1,672 research outputs found
Swapping trajectories: a new wall-induced cross-streamline particle migration mechanism in a dilute suspension of spheres
Binary encounters between spherical particles in shear flow are studied for a
system bounded by a single planar wall or two parallel planar walls under
creeping flow conditions. We show that wall proximity gives rise to a new class
of binary trajectories resulting in cross-streamline migration of the
particles. The spheres on these new trajectories do not pass each other (as
they would in free space) but instead they swap their cross-streamline
positions. To determine the significance of the wall-induced particle
migration, we have evaluated the hydrodynamic self-diffusion coefficient
associated with a sequence of uncorrelated particle displacements due to binary
particle encounters. The results of our calculations quantitatively agree with
the experimental value obtained by \cite{Zarraga-Leighton:2002} for the
self-diffusivity in a dilute suspension of spheres undergoing shear flow in a
Couette device. We thus show that the wall-induced cross-streamline particle
migration is the source of the anomalously large self-diffusivity revealed by
their experiments.Comment: submited to JF
Probing clumpy stellar winds with a neutron star
INTEGRAL, the European Space Agency's gamma-ray observatory, tripled the
number of super-giant high-mass X-ray binaries (sgHMXB) known in the Galaxy by
revealing absorbed and fast transient (SFXT) systems. In these sources,
quantitative constraints on the wind clumping of the massive stars could be
obtained from the study of the hard X-ray variability of the compact accreting
object. Hard X-ray flares and quiescent emission of SFXT systems have been
characterized and used to derive wind clump parameters. A large fraction of the
hard X-ray emission is emitted in the form of flares with a typical duration of
3 ks, frequency of 7 days and luminosity of 1E36 erg/s. Such flares are most
probably emitted by the interaction of a compact object orbiting at ~10 R* with
wind clumps (1E(22-23) g) representing a large fraction of the stellar
mass-loss rate. The density ratio between the clumps and the inter-clump medium
is 1E(2-4) in SFXT systems. The parameters of the clumps and of the inter-clump
medium, derived from the SFXT flaring behavior, are in good agreement with
macro-clumping scenario and line driven instability simulations. SFXT have
probably a larger orbital radius than classical sgHMXB.Comment: 8 page
A search for near infrared counterparts of 3 pulsar wind nebulae
While pulsar wind nebulae (PWNe) and their associated isolated pulsars are
commonly detected at X-ray energies, they are much rarer at near infrared (nIR)
and optical wavelengths. Here we examine three PWN systems in the Galactic
plane - IGR J14003-6326, HESS J1632-478 and IGR J18490-0000 - in a bid to
identify optical/nIR emission associated with either the extended PWNe or their
previously detected X-ray point sources. We obtain optical/nIR images of the
three fields with the ESO - New Technology Telescope and apply standard
photometric and astrometric calibrations. We find no evidence of any extended
emission associated with the PWNe in any of the fields; neither do we find any
new counterparts to the X-ray point sources, except to confirm the magnitude of
the previously identified counterpart candidate of IGR J18490-0000. Further
observations are required to confirm the association of the nIR source to IGR
J18490-0000 and to detect counterparts to IGR J14003-6326 and HESS J1632-478,
while a more accurate X-ray position is required to reduce the probability of a
chance superposition in the field of the latter.Comment: Accepted to A&A (4 pages, 1 figure
Does grade retention affect achievement? Some evidence from PISA
Grade retention practices are at the forefront of the educational debate. In this paper, we use PISA 2009 data for Spain to measure the effect of grade retention on students achievement. One important problem when analyzing this question is that school outcomes and the propensity to repeat a grade are likely to be determined simultaneously. We address this problem by estimating a Switching Regression Model. We
find that grade retention has a negative impact on educational outcomes, but we confi
rm the importance of endogenous selection, which makes observed differences between repeaters and non-repeaters appear 14.6% lower than they actually are. The effect on PISA scores of repeating is much smaller (-10% of non-repeaters average) than the counterfactual reduction that non-repeaters would suffer had they been retained as repeaters (-24% of their average). Furthermore, those who repeated a grade during primary education suffered more than those who repeated a grade of secondary school, although the effect of repeating at both times is, as expected, much larger.Grade retention, educational scores, PISA
Does grade retention affect achievement? Some evidence from Pisa
Grade retention practices are at the forefront of the educational debate. In this paper, we use PISA 2009 data for Spain to measure the effect of grade retention on students’achievement. One important problem when analyzing this question is that school outcomes and the propensity to repeat a grade are likely to be determined simultaneously. We address this problem by estimating a Switching Regression Model. We find that grade retention has a negative impact on educational outcomes, but we confirm the importance of endogenous selection, which makes observed differences between repeaters and non-repeaters appear 14.6% lower than they actually are. The effect on PISA scores of repeating is much smaller (-10% of non-repeaters’average) than the counterfactual reduction that non-repeaters would suffer had they been retained as repeaters (-24% of their average). Furthermore, those who repeated a grade during primary education suffered more than those who repeated a grade of secondary school, although the effect of repeating at both times is, as expected, much larger.Grade retention, educational scores, PISA
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