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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 307 records · Page 17
All sourcesNASA NTRSDOE OSTI
NASA NTRS1991

Three-dimensional compressible hydrodynamic convection in the sun and stars

A pseudospectral code is used to obtain numerical solutions of 3D compressible hydrodynamic convection in a stratified medium where the boundaries are open and where radiative losses are strongest at the top. Results are presented for a series of three simulations in progressively deeper boxes. The code does not make the anelastic approximation, but follows sound waves explicitly. Spatial organization is found in the form of vertical 'stacking' of smaller structures on top of larger ones, with each structure being roughly Hp to a vertical extent. Compressibility effects are apparent in the density: the snapshots indicate that, on a horizontal plane, the density at certain points may be of an order of 10 percent larger or smaller than elsewhere on that plane.

Hossain, Murshed↗
NASA NTRS1992

Lightweight Shield Against Space Debris

Report presents concept for lightweight, deployable shield protecting orbiting spacecraft against meteoroids and debris, and functions as barrier to conductive and radiative losses of heat. Shield made in four segments providing 360 degree coverage of cylindrical space-station module.

Redmon, John W., Jr.↗
NASA NTRS1991

Los Alamos NEP research in advanced plasma thrusters

Research was initiated in advanced plasma thrusters that capitalizes on lab capabilities in plasma science and technology. The goal of the program was to examine the scaling issues of magnetoplasmadynamic (MPD) thruster performance in support of NASA's MPD thruster development program. The objective was to address multi-megawatt, large scale, quasi-steady state MPD thruster performance. Results to date include a new quasi-steady state operating regime which was obtained at space exploration initiative relevant power levels, that enables direct coaxial gun-MPD comparisons of thruster physics and performance. The radiative losses are neglible. Operation with an applied axial magnetic field shows the same operational stability and exhaust plume uniformity benefits seen in MPD thrusters. Observed gun impedance is in close agreement with the magnetic Bernoulli model predictions. Spatial and temporal measurements of magnetic field, electric field, plasma density, electron temperature, and ion/neutral energy distribution are underway. Model applications to advanced mission logistics are also underway.

Schoenberg, Kurt↗
NASA NTRS1992

Dynamics of wind bubbles and superbubbles. I - Slow winds and fast winds. II - Analytic theory

The paper describes the overall evolution of wind-blown bubbles in a uniform medium from the initial, free-expansion stage to the final stage in which the pressure of the ambient medium is significant. The concepts of slow and fast winds, which naturally arise from consideration of radiative losses at the free-expansion stage, are introduced. The evolution of bubbles in a plane-parallel disk, where the density decreases steeply along a vertical direction, is considered. The questions of when a bubble can break out of a thin galactic disk and how they evolve after the breakout are discussed. After breakout, bubbles can evolve into jets. Steady, collimated jets can form only over a limited range of wind luminosity and Mach number; astronomical jets are likely to be unsteady and/or hydromagnetic. The results are applied to the neutral stellar wind in the HH 7-11 region, to the north polar spur, and to the galactic winds in starburst galaxies. The evolution of wind-blown bubbles in a power-law density distribution is investigated. Characteristic evolutionary time scales, as well as the equation of motion for both the swept-up gas and the wind shock in each evolutionary stage are obtained.

Koo, Bon-Chul↗
NASA NTRS1992

Proton acceleration in neutron star magnetospheres

To explain the emission of TeV and PeV gamma rays from accreting X-ray binary sources, protons must be accelerated to several times the gamma-ray energy. It is shown here that at certain times, the plasma in the accretion column of the neutron star may form a deep enough pool that the top portion becomes unstable to convective motions in spite of the strong magnetic field. The resulting turbulence produces fluctuations in the strength of the magnetic field that travel up the accretion column, taking energy out to the region of the energetic protons. The protons resonantly absorb this energy and are accelerated to high energies. Including the synchrotron radiation losses of the protons, it is shown that they can be accelerated to energies that are high enough to explain the gamma-ray observations.

