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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 217 records · Page 12
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NASA NTRS1986

Time-dependent numerical simulation of hot ion outflow from the polar ionosphere

The time-dependent hydrodynamic model of Gombosi et al. (1985) was used to investigate the effects of bulk plasma heating on ion flows in the polar ionosphere. In the analysis, coupled time-dependent continuity, momentum, and energy equations of a two-ion (H/+/ and O/+/) quasi-neutral plasma were solved between 200 and 8000 km for polar wind conditions. In spite of its several limitations, the model can serve to indicate the gross behavior of polar region plasma flows. Two specific cases, ion heating and electron heating, were considered. The calculated temperature and flow profiles suggest that ion heating, rather than electron heating, generates the upwelling ion events, for which large transient O(+) outflows accompanied by elevated ion temperatures were observed.

Gombosi, T. I.↗
NASA NTRS1987

The progenitors and products of supernovae

Much of the recent radio and X-ray observational work relevant to the questions of the progenitor populations and explosion products of supernovae is reviewed. Several illustrative examples are given of how X-ray data on shell-type supernova remnants, coupled with analytic and numerical hydrodynamic models, can constrain the mass and evolutionary state of the stellar precursor. The population of Crab-like remnants is examined, pointing out the restrictions imposed on explosion models by the statistics of collapsed remnant cores.

Helfand, David J.↗
NASA NTRS1987

A numerical study of transient, thermally-conductive solar wind

A numerical analysis of transient solar wind starting at the solar surface and arriving at 1 AU is performed by an implicit numerical method. The model hydrodynamic equations include thermal conduction terms for both steady and unsteady simulations. Simulation results show significant influence of thermal conduction on both steady and time-dependent solar wind. Higher thermal conduction results in higher solar wind speed, higher temperature, but lower plasma density at 1 AU. Higher base temperature at the solar surface gives lower plasma speed, lower temperature, but higher density at 1 AU. Higher base density, on the other hand, gives lower velocity, lower temperature, but higher density at 1 AU.

Han, S. M.↗
NASA NTRS1987

Cosmic ray-modified stellar winds. I - Solution topologies and singularities

In the present two-fluid hydrodynamical model for stellar wind flow modification due to its interaction with Galactic cosmic rays, these rays are coupled to the stellar wind by either hydromagnetic wave scattering or background flow irregularity propagation. The background flow is modified by the cosmic rays via their pressure gradient. The system of equations used possesses a line of singularities in (r, u, P sub c)-space, or a two-dimensional hypersurface of singularities in (r, u, P sub c, dP sub c/dr)-space, where r, u, and P sub c are respectively the radial distance from the star, the radial wind flow speed, and the cosmic ray pressure. The singular points may be nodes, foci, or saddle points.

Ko, C. M.↗
NASA NTRS1988

Modified non-linear Burgers' equations and cosmic ray shocks

A reductive perturbation scheme is used to derive a generalized non-linear Burgers' equation, which includes the effects of dispersion, in the long wavelength regime for the two-fluid hydrodynamical model used to describe cosmic ray acceleration by the first-order Fermi process in astrophysical shocks. The generalized Burger's equation is derived for both relativistic and non-relativistic cosmic ray shocks, and describes the time evolution of weak shocks in the theory of diffusive shock acceleration. The inclusion of dispersive effects modifies the phase velocity of the shock obtained from the lower order non-linear Burger's equation through the introduction of higher order terms from the long wavelength dispersion equation. The travelling wave solution of the generalized Burgers' equation for a single shock shows that larger cosmic ray pressures result in broader shock transitions. The results for relativistic shocks show a steepening of the shock as the shock speed approaches the relativistic cosmic ray sound speed. The dependence of the shock speed on the cosmic ray pressure is also discussed.

Zank, G. P.↗
NASA NTRS1989

Temporal features of the outflow of heavy ionospheric ions in response to a high altitude plasma cavity

Using a hydrodynamic model for the plasma, it is demonstrated that the auroral plasma cavity is capable of drawing an appreciably large flux of oxygen ions, which are normally gravitationally bound. This escape mechanism of O(+) does not involve any additional heating or acceleration of the plasma in the ionosphere. The temporal evolution of the outflow shows that it starts near the cavity and penetrates into the ionosphere with a flux front moving down with the ion-acoustic speed. A steady outflow is reached in a few hours for a cavity at a height of one R(e) if the cavity is maintained. This time reduces to less than 1 hour for cavity heights less than 3000 km. During the transient state the outflux can appreciably exceed that in the steady state. In view of the horizontal convection, the transient outflow is of special significance. As the cavity descends, the ionospheric flux of O(+) increases; for the lower edge of the cavity in the altitude range 3000-10,000 km, the steady flux is found to be in the range 10 to the 7th to 10 to the 9th. An analysis for the steady state shows that outflow is controlled by the density scale height in the cavity.

