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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 505 records · Page 28
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NASA NTRS1992

Turbulence modelling in CFD: Present status, future prospects

Information is given in viewgraph form for turbulence modeling in computational fluid dynamics (CFD). The Eddy Viscosity Models (EVM), Algebraic Second Moment Closures (ASM), and Differential Second-Moment Closures (DSM) are considered. It is concluded that EVM's, ASM's, and DSM's will remain in use, though with a steady decline in importance of EVM's and ASM's in favor of DSM's. Improved versions of low-Re two-equation EVM's should lead to more reliable predictions of separated flows than are achievable at present. Further refinement of sub-models in second moment closures can be expected throughout this decade. There will be increasing attention given to interfacing SMC with higher order approaches such as LES, and an increased use of two-time-scale schemes providing distinct time scales for large and fairly small eddies.

Launder, Brian E.↗
NASA NTRS1991

Cosmic rays in the Small Magellanic Cloud

The SMC is investigated in order to determine whether this galaxy is in dynamic quasi-stable equilibrium between the expansive pressure of the hot cosmic ray gas, the magnetic fields, and the kinetic motion of interstellar gas and the gravitational attraction of matter. It is argued that the cosmic ray density level is three to five times smaller than that which would exist if the galaxy were in approximate dynamic equilibrium and cosmic ray sources were adequate. The scale of coupling between the cosmic rays and matter in the projected dimension that can be observed is less than 0.5 kpc, which is rather small to be consistent with a stable cosmic ray distribution. The observed matter distribution and the calculated cosmic ray density distribution are used to determine the expected gamma ray intensity distribution.

Sreekumar, P.↗
NASA NTRS1992

The atmospheric structures of the companion stars of eclipsing binary x ray sources

This investigation was aimed at determining structural features of the atmospheres of the massive early-type companion stars of eclipse x-ray pulsars by measurement of the attenuation of the x-ray spectrum during eclipse transitions and in deep eclipse. Several extended visits were made to ISAS in Japan by G. Clark and his graduate student, Jonathan Woo to coordinate the Ginga observations and preliminary data reduction, and to work with the Japanese host scientist, Fumiaki Nagase, in the interpretation of the data. At MIT extensive developments were made in software systems for data interpretation. In particular, a Monte Carlo code was developed for a 3-D simulation of the propagation of x-rays from the neutron star through the ionized atmosphere of the companion. With this code it was possible to determine the spectrum of Compton-scattered x-rays in deep eclipse and to subtract that component from the observed spectra, thereby isolating the software component that is attributable in large measure to x-rays that have been scattered by interstellar grains. This research has culminated in the submission of paper to the Astrophysical Journal on the determination of properties of the atmosphere of QV Nor, the BOI companion of 4U 1538-52, and the properties of interstellar dust grains along the line of sight from the source. The latter results were an unanticipated byproduct of the investigation. Data from Ginga observations of the Magellanic binaries SMC X-1 and LMC X-4 are currently under investigation as the PhD thesis project of Jonathan Woo who anticipated completion in the spring of 1993.

Clark, George W.↗
NASA NTRS1993

Integrated oxygen recovery system

Life Systems has conceptualized an innovative Integrated Oxygen Recovery System (IORS) applicable to advanced mission air revitalization. The IORS provides the capability to electrochemically generate metabolic oxygen (O2) and recover O2 from the space habitat atmosphere via a carbon dioxide (CO2) reduction process within a single assembly. To achieve this capability, the IORS utilizes a Solid Metal Cathode (SMC) water electrolysis unit that simultaneously serves as the Sabatier CO2 reduction reactor. The IORS enables two major life support systems currently baselined in closed loop air revitalization systems to be combined into one smaller, less complex system. This concept reduces fluidic and electrical interface requirements and eliminates a hydrogen (H2) interface. Life Systems is performing an evaluation of the IORS process directed at demonstrating performance and quantifying key physical characteristics including power, weight, and volume. Technical progress achieved during the first two months of the program is summarized.

Lee, M. Gene↗
NASA NTRS1993

Integrated oxygen recovery system

Life Systems has conceptualized an innovative Integrated Oxygen Recovery System (IORS) applicable to advanced mission air revitalization. The IORS provides the capability to electrochemically generate metabolic oxygen (O2) and recover O2 from the space habitat atmosphere via a carbon dioxide (CO2) reduction process within a single assembly. To achieve this capability, the IORS utilizes a Solid Metal Cathode (SMC) water electrolysis unit that simultaneously serves as the Sabatier CO2 reduction reactor. The IORS enables two major life support systems currently baselined in closed loop air revitalization systems to be combined into one smaller, less complex system. This concept reduces fluidic and electrical interface requirements and eliminates a hydrogen (H2) interface. Life Systems is performing an evaluation of the IORS process directed at demonstrating performance and quantifying key physical characteristics including power, weight, and volume. The results of the checkout, shakedown, and initial parametric tests are summarized.