Smith, I. A.↗
NASA NTRS1991

A fine-wire thermocouple probe for measurement of stagnation temperatures in real gas hypersonic flows of nitrogen

A fine-wire thermocouple probe was used to determine freestream stagnation temperatures in hypersonic flows. Data were gathered in a N2 blowdown wind tunnel with runtimes of 1-5 s. Tests were made at supply pressures between 30 and 1400 atm and supply temperatures between 700 and 1900 K, with Mach numbers of 14 to 16. An iterative procedure requiring thermocouple data, pilot pressure measurements, and supply conditions was used to determine test cell stagnation temperatures. Probe conduction and radiation losses, as well as real gas behavior of N2, were accounted for during analysis. Temperature measurement error was found to be 5 to 10 percent. A correlation was drawn between thermocouple diameter Reynolds number and temperature recovery ratio. Transient probe behavior was studied and was found to be adequate in temperature gradients up to 1000 K/s.

Hollis, Brian R.↗
NASA NTRS1993

Radio jets in colliding galaxies: Testing the interaction-activity connection

The idea that galaxy interactions and merging are related to the generation of starburst and AGN activity in galactic nuclei has been the subject of intensive investigations over the past several years and is still a matter of lively debate. Peculiar morphologies, indicative of tidal interactions, have been detected in high-luminosity radio galaxies, in quasars, and in ultraluminous IRAS galaxies. In addition, low-luminosity radio and active galaxies show similar evidence for a recent merger or for nearby companions. In a recent CCD optical study of galaxies selected on the basis that they all contain well defined radio jets, it was found that almost half of the sample consists of pairs of elliptical galaxies. Many of these low-luminosity radio galaxies with companions show a well defined distorted radio jet structure at the VLA scale with an S- or C-shaped morphology. We have developed a general numerical simulation algorithm for ballistic radio jets with the intention of applying this model to the study of the bent jets seen in colliding pairs of galaxies and with the hope of testing the well documented interaction-activity connection. In our model the morphological evolution of the jets is determined by their response to the simple mechanical forces (i.e., gravity and ram pressure) imposed on them from both the host and the companion galaxies. Radiative losses, jet precession, magnetic effects, relativistic terms, and hydrodynamic instabilities have all been ignored. Starting with a previously derived collision model for the interacting pair of elliptical galaxies NGC 4782/4783, we have used our algorithm to simulate the specific two-sided jet morphology seen in the radio source 3C 278, associated with NGC 4782. This is the first time that such jet simulations have been produced for a galaxy pair whose relative orbit was determined independently from the jet modeling.

Borne, Kirk D.↗
NASA NTRS1993

Mergers, cooling flows, and evaporation

Mergers (the capture of cold gas, especially) can have a profound influence on the hot coronal gas of early-type galaxies and clusters, potentially inducing symptoms hitherto attributed to a cooling flow, if thermal conduction is operative in the coronal plasma. Heat can be conducted from the hot phase into the cold phase, simultaneously ionizing the cold gas to make optical filaments, while locally cooling the coronal gas to mimic a cooling-flow. If there is heat conduction, though, there is no standard cooling-flow since radiative losses are balanced by conduction and not mass deposition. Amongst the strongest observational support for the existence of cooling-flows is the presence of intermediate temperature gas with x-ray emission-line strengths in agreement with cooling-flow models. Here, x-ray line strengths are calculated for this alternative model, in which mergers are responsible for the observed optical and x-ray properties. Since gas around 10(exp 4) K is thermally stable, the cold cloud need not necessarily evaporate and hydrostatic solutions exist. Good agreement with the x-ray data is obtained. The relative strengths of intermediate temperature x-ray emission lines are in significantly better agreement with a simple conduction model than with published cooling-flow models. The good agreement of the conduction model with optical, infrared and x-ray data indicates that significantly more theoretical effort into this type of solution would be profitable.

Sparks, W. B.↗
NASA NTRS1992

Variable EUV emission in the quiet sun and coronal heating

We review the characteristics of the variable emission from the small scale structure in the quiet sun, in view of the recent theoretical proposals that microflares are responsible for the heating of the corona and the solar wind. We consider the observational properties of the variable emission in quiet regions and coronal holes. Our results are based primarily on simultaneous multiwavelength EUV observations, supplemented by combinations of simultaneous cm radio, He I 10830 A, X-ray and line of sight photospheric magnetic field measurements. We show that the variable emission from the small-scale structure has surprisingly well-defined properties. Yet, within the limit of the temporal and spatial resolution of data currently available, the radiative losses, from this component of the emission, are a factor of ten smaller than that in the quiet sun. Hence, the theory of microflare heating cannot rely on these observations for support.

