7,490 research outputs found
Instability of the symmetric Couette-flow in a granular gas: hydrodynamic field profiles and transport
We investigate the inelastic hard disk gas sheared by two parallel bumpy
walls (Couette-flow). In our molecular dynamic simulations we found a
sensitivity to the asymmetries of the initial condition of the particle places
and velocities and an asymmetric stationary state, where the deviation from
(anti)symmetric hydrodynamic fields is stronger as the normal restitution
coefficient decreases. For the better understanding of this sensitivity we
carried out a linear stability analysis of the former kinetic theoretical
solution [Jenkins and Richman: J. Fluid. Mech. {\bf 171} (1986)] and found it
to be unstable. The effect of this asymmetry on the self-diffusion coefficient
is also discussed.Comment: 9 pages RevTeX, 14 postscript figures, sent to Phys. Rev.
The conservation status of the world’s freshwater molluscs
With the biodiversity crisis continuing unchecked, we need to establish levels and drivers of extinction risk, and reassessments over time, to effectively allocate conservation resources and track progress towards global conservation targets. Given that threat appears particularly high in freshwaters, we assessed the extinction risk of 1428 randomly selected freshwater molluscs using the IUCN Red List Categories and Criteria, as part of the Sampled Red List Index project. We show that close to one-third of species in our sample are estimated to be threatened with extinction, with highest levels of threat in the Nearctic, Palearctic and Australasia and among gastropods. Threat levels were higher in lotic than lentic systems. Pollution (chemical and physical) and the modification of natural systems (e.g. through damming and water abstraction) were the most frequently reported threats to freshwater molluscs, with some regional variation. Given that we found little spatial congruence between species richness patterns of freshwater molluscs and other freshwater taxa, apart from crayfish, new additional conservation priority areas emerged from our study. We discuss the implications of our findings for freshwater mollusc conservation, the adequacy of a sampled approach and important next steps to estimate trends in freshwater mollusc extinction risk over time
Long distance effects in weak radiative decays of D-mesons
We present a detailed analysis of the transitions, using a
model which combines heavy quark effective theory and the chiral Lagrangian
approach and includes symmetry breaking. We notice that in addition to the
previously considered s - channel annihilation and t - channel W - exchange,
there is a long distance penguin - like contribution in the t
- channel of Cabibbo - suppressed modes. Its magnitude is determined by the
size of symmetry breaking which we calculate with a vector dominance approach.
Although smaller in magnitude, the penguin - like contribution would lead to
sizeable effects in case of cancellations among the other contributions to the
amplitude. Thus, it may invalidate suggested tests for beyond the standard
model effects in these decays. We also indicate the range of expectations for
the branching ratios of various modes.Comment: 28 pages, Latex, 2 Figure
Semileptonic B decays to excited charmed mesons
Exclusive semileptonic B decays into excited charmed mesons are investigated
at order in the heavy quark effective theory. Differential
decay rates for each helicity state of the four lightest excited mesons
(, , , and ) are examined. At zero recoil,
corrections to the matrix elements of the weak currents can
be written in terms of the leading Isgur-Wise functions for the corresponding
transition and meson mass splittings. A model independent prediction is found
for the slope parameter of the decay rate into helicity zero at zero
recoil. The differential decay rates are predicted, including
corrections with some model dependence away from zero
recoil and including order corrections. Ratios of various exclusive
branching ratios are computed. Matrix elements of the weak currents between
mesons and other excited charmed mesons are discussed at zero recoil to order
. These amplitudes vanish at leading order, and can be
written at order in terms of local matrix elements.
Applications to decay sum rules and factorization are presented.Comment: 39 pages revtex including 10 figures, uses epsf. Substantial
improvements throughout the pape
Multiple time scales in cataclysmic binaries. The low-field magnetic dwarf nova DO Draconis
We study the variability of the cataclysmic variable DO Dra, on time-scales
of between minutes and decades.