Lee, M. Gene↗
NASA NTRS1991

Cosmic rays in the Large and Small Magellanic Clouds and the relationship to our Galaxy

Information on the cosmic rays in the Large and Small Magellanic Clouds may be obtained from measurements of the synchrotron radiation related to the energetic electrons, which are believed to contain only about 1 percent of the cosmic rays energy. Assuming the same ratio as in our Galaxy between the cosmic ray electrons and nucleons, the energy density distribution may be estimated. This prediction is compared to that deduced from the matter density distribution, the concept of dynamic balance, and an appropriate coupling scale. For the LMC, the condition of quasi-equilibrium seems satisfied as is required for dynamic balance to be a valid concept, but for the SMC, this seems not to be the case.

Fichtel, Carl E.↗
NASA NTRS1991

Imaging the sun in hard X-rays - Spatial and rotating modulation collimators

Several approaches to imaging hard X-rays emitted from solar flares have been proposed or are planned for the nineties including the spatial modulation collimator (SMC) and the rotating modulation collimator (RMC). A survey of current solar flare theoretical literature indicates the desirability of spatial resolutions down to 1 arcsecond, field of views greater than the full solar disk (i.e., 32 arcminutes), and temporal resolutions down to 1 second. Although the sun typically provides relatively high flux levels, the requirement for 1 second temporal resolution raises the question as to the viability of Fourier telescopes subject to the aforementioned constraints. A basic photon counting, Monte Carlo 'end-to-end' model telescope was employed using the Astronomical Image Processing System (AIPS) for image reconstruction. The resulting solar flare hard X-ray images compared against typical observations indicated that both telescopes show promise for the future.

Campbell, Jonathan W.↗
NASA NTRS1993

Spectrophotometry of Wolf-Rayet stars. I - Continuum energy distributions

All available low-resolution IUE spectra are assembled for Galactic, LMC, and SMC W-R stars and are merged with ground-based optical and NIR spectra in order to collate in a systematic fashion the shapes of these energy distributions over the wavelength range 0.1-1 micron. They can be consistently fitted by a power law of the form F(lambda) is approximately equal to lambda exp -alpha over the range 1500-9000 A to derive color excesses E(B-V) and spectral indices by removing the 2175-A interstellar absorption feature. The WN star color excesses derived are found to be in good agreement with those of Schmutz and Vacca (1991) and Koesterke et al. (1991). Significant heterogeneity in spectral index values was generally seen with any given subtype, but the groups consisting of the combined set of Galactic and LMC W-R stars, the separate WN and WC sequences, and the Galactic and LMC W-R stars all showed a striking and consistent Gaussian-like frequency distribution of values.

Morris, Patrick W.↗
NASA NTRS1992

Pulse-Timing Studies of X Ray Pulsars

The pulse-timing projects supported by NASA Grant NAG8-695 were motivated in large part by our long-standing interest in the physics of rotating neutron stars and particularly the variations in rotation due to internal and external fluctuating torques. For accretion-powered pulsars, observed as compact galactic X-ray sources, our work has been motivated by questions regarding the physics of matter accretion, mass transfer, and mass loss in these X-ray binary systems. For rotation-powered pulsars using X-ray, optical, and radio observations, we have examined the internal structure of neutron stars and mechanisms for secular spin-down. These issues are still central to the continuing process of understanding the complex behavior of these fascinating systems. The work supported by this grant is based mainly on Ginga observations of three pulsating X-ray sources. We observed two of these sources (Her X-1 and SMC X-1) within the framework of a NASA-ISAS program for U.S.-Japan collaborations to obtain and analyze Ginga observations. In addition, we joined with members of the Ginga team to apply our pulse-timing methods to PSR 0540-69 data obtained as a collateral benefit of the regular monitoring by Ginga of SN 1987A. In addition to these Ginga observations, there exist other relevant data on all three of these sources. The direction of our investigations has been affected by the necessity of including these supporting data in our analyses, and setting up the necessary collaborations has sometimes entailed extra work that was not anticipated in our proposals. Indeed, we have often taken the initiative in establishing these joint projects.

