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At least 1,351 records · Page 75

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NASA NTRS1979

Radio emission from nova shells

This paper presents data from multifrequency radio observations of the novae HR Del 1967 and FH Ser 1970 made from June 1970 to December 1978 and of V1500 Cyg 1975 made from September 1975 to December 1978. The data for FH Ser and V1500 Cyg constitute a complete history of the important phases of their radio 'light' curves, while all but the rising portion of the optically thick phase of an expanding nova shell is present for HR Del. A simple solution of the spherically symmetric equations of mass and momentum conservation for an isothermal gas, for which pressure gradients are effectively decoupled from the dynamics, is shown to fit the radio data for all three novae quite well. The main features of this model are strong density and velocity gradients that lead to long periods when the 'observable' shell evolves from an optically thick 'disk' to a completely optically thin shell.

Hjellming, R. M.↗

NASA NTRS1981

Formation of a conical distribution and intense ion heating in the presence of hydrogen cyclotron waves

In the considered investigation, it is assumed that the field aligned currents are responsible for producing electrostatic harmonic cyclotron waves (EHC). Using a one-dimensional simulation model in which the electron velocity distribution is maintained by a constant injection of the initial distribution, it is shown that, in contrast to earlier initial value simulations, EHC waves grow to a large amplitude, resulting in the formation of an anisotropic ion velocity distribution. Both the heating rate and the anisotropy are in reasonable agreement with the quasi-linear theory, taking into account the cyclotron resonance. The results show that the saturation is due to the combined effects of wave induced diffusion in an electron velocity space and the heating of ions perpendicularly. Both these effects reduce the growth rate.

Okuda, H.↗

NASA NTRS1982

Numerical simulation of the interaction of the plasma sheet with the lobes of the Earth's magnetotail

Codes involving one and two spatial dimensions and three velocity dimensions were used to model the Earth's magnetotail. It was shown that the magnetotail can become inflated as a consequence of low energy plasma convection toward the neutral plane. The computer study exhibits a conversion of both magnetic field energy and of energy supplied by the convection electric field into particle energy. The numerical simulations suggest that much of the magnetotail substorm morphology may be a simple consequence of an increase, followed by a decrease, in the convection electric field, without the requirement of any magnetospheric size scale plasma instability or other disruptive processes. It is also concluded that the presence of the convection electric field and a continuing replenishment of low energy particles in the magnetotail are both necessary for maintenance of the magnetotail.

Swift, D. W.↗

NASA NTRS1982

The energy dependence of cosmic ray propagation at low energy

The interstellar propagation of cosmic rays is investigated using current 'best estimates' for the partial and total cross sections and their energy dependence and the current model of solar modulation. The experimental boron to carbon ratio is reproduced if the mean of the path length distribution decreases with decreasing energy below approximately 1 GeV/nucleon. This energy dependence is compared to shock acceleration models and dynamical halo models using different galactic wind velocities.

Garcia-Munoz, M.↗

NASA NTRS1982

Increased capabilities of the Langley Mach 7 Scramjet Test Facility

An experimental research program which explored the potential for expanding the operating range of NASA Langley's Mach 7 Scramjet Test Facility is described. The facility uses electric arc-heated air as the test gas and is configured for testing airframe-integrated scramjet engine models at conditions duplicating flight velocities and temperatures. Previous scramjet testing in this facility was limited to a single simulated flight condition of Mach 6.9 at 115,300 ft. altitude. The present arc heater research demonstrated that the facility can be used for scramjet testing at simulated flight conditions from Mach 4 (at altitudes from 77,000 to 114,000 ft.) to Mach 7 (at altitudes from 108,000 to 149,000 ft.). Flow quality was established from the uniformity of measured total temperature profiles. In addition, nitrogen oxide levels in the test stream were determined for relating engine test data to expected flight performance. The test frequency and reliability of the facility were also improved by a novel redesign of the heater downstream electrode and by defining limits for stable arc operation.

Thomas, S. R.↗

NASA NTRS1982

A comprehensive method for preliminary design optimization of axial gas turbine stages

A method is presented that performs a rapid, reasonably accurate preliminary pitchline optimization of axial gas turbine annular flowpath geometry, as well as an initial estimate of blade profile shapes, given only a minimum of thermodynamic cycle requirements. No geometric parameters need be specified. The following preliminary design data are determined: (1) the optimum flowpath geometry, within mechanical stress limits; (2) initial estimates of cascade blade shapes; (3) predictions of expected turbine performance. The method uses an inverse calculation technique whereby blade profiles are generated by designing channels to yield a specified velocity distribution on the two walls. Velocity distributions are then used to calculate the cascade loss parameters. Calculated blade shapes are used primarily to determine whether the assumed velocity loadings are physically realistic. Model verification is accomplished by comparison of predicted turbine geometry and performance with four existing single stage turbines.

