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At least 199 records · Page 11
All sourcesNASA NTRSDOE OSTI
NASA NTRS1982

Drop rebound in clouds and precipitation

The possibility of rebound for colliding cloud drops was measured by determining the collection efficiency. The collection efficiency for 17 size pairs of relatively uncharged drops in over 500 experimental runs was measured using two techniques. The collection efficiencies fall in a narrow range of 0.60 to 0.70 even though the collection drop was varied between 63 and 326 microns and the size ratio from 0.05 to 0.33. In addition the measured values of collection efficiencies (Epsilon) were below the computed values of collision efficiencies (E) for rigid spheres. Therefore it was concluded that rebound was occurring for these sizes since inferred coalescence (epsilon = Epsilon/E) efficiencies are about 0.6 yo 0.8. At a very small size ratio (r/R = p = 0.05, R = 326 microns) the coalescence efficiency inferred is in good agreement with the experimental findings for a supported collector drop. At somewhat large size ratios the inferred values of epsilon are well above results of supported drop experiments, but show a slight correspondence in collected drop size dependency to two models of drop rebound. At a large size ratio (p = 0.73, R = 275) the inferred coalescence efficiency is significantly different from all previous results.

Ochs, H. T., III↗
NASA NTRS1981

Eikonal approximation for charge transfer from a multielectron atom to fast projectiles

The eikonal approach developed previously for calculating electron-capture cross sections for bare projectiles colliding with hydrogenic targets is extended here to allow for multielectron targets. Both the impact and wave pictures are employed and their equivalence is discussed. As a first approximation, each atomic orbital is specified by the three hydrogenic quantum numbers, an effective nuclear charge Z sub t, and an energy eigenvalue in the impact picture, or ionization potential in the wave picture. The Z sub t prime appearing in the eikonal phase factor is left undetermined because of incomplete information on the many-body target. However, analytic expressions are derived for the theoretical cross sections, and numerical values are calculated for simple choices of Z sub t prime. Those results are compared with existing experimental data for C, Ne, Ar, N2, O2, and He targets.

Ho, T. S.↗
NASA NTRS1982

The motion and distribution of the vibrationally excited H2 in the Orion molecular cloud

Observations of the v=1-0 S(1) and S(0), and v=2-1 S(1) emission lines of H2 in the Orion molecular cloud are presented, showing that the emission region has an approximately circular symmetry, which may be divided into a central and a peripheral region. The emission lines in the central region have a large range of velocities, and predominantly occur at the negative velocities of 0 to -100 km/sec, while those at the periphery are symmetric. The brightness of the peak emission is generally higher at positions in the periphery than in the central region. These results lead to a model of H2 line emission in which there is gas undergoing radial expansion at velocities of up to about 100 km/sec within a region of some 2.5 x 10 to the 17th cm in diameter around the cluster of IR sources. A substantial part of the H2 line emission comes from the outside boundary of this expansion region, where the flow collides with the gas in the molecular cloud.

Nadeau, D.↗
NASA NTRS1982

Observations and interpretation of the line profiles of excited H2 in Orion

The profiles of the V=1-0 S(1) line of H2 in the Orion molecular cloud show a variation from the center of the emission region where the profiles are broad and have extended blue wings, to the periphery where the profiles are narrow and have a peak at the velocity of the molecular cloud. These observations can be understood by a model of a radially expanding supersonic flow of gas colliding with the surrounding molecular cloud. It is concluded that the high velocity emission originates in the flow and the low velocity emission originates in the molecular cloud. Density variations in the flow and the surrounding cloud are necessary to explain the detailed line profiles and morphology.

Geballe, T. R.↗
NASA NTRS1982

Improvements to the Langley HZE abrasion model

Improvements to a previously developed high charge energy abrasion model are made by incorporating more realistic values for the constituent Fermi momentum and nucleon root-mean-square charge radius. The theoretical predictions for neon projectiles at 2.1 GeV/nucleon colliding with carbon and molybdenum targets are in excellent agreement with recent experiment results.

Townsend, L. W.↗
NASA NTRS1982

Interaction between a magnetized plasma flow and a strongly magnetized celestial body with an ionized atmosphere - Energetics of the magnetosphere

Findings on the interaction between a magnetized plasma flow and a strongly magnetized celestial body are described, emphasizing the energetics of the magnetosphere and some astrophysical implications. It is shown that the interaction between the solar wind and the magnetosphere constitutes a dynamo whose power is modulated by the magnetized plasma flow. The varying with time of the flow speed, the magnetic field magnitude, and the latter's orientation are studied along with the reasons for the variation. The mode of dissipation of the generated power in the magnetosphere is investigated. As a preliminary, the basic solar wind conditions in the heliosphere are analyzed. It is shown how a flare-generated disturbance propagates in the heliosphere and how the dynamo power is modulated as the solar wind disturbance collides with the magnetosphere. The origin of geomagnetic storms and auroral phenomena in the dissipation of power in the magnetosphere is detailed.

