139,309 research outputs found
Analysis of Granular Packing Structure by Scattering of THz Radiation
Scattering methods are widespread used to characterize the structure and
constituents of matter on small length scales. This motivates this introductory
text on identifying prospective approaches to scattering-based methods for
granular media. A survey to light scattering by particles and particle
ensembles is given. It is elaborated why the established scattering methods
using X-rays and visible light cannot in general be transferred to granular
media. Spectroscopic measurements using Terahertz radiation are highlighted as
they to probe the scattering properties of granular media, which are sensitive
to the packing structure. Experimental details to optimize spectrometer for
measurements on granular media are discussed. We perform transmission
measurements on static and agitated granular media using Fourier-transform
spectroscopy at the THz beamline of the BessyII storage ring. The measurements
demonstrate the potential to evaluate degrees of order in the media and to
track transient structural states in agitated bulk granular media.Comment: 12 Pages, 9 Figures, 56 Reference
Entangled granular media
We study the geometrically induced cohesion of ensembles of granular
"u-particles" which mechanically entangle through particle interpenetration. We
vary the length-to-width ratio of the u-particles and form them into
free-standing vertical columns. In laboratory experiment we monitor the
response of the columns to sinusoidal vibration (frequency , peak
acceleration ). Column collapse occurs in a characteristic time,
, which follows the relation .
resembles an activation energy and is maximal at intermediate .
Simulation reveals that optimal strength results from competition between
packing and entanglement.Comment: 4 pages, 5 figure
Plug flow and the breakdown of Bagnold scaling in cohesive granular flows
Cohesive granular media flowing down an inclined plane are studied by
discrete element simulations. Previous work on cohesionless granular media
demonstrated that within the steady flow regime where gravitational energy is
balanced by dissipation arising from intergrain forces, the velocity profile in
the flow direction scales with depth in a manner consistent with the
predictions of Bagnold. Here we demonstrate that this Bagnold scaling does not
hold for the analogous steady-flows in cohesive granular media. We develop a
generalization of the Bagnold constitutive relation to account for our
observation and speculate as to the underlying physical mechanisms responsible
for the different constitutive laws for cohesive and noncohesive granular
media.Comment: 8 pages, 10 figure
Modeling of the interaction of rigid wheels with dry granular media
We analyze the capabilities of various recently developed techniques, namely
Resistive Force Theory (RFT) and continuum plasticity implemented with the
Material Point Method (MPM), in capturing dynamics of wheel--dry granular media
interactions. We compare results to more conventionally accepted methods of
modeling wheel locomotion. While RFT is an empirical force model for
arbitrarily-shaped bodies moving through granular media, MPM-based continuum
modeling allows the simulation of full granular flow and stress fields. RFT
allows for rapid evaluation of interaction forces on arbitrary shaped intruders
based on a local surface stress formulation depending on depth, orientation,
and movement of surface elements. We perform forced-slip experiments for three
different wheel types and three different granular materials, and results are
compared with RFT, continuum modeling, and a traditional terramechanics
semi-empirical method. Results show that for the range of inputs considered,
RFT can be reliably used to predict rigid wheel granular media interactions
with accuracy exceeding that of traditional terramechanics methodology in
several circumstances. Results also indicate that plasticity-based continuum
modeling provides an accurate tool for wheel-soil interaction while providing
more information to study the physical processes giving rise to resistive
stresses in granular media
Wet Granular Materials
Most studies on granular physics have focused on dry granular media, with no
liquids between the grains. However, in geology and many real world
applications (e.g., food processing, pharmaceuticals, ceramics, civil
engineering, constructions, and many industrial applications), liquid is
present between the grains. This produces inter-grain cohesion and drastically
modifies the mechanical properties of the granular media (e.g., the surface
angle can be larger than 90 degrees). Here we present a review of the
mechanical properties of wet granular media, with particular emphasis on the
effect of cohesion. We also list several open problems that might motivate
future studies in this exciting but mostly unexplored field.Comment: review article, accepted for publication in Advances in Physics;
tex-style change
Granular Pressure and the Thickness of a Layer Jamming on a Rough Incline
Dense granular media have a compaction between the random loose and random
close packings. For these dense media the concept of a granular pressure
depending on compaction is not unanimously accepted because they are often in a
"frozen" state which prevents them to explore all their possible microstates, a
necessary condition for defining a pressure and a compressibility
unambiguously. While periodic tapping or cyclic fluidization have already being
used for that exploration, we here suggest that a succession of flowing states
with velocities slowly decreasing down to zero can also be used for that
purpose. And we propose to deduce the pressure in \emph{dense and flowing}
granular media from experiments measuring the thickness of the granular layer
that remains on a rough incline just after the flow has stopped.Comment: 10 pages, 2 figure
From Elasticity to Hypoplasticity: Dynamics of Granular Solids
"Granular elasticity," useful for calculating static stress distributions in
granular media, is generalized by including the effects of slowly moving,
deformed grains. The result is a hydrodynamic theory for granular solids that
agrees well with models from soil mechanics
Swimming in Granular Media
We study a simple model of periodic contraction and extension of large
intruders in a granular bed to understand the mechanism for swimming in an
otherwise solid media. Using an event-driven simulation, we find optimal
conditions that idealized swimmers must use to critically fluidize a sand bed
so that it is rigid enough to support a load when needed, but fluid enough to
permit motion with minimal resistance. Swimmers - or other intruders - that
agitate the bed too rapidly produce large voids that prevent traction from
being achieved, while swimmers that move too slowly cannot travel before the
bed re-solidifies around them i.e., the swimmers locally probe the fundamental
time-scale in a granular packing
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