13,661 research outputs found
Sedimentation of Oblate Ellipsoids at low and Moderate Reynolds numbers
In many applications to biophysics and environmental engineering,
sedimentation of non-spherical particles for example: ellipsoids, is an
important problem. In our work, we simulate the dynamics of oblate ellipsoids
under gravity. We study the settling velocity and the average orientation of
the ellipsoids as a function of volume fraction. We see that the settling
velocity shows a local maximum at the intermmediate densities unlike the
spheres. The average orientation of the ellipsoids also shows a similar local
maximum and we observe that this local maximum disappears as the Reynolds
number is increased. Also, at small volume fractions, we observe that the
oblate ellipsoids exhibit an orientational clustering effect in alignment with
gravity accompanied by strong density fluctuations. The vertical and horizontal
fluctuations of the oblate ellipsoids are small compared to that of the
spheres
A simple beam model for the shear failure of interfaces
We propose a novel model for the shear failure of a glued interface between
two solid blocks. We model the interface as an array of elastic beams which
experience stretching and bending under shear load and break if the two
deformation modes exceed randomly distributed breaking thresholds. The two
breaking modes can be independent or combined in the form of a von Mises type
breaking criterion. Assuming global load sharing following the beam breaking,
we obtain analytically the macroscopic constitutive behavior of the system and
describe the microscopic process of the progressive failure of the interface.
We work out an efficient simulation technique which allows for the study of
large systems. The limiting case of very localized interaction of surface
elements is explored by computer simulations.Comment: 11 pages, 13 figure
Calculation of the incremental stress-strain relation of a polygonal packing
The constitutive relation of the quasi-static deformation on two dimensional
packed samples of polygons is calculated using molecular dynamic simulations.
The stress values at which the system remains stable are bounded by a failure
surface, that shows a power law dependence on the pressure. Below the failure
surface, non linear elasticity and plastic deformation are obtained, which are
evaluated in the framework of the incremental linear theory. The results shows
that the stiffness tensor can be directly related to the micro-contact
rearrangements. The plasticity obeys a non-associated flow rule, with a plastic
limit surface that does not agree with the failure surface.Comment: 11 pages, 20 figur
Chromosome condensation in mitosis and meiosis of rye (Secale cereale L.)
Structural investigation and morphometry of meiotic chromosomes by scanning electron microscopy (in comparison to light microscopy) of all stages of condensation of meiosis I + II show remarkable differences during chromosome condensation in mitosis and meiosis I of rye (Secale cereale) with respect to initiation, mode and degree of condensation. Mitotic chromosomes condense in a linear fashion, shorten in length and increase moderately in diameter. In contrast, in meiosis I, condensation of chromosomes in length and diameter is a sigmoidal process with a retardation in zygotene and pachytene and an acceleration from diplotene to diakinesis. The basic structural components of mitotic chromosomes of rye are ``parallel fibers{''} and ``chromomeres{''} which become highly compacted in metaphase. Although chromosome architecture in early prophase of meiosis seems similar to mitosis in principle, there is no equivalent stage during transition to metaphase I when chromosomes condense to a much higher degree and show a characteristic ``smooth{''} surface. No indication was found for helical winding of chromosomes either in mitosis or in meiosis. Based on measurements, we propose a mechanism for chromosome dynamics in mitosis and meiosis, which involves three individual processes: (i) aggregation of chromatin subdomains into a chromosome filament, (ii) condensation in length, which involves a progressive increase in diameter and (iii) separation of chromatids. Copyright (C) 2003 S. Karger AG, Basel
Break-up of shells under explosion and impact
A theoretical and experimental study of the fragmentation of closed thin
shells made of a disordered brittle material is presented. Experiments were
performed on brown and white hen egg-shells under two different loading
conditions: fragmentation due to an impact with a hard wall and explosion by a
combustion mixture giving rise to power law fragment size distributions. For
the theoretical investigations a three-dimensional discrete element model of
shells is constructed. Molecular dynamics simulations of the two loading cases
resulted in power law fragment mass distributions in satisfactory agreement
with experiments. Based on large scale simulations we give evidence that power
law distributions arise due to an underlying phase transition which proved to
be abrupt and continuous for explosion and impact, respectively. Our results
demonstrate that the fragmentation of closed shells defines a universality
class different from that of two- and three-dimensional bulk systems.Comment: 11 pages, 14 figures in eps forma
New universality class for the fragmentation of plastic materials
We present an experimental and theoretical study of the fragmentation of
polymeric materials by impacting polypropylene particles of spherical shape
against a hard wall. Experiments reveal a power law mass distribution of
fragments with an exponent close to 1.2, which is significantly different from
the known exponents of three-dimensional bulk materials. A 3D discrete element
model is introduced which reproduces both the large permanent deformation of
the polymer during impact, and the novel value of the mass distribution
exponent. We demonstrate that the dominance of shear in the crack formation and
the plastic response of the material are the key features which give rise to
the emergence of the novel universality class of fragmentation phenomena.Comment: 4 pages, 4 figures, appearing in Phys. Rev. Let
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