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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 1,189 records · Page 66

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NASA NTRS1972

The fluid mechanics of thrombus formation

Experimental data are presented for the growth of thrombi (blood clots) in a stagnation point flow of fresh blood. Thrombus shape, size and structure are shown to depend on local flow conditions. The evolution of a thrombus is described in terms of a physical model that includes platelet diffusion, a platelet aggregation mechanism, and diffusion and convection of the chemical species responsible for aggregation. Diffusion-controlled and convection-controlled regimes are defined by flow parameters and thrombus location, and the characteristic growth pattern in each regime is explained. Quantitative comparisons with an approximate theoretical model are presented, and a more general model is formulated.

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NASA NTRS1975

Two classes of cosmic ray decrease

In a study of galactic cosmic ray intensity variations it had been found that there are two distinct classes of transient intensity decrease, which have been called type I and type II. Type I events are flare-associated Forbush decreases. Type II events appear to be associated with 27-day recurrences. It is suggested, on the basis of several characteristics of the decreases, that type II events are simply the subsequent evolution of type I events or quasi-stationary 'corotating' structures loosely associated with active regions.

Verschell, H. J.↗

NASA NTRS1977

Absorption of X-rays in the interstellar medium

The X-ray photoabsorption cross sections (per hydrogen atom) of interstellar clouds and the intercloud medium are calculated separately, taking into account abundance evolution, self-blanketing by grains, molecule formation, and ionization structure. A reference cross section describing a neutral monatomic gas of elements in solar abundances is computed for purposes of comparison, the cross section of the intercloud medium is calculated by assuming that 15% of the hydrogen is ionized, and the cross section of the clouds is computed for the cases where all hydrogen is in either molecular or atomic form. A comparison of the reference cross section with the others shows that a cross section is increased relative to the reference if abundances are enhanced or if molecular hydrogen exists and is decreased if atoms exist in grains or if hydrogen is ionized. It is noted that the total cross section will be a linear combination of the cross sections of the clouds and the intercloud medium and can reflect the characteristics of a particular line of sight.

Ride, S. K.↗

NASA NTRS1978

Relativistic blast-wave model for the rapid flux variations of AO 0235+164 and other compact radio sources

A relativistic blast-wave version of a signal-screen model is developed which can adequately explain the details of the flux-density and structural variations of compact extragalactic radio sources. The relativistic motion implied by flux variations is analyzed with respect to the synchrotron spectrum of the BL Lac object AO 0235+164 observed during outbursts, and a signal-screen model for rapidly expanding shells produced by ultrarelativistic blast waves is examined. The approximate observed structure of the blast wave at three stages in its evolution is illustrated, each stage is described, and the model is applied to the flux density outburst in AO 0235+164 observed in late 1975. The results show that a relativistic blast-wave model can in general reproduce the main features of the observed flux variations in compact sources. Some problems with the proposed model are briefly discussed.

Marscher, A. P.↗

NASA NTRS1981

Cooling of young neutron stars and the Einstein X-ray observations

Cooling of neutron stars is calculated using an exact stellar evolution code. The full general relativistic version of the stellar structure equations are solved, with the best physical input available. For neutron stars with a stiff equation of state, it is found that the deviation from the isothermality in the interior is significant and that it takes at least a few thousand years to reach the isothermal state. By comparing theoretical and observational results, it is concluded that for Cas A, SN1006, and probably Tycho, standard cooling is inconsistent with the results from the Einstein Observatory, if neutron stars are assumed to be present in these objects. On the other hand, the detection points for RCW103 and the Crab are consistent with these theoretical results.

Nomoto, K.↗

NASA NTRS1981

The LAMAR: A high throughput X-ray astronomy facility for a moderate cost mission

The performance of a large area modular array of reflectors (LAMAR) is considered in several hypothetical observations relevant to: (1) cosmology, the X-ray background, and large scale structure of the universe; (2) clusters of galaxies and their evolution; (3) quasars and other active galactic nuclei; (4) compact objects in our galaxy; (5) stellar coronae; and (6) energy input to the interstellar medium.

Gorenstein, P.↗

NASA NTRS1981

Membrane lipids and the origin of life

The current state of knowledge regarding the development of biological systems is briefly reviewed. At a crucial stage concerning the evolution of such systems, the mechanisms leading to more complex structures must have evolved within the confines of a protected microenvironment, similar to those provided by the contemporary cell membranes. The major components found normally in biomembranes are phospholipids. The structure of the biomembrane is examined, and attention is given to questions concerning the availability of the structural components which are necessary in the formation of primitive lipid membranes. Two approaches regarding the study of protomembranes are discussed. The probability of obtaining ether lipids under prebiotic conditions is considered, taking into account the formation of cyclic and acyclic isoprenoids by the irradiation of isoprene with UV.

