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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 649 records · Page 36
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NASA NTRS1980

Galaxy collisions - A preliminary study

Collisions of spherical galaxies were studied in a series of numerical experiments to see what happens when galaxies collide. Each experiment starts with two model galaxies, each consisting of 50,000 stars, moving toward each other along a specified orbit. The series of experiments provides a systematic sampling of the parameter space spanned by the initial orbital energy and the initial angular momentum. Deeply penetrating collisions are emphasized. The collisions reported here scale to relative velocities as great as 500 km/s, well into the range for collisions within clusters of galaxies. It is found that: (1) the galaxies contract momentarily to about half their original sizes shortly after close passage; and (2) the initial galaxies blend into a single dynamical system while they are near each other.

Miller, R. H.↗
NASA NTRS1979

Star and planetary system formation in collapsing, viscous, rotating clouds

The results of a preliminary investigation of several processes that are of interest both for the formation of stars and for the evolution of planetary systems are presented. It is shown that turbulent viscosity is capable of playing an important role in conveying angular momentum over time scales that are short enough to be significant. Meridional circulation can also act in this fashion during some phases of the evolution. This transport may reduce the probability of formation of the rings that have been found by most earlier investigators. Transport and mixing on a faster than cooling time scale should also inhibit usual modes of fragmentation and the present work casts some doubt on the multiple successive fragmentation scenarios that lead from a massive molecular cloud to a collection of roughly solar mass protostars. The conditions that are probable in the disks that would exist at varying phases of collapse are examined and it is concluded that turbulent viscosity would be very important in a pre-solar nebula.

Wiita, P. J.↗
NASA NTRS1980

Ring formation in rotating protostellar clouds

The formation of a ring during the dynamic collapse of a rotating gas cloud is shown to be an understandable physical phenomenon. By analytically integrating the equation of motion for particles in the equatorial plane of a rotating cloud which collapses in a gravitational potential well defined by a (1 - r-squared) mass density distribution the mechanism which initiates the growth of the toroidal structure is demonstrated. An analysis of the ring formation process indicates that the ring should develop in rotating, self-gravitating gas clouds which collapse from a wide range of axisymmetric intial conditions; the degree of central condensation and the initial distribution of angular momentum in a cloud should affect only the position and size of the developing ring. Ring formation, being a dynamic process in collapsing gas clouds, cannot be explained in terms of the classical ring instability that arises in rapidly rotating, equilibrium spheroids. Conditions in a cloud which should inhibit ring formation are also discussed.

Tohline, J. E.↗
NASA NTRS1980

Nonlinear axially symmetric circulations in a nearly inviscid atmosphere

The structure of certain axially symmetric circulations in a stably stratified, differentially heated, rotating Boussinesq fluid on a sphere is analyzed. A simple approximate theory (similar to that introduced by Schneider (1977)) is developed for the case in which the fluid is sufficiently inviscid that the poleward flow in the Hadley cell is nearly angular momentum conserving. The theory predicts the width of the Hadley cell, the total poleward heat flux, the latitude of the upper level jet in the zonal wind, and the distribution of surface easterlies and westerlies. Fundamental differences between such nearly inviscid circulations and the more commonly studied viscous axisymmetric flows are emphasized. The theory is checked against numerical solutions to the model equations.

Held, I. M.↗
NASA NTRS1980

On the capture of the Moon

This paper studies the possibility of lunar capture depending on variations of the solar mass under certain well specified conditions and assumptions regarding the behaviour of the three-body dynamical system formed by the Sun, Earth and Moon. It is found that a large amount of decrease in the solar mass (approximately 37%) would be required to allow capture if the model of the planar restricted problem of three bodies is assumed, if the masses of the Earth and Moon did not change and if the angular momentum of the Sun-Earth system did not change. Such large mass-changes of the Sun can not be associated with radiation mass losses only with catastrophic events, such as stellar close approaches.

Szebehely, V.↗
NASA NTRS1980

NGC 1275 and the Perseus cluster - The formation of optical filaments in cooling gas in X-ray clusters

The paper generalizes previous discussions of the steady-flow models by allowing for the effects of angular momentum, which may be important in the central cooled regions of rich clusters of galaxies. The thermal stability of the flow is studied in a general way by relaxing the assumption that the perturbations comove. It is found that they are only unstable for a restricted range of initial parameters. Situations in which cold filaments can form are considered, and the subsequent evolution and excitation of such filaments are discussed. An estimate of the mass-to-light ratio of NGC 1275 is obtained by comparing the obtained results with the low-velocity filamentation system surrounding NGC 1275 in the Perseus cluster. Finally, the various aspects of cooling accretion flows in clusters are summarized.

