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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 361 records · Page 20
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NASA NTRS1981

Radiative transfer in the surfaces of atmosphereless bodies. I - Theory. II - Interpretation of phase curves

A generalized theory of radiative transfer is presented which includes single and multiple scattering of light in particulate surfaces, and is applicable to both the surfaces of atmosphereless bodies and to laboratory samples. Single scattering is described in terms of the effects of porosity and roughness, and is formulated by means of a probabilistic method. It is shown that, for low-albedo surfaces, the effects of porosity and roughness are separable, the opposition effect is caused by the former, and the slope of the linear part of the phase curve is mainly controlled by the latter. The theory, which is applicable to all albedos and phase angles, may be used in both surface brightness and integrated brightness studies. The limiting case of roughness and volume density tending to zero yields the results of classical radiative transfer theory.

Lumme, K.↗
NASA NTRS1982

Quantitative tests of a steady state theory of solar wind electrons

A comparison is made of IMP 6, 7 and 8 electron data with the predictions of a solar wind electron steady state theory in which the control of transport by the macroscopic interplanetary electric and magnetic fields, as well as elastic Coulomb collisions with solar wind protons and thermal electrons, is assumed. While a ratio of forward to backward phase density for field-aligned extrathermal electrons of 6:1 is predicted, electron distribution measurements within the high speed solar wind show this ratio to be typically about an order of magnitude larger. A set of solar wind bulk speed anticorrelations predicted by the theory on the basis of a larger set of assumptions cannot be found in the IMP electron data set, so that improved agreement may require such modifications of the theory's assumptions as the inclusion of inelastic Coulomb and/or wave electron collisions.

Feldman, W. C.↗
NASA NTRS1983

Comparison of forward flight effects theory of A. Michalke and U. Michel with measured data

The scaling laws of a Michalke and Michel predict flyover noise of a single stream shock free circular jet from static data or static predictions. The theory is based on a farfield solution to Lighthill's equation and includes density terms which are important for heated jets. This theory is compared with measured data using two static jet noise prediction methods. The comparisons indicate the theory yields good results when the static noise levels are accurately predicted.

Rawls, J. W., Jr.↗
NASA NTRS1983

A Nonvolume Preserving Plasticity Theory with Applications to Powder Metallurgy

A plasticity theory has been developed to predict the mechanical response of powder metals during hot isostatic pressing. The theory parameters were obtained through an experimental program consisting of hydrostatic pressure tests, uniaxial compression and uniaxial tension tests. A nonlinear finite element code was modified to include the theory and the results of themodified code compared favorably to the results from a verification experiment.

Cassenti, B. N.↗
NASA NTRS1983

Theories of radio emissions and plasma waves

The complex region of Jupiter's radio emissions at decameter wavelengths, the so-called DAM, is considered, taking into account the basic theoretical ideas which underly both the older and newer theories and models. Linear theories are examined, giving attention to direct emission mechanisms, parallel propagation, perpendicular propagation, and indirect emission mechanisms. An investigation of nonlinear theories is also conducted. Three-wave interactions are discussed along with decay instabilities, and three-wave up-conversio. Aspects of the Io and plasma torus interaction are studied, and a mechanism by which Io can accelerate electrons is reviewed.

Goldstein, M. L.↗
NASA NTRS1983

Vibrations of cantilevered circular cylindrical shells Shallow versus deep shell theory

Free vibrations of cantilevered circular cylindrical shells having rectangular planforms are studied in this paper by means of the Ritz method. The deep shell theory of Novozhilov and Goldenveizer is used and compared with the usual shallow shell theory for a wide range of shell parameters. A thorough convergence study is presented along with comparisons to previously published finite element solutions and experimental results. Accurately computed frequency parameters and mode shapes for various shell configurations are presented. The present paper appears to be the first comprehensive study presenting rigorous comparisons between the two shell theories in dealing with free vibrations of cantilevered cylindrical shells.

Lee, J. K.↗
NASA NTRS1982

Nucleation theory - Is replacement free energy needed?

It has been suggested that the classical theory of nucleation of liquid from its vapor as developed by Volmer and Weber (1926) needs modification with a factor referred to as the replacement free energy and that the capillary approximation underlying the classical theory is in error. Here, the classical nucleation equation is derived from fluctuation theory, Gibb's result for the reversible work to form a critical nucleus, and the rate of collision of gas molecules with a surface. The capillary approximation is not used in the derivation. The chemical potential of small drops is then considered, and it is shown that the capillary approximation can be derived from thermodynamic equations. The results show that no corrections to Volmer's equation are needed.

Doremus, R. H.↗
NASA NTRS1983

Statistical mechanics of light elements at high pressure. V Three-dimensional Thomas-Fermi-Dirac theory

A numerical technique for solving the Thomas-Fermi-Dirac (TED) equation in three dimensions, for an array of ions obeying periodic boundary conditions, is presented. The technique is then used to calculate deviations from ideal mixing for an alloy of hydrogen and helium at zero temperature and high presures. Results are compared with alternative models which apply perturbation theory to calculation of the electron distribution, based upon the assumption of weak response of the electron gas to the ions. The TFD theory, which permits strong electron response, always predicts smaller deviations from ideal mixing than would be predicted by perturbation theory. The results indicate that predicted phase separation curves for hydrogen-helium alloys under conditions prevailing in the metallic zones of Jupiter and Saturn are very model dependent.

Macfarlane, J. J.↗
NASA NTRS1984

Consistency tests of cosmogonic theories from models of Uranus and Neptune

The planetary ratios of ice to rock (I/R) abundances expected in Uranus and Neptune are derived on the basis of several cosmogonic theories. For both Uranus and Neptune, the value of I/R lies between about 1.0 and 3.6. This value is difficult to reconcile with a scenario in which N and C are accreted primarily in the form of N2 and CO. It is consistent with some versions of both giant protoplanet theories and equilibrium accretion theories.