Akasofu, S.-I.↗
NASA NTRS1982

Coronagraphic observations of two new sungrazing comets

The discovery has recently been reported of a comet (Howard-Koomen-Michels: 1979 XI) that apparently collided with the sun on 30 August 1979. A report is presented of observations of two additional sungrazers that encountered the sun on 27 January 1981 and 20 July 1981, respectively. Like comet 1979 XI, these two new comets seem to have been members of the Kreutz group of sungrazers, and like 1979 XI the new comets did not reappear after their encounters with the sun. The discovery of three previously unreported comets during the initial 2.3 yr of satellite coronal observations suggests that sungrazers are much more common than one might suppose from the list of only nine known sungrazers observed during the years 1668-1970.

Sheeley, N. R., Jr.↗
NASA NTRS1982

Internal stresses in Phobos and other triaxial bodies

The unusual dynamical behavior of Phobos, its strange appearance, and its mysterious network of grooves all make it an intriguing object. Geophysical studies, though, have been hampered by the lack of suitable theories applicable to nonspherical bodies. In this paper the Martian satellites are modeled as homogeneous, elastic triaxial ellipsoids subject to tidal, rotational, and self-gravitational stresses. A novel semianalytical treatment then gives the stress and strain fields throughout their interiors. Yield phenomena and their possible surface expressions are also investigated. The results indicate that Phobos and Deimos have always been stable with respect to tidal fracture or disruption, but that Phobos will probably break up before colliding with Mars. Applications of the new formulation to other nonspherical bodies in the solar system are also discussed.

Dobrovolskis, A. R.↗
NASA NTRS1982

Electron energy distribution produced by beam-plasma discharge

In an investigation of a beam-plasma discharge (BPD), the electron energy distribution of an electron beam moving through a partially ionized gas is analyzed. Among other results, it is found that the occurrence of BPD heats the initially cold electron beam from the accelerator. The directional intensity of electrons measured outside the beam core indicates that most particles suffer a single scattering in energy and pitch angle. At low currents this result is expected as beam particles collide with the neutral atmosphere, while in BPD the majority of particles is determined to still undergo a single scattering near the original beam core. The extended energy spectra at various beam currents show two rather distinct plasma populations, one centered at the initial beam energy (approximately 1500 eV) and the other at approximately 150 eV.

Anderson, H. R.↗
NASA NTRS1983

Speckle interferometry of IRC +10216 in the fundamental vibration-rotation lines of CO

The largest fraction of the matter returned by stars to the interstellar medium is probably provided by red giants. The carbon star IRC +10216 is an example of an evolved giant with a large mass loss rate. One plausible mechanism for the acceleration of the gas in stars like IRC +10216 is radiation pressure on dust grains which then collide with and transfer their momentum to the gas. However, at the present time neither infrared nor microwave observations provide a clear picture of the distribution of matter near cool red giant stars. There exists one method which may be used to obtain more information about the distribution of matter very close to the star. This method involves the measurement of the spatial extent of near-infrared lines by employing a combination of very high spatial and high spectral resolution. The present investigation is concerned with an application of this method. Speckle interferometry is used to measure the radial distribution of CO molecules on angular scales of 1 sec near IRC +10216.

Dyck, H. M.↗
NASA NTRS1984

An intersection algorithm for moving parts

The problem of deciding whether moving mechanical parts will collide during motion arises frequently in computer-aided design of machinery and in robotics. In machinery design, the problem becomes acute when there are various complex curved mechanical parts that will follow planned trajectories. There is a need to know at an early stage in the design process whether there will be collisions between parts necessitating changes in part shapes, trajectories, or in the overall design. This paper describes a collision detection algorithm that is guaranteed to detect a collision if one will occur, and it also computes the earliest point of contact.

Esterling, D. M.↗
NASA NTRS1984

An abrasion-ablation model description of galactic heavy-ion fragmentation

The fragmentation of high-energy galactic heavy ions by nuclear interactions with arbitrary target nuclei is described within the context of a simple abrasion-ablation fragmentation model. The abrasion part of the theory utilizes a quantum-mechanical formalism based upon an optical model potential approximation to the exact nucleus-nucleus multiple-scattering series. Nuclear charge distributions of the excited prefragments are calculated using either a hypergeometric distribution or a method based upon the zero-point oscillations of the giant dipole resonance. The excitation energy of the prefragment is estimated from the geometric clean-cut abrasion-ablation model. The decay probabilities for the various particle emission channels, in the ablation stage of the fragmentation, are obtained from the EVAP-4 Monte Carlo computer program. Elemental production cross sections for 1.88-GeV/nucleon iron colliding with carbon, silver, and lead targets are calculated and compared with experimental data and with the predictions from the semiempirical relations of Silberberg and Tsao.