Deeter, John E.↗
NASA NTRS1990

Spectrophotometry of emission-line stars in the magellanic clouds

The strong emission lines in the most luminous stars in the Magellanic Clouds indicate that these stars have such strong stellar winds that their photospheres are so masked that optical absorption lines do not provide an accurate measure of photospheric conditions. In the research funded by this grant, temperatures and gravities of emission-line stars both in the Large (LMC) and Small Magellanic Clouds (SMC) have been measured by fitting of continuum ultraviolet-optical fluxes observed with IUE with theoretical model atmospheres. Preliminary results from this work formed a major part of an invited review 'The Distribution of Types of Luminous Blue Variables'. Interpretation of the IUE observations obtained in this grant and archive data were also included in a talk at the First Boulder-Munich Hot Stars Workshop. Final results of these studies are now being completed for publication in refereed journals.

Bohannan, Bruce↗
NASA NTRS1994

ISTAR: Intelligent System for Telemetry Analysis in Real-time

The intelligent system for telemetry analysis in real-time (ISTAR) is an advanced vehicle monitoring environment incorporating expert systems, analysis tools, and on-line hypermedia documentation. The system was developed for the Air Force Space and Missile Systems Center (SMC) in Los Angeles, California, in support of the inertial upper stage (IUS) booster vehicle. Over a five year period the system progressed from rapid prototype to operational system. ISTAR has been used to support five IUS missions and countless mission simulations. There were a significant number of lessons learned with respect to integrating an expert system capability into an existing ground system.

Simmons, Charles↗
NASA NTRS1994

Opacity, metallicity, and Cepheid period ratios in the galaxy and Magellanic Clouds

Linear pulsation calculations are employed to reproduce the bump Cepheid resonance (P(sub 2)/P(sub 0) = 0.5 at P(sub 0) approximately equal to 10 days) and to model, individually, the P(sub 1)/P(sub 0) period ratios for the dozen known Galactic beat Cepheids. Convection is ignored. The results point to a range of metallicity among the Cepheids, perhaps as large as 0.01 approximately less than Z approximately less than 0.02, with no evidence for any star exceeding Z = 0.02. We find masses and luminosities which range from M approximately less than 4 solar mass, log(base 10) approximately less than 3.0 at P(sub 0) approximately equal to 3 days to M approximately less than 6 solar mass, log(base 10) L approximately greater than 3.5 at P(sub 0) approximately equal to 10 days. Similar parameters are indicated for the P(sub 0) approximately equal to 10 days Cepheids in the LMC and SMC, provided that the resonance for these stars occurs at a slightly longer period, P(sub 0) days, as has been suggested in the literature. Our calculations were performed mainly using OPAL opacities, but also with new opacities from the Opacity project (OP). Only small differences were found between the OPAL results and those from OP. Finally, some suggestions are made for possible future work, including evolution and pulsation calculations, and more precise observations of Cepheids in the Magellanic Clouds.

Simon, Norman R.↗
NASA NTRS1994

Can MACHOs probe the shape of the galaxy halo ?

Microlensing searches in our galaxy have recently discovered several candidates in the direction of the Large Magellanic Cloud (LMC). We study the prospects for such searches to yield useful information about the flattening of the Galaxy dark matter halo, using a self-consistent oblate halo model and allowing for the possibility of misalignment between the disk and halo symmetry axes. The microlensing optical depth for the LMC, tau(LMC), depends sensitively on the disk-halo tilt angle in the Milky Way, as does the ratio tau(SMC)/tau(LMC). If the tilt angle is as large as 30 deg, a much larger spread in values for tau(LMC) is consistent with rotation curve constraints than previously thought. Disk-halo tilt and halo flattening do not significantly affect the massive compact halo object (MACHO) masses inferred from event durations.

Frieman, Joshua↗
NASA NTRS1994

A new transient pulsar in the Small Magellanic Cloud with an unusual x-ray spectrum

