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At least 433 records · Page 24

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NASA NTRS1981

A stochastic-dynamical approach to the study of the natural variability of the climate

A method, suggested by Leith (1975), which employed stochastic-dynamic forecasts obtained from a general circulation model in such a way as to satisfy the definition of climatic noise, was used to validate assumptions accounting for the effects of external influences in estimating the climatic noise. Two assumptions were investigated: (1) that the weather fluctuations can be represented as a Markov process, and (2) that changing external conditions do not influence the atmosphere's statistical properties on short time scales. The general circulation model's simulation of the daily weather fluctuations was generated by performing integrations with prescribed climatological boundary conditions for random initial atmospheric states, with resulting dynamical forecasts providing an ensemble of simulated data for the autoregressive modeling of weather fluctuations. To estimate the climatic noise from the observational data (consisting of hourly values of sea level pressure and surface temperature at 54 U.S. stations for the month of January for the years 1949-1975) use of the short time-scale assumption is made. The simulated and observed data were found not to be consistent with either white noise or a Markov process of weather fluctuations. Good agreement was found between the results of the hypothetical testing of the simulated and the observed surface temperatures; and only partial support was found for the short time-scale assumption, i.e., for sea level pressure.

Straus, D. M.↗

NASA NTRS1981

On the importance of reverse current ohmic losses in electron-heated solar flare atmospheres

The paper considers the passage of a beam of nonthermal electrons through the flaring solar atmosphere, paying particular attention to the requirement that the beam be stable to the generation of plasma turbulence. The ratio of energy losses due to reverse current ohmic heating and heating by Coulomb collisions, respectively, for the greatest flux which can pass stably through the atmosphere is computed. It is shown that this ratio is determined by the low energy cutoff of the beam, by the electron temperature of the ambient atmosphere, and by the electron to ion temperature ratio theta. It is also independent of the atmospheric density. The results show that ohmic energy losses are undoubtedly important in the initial transient state, in agreement with other authors, but that their role is debatable in the flare atmosphere, depending on the value of theta appropriate.

Emslie, A. G.↗

NASA NTRS1981

Ion-neutral momentum coupling near discrete high-latitude ionospheric features

A two-dimensional numerical model is developed to study the momentum coupling between the ionosphere and neutral atmosphere in the vicinity of discrete high-latitude features, such as convection channels and plasma density troughs. Based on generalized magnetohydrodynamic equations the model takes account of global pressure gradients, viscous dissipation, ion drag, the Coriolis force, and electrodynamic drifts. Among the findings of an initial steady state investigation are the following: (1) in convection channels, significant shears and rotations of the thermospheric flow can occur below 200 km if a minimum in the electron density profile is present between the E and F regions; (2) in convection channels, the thermospheric wind decreases with height in the F region owing to the effects of horizontal viscosity; and (3) at low altitudes, the boundaries of convection channels may produce Ekman spirals.

St-Maurice, J.-P.↗

NASA NTRS1982

Automotive Stirling Engine Development Program

Activities performed on Mod I engine testing and test results; the manufacture, assembly, and test of a Mod I engine in the United States; design initiation of the Mod I-A engine system; transient performance testing; Stirling reference engine manufacturing and reduced size studies; components and subsystems; and the study and test of low cost alloys are summarized.

Nightingale, N.↗

NASA NTRS1982

A mechanism for plasma waves at the harmonics of the plasma frequency foreshock boundary

A bump-on-tail unstable reduced velocity distribution, constructed from data obtained at the upstream boundary of the electron foreshock by the GSFC electron spectrometer experiment on the ISEE-1 satellite, is used as the initial plasma state for a numerical integration of the 1D-Vlasov-Maxwell system of equations. The integration is carried through the growth of the instability, beyond its saturation, and well into the stabilized plasma regime. A power spectrum computed for the electric field of the stabilized plasma is dominated by a narrow peak at the Bohm-Gross frequency of the unstable field mode but also contains significant power at the harmonics of the Bohm-Gross frequency. The harmonic power is in sharp peaks which are split into closely spaced doublets. The fundamental peak at the Bohm-Gross frequency is split into a closely spaced triplet. The mechanism for excitation of the second harmonic is shown to be second order wave-wave coupling.

Klimas, A. J.↗

NASA NTRS1982

The ORSER LANDSAT Data Base of Pennsylvania

A mosaicked LANDSAT data base for Pennsylvania, installed at the computation center of the Pennsylvania State University is described. Initially constructed by Penn State's Office for Remote Sensing of Earth Resources (ORSER) for the purpose of assisting in state-wide mapping of gypsy moth defoliation, the data base will be available to a variety of potential users. It will provide geometrically correct LANDSAT data accessible by political, jurisdictional, or arbitrary boundaries.