Habbal, S. R.↗
NASA NTRS1993

Mg II and Ly-alpha fluxes in M dwarfs - Evaluation of an acoustic model

Surface fluxes of Mg II and Ly-alpha from a sample of M dwarfs have been reported by Panagi and Mathioudakis (PM, 1992). In order to determine if acoustic effects can reproduce the fluxes in the least active ('basal') stars in the PM sample, we have constructed steady state models of plane-parallel atmospheres with T(eff) = 3000 and 4000 K including heating by shock dissipation. The models incorporate radiative losses in the chromosphere by solving the transfer equation for two strong lines (Mg II k and Ly-alpha). Predictions of standard weak shock theory are expected to overestimate the rate of energy deposition in the chromospheres of M dwarfs compared to the rate of deposition in F, G, or K dwarfs. In order to incorporate this effect in M dwarfs, we compute a series of atmospheric models in which, for a given T(eff), the standard weak shock dissipation rate is reduced by a series of factors F(r). Although the input acoustic flux F(mech) may in principle be determined uniquely by T(eff) and log g, the correct value to use in any particular case is not well known. We therefore use F(mech) and F(r) as parameters to construct a family of M dwarf models at various T(eff) and g. Comparing with the observed fluxes in the least active M dwarfs of the PM sample, we find that our models reproduce the observed fluxes of both Mg II and Ly-alpha best if F(mech) is close to the value predicted by Bohn (1984) and if F(r) is approximately equal to 10-20.

Mullan, D. J.↗
NASA NTRS1993

Chromospheric heating by acoustic shock waves

Work by Anderson & Athay (1989) suggests that the mechanical energy required to heat the quiet solar chromosphere might be due to the dissipation of weak acoustic shocks. The calculations reported here demonstrate that a simple picture of chromospheric shock heating by acoustic waves propagating upward through a model solar atmosphere, free of both magnetic fields and local inhomogeneities, cannot reproduce their chromospheric model. The primary reason is the tendency for vertically propagating acoustic waves in the range of allowed periods to dissipate too low in the atmosphere, providing insufficient residual energy for the middle chromosphere. The effect of diverging magnetic fields and the corresponding expanding acoustic wavefronts on the mechanical dissipation length is then discussed as a means of preserving a quasi-acoustic heating hypothesis. It is argued that this effect, in a canopy that overlies the low chromosphere, might preserve the acoustic shock hypothesis consistent with the chromospheric radiation losses computed by Anderson & Athay.

Jordan, Stuart D.↗
NASA NTRS1993

Fluid absorption solar energy receiver

A conventional solar dynamic system transmits solar energy to the flowing fluid of a thermodynamic cycle through structures which contain the gas and thermal energy storage material. Such a heat transfer mechanism dictates that the structure operate at a higher temperature than the fluid. This investigation reports on a fluid absorption receiver where only a part of the solar energy is transmitted to the structure. The other part is absorbed directly by the fluid. By proportioning these two heat transfer paths the energy to the structure can preheat the fluid, while the energy absorbed directly by the fluid raises the fluid to its final working temperature. The surface temperatures need not exceed the output temperature of the fluid. This makes the output temperature of the gas the maximum temperature in the system. The gas can have local maximum temperatures higher than the output working temperature. However local high temperatures are quickly equilibrated, and since the gas does not emit radiation, local high temperatures do not result in a radiative heat loss. Thermal radiation, thermal conductivity, and heat exchange with the gas all help equilibrate the surface temperature.

Bair, Edward J.↗
NASA NTRS1993

A comprehensive theoretical and experimental study of coplanar waveguide shunt stubs

A comprehensive theoretical and experimental study of straight and bent coplanar waveguide (CPW) shunt stubs is presented. In the theoretical analysis, the CPW is assumed to be inside a cavity, while, the experiments are performed on open structures. A hybrid technique was developed to analyze the CPW discontinuities which proved to be accurate since the theoretical and experimental results agree very well. The effect of the cavity resonances on the behavior of the stubs with and without air-bridges is investigated. In addition, the encountered radiation loss due to the discontinuities is evaluated experimentally.

Dib, Nihad I.↗
NASA NTRS1993

A theoretical and experimental study of coplanar waveguide shunt stubs

A comprehensive theoretical and experimental study of straight and bent coplanar waveguide (CPW) shunt stubs is presented. In the theoretical analysis, the CPW is assumed to be inside a cavity while, the experiments are performed on open structures. For the analysis of CPW discontinuities with air-bridges, a hybrid technique was developed which was validated through extensive theoretical and experimental comparisons. The effect of the cavity resonances on the behavior of the stubs with and without air-bridges is investigated. In addition, the encountered radiation loss due to the discontinuities is evaluated experimentally.

