4,770 research outputs found
Average balance equations, scale dependence, and energy cascade for granular materials
A new averaging method linking discrete to continuum variables of granular
materials is developed and used to derive average balance equations. Its
novelty lies in the choice of the decomposition between mean values and
fluctuations of properties which takes into account the effect of gradients.
Thanks to a local homogeneity hypothesis, whose validity is discussed,
simplified balance equations are obtained. This original approach solves the
problem of dependence of some variables on the size of the averaging domain
obtained in previous approaches which can lead to huge relative errors (several
hundred percentages). It also clearly separates affine and nonaffine fields in
the balance equations. The resulting energy cascade picture is discussed, with
a particular focus on unidirectional steady and fully developed flows for which
it appears that the contact terms are dissipated locally unlike the kinetic
terms which contribute to a nonlocal balance. Application of the method is
demonstrated in the determination of the macroscopic properties such as volume
fraction, velocity, stress, and energy of a simple shear flow, where the
discrete results are generated by means of discrete particle simulation.Comment: Accepted forpublication in Physical Review
Experiments and DEM Simulations of Granular Ratcheting
In this work we studied the effect of cyclic loading on a granular packing by means of numerical simulations and experiments. A confined packing of glass beads was prepared and one of the walls was moved cyclically with a prescribed amplitude of the order of the particle diameter. Different amplitudes were tested, and their effect on the free surface evolution, the force transmitted to the moving wall and the displacement patterns in the material was characterized. Discrete numerical simulations were also carried out with the specific purpose of evaluating the effect of the particle shape on the dynamics of the system. The displacement amplitude of the moving wall was shown to increase the maximum force experienced at the end of the compressive phase of the wall movement; the angularity of the particles had a similar effect. Force-wall displacement curves displayed a peculiar hysteretic behavior. The evolution of the system towards an asymptotic state was shown to be faster for spheres than for angular particles; the latter displayed an interesting long-time evolution of the force-displacement paths which deserves deeper investigations
Generation Engineering of Heralded Narrowband Colour Entangled States
Efficient heralded generation of entanglement together with its manipulation
is of great importance for quantum communications. In addition, states
generated with bandwidths naturally compatible with atomic transitions allow a
more efficient mapping of light into matter which is an essential requirement
for long distance quantum communications. Here we propose a scheme where the
indistinguishability between two spontaneous four-wave mixing processes is
engineered to herald generation of single-photon frequency-bin entangled
states, i.e., single-photons shared by two distinct frequency modes. We show
that entanglement can be optimised together with the generation probability,
while maintaining absorption negligible. Besides, the scheme illustrated for
cold rubidium atoms is versatile and can be implemented in several other
physical systems
Condition for equivalence of q-deformed and anharmonic oscillators
The equivalence between the q-deformed harmonic oscillator and a specific anharmonic oscillator model, by which some new insight into the problem of the physical meaning of the parameter q can be attained, are discussed
Radiation 'damping' in atomic photonic crystals
The force exerted on a material by an incident beam of light is dependent
upon the material's velocity in the laboratory frame of reference. This
velocity dependence is known to be diffcult to measure, as it is proportional
to the incident optical power multiplied by the ratio of the material velocity
to the speed of light. Here we show that this typically tiny effect is greatly
amplified in multilayer systems composed of resonantly absorbing atoms (e.g.
optically trapped 87Rb), which may exhibit ultra-narrow photonic band gaps. The
amplification of the effect is shown to be three orders of magnitude greater
than previous estimates for conventional photonic-band-gap materials, and
significant for material velocities of a few ms/s.Comment: 5 pages, 3 figure
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