Turner, B. J.↗

NASA NTRS1983

A mechanism for plasma waves at the harmonics of the plasma frequency in the electron foreshock boundary

Klimas, A. J.↗

NASA NTRS1984

A full-implicit-continuous-Eulerian (FICE) scheme for multidimensional transient magnetohydrodynamic (MHD) flows

A full implicit continuous Eulerian (FICE) scheme is developed for solving multidimensional transient MHD flow problems. The physical system under consideration is a general case of a transient MHD flow in which an initial steady state is subject to a finite amplitude disturbance. The governing equations are described, their finite difference formulation is presented and the FICE algorithm is given. The boundary conditions are treated by classifying them into physical and computational ones. The usefulness of the FICE algorithm is demonstrated using a physical example concerning the dynamical response of the static solar atmosphere due to a representative photospheric disturbance.

Wu, S. T.↗

NASA NTRS1985

Modeling of transient heat pipe operation

The overall goal is to gain a better understanding of the transient behavior of heat pipes operating under both normal and adverse conditions. Normal operation refers to cases where the capillary structure remains fully wetted. Adverse operation occurs when drying, re-wetting, choking, noncontinuum flow, freezing, thawing etc., occur within the heat pipe. The work was redirected towards developing the capability to predict operational behavior of liquid metal heat pipes used for cooling aerodynamic structures. Of particular interest is the startup of such heat pipes from an initially frozen state such as might occur during re-entry of a space vehicle into the Earth's atmosphere or during flight of hypersonic aircraft.

Colwell, G. T.↗

NASA NTRS1983

The origin of density fluctuations in the 'new inflationary universe'

Cosmological mysteries which are not explained by the Big Bang hypothesis but may be approached by a revamped inflationary universe model are discussed. Attention is focused on the isotropy, the large-scale homogeneity, small-scale inhomogeneity, the oldness/flatness of the universe, and the baryon asymmetry. The universe is assumed to start in the lowest energy state, be initially dominated by false vacuum energy, enter a de Sitter phase, and then cross a barrier which is followed by the formation of fluctuation regions that lead to structure. The scalar fields (perturbation regions) experience quantum fluctuations which produce spontaneous symmetry breaking on a large scale. The scalar field value would need to be much greater than the expansion rate during the de Sitter epoch. A supersymmetric (flat) potential which satisfies the requirement, yields fluctuations of the right magnitude, and allows inflation to occur is described.

Turner, M. S.↗

NASA NTRS1985

The Hadley and Rossby regimes in a spherical atmosphere

The properties of the steady Hadley and Rossby regimes for a thermally forced rotating fluid on a sphere are studied. The two layer modified geostrophic model is employed which allows for thermal advection by the divergent wind and time dependent static stability. Heating processes are parameterized using the Newtonian approximation and Rayleigh friction is accounted for. The equations are transformed to spectral form using spherical harmonics and then truncated retaining a simple axisymmetric state and initial, one wave. A time independent Hadley circulation is obtained which is neutral to axisymmetric disturbances but unstable to wave like perturbations for intermediate values of the meridional temperature gradient, indicating the existence of both an upper and lower symmetric Hadley regime. An analytical solution for the steady Rossby circulation is determined for values of the meridional temperature gradient where the Hadley regime is unstable. Linear perturbation theory is used to show that within the steady Rossby regime two or more waves cannot exist simultaneously.

Feldstein, S. B.↗

NASA NTRS1985

Photodetachment of the positronium negative ion

The photodetachment cross section of the negative positronium ion is calculated. The description of the initial bound state is simplified by representing it by an asymptotic form whose normalization comes from the most accurate Hylleraas wave function of the ion. The final state is assumed to be a plane wave.

Bhatia, A. K.↗

NASA NTRS1986

Electron-nitrogen molecular collisions in high-temperature nonequilibrium air

Ab initio calculation of vibrational excitation and deexcitation cross sections of N2 by low energy resonant electron impact have been carried out. The calculation includes initial target state in v = 0-12 and final target state in v = 0-17. The calculated cross sections are found to agree with available experimental data to within reported experimental error. Effect of target rotation has been investigated. Comparison of vibrational excitation cross sections at J = 0, 50, and 150 shows that the major effect of target rotation is to lower the resonance energy. Vibrational excitation and deexcitation rate coefficients are tabulated for electron temperatures of 1100-55,000 K. The present tabulation supercedes an earlier compilation by the Naval Research Laboratory that was derived from experimental data with a factor-of-two error. The tabulated rate coefficients are suited for flowfield modeling of aeroassisted orbital transfer vehicles.

