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At least 397 records · Page 22
All sourcesNASA NTRSDOE OSTI
NASA NTRS1972

Explicit expressions of the potential and its derivatives at the origin in terms of the scattering data

The quantum mechanical theory of scattering of a particle by a spherically symmetrical potential is presented. As in the inverse scattering problem, the input of the calculation is the scattering and bound-state data, and the output is data on the potential. The results discussed are explicit expressions for the values of the potential and its derivatives at the origin in terms of the scattering and boundstate data. Various methods to obtain these results are outlined. The presentation is aimed at introducing these various approaches. The simplest scattering problem (nonrelativistic S-wave scattering on a holomorphic potential without bound states) is used as the basis for discussion, and technicalities are omitted whenever possible without loss of clarity. A complete compilation is given of the results obtained to date in this field, including the treatment of higher partial waves and the Klein-Gordon and Dirac equations.

Calogero, F.↗
NASA NTRS1974

Rarefied gas dynamics; Proceedings of the Eighth International Symposium, Stanford University, Stanford, Calif., July 10-14, 1972

Recent studies in the theory of rarefied gases (kinetic theory, shock structure, molecular forces and interactions, evaporation and condensation, expansions and jets, flow past bodies, gas-solid interactions) are presented along with descriptions of numerous experiments in these areas. Some of the topics covered include: singular limits of solutions of Boltzmann's equations, the Boltzmann and Klimontovich formalisms with reference to turbulent resolution, higher-order kinetic theory for the Lorentz gas, intermolecular potentials from shock structure experiments, cooling of particles in aerosol beams, hypersonic-subsonic transition in spherically symmetric expanding flows, absolute velocity measurements in a rarefied gas flow by an ion time-of-flight technique, the product distribution of gas-solid reactions from absolute rate theory, and relaxation of a gas with internal degrees of freedom. Individual items are announced in this issue.

Karamcheti, K.↗
NASA NTRS1974

Perturbation theory of structure in classical liquid mixtures: Application to metallic systems near phase separation

The partial structure factors of classical simple liquid mixtures near phase separation are dicussed. The theory is developed for particles interacting through pair potentials, and is thus appropriate both to insulating fluids, and also to metallic systems if these may be described by an effective ion-ion pair interaction. The motivation arose from consideration of metallic liquid mixtures, in which resistive anomalies have been observed near phase separation. A mean field theory correction appropriate to 3 pair potential for the effects of correlated motions in the reference fluid is studied. The work is cast in terms of functions which are closely related to the direct correlation functions of Ornstein and Zernike. The results are qualitatively in accord with physical expectations. Quantitative agreement with experiment seems to turn on the selection of the hard core reference potential in terms of the metallic effective pair potential. It is suggested that the present effective pair potentials are perhaps not properly used to calculate the metallic structure factors at long wavelength.

Henderson, R. L.↗
NASA NTRS1960

Applications of a General Finite-Difference Method for Calculating Bending Deformations of Solid Plates

This paper reports the findings of an investigation of a finite - difference method directly applicable to calculating static or simple harmonic flexures of solid plates and potentially useful in other problems of structural analysis. The method, which was proposed in doctoral thesis by John C. Houbolt, is based on linear theory and incorporates the principle of minimum potential energy. Full realization of its advantages requires use of high-speed computing equipment. After a review of Houbolt's method, results of some applications are presented and discussed. The applications consisted of calculations of the natural modes and frequencies of several uniform-thickness cantilever plates and, as a special case of interest, calculations of the modes and frequencies of the uniform free-free beam. Computed frequencies and nodal patterns for the first five or six modes of each plate are compared with existing experiments, and those for one plate are compared with another approximate theory. Beam computations are compared with exact theory. On the basis of the comparisons it is concluded that the method is accurate and general in predicting plate flexures, and additional applications are suggested. An appendix is devoted t o computing procedures which evolved in the progress of the applications and which facilitate use of the method in conjunction with high-speed computing equipment.

