3,603 research outputs found
Extended phase diagram of the Lorenz model
The parameter dependence of the various attractive solutions of the three
variable nonlinear Lorenz model equations for thermal convection in
Rayleigh-B\'enard flow is studied. Its bifurcation structure has commonly been
investigated as a function of r, the normalized Rayleigh number, at fixed
Prandtl number \sigma. The present work extends the analysis to the entire
(r,\sigma) parameter plane. An onion like periodic pattern is found which is
due to the alternating stability of symmetric and non-symmetric periodic
orbits. This periodic pattern is explained by considering non-trivial limits of
large r and \sigma. In addition to the limit which was previously analyzed by
Sparrow, we identify two more distinct asymptotic regimes in which either
\sigma/r or \sigma^2/r is constant. In both limits the dynamics is
approximately described by Airy functions whence the periodicity in parameter
space can be calculated analytically. Furthermore, some observations about
sequences of bifurcations and coexistence of attractors, periodic as well as
chaotic, are reported.Comment: 36 pages, 20 figure
Aging and intermittency in a p-spin model of a glass
We numerically analyze the statistics of the heat flow between an aging
system and its thermal bath, following a method proposed and tested for a
spin-glass model in a recent Letter (P. Sibani and H.J. Jensen, Europhys.
Lett.69, 563 (2005)). The present system, which lacks quenched randomness,
consists of Ising spins located on a cubic lattice, with each plaquette
contributing to the total energy the product of the four spins located at its
corners. Similarly to our previous findings, energy leaves the system in rare
but large, so called intermittent, bursts which are embedded in reversible and
equilibrium-like fluctuations of zero average. The intermittent bursts, or
quakes, dissipate the excess energy trapped in the initial state at a rate
which falls off with the inverse of the age. This strongly heterogeneous
dynamical picture is explained using the idea that quakes are triggered by
energy fluctuations of record size, which occur independently within a number
of thermalized domains. From the temperature dependence of the width of the
reversible heat fluctuations we surmise that these domains have an exponential
density of states. Finally, we show that the heat flow consists of a
temperature independent term and a term with an Arrhenius temperature
dependence. Microscopic dynamical and structural information can thus be
extracted from numerical intermittency data. This type of analysis seems now
within the reach of time resolved micro-calorimetry techniques.Comment: 9 pages, 6 figures, europhysics letter style, to appear in Physical
Review
Convective intensification of magnetic fields in the quiet Sun
Kilogauss-strength magnetic fields are often observed in intergranular lanes at the photosphere in the quiet Sun. Such fields are stronger than the equipartition field B_e, corresponding to a magnetic energy density that matches the kinetic energy density of photospheric convection, and comparable with the field B_p that exerts a magnetic pressure equal to the ambient gas pressure. We present an idealised numerical model of three-dimensional compressible magnetoconvection at the photosphere, for a range of values of the magnetic Reynolds number. In the absence of a magnetic field, the convection is highly supercritical and is characterised by a pattern of vigorous, time-dependent, “granular” motions. When a weak magnetic field is imposed upon the convection, magnetic flux is swept into the convective downflows where it forms localised concentrations. Unless this process is significantly inhibited by magnetic diffusion, the resulting fields are often much greater than B_e, and the high magnetic pressure in these flux elements leads to their being partially evacuated. Some of these flux elements contain ultra-intense magnetic fields that are significantly greater than B_p. Such fields are contained by a combination of the thermal pressure of the gas and the dynamic pressure of the convective motion, and they are constantly evolving. These ultra-intense fields develop owing to nonlinear interactions between magnetic fields and convection; they cannot be explained in terms of “convective collapse” within a thin flux tube that remains in overall pressure equilibrium with its surroundings
Classification of phase transitions of finite Bose-Einstein condensates in power law traps by Fisher zeros
We present a detailed description of a classification scheme for phase
transitions in finite systems based on the distribution of Fisher zeros of the
canonical partition function in the complex temperature plane. We apply this
scheme to finite Bose-systems in power law traps within a semi-analytic
approach with a continuous one-particle density of states for different values of and to a three dimensional harmonically
confined ideal Bose-gas with discrete energy levels. Our results indicate that
the order of the Bose-Einstein condensation phase transition sensitively
depends on the confining potential.Comment: 7 pages, 9 eps-figures, For recent information on physics of small
systems see "http://www.smallsystems.de
Tunneling in a cavity
The mechanism of coherent destruction of tunneling found by Grossmann et al.
