1,071 research outputs found
Noise Rectification and Fluctuations of an Asymmetric Inelastic Piston
We consider a massive inelastic piston, whose opposite faces have different
coefficients of restitution, moving under the action of an infinitely dilute
gas of hard disks maintained at a fixed temperature. The dynamics of the piston
is Markovian and obeys a continuous Master Equation: however, the asymmetry of
restitution coefficients induces a violation of detailed balance and a net
drift of the piston, as in a Brownian ratchet. Numerical investigations of such
non-equilibrium stationary state show that the velocity fluctuations of the
piston are symmetric around the mean value only in the limit of large piston
mass, while they are strongly asymmetric in the opposite limit. Only taking
into account such an asymmetry, i.e. including a third parameter in addition to
the mean and the variance of the velocity distribution, it is possible to
obtain a satisfactory analytical prediction for the ratchet drift velocity.Comment: 6 pages, 5 figures, to be published on Europhysics Letters; some
references have been adde
Irreversible effects of memory
The steady state of a Langevin equation with short ranged memory and coloured
noise is analyzed. When the fluctuation-dissipation theorem of second kind is
not satisfied, the dynamics is irreversible, i.e. detailed balance is violated.
We show that the entropy production rate for this system should include the
power injected by ``memory forces''. With this additional contribution, the
Fluctuation Relation is fairly verified in simulations. Both dynamics with
inertia and overdamped dynamics yield the same expression for this additional
power. The role of ``memory forces'' within the fluctuation-dissipation
relation of first kind is also discussed.Comment: 6 pages, 1 figure, publishe
Wave Energy: a Pacific Perspective
This is the author's peer-reviewed final manuscript, as accepted by the publisher. The published article is copyrighted by The Royal Society and can be found at: http://rsta.royalsocietypublishing.org/.This paper illustrates the status of wave energy development in Pacific Rim countries by characterizing the available resource and introducing the region‟s current and potential future leaders in wave energy converter development. It also describes the existing licensing and permitting process as well as potential environmental concerns. Capabilities of Pacific Ocean testing facilities are described in addition to the region‟s vision of the future of wave energy
Shaken Granular Lasers
Granular materials have been studied for decades, also driven by industrial
and technological applications. These very simple systems, composed by
agglomerations of mesoscopic particles, are characterized, in specific regimes,
by a large number of metastable states and an extreme sensitivity (e.g., in
sound transmission) on the arrangement of grains; they are not substantially
affected by thermal phenomena, but can be controlled by mechanical
solicitations. Laser emission from shaken granular matter is so far unexplored;
here we provide experimental evidence that it can be affected and controlled by
the status of motion of the granular, we also find that competitive random
lasers can be observed. We hence demonstrate the potentialities of gravity
affected moving disordered materials for optical applications, and open the
road to a variety of novel interdisciplinary investigations, involving modern
statistical mechanics and disordered photonics.Comment: 4 pages, 3 figures. To be published in Physical Review Letter
Velocity fluctuations in a one dimensional Inelastic Maxwell model
We consider the velocity fluctuations of a system of particles described by
the Inelastic Maxwell Model. The present work extends the methods, previously
employed to obtain the one-particle velocity distribution function, to the
study of the two particle correlations. Results regarding both the homogeneous
cooling process and the steady state driven regime are presented. In particular
we obtain the form of the pair correlation function in the scaling region of
the homogeneous cooling process and show that some of its moments diverge. This
fact has repercussions on the behavior of the energy fluctuations of the model.Comment: 16 pages, 1 figure, to be published on Journal of Statistical
Mechanics: Theory and Experiment
Violation of the Einstein relation in Granular Fluids: the role of correlations
We study the linear response in different models of driven granular gases. In
some situations, even if the the velocity statistics can be strongly
non-Gaussian, we do not observe appreciable violations of the Einstein formula
for diffusion versus mobility. The situation changes when strong correlations
between velocities and density are present: in this case, although a form of
fluctuation-dissipation relation holds, the differential velocity response of a
particle and its velocity self-correlation are no more proportional. This
happens at high densities and strong inelasticities, but still in the
fluid-like (and ergodic) regime.Comment: 18 pages, 6 figures, submitted for publicatio
A note on the violation of the Einstein relation in a driven moderately dense granular gas
The Einstein relation for a driven moderately dense granular gas in
-dimensions is analyzed in the context of the Enskog kinetic equation. The
Enskog equation neglects velocity correlations but retains spatial correlations
arising from volume exclusion effects. As expected, there is a breakdown of the
Einstein relation relating diffusion and
mobility , being the temperature of the impurity. The kinetic theory
results also show that the violation of the Einstein relation is only due to
the strong non-Maxwellian behavior of the reference state of the impurity
particles. The deviation of from unity becomes more significant as
the solid volume fraction and the inelasticity increase, especially when the
system is driven by the action of a Gaussian thermostat. This conclusion
qualitatively agrees with some recent simulations of dense gases [Puglisi {\em
et al.}, 2007 {\em J. Stat. Mech.} P08016], although the deviations observed in
computer simulations are more important than those obtained here from the
Enskog kinetic theory. Possible reasons for the quantitative discrepancies
between theory and simulations are discussed.Comment: 6 figure
Towards a continuum theory of clustering in a freely cooling inelastic gas
We performed molecular dynamics simulations to investigate the clustering
instability of a freely cooling dilute gas of inelastically colliding disks in
a quasi-one-dimensional setting. We observe that, as the gas cools, the shear
stress becomes negligibly small, and the gas flows by inertia only. Finite-time
singularities, intrinsic in such a flow, are arrested only when close-packed
clusters are formed. We observe that the late-time dynamics of this system are
describable by the Burgers equation with vanishing viscosity, and predict the
long-time coarsening behavior.Comment: 7 pages, 5 eps figures, to appear in Europhys. Let
Attempted density blowup in a freely cooling dilute granular gas: hydrodynamics versus molecular dynamics
It has been recently shown (Fouxon et al. 2007) that, in the framework of
ideal granular hydrodynamics (IGHD), an initially smooth hydrodynamic flow of a
granular gas can produce an infinite gas density in a finite time. Exact
solutions that exhibit this property have been derived. Close to the
singularity, the granular gas pressure is finite and almost constant. This work
reports molecular dynamics (MD) simulations of a freely cooling gas of nearly
elastically colliding hard disks, aimed at identifying the "attempted" density
blowup regime. The initial conditions of the simulated flow mimic those of one
particular solution of the IGHD equations that exhibits the density blowup. We
measure the hydrodynamic fields in the MD simulations and compare them with
predictions from the ideal theory. We find a remarkable quantitative agreement
between the two over an extended time interval, proving the existence of the
attempted blowup regime. As the attempted singularity is approached, the
hydrodynamic fields, as observed in the MD simulations, deviate from the
predictions of the ideal solution. To investigate the mechanism of breakdown of
the ideal theory near the singularity, we extend the hydrodynamic theory by
accounting separately for the gradient-dependent transport and for finite
density corrections.Comment: 11 pages, 9 figures, accepted for publication on Physical Review
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