280,226 research outputs found
Recommended from our members
An Investigation of Gas-Powder Flow in Laser-Based Direct Metal Deposition
Laser-Based Direct Metal Deposition (LBDMD) is a blown-powder laser deposition process
which can produce fully-dense and metallurgicaly sound parts by a layered manufacturing
method. Since a deposition head equipped with discontinuous radially symmetric nozzles has the
potential to be tilted without influence of the gravity on the powder stream shape, it can be used
for multi-axis deposition. The shape of the gas-powder stream with respect to the shape of laser
beam and the size of the molten pool, have a large influence on the size and shape of the buildup.
They determine the geometrical accuracy and the surface quality of the buildup. This paper
examines gas-powder flow from radially symmetric nozzles using computational fluid dynamics
method. For verification purpose the powder flow was investigated by a visualization method
and powder concentration distribution was analyzed using image processing technique. The
obtained results are in good agreement with numerical model.Mechanical Engineerin
Design and loading of dragline buckets
Draglines are an expensive and essential part of open cut coal mining. Small improvements in performance can produce substantial savings. The design of the bucket and the way in which it fills with overburden are very important to the overall dragline performance. Here we use a numerical model to simulate this filling process and to differentiate between the flow patterns of two different buckets. Extensions to the model are explored
Grain size limits derived from 3.6 {\mu}m and 4.5 {\mu}m coreshine
Recently discovered scattered light from molecular cloud cores in the
wavelength range 3-5 {\mu}m (called "coreshine") seems to indicate the presence
of grains with sizes above 0.5 {\mu}m. We aim to analyze 3.6 and 4.5 {\mu}m
coreshine from molecular cloud cores to probe the largest grains in the size
distribution. We analyzed dedicated deep Cycle 9 Spitzer IRAC observations in
the 3.6 and 4.5 {\mu}m bands for a sample of 10 low-mass cores. We used a new
modeling approach based on a combination of ratios of the two background- and
foreground-subtracted surface brightnesses and observed limits of the optical
depth. The dust grains were modeled as ice-coated silicate and carbonaceous
spheres. We discuss the impact of local radiation fields with a spectral slope
differing from what is seen in the DIRBE allsky maps. For the cores L260,
ecc806, L1262, L1517A, L1512, and L1544, the model reproduces the data with
maximum grain sizes around 0.9, 0.5, 0.65, 1.5, 0.6, and > 1.5 {\mu}m,
respectively. The maximum coreshine intensities of L1506C, L1439, and L1498 in
the individual bands require smaller maximum grain sizes than derived from the
observed distribution of band ratios. Additional isotropic local radiation
fields with a spectral shape differing from the DIRBE map shape do not remove
this discrepancy. In the case of Rho Oph 9, we were unable to reliably
disentangle the coreshine emission from background variations and the strong
local PAH emission. Considering surface brightness ratios in the 3.6 and 4.5
{\mu}m bands across a molecular cloud core is an effective method of
disentangling the complex interplay of structure and opacities when used in
combination with observed limits of the optical depth.Comment: 23 pages, 18 figures, accepted for publication in A&
Measurement of an AGN Central Mass on Centiparsec Scales: Results of Long-Term Optical Monitoring of Arp 102B
The optical spectrum of the broad-line radio galaxy Arp 102B has been
monitored for more than thirteen years to investigate the nature of the source
of its broad, double-peaked hydrogen Balmer emission lines. The shape of the
lines varied subtly; there was an interval during which the variation in the
ratio of the fluxes of the two peaks appeared to be sinusoidal, with a period
of 2.16 years and an amplitude of about 16% of the average value. The variable
part of the broad H-alpha line is well fit by a model in which a region of
excess emission (a quiescent ``hot spot'') within an accretion disk (fitted to
the non-varying portion of the double-peaked line) completes at least two
circular orbits and eventually fades. Fits to spectra from epochs when the hot
spot is not present allow determination of the disk inclination, while fits for
epochs when it is present provide a measurement of the radius of the hot spot's
orbit. From these data and the period of variation, we find that the mass
within the hot spot's orbit is 2.2 +0.2/-0.7 times 10^8 solar masses, within
the range of previous estimates of masses of active galactic nuclei. Because
this mass is determined at a relatively small distance (~1000 AU) from the
central body, it is extremely difficult to explain without assuming that a
supermassive black hole lies within Arp 102B. The lack of any systematic change
in the velocity of the blue peak over time yields a lower limit on the combined
mass of the two bodies in a binary black hole model like that of Gaskell (1983)
of 10^10 solar masses.Comment: 29 pages, including 6 figures; to appear in the Astrophysical Journal
199
Effects of habitat composition and landscape structure on worker foraging distances of five bumblebee species