The characteristic decay time dt/dm=0.902(3) days/mag was estimated from our
3 nights of CCD R observations. The quiescent data show a photometric wave with
a cycle about 303(15)d. We analyzed the profile of the composite (or mean)
outburst. We discovered however, that a variety of different outburst heights
and durations had occurred, contrary to theoretical predictions. With
increasing maximum brightness, we find that the decay time also increases; this
is in contrast to the model predictions, which indicate that outbursts should
have a constant shape. This is interpreted as representing the presence of
outburst-to-outburst variability of the magnetospheric radius. A presence of a
number of missed weak narrow outbursts is predicted from this statistical
relationship. A new type of variability is detected, during 3 subsequent nights
in 2007: periodic (during one nightly run) oscillations with rapidly-decreasing
frequency from 86 to 47 cycles/day and a semi-amplitude increasing from 0.06 to
0. 10, during a monotonic brightness increase from 14. 27 to 14. 13. This
phenomenon was observed only during an unusually prolonged event of about 1 mag
brightening in 2007 (lasting till autumn), during which no (expected) outburst
was detected. We refer to this behaviour as to the transient periodic
oscillations (TPO). To study this new and interesting phenomenon, new regular
photometric and spectral (in a target of opportunity mode) observations are
required.Comment: 12pages, 8figures, accepted in Astronomy and Astrophysic
First normal stress difference and crystallization in a dense sheared granular fluid
The first normal stress difference () and the microstructure
in a dense sheared granular fluid of smooth inelastic hard-disks are probed
using event-driven simulations. While the anisotropy in the second moment of
fluctuation velocity, which is a Burnett-order effect, is known to be the
progenitor of normal stress differences in {\it dilute} granular fluids, we
show here that the collisional anisotropies are responsible for the normal
stress behaviour in the {\it dense} limit. As in the elastic hard-sphere
fluids, remains {\it positive} (if the stress is defined in
the {\it compressive} sense) for dilute and moderately dense flows, but becomes
{\it negative} above a critical density, depending on the restitution
coefficient. This sign-reversal of occurs due to the {\it
microstructural} reorganization of the particles, which can be correlated with
a preferred value of the {\it average} collision angle in the direction opposing the shear. We also report on the shear-induced
{\it crystal}-formation, signalling the onset of fluid-solid coexistence in
dense granular fluids. Different approaches to take into account the normal
stress differences are discussed in the framework of the relaxation-type
rheological models.Comment: 21 pages, 13 figure
Quantum value indefiniteness
The indeterministic outcome of a measurement of an individual quantum is
certified by the impossibility of the simultaneous, definite, deterministic
pre-existence of all conceivable observables from physical conditions of that
quantum alone. We discuss possible interpretations and consequences for quantum
oracles.Comment: 19 pages, 2 tables, 2 figures; contribution to PC0
Adjacent thoracic lymph node metastases originating from two separate primary cancers: case report
Reported is an unusual case of adjacent thoracic lymph nodes demonstrating metastases from two different primary malignancies. A 51 year-old woman with a previous history of bilateral breast cancer underwent a radical gastro-oesophagectomy for adenocarcinoma of the lower third of the oesophagus. The resection specimen demonstrated breast and oesophageal metastases in adjacent thoracic lymph nodes. Mechanisms for this phenomenon, including the known local immune suppression on lymphoid cells by oesophageal carcinoma cells, are discussed
Hydrodynamic theory for granular gases
A granular gas subjected to a permanent injection of energy is described by
means of hydrodynamic equations derived from a moment expansion method. The
method uses as reference function not a Maxwellian distribution but
a distribution , such that adds a fourth cumulant
to the velocity distribution. The formalism is applied to a stationary
conductive case showing that the theory fits extraordinarily well the results
coming from our molecular dynamic simulations once we determine as a
function of the inelasticity of the particle-particle collisions. The shape of
is independent of the size of the system.Comment: 10 pages, 9 figures, more about our research in
http://www.cec.uchile.cl/cinetica
A stochastic model for heart rate fluctuations
Normal human heart rate shows complex fluctuations in time, which is natural,
since heart rate is controlled by a large number of different feedback control
loops. These unpredictable fluctuations have been shown to display fractal
dynamics, long-term correlations, and 1/f noise. These characterizations are
statistical and they have been widely studied and used, but much less is known
about the detailed time evolution (dynamics) of the heart rate control
mechanism. Here we show that a simple one-dimensional Langevin-type stochastic
difference equation can accurately model the heart rate fluctuations in a time
scale from minutes to hours. The model consists of a deterministic nonlinear
part and a stochastic part typical to Gaussian noise, and both parts can be
directly determined from the measured heart rate data. Studies of 27 healthy
subjects reveal that in most cases the deterministic part has a form typically
seen in bistable systems: there are two stable fixed points and one unstable
one.Comment: 8 pages in PDF, Revtex style. Added more dat
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