Oro, J.↗

NASA NTRS1984

Large-scale inhomogeneities and galaxy statistics

The density fluctuations associated with the formation of large-scale cosmic pancake-like and filamentary structures are evaluated using the Zel'dovich approximation for the evolution of nonlinear inhomogeneities in the expanding universe. It is shown that the large-scale nonlinear density fluctuations in the galaxy distribution due to pancakes modify the standard scale-invariant correlation function xi(r) at scales comparable to the coherence length of adiabatic fluctuations. The typical contribution of pancakes and filaments to the J3 integral, and more generally to the moments of galaxy counts in a volume of approximately (15-40 per h Mpc)exp 3, provides a statistical test for the existence of large scale inhomogeneities. An application to several recent three dimensional data sets shows that despite large observational uncertainties over the relevant scales characteristic features may be present that can be attributed to pancakes in most, but not all, of the various galaxy samples.

Schaeffer, R.↗

NASA NTRS1984

Mercury

The geology of the planet Mercury is considered herein. The surface of the planet, including craters, basins, tectonics, and evolution, is discussed. The orbit, rotation, magnetic fields, atmosphere, and structure of Mercury are also addressed.

Robert G Strom↗

NASA NTRS1986

New high-resolution rocket-ultraviolet filtergrams of the solar disc

A rocket-borne solar ultraviolet telescope named Transition Region Camera was launched successfully for the third on July 13, 1982. High quality calibrated photographic images of the sun were obtained at Lyman alpha and in the continuum at 160 nm and 220 nm. The angular resolution achieved is better than one arcsec. A flare, active regions, sunspots, the 8 Mm mesostructure, the chromospheric network, bright UV grains and coronal loops were observed during the flight. The results are presented and the evolution with height in the solar atmosphere of the various structures observed is followed from one wavelength to the other, showing distinct differences. The value of the field's intensity of magnetic flux tubes is deduced from the observations.

Foing, B.↗

NASA NTRS1987

Circumstellar interaction and a pulsar nebula in the supenova 1986j

It is shown here that the circumstellar interaction model for radio supernovae provides an adequate fit to the existing data on supernova SN1986j. The model implies a time of explosion of about January 1983 and provides a guide for the future evolution of the radio source. A shell-like source structure is expected. The model, together with VLBI observations of the radio source, implies a shock velocity of about 10,000 km/s and a presupernova mass loss rate of about 0.0001 solar mass/yr for a wind velocity of 10 km/s. The time behavior of the optical flux and its high luminosity suggest that a central pulsar nebula is the energy source. The required properties are comparable to those expected in the early phases of the Crab nebula.

Chevalier, Roger A.↗

NASA NTRS1990

Instabilities in line-driven stellar winds. IV - Linear perturbations in three dimensions

Nonradial wave propagation in line-driven stellar winds is analyzed including both finite disk effects and the line-drag effect of scattered line radiation. Within the local (WKB) analysis the results apply to wavelengths both longer and shorter than the Sobolev length. The finite disk causes short-wavelength waves with lateral velocity polarization to be unstable in the idealized case of pure absorption; however, the growth rates are smaller than for radially polarized waves, and the instability is damped by a relatively small amount of scattering. Hence, in realistic stellar winds, where the driving is primarily by scattering lines, perturbations with an arbitrary mixture of lateral and radial polarizations at the wind base should quickly become nearly radially polarized farther out in the wind. The implications of these results are discussed, both for interpretation of observational signatures of wind structure and for theoretical calculations aimed at modeling the nonlinear evolution of wind instabilities.

Rybicki, G. B.↗

NASA NTRS1990

The nonlinear evolution of magnetized solar filaments

Thermal instability driven by optically thin radiation is believed to initiate the formation of plasma filaments in the solar corona. The fact that filaments are observed generally to separate regions of opposite, line-of-sight, magnetic polarity in the underlying photosphere suggests that filament formation requires the presence of a highly sheared, local magnetic field. Two-dimensional, nonlinear, magnetohydrodynamic simulations of the local genesis and growth of solar filaments in a force-free, sheared, magnetic field were performed, and the evolution of generic perturbations possessing broad spatial profiles was traced. It was found that simulations of the evolution of initial random-noise perturbations produce filamentary plasma structures that exhibit densities and temperatures characteristic of observed solar filaments. Furthermore, in each of these simulations, the filament axis lies at a finite angle with respect to the local magnetic field, consistent with solar observations.

