl with Sulfuric Acid Tetrahydrate: Implications for the Stratosphere

The reaction probabilities for ClONO2+H2O- HOCl + HNO3 and ClONO2+ HCl Cl2 +HNO3 have been investigated on sulfuric acid tetrahydrate (SAT, H2SO4-4H2O)surfaces at temperatures between 190 and 230 K and at reactant concentrations that are typical in the lower stratosphere, using a fast-flow reactor coupled to a quadrupole mass spectrometer. The results indicate that the reaction probabilities as well as HCl uptake depend strongly on the thermodynamic state of SAT surface: they decrease significantly with decreasing H2O partial pressure at a given temperature, and decrease with increasing temperature at a given H2O partial pressure, as the SAT changes from the H2O-rich form to the H2SO4-rich form. For H2O-rich SAT at 195 K gamma(sub 1) approx. = -0.01 and gamma(sub 2) greater or equal to 0.1, whereas the values for H2SO4-rich SAT decrease by more than 2 orders of magnitude. At low concentrations of HCl, close to those found in the stratosphere, the amount of HCl taken up by H2O-rich SAT films corresponds to a coverage of the order of a tenth of a monolayer (approx. = 10(exp 14) molecules/sq cm); H2SO4-rich SAT films take up 2 orders of magnitude less HCl (les than 10(exp 12) molecules/sq cm). Substantial HCl uptake at high HCl concentrations is also observed, as a result of surface melting. The data reveal that frozen stratospheric sulfate aerosols may play an important role in chlorine activation in the winter polar stratosphere via processes similar to those occurring on the surfaces of polar stratospheric cloud particles.

Zhang, Renyi↗
NASA NTRS2002

Pulsed Electromagnetic Acceleration of Plasmas

A major shift in paradigm in driving pulsed plasma thruster is necessary if the original goal of accelerating a plasma sheet efficiently to high velocities as a plasma "slug" is to be realized. Firstly, the plasma interior needs to be highly collisional so that it can be dammed by the plasma edge layer not (upstream) adjacent to the driving 'vacuum' magnetic field. Secondly, the plasma edge layer needs to be strongly magnetized so that its Hall parameter is of the order of unity in this region to ensure excellent coupling of the Lorentz force to the plasma. Thirdly, to prevent and/or suppress the occurrence of secondary arcs or restrike behind the plasma, the region behind the plasma needs to be collisionless and extremely magnetized with sufficiently large Hall parameter. This places a vacuum requirement on the bore conditions prior to the shot. These requirements are quantified in the paper and lead to the introduction of three new design parameters corresponding to these three plasma requirements. The first parameter, labeled in the paper as gamma (sub 1), pertains to the permissible ratio of the diffusive excursion of the plasma during the course of the acceleration to the plasma longitudinal dimension. The second parameter is the required Hall parameter of the edge plasma region, and the third parameter the required Hall parameter of the region behind the plasma. Experimental research is required to quantify the values of these design parameters. Based upon fundamental theory of the transport processes in plasma, some theoretical guidance on the choice of these parameters are provided to help designing the necessary experiments to acquire these data.

Thio, Y. C. Francis↗
NASA NTRS2003

Nonlinear Flutter Aspects of the Flexible HSCT Semispan Model

The nonlinear aspects that lead to the flutter of an High-Speed Civil Transport (HSCT) Flexible Semispan Model are analyzed. A hierarchy of spectral moments was used to determine the characteristics of the aerodynamic loading and structural strains and motions. The results show that the frequency of the bending motion of the wing varied significantly as the Mach number was increased between 0.90 and 0.97. Examination of the pressure coefficients in terms of mean value and fluctuations showed that the flow characteristics over the wing changed significantly around a Mach number of 0.97. A strong shock was identified near the trailing edge. Nonlinear analysis of the pressure fluctuations, under these conditions, showed nonlinear coupling involving low-frequency components at pressure locations where the mean value was at a local minimum. This shows that the aerodynamic forces acting on the model had nonlinearly coupled frequency components. The results presented here show how nonlinear analysis tools can be used to identify nonlinear aspects of the flutter phenomenon which are needed in the validation of nonlinear computational methodologies. Keywords: Nonlinear aeroelasticity, Flutter, Bispectrum.

