9,101 research outputs found
Fundamentals of Traffic Flow
From single vehicle data a number of new empirical results concerning the
density-dependence of the velocity distribution and its moments as well as the
characteristics of their temporal fluctuations have been determined. These are
utilized for the specification of some fundamental relations of traffic flow
and compared with existing traffic theories.Comment: For related work see
http://www.theo2.physik.uni-stuttgart.de/helbing.htm
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
Discrete stochastic models for traffic flow
We investigate a probabilistic cellular automaton model which has been
introduced recently. This model describes single-lane traffic flow on a ring
and generalizes the asymmetric exclusion process models. We study the
equilibrium properties and calculate the so-called fundamental diagrams (flow
vs.\ density) for parallel dynamics. This is done numerically by computer
simulations of the model and by means of an improved mean-field approximation
which takes into account short-range correlations. For cars with maximum
velocity 1 the simplest non-trivial approximation gives the exact result. For
higher velocities the analytical results, obtained by iterated application of
the approximation scheme, are in excellent agreement with the numerical
simulations.Comment: Revtex, 30 pages, full postscript version (including figures)
available by anonymous ftp from "fileserv1.mi.uni-koeln.de" in the directory
"pub/incoming/" paper accepted for publication in Phys.Rev.
SDSS J212531.92–010745.9 : the first definite PG 1159 close binary system
Aims. The archival spectrum of SDSS J212531.92−010745.9 shows not only the typical signature of a PG 1159 star, but also indicates the presence of a companion. Our aim was the proof of the binary nature of this object and the determination of its orbital period.
Methods. We performed time-series photometry of SDSS J212531.92−010745.9. We observed the object during 10 nights, spread over one month, with the Tübingen 80 cm and the Göttingen 50 cm telescopes. We fitted the observed light curve with a sine and simulated the light curve of this system with the nightfall program. Furthermore, we compared the spectrum of SDSS J212531.92−010745.9 with
NLTE models, the results of which also constrain the light curve solution.
Results. An orbital period of 6.95616(33) h with an amplitude of 0.354(3) mag is derived from our observations. A pulsation period could not be detected. For the PG 1159 star we found, as preliminary results from comparison with our NLTE models, T eff ∼ 90 000 K, log g ∼ 7.60, and the abundance ratio C/He ∼ 0.05 by number fraction. For the companion we obtained with a mean radius of 0.4 ± 0.1 R, a mass of 0.4 ± 0.1 M, and a temperature of 8200 K on the irradiated side, good agreement between the observed light curve and the nightfall simulation, but we do not regard those values as final
Far-infrared spectroscopy of spin excitations and Dzyaloshinskii-Moriya interactions in a Shastry-Sutherland compound SrCu(BO)$_2
We have studied spin excitation spectra in the Shastry-Sutherland model
compound SrCu(BO) in magnetic fields using far-infrared Fourier
spectroscopy. The transitions from the ground singlet state to the triplet
state at 24 cm and to several bound triplet states are induced by the
electric field component of the far-infrared light. To explain the light
absorption in the spin system we invoke a dynamic Dzyaloshinskii-Moriya (DM)
mechanism where light couples to a phonon mode, allowing the DM interaction.