Singh, Nagendra↗
NASA NTRS1989

On wave stability in relativistic cosmic-ray hydrodynamics

Wave stability of a two-fluid hydrodynamical model describing the acceleration of cosmic rays by the first-order Fermi mechanism in relativistic, cosmic-ray-modified shocks is investigated. For a uniform background state, the short- and long-wavelength wave speeds are shown to interlace, thus assuring wave stability in this case. A JWKB analysis is performed to investigate the stability of short-wavelength thermal gas sound waves in the smooth, decelerating supersonic flow upstream of a relativistic, cosmic-ray-modified shock. The stability of the waves is assessed both in terms of the fluid velocity and density perturbations, as well as in terms of the wave action. The stability and interaction of the short-wavelength cosmic-ray coherent mode with the background flow is also studied.

Webb, G. M.↗
NASA NTRS1989

Io meteorology - How atmospheric pressure is controlled locally by volcanos and surface frosts

The present modification of the Ingersoll et al. (1985) hydrodynamic model of the SO2 gas sublimation-driven flow from the day to the night side of Io includes the effects of nonuniform surface properties noted in observational studies. Calculations are conducted for atmospheric pressures, horizontal winds, sublimation rates, and condensation rates for such surface conditions as patchy and continuous frost cover, volcanic venting, surface temperature discontinuities, subsurface cold trapping, and the propagation of insolation into the frost. While pressure is found to follow local vapor pressure away from the plumes, it becomes higher inside them.

Ingersoll, Andrew P.↗
NASA NTRS1989

An overview of solar and stellar flare research

A overview of the topics concerning solar and stellar flares at the IAU Colloquium No. 104 is presented. Work being done on the location of flares, the characteristics of solar flares, the optical or white-light flare, and the question of microflaring is reviewed. Theoretical questions and hydrodynamic models are examined, and suggested areas for future investigations are addressed.

Haisch, Bernhard M.↗
NASA NTRS1990

Herbig-Haro objects

The IUE (International Ultraviolet Explorer) spectra of Herbig-Haro (HH) objects and their relation to the very detailed optical spectra available for these objects are studied. Useful information about the physics of HH objects in general and especially about their hydrodynamic models which are contained in these IUE observations is outlined. The merged spectra of high excitation, low excitation HH objects (like HH43 and HH47), and peculiar HH objects are discussed. Results of the spatial variation of lines and continua in HH objects are discussed. Their compatibility with predictions for aerodynamic and especially bow shock models is discussed. The problems arising in the interpretation of the continuous energy distribution in the short wavelength range are discussed. They require the presence of other emission mechanisms in addition to the collisionally enhanced two photon continuum. In HH43 the fluorescent H2 line emission comes from a surprisingly small spatial region. Implications of these results are discussed.

Boehm, K. H.↗
NASA NTRS1990

Line asymmetries in G and K dwarfs - Dependence on spectral type and activity

Observations of photospheric line asymmetries in 12 G and 11 K dwarfs, particularly the Fe I 6151.62 A line, are examined to determine the amplitudes and shapes of the bisectors. Bisector shapes are compared for magnetically inactive and active stars, and correlations between the slope and the stellar angular velocity are investigated. G stars have increasingly negative slopes with larger stellar angular velocity while K-star slopes become more positive, an effect which is shown to be consistent with recent hydrodynamical models.

Saar, Steven H.↗
NASA NTRS1992

Effects of equatorially trapped ions on refilling of the plasmasphere

The kinetic aspects of shock formation in response to the equatorial heating of ions are reported by means of small-scale kinetic simulations of countersteaming plasma flows along a spatially varying magnetic field having the features of the dipolar magnetic field lines. Attention is given to the trapping of the field-aligned flowing ions in response to the simulated equatorial heating of ions and the associated self-consistent structures in the electric potentials and the f-plasma flow pattern. Trapping is found to lead to the formation of electrostatic shocks, which propagate away from the 'equator' as seen in the hydrodynamic model of Singh and Torr (1990). The shocks form near the mirror points of the 'equatorially' heated ions. The second stage of the refilling with equatorially trapped ions is reported. A variety of structures in density and temperature distributions as the refilling proceeds slowly is found.