Cowie, L. L.↗
NASA NTRS1980

Protostellar formation in rotating interstellar clouds. II - Axially symmetric collapse

A two spatial dimension gravitational hydrodynamics code has been used to calculate the initial isothermal dynamic collapse phase of axially symmetric, rotating interstellar clouds. The Eulerian code has been constructed so as to conserve angular momentum both locally (approximately) and globally (exactly). An axially symmetric, rotating cloud collapses to form a rotating, near-equilibrium ring. The rings so formed are compared with those found previously by Black and Bodenheimer, and Bodenheimer and Tscharnuter, and found to agree in structure fairly well. Numerical tests with the code, as well as an analytic calculation of the collapse of a pressureless, rotating, axially symmetric cloud in a fixed gravitational potential, support the assertion that the observed ring formation is physically realistic.

Boss, A. P.↗
NASA NTRS1980

Protostellar formation in rotating interstellar clouds. III - Nonaxisymmetric collapse

The paper discusses a full three spatial-dimension gravitational hydrodynamic code used to follow the collapse of isothermal rotating clouds subjected to various nonaxially symmetric perturbations (NAP). An initially axially symmetric cloud collapsed to form a ring which then fragmented into a binary protostellar system; a low thermal energy cloud with a large bar-shaped NAP collapsed and fragmented into a binary, and higher thermal energy clouds damp out such NAPs while higher rotational energy clouds produce binaries with wider separations. The three-dimensional calculations indicate that isothermal interstellar clouds may fragment into protostellar objects while still in the isothermal regime. Interstellar clouds and their fragments may pass through collapse phases with fragmentation and reduction of spin angular momentum terminating in the formation of pre-main-sequence stars with the observed pre-main-sequence rotation rates.

Boss, A. P.↗
NASA NTRS1980

The shape and stability of rotating liquid drops

Equilibrium shapes and stability of rotating drops held together by surface tension are found by computer-aided analysis that uses expansions in finite-element basis functions. Shapes are calculated as extrema of appropriate energies. Stability and relative stability are determined from curvatures of the energy surface in the neighborhood of the extremum. Families of axisymmetric, two-, three-, and four-lobed drop shapes are traced systematically. Bifurcation and turning points are located and the principle of exchange of stabilities is tested. The axisymmetric shapes are stable at low rotation rates but lose stability at the bifurcation to two-lobed shapes. Two-lobed drops isolated with constant angular momentum are stable. The results bear on experiments designed to further those of Plateau (1863).

Brown, R. A.↗
NASA NTRS1979

Can a macroscopic gyroscope feel torsion

We demonstrate that for a large class of Lagrangian-based torsion theories a macroscopic gyroscope is insensitive to the torsion field: there can be no coupling of the torsion to the gyroscope's angular momentum of rotation. To detect torsion a polarized system with a net elementary particle spin is needed. These conclusions are evident from the conservation laws, which form the basis for deriving the equations of motion.

Stoeger, W. R.↗
NASA NTRS1980

The equatorial rotation rate of the supergranulation cells

The equatorial rotation rate of the supergranulation cells has been observed to be 14.72 plus or minus 0.07 deg/day. Velocity patterns observed at different times are cross-correlated to derive the rotation rate. The observed rate is 3% faster than recent observations of the surface rotation rate by Doppler shifts. The difference between the cell rate and surface rate is consistent with a model of the supergranular convection in which angular momentum per unit mass is conserved in the radial flow (Foukal, 1977).

Duvall, T. L., Jr.↗
NASA NTRS1980

New class of asymmetric shapes of rotating liquid drops

Shapes and stability of surface-tension-endowed drops rotating rigidly at fixed angular momentum are calculated by finite-element analysis. A new family of asymmetric two-lobed drop shapes is discovered that branches from, and rejoins, the Pik-Pichak family of symmetric two-lobed shapes. The computations are verified for axisymmetric and symmetric two-lobed drop shape by comparison with previous approximations.

Brown, R. A.↗
NASA NTRS1980

Time-dependent, optically thick accretion onto a black hole

A fully relativistic hydrodynamics code which incorporates diffusive radiation transport is used to study time-dependent, spherically symmetric, optically thick accretion onto a black hole. It is found that matter free-falls into the hole regardless of whether the diffusion time scale is longer or shorter than the dynamical time. Nonadiabatic heating due to magnetic field reconnection is included. The internal energy thus generated affects the flow in a purely relativistic way, again ensuring free-fall collapse of the inflowing matter. Any matter enveloping a black hole will thus be swallowed on a dynamical time scale with relatively small net release of energy. The inclusion of angular momentum will not necessarily affect this conclusion.

