43,356 research outputs found
A Cellular Automaton Model for the Traffic Flow in Bogota
In this work we propose a car cellular automaton model that reproduces the
experimental behavior of traffic flows in Bogot\'a. Our model includes three
elements: hysteresis between the acceleration and brake gaps, a delay time in
the acceleration, and an instantaneous brake. The parameters of our model were
obtained from direct measurements inside a car on motorways in Bogot\'a. Next,
we simulated with this model the flux-density fundamental diagram for a
single-lane traffic road and compared it with experimental data. Our
simulations are in very good agreement with the experimental measurements, not
just in the shape of the fundamental diagram, but also in the numerical values
for both the road capacity and the density of maximal flux. Our model
reproduces, too, the qualitative behavior of shock waves. In addition, our work
identifies the periodic boundary conditions as the source of false peaks in the
fundamental diagram, when short roads are simulated, that have been also found
in previous works. The phase transition between free and congested traffic is
also investigated by computing both the relaxation time and the order
parameter. Our work shows how different the traffic behavior from one city to
another can be, and how important is to determine the model parameters for each
city.Comment: 14 pages and 13 figures (gzipped tar file). Submitted to
Int.J.Mod.Phys.C. Minor changes, specially at references and typoes, plus a
clearer summary of the CA rule
Shear band formation in granular media as a variational problem
Strain in sheared dense granular material is often localized in a narrow
region called shear band. Recent experiments in a modified Couette cell
provided localized shear flow in the bulk away from the confining walls. The
non-trivial shape of the shear band was measured as the function of the cell
geometry. First we present a geometric argument for narrow shear bands which
connects the function of their surface position with the shape in the bulk.
Assuming a simple dissipation mechanism we show that the principle of minimum
dissipation of energy provides a good description of the shape function.
Furthermore, we discuss the possibility and behavior of shear bands which are
detached from the free surface and are entirely covered in the bulk.Comment: 4 pages, 5 figures; minor changes, typos and journal-ref adde
Electrically Conductive Paints for Satellites
A program was conducted to develop and test electrically conductive paint coatings for spacecraft. A wide variety of organic and inorganic coatings were formulated using conductive binders, conductive pigments, and similar approaches. Z-93, IITRI's standard specification inorganic thermal control coating, exhibits good electrical properties and is a very space-stable coating system. Several coatings based on a conductive pigment (antimony-doped tin oxide) in silicone and silicate binders offer considerable promise. Paint systems using commercially available conductive polymers also appear to be of interest, but will require substantial development. Evaluations were made based on electrical conductivity, paint physical properties, and the stability of spectral reflectance in space environment testing
Growth of Patterned Surfaces
During epitaxial crystal growth a pattern that has initially been imprinted
on a surface approximately reproduces itself after the deposition of an integer
number of monolayers. Computer simulations of the one-dimensional case show
that the quality of reproduction decays exponentially with a characteristic
time which is linear in the activation energy of surface diffusion. We argue
that this life time of a pattern is optimized, if the characteristic feature
size of the pattern is larger than , where is the surface
diffusion constant, the deposition rate and the surface dimension.Comment: 4 pages, 4 figures, uses psfig; to appear in Phys. Rev. Let
Damping of Oscillations in Layer-by-Layer Growth
We present a theory for the damping of layer-by-layer growth oscillations in
molecular beam epitaxy. The surface becomes rough on distances larger than a
layer coherence length which is substantially larger than the diffusion length.
The damping time can be calculated by a comparison of the competing roughening
and smoothening mechanisms. The dependence on the growth conditions,
temperature and deposition rate, is characterized by a power law. The
theoretical results are confirmed by computer simulations.Comment: 19 pages, RevTex, 5 Postscript figures, needs psfig.st
Identification of type A and B isolates of Epstein-Barr virus by polymerase chain reaction
A method is described for the identification of type A and type B isolates of Epstein-Barr virus (EBV) by means of the polymerase chain reaction. The use of three pairs of primers specific for genomic sequences coding for the two forms of EBV nuclear antigen (EBNA), 2A and 2B, and for a DNA sequence from the BamZ/BamR region allows the reliable and rapid detection of type A and B viruses in as little as 1000 EBV positive cells
Spectral Dependence of Polarized Radiation due to Spatial Correlations
We study the polarization of light emitted by spatially correlated sources.
We show that in general polarization acquires nontrivial spectral dependence
due to spatial correlations. The spectral dependence is found to be absent only
for a special class of sources where the correlation length scales as the
wavelength of light. We further study the cross correlations between two
spatially distinct points that are generated due to propagation. It is found
that such cross correlation leads to sufficiently strong spectral dependence of
polarization which can be measured experimentally.Comment: 5 pages, 4 figure
Mathematical modelling of promoter occupancies in MYC-dependent gene regulation
The human MYC proto-oncogene protein (MYC) is a transcription factor that plays a major role in the regulation of cell proliferation. Deregulation of MYC expression is often found in cancer. In the last years, several hypotheses have been proposed to explain cell type specific MYC target gene expression patterns despite genome wide DNA binding of MYC. In a recent publication, a mathematical modelling approach in combination with experimental data demonstrated that differences in MYC-DNA-binding affinity are sufficient to explain distinct promoter occupancies and allow stratification of distinct MYC-regulated biological processes at different MYC concentrations. Here, we extend the analysis of the published mathematical model of DNA-binding behaviour of MYC to demonstrate that the insights gained in the investigation of the human osteosarcoma cell line U2OS can be generalized to other human cell types
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