Hajj, Muhammad R.↗
NASA NTRS2010

On the Relationship Between the Brewer-Dobson Circulation and the Southern Annular Mode During Austral Summer in Couple Chemistry-Climate Model Simulations

The Brewer-Dobson circulation (BDC) is the mean transport circulation in the stratosphere. It consists of an upwelling branch in the tropics, poleward flows from the tropics to the extratropics, and downward flows in the extratropics. The BDC plays a crucial role in the distribution of important stratospheric trace gases, such as ozone and water vapor. Therefore changes in the strength of the BDC under global warming could have significant impact on stratospheric ozone depletion and recovery. For example, all climate models that are used by the World Meteorological Organization to predict ozone evolution in the 21 st century project a strengthening of the BDC that leads to ozone superrecovery in the mid-latitudes. On the other hand, ozone changes could also affect the strength of the BDC. This work investigates an outstanding question: whether and how changes in the Brewer-Dobson circulation are connected to climate change in the troposphere, in particular, the annular modes. The annular modes are the leading variability in the extratropical troposphere, which describes a seesaw pattern of circulation fluctuations between the polar and middle latitudes. Using simulations from the Goddard Earth Observing System Coupled Chemistry Climate Model (GEOS CCM), we found the strengthening of the BDC in the summer Southern Hemisphere is strongly correlated with a shift of the Southern Hemisphere Annular Mode (SAM) toward its positive phase for the last 4 decades of the 20th century. This relationship is only present in model runs that simulate the stratospheric ozone depletion. Therefore it is concluded that the BDC-SAM relationship is driven by Antarctic ozone depletion. The ozone hole significantly cools the Antarctic stratosphere in late spring/early summer, which leads to a delayed breakdown of the polar vortex: strong circumpolar eastward flows that usually shift to westward winds in late spring. The prolonged persistence of stratospheric eastward flow enhances upward propagation of tropospheric waves into the stratosphere and strengthens the BDC. The increased wave flux in the stratosphere in turn drives a SAM trend toward its positive phase. Our results also show that the BDC-SAM relationship is robust on the interannual timescale

Li, Feng↗
NASA NTRS2012

An Assessment of CFD/CSD Prediction State-of-the-Art by Using the HART II International Workshop Data

Over the past decade, there have been significant advancements in the accuracy of rotor aeroelastic simulations with the application of computational fluid dynamics methods coupled with computational structural dynamics codes (CFD/CSD). The HART II International Workshop database, which includes descent operating conditions with strong blade-vortex interactions (BVI), provides a unique opportunity to assess the ability of CFD/CSD to capture these physics. In addition to a baseline case with BVI, two additional cases with 3/rev higher harmonic blade root pitch control (HHC) are available for comparison. The collaboration during the workshop permits assessment of structured, unstructured, and hybrid overset CFD/CSD methods from across the globe on the dynamics, aerodynamics, and wake structure. Evaluation of the plethora of CFD/CSD methods indicate that the most important numerical variables associated with most accurately capturing BVI are a two-equation or detached eddy simulation (DES)-based turbulence model and a sufficiently small time step. An appropriate trade-off between grid fidelity and spatial accuracy schemes also appears to be pertinent for capturing BVI on the advancing rotor disk. Overall, the CFD/CSD methods generally fall within the same accuracy; cost-effective hybrid Navier-Stokes/Lagrangian wake methods provide accuracies within 50% the full CFD/CSD methods for most parameters of interest, except for those highly influenced by torsion. The importance of modeling the fuselage is observed, and other computational requirements are discussed.

Smith, Marilyn J.↗
NASA NTRS2023

Importance of Atmospheric Feedbacks in Simulating the Seasonal Cycle of the Antarctic Sea Ice and Its Response to Perturbations

The seasonal cycle of the Antarctic sea ice extent is largely controlled by the evolution of the insolation received at the top of the atmosphere. However, sea ice processes and feedbacks with the ocean and the atmosphere can modulate this seasonal cycle. Here, the atmospheric feedbacks are quantified in a series of idealized sensitivity experiments performed with an eddy-permitting (1/4°) NEMO-LIM3 Southern Ocean configuration, including a representation of ice shelf cavities, in which the model was either driven by an atmospheric reanalysis or coupled to the COSMO-CLM 2 regional atmospheric model. In these experiments, sea ice thermodynamics and dynamics as well as the exchanges with the ocean and atmosphere are strongly perturbed. This perturbation is achieved by modifying snow and ice thermal conductivities, the vertical mixing in the ocean top layers, the effect of freshwater uptake/release upon sea ice growth/melt, ice dynamics and surface albedo. We show that the changes in surface heat fluxes are very different between the configurations driven by the reanalysis and those coupled to the atmosphere. Atmospheric feedbacks enhance the response of the modelled winter ice extent to any of the perturbed processes, and the enhancement is strongest when the albedo is modified. The response of sea ice volume and extent to changes in entrainment of subsurface warm waters to the ocean surface is also greatly amplified by the coupling with the atmosphere. By contrast, the atmospheric feedbacks can damp the impact of the perturbations affecting the heat conductivity through sea ice.

