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At least 1,693 records · Page 94
All sourcesNASA NTRSDOE OSTI
NASA NTRS1990

Comparison of analytical and physical modeling of planetesimal accumulation

Three cases for which the analytic solutions to the planetary embryo coagulation equation are known, namely those of constant coagulation rate, of rate proportional to the sum of the masses of the two colliding bodies, and of rate proportional to the product of their masses, are presently used to test the mathematical validity of the Wetherill and Stewart (1989) physical model for the evolution of planetary embryos' mass distribution. In all cases, excellent agreement is found between numerical physical modeling results and those of the analytic solutions. The treatment of the runaway case can proceed via extension of the efforts of Trubnikov (1971).

Wetherill, George W.↗
NASA NTRS1991

The physics of the Harang discontinuity

It is found that the Harang discontinuity results from a dawn/dusk asymmetry in total energetic plasma content across the nightside of the magnetosphere. This effect, referred to as the dawnside depletion effect, is discussed in detail. The basic physical assumptions usually regarded as appropriate for the description of the closed field line plasma sheet are outlined and a quantitative, bounce-averaged description of the particle drifts is presented. A simplified, illustrative model is given of the magnetospheric geometry and convection electric field in order to demonstrate the dawnside depletion effect and examine its ionospheric and magnetospheric implications. For a quantitative examination of the implications of the dawnside depletion effect and the resulting ionospheric electric field structure, the Rice convection model Z(RCM) is used. A brief description is provided of the RCM, the specific run setups, and the RCM results. A general discussion of the basic physical connection between cross-tail gradients in plasma contents and the Harang discontinuity is presented.

Erickson, G. M.↗
NASA NTRS1991

Physics in strong magnetic fields near neutron stars

Electromagnetic phenomena occurring in the strong magnetic fields of neutron stars are currently of great interest in high-energy astrophysics. Observations of rotation rate changes and cyclotron lines in pulsars and gamma-ray bursts indicate that surface magnetic fields of neutron stars often exceed a trillion gauss. In fields this strong, where electrons behave much as if they were in bound atomic states, familiar processes undergo profound changes, and exotic processes become important. Strong magnetic fields affect the physics in several fundamental ways: energies perpendicular to the field are quantized, transverse momentum is not conserved, and electron-positron spin is important. Neutron stars therefore provide a unique laboratory for the study of physics in extremely high fields that cannot be generated on earth.

Harding, Alice K.↗
NASA NTRS1990

Numerical simulation of instability and transition physics

The study deals with the algorithm technology used in instability and transition simulations. Discretization methods are outlined, and attention is focused on high-order finite-difference methods and high-order centered-difference formulas. One advantage of finite-difference methods over spectral methods is thought to be in implementation of nonrigorous boundary conditions. It is suggested that the next significant advances in the understanding of transition physics and the ability to predict transition will come with more physically-realistic simulations. Compressible-flow algorithms are discussed, and it is noted that with further development, exploration of bypass mechanism on simple bodies at high speed would be possible.

Streett, C. L.↗
NASA NTRS1990

Flare physics at high energies

High-energy processes, involving a rich variety of accelerated particle phenomena, lie at the core of the solar flare problem. The most direct manifestation of these processes are high-energy radiations, gamma rays, hard X-rays and neutrons, as well as the accelerated particles themselves, which can be detected in interplanetary space. In the study of astrophysics from the moon, the understanding of these processes should have great importance. The inner solar system environment is strongly influenced by activity on the sun; the physics of solar flares is of great intrinsic interest; and much high-energy astrophysics can be learned from investigations of flare physics at high energies.

Ramaty, R.↗
NASA NTRS1990

Particle astronomy and particle physics from the moon - The particle observatory

Promising experiments from the moon using particle detectors are discussed, noting the advantage of the large flux collecting power Pc offered by the remote, stable environment of a lunar base. An observatory class of particle experiments is presented, based upon proposals at NASA's recent Stanford workshop. They vary from neutrino astronomy, particle astrophysics, and cosmic ray experiments to space physics and fundamental physics experiments such as proton decay and 'table-top' arrays. This research is background-limited on earth, and it is awkward and unrealistic in earth orbit, but is particularly suited for the moon where Pc can be quite large and the instrumentation is not subject to atmospheric erosion as it is (for large t) in low earth orbit.

