469 research outputs found
Synchronized flow and wide moving jams from balanced vehicular traffic
Recently we proposed an extension to the traffic model of Aw, Rascle and
Greenberg. The extended traffic model can be written as a hyperbolic system of
balance laws and numerically reproduces the reverse shape of the
fundamental diagram of traffic flow. In the current work we analyze the steady
state solutions of the new model and their stability properties. In addition to
the equilibrium flow curve the trivial steady state solutions form two
additional branches in the flow-density diagram. We show that the
characteristic structure excludes parts of these branches resulting in the
reverse shape of the flow-density relation. The upper branch is
metastable against the formation of synchronized flow for intermediate
densities and unstable for high densities, whereas the lower branch is unstable
for intermediate densities and metastable for high densities. Moreover, the
model can reproduce the typical speed of the downstream front of wide moving
jams. It further reproduces a constant outflow from wide moving jams, which is
far below the maximum free flow. Applying the model to simulate traffic flow at
a bottleneck we observe a general pattern with wide moving jams traveling
through the bottleneck.Comment: 10 pages, 12 figure
A multi-scale study on the bubble dynamics of cryogenic cavitation
This study aims to construct a multi-scale cavitation model for unsteady cryogenic cavitation CFD. Many elementary physical processes of bubbles (i.e, nucleation, growth/shrink, evaporation/condensation, coalescence/fission, collapse, bubblebubble interaction, bubble-turbulence interaction, and so on) emerge in cryogenic cavitation where some of the processes have not been understood well. In this paper, we mainly focused the molecular processes in homogeneous liquid-vapor nucleation with noncondensable gas solution by using Molecular Dynamics (MD) method. Bubble nucleation in liquid oxygen including helium, nitrogen, or argon was simulated. Molecular interaction was given by Lennard-Jones potential, and basically, each potential parameter was defined so that a saturation curve obtained by MD data was consistent with an experimental value. In the case that helium was impurity, a bubble nucleus was formed by density fluctuation at a lower concentration while a cluster constituted with helium molecules formed a bubble nucleus at a higher concentration, and the nucleation point becomes closer to the saturation point of pure oxygen when helium molecules form clusters. On the other hand, in the case that nitrogen or argon was the impurity, the above-mentioned clustering was not observed clearly at a concentration where helium made clusters, and these impurities have weaker action to make clusters and cavitation bubble nuclei compared with helium.http://deepblue.lib.umich.edu/bitstream/2027.42/84285/1/CAV2009-final102.pd
An Alternative Model of Amino Acid Replacement
The observed correlations between pairs of homologous protein sequences are
typically explained in terms of a Markovian dynamic of amino acid substitution.
This model assumes that every location on the protein sequence has the same
background distribution of amino acids, an assumption that is incompatible with
the observed heterogeneity of protein amino acid profiles and with the success
of profile multiple sequence alignment. We propose an alternative model of
amino acid replacement during protein evolution based upon the assumption that
the variation of the amino acid background distribution from one residue to the
next is sufficient to explain the observed sequence correlations of homologs.