Atmospheric feedbacks↗
NASA NTRS1995

GPS detection of ionospheric perturbations following the January 17, 1994, northridge earthquake

Sources such as atmospheric or buried explosions and shallow earthquakes producing strong vertical ground displacements produce pressure waves that propagate at infrasonic speeds in the atmosphere. At ionospheric altitudes low frequency acoustic waves are coupled to ionispheric gravity waves and induce variations in the ionoispheric electron density. Global Positioning System (GPS) data recorded in Southern California were used to compute ionospheric electron content time series for several days preceding and following the January 17, 1994, M(sub w) = 6.7 Northridge earthquake. An anomalous signal beginning several minutes after the earthquake with time delays that increase with distance from the epicenter was observed. The signal frequency and phase velocity are consistent with results from numerical models of atmospheric-ionospheric acoustic-gravity waves excited by seismic sources as well as previous electromagnetic sounding results. It is believed that these perturbations are caused by the ionospheric response to the strong ground displacement associated with the Northridge earthquake.

Calais, Eric↗
NASA NTRS1994

Coherent variations of monthly mean total ozone and lower stratospheric temperature

Space-time patterns of correlation between total ozone and lower stratospheric temperature are documented, based on 14 years (1979-1992) of global monthly mean observations. Data are obtained from the total ozone mapping spectrometer (TOMS) and microwave sounding unit (MSU) channel 4, the latter being a weighted mean temperature of the 150- to 50-mbar layer. These data are analyzed (separately) for linear trend, solar cycle, quasi-biennial oscillation (QBO), and El Nino-Southern Oscillation (ENSO) variations via linear regression: significant signals are identified for each term, and the corresponding structures in ozone and temperature are found to be highly coherent. The temperature trends derived here show significant cooling of the lower stratosphere over Northern Hemisphere (NH) midlatitudes in winter-spring and over Antarctica in Southern Hemisphere (SH) spring; the overall space-time patterns are similar to those determined for ozone trends. Interestingly, temperatures do not decrease over SH midlatitudes during midwinter, in spite of large ozone losses. These data furthermore show globally coherent ozone and temperature perturbations associated with both QBO and ENSO variations; a new result here show large total ozone anomalies in middle-to-high latitudes of both hemispheres associated with ENSO events. Residuals from the ozone and temperature time series (defined as the deseasonalized total minus the regression fits) show strong positive correlation in middle-to-high latitudes but weak correlations in the trop ics. Time periods following the volcanic eruptions of El Chichon and Pinatubo are clearly identified from the coupled signatures of decreased ozone and increased temperature, opposite to the positive ozone-temperature correlations observed at other times. The ratios of ozone to temperature anomalies derived here show quantitative signatures indicating that either radiative (trend, solar, and QBO) or dynamical (ENSO and residuals) processes are responsible for the strong ozone-temperature correlations.

Randel, William J.↗
NASA NTRS1979

Convergent perturbation analysis of intense coherent multiphoton interactions

Use has been made of flow graphs to deduce Feenberg perturbation expansions for radiative interactions. It is demonstrated that these expansions can in certain cases be summed to provide closed form expressions for the molecular response. In particular, it is shown that the coherent state response can be obtained by the summation of a continued fraction perturbation expansion for the harmonic oscillator. Anharmonicity in the lower levels is treated and its shown to introduce Rabi flopping identifiable with multiphoton transitions among isolated tightly coupled subsystems of levels. Relevance to laser induced multiphoton excitation and energy level shift calculations in the presence of a strong field are also discussed.

Gower, M. C.↗
NASA NTRS1982

Detailed observations of the source of terrestrial narrowband electromagnetic radiation

Detailed observations are presented of a region near the terrestrial plasmapause where narrowband electromagnetic radiation (previously called escaping nonthermal continuum radiation) is being generated. These observations show a direct correspondence between the narrowband radio emissions and electron cyclotron harmonic waves near the upper hybrid resonance frequency. In addition, electromagnetic radiation propagating in the Z-mode is observed in the source region which provides an extremely accurate determination of the electron plasma frequency and, hence, density profile of the source region. The data strongly suggest that electrostatic waves and not Cerenkov radiation are the source of the banded radio emissions and define the coupling which must be described by any viable theory.

