794 research outputs found
Velocity distribution of fluidized granular gases in presence of gravity
The velocity distribution of a fluidized dilute granular gas in the direction
perpendicular to the gravitational field is investigated by means of Molecular
Dynamics simulations. The results indicate that the velocity distribution can
be exactly described neither by a Gaussian nor by a stretched exponential law.
Moreover, it does not exhibit any kind of scaling. In fact, the actual shape of
the distribution depends on the number of monolayers at rest, on the
restitution coefficient and on the height at what it is measured. The role
played by the number of particle-particle collisions as compared with the
number of particle-wall collisions is discussed
Clustering transitions in vibro-fluidized magnetized granular materials
We study the effects of long range interactions on the phases observed in
cohesive granular materials. At high vibration amplitudes, a gas of magnetized
particles is observed with velocity distributions similar to non-magnetized
particles. Below a transition temperature compact clusters are observed to form
and coexist with single particles. The cluster growth rate is consistent with a
classical nucleation process. However, the temperature of the particles in the
clusters is significantly lower than the surrounding gas, indicating a
breakdown of equipartition. If the system is quenched to low temperatures, a
meta-stable network of connected chains self-assemble due to the anisotropic
nature of magnetic interactions between particles.Comment: 4 pages, 5 figure
Structure formation in binary colloids
A theoretical study of the structure formation observed very recently [Phys.
Rev. Lett. 90, 128303 (2003)] in binary colloids is presented. In our model
solely the dipole-dipole interaction of the particles is considered,
electrohidrodynamic effects are excluded. Based on molecular dynamics
simulations and analytic calculations we show that the total concentration of
the particles, the relative concentration and the relative dipole moment of the
components determine the structure of the colloid. At low concentrations the
kinetic aggregation of particles results in fractal structures which show a
crossover behavior when increasing the concentration. At high concentration
various lattice structures are obtained in a good agreement with experiments.Comment: revtex, 4 pages, figures available from authors due to size problem
Velocity Fluctuations in Electrostatically Driven Granular Media
We study experimentally the particle velocity fluctuations in an
electrostatically driven dilute granular gas. The experimentally obtained
velocity distribution functions have strong deviations from Maxwellian form in
a wide range of parameters. We have found that the tails of the distribution
functions are consistent with a stretched exponential law with typical
exponents of the order 3/2. Molecular dynamic simulations shows qualitative
agreement with experimental data. Our results suggest that this non-Gaussian
behavior is typical for most inelastic gases with both short and long range
interactions.Comment: 4 pages, 4 figure
Stress Transmission through Three-Dimensional Ordered Granular Arrays
We measure the local contact forces at both the top and bottom boundaries of
three-dimensional face-centered-cubic and hexagonal-close-packed granular
crystals in response to an external force applied to a small area at the top
surface. Depending on the crystal structure, we find markedly different results
which can be understood in terms of force balance considerations in the
specific geometry of the crystal. Small amounts of disorder are found to create
additional structure at both the top and bottom surfaces.Comment: 9 pages including 9 figures (many in color) submitted to PR
Effect of boundaries on the force distributions in granular media
The effect of boundaries on the force distributions in granular media is
illustrated by simulations of 2D packings of frictionless, Hertzian spheres. To
elucidate discrepancies between experimental observations and theoretical
predictions, we distinguish between the weight distribution {\cal P} (w)
measured in experiments and analyzed in the q-model, and the distribution of
interparticle forces P(f). The latter one is robust, while {\cal P}(w) can be
obtained once the local packing geometry and P(f) are known. By manipulating
the (boundary) geometry, we show that {\cal P}(w) can be varied drastically.Comment: 4 pages, 4 figure
Collision statistics of driven granular materials
We present an experimental investigation of the statistical properties of
spherical granular particles on an inclined plane that are excited by an
oscillating side-wall. The data is obtained by high-speed imaging and particle
tracking techniques. We identify all particles in the system and link their
positions to form trajectories over long times. Thus, we identify particle
collisions to measure the effective coefficient of restitution and find a broad
distribution of values for the same impact angles. We find that the energy