[Phys. Rev. Lett. 67, 516 (1991)] is studied from the viewpoint of quantum
optics by considering the photon statistics of a single mode cavity field which
is strongly coupled to a two-level tunneling system (TS). As a function of the
interaction time between TS and cavity the photon statistics displays the
tunneling dynamics. In the semi-classical limit of high photon occupation
number , coherent destruction of tunneling is exhibited in a slowing down of
an amplitude modulation for certain parameter ratios of the field. The
phenomenon is explained as arising from interference between displaced number
states in phase space which survives the large limit due to identical
scaling between orbit width and displacement.Comment: 4 pages Revtex, 2 PS-figures, appears in The Physical Review
Semi-autonomous Intersection Collision Avoidance through Job-shop Scheduling
In this paper, we design a supervisor to prevent vehicle collisions at
intersections. An intersection is modeled as an area containing multiple
conflict points where vehicle paths cross in the future. At every time step,
the supervisor determines whether there will be more than one vehicle in the
vicinity of a conflict point at the same time. If there is, then an impending
collision is detected, and the supervisor overrides the drivers to avoid
collision. A major challenge in the design of a supervisor as opposed to an
autonomous vehicle controller is to verify whether future collisions will occur
based on the current drivers choices. This verification problem is particularly
hard due to the large number of vehicles often involved in intersection
collision, to the multitude of conflict points, and to the vehicles dynamics.
In order to solve the verification problem, we translate the problem to a
job-shop scheduling problem that yields equivalent answers. The job-shop
scheduling problem can, in turn, be transformed into a mixed-integer linear
program when the vehicle dynamics are first-order dynamics, and can thus be
solved by using a commercial solver.Comment: Submitted to Hybrid Systems: Computation and Control (HSCC) 201
Coherent transport in a two-electron quantum dot molecule
We investigate the dynamics of two interacting electrons confined to a pair
of coupled quantum dots driven by an external AC field. By numerically
integrating the two-electron Schroedinger equation in time, we find that for
certain values of the strength and frequency of the AC field we can cause the
electrons to be localised within the same dot, in spite of the Coulomb
repulsion between them. Reducing the system to an effective two-site model of
Hubbard type and applying Floquet theory leads to a detailed understanding of
this effect. This demonstrates the possibility of using appropriate AC fields
to manipulate entangled states in mesoscopic devices on extremely short
timescales, which is an essential component of practical schemes for quantum
information processing.Comment: 4 pages, 3 figures; the section dealing with the perturbative
treatment of the Floquet states has been substantially expanded to make it
easier to follo
Efeito do processamento e das condições ambientais nas propriedades de materiais biodegradáveis de amido de aveia.
Influence of thickness on properties of plasticized oat starch films.
The aim of this study was to investigate the effect of thickness (between 80 and 120 µm) on apparent opacity, water vapor permeability and mechanical properties (tensile and puncture) of oat starch films plasticized with glycerol, sorbitol, glycerol:sorbitol mixture, urea and sucrose. Films were stored under 11, 57, 76 and 90% relative humidity (RH) to study the mechanical properties. It was observed that the higher the thickness, the higher was the opacity values. Films without the plasticizer were more opaque in comparison with the plasticized ones. Glycerol:sorbitol films presented increased elongation with increasing thickness at all RH. Puncture force showed a strong dependence on the film thickness, except for the films plasticized with sucrose. In general, thickness did not affect the water permeability
Developed turbulence: From full simulations to full mode reductions
Developed Navier-Stokes turbulence is simulated with varying wavevector mode
reductions. The flatness and the skewness of the velocity derivative depend on
the degree of mode reduction. They show a crossover towards the value of the
full numerical simulation when the viscous subrange starts to be resolved. The
intermittency corrections of the scaling exponents of the pth order velocity
structure functions seem to depend mainly on the proper resolution of the
inertial subrange. Universal scaling properties (i.e., independent of the
degree of mode reduction) are found for the relative scaling exponents rho
which were recently defined by Benzi et al.Comment: 4 pages, 5 eps-figures, replaces version from August 5th, 199
- …