Bumblebees (Bombus spp.) are important pollinators of both crops and wild flowers. Their contribution to this essential ecosystem service has been threatened over recent decades by changes in land use, which have led to declines in their populations. In order to design effective conservation measures it is important to understand the effects of variation in landscape composition and structure on the foraging activities of worker bumblebees. This is because the viability of individual colonies is likely to be affected by the trade-off between the energetic costs of foraging over greater distances and the potential gains from access to additional resources. We used field surveys, molecular genetics and fine resolution remote sensing to estimate the locations of wild bumblebee nests and to infer foraging distances across a 20 km2 agricultural landscape in southern England. We investigated five species, including the rare B. ruderatus and ecologically similar but widespread B. hortorum. We compared worker foraging distances between species and examined how variation in landscape composition and structure affected foraging distances at the colony level. Mean worker foraging distances differed significantly between species. Bombus terrestris, B. lapidarius and B. ruderatus exhibited significantly greater mean foraging distances (551 m, 536 m, 501 m, respectively) than B. hortorum and B. pascuorum (336 m, 272 m, respectively). There was wide variation in worker foraging distances between colonies of the same species, which was in turn strongly influenced by the amount and spatial configuration of available foraging habitats. Shorter foraging distances were found for colonies where the local landscape had high coverage and low fragmentation of semi-natural vegetation, including managed agri-environmental field margins. The strength of relationships between different landscape variables and foraging distance varied between species, for example the strongest relationship for B. ruderatus being with floral cover of preferred forage plants. Our findings suggest that favourable landscape composition and configuration has the potential to minimise foraging distances across a range of bumblebee species. There is thus potential for improvements in the design and implementation of landscape management options, such as agri-environment schemes, aimed at providing foraging habitat for bumblebees and enhancing crop pollination services
The formation of spiral arms and rings in barred galaxies
In this and in a previous paper (Romero-Gomez et al. 2006) we propose a
theory to explain the formation of both spirals and rings in barred galaxies
using a common dynamical framework. It is based on the orbital motion driven by
the unstable equilibrium points of the rotating bar potential. Thus, spirals,
rings and pseudo-rings are related to the invariant manifolds associated to the
periodic orbits around these equilibrium points. We examine the parameter space
of three barred galaxy models and discuss the formation of the different
morphological structures according to the properties of the bar model. We also
study the influence of the shape of the rotation curve in the outer parts, by
making families of models with rising, flat, or falling rotation curves in the
outer parts. The differences between spiral and ringed structures arise from
differences in the dynamical parameters of the host galaxies. The results
presented here will be discussed and compared with observations in a
forthcoming paper.Comment: 16 pages, 13 figures, accepted in A&A. High resolution version
available at http://www.oamp.fr/dynamique/pap/merce.htm
Structure of sunspot penumbral filaments: a remarkable uniformity of properties
The sunspot penumbra comprises numerous thin, radially elongated filaments
that are central for heat transport within the penumbra, but whose structure is
still not clear. To investigate the fine-scale structure of these filaments, we
perform a depth-dependent inversion of spectropolarimetric data of a sunspot
very close to solar disk center obtained by Hinode (SOT/SP). We have used a
recently developed spatially coupled 2D inversion scheme which allows us to
analyze the fine structure of individual penumbral filaments up to the
diffraction limit of the telescope. Filaments of different sizes in all parts
of penumbra display very similar magnetic field strengths, inclinations and
velocity patterns. The similarities allowed us to average all these filaments
and to extract the physical properties common to all of them. This average
filament shows upflows associated with an upward pointing field at its inner,
umbral end and along its axis, downflows along the lateral edge and strong
downflows in the outer end associated with a nearly vertical, strong and
downward pointing field. The upflowing plasma is significantly hotter than the
downflowing plasma. The hot, tear-shaped head of the averaged filament can be
associated with a penumbral grain. The central part of the filament shows
nearly horizontal fields with strengths of ~1kG. The field above the filament
converges, whereas a diverging trend is seen in the deepest layers near the
head of the filament. We put forward a unified observational picture of a
sunspot penumbral filament. It is consistent with such a filament being a
magneto-convective cell, in line with recent MHD simulations. The uniformity of
its properties over the penumbra sets constraints on penumbral models and
simulations. The complex and inhomogeneous structure of the filament provides a
natural explanation for a number of long-running controversies in the
literature.Comment: 19 pages; 12 figures; accepted for publication in A&
- …