Podolak, M.↗
NASA NTRS1983

Natural vibrations of laminated anisotropic plates using three-dimensional elasticity theory

The paper contains a description of the three-dimensional elasticity equations and the associated finite element model for natural vibrations of laminated anisotropic rectangular plates. The numerical results for natural frequencies are compared with those obtained by a shear deformable plate theory. A number of cross-ply and angle-ply rectangular plates are analyzed for natural frequencies. For relatively thick plates, the shear deformable-plate theory element predicts frequencies higher than those predicted by the three-dimensional elasticity theory element.

Reddy, J. N.↗
NASA NTRS1984

Dynamic analysis of laminated plates using a higher-order theory

This paper deals with the dynamic analog of the higher-order shear deformation plate theory developed by the senior author. The theory is based on a displacement field in which the inplane displacements are expanded as cubic functions of the thicknes coordinate and the transverse deflection is assumed to be constant through the thickness. The additional dependent unkowns introduced with the quadratic and cubic terms of the thickness coordinate are eliminated by requiring the transverse shear stresses to vanish on the bounding planes of the plate. The theory accounts for the parabolic distribution of the transverse shear stresses, and hence no shear correction coefficients are required.

Reddy, J. N.↗
NASA NTRS1984

Variational principle for a prototype Rastall theory of gravitation

A prototype of Rastall's theory of gravity, in which the divergence of the energy-momentum tensor is proportional to the gradient of the scalar curvature, is shown to be derivable from a variational principle. Both the proportionality factor and the unrenormalized gravitational constant are found to be covariantly constant, but not necessarily constant. The prototype theory is, therefore, a gravitational theory with variable gravitational constant.

Smalley, L. L.↗
NASA NTRS1985

The second-order theory of electromagnetic hot ion beam instabilities

The present investigation is concerned with the application of a second-order theory for electromagnetic instabilities in a collisionless plasma to two modes which resonate with hot ion beams. The application of the theory is strictly limited to the linear growth phase. However, the application of the theory may be extended to obtain a description of the beam at postsaturation if the wave-beam resonance is sufficiently broad in velocity space. Under the considered limitations, it is shown that, as in the cold beam case, the fluctuating fields do not gain appreciable momentum and that the primary exchange of momentum is between the beam and main component.

Gary, S. P.↗
NASA NTRS1985

Does nucleation theory apply to the formation of refractory circumstellar grains?

It is pointed out that kinetic factors inherent in cosmic systems will prevent the attainment of an approximate equilibrium precondensation cluster distribution, and that therefore such systems will violate one of the most fundamental assumptions of nucleation theory. More importantly, it is shown that of the 11 refractory metal/metal oxide systems which have been studied experimentally none are consistent with the predictions of either classical nucleation theory or the Lothe-Pound modification. Since no valid method is known by which these deficiencies can be circumvented, it is concluded that nucleation theory cannot be made to work in expanding circumstellar shells. However, a stochastic, kinetic treatment which makes use of experimental and theoretical data for specific refractory systems does appear to be feasible.

Donn, B.↗
NASA NTRS1984

Dust in magnetised plasmas - Basic theory and some applications

In this paper the theory of charged test particle motion in magnetic fields is reviewed. This theory is then extended to charged dust particles, for which gravity and charge fluctuations play an important role. It is shown that systematic drifts perpendicular to the magnetic field and stochastic transport effects may then have to be considered none of which occur in the case of atomic particles (with the exception of charge exchange reactions). Some applications of charged dust particle transport theory to planetary rings are then briefly discussed.

Northrop, T. G.↗
NASA NTRS1985

A higher-order shear deformation theory of laminated elastic shells

A higher-order shear deformation theory of elastic shells is developed for shells laminated of orthotropic layers. The theory is a modification of the Sanders' theory and accounts for parabolic distribution of the transverse shear strains through thickness of the shell and tangential stress-free boundary conditions on the boundary surfaces of the shell. The Navier-type exact solutions for bending and natural vibration are presented for cylindrical and spherical shells under simply supported boundary conditions.

Reddy, J. N.↗
NASA NTRS1984

Cell symbiosis theory - Status and implications for the fossil record

Smith (1981) has presented three alternative models of the origin of eukaryotes. In the present investigation, alternative theories are discussed along with the status of serial endosymbiotic theory. It is pointed out that the usefulness of the serial endosymbiotic theory is attested by the great flourishing of productive scientific studies it has generated. Attention is given to the origin of mitochondria and plastids from free-living bacteria, the polyphyly of mitochondria, the origin of the nuclear membrane, the origin of undulipodia (cilia and other 9 + 2 organelles), the sequence of acquisition of organelles, large ribosomes, genome organization, animals and plants, and systematics and axonomy of the microorganisms.

Margulis, L.↗
NASA NTRS1985

A beam theory for anisotropic materials

Beam theory plays an important role in structural analysis. The basic assumption is that initially plane sections remain plane after deformation, neglecting out-of-plane warpings. Predictions based on these assumptions are accurate for slender, solid, cross-sectional beams made out of isotropic materials. The beam theory derived in this paper from variational principles is based on the sole kinematic assumption that each section is infinitely rigid in its own plane, but free to warp out of plane. After a short review of the Bernoulli and Saint-Venant approaches to beam theory, a set of orthonormal eigenwarpings is derived. Improved solutions can be obtained by expanding the axial displacements or axial stress distribution in series of eigenwarpings and using energy principles to derive the governing equations. The improved Saint-Venant approach leads to fast converging solutions and accurate results are obtained considering only a few eigenwarping terms.

Bauchau, O. A.↗