31,832 research outputs found
Subpicosecond (320 fs) pulses from CW passively mode-locked external cavity two-section multiquantum well lasers
Pulses from a passively mode-locked two-section multi-quantum well laser coupled to an external cavity are compressed to subpicosecond pulse widths using an external grating telescope compressor. A minimum deconvolved pulse width of 0.32 ps is measured, close to the transform limit, with peak powers of 1.9 W
Resolving the virial discrepancy in clusters of galaxies with modified Newtonian dynamics
A sample of 197 X-ray emitting clusters of galaxies is considered in the
context of Milgrom's modified Newtonian dynamics (MOND). It is shown that the
gas mass, extrapolated via an assumed model to a fixed radius of 3 Mpc,
is correlated with the gas temperature as predicted by MOND (). The observed temperatures are generally consistent with the inferred
mass of hot gas; no substantial quantity of additional unseen matter is
required in the context of MOND. However, modified dynamics cannot resolve the
strong lensing discrepancy in those clusters where this phenomenon occurs. The
prediction is that additional baryonic matter may be detected in the central
regions of rich clusters.Comment: Submitted to A&A, 4 pages, 3 figures, A&A macro
Effect of thermal cycling in a Mach 0.3 burner rig on properties and structure of directionally solidified gamma/gamma prime - delta eutectic
Tensile and stress rupture properties at 1040 C of a thermally cycled gamma/gamma prime - delta eutectic were essentially equivalent to the as-grown properties. Tensile strength and rupture life at 760 C appeared to decrease slightly by thermal cycling. Thermal cycling resulted in gamma prime coarsening and Widmanstatten delta precipitation in the gamma phase. An unidentified precipitate, presumably gamma prime, was observed within the delta phase. The eutectic alloy exhibited a high rate of oxidation-erosion weight loss during thermal cycling in the Mach 0.3 burner rig
Modification of a three-dimensional supersonic nozzle analysis and comparison with experimental data
A computer program previously developed to analyze three-dimensional supersonic nozzles by the method of characteristics has been modified to study less restrictive nozzle geometries and nonuniform inlet conditions. An example indicates that a one-dimensional calculation that uses an averaged initial profile may be significantly in error. A comparison between the analysis and the data from a three-dimensional experiment shows generally good agreement between the two
High Velocity Burner Rig Oxidation and Thermal Fatigue Behavior of Si3N4- and SiC Base Ceramics to 1370 Deg C
One SiC material and three Si3N4 materials including hot-pressed Si3N4 as a baseline were exposed in a Mach-1-gas-velocity burner rig simulating a turbine engine environment. Criteria for the materials selection were: potential for gas-turbine usage, near-net-shape fabricability and commercial/domestic availability. Cyclic exposures of test vanes up to 250 cycles (50 hr at temperature) were at leading-edge temperatures to 1370 C. Materials and batches were compared as to weight change, surface change, fluorescent penetrant inspection, and thermal fatigue behavior. Hot-pressed Si3N4 survived the test to 1370 C with slight weight losses. Two types of reaction-sintered Si3N4 displayed high weight gains and considerable weight-change variability, with one material exhibiting superior thermal fatigue behavior. A siliconized SiC showed slight weight gains, but considerable batch variability in thermal fatigue
Numerical methods and calculations for droplet flow, heating and ignition
A numerical method was devised and employed to solve a variety of problems related to liquid droplet combustion. The basic transport equations of mass, momentum and energy were formulated in terms of generalized nonorthogonal coordinates, which allows for adaptive griding and arbitrary particle shape. Example problems are solved for internal droplet heating, droplet ignition and high Reynolds number flow over a droplet
Fine structure of distributions and central limit theorem in diffusive billiards
We investigate deterministic diffusion in periodic billiard models, in terms
of the convergence of rescaled distributions to the limiting normal
distribution required by the central limit theorem; this is stronger than the
usual requirement that the mean square displacement grow asymptotically
linearly in time. The main model studied is a chaotic Lorentz gas where the
central limit theorem has been rigorously proved. We study one-dimensional
position and displacement densities describing the time evolution of
statistical ensembles in a channel geometry, using a more refined method than
histograms. We find a pronounced oscillatory fine structure, and show that this
has its origin in the geometry of the billiard domain. This fine structure
prevents the rescaled densities from converging pointwise to gaussian
densities; however, demodulating them by the fine structure gives new densities
which seem to converge uniformly. We give an analytical estimate of the rate of
convergence of the original distributions to the limiting normal distribution,
based on the analysis of the fine structure, which agrees well with simulation
results. We show that using a Maxwellian (gaussian) distribution of velocities
in place of unit speed velocities does not affect the growth of the mean square
displacement, but changes the limiting shape of the distributions to a
non-gaussian one. Using the same methods, we give numerical evidence that a
non-chaotic polygonal channel model also obeys the central limit theorem, but
with a slower convergence rate.Comment: 16 pages, 19 figures. Accepted for publication in Physical Review E.
