124,707 research outputs found
Formalism of collective electron excitations in fullerenes
We present a detailed formalism for the description of collective electron
excitations in fullerenes in the process of the electron inelastic scattering.
Considering the system as a spherical shell of a finite width, we show that the
differential cross section is defined by three plasmon excitations, namely two
coupled modes of the surface plasmon and the volume plasmon. The interplay of
the three plasmons appears due to the electron diffraction of the fullerene
shell. Plasmon modes of different angular momenta provide dominating
contributions to the differential cross section depending on the transferred
momentum.Comment: 11 pages, 2 figures; submitted to the special issue "Atomic Cluster
Collisions: Structure and Dynamics from the Nuclear to the Biological Scale"
of Eur. Phys. J.
A copula model for dependent competing risks
Many popular estimators for duration models require independent competing risks or independent censoring. In contrast, copula based estimators are also consistent in presence of dependent competing risks. In this paper we suggest a computationally convenient extension of the Copula Graphic Estimator (Zheng and Klein, 1995) to a model with more than two dependent competing risks. We analyse the applicability of this estimator by means of simulations and real world unemployment duration data from Germany. We obtain evidence that our estimator yields nice results if the dependence structure is known and that it is a powerful tool for the assessment of the relevance of (in-)dependence assumptions in applied duration research.Archimedean copula, dependent censoring, unemployment duration
Dynamical screening of an endohedral atom
The present work is a generalisation of the dynamical screening factor
presented in [1] to consider an atom located at an arbitrary position within
the fullerene. A more elaborated investigation into the case where the atom is
located at the centre is performed and compared with quantum mechanical
calculations for dynamical screening factor of Ar@C [2] and Mg@C
[3]. The and plasmons of the fullerene are accounted for in a
modified screening factor to improve correspondence with the quantum
calculations. The spatial dependence of the screening factor was explored with
Ar@C and Ar@C and found to depend significantly on the radial
distance of the atom from the centre of the fullerene. A spatial averaging of
the screening factor is presented.Comment: 18 pages, 7 figure
Comparison of experimental and Computational Fluid Dynamics (CFD) studies of slug flow in a vertical riser
This paper presents a comparison of the results obtained from experiments and CFD studies of slug flow in a vertical riser. A series of two experimental investigations were carried out on a 6 m vertical pipe with a 0.067 m internal diameter charged with an air–silicone oil mixture. For the first set of experiments, the riser was initially full of air, and then liquid and gas flows set to liquid and gas superficial velocities = 0.05 and 0.344 m/s, respectively, electrical capacitance tomography (ECT) and wire mesh sensor (WMS) transducers were employed. In the second one, the riser was initially full of (static) liquid, and then liquid and gas flows set to liquid and gas superficial velocities = 0.05 and 0.344 m/s, respectively, only ECT was used. A characterisation of the observed slug flow regimes was carried out. This includes the evaluation of the instantaneous distribution of the phases over the pipe cross-section, the Probability Density Function (PDF) of void fraction, time series of cross-sectional void fraction, Power Spectral Density (PSD), structure velocity of the Taylor bubble, lengths of the liquid slug and Taylor bubble and void fractions in the liquid slug and Taylor bubble. The simulation results were validated both qualitatively and quantitatively against experimental data. A reasonably good agreement was observed between the results of the experiment and CFD
Self-Segregation vs. Clustering in the Evolutionary Minority Game
Complex adaptive systems have been the subject of much recent attention. It
is by now well-established that members (`agents') tend to self-segregate into
opposing groups characterized by extreme behavior. However, while different
social and biological systems manifest different payoffs, the study of such
adaptive systems has mostly been restricted to simple situations in which the
prize-to-fine ratio, , equals unity. In this Letter we explore the dynamics
of evolving populations with various different values of the ratio , and
demonstrate that extreme behavior is in fact {\it not} a generic feature of
adaptive systems. In particular, we show that ``confusion'' and
``indecisiveness'' take over in times of depression, in which case cautious
agents perform better than extreme ones.Comment: 4 pages, 4 figure
Drag Reduction by Bubble Oscillations
Drag reduction in stationary turbulent flows by bubbles is sensitive to the
dynamics of bubble oscillations. Without this dynamical effect the bubbles only
renormalize the fluid density and viscosity, an effect that by itself can only
lead to a small percentage of drag reduction. We show in this paper that the
dynamics of bubbles and their effect on the compressibility of the mixture can
lead to a much higher drag reduction.Comment: 7 pages, 1 figure, submitted to Phys. Rev.
Turbulent Drag Reduction by Flexible and Rodlike Polymers: Crossover Effects at Small Concentrations
Drag reduction by polymers is bounded between two universal asymptotes, the
von-K\'arm\'an log-law of the law and the Maximum Drag Reduction (MDR)
asymptote. It is theoretically understood why the MDR asymptote is universal,
independent of whether the polymers are flexible or rodlike. The cross-over
behavior from the Newtonian von-K\'arm\'an log-law to the MDR is however not
universal, showing different characteristics for flexible and rodlike polymers.
In this paper we provide a theory for this cross-over phenomenology.Comment: 5 pages, 4 figures, submitted to Physical Review
Nonequilibrium Phase Transition in the Kinetic Ising model: Critical Slowing Down and Specific-heat Singularity
The nonequilibrium dynamic phase transition, in the kinetic Ising model in
presence of an oscillating magnetic field, has been studied both by Monte Carlo
simulation and by solving numerically the mean field dynamic equation of motion
for the average magnetisation. In both the cases, the Debye 'relaxation'
behaviour of the dynamic order parameter has been observed and the 'relaxation
time' is found to diverge near the dynamic transition point. The Debye
relaxation of the dynamic order parameter and the power law divergence of the
relaxation time have been obtained from a very approximate solution of the mean
field dynamic equation. The temperature variation of appropiately defined
'specific-heat' is studied by Monte Carlo simulation near the transition point.
The specific-heat has been observed to diverge near the dynamic transition
point.Comment: Revtex, Five encapsulated postscript files, submitted to Phys. Rev.
Pearling instability of nanoscale fluid flow confined to a chemical channel
We investigate the flow of a nano-scale incompressible ridge of
low-volatility liquid along a "chemical channel": a long, straight, and
completely wetting stripe embedded in a planar substrate, and sandwiched
between two extended less wetting solid regions. Molecular dynamics
simulations, a simple long-wavelength approximation, and a full stability
analysis based on the Stokes equations are used, and give qualitatively
consistent results. While thin liquid ridges are stable both statically and
during flow, a (linear) pearling instability develops if the thickness of the
ridge exceeds half of the width of the channel. In the flowing case periodic
bulges propagate along the channel and subsequently merge due to nonlinear
effects. However, the ridge does not break up even when the flow is unstable,
and the qualitative behavior is unchanged even when the fluid can spill over
onto a partially wetting exterior solid region.Comment: 17 pages, 12 figures, submitted to Physics of Fluids, fixed equation
numbering after Eq. (17
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