28,739 research outputs found
Athermal Phase Separation of Self-Propelled Particles with no Alignment
We study numerically and analytically a model of self-propelled polar disks
on a substrate in two dimensions. The particles interact via isotropic
repulsive forces and are subject to rotational noise, but there is no aligning
interaction. As a result, the system does not exhibit an ordered state. The
isotropic fluid phase separates well below close packing and exhibits the large
number fluctuations and clustering found ubiquitously in active systems. Our
work shows that this behavior is a generic property of systems that are driven
out of equilibrium locally, as for instance by self propulsion.Comment: 5 pages, 4 figure
Wind speed statistics for Goldstone, California, anemometer sites
An exploratory wind survey at an antenna complex was summarized statistically for application to future windmill designs. Data were collected at six locations from a total of 10 anemometers. Statistics include means, standard deviations, cubes, pattern factors, correlation coefficients, and exponents for power law profile of wind speed. Curves presented include: mean monthly wind speeds, moving averages, and diurnal variation patterns. It is concluded that three of the locations have sufficiently strong winds to justify consideration for windmill sites
Exchange Monte Carlo Method and Application to Spin Glass Simulations
We propose an efficient Monte Carlo algorithm for simulating a
``hardly-relaxing" system, in which many replicas with different temperatures
are simultaneously simulated and a virtual process exchanging configurations of
these replica is introduced. This exchange process is expected to let the
system at low temperatures escape from a local minimum. By using this algorithm
the three-dimensional Ising spin glass model is studied. The ergodicity
time in this method is found much smaller than that of the multi-canonical
method. In particular the time correlation function almost follows an
exponential decay whose relaxation time is comparable to the ergodicity time at
low temperatures. It suggests that the system relaxes very rapidly through the
exchange process even in the low temperature phase.Comment: 10 pages + uuencoded 5 Postscript figures, REVTe
Scheduling strategies for LTE uplink with flow behaviour analysis
Long Term Evolution (LTE) is a cellular technology developed to support\ud
diversity of data traffic at potentially high rates. It is foreseen to extend the capacity and improve the performance of current 3G cellular networks. A key\ud
mechanism in the LTE traffic handling is the packet scheduler, which is in charge of allocating resources to active flows in both the frequency and time dimension. In this paper we present a performance comparison of two distinct scheduling schemes for LTE uplink (fair fixed assignment and fair work-conserving) taking into account both packet level characteristics and flow level dynamics due to the random user behaviour. For that purpose, we apply a combined analytical/simulation approach which enables fast evaluation of performance measures such as mean flow transfer times manifesting the impact of resource allocation strategies. The results show that the resource allocation strategy has a crucial impact on performance and that some trends are observed only if flow level dynamics are considered
Dust Storms in Space?
Primarily from the Pioneer 8 and 9 results, it is concluded that the flux of picogram sized dust particles near the earth's orbit has been constant to within the observational limits over three years of observation. In particular, since dust streams are not observed, they cannot explain microphone detected events. However, the possibility of rare events due to dust blown directly off a cometary nucleus (such as that reported for Comet Bennett) cannot be completely ruled out
Run-and-tumble particles with hydrodynamics: sedimentation, trapping and upstream swimming
We simulate by lattice Boltzmann the nonequilibrium steady states of
run-and-tumble particles (inspired by a minimal model of bacteria), interacting
by far-field hydrodynamics, subject to confinement. Under gravity, hydrodynamic
interactions barely perturb the steady state found without them, but for
particles in a harmonic trap such a state is quite changed if the run length is
larger than the confinement length: a self-assembled pump is formed. Particles
likewise confined in a narrow channel show a generic upstream flux in
Poiseuille flow: chiral swimming is not required
The unreasonable effectiveness of equilibrium-like theory for interpreting non-equilibrium experiments
There has been great interest in applying the results of statistical
mechanics to single molecule experiements. Recent work has highlighted
so-called non-equilibrium work-energy relations and Fluctuation Theorems which
take on an equilibrium-like (time independent) form. Here I give a very simple
heuristic example where an equilibrium result (the barometric law for colloidal
particles) arises from theory describing the {\em thermodynamically}
non-equilibrium phenomenon of a single colloidal particle falling through
solution due to gravity. This simple result arises from the fact that the
particle, even while falling, is in {\em mechanical} equilibrium (gravitational
force equal the viscous drag force) at every instant. The results are
generalized by appeal to the central limit theorem. The resulting time
independent equations that hold for thermodynamically non-equilibrium (and even
non-stationary) processes offer great possibilities for rapid determination of
thermodynamic parameters from single molecule experiments.Comment: 6 page
A New Approach to Spin Glass Simulations
We present a recursive procedure to calculate the parameters of the recently
introduced multicanonical ensemble and explore the approach for spin glasses.
Temperature dependence of the energy, the entropy and other physical quantities
are easily calculable and we report results for the zero temperature limit. Our
data provide evidence that the large increase of the ergodicity time is
greatly improved. The multicanonical ensemble seems to open new horizons for
simulations of spin glasses and other systems which have to cope with
conflicting constraints
Effects of thermal fluctuation and the receptor-receptor interaction in bacterial chemotactic signalling and adaptation
Bacterial chemotaxis is controlled by the conformational changes of the
receptors, in response to the change of the ambient chemical concentration. In
a statistical mechanical approach, the signalling due to the conformational
changes is a thermodynamic average quantity, dependent on the temperature and
the total energy of the system, including both ligand-receptor interaction and
receptor-receptor interaction. This physical theory suggests to biology a new
understanding of cooperation in ligand binding and receptor signalling
problems. How much experimental support of this approach can be obtained from
the currently available data? What are the parameter values? What is the
practical information for experiments? Here we make comparisons between the
theory and recent experimental results. Although currently comparisons can only
be semi-quantitative or qualitative, consistency is clearly shown. The theory
also helps to sort a variety of data.Comment: 26 pages, revtex. Journal version. Analysis on another set of data on
adaptation time is adde
Critical light scattering in liquids
We compare theoretical results for the characteristic frequency of the
Rayleigh peak calculated in one-loop order within the field theoretical method
of the renormalization group theory with experiments and other theoretical
results. Our expressions describe the non-asymptotic crossover in temperature,
density and wave vector. In addition we discuss the frequency dependent shear
viscosity evaluated within the same model and compare our theoretical results
with recent experiments in microgravity.Comment: 17 pages, 12 figure
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