Gilden, D. L.↗
NASA NTRS1980

Radial accretion of H-rich material onto a He white dwarf

The nova outburst is modeled by spherically accreting H-rich material onto a 1 solar mass He white dwarf at a rate of 10 to the -8th solar mass/yr. The star accretes for 5848 years, when the nuclear reactions run away near the base of the accreted envelope. The nuclear-energy generation rate rises to 4.6(8) solar luminosities, and the envelope expands in response to it. However, nova-like mass ejection does not occur because the envelope is of insufficient mass, the base of the envelope is only mildly electron degenerate, and there is no enrichment of the CNO abundance. To overcome these limiting conditions, it is suggested that the H-rich material be accreted either more slowly than 10 to the -10th solar mass/yr or with angular momentum. Outbursts resulting in the former case should be similar to the nova models computed by Starrfield, Truran, and Sparks. Outbursts in the latter case should be strengthened, and novae might result because the H-rich material will be mixed into the surface layers of the white dwarf, as first suggested by Kippenhahn and Thomas (1978).

Kutter, G. S.↗
NASA NTRS1980

Transonic disk accretion onto black holes

The solution for the radial drift velocity of thin disk accretion onto black holes must be transonic, and is analogous to the critical solution in spherical Bondi accretion, except for the presence of angular momentum. The transonic requirement yields a correct treatment of the inner region of the disk not found in the conventional Keplerian models and may lead to significantly different overall disk structures. Possible observational consequences, relevant to the black hole hypothesis for Cyg X-1 and other candidates, are discussed.

Liang, E. P. T.↗
NASA NTRS1980

Stellar coronae from Einstein - Observations and theory

Einstein Observatory observations of stellar X-ray emission are presented and their implications for the formation of stellar coronae and the problem of stellar angular momentum loss are discussed. Solar coronal X-ray observations and observations of stellar coronae made prior to Einstein are reviewed, and it is noted that they already suggest that the standard theory of acoustic coronal heating is inadequate. The principal results of the Einstein/CfA stellar survey are summarized, with attention given to variations of the level of X-ray flux detected along the main sequence, the decline of X-ray flux with increasing age of giants and supergiants, and indications of a large range of X-ray emission levels within a given type, which are clearly incompatible with models for acoustic flux generation. A new theory to explain stellar coronae and hence X-ray emission from them is then proposed in which stellar magnetic fields play the key role in determining the level of coronal emission, and the modulation of the surface magnetic flux level and the level of stressing of surface magnetic fields essentially determine the variation of mean coronal activity in the H-R diagram.

Rosner, R.↗
NASA NTRS1980

A gravitational kinetic theory for planetesimals

An analytical theory is developed for the velocity evolution of nonaccreting planetesimal populations, based on the Boltzmann and Fokker-Planck equations. Adapting Shkarofsky's calculation of plasma viscosities, the rate of increase in random velocities due to gravitational encounters between planetesimals of equal mass is found to be one-third to one-half Safronov's result. Comparison with Wetherill's numerical experiments suggests that the Fokker-Planck equation underestimates the effectiveness of encounters and that Safronov's value is approximately correct. For populations of nonuniform sizes, the Fokker-Planck equation indicates an efficient redistribution of energy from the largest bodies to the smaller ones. By conserving angular momentum, the rate of radial spreading of orbits is also derived.

Stewart, G. R.↗
NASA NTRS1980

Limits on the possible intrinsic magnetic field of Venus

Magnetic field measurements obtained by the Pioneer Venus orbiter at low altitudes in the solar wind wake region are examined for possible surface-correlated features and any possible intrinsic magnetic moment. The field variations observed in the wake do not resemble those expected for a solar wind interaction with even a weak intrinsic magnetic field. Little orbit-to-orbit persistence of features is found in the magnetic records. The magnetic field measurements in the wake are averaged in 10 deg x 10 deg bins to minimize the effects of external field sources. In these 37 bins, the average fields appear to be randomly oriented and consistent with zero mean in the region mapped. Using these 37 averaged vector fields, a maximum intrinsic magnetic dipole moment is obtained of 4.3 + or - 2.0 x 10 to the 21st G cu cm, approximately an order of magnitude less than previous estimates. It is noted that a more conservative estimate of the probable error of the mean is 5.5 x 10 to the 21st G cu cm. The Pioneer Venus measurements are thus consistent with zero planetary moment. The present measurements are found to be far below estimates made on the basis of angular momentum, the so-called magnetic Bode's law, and far below the dynamo scaling law of Busse.

Russell, C. T.↗