3,754 research outputs found
Embedding relatively hyperbolic groups in products of trees
We show that a relatively hyperbolic group quasi-isometrically embeds in a
product of finitely many trees if the peripheral subgroups do, and we provide
an estimate on the minimal number of trees needed. Applying our result to the
case of 3-manifolds, we show that fundamental groups of closed 3-manifolds have
linearly controlled asymptotic dimension at most 8. To complement this result,
we observe that fundamental groups of Haken 3-manifolds with non-empty boundary
have asymptotic dimension 2.Comment: v1: 18 pages; v2: 20 pages, minor change
Generalized Centrifugal Force Model for Pedestrian Dynamics
A spatially continuous force-based model for simulating pedestrian dynamics
is introduced which includes an elliptical volume exclusion of pedestrians. We
discuss the phenomena of oscillations and overlapping which occur for certain
choices of the forces. The main intention of this work is the quantitative
description of pedestrian movement in several geometries. Measurements of the
fundamental diