19,292 research outputs found
Transition from single-file to two-dimensional diffusion of interacting particles in a quasi-one-dimensional channel
Diffusive properties of a monodisperse system of interacting particles
confined to a \textit{quasi}-one-dimensional (Q1D) channel are studied using
molecular dynamics (MD) simulations. We calculate numerically the mean-squared
displacement (MSD) and investigate the influence of the width of the channel
(or the strength of the confinement potential) on diffusion in finite-size
channels of different shapes (i.e., straight and circular). The transition from
single-file diffusion (SFD) to the two-dimensional diffusion regime is
investigated. This transition (regarding the calculation of the scaling
exponent () of the MSD ) as a
function of the width of the channel, is shown to change depending on the
channel's confinement profile. In particular the transition can be either
smooth (i.e., for a parabolic confinement potential) or rather sharp/stepwise
(i.e., for a hard-wall potential), as distinct from infinite channels where
this transition is abrupt. This result can be explained by qualitatively
different distributions of the particle density for the different confinement
potentials.Comment: 13 pages, 11 figure
Magnetized Accretion-Ejection Structures: 2.5D MHD simulations of continuous Ideal Jet launching from resistive accretion disks
We present numerical magnetohydrodynamic (MHD) simulations of a magnetized
accretion disk launching trans-Alfvenic jets. These simulations, performed in a
2.5 dimensional time-dependent polytropic resistive MHD framework, model a
resistive accretion disk threaded by an initial vertical magnetic field. The
resistivity is only important inside the disk, and is prescribed as eta =
alpha_m V_AH exp(-2Z^2/H^2), where V_A stands for Alfven speed, H is the disk
scale height and the coefficient alpha_m is smaller than unity. By performing
the simulations over several tens of dynamical disk timescales, we show that
the launching of a collimated outflow occurs self-consistently and the ejection
of matter is continuous and quasi-stationary. These are the first ever
simulations of resistive accretion disks launching non-transient ideal MHD
jets. Roughly 15% of accreted mass is persistently ejected. This outflow is
safely characterized as a jet since the flow becomes super-fastmagnetosonic,
well-collimated and reaches a quasi-stationary state. We present a complete
illustration and explanation of the `accretion-ejection' mechanism that leads
to jet formation from a magnetized accretion disk. In particular, the magnetic
torque inside the disk brakes the matter azimuthally and allows for accretion,
while it is responsible for an effective magneto-centrifugal acceleration in
the jet. As such, the magnetic field channels the disk angular momentum and
powers the jet acceleration and collimation. The jet originates from the inner
disk region where equipartition between thermal and magnetic forces is
achieved. A hollow, super-fastmagnetosonic shell of dense material is the
natural outcome of the inwards advection of a primordial field.Comment: ApJ (in press), 32 pages, Higher quality version available at
http://www-laog.obs.ujf-grenoble.fr/~fcass
Time-dependent magnetohydrodynamic self-similar extragalactic jets
Extragalactic jets are visualized as dynamic erruptive events modelled by
time-dependent magnetohydrodynamic (MHD) equations. The jet structure comes
through the temporally self-similar solutions in two-dimensional axisymmetric
spherical geometry. The two-dimensional magnetic field is solved in the finite
plasma pressure regime, or finite regime, and it is described by an
equation where plasma pressure plays the role of an eigenvalue. This allows a
structure of magnetic lobes in space, among which the polar axis lobe is
strongly peaked in intensity and collimated in angular spread comparing to the
others. For this reason, the polar lobe overwhelmes the other lobes, and a jet
structure arises in the polar direction naturally. Furthermore, within each
magnetic lobe in space, there are small secondary regions with closed
two-dimensional field lines embedded along this primary lobe. In these embedded
magnetic toroids, plasma pressure and mass density are much higher accordingly.