Dib, Nihad I.↗
NASA NTRS1993

Controls on the CO2 seasonal cycle

Surface pressure measurement performed by the Viking landers show substantial variations in pressure on seasonal timescales that are characterized by two local minima and two local maxima. These variations have widely been attributed to the seasonal condensation and sublimation of CO2 in the two polar regions. It has been somewhat of a surprise that the amplitude of the minimum and maximum that is dominated by the CO2 cycle in the north was much weaker than the corresponding amplitude of the south-dominated extrema. Another surprise was that the seasonal pressure cycle during years 2 and 3 of the Viking mission was so similar to that for year 1, despite the occurrence of two global dust storms during year 1 and none during years 2 and 3. An energy balance model that incorporates dynamical factors from general circulation model (GCM) runs in which the atmospheric dust opacity and seasonal date were systematically varied was used to model the observed seasonal pressure variations. The energy balance takes account of the following processes in determining the rates of CO2 condensation and sublimation at each longitudinal and latitudinal grid point: solar radiation, infrared radiation from the atmosphere and surface, subsurface heat conduction, and atmospheric heat advection. Condensation rates are calculated both at the surface and in the atmosphere. In addition, the energy balance model also incorporates information from the GCM runs on seasonal redistribution of surface pressure across the globe. Estimates of surface temperature of the seasonal CO2 caps were used to define the infrared radiative losses from the seasonal polar caps. The seasonal pressure variations measured at the Viking lander sites were closely reproduced.

Pollack, J. B.↗
NASA NTRS1994

Analytic Lyman-alpha wing diagnostics and the chromospheric excitation balance in cool dwarfs

I show that the Lyman alpha wings of cool dwarfs can be understood in terms of a very simple model, based on a simplified representation of the chromospheric hydrogen excitation balance and approximate analytic wing diagnostics in partial redistribution. Much of the complexity of the radiation transfer in the partially coherent and steeply temperature-sensitive Lyman alpha line is circumvented by this technique. The result is an expedient scheme for inverting the Lyman alpha wing profile to determine the characteristic free electron density and its gradient in the middle chromosphere. This represents an important new diagnostic for constraining chromospheric models and their radiative losses. I apply this diagnostic to the Lyman alpha wing profiles of the Sun and AU Mic and show that current instrumentation, most notably the Goddard High Resolution Spectrograph (GHRS) on the Hubble Space Telescope, is capable of providing the necessary data.

Gayley, K. G.↗
NASA NTRS1994

Acoustic waves in M dwarfs: Maintaining a corona

We use a time-dependent hydrodynamics code to follow the propagation of acoustic waves into the corona of an M dwarf star. An important qualitative difference between M dwarfs and stars such as the Sun is that the acoustic spectrum in M dwarfs is expected to peak at periods close to the acoustic cutoff P(sub A): this allows more effective penetration of waves into the corona. In our code, radiative losses in the photosphere, chromosphere, and corona are computed using Rosseland mean opacities, Mg II kappa and Ly alpha emission, and optically thin emissivities respectively. We find that acoustic heating can maintain a corona with a temperature of order 0.7-1 x 10(exp 6) K and a surface X-ray flux as large as 10(exp 5)ergs/sq cm/s. In a recent survey of X-rays from M dwarfs, some (20%-30%) of the stars lie at or below this limiting X-ray flux: we suggest that such stars may be candidates for acoustically maintained coronae.

Mullan, D. J.↗
NASA NTRS1994

Particle acceleration mechanisms

High-energy particle acceleration is observed to proceed in a diverse variety of astrophysical sites ranging from the terrestrial aurorae to the most distant quasars. Particle acceleration is a fairly common channel for the release of large-scale kinetic, rotational, and magnetic energy. Physical mechanisms include electrostatic acceleration, stochastic processes, and diffusive shock energization. Cosmic-ray energy spectra have shapes which reflect escape, collisional, and radiative losses. The overall acceleration efficiency is controlled by the low-energy particle injection which may, in turn, feed back into the energization. Recent observational developments, which illustrate these general principles and raise fresh questions, are briefly summarized.

Blandford, R. D.↗