Kurth, W. S.↗
NASA NTRS1985

Magnetospheric impulse response for many levels of geomagnetic activity

The temporal relationship between the solar wind and magnetospheric activity has been studied using 34 intervals of high time resolution IMP 8 solar wind data and the corresponding AL auroral activity index. The median values of the AL index for each interval were utilized to rank the intervals according to geomagnetic activity level. The linear prediction filtering technique was then applied to model magnetospheric response as measured by the AL index to the solar wind input function VB(s). The linear prediction filtering routine produces a filter of time-lagged response coefficients which estimates the most general linear relationship between the chosen input and output parameters of the magnetospheric system. It is found that the filters are composed of two response pulses speaking at time lags of 20 and 60 min. The amplitude of the 60-min pulse is the larger for moderate activity levels, while the 20-min pulse is the larger for strong activity levels. A possible interpretation is that the 20-min pulse represents magnetospheric activity driven directly by solar wind coupling and that the 60-min pulse represents magnetospheric activity driven by the release of energy previously stored in the magnetotail. If this interpretation is correct, the linear filtering results suggest that both the driven and the unloading models of magnetospheric response are important facets of a more comprehensive response model.

Bargatze, L. F.↗
NASA NTRS1992

Gravitational influence on eutectic solidification

The effect of gravity on eutectic solidification was investigated experimentally for Pb-Sn binary eutectic alloys. It is found that the separation of the primary crystals of the binary system from the coupled eutectic occurs over a wide range of cooling rates and gravitational fields (100-100,000 g). The separation is strongly influenced by the nucleation behavior of the particular alloy system. A mechanism for the strong phase separation is postulated on the basis of cloud convection models in atmospheric convection.

Sokolowski, Robert S.↗
NASA NTRS1992

Theoretical study of Cr(+) and Co(+) bound to H2 and N2

Binding energies of Co(H2)n(+) (n = 1-3) and CrN2(+) computed using the modified coupled-pair functional approach agree well with measured values. The calculations show that both H2 and N2 bond much more strongly to Co(+) than to Cr(+). Theoretical studies also show that despite the similar polarizabilities of N2 and Ar, the CrN2(+) binding energy is twice as large as that of CrAr(+) due to the charge-quadrupole contribution. Finally, the bonding of H2 and N2 with Na(+) and Mg(+) is contrasted with that for Cr(+) and Co(+).

Bauschlicher, Charles W., Jr.↗
NASA NTRS2000

Modeling Ultraviolet Emissions Near Io

In this report, we describe work awarded to Science Applications International Corporation, for the period 6/l/99 to 5/31/00. During this time period, we have investigated the interaction of Io, Jupiter's innermost Galilean satellite, with the Io plasma torus, and the role this interaction plays in producing ultraviolet (UV) emissions from neutral oxygen and sulfur. Io, the innermost of Jupiter's Galilean satellites, plays a unique role in the jovian magnetosphere. Neutral material that escapes from Io is ionized to form the lo torus, a dense, heavy-ion plasma that corotates with Jupiter and interacts with Io. Io supplies not only the torus, but is a major source of plasma for the entire magnetosphere. Ionization and charge-exchange of neutrals near lo strongly influences the plasma interaction, and Io's neutral atmosphere plays an important role in the generation of currents that couple Io to Jupiter. There have been no in situ measurements of the neutral density near Io, but remote observations of neutrals near lo have been performed for many years. Recent observations from the Hubble Space Telescope (HST) have shown detailed structure in UV emissions from neutral species near Io. Electron-impact of the neutrals by the Io torus plasma is the primary mechanism responsible for exciting these emissions. Previously, we have modeled the Io plasma environment using three-dimensional magnetohydrodynamic (MHD) simulations, and we have shown that the interaction between Io and the plasma torus plays an important role in producing the morphology of the observed emissions. In the past year, we have extended these studies to use both UV observations and Galileo particle and field measurements to investigate the Io interaction.

Linker, Jon A.↗
NASA NTRS1999

Phase Diagram of the Two-Chain Hubbard Model

We have calculated the charge gap and spin gap for the two-chain Hubbard model as a function of the on-site Coulomb interaction and the interchain hopping amplitude. We used the density matrix renormalization group method and developed a method to calculate separately the gaps numerically for the symmetric and antisymmetric modes with respect to the exchange of the chain indices. We have found very different behaviors for the weak and strong interaction cases. Our calculated phase diagram is compared to the one obtained by Balents and Fisher using the weak coupling renormalization group technique.

Park, Youngho↗