Wilson, Thomas L.↗
NASA NTRS1992

Analysis of a generalized dual reflector antenna system using physical optics

Reflector antennas are widely used in communication satellite systems because they provide high gain at low cost. Offset-fed single paraboloids and dual reflector offset Cassegrain and Gregorian antennas with multiple focal region feeds provide a simple, blockage-free means of forming multiple, shaped, and isolated beams with low sidelobes. Such antennas are applicable to communications satellite frequency reuse systems and earth stations requiring access to several satellites. While the single offset paraboloid has been the most extensively used configuration for the satellite multiple-beam antenna, the trend toward large apertures requiring minimum scanned beam degradation over the field of view 18 degrees for full earth coverage from geostationary orbit may lead to impractically long focal length and large feed arrays. Dual reflector antennas offer packaging advantages and more degrees of design freedom to improve beam scanning and cross-polarization properties. The Cassegrain and Gregorian antennas are the most commonly used dual reflector antennas. A computer program for calculating the secondary pattern and directivity of a generalized dual reflector antenna system was developed and implemented at LeRC. The theoretical foundation for this program is based on the use of physical optics methodology for describing the induced currents on the sub-reflector and main reflector. The resulting induced currents on the main reflector are integrated to obtain the antenna far-zone electric fields. The computer program is verified with other physical optics programs and with measured antenna patterns. The comparison shows good agreement in far-field sidelobe reproduction and directivity.

Acosta, Roberto J.↗
NASA NTRS1992

Cosmological texture is incompatible with Planck-scale physics

Nambu-Goldstone modes are sensitive to the effects of physics at energies comparable to the scale of spontaneous symmetry breaking. We show that as a consequence of this the global texture proposal for structure formation requires rather severe assumptions about the nature of physics at the Planck scale.

Holman, Richard↗
NASA NTRS1991

Analyzing high energy physics data using database computing: Preliminary report

A proof of concept system is described for analyzing high energy physics (HEP) data using data base computing. The system is designed to scale up to the size required for HEP experiments at the Superconducting SuperCollider (SSC) lab. These experiments will require collecting and analyzing approximately 10 to 100 million 'events' per year during proton colliding beam collisions. Each 'event' consists of a set of vectors with a total length of approx. one megabyte. This represents an increase of approx. 2 to 3 orders of magnitude in the amount of data accumulated by present HEP experiments. The system is called the HEPDBC System (High Energy Physics Database Computing System). At present, the Mark 0 HEPDBC System is completed, and can produce analysis of HEP experimental data approx. an order of magnitude faster than current production software on data sets of approx. 1 GB. The Mark 1 HEPDBC System is currently undergoing testing and is designed to analyze data sets 10 to 100 times larger.

Baden, Andrew↗
NASA NTRS1991

Review of the physics of enhancing vortex lift by unsteady excitation

A review aimed at providing a physical understanding of the crucial mechanisms for obtaining super lift by means of unsteady excitations is presented. Particular attention is given to physical problems, including rolled-up vortex layer instability and receptivity, wave-vortex interaction and resonance, nonlinear streaming, instability of vortices behind bluff bodies and their shedding, and vortex breakdown. A general theoretical framework suitable for handling the unsteady vortex flows is introduced. It is suggested that wings with swept and sharp leading edges, equipped with devices for unsteady excitations, could yield the first breakthrough of the unsteady separation barrier and provide super lift at post-stall angle of attack.

Wu, J. Z.↗
NASA NTRS1990

Cosmic ray studies of solar and heliospheric physics - Goals for the 1990's and beyond

Recommendations are made for programs to meet high-priority objectives for solar and heliospheric physics. These priority areas include: the physics of the ISM and the heliospheric boundary; solar particle acceleration, transport, and composition; the ultraheavy composition of solar energetic particles; the large-scale structure of the heliosphere and solar modulation; the microphysics of interplanetary acceleration and shock acceleration. Measures that should be taken to continue and enhance ongoing programs are summarized.

Wiedenbeck, Mark E.↗
NASA NTRS1992

The secondary outburst maximum of T Coronae Borealis - Implications for the physics of accretion disks

The hypothesis of Webbink (1976) that the accretion of a torus of matter onto a main-sequence star caused secondary maxima during the 1866 and 1946 eruptions of the recurrent nova T CrB is examined. Simple 1D calculations show that the accretion disk viscosity must increase with time to produce light curves resembling observations. It is inferred that the physical mechanism responsible for producing the viscous dissipation in the accretion disk has a fast growth rate and saturates to a viscosity parameter of order unity. The good agreement between parameters estimated from observations and those derived from a physical mechanism suggests that the growth rate instability identified by Balbus and Hawley (1991) is a promising source for accretion disk viscosity.

Cannizzo, John K.↗
NASA NTRS1992

Review of the physical and mechanical properties and potential applications of the B2 compound NiAl: Unabridged version of a paper published in International materials review

Considerable work has been performed on NiAl over the last three decades, with an extremely rapid growth in research on this intermetallic occurring in the last few years due to recent interest in this material for electronic and high temperature structural applications. However, many physical properties and the controlling fracture and deformation mechanisms over certain temperature regimes are still in question. Part of this problem lies in the incomplete characterization of many of the alloys previously investigated. Fragmentary data on processing conditions, chemistry, microstructure and the apparent difficulty in accurately measuring composition has made direct comparison between individual studies sometimes tenuous. Therefore, the purpose of this review is to summarize all available mechanical and pertinent physical properties on NiAl, stressing the most recent investigations, in an attempt to understand the behavior of NiAl and its alloys over a broad temperature range.

