2,492 research outputs found
Excitable Delaunay triangulations
In an excitable Delaunay triangulation every node takes three states
(resting, excited and refractory) and updates its state in discrete time
depending on a ratio of excited neighbours. All nodes update their states in
parallel. By varying excitability of nodes we produce a range of phenomena,
including reflection of excitation wave from edge of triangulation, backfire of
excitation, branching clusters of excitation and localized excitation domains.
Our findings contribute to studies of propagating perturbations and waves in
non-crystalline substrates
Theoretical Study of the Conditions and the Mechanism of Shear Crack Acceleration towards the Longitudinal Wave Velocity
AbstractThe question about physically admissible velocity of dynamic crack growth is of significance to safety engineering as well as to earthquake dynamics. Recent researches including numerical simulations, experimental observations and the analysis of strong earthquakes have shown a possibility of propagation of shear cracks in supershear regime, namely at velocities comparable with dilatational wave speed. The present paper is devoted to the theoretical (numerical) study of some fundamental aspects of this problem. It is shown that development of a sub Raleigh shear crack is connected with a vortex traveling ahead of the crack tip at a shear wave velocity. The stress concentration area ahead of the crack tip revealed by different authors (Burridge, Andrews, Geubelle, Rosakis and others) is connected with this vortex. Acceleration of a shear crack towards the longitudinal wave velocity is concerned with formation of a daughter crack by the mechanism of shearing (the daughter crack is formed in the center of vortex). Analysis of sub Raleigh to intersonic transition has shown that development of shear cracks is self-similar and depends on dimensionless parameters
A study of fragmentation processes using a discrete element method
We present a model of solids made from polygonal cells connected via beams.
We calculate the macroscopic elastic moduli from the beam and cell parameters.
This modellisation is particularly suited for the simulation of fragmentation
processes. We study the effects of an explosion inside a circular disk and the
impact of a projectile and obtain the fragment size distribution. We find that
if breaking only happens under tensile forces a layer on the free wall opposed
to impact is first ejected. In that case the distribution follows a power-law
with an exponent that in most cases is around two.Comment: 16 pages in LaTex format, 17 PostScript figures. Figures are
available upon request from the authors. Submitted to Int. J. of Mod. Phys.
Cellular Automaton for Realistic Modelling of Landslides
A numerical model is developed for the simulation of debris flow in
landslides over a complex three dimensional topography. The model is based on a
lattice, in which debris can be transferred among nearest neighbors according
to established empirical relationships for granular flows. The model is then
validated by comparing a simulation with reported field data. Our model is in
fact a realistic elaboration of simpler ``sandpile automata'', which have in
recent years been studied as supposedly paradigmatic of ``self-organized
criticality''.
Statistics and scaling properties of the simulation are examined, and show
that the model has an intermittent behavior.Comment: Revised version (gramatical and writing style cleanup mainly).
Accepted for publication by Nonlinear Processes in Geophysics. 16 pages, 98Kb
uuencoded compressed dvi file (that's the way life is easiest). Big (6Mb)
postscript figures available upon request from [email protected] /
[email protected]
Numerical analysis of the geometrical and material criteria of acceleration of shear crack to supershear velocity in brittle nanoporous solids
The paper is devoted to the study of dynamic propagation of mode II cracks in porous brittle materials with nanoscale pore size. We compared static (shear strength) and dynamic parameters of crack growth in dry and fluid saturated nanoporous brittle materials at different degrees of confinement. We have shown that pore fluid in nanoporous brittle materials influences mainly the dynamics of crack propagation. This leads in particular to pronounced peculiarities of the dependence of the critical value of dimensionless geometrical parameter of the initial crack (it majorizes the interval of the ratios of length to thickness for the cracks that are capable to accelerate to intersonic velocity) on applied crack normal stress. The results of the study are relevant for understanding the conditions of supershear regime of propagation of mode II cracks as well as for assessment of the ability of mode II cracks in brittle materials (including nanoporous fluid-saturated solids) to develop in supershear regime
Role of vortex-like motion in fracture of coating-substrate system under contact loading
Deformation of a heterogeneous material containing internal interfaces or/and free surfaces is accompanied by collective vortex motion near these boundaries. One should expect that rotational motion in nanomaterials takes place at different scales, from the atomic scale to the macroscopic one. Nevertheless such a fundamental factor as elastic vortex motion in material formed during dynamic loading still remains out of discussion. The aim of this paper is revealing the role of vortex displacements in contact interaction of the strengthening coating with a hard counter-body by means of 3D modeling using movable cellular automata (MCA). MCA method is an efficient numerical method in particle mechanics, which assumes that the material is composed of a certain amount of elementary objects interacting among each other according to many-particle forces. In this paper MCA method is applied to 3D modeling deformation of the coating-substrate system under its contact loading by the rigid indenter. Main attention of the research is focused on the role of vortex structures in the velocity fields in elastic and non-elastic deformation of the strengthening coating and substrate. The mechanical properties of the model coating correspond to multifunctional nanostructured film and the properties of the substrate, to nanostructured titanium. The loading is performed by a hard conical indenter with various ratios of normal and tangential components. The peculiarities of the velocity vortex formation and propagation, as well as interaction with the structural elements are studied
Validation and Calibration of Models for Reaction-Diffusion Systems
Space and time scales are not independent in diffusion. In fact, numerical
simulations show that different patterns are obtained when space and time steps
( and ) are varied independently. On the other hand,
anisotropy effects due to the symmetries of the discretization lattice prevent
the quantitative calibration of models. We introduce a new class of explicit
difference methods for numerical integration of diffusion and
reaction-diffusion equations, where the dependence on space and time scales
occurs naturally. Numerical solutions approach the exact solution of the
continuous diffusion equation for finite and , if the
parameter assumes a fixed constant value,
where is an odd positive integer parametrizing the alghorithm. The error
between the solutions of the discrete and the continuous equations goes to zero
as and the values of are dimension
independent. With these new integration methods, anisotropy effects resulting
from the finite differences are minimized, defining a standard for validation
and calibration of numerical solutions of diffusion and reaction-diffusion
equations. Comparison between numerical and analytical solutions of
reaction-diffusion equations give global discretization errors of the order of
in the sup norm. Circular patterns of travelling waves have a maximum
relative random deviation from the spherical symmetry of the order of 0.2%, and
the standard deviation of the fluctuations around the mean circular wave front
is of the order of .Comment: 33 pages, 8 figures, to appear in Int. J. Bifurcation and Chao
Simulating Three-Dimensional Hydrodynamics on a Cellular-Automata Machine
We demonstrate how three-dimensional fluid flow simulations can be carried
out on the Cellular Automata Machine 8 (CAM-8), a special-purpose computer for
cellular-automata computations. The principal algorithmic innovation is the use
of a lattice-gas model with a 16-bit collision operator that is specially
adapted to the machine architecture. It is shown how the collision rules can be
optimized to obtain a low viscosity of the fluid. Predictions of the viscosity
based on a Boltzmann approximation agree well with measurements of the
viscosity made on CAM-8. Several test simulations of flows in simple geometries
-- channels, pipes, and a cubic array of spheres -- are carried out.
Measurements of average flux in these geometries compare well with theoretical
predictions.Comment: 19 pages, REVTeX and epsf macros require
- …