Townsend, L. W.↗
NASA NTRS1984

Coalescence of two pressure waves associated with stream interactions

An MHD unsteady 1-D model is used to simulate the interaction and coalescence of two pressure waves in the outer heliosphere. Each of the two pressure waves was a compression region bounded by a shock pair. Computer simulation using Voyager data as input demonstrates the interaction and coalescence process involving one pressure wave associated with a fast stream and the other pressure wave without a fast stream. The process produced a significant change in the magnetic field and plasma signatures. The propagation of the forward and reverse shocks first widened the radial dimension of the shock compression region with increasing heliocentric distances. The shocks belonging to two neighboring compression regions eventually collided and two compression regions began to overlap with each other. This type of interaction is a dominant dynamical process in the outer heliosphere, and significantly and irreversible alters the structure of the medium.

Whang, Y. C.↗
NASA NTRS1984

Physics of Nonmagnetic Relativistic Thermal Plasmas

A detailed treatment of the kinematics of relativistic systems of particles and photons is presented. In the case of a relativistic Maxwell-Boltzmann distribution of particles, the reaction rate and luminosity are written as single integrals over the invariant cross section, and the production spectrum is written as a double integral over the cross section differential in the energy of the produced particles (or photons) in the center-of-momentum system of two colliding particles. The results are applied to the calculation of the annihilation spectrum of a thermal electron-positron plasma, confirming previous numerical and analytic results. Relativistic thermal electron-ion and electron-electron bremsstrahlung are calculated exactly to lowest order, and relativistic thermal electron-positron bremsstrahlung is calculated in an approximate fashion. An approximate treatment of relativistic Comptonization is developed. The question of thermalization of a relativistic plasma is considered. A formula for the energy loss or exchange rate from the interaction of two relativistic Maxwell-Boltzmann plasmas at different temperatures is derived. Application to a stable, uniform, nonmagnetic relativistic thermal plasma is made. Comparison is made with other studies.

Dermer, C. D.↗
NASA NTRS1984

Navier-Stokes calculations for unsteady three-dimensional vortical flows in unbounded domains

Finite-difference Navier-Stokes calculations for unsteady, three-dimensional, incompressible, viscous flows induced by initial vorticity distributions are presented and discussed in this paper. The initial vorticity distributions are assumed to be embedded in a flow field of infinite extent that is quiescent at infinity. These vorticity distributions are typical of vortex rings and other closed vortical tubes or structures. Such structures are important elements in fluid flows such as jets, atmospheric convection and the far-field wakes of aircraft; studies of their interaction may aid in an understanding of complex fluid flows. The calculations employ a method recently proposed by Ting to approximate the infinite-domain boundary value problem with a finite boundary computational domain, and this method is shown to yield accurate three-dimensional results for reasonable expenditures of computer time. Because of the efficiency of the boundary condition technique and the resulting Navier-Stokes code, a 16-bit minicomputer with virtual memory was capable of performing the calculations for the unsteady motion of two obliquely colliding vortex rings. The results of these calculations are presented in the paper.

Chamberlain, J. P.↗
NASA NTRS1983

Impact versus internal origins for mesosiderites

The textures, mineral compositions, and thermal history of mesosiderites are examined to see whether they are compatible with any kind of impact process or internal process which might produce stony-iron breccias. It was proposed that cratering by metal projectiles is not an adequate origin mechanism since it would not deposit enough metal in the stony-iron breccias. It was also proposed that such mechanisms as accretion of differentiated planitesimals must be considered for mesosiderite origins. But no single model for origins is fully satisfactory. Disruption and reassembly of two colliding bodies could form stony-iron breccias at depth, but, as with the previous models, the easiest way to explain the low olivine of mesosiderites is to postulate derivation from olivine-poor chondritic material. Turbulent metal-crust mixing during core formation and crustal blocks sinking into the core withhold olivine from the breccia but do not explain how the silicate and metal liquids in subgroup IV could have failed to segregate.

Hewins, R. H.↗
NASA NTRS1983

Chondrules and other components in C, O, and E chondrites Similarities in their properties and origins

Three types of chondrules are described that are common to H3, LL3, CM2, CO3, and CV3 chondrites. Low- and high-FeO, porphyritic olivine chondrules contain olivine with Fa0.3-8 and Fa5-50 respectively, and can easily be distinguished petrographically. Poikilitic pyroxene chondrules have 1-20 vol pct olivine (Fa0.2-8) enclosed by low-Ca pyroxene (Fs0.5-7), and also occur in E chondrites. These three types formed in separate nebular regions which had dimensions and spacings such that a few percent of the chondrules that collided to form compound chondrules were of different types. Sorting of chondrule precursor material and mixing of chondrule types probably account for most variations in bulk and mineral chemistry among chondrite groups. Metallic Fe,Ni grains containing 0.1-1 percent Cr, Si, and P are present in low-FeO olivine chondrules from all type 2 and least metamorphosed type 3 chondrites. Metal compositions reflect reduction during chondrule formation in the nebula, not nebular condensation. Opaque matrices in type 3 ordinary and carbonaceous chondrites are somewhat similar in composition and mineralogy, and probably have related origins. It is concluded that chondrules in all known chondrite groups share similar nebular origins.

Scott, E. R. D.↗