9,900 research outputs found
A finite-strain hyperviscoplastic model and undrained triaxial tests of peat
This paper presents a finite-strain hyperviscoplastic constitutive model
within a thermodynamically consistent framework for peat which was categorised
as a material with both rate-dependent and thermodynamic equilibrium hysteresis
based on the data reported in the literature. The model was implemented
numerically using implicit time integration and verified against analytical
solutions under simplified conditions. Experimental studies on the undrained
relaxation and loading-unloading-reloading behaviour of an undisturbed fibrous
peat were carried out to define the thermodynamic equilibrium state during
deviatoric loading as a prerequisite for further modelling, to fit particularly
those model parameters related to solid matrix properties, and to validate the
proposed model under undrained conditions. This validation performed by
comparison to experimental results showed that the hyperviscoplastic model
could simulate undrained triaxial compression tests carried out at five
different strain rates with loading/unloading relaxation steps.Comment: 30 pages, 16 figures, 4 tables. This is a pre-peer reviewed version
of manuscript submitted to the International Journal of Numerical and
Analytical Methods in Geomechanic
Kepler super-flare stars: what are they?
The Kepler mission has led to the serendipitous discovery of a significant
number of `super flares' - white light flares with energies between 10^33 erg
and 10^36 erg - on solar-type stars. It has been speculated that these could be
`freak' events that might happen on the Sun, too. We have started a programme
to study the nature of the stars on which these super flares have been
observed. Here we present high-resolution spectroscopy of 11 of these stars and
discuss our results. We find that several of these stars are very young,
fast-rotating stars where high levels of stellar activity can be expected, but
for some other stars we do not find a straightforward explanation for the
occurrence of super flares.Comment: Accepted for publication in A&A, 9 pages, 4 figure
Non-concave fundamental diagrams and phase transitions in a stochastic traffic cellular automaton
Within the class of stochastic cellular automata models of traffic flows, we
look at the velocity dependent randomization variant (VDR-TCA) whose parameters
take on a specific set of extreme values. These initial conditions lead us to
the discovery of the emergence of four distinct phases. Studying the
transitions between these phases, allows us to establish a rigorous
classification based on their tempo-spatial behavioral characteristics. As a
result from the system's complex dynamics, its flow-density relation exhibits a
non-concave region in which forward propagating density waves are encountered.
All four phases furthermore share the common property that moving vehicles can
never increase their speed once the system has settled into an equilibrium
A Simplified Cellular Automaton Model for City Traffic
We systematically investigate the effect of blockage sites in a cellular
automaton model for traffic flow. Different scheduling schemes for the blockage
sites are considered. None of them returns a linear relationship between the
fraction of ``green'' time and the throughput. We use this information for a
fast implementation of traffic in Dallas.Comment: 12 pages, 18 figures. submitted to Phys Rev
Vibrational entropy and microstructural effects on the thermodynamics of partially disordered and ordered Ni3V
Samples of Ni3V were prepared with two microstructures: (1) with equilibrium D022 order, and (2) with partial disorder (having a large D022 chemical order parameter, but without the tetragonality of the unit cell). For both materials, we measured the difference in their heat capacities from 60 to 325 K, inelastic neutron-scattering spectra at four values of Q at 11 and at 300 K, and Young's moduli and coefficients of thermal expansion. The difference in heat capacity at low temperatures was consistent with a harmonic model using the phonon density of states (DOS) curves determined from the inelastic neutron-scattering spectra. In contrast, at temperatures greater than 160 K the difference in heat capacity did not approach zero, as expected of harmonic behavior. The temperature dependence of the phonon DOS can be used to approximately account for the anharmonic contributions to the differential heat capacity. We also argue that some of the anharmonic behavior should originate with a microstructural contribution to the heat capacity involving anisotropic thermal contractions of the D022 structure. We estimate the difference in vibrational entropy between partially disordered and ordered Ni3V to be Spdis -Sord =(+0.038±0.015)kB /atom at 300 K
Two-lane traffic rules for cellular automata: A systematic approach
Microscopic modeling of multi-lane traffic is usually done by applying
heuristic lane changing rules, and often with unsatisfying results. Recently, a
cellular automaton model for two-lane traffic was able to overcome some of
these problems and to produce a correct density inversion at densities somewhat
below the maximum flow density. In this paper, we summarize different
approaches to lane changing and their results, and propose a general scheme,
according to which realistic lane changing rules can be developed. We test this
scheme by applying it to several different lane changing rules, which, in spite
of their differences, generate similar and realistic results. We thus conclude
that, for producing realistic results, the logical structure of the lane
changing rules, as proposed here, is at least as important as the microscopic
details of the rules
Presence of Many Stable Nonhomogeneous States in an Inertial Car-Following Model
A new single lane car following model of traffic flow is presented. The model
is inertial and free of collisions. It demonstrates experimentally observed
features of traffic flow such as the existence of three regimes: free,
fluctuative (synchronized) and congested (jammed) flow; bistability of free and
fluctuative states in a certain range of densities, which causes the hysteresis
in transitions between these states; jumps in the density-flux plane in the
fluctuative regime and gradual spatial transition from synchronized to free
flow. Our model suggests that in the fluctuative regime there exist many stable
states with different wavelengths, and that the velocity fluctuations in the
congested flow regime decay approximately according to a power law in time.Comment: 4 pages, 4 figure
Solitons in the one-dimensional forest fire model
Fires in the one-dimensional Bak-Chen-Tang forest fire model propagate as
solitons, resembling shocks in Burgers turbulence. The branching of solitons,
creating new fires, is balanced by the pair-wise annihilation of oppositely
moving solitons. Two distinct, diverging length scales appear in the limit
where the growth rate of trees, , vanishes. The width of the solitons, ,
diverges as a power law, , while the average distance between solitons
diverges much faster as .Comment: 4 pages with 2 figures include
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