Walton, William C., Jr.↗
NASA NTRS1985

Conducting a thermal conductivity survey

A physically transparent approximate theory of phonon decay rates is presented starting from a pair potential model of the interatomic forces in an insulator or semiconductor. The theory applies in the classical regime and relates the 3-phonon decay rate to the third derivative of the pair potential. Phonon dispersion relations do not need to be calculated, as sum rules relate all the needed quantities directly to the pair potential. The Brillouin zone averaged phonon lifetime turns out to involve a dimensionless measure of the anharmonicity multiplied by an effective density of states for 3-phonon decay. Results are given for rare gas and alkali halide crystals. For rare gases, the results are in good agreement with more elaborate perturbation calculations. Comparison to experimental data on phonon linewidths and thermal conductivity are made.

Allen, P. B.↗
NASA NTRS1986

Contribution of three-body potentials to the binding energy of heavy atoms

The conversion of quantum electrodynamics to a configuration-space Hamiltonian formalism introduces three-electron potentials of relativistic origin. For heavy atoms, it is found that the contribution of these potentials to the inner-shell binding energy is no more than 0.21 eV. This is too small to explain the discrepancy between current theory and experiment. The uniqueness of the potentials obtained in the configuration-space Hamiltonian is also discussed.

Zygelman, B.↗
NASA NTRS1992

All-electron molecular Dirac-Hartree-Fock calculations - Properties of the XH4 and XH2 molecules and the reaction energy XH4 yields XH2 + H2, X = Si, Ge, Sn, Pb

Relativistic corrections to a number of properties of the Group IV hydrides are calculated using the Dirac-Hartree-Fock method. The use of first-order perturbation theory is sufficient to obtain relativistic corrections for Ge, but the effects of spin-orbit interaction and other higher-order effects begin to show for Sn and become important for Pb. The energy of the reaction XH4 yields XH2 + H2 (X = Si, Ge, Sn, and Pb) is also calculated. The results are compared with relativistic effective core potential calculations, first-order perturbation theory calculations, and limited experimental data.

Dyall, Kenneth G.↗
NASA NTRS1989

A Van der Waals-like theory of plasma double layers

A theory describing plasma double layers in terms of multiple roots of the charge density expression is presented. The theory presented uses the fact that equilibrium plasmas shield small potential perturbations linearly; for high potentials, the shielding decreases. The approach is analogous to Van der Waals' theory of simple fluids in which inclusion of approximate expressions for both excluded volume and long range attractive forces sufficiently describes the first-order liquid-gas phase transition.

Katz, Ira↗
NASA NTRS1991

Unified Aeroacoustics Analysis for High Speed Turboprop Aerodynamics and Noise: Development of Theory for Blade Loading, Wakes, and Noise - Volume 1

A unified theory for the aerodynamics and noise of advanced turboprops are presented. Aerodynamic topics include calculation of performance, blade load distribution, and non-uniform wake flow fields. Blade loading can be steady or unsteady due to fixed distortion, counter-rotating wakes, or blade vibration. The aerodynamic theory is based on the pressure potential method and is therefore basically linear. However, nonlinear effects associated with finite axial induction and blade vortex flow are included via approximate methods. Acoustic topics include radiation of noise caused by blade thickness, steady loading (including vortex lift), and unsteady loading. Shielding of the fuselage by its boundary layer and the wing are treated in separate analyses that are compatible but not integrated with the aeroacoustic theory for rotating blades.

Hanson, D. B.↗
NASA NTRS1971

Experimental tests of relativistic gravitation theories

Experimental tests were studied for determining the potential uses of future deep space missions in studies of relativistic gravity. The extensions to the parametrized post-Newtonian framework to take explicit account of the solar system's center of mass relative to the mean rest frame of the Universe is reported. Discoveries reported include the Machian effects of motion relative to the universal rest frame. Summaries of the JPL research are included.

Anderson, J. D.↗
NASA NTRS1986

A new approach for solving the three-dimensional steady Euler equations. I - General theory

The present iterative procedure combines the Clebsch potentials and the Munk-Prim (1947) substitution principle with an extension of a semidirect Cauchy-Riemann solver to three dimensions, in order to solve steady, inviscid three-dimensional rotational flow problems in either subsonic or incompressible flow regimes. This solution procedure can be used, upon discretization, to obtain inviscid subsonic flow solutions in a 180-deg turning channel. In addition to accurately predicting the behavior of weak secondary flows, the algorithm can generate solutions for strong secondary flows and will yield acceptable flow solutions after only 10-20 outer loop iterations.

