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At least 649 records · Page 36

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NASA NTRS1986

Radiative coupled nonequilibrium flow fields associated with aeroassisted orbital transfer

Development of an algorithm which will allow nonequilibrium flow field codes to be coupled to radiative transfer codes (currently limited to equilibrium applications) was initiated. The algorithm consists of an equation for the prediction of electronic state population fractions (electronic state temperature) and equations for the computation of spectral absorption coefficients and source functions. These expressions represent a thorough treatment in the context of the bandless model which in the past has proven adequate for equilibrium radiative transfer predictions. Perhaps the approach is more than adequate as the current algorithm requires four temperatures for each transition. Clearly, a more sophisticated model can be developed; however, it would require more information, such as rotational temperatures of the electronic levels.

Horton, T. E.↗

NASA NTRS1986

Universality in the compressive behavior of solids

It was discovered that the isothermal equation of state for solids in compression is a simple, universal form. This single form accurately describes the pressure and bulk modulus as a function of volume for tonic, metallic, covalent, and rare gas solids.

Vinet, P.↗

NASA NTRS1984

Astrophysical materials science: Theory

A method of structural expansions for use in determining the equation of state of metallic hydrogen (and indeed other metals) up to the 4th order in the perturbation theory was developed. The electrical and thermal transport properties of the planetary interior of Jupiter were calculated. The nature of the interaction between molecules at short range and the importance of multicenter terms in arriving at an adequate description of the thermodynamic functions of condensed molecular hydrogen were also investigated.

Ashcroft, N. W.↗

NASA NTRS1986

Existence of needle crystals in local models of solidification

The way in which surface tension acts as a singular perturbation to destroy the continuous family of needle-crystal solutions of the steady-state growth equations is analyzed in detail for two local models of solidification. All calculations are performed in the limit of small surface tension or, equivalently, small velocity. The basic mathematical ideas are introduced in connection with a quasilinear, isotropic version of the geometrical model of Brower et al., in which case the continuous family of solutions dissappears completely. The formalism is then applied to a simplified boundary-layer model with an anisotropic kinetic attachment coefficient. In the latter case, the solvability condition for the existence of needle crystals can be satisfied whenever the coefficient of anisotropy is arbitrarily small but nonzero.

Langer, J. S.↗

NASA NTRS1986

Estimating unsteady aerodynamic forces on a cascade in a three-dimensional turbulence field

An analytical method has been developed to estimate tne unsteady aerodynamic forces caused by flow field turbulence on a wind tunnel turning vane cascade system (vane set). This method approximates dynamic lift and drag by linearly perturbing the appropriate steady state force equations, assuming that the dynamic loads are due only to free stream turbulence and that this turbulence is homogeneous, isotropic, and Gaussian. Correlation and unsteady aerodynamic effects are also incorporated into the analytical model. Using these assumptions, equations relating dynamic lift and drag to flow turbulence, mean velocity, and vane set geometry are derived. From these equations, estimates for the power spectra and rms (root mean squared value, delta) loading of both lift and drag can be determined.

Norman, T. R.↗

NASA NTRS1986

The X-ray spectrm of the Coma Cluster of galaxies

The HEAO 1 A-2 and OSO 8 observations of the Coma Cluster of galaxies in the 2-60 keV energy band are presented. The spectrum is not well fitted by an isothermal bremsstrahlung model with an Fe line. To model the spectrum more accurately, we have performed fits of polytropic models and find that the gas is well fitted by an equation of state with polytropic index ranging between 1.38 and 1.6. This analysis reduces the total cluster mass by a factor of 3-5 below the optically derived virial mass. Using the best-fitting parameters, the gas is found to be a significant fraction (25 percent) of the total cluster mass. The Fe K-alpha line blend is strongly detected, from which we infer 0.18-0.36 solar abundance in the X-ray emitting gas. The Fe K-beta line blend is also weakly detected. The spectrum is not consistent with the existence of a hard X-ray tail at the flux level reported by Bazzano el al. (1984). In addition, the HEAO I scanning mode data place strong constraints on possible contaminating sources in the region surrounding Coma.