Sparks, L.↗

NASA NTRS1992

Investigation of forced unsteady separated flows using velocity-vorticity form of Navier-Stokes equations

The phenomenon of forced unsteady separation and eruption of boundary-layer vorticity is a highly-complex, high-Reynolds number flow phenomenon, which abruptly leads to the formation of a dynamic stall vortex as demonstrated earlier by the authors for a NACA 0015 airfoil undergoing constant rate pitch-up motion. This, as well as the results of other researchers, have convincingly demonstrated a complex vortical structure within the state of unsteady separation prior to the evolution of dynamic stall. This phenomenon of vortex eruption, although observed in studying dynamic stall phenomena, is also associated with transition from laminar to turbulence flow and its generic nature has been stressed by many researchers including the present investigators. An unsteady Navier-Stokes (NS) analysis is developed for arbitrarily maneuvering bodies using velocity-vorticity variables; this formulation is nearly form-invariant under a generalized non-inertial coordinate transformation. A fully-implicit uniformly second-order accurate method is used, with the nonlinear convective terms approximated using a biased third-order upwind differencing scheme to be able to simulate higher-Re flows. No explicit artificial dissipation is added. The numerical method is fully vectorized and currently achieves a computational index of 7 micro-seconds per time step per mesh point, using a single processor on a CRAY Y-MP. The simulation results show that the energetic free shear from the leading edge is responsible for the wall viscous layer to abruptly erupt near the center of the counterclockwise rotating eddy in the unsteady boundary layer. Primary, secondary, tertiary and quaternary vortices have been observed before the dynamic stall vortex evolves and gathers its maximum strength. This study will discuss the simulation results of Reynolds number up to Re = 45,000 and will also discuss the efforts of initial acceleration in a specific maneuver, on the evolution of the stall vortex.

Ghia, K. N.↗

NASA NTRS1992

Mars

The present volume on Mars discusses visual, photographic and polarimetric telescopic observations, spacecraft exploration of Mars, the origin and thermal evolution of Mars, and the bulk composition, mineralogy, and internal structure of the planet. Attention is given to Martian gravity and topography, stress and tectonics on Mars, long-term orbital and spin dynamics of Mars, and Martian geodesy and cartography. Topics addressed include the physical volcanology of Mars, the canyon system on planet, Martian channels and valley networks, and ice in the Martian regolith. Also discussed are Martian aeolian processes, sediments, and features, polar deposits of Mars, dynamics of the Martian atmosphere, and the seasonal behavior of water on Mars.

Kieffer, Hugh H.↗

NASA NTRS1992

The universe at moderate redshift

The Final Report on the universe at moderate redshift covering the period from 1 Mar. 1988 to 28 Feb. 1991 is presented. Areas of research included: galaxy formation and large-scale structure; intergalactic medium and background radiation fields; quasar statistics and evolution; and gravitational lenses.

Ostriker, Jeremiah P.↗

NASA NTRS1994

Scientific investigations of atmospheric processes

Research was performed in atmospheric, dynamical, and thermodynamical processes and in other disciplines necessary to accomplish the following tasks: develop procedures for combining generalized radiative transfer codes with dynamic atmospheric model codes; perform diagnostic analysis of atmospheric processes to gain a better understanding of the evolution and development of mesoscale circulation systems and their precipitation structures; and to develop algorithms and software necessary to graphically display diagnostic sets on the MSFC McIDAS and EADS to facilitate scientific study and sensor capability evaluation. Research activities during this reporting period are detailed.

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NASA NTRS1994

Protostellar collapse in a self-gravitating sheet

We present preliminary calculations of protostellar cloud collapse starting from an isothermal, self-gravitating gaseous layer in hydrostatic equilibrium. This gravitationally unstable layer collapses into a flattened or toroidal density distribution, even in the absence of rotation or magnetic fields. We suggest that the flat infalling envelope recently observed in HL Tau by Hayashi et al.is the result of collapse from an initially nonspherical layer. We also speculate that the later evolution of such a flattened, collapsing envelope can produce a structure similar to the 'flared disk' invoked by Kenyon and Hartmann to explain the infrared excesses of many T Tauri stars.

Hartmann, Lee↗