This article reports the discovery of a luminous (3.5 x 10(exp 37) ergs/sec over the 0.2 to 2 keV band) transient X-ray pulsar in the Small Magellanic Cloud (SMC) with an extremely soft component to its X-ray spectrum. This is the first time that a spectrum of this type has been seen in this class of X-ray source. The pulse period is 2.7632 s, and the pulse modulation appears to vary with energy from nearly unpulsed in the low-energy band of the ROSAT Position Sensitive Proportional Counter (PSPC) (0.07 to 0.4 keV) to about 50% in the high-energy band (1.0 to 2.4 keV). The object, RX J0059.2-7138, also shows flickering variability in its X-ray emission on timescales of 50 to 100s. The pulse-phase-averaged PSPC X-ray spectrum can be well described by a two-component source model seen through an absorbing column density of approximately 10(exp 21) atoms cm(exp -2). One spectral component is a power law with photon index 2.4. The other component is significantly softer and can be described by either a steeply falling power law or a blackbody with a temperature KT(sub BB) approximately 35 eV. Ths component is transient, but evidently upulsed, and, for the blackbody model fits, requires a large bolometric luminosity: near, or even several times greater than, the Eddington luminosity for a 1.4 solar mass object. When these characteristics of its soft emission are considered, RX J0059.2-7138 appears quite similar to other X-ray sources in the magellanic Clouds, such as CAL 83, CAL 87, and RX J0527.8-6954, which show only extreme ultrasoft (EUS) X-ray spectra. The discovery of RX J0059.2-7138, a probably high-mass X-ray binary, clearly indicates that EUS spectra may arise from accretion-powered neutron-star X-ray sources. This result lends support to the idea that some of the 'pure' EUS sources may be shrouded low-mass X-ray binaries rather than accreting white dwarfs.

Hughes, John P.↗
NASA NTRS1994

X-ray spectroscopy of SNR E0102-72 with the ASCA satellite

The Advanced Satellite for Cosmology and Astrophysics (ASCA) satellite has obtained a moderate-resolution energy spectrum of E0102-72, the brightest Supernova Remnant (SNR) in the Small Magellanic Cloud (SMC). This paper reports on the first results of the analysis of the high quality spectrum of E0102-72. The spectrum shows resolved emission lines of He-like K alpha, H-like K alpha and K beta from oxygen, neon, and magnesium. The intensity ratios of these lines cannot be explained by a multi-component plasma model with uniform abundances, but requires abundance inhomogeneity in the plasma. We demonstrate how the spectral capabilities of the ASCA SIS make available new diagnostics of X-ray plasmas in a state of non-equilibrium ionization. Some interpretation based on the spectral analysis is also given.

Hayashi, Ichizo↗
NASA NTRS1994

The two period-luminosity relations for population I Cepheids

We summarize the evidence that most Population I Cepheids with periods less than approximately 8 days pulsate in the first overtone mode. Fundamental model and first overtone pulsators must follow different period-luminosity (P - L) relations. We demonstrate these different relations for different stellar systems, especially for the calibrating Cepheids in clusters and for Cepheids in the Large Magellanic Cloud (LMC), the Small Magellanic Cloud (SMC), M31, and IC 4182.

Bohm-Vitense, Erika↗
NASA NTRS1994

The initial mass function for massive stars in the Magellanic Clouds. 2: Interstellar reddening toward 14 OB associations

We have used UBV CCD photometry to determine the interstellar reddening toward 14 OB associations in the Magellanic Clouds. The two reddening-free indices available in the UBV system were used to obtain the reddening estimates. The mean color excesses of the associations range from E(B-V) = 0.01-0.26 mag in the Large Magellanic Cloud (LMC) and from E(B-V) = 0.06-0.25 mag in the Small Magellanic Cloud (SMC). We have modeled the observed scatter in the color excesses of individual stars within the associations and find that statisically significant differential reddening exists in at least some of the associations.

Hill, Robert J.↗
NASA NTRS1994

The initial mass function for massive stars in the Magellanic Clouds. 3: Luminosity and mass functions for 14 OB associations

We have used UBV photometry of stars in 14 associations in the Large and Small Magellanic Clouds (LMC/SMS) (Hill, Madore, & Freedman) to derive luminosity and mass functions for the most massive stars. The main-sequence luminosity functions for the associations are quite similar, having an average slope of s = 0.03 +/- 0.06. The mass functions for the associations have slopes that span similar ranges about a common mean for both the LMC and SMC and there is no strong evidence for a significant variation in the slopes from one association to another. Accordingly, metal abundance does not appear to have a strong effect on the initial mass function (IMF), at least for the range in metallicity observed between the Magellanic Clouds. The average slope of the IMF for the Magellanic Cloud associations is Gamma = -2.0 +/- 0.5 for M greater than 9 Solar Masses. The range in the derived slopes is more likely due to the large uncertainties associated with the calculations of the mass functions, rather than to real variations in the IMF. There may be some evidence for a decrease in the slope of the IMF at masses below 9 Solar Masses, but incompleteness and the larger photometric errors associated with the faintest stars make this conclusion tentative.

Hill, Robert J.↗