Singh, Nagendra↗
NASA NTRS1993

Static galactic halo and galactic wind

Although the exact state of the interstellar medium (ISM) in our Galaxy (other galaxies as well) is not clear at all, the 'common consensus' is that a rough pressure balance (or equipartition of energy) exists between different components and phases: cold, warm, hot phases of the ISM, magnetic field, cosmic rays, etc. If the halo of a galaxy is taken to be an extension of the ISM, then its structure is influenced by various ISM components. A 'complete' description of the halo is evidently very complicated. This paper gives a brief account on cosmic ray halo, which emphasizes the role played by cosmic rays. The interaction between cosmic rays and thermal plasma is facilitated by magnetic field. The cosmic rays are scattered by hydromagnetic waves (e.g., Alfven waves) which in turn can be generated by cosmic ray streaming instability. This constitutes a self-consistent picture. Since we are interested in the structure of the halo, we adopted a hydrodynamic model in which the cosmic rays and waves are described by their pressures. In general there are two classes of halos: static and dynamic.

Ko, Chung-Ming↗
NASA NTRS1992

Lewis number effect on the propagation of premixed flames in closed tubes

The evolution of a premixed flame under conditions of confinement is studied theoretically. The analysis is based on a hydrodynamic model in which the flame is treated as a surface of discontinuity. The flame structure is assumed to be quasi-steady with a high activation energy and a large heat release. Its resolution in the postignition period yields a coupled system of equations for the determination of the pressure and the burning rate. The analysis also resolves the thermal and flow fields on either side of the flame and determines the instantaneous location of the flame front together with the overall time required for the flame to reach the end of the tube. The results indicate that qualitatively distinct behaviors are possible for mixtures depending on whether their Lewis numbers Le are less or greater than one.

Mcgreevy, J. L.↗
NASA NTRS1993

Numerical simulation of plasma processes driven by transverse ion heating

The plasma processes driven by transverse ion heating in a diverging flux tube are investigated with numerical simulation. The heating is found to drive a host of plasma processes, in addition to the well-known phenomenon of ion conics. The downward electric field near the reverse shock generates a doublestreaming situation consisting of two upflowing ion populations with different average flow velocities. The electric field in the reverse shock region is modulated by the ion-ion instability driven by the multistreaming ions. The oscillating fields in this region have the possibility of heating electrons. These results from the simulations are compared with results from a previous study based on a hydrodynamical model. Effects of spatial resolutions provided by simulations on the evolution of the plasma are discussed.

Singh, Nagendra↗
NASA NTRS1991

Radiative transfer in the dynamic atmospheres of Mira-type variables

This NASA grant covers our ADP research program, which involved detailed radiative transfer calculations of hydrodynamic models of pulsating asymptotic giant branch stars. Synthetic spectra resulting from these calculations are compared with International Ultraviolet Explorer (IUE) observations of these stars to test the validity of the models.

Linsky, Jeffrey L.↗
NASA NTRS1993

Near grazing scattering from non-Gaussian ocean surfaces

We investigate the behavior of the scattered electromagnetic waves from non-Gaussian ocean surfaces at near grazing incidence. Even though the scattering mechanisms at moderate incidence angles are relatively well understood, the same is not true for near grazing rough surface scattering. However, from the experimental ocean scattering data, it has been observed that the backscattering cross section of a horizontally polarized wave can be as large as the vertical counterpart at near grazing incidence. In addition, these returns are highly intermittent in time. There have been some suggestions that these unexpected effects may come from shadowing or feature scattering. Using numerical scattering simulations, it can be shown that the horizontal backscattering cannot be larger than the vertical one for the Gaussian surfaces. Our main objective of this study is to gain a clear understanding of scattering mechanisms underlying the near grazing ocean scattering. In order to evaluate the backscattering cross section from ocean surfaces at near grazing incidence, both the hydrodynamic modeling of ocean surfaces and an accurate near grazing scattering theory are required. For the surface modeling, we generate Gaussian surfaces from the ocean surface power spectrum which is derived using several experimental data. Then, weakly nonlinear large scale ocean surfaces are generated following Longuet-Higgins. In addition, the modulation of small waves by large waves is included using the conservation of wave action. For surface scattering, we use MOM (Method of Moments) to calculate the backscattering from scattering patches with the two scale shadowing approximation. The differences between Gaussian and non-Gaussian surface scattering at near grazing incidence are presented.

Kim, Yunjin↗
NASA NTRS1994

The dynamical settling of warped disks and angular momentum transport in galaxies

We present results of three-dimensional, hydrodynamic models of gaseous disks settling in a nonspherical potential. As the gas settles, differential precession creates a warped disk similar to the warps seen in spiral galaxies. A logarithmic potential, indicative of a massive halo, seems to induce warps more extreme than those produced by a l/r potential with a quadrupole distortion.

Fisher, P.↗