The resulting dynamical model of independent replacements drawn from
heterogeneous backgrounds is simple and consistent, and provides a unified
homology match score for sequence-sequence, sequence-profile and
profile-profile alignment.Comment: Minor improvements. Added figure and reference
Collisions of F+ with Ne
Measurements of inelastic collisions of F+ with Ne have been made. Transitions between 3P and 1D terms of F+ are seen, with the inelastically scattered ions sharply focused in the forward direction. Potential energy curves of (FNe)+ have been calculated. The 3Σ and 3Π curves correlating to F+(3P) are repulsive, while the 1Σ correlating to F+(1D) is attractive. Several curve crossings are identified, where transitions occur through spin-orbit coupling. Scattering angles and differential cross sections have been calculated, and they show the presence of a ‘‘glory’’ (or halo) effect, which accounts for the forward scattering of ions
Microscopic features of moving traffic jams
Empirical and numerical microscopic features of moving traffic jams are
presented. Based on a single vehicle data analysis, it is found that within
wide moving jams, i.e., between the upstream and downstream jam fronts there is
a complex microscopic spatiotemporal structure. This jam structure consists of
alternations of regions in which traffic flow is interrupted and flow states of
low speeds associated with "moving blanks" within the jam. Empirical features
of the moving blanks are found. Based on microscopic models in the context of
three-phase traffic theory, physical reasons for moving blanks emergence within
wide moving jams are disclosed. Structure of moving jam fronts is studied based
in microscopic traffic simulations. Non-linear effects associated with moving
jam propagation are numerically investigated and compared with empirical
results.Comment: 19 pages, 12 figure
Single-vehicle data of highway traffic - a statistical analysis
In the present paper single-vehicle data of highway traffic are analyzed in
great detail. By using the single-vehicle data directly empirical time-headway
distributions and speed-distance relations can be established. Both quantities
yield relevant information about the microscopic states. Several fundamental
diagrams are also presented, which are based on time-averaged quantities and
compared with earlier empirical investigations. In the remaining part
time-series analyses of the averaged as well as the single-vehicle data are
carried out. The results will be used in order to propose objective criteria
for an identification of the different traffic states, e.g. synchronized
traffic.Comment: 12 pages, 19 figures, RevTe
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A Kinetic Modeling study on the Oxidation of Primary Reference Fuel?Toluene Mixtures Including Cross Reactions between Aromatics and Aliphatics
A detailed chemical kinetic model for the mixtures of Primary Reference Fuel (PRF: n-heptane and iso-octane) and toluene has been proposed. This model is divided into three parts; a PRF mechanism [T. Ogura et al., Energy & Fuels 21 (2007) 3233-3239], toluene sub-mechanism and cross reactions between PRF and toluene. Toluene sub-mechanism includes the low temperature kinetics relevant to engine conditions. A chemical kinetic mechanism proposed by Pitz et al. [Proc. the 2nd Joint Meeting of the U.S. Combust. Institute (2001)] was used as a starting model and modified by updating rate coefficients. Theoretical estimations of rate coefficients were performed for toluene and benzyl radical reactions important at low temperatures. Cross-reactions between alkane, alkene, and aromatics were also included in order to account for the acceleration by the addition of toluene into iso-octane recently found in the shock tube study of the ignition delay [Y. Sakai et al, SAE 2007-01-4014 (2007)]. Validations of the model were performed with existing shock tube and flow tube data. The model well predicts the ignition characteristics of toluene and PRF/Toluene mixtures under the wide range of temperatures (500-1700 K) and pressures (2-50 atm). It is found that reactions of benzyl radical with oxygen molecule determine the reactivity of toluene at low temperature. Although the effect of toluene addition to iso-octane is not fully resolved, the reactions of alkene with benzyl radical have the possibility to account for the kinetic interactions between PRF and toluene
The Fundamental Diagram of Pedestrian Movement Revisited
The empirical relation between density and velocity of pedestrian movement is
not completely analyzed, particularly with regard to the `microscopic' causes
which determine the relation at medium and high densities. The simplest system
for the investigation of this dependency is the normal movement of pedestrians
along a line (single-file movement). This article presents experimental results
for this system under laboratory conditions and discusses the following
observations: The data show a linear relation between the velocity and the
inverse of the density, which can be regarded as the required length of one
pedestrian to move. Furthermore we compare the results for the single-file
movement with literature data for the movement in a plane. This comparison
shows an unexpected conformance between the fundamental diagrams, indicating
that lateral interference has negligible influence on the velocity-density
relation at the density domain . In addition we test a
procedure for automatic recording of pedestrian flow characteristics. We
present preliminary results on measurement range and accuracy of this method.Comment: 13 pages, 9 figure
Macroscopic traffic models from microscopic car-following models
We present a method to derive macroscopic fluid-dynamic models from
microscopic car-following models via a coarse-graining procedure. The method is
first demonstrated for the optimal velocity model. The derived macroscopic
model consists of a conservation equation and a momentum equation, and the
latter contains a relaxation term, an anticipation term, and a diffusion term.
Properties of the resulting macroscopic model are compared with those of the
optimal velocity model through numerical simulations, and reasonable agreement
is found although there are deviations in the quantitative level. The
derivation is also extended to general car-following models.Comment: 12 pages, 4 figures; to appear in Phys. Rev.
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