9,266 research outputs found
Structure of plastically compacting granular packings
The developing structure in systems of compacting ductile grains were studied
experimentally in two and three dimensions. In both dimensions, the peaks of
the radial distribution function were reduced, broadened, and shifted compared
with those observed in hard disk- and sphere systems. The geometrical
three--grain configurations contributing to the second peak in the radial
distribution function showed few but interesting differences between the
initial and final stages of the two dimensional compaction. The evolution of
the average coordination number as function of packing fraction is compared
with other experimental and numerical results from the literature. We conclude
that compaction history is important for the evolution of the structure of
compacting granular systems.Comment: 12 pages, 12 figure
High-speed ultrasound imaging in dense suspensions reveals impact-activated solidification due to dynamic shear jamming
A remarkable property of dense suspensions is that they can transform from
liquid-like at rest to solid-like under sudden impact. Previous work showed
that this impact-induced solidification involves rapidly moving jamming fronts;
however, details of this process have remained unresolved. Here we use
high-speed ultrasound imaging to probe non-invasively how the interior of a
dense suspension responds to impact. Measuring the speed of sound we
demonstrate that the solidification proceeds without a detectable increase in
packing fraction, and imaging the evolving flow field we find that the shear
intensity is maximized right at the jamming front. Taken together, this
provides direct experimental evidence for jamming by shear, rather than
densification, as driving the transformation to solid-like behavior. Based on
these findings we propose a new model to explain the anisotropy in the
propagation speed of the fronts and delineate the onset conditions for dynamic
shear jamming in suspensions.Comment: 9 pages, 3 figure
Digital predictions of complex cylinder packed columns
A digital computational approach has been developed to simulate realistic structures of packed beds. The underlying principle of the method is digitisation of the particles and packing space, enabling the generation of realistic structures. Previous publications [Caulkin, R., Fairweather, M., Jia, X., Gopinathan, N., & Williams, R.A. (2006). An investigation of packed columns using a digital packing algorithm. Computers & Chemical Engineering, 30, 1178–1188; Caulkin, R., Ahmad, A., Fairweather, M., Jia, X., & Williams, R. A. (2007). An investigation of sphere packed shell-side columns using a digital packing algorithm. Computers & Chemical Engineering, 31, 1715–1724] have demonstrated the ability of the code in predicting the packing of spheres. For cylindrical particles, however, the original, random walk-based code proved less effective at predicting bed structure. In response to this, the algorithm has been modified to make use of collisions to guide particle movement in a way which does not sacrifice the advantage of simulation speed. Results of both the original and modified code are presented, with bulk and local voidage values compared with data derived by experimental methods. The results demonstrate that collisions and their impact on packing structure cannot be disregarded if realistic packing structures are to be obtained
Granular packings with moving side walls
The effects of movement of the side walls of a confined granular packing are
studied by discrete element, molecular dynamics simulations. The dynamical
evolution of the stress is studied as a function of wall movement both in the
direction of gravity as well as opposite to it. For all wall velocities
explored, the stress in the final state of the system after wall movement is
fundamentally different from the original state obtained by pouring particles
into the container and letting them settle under the influence of gravity. The
original packing possesses a hydrostatic-like region at the top of the
container which crosses over to a depth-independent stress. As the walls are
moved in the direction opposite to gravity, the saturation stress first reaches
a minimum value independent of the wall velocity, then increases to a
steady-state value dependent on the wall-velocity. After wall movement ceases
and the packing reaches equilibrium, the stress profile fits the classic
Janssen form for high wall velocities, while it has some deviations for low
wall velocities. The wall movement greatly increases the number of
particle-wall and particle-particle forces at the Coulomb criterion. Varying
the wall velocity has only small effects on the particle structure of the final
packing so long as the walls travel a similar distance.Comment: 11 pages, 10 figures, some figures in colo
Automated pebble mosaic stylization of images
Digital mosaics have usually used regular tiles, simulating the historical
"tessellated" mosaics. In this paper, we present a method for synthesizing
pebble mosaics, a historical mosaic style in which the tiles are rounded
pebbles. We address both the tiling problem, where pebbles are distributed over
the image plane so as to approximate the input image content, and the problem
of geometry, creating a smooth rounded shape for each pebble. We adapt SLIC,
simple linear iterative clustering, to obtain elongated tiles conforming to
image content, and smooth the resulting irregular shapes into shapes resembling
pebble cross-sections. Then, we create an interior and exterior contour for
each pebble and solve a Laplace equation over the region between them to obtain
height-field geometry. The resulting pebble set approximates the input image
while presenting full geometry that can be rendered and textured for a highly
detailed representation of a pebble mosaic
Geometric reasoning via internet crowdsourcing
The ability to interpret and reason about shapes is a peculiarly human capability that has proven difficult to reproduce algorithmically. So despite the fact that geometric modeling technology has made significant advances in the representation, display and modification of shapes, there have only been incremental advances in geometric reasoning. For example, although today's CAD systems can confidently identify isolated cylindrical holes, they struggle with more ambiguous tasks such as the identification of partial symmetries or similarities in arbitrary geometries. Even well defined problems such as 2D shape nesting or 3D packing generally resist elegant solution and rely instead on brute force explorations of a subset of the many possible solutions. Identifying economic ways to solving such problems would result in significant productivity gains across a wide range of industrial applications. The authors hypothesize that Internet Crowdsourcing might provide a pragmatic way of removing many geometric reasoning bottlenecks.This paper reports the results of experiments conducted with Amazon's mTurk site and designed to determine the feasibility of using Internet Crowdsourcing to carry out geometric reasoning tasks as well as establish some benchmark data for the quality, speed and costs of using this approach.After describing the general architecture and terminology of the mTurk Crowdsourcing system, the paper details the implementation and results of the following three investigations; 1) the identification of "Canonical" viewpoints for individual shapes, 2) the quantification of "similarity" relationships with-in collections of 3D models and 3) the efficient packing of 2D Strips into rectangular areas. The paper concludes with a discussion of the possibilities and limitations of the approach
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