Huo, W. M.↗

NASA NTRS1988

On the cyclic stress-strain behaviour of a Ni-base superalloy at room temperature

The cyclic stress-strain behavior of Nimonic alloy PE16 was studied at room temperature and at different aging conditions to determine whether the plateau in the cyclic stress-strain curve (CSSC) reported by Arbuthnot (1982) is typical of the room temperature behavior and/or some specific initial microstructural states. Specimen blanks were heat-treated in batches in Ar/H2 (98/2) atmosphere to produce gamma-prime precipitates of different average sizes, but with the volume fraction of gamma-prime precipitates kept constant at about 7 percent at all the heat-treatment conditions. Total axial strain controlled LCF tests were conducted under fully reversed loading (R = -1) at a constant strain rate of 0.004/s, using a servohydraulic machine. The load response in tension and compression was recorded continually, and stress-strain hysteresis loops were recorded at frequent intervals. In the present investigation, the CSSCs of the P16 alloy at room temperature did not display the plateaus reported by Arbuthnot.

Singh, Vakil↗

NASA NTRS1988

On wind-type flows in astrophysical jets. III - Temporal evolution of perturbations and the formation of shocks

The temporal evolution of disturbances in a spherically symmetric polytropic wind from a central object is studied. Such disturbances may be due to localized momentum addition/subtraction, as, for example, by MHD waves, heating/cooling mechanisms in the outflow, or localized deviations from spherical symmetric expansion. The evolution of an initial perturbed state to a continuous or discontinuous final equilibrium state, as predicted by previous analytic calculations for stationary flows, is followed. It is shown that some of the predicted discontinuous equilibrium states are not physically accessible, while the attainment of the remaining equilibrium states depends on both the temporal and the spatial parameters characterizing the perturbation. The results are derived for solar conditions, but in fact can be applied to outflows in other astrophysical systems. In particular, applications to the solar wind and flows in astrophysical jets are discussed.

Trussoni, E.↗

NASA NTRS1987

Application of optimization techniques to spacecaft fuel usage minimization in deep space navigation

Mathematical analysis of the minimization of spacecraft fuel usage for both impulsive and finite motor burns is presented. A high precision integrated trajectory search program (SEPV) and several optimization software libraries are used to solve minimum fuel usage problems. The SEPV program has the capacity to vary either the initial spacecraft state or the finite burn parameters to acquire a specified set of target values. Several test examples for the Voyager 2 Uranus Encounter and the Galileo Jupiter Orbiter are presented to show that spacecraft fuel consumption can be minimized in targeting maneuver strategies. The fuel savings achieved by the optimum solution can be significant.

Wang, Tseng-Chan↗

NASA NTRS1989

The formation of topological defects in phase transitions

It was argued, and fought through numerical work that the results of non-dynamical Monte Carlo computer simulations cannot be applied to describe the formation of topological defects when the correlation length at the Ginzburg temperature is significantly smaller than the horizon size. To test the current hypothesis that infinite strings at formation are essentially described by Brownian walks of size the correlation length at the Ginzburg temperature, fields at the Ginzburg temperature were equilibrated. Infinite structure do not exist in equilibrium for reasonable definitions of the Ginzburg temperature, and horizons must be included in a proper treatment. A phase transition, from small-scale to large-scale string or domain wall structure, is found to occur very close to the Ginzburg temperature, in agreement with recent work. The formation process of domain walls and global strings were investigated through the breaking of initially ordered states. To mimic conditions in the early Universe, cooling times are chosen so that horizons exist in the sample volume when topological structure formation occurs. The classical fields are evolved in real-time by the numerical solution of Langevin equations of motion on a three dimensional spatial lattice. The results indicate that it is possible for most of the string energy to be in small loops, rather than in long strings, at formation.

Hodges, Hardy M.↗

NASA NTRS1988

Effect of orbital and rotational angular momentum averaging on branching ratios of dynamical resonances in the reaction H + p-H2 yields o-H2 + H

The paper reports extensive quantum mechanical calculations of the product vibrational branching ratios in the reaction H + p-H2 yields o-H2 + H. The calculations involve total angular momentum up to 2 and excited as well as ground initial rotational states, and they are completely converged with up to 513 channels in individual total angular momentum/parity blocks. Comparisons are made with recent experiments by Nieh and Valentini.

Mladenovic, Mirjana↗