Henriksen, M. J.↗

NASA NTRS1986

Dynamic compression and volatile release of carbonates

Particle velocity profiles upon shock compression and isentropic releases were measured for polycrystalline calcite. The Solenhofen limestone release paths lie close to the Hugoniot. Calcite 3 to 2 transition, upon release was observed, but rarefaction shocks were not detected. The equation of state is used to predict the fraction of material devolatilized upon isentropic release as a function of shock pressure. The effect of ambient partial pressure of CO2 on the calculations is demonstrated and considered in models of atmospheric evolution by impact induced mineral devolatilization. The radiative characteristics of shocked calcite indicate that localization of thermal energy occurs under shock compression. Shock entropy calculations result in a minimum estimate of 90 percent devolatilization upon complete release from 10 GPa. Isentropic release paths from calculated continuum Hugoniot temperatures cross into the CaO (solid) + CO2 (vapor) field at improbably low pressures. It is found that release paths from measured shock temperatures cross into the melt plus vapor field at pressures greater than .5 GPa, which suggests that devolatilization is initiated at the shear banding sites.

Tyburczy, J. A.↗

NASA NTRS1986

High pressure cosmochemistry applied to major planetary interiors: Experimental studies

The overall goal of this project is to determine properties of the H-He-C-N-O system, as represented by small molecules composed of these elements, that are needed to constrain theoretical models of the interiors of the major planets. Much of our work now concerns the H2O-NH3 system. This project is the first major effort to measure phase equilibria in binary fluid-solid systems in diamond anvil cells. Vibrational spectroscopy, direct visual observations, and X-ray crystallography of materials confined in externally heated cells are our primary experimental probes. We also are collaborating with the shockwave physics group at Lawrence Livermore Laboratory in studies of the equation of state of a synthetic Uranus fluid and molecular composition of this and other H-C-N-O materials under planetary conditions.

Nicol, M. F.↗

NASA NTRS1986

Interiors of the giant planets

The interior structure of the Jovian planets was studied by means of observational data obtained by ground-based astronomy. Recent work was oriented toward a determination of oblateness from occultation measurements of the shape of planetary atmospheres. This provides a determination of the degree of central condensation of the planet, which is then compared with interior models and theoretical equations of state. As a serendipitous result of the attempts to measure the oblateness of Neptune, a partial Neptune ring (or arc) was discovered following an occultation observation on July 22, 1984. During 1985, older observations were reexamined to determine whether Neptune arcs had been detected (but not identified) during previous occultations. It appears that Neptunian arcs may locally imitate Uranian rings, with an 1981 event resembling Uranus' epsilon ring, while an 1984 event was more similar to the thinner Uranian rings such as the alpha ring.

Hubbard, W. B.↗

NASA NTRS1987

Mechanical and thermal properties of planetologically important ices

Two squences of ice composition were proposed for the icy satellites: a dense nebula model and a solar nebula model. Careful modeling of the structure, composition, and thermal history of satellites composed of these various ices requires quantitative information on the density, compressibility, thermal expansion, heat capacity, and thermal conductivity. Equations of state were fitted to the density data of the molecular ices. The unusual thermal and mechanical properties of the molecular and binary ices suggest a larger range of phenomena than previously anticipated, sufficiently complex perhaps to account for many of the unusual geologic phenomena found on the icy satellites.

Croft, Steven K.↗

NASA NTRS1987

High pressure cosmochemistry of major planetary interiors: Laboratory studies of the water-rich region of the system ammonia-water

Several studies relative to high pressure cosmochemistry of major planetary interiors are summarized. The behavior of gas-ice mixtures at very high pressures, studies of the phase diagram of (NH3) sub x (H2O) sub 1-x at pressures to 5GPa and temperatures from 240 to 370 K, single crystal growth of ammonia dihydrate at room temperature in order to determine their structures by x-ray diffraction, spectroscopy of chemical reactions during shock compression in order to evaluate how the reactions affect the interpretation of equation of state data obtained by shock methods, and temperature and x-ray diffraction measurements made on resistively heated wire in diamond anvil cells in order to obtain phase and structural data relevant to the interiors of terrestrial planets are among the studies discussed.

Nicol, Malcolm↗

NASA NTRS1987

Properties of planetary fluids at high pressure and temperature

In order to derive models of the interiors of Uranus, Neptune, Jupiter and Saturn, researchers studied equations of state and electrical conductivities of molecules at high dynamic pressures and temperatures. Results are given for shock temperature measurements of N2 and CH4. Temperature data allowed demonstration of shock induced cooling in the the transition region and the existence of crossing isotherms in P-V space.