Chang, S.-C.↗
NASA NTRS1979

Spacecraft charging results for the DSCS-3 satellite

Spacecraft charging results are presented for the DSCS-3 satellite for a severe geomagnetic substorm. Spacecraft charging results were obtained by the use of the electrostatic charging analysis program (ESCAP). The ESCAP computer code which can determine both the transient or steady state differential charging potentials was an engineering design tool that utilized a circuit theory approach to spacecraft charging. Using the ESCAP code, the steady state (static) differential potentials of the outer spacecraft surfaces and metallic structure were obtained for the DSCS-3 satellite when under the influence of a severe geomagnetic substorm during the local midnight-to-dawn quadrant of its geosynchronous orbital path. The results indicated that, in the steady state, most of the DSCS-3 outer surface materials will not achieve differential potentials large enough to produce an electrostatic discharge.

Massaro, M. J.↗
NASA NTRS2009

Interstellar Propulsion Research: Realistic Possibilities and Idealistic Dreams

Though physically possible, interstellar travel will be exceedingly difficult. Both the known laws of physics and the limits of our current understanding of engineering place extreme limits on what may actually be possible. Our remote ancestors looked at the night sky and assumed those tiny points of light were campfires around which other tribes were gathered -- and they dreamed of someday making the trip to visit them. In our modern era, we've grown accustomed to humans regularly traveling into space and our robots voyaging ever-deeper into the outer edges of our solar system. Traveling to those distant campfires (stars) has been made to look easy by the likes of Captains Kirk and Picard as well as Han Solo and Commander Adama. Our understanding of physics and engineering has not kept up with our imaginations and many are becoming frustrated with the current pace at which we are exploring the universe. Fortunately, there are ideas that may one day lead to new physical theories about how the universe works and thus potentially make rapid interstellar travel possible -- but many of these are just ideas and are not even close to being considered a scientific theory or hypothesis. Absent any scientific breakthroughs, we should not give up hope. Nature does allow for interstellar travel, albeit slowly and requiring an engineering capability far beyond what we now possess. Antimatter, fusion and photon sail propulsion are all candidates for relatively near-term interstellar missions. The plenary lecture will discuss the dreams and challenges of interstellar travel, our current understanding of what may be possible and some of the "out of the box" ideas that may allow us to become an interstellar species someday in the future.

Johnson, Les↗
NASA NTRS1988

Intermolecular potential for thermal H2O-He collisions

Theoretical potentials for rotational excitation of H2O by He were constructed via several methods, all of which start with a large basis set SCF interaction. The semiempirical Hartree-Fock with damped dispersion model adds a damped long-range attraction with parameters adjusted to fit experimental total differential cross sections. Purely ab initio potentials add correlation energies obtained via perturbation theory (MP2 and MP4) or a variational method (ICF1). Scattering calculations were performed on all surfaces to compare wih available beam scattering and pressure broadening data and to assess sensitivity of state-to-state rates to uncertainties in the potential. From comparison with the limited experimental data, the ICF1 surface appears to be marginally better than the MP4 surface. Thermal rates calculated from this surface should be accurate to better than 50 percent, at least for the larger, more important rates.

Palma, Amedeo↗
NASA NTRS1973

Recovery of sounding rocket payloads by center-of-gravity position control.

A computerized procedure has been developed to predict dynamic pressure at parachute deployment altitude. Inputs to the program are payload geometry, center-of-gravity position, velocity, and altitude at start of reentry. Payload is assumed to be trimmed, and potential and cross-flow drag theory is used to compute trim angle-of-attack and trim drag coefficient as a function of free-stream Mach number. The computations involve only simple geometric quantities and closed-form equations. Payload flight path is assumed to be vertical, and equations of motion of the payload are integrated from start of reentry to chute deployment altitude.

Mcgarvey, J. F.↗