3,477 research outputs found
Elastic theory of icosahedral quasicrystals - application to straight dislocations
In quasicrystals, there are not only conventional, but also phason
displacement fields and associated Burgers vectors. We have calculated
approximate solutions for the elastic fields induced by two-, three- and
fivefold straight screw- and edge-dislocations in infinite icosahedral
quasicrystals by means of a generalized perturbation method. Starting from the
solution for elastic isotropy in phonon and phason spaces, corrections of
higher order reflect the two-, three- and fivefold symmetry of the elastic
fields surrounding screw dislocations. The fields of special edge dislocations
display characteristic symmetries also, which can be seen from the
contributions of all orders.Comment: 13 pages, 11 figure
Study of liquid jet impingement on screens
A model is presented for an unconfined flow, such as a free jet, impinging on a screen which incorporates the influence of liquid deflection by the screen. The boundary layer blockage coefficient is introduced. This coefficient depends on the screen weave geometry and the jet impingement angle, and essentially accounts for the increase in fluid particle trajectory length through the screen resulting from the flow deflection. Comparisons were made with previous experimental studies to determine empirical values of the blockage coefficient. It is concluded that the new model reliably predicts the bulk flow and penetration characteristics of an impinging liquid jet interacting with a screen
A mercuric detector system for X-ray astronomy. 2. Results from flight tests of a balloon borne instrument
To establish the expected sensitivity of a new hard X-ray telescope design, an experiment was conducted to measure the background counting rate at balloon altitudes (40 km) of mercuric iodide, a room temperature solid state X-ray detector. The prototype detector consisted of two thin mercuric iodide (HgI2) detectors surrounded by a large bismuth germanate (Bi4Ge3O12) scintillator operated in anticoincidence. The bismuth germanate shield vetoed most of the background counting rate induced by atmospheric gamma-rays, neutrons and cosmic rays. A balloon-borne gondola containing a prototype detector assembly was designed, constructed and flown twice in the spring of 1982 from Palestine, Texas. The second flight of this instrument established a differential background counting rate of 4.2 O.7 x 10-5 counts/sec cm keV over the energy range of 40 to 80 keV. This measurement was within 50% of the predicted value. The measured rate is approx 5 times lower than previously achieved in shielded NaI/CsI or Ge systems operating in the same energy range. The prediction was based on a Monte Carlo simulation of the detector assembly in the radiation environment at float altitude
An orientable, stabilized balloon-borne gondola for around-the-world flights
A system capable of pointing a balloon-borne telescope at selected celestial objects to an accuracy of approximately 10 arc minutes for an extended period (weeks to months) without reliance on telemetry is described. A unique combination of a sun/star tracker, an on-board computer, and a gyrocompass is utilized for navigation, source acquisition and tracking, and data compression and recording. The possibilities for intelligent activities by the computer are also discussed
The Impact of Type Ia Supernova Ejecta on Binary Companions
We present adaptive mesh refinement (AMR) hydrodynamical simulations of the
interaction between Type Ia supernovae and their companion stars within the
context of the single-degenerate model. Results for 3D red-giant companions
without binary evolution agree with previous 2D results by Marietta et al. We
also consider evolved helium-star companions in 2D. For a range of helium-star
masses and initial binary separations, we examine the mass unbound by the
interaction and the kick velocity delivered to the companion star. We find that
unbound mass versus separation obeys a power law with index between -3.1 and
-4.0, consistent with previous results for hydrogen-rich companions. Kick
velocity also obeys a power-law relationship with binary separation, but the
slope differs from those found for hydrogen-rich companions. Assuming accretion
via Roche-lobe overflow, we find that the unbound helium mass is consistent
with observational limits. Ablation (shock heating) appears to be more
important in removing gas from helium-star companions than from hydrogen-rich
ones, though stripping (momentum transfer) dominates in both cases.Comment: 6 pages, 2 figures, to appear in the proceedings of the conference
"Binary Star Evolution: Mass Loss, Accretion, and Mergers" at Mykonos,
Greece, June 22-25, 201
A Direct Multigrid Poisson Solver for Oct-Tree Adaptive Meshes
We describe a finite-volume method for solving the Poisson equation on
oct-tree adaptive meshes using direct solvers for individual mesh blocks. The
method is a modified version of the method presented by Huang and Greengard
(2000), which works with finite-difference meshes and does not allow for shared
boundaries between refined patches. Our algorithm is implemented within the
FLASH code framework and makes use of the PARAMESH library, permitting
efficient use of parallel computers. We describe the algorithm and present test
results that demonstrate its accuracy.Comment: 10 pages, 6 figures, accepted by the Astrophysical Journal; minor
revisions in response to referee's comments; added char
Glassy behavior of molecular crystals: A comparison between results from MD-simulation and mode coupling theory
We have investigated the glassy behavior of a molecular crystal built up with
chloroadamantane molecules. For a simple model of this molecule and a rigid fcc
lattice a MD simulation was performed from which we obtained the dynamical
orientational correlators and the ``self''
correlators , with ,
. Our investigations are for the diagonal correlators
. Since the lattice constant decreases with decreasing
temperature which leads to an increase of the steric hindrance of the
molecules, we find a strong slowing down of the relaxation. It has a high
sensitivity on , . For most , there is a two-step
relaxation process, but practically not for , ,
and . Our results are consistent with the -relaxation
scaling laws predicted by mode coupling theory from which we deduce the glass
transition temperature . From a first principle solution
of the mode coupling equations we find . Furthermore mode
coupling theory reproduces the absence of a two-step relaxation process for
, , and , but underestimates the critical
nonergodicity parameters by about 50 per cent for all other . It is
suggested that this underestimation originates from the anisotropic crystal
field which is not accounted for by mode coupling theory. Our results also
imply that phonons have no essential influence on the long time relaxation
Robust Emergent Activity in Dynamical Networks
We study the evolution of a random weighted network with complex nonlinear
dynamics at each node, whose activity may cease as a result of interactions
with other nodes. Starting from a knowledge of the micro-level behaviour at
each node, we develop a macroscopic description of the system in terms of the
statistical features of the subnetwork of active nodes. We find the asymptotic
characteristics of this subnetwork to be remarkably robust: the size of the
active set is independent of the total number of nodes in the network, and the
average degree of the active nodes is independent of both the network size and
its connectivity. These results suggest that very different networks evolve to
active subnetworks with the same characteristic features. This has strong
implications for dynamical networks observed in the natural world, notably the
existence of a characteristic range of links per species across ecological
systems.Comment: 4 pages, 5 figure
Rapid fluctuations in the high-energy X-ray flux from a source in Crux
Balloonborne X ray telescopic observations of two point sources in Cru
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