Some higher quality figures at http://www.maths.warwick.ac.uk/~dsander
Gravitational polarization and the phenomenology of MOND
The modified Newtonian dynamics (MOND) has been proposed as an alternative to
the dark matter paradigm; the philosophy behind is that there is no dark matter
and we witness a violation of the Newtonian law of dynamics. In this article,
we interpret differently the phenomenology sustaining MOND, as resulting from
an effect of "gravitational polarization", of some cosmic fluid made of dipole
moments, aligned in the gravitational field, and representing a new form of
dark matter. We invoke an internal force, of non-gravitational origin, in order
to hold together the microscopic constituents of the dipole. The dipolar
particles are weakly influenced by the distribution of ordinary matter; they
are accelerated not by the gravitational field, but by its gradient, or tidal
gravitational field.Comment: 14 pages, 1 figure, to appear in Classical and Quantum Gravit
Observations of fast anisotropic ion heating, ion cooling, and ion recycling in large-amplitude drift waves
Large-amplitude drift wave fluctuations are observed to cause severe ion temperature oscillations in plasmas of the Caltech Encore tokamak [J. M. McChesney, P. M. Bellan, and R. A. Stern, Phys. Fluids B 3, 3370 (1991)]. Experimental investigations of the complete ion dynamical behavior in these waves are presented. The wave electric field excites stochastic ion orbits in the plane normal (perpendicular to) to B, resulting in rapid perpendicular to heating. Ion-ion collisions impart energy along (parallel to) B, relaxing the perpendicular to-parallel to temperature anisotropy. Hot ions with large orbit radii escape confinement, reaching the chamber wall and cooling the distribution. Cold ions from the plasma edge convect back into the plasma (i.e., recycle), causing further cooling and significantly replenishing the density depleted by orbit losses. The ion-ion collision period tau(ii)similar to Tau(3/2)/n fluctuates strongly with the drift wave phase, due to intense (approximate to 50%) fluctuations in n and Tau. Evidence for particle recycling is given by observations of bimodal ion velocity distributions near the plasma edge, indicating the presence of cold ions (0.4 eV) superposed atop the hot (4-8 eV) plasma background. These appear periodically, synchronous with the drift wave phase at which ion fluid flow from the wall toward the plasma center peaks. Evidence is presented that such a periodic heat/loss/recycle/cool process is expected in plasmas with strong stochastic heating
Constraints on turbulent velocity broadening for a sample of clusters, groups and elliptical galaxies using XMM-Newton
Using the width of emission lines in XMM-Newton Reflection Grating
Spectrometer spectra, we place direct constraints on the turbulent velocities
of the X-ray emitting medium in the cores of 62 galaxy clusters, groups and
elliptical galaxies. We find five objects where we can place an upper limit on
the line-of-sight broadening of 500 km/s (90 per cent confidence level), using
a single thermal component model. Two other objects are lower than this limit
when two thermal components are used. Half of the objects examined have an
upper limit on the velocity broadening of less than 700 km/s. To look for
objects which have significant turbulent broadening, we use Chandra spectral
maps to compute the expected broadening caused by the spatial extent of the
source. Comparing these with our observed results, we find that Klemola 44 has
extra broadening at the level of 1500 km/s. RX J1347.5-1145 shows weak evidence
for turbulent velocities at 800 km/s. In addition we obtain limits on
turbulence for Zw3146, Abell 496, Abell 1795, Abell 2204 and HCG 62 of less
than 200 km/s. After subtraction of the spatial contribution and including a 50
km/s systematic uncertainty, we find at least 15 sources with less than 20 per
cent of the thermal energy density in turbulence.Comment: 17 pages, 17 figures, accepted by MNRAS. Includes minor edits to
proo
- …