inelasticity can take on values greater than one, which implies that the
rotational degrees play an important role in energy transfer. We also measure
the distance and the time between collision events in order to directly
determine the distribution of path lengths and the free times. These
distributions are shown to deviate from expected theoretical forms for elastic
spheres, demonstrating the inherent clustering in this system. We describe the
data with a two-parameter fitting function and use it to calculated the mean
free path and collision time. We find that the ratio of these values is
consistent with the average velocity. The velocity distribution are observed to
be strongly non-Gaussian and do not demonstrate any apparent universal
behavior. We report the scaling of the second moment, which corresponds to the
granular temperature, and higher order moments as a function of distance from
the driving wall. Additionally, we measure long time correlation functions in
both space and in the velocities to probe diffusion in a dissipative gas.Comment: 12 pages, 4 figures, uses revtex
Space Vehicle Terrestrial Environment Design Requirements Guidelines
The terrestrial environment is an important driver of space vehicle structural, control, and thermal system design. NASA is currently in the process of producing an update to an earlier Terrestrial Environment Guidelines for Aerospace Vehicle Design and Development Handbook. This paper addresses the contents of this updated handbook, with special emphasis on new material being included in the areas of atmospheric thermodynamic models, wind dynamics, atmospheric composition, atmospheric electricity, cloud phenomena, atmospheric extremes, and sea state. In addition, the respective engineering design elements are discussed relative to terrestrial environment inputs that require consideration. Specific lessons learned that have contributed to the advancements made in the application and awareness of terrestrial environment inputs for aerospace engineering applications are presented
A "superstorm": When moral panic and new risk discourses converge in the media
This is an Author's Accepted Manuscript of an article published in Health, Risk and Society, 15(6), 681-698, 2013, copyright Taylor & Francis, available online at: http://www.tandfonline.com/10.1080/13698575.2013.851180.There has been a proliferation of risk discourses in recent decades but studies of these have been polarised, drawing either on moral panic or new risk frameworks to analyse journalistic discourses. This article opens the theoretical possibility that the two may co-exist and converge in the same scare. I do this by bringing together more recent developments in moral panic thesis, with new risk theory and the concept of media logic. I then apply this theoretical approach to an empirical analysis of how and with what consequences moral panic and new risk type discourses converged in the editorials of four newspaper campaigns against GM food policy in Britain in the late 1990s. The article analyses 112 editorials published between January 1998 and December 2000, supplemented with news stories where these were needed for contextual clarity. This analysis shows that not only did this novel food generate intense media and public reactions; these developed in the absence of the type of concrete details journalists usually look for in risk stories. Media logic is important in understanding how journalists were able to engage and hence how a major scare could be constructed around convergent moral panic and new risk type discourses. The result was a media ‘superstorm’ of sustained coverage in which both types of discourse converged in highly emotive mutually reinforcing ways that resonated in a highly sensitised context. The consequence was acute anxiety, social volatility and the potential for the disruption of policy and social change
Confined granular packings: structure, stress, and forces
The structure and stresses of static granular packs in cylindrical containers
are studied using large-scale discrete element molecular dynamics simulations
in three dimensions. We generate packings by both pouring and sedimentation and
examine how the final state depends on the method of construction. The vertical
stress becomes depth-independent for deep piles and we compare these stress
depth-profiles to the classical Janssen theory. The majority of the tangential
forces for particle-wall contacts are found to be close to the Coulomb failure
criterion, in agreement with the theory of Janssen, while particle-particle
contacts in the bulk are far from the Coulomb criterion. In addition, we show
that a linear hydrostatic-like region at the top of the packings unexplained by
the Janssen theory arises because most of the particle-wall tangential forces
in this region are far from the Coulomb yield criterion. The distributions of
particle-particle and particle-wall contact forces exhibit
exponential-like decay at large forces in agreement with previous studies.Comment: 11 pages, 11 figures, submitted to PRE (v2) added new references,
fixed typo
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