Jenkins, R. M.↗

NASA NTRS1982

The plumes of Titan

The plasma density enhancements observed during the Voyager 1 Titan encounter are interpreted in terms of a single plume of plasma wrapped around Saturn by corotation. Consideration is given to the radial motions of the plume driven by fluctuations in solar wind pressure and the dispersal of the plasma by the centrifugal interchange instability, by heating, and by azimuthal acceleration. It is shown that Saturn cannot readily impose corotation directly on the plume and, incidentally, that the total dissipation associated with the Titan-magnetosphere interaction is insufficient to supply the power to produce the observed Titan ultraviolet airglow. A pickup velocity of 8 km/s is inferred on the basis of the observed velocity and a standing wave model. It is found that the Voyager plasma observations are consistent with the predictions of the model.

Eviatar, A.↗

NASA NTRS1983

The effect of matrix properties and fiber properties on impact failure mechanics

The low-velocity impact problem in graphite/epoxy composite sheets must be solved before large amounts of that material can be used in commercial aircraft. Many of the low-velocity impacts that affect aircraft parts occur during normal ground operations and maintenance. Service equipment and tools have masses above 1 kg, and at velocities of less than 3 m/s can impact structural parts with energies higher than composites can endure without degradation of stiffness or strength. Simple solutions were developed for large-mass, low-velocity impacts which can be modeled as quasi-static events. Static test data and impact data show that the fiber properties control the impact energy which can be absorbed before penetration. Matrix shear strength and peel resistance control the extent of delamination. Comparison of results from tough matrix and brittle matrix composites show that although tough matrices reduce the extent of delamination, they lead to more fiber damage in the contact area.

Elber, W.↗

NASA NTRS1983

Stellar evolution in N-body simulations of disk galaxies. I

The Kalnajs (1972, 1976) Omega models of global mass and velocity distributions are employed in the present two-dimensional N-body simulation, which allows for a spectrum of particle masses, stellar explosions, explosion remnant interactions with an interstellar medium, and the creation of new stars from the gas. Two sequences of runs using the Omega values of 0.8 and 0.9 examine the separate and combined effects of particle mass distribution, the gravitational influence of an interstellar gas distribution on the N-body particles, and stellar evolution, allowing for stellar explosions and star formation from the gas. It is found that both Omega values' nonequilibrium results dramatically change when evolution is allowed to occur. These results call for more realistic coupled N-body and evolution simulations in order to improve the understanding of disk galaxy evolution.

Comins, N. F.↗

NASA NTRS1985

Analysis of turbulent underexpanded jets. I - Parabolized Navier-Stokes model, SCIPVIS

A new computational model (SCIPVIS) is described which predicts the multiple-cell wave/shock structure in underexpanded or overexpanded turbulent jets. SCIPVIS solves the parabolized Navier-Stokes jet-mixing equations utilizing a shock-capturing approach in supersonic regions of the jet and a pressure-split approach in subsonic regions. Turbulence processes are represented by the solution of compressibility-corrected two-equation turbulence models. SCIPVIS presently analyzes jets exhausting into a quiescent or supersonic external stream for which a single-pass spatial-marching solution can be obtained. The features of SCIPVIS are reviewed, and calculations are described exhibiting the influence of turbulence modelling, jet temperature, and flight velocity on the jet shock structure.

Dash, S. M.↗

NASA NTRS1986

A model of Comet P/Giacobini-Zinner

A computer model of Comet P/Giacobini-Zinner is presented which contains photoprocesses, gas-phase chemical kinetics, energy balance, multifluid hydrodynamics with a transition to free molecular flow, and solar wind interaction. Recently the physics for electrons in the model has been improved by including electron impact ionization and dissociation and separately accounting for electron energetics. Electron heating and cooling mechanisms include photoprocesses, recombination processes, inelastic and elastic collisions with heavy molecules, and expansion cooling. The model incorporates an internally consistent interaction of the solar wind with the coma gas using the axisymmetric ideal fluid dynamic equations. The nuclear size and composition have been chosen to make the calculations relevant to the September 11, 1985 International Cometary Explorer (ICE) encounter with Comet P/Giacobini-Zinner. Model profiles of the temperature, velocity, and number density of the electrons are in good agreement with measurements along ICE's trajectory. The results indicate the probe passed through a region of the coma at the onset of the plasma tail.

Boice, D. C.↗

NASA NTRS1985

Phase shifting two coupled circadian pacemakers - Implications for jet lag

Two Van der Pol oscillators with reciprocal linear velocity coupling are utilized to model the response of the human circadian timing system to abrupt displacements of the environmental time cues (zeitgebers). The core temperature rhythm and sleep-wake cycle simulated by the model are examined. The relationship between the masking of circadian rhythms by environmental variables and behavioral and physiological events and the rates of resynchronization is studied. The effects of zeitgeber phase shifts and zeitgeber strength on the resynchronization rates are analyzed. The influence of intrinsic pacemakers periods and coupling strength on resynchronization are investigated. The simulated data reveal that: resynchronization after a time zone shift depends on the magnitude of the shift; the time of day of the shift has little influence on resynchronization; the strength of zeitgebers affects the rate and direction of the resynchronization; the intrinsic pacemaker periods have a significant effect on resynchronization; and increasing the coupling between the oscillators results in an increase in the rate of resynchronization. The model data are compared to transmeridian flight studies data and similar resynchronization patterns are observed.