These are termed as secondary plasmoids. The magnetic field lines in these
secondary plasmoids circle in alternating sequence such that adjacent plasmoids
have opposite field lines. In particular, along the polar primary lobe, such
periodic plasmoid structure happens to be compatible with radio observations
where islands of high radio intensities are mapped
Two-component mixture of charged particles confined in a channel: melting
The melting of a binary system of charged particles confined in a {\it
quasi}-one-dimensional parabolic channel is studied through Monte Carlo
simulations. At zero temperature the particles are ordered in parallel chains.
The melting is anisotropic and different melting temperatures are obtained
according to the spatial direction, and the different types of particles
present in the system. Melting is very different for the single-, two- and
four-chain configurations. A temperature induced structural phase transition is
found between two different four chain ordered states which is absent in the
mono-disperse system. In the mixed regime, where the two types of particles are
only slightly different, melting is almost isotropic and a thermally induced
homogeneous distribution of the distinct types of charges is observed.Comment: To appear in Journal of Physics: condensed matter ; (13 pages, 12
figures
The complex Sine-Gordon equation as a symmetry flow of the AKNS Hierarchy
It is shown how the complex sine-Gordon equation arises as a symmetry flow of
the AKNS hierarchy. The AKNS hierarchy is extended by the ``negative'' symmetry
flows forming the Borel loop algebra. The complex sine-Gordon and the vector
Nonlinear Schrodinger equations appear as lowest negative and second positive
flows within the extended hierarchy. This is fully analogous to the well-known
connection between the sine-Gordon and mKdV equations within the extended mKdV
hierarchy.
A general formalism for a Toda-like symmetry occupying the ``negative''
sector of sl(N) constrained KP hierarchy and giving rise to the negative Borel
sl(N) loop algebra is indicated.Comment: 8 pages, LaTeX, typos corrected, references update
New type II Cepheids from VVV data towards the Galactic center
The Galactic center (GC) is the densest region of the Milky Way. Variability
surveys towards the GC potentially provide the largest number of variable stars
per square degree within the Galaxy. However, high stellar density is also a
drawback due to blending. Moreover, the GC is affected by extreme reddening,
therefore near infrared observations are needed. We plan to detect new variable
stars towards the GC, focusing on type II Cepheids (T2Cs) which have the
advantage of being brighter than RR Lyrae stars. We perform parallel
Lomb-Scargle and Generalized Lomb-Scargle periodogram analysis of the
-band time series of the VISTA variables in the Via Lactea survey, to
detect periodicities. We employ statistical parameters to clean our sample. We
take account of periods, light amplitudes, distances, and proper motions to
provide a classification of the candidate variables. We detected 1,019 periodic
variable stars, of which 164 are T2Cs, 210 are Miras and 3 are classical
Cepheids. We also found the first anomalous Cepheid in this region. We compare
their photometric properties with overlapping catalogs and discuss their
properties on the color-magnitude and Bailey diagrams. We present the most
extensive catalog of T2Cs in the GC region to date. Offsets in E() and
in the reddening law cause very large (1-2 kpc) uncertainties on
distances in this region. We provide a catalog which will be the starting point
for future spectroscopic surveys in the innermost regions of the Galaxy.Comment: A&A, accepte
Fatigue life predictions in polymer particle composites
This paper presents a study on fatigue life predictions in three polymer particle composites with different volume fractions of filler and different particle sizes. Central hole notched specimens were analysed using a fracture mechanics approach. A solution for the stress intensity factor of corner cracks at a hole was obtained using the finite element method and considering quarter-circular and quarter-elliptical cracks of different sizes. The solution was compared with a literature solution and significant differences were found. Fatigue crack propagation tests were performed at room temperature and constant loading amplitude, for stress ratios R=0 and R=-0.75. Finally, fatigue lives, crack shape evolution and final crack length were predicted assuming an initial crack size and considering that the crack maintains a quarter-elliptical shape. The comparison with experimental fatigue lives indicated the presence of initial defects larger than the silica particles; however, these large sizes can be explained by the residual stresses measured near the hole.http://www.sciencedirect.com/science/article/B6V35-454FDJ5-7/1/4231c9abb6a4b1b364cde431359b1e1
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