Two optical phonons couple light to the singlet to triplet transition in
SrCu(BO). One is -polarized and creates an intra-dimer dynamic
DM along the c axis. The other is -polarized and creates an intra-dimer
dynamic DM interaction, it is in the plane and perpendicular to the
dimer axis. Singlet levels at 21.5 and 28.6 cm anti-cross with the first
triplet as is seen in far-infrared spectra. We used a cluster of two dimers
with a periodic boundary condition to perform a model calculation with scaled
intra- and inter-dimer exchange interactions. Two static DM interactions are
sufficient to describe the observed triplet state spectra. The static
inter-dimer DM in the c-direction cm splits the triplet state
sub-levels in zero field [C\'{e}pas et al., Phys. Rev. Lett. \textbf{87},
167205 (2001)]. The static intra-dimer DM in the plane (perpendicular to
the dimer axis) cm, allowed by the buckling of CuBO
planes, couples the triplet state to the 28.6 cm singlet as is seen from
the avoided crossing.Comment: 12 pages with 7 figures, some references correcte
Dynamic masses for the close PG1159 binary SDSSJ212531.92-010745.9
SDSSJ212531.92-010745.9 is the first known PG1159 star in a close binary with
a late main sequence companion allowing a dynamical mass determination. The
system shows flux variations with a peak-to-peak amplitude of about 0.7 mag and
a period of about 6.96h. In August 2007, 13 spectra of SDSSJ212531.92-010745.9
covering the full orbital phase range were taken at the TWIN 3.5m telescope at
the Calar Alto Observatory (Alm\'{e}ria, Spain). These confirm the typical
PG1159 features seen in the SDSS discovery spectrum, together with the Balmer
series of hydrogen in emission (plus other emission lines), interpreted as
signature of the companion's irradiated side. A radial velocity curve was
obtained for both components. Using co-added radial-velocity-corrected spectra,
the spectral analysis of the PG1159 star is being refined.
The system's lightcurve, obtained during three seasons of photometry with the
G\"ottingen 50cm and T\"ubingen 80cm telescopes, was fitted with both the
NIGHTFALL and PHOEBE binary simulation programs. An accurate mass determination
of the PG1159 component from the radial velocity measurements requires to first
derive the inclination, which requires light curve modelling and yields further
constraints on radii, effective temperature and separation of the system's
components. From the analysis of all data available so far, we present the
possible mass range for the PG1159 component of SDSSJ212531.92-010745.9.Comment: 8 pages, in "White dwarfs", proceedings of the 16th European White
Dwarf Workshop, eds. E. Garcia-Berro, M. Hernanz, J. Isern, S. Torres, to be
published in J. Phys.: Conf. Se
Breakdown and recovery in traffic flow models
Most car-following models show a transition from laminar to ``congested''
flow and vice versa. Deterministic models often have a density range where a
disturbance needs a sufficiently large critical amplitude to move the flow from
the laminar into the congested phase. In stochastic models, it may be assumed
that the size of this amplitude gets translated into a waiting time, i.e.\
until fluctuations sufficiently add up to trigger the transition. A recently
introduced model of traffic flow however does not show this behavior: in the
density regime where the jam solution co-exists with the high-flow state, the
intrinsic stochasticity of the model is not sufficient to cause a transition
into the jammed regime, at least not within relevant time scales. In addition,
models can be differentiated by the stability of the outflow interface. We
demonstrate that this additional criterion is not related to the stability of
the flow. The combination of these criteria makes it possible to characterize
commonalities and differences between many existing models for traffic in a new
way
Stochastic Description of a Bistable Frustrated Unit
Mixed positive and negative feedback loops are often found in biological
systems which support oscillations. In this work we consider a prototype of
such systems, which has been recently found at the core of many genetic
circuits showing oscillatory behaviour. Our model consists of two interacting
species A and B, where A activates not only its own production, but also that
of its repressor B. While the self-activation of A leads already to a bistable
unit, the coupling with a negative feedback loop via B makes the unit
frustrated. In the deterministic limit of infinitely many molecules, such a
bistable frustrated unit is known to show excitable and oscillatory dynamics,
depending on the maximum production rate of A which acts as a control
parameter. We study this model in its fully stochastic version and we find
oscillations even for parameters which in the deterministic limit are deeply in
the fixed-point regime. The deeper we go into this regime, the more irregular
these oscillations are, becoming finally random excitations whenever
fluctuations allow the system to overcome the barrier for a large excursion in
phase space. The fluctuations can no longer be fully treated as a perturbation.
The smaller the system size (the number of molecules), the more frequent are
these excitations. Therefore, stochasticity caused by demographic noise makes
this unit even more flexible with respect to its oscillatory behaviour.Comment: 28 pages, 17 figure
- …