Nellis, W. J.↗

NASA NTRS1986

The origin of cosmic rays

Data related to the development of cosmic rays are discussed. The relationship between cosmic ray production and the steady-state Boltzmann equation is analyzed. The importance of the power-law spectrum, the scattering rate, the theory of shock acceleration, anisotropic instabilities, and cosmic ray diffusion in the formation of cosmic rays is described. It is noted that spacecraft observations at the earth's bow shock are useful for studying cosmic rays and that the data support the collisionless shock-wave theory of cosmic ray origin.

Eichler, D.↗

NASA NTRS1986

Molecular dissociation and shock-induced cooling in fluid nitrogen at high densities and temperatures

Radiative temperatures and electrical conductivities were measured for fluid nitrogen compressed dynamically to pressures of 18-90 GPa, temperatures of 4000-14,000 K, and densities of 2-3 g/cu cm. The data show a continuous phase transition above 30 GPa shock pressure and confirm that (delta-P/delta-T)v is less than 0, as indicated previously by Hugoniot equation-of-state experiments. The first observation of shock-induced cooling is also reported. The data are interpreted in terms of molecular dissociation, and the concentration of dissociated molecules is calculated as a function of density and temperature.

Radousky, H. B.↗

NASA NTRS1986

Thermal catastrophe in the plasma sheet boundary layer

This letter presents a first step towards a substorm model including particle heating and transport in the plasma sheet boundary layer (PSBL). The heating mechanism discussed is resonant absorption of Alfven waves. For some assumed MHD perturbation incident from the tail lobes onto the plasma sheet, the local heating rate in the PSBL has the form of a resonance function of the one-fluid plasma temperature. Balancing the local heating by convective transport of the heated plasma toward the central plasma sheet, an 'equation of state" is found for the steady-state PSBL whose solution has the form of a mathematical catastrophe: at a critical value of a parameter containing the incident power flux, the local density, and the convection velocity, the equilibrium temperature jumps discontinuously. Associating this temperature increase with the abrupt onset of the substorm expansion phase, the catastrophe model indicates at least three ways in which the onset may be triggered. Several other consequences related to substorm dynamics are suggested by the simple catastrophe model.

Smith, Robert A.↗

NASA NTRS1987

Experimentation in planetary geology

Laboratory simulations of geological processes on the terrestrial planets are described, summarizing results published during the period 1983-1986. Included are studies of wind-driven processes on Mars and Venus (using the special wind-tunnel facilities at NASA Ames); simulations of shock-induced loss of volatiles from solids; equation-of-state determinations; impact experiments simulating cratering, spallation, regolith formation, and disruption; fluid-flow simulations of channel formation on Mars; and dust studies. The use of the microgravity environment of the Space Station for planetary-geology experiments is briefly considered.

Cintala, Mark J.↗

NASA NTRS1987

Constraints on the mass-radius relation of the neutron star in 4U 1746-37/NGC 6441

An analysis of 12 hours of continuous EXOSAT observations of 4U 1746-37 in NGC 6441, obtained between September 9 and 10, 1985, puts constraints on its mass-radius relation. The spectral shape of the persistent X-ray flux is well fitted by a thermal bremsstrahlung spectrum, with a kT temperature varying between 7.5 and 9.8 keV, and an interstellar column density of about 2.8 x 10 to the 21st/sq cm. Two X-ray bursts noted during the observation are found to be type I, with radius expansion of the neutron star photosphere during their peaks. If the neutron star has a canonical mass of 1.4 solar masses, the observed burst fluxes must be reduced as a result of anisotropy of the burst by a factor of at least 1.3-2.2. The present results are consistent with the very soft and intermediately stiff equations of state.

Sztajno, M.↗

NASA NTRS1987

Cell model and elastic moduli of disordered solids - Low temperature limit

The cell theory has been previously employed to compute the equation of state of a disordered condensed system. It is now generalized to include anisotropic stresses. The condition of affine deformation is adopted, transforming an orginally spherical into an ellipsoidal cell. With a Lennard-Jones n-m potential between nonbonded centers, the formal expression for the deformational free energy is derived. It is to be evaluated in the limit of the linear elastic range. Since the bulk modulus in this limit is already known, it is convenient to consider a uniaxial deformation. To begin with, restrictions are made to the low-temperature limit in the absence of entropy contributions. Young's modulus and Poisson's ratio then follow.

Peng, S. T. J.↗