Noebe, Ronald D.↗
NASA NTRS1992

Physical interpretation and development of ultrasonic nondestructive evaluation techniques applied to the quantitative characterization of textile composite materials

The development and implementation of advanced ultrasonic nondestructive evaluation methods applied to the characterization of composite materials requires a better understanding of the physics underlying the interaction of ultrasound with the material. The purpose of this investigation is to identify and characterize the features of complex, three dimensional materials that limit the ability of ultrasound to detect flaws in this broad class of emerging materials. In order to explore the interaction of ultrasound with such complex media, we investigate the characteristics of ultrasonic fields which have propagated through samples with complex geometries and/or internal architecture. We focus on the physics that underlies the detection of flaws in such materials.

Miller, James G.↗
NASA NTRS1992

Theoretical studies of the physics of the solar atmosphere

Significant advances in our theoretical basis for understanding several physical processes related to dynamical phenomena on the sun were achieved. We have advanced a new model for spicules and fibrils. We have provided a simple physical view of resonance absorption of MHD surface waves; this allowed an approximate mathematical procedure for obtaining a wealth of new analytical results which we applied to coronal heating and p-mode absorption at magnetic regions. We provided the first comprehensive models for the heating and acceleration of the transition region, corona, and solar wind. We provided a new view of viscosity under coronal conditions. We provided new insights into Alfven wave propagation in the solar atmosphere. And recently we have begun work in a new direction: parametric instabilities of Alfven waves.

Hollweg, Joseph V.↗
NASA NTRS1992

Microgravity fluid physics research in the Space Station Freedom era

Microgravity fluid physics covers an exciting range of established and potential fields of scientific research. Areas in which the Microgravity Science and Applications Division of NASA's Office of Space Science and Applications is currently supporting research include: multiphase flow and phase change heat transfer, behavior of granular media and colloids; and interface dynamics, morphological stability, and contact line phenomena. As they contribute to our knowledge of fluid behavior, advances in these areas will enhance our understanding of materials processing on Earth and in space, and will contribute to technologies as diverse as chemical extraction, the prediction of soil behavior in earthquakes, and the production of oil reservoirs. NASA' s primary platform for research in microgravity fluid physics will soon be the Fluid Physics/Dynamics Facility on Space Station Freedom. This facility shares a rack for control and utilities with the Modular Combustion Facility, and has one rack for experiment-unique instruments. It is planned to change out the content of the experiment-unique rack at intervals on the order of one year. In order to obtain a maximum return on the operation of the facility during these intervals, the research community must carefully plan and coordinate an effort that brings the efforts of many investigators to bear on problems of particular importance. NASA is currently working with the community to identify research areas in which microgravity can make a unique and valuable contribution, and to build a balanced program of research around these areas or thrusts. Selections will soon be made from our first solicitation for research in fluid dynamics and transport phenomena. These solicitations will build the research community that will make Space Station Freedom a catalyst for scientific and technological discovery, and offer U.S. scientists in many disciplines a unique opportunity to participate in space science.

Carpenter, Bradley M.↗
NASA NTRS1992

Near-Earth asteroids: Observer alert network and physical observations

This project strives to obtain physical observations on newly discovered Near-Earth Objects (NEO's) in order to provide fundamental data needed to assess the resources available in the population. The goal is acquiring data on all objects brighter than magnitude V= 17.0. To accomplish this, an electronic mail alert and observer information service that informs observers around the world as to the status of physical observations on currently observable NEO's was established. Such data is also acquired ourselves through a cooperative program with European colleagues that uses telescopes on La Palma to obtain spectra of NEO's and through observations made from a local telescope on Tumamoc Hill. This latter telescope has the advantage that large amounts of observing time are available, so that whenever a new NEO's discovered, we can be assured of getting time to observe it.

Davis, Donald R.↗
NASA NTRS1993

The harmonic oscillator and nuclear physics

The three-dimensional harmonic oscillator plays a central role in nuclear physics. It provides the underlying structure of the independent-particle shell model and gives rise to the dynamical group structures on which models of nuclear collective motion are based. It is shown that the three-dimensional harmonic oscillator features a rich variety of coherent states, including vibrations of the monopole, dipole, and quadrupole types, and rotations of the rigid flow, vortex flow, and irrotational flow types. Nuclear collective states exhibit all of these flows. It is also shown that the coherent state representations, which have their origins in applications to the dynamical groups of the simple harmonic oscillator, can be extended to vector coherent state representations with a much wider range of applicability. As a result, coherent state theory and vector coherent state theory become powerful tools in the application of algebraic methods in physics.

Rowe, D. J.↗