Gander, P. H.↗

NASA NTRS1988

Phase-resolved IUE observations of AM Herculis system E1405-451

The first phase-resolved IUE observations of the AM Herculis system E1405-451 is reported. The spectra show bright emission lines from N V, C IV, and He II, whose centroids vary drastically during the 101.5 minute period. The radial velocities measured from the line centroids show a sinusoidal variation with a semiamplitude of 300 km/s. An explanation for the observed emission line velocities is considered using a model for the system having inclination i = 60 deg and magnetic colatitude Delta = 20 deg.

Takalo, L. O.↗

NASA NTRS1989

Electron velocity space hole modes and narrowband electrostatic noise in the distant tail

Narrowband electrostatic noise (NEN) is frequently observed upstream of the slow shocks in the distant tail. It is suggested that NEN represents a nonstandard plasma wave mode which results from a hole in the low energy electron velocity distribution. A very simple model which plausibly indicates how a hole might be formed by the interaction of the electrons with the slow shock is presented. The hole mode is destabilized by a positive slope in the parallel electron distribution and/or by a weak beam of energetic ions which escape upstream from the slow shock.

Coroniti, Ferdinand V.↗

NASA NTRS1990

The formation and structure of fluffy cometary nuclei from random accumulation of grains

It is now generally accepted that solid bodies in the solar system are formed by accumulation of solid grains. The mechanism of formation of smaller bodies, less than some tens of kilometers in radius, is investigated. These include planetesimals and comets. Results indicate that essentially complete agglomeration occurs for relative velocities of less than about 1000 cm/sec and fairly efficient mass build-up occurs up to 5000/cm. These velocities are consistent with current models of the primordial solar nebula. The temperature produced in a collision will be under 20 K. For ice/dust grains, the resultant aggregates will be of low density with their volatiles preserved. Cometary nuclei with radii of a few km will be irregularly shaped and fragile with low mean density and significant interanl density variations. The characteristics of such a structure could account for the observed properties of comets.

Donn, B. D.↗

NASA NTRS1991

Tidal disruption of inviscid planetesimals

In view of previous efforts' demonstration that strongly dissipative planetesimals are immune to tidal disruption, an examination is presently conducted of the complementary case of inviscid planetesimals arising from collisions that are sufficiently energetic to entirely melt the resulting planetesimal and debris. The tidal disruption is numerically simulated by means of the smoothed particle hydrodynamics (SPH) code of Cameron and Benz (1991), concentrating on the tidal disruption of 0.01 earth-mass planetesimals passing by the earth with variations in the impact parameter at perigee and velocity at infinity. The SPH models show that tidal forces during a close encounter can efficiently convert orbital angular momentum into spin angular momentum, thereby initiating equatorial mass-shedding to inviscid planetesimals that have been spun up beyond the limit of rotational stability.

Boss, A. P.↗

NASA NTRS1991

Is the kinematically distinct core in NGC 5982 necessarily the remnant of an accreted companion?

NGC 5982 is an elliptical galaxy with a 'kinematically distinct core' whose projected rotation axis is about 90 deg away from that of the galaxy's outer parts. Distinct cores in other ellipticals have been attributed to dynamically distinct subsystems that are the remnants of accreted companions. Here, a model for NGC 5982 is presented in which the kinematic twist in the core results simply from the projection of the major families of circulating orbits in a triaxial potential. Orbits in the model are populated smoothly, and the core is not a dynamical subsystem. The model predicts a complicated overall velocity field for the galaxy, with particularly low velocity amplitudes (15 km/s or less) along a direction 150 deg counterclockwise from the major axis, a direction not yet observed spectroscopically.

Statler, Thomas S.↗

NASA NTRS1992

Hubble Space Telescope Wide Field Camera imaging of the gravitational lens 2237 + 0305

Images of the gravitational lens system 2237 + 0305, taken with the HST Wide Field Camera, are analyzed. Positions for the four quasar images, accurate to +/-0.015 arcsec, and relative magnitudes in U and R, accurate to +/-0.06 and 0.04 mag, respectively, are determined. The upper limits on the observed brightness of the fifth image are found to be less than or approximately equal to 7 percent of the brightest quasar image. The mass of the lens inside 0.9 arcsec is found to be 1.08 +/-0.02 x 10 exp 10 solar masses/h100 corresponding to a mass-to-light ratio in B of 12.3h100. This solar mass/solar luminosity estimate agrees with values obtained from stellar dynamics for other elliptical galaxies. A comparison of predictions from this mass model with the measured central velocity dispersion yields a distance-independent agreement to within 10 percent, assuming isotropic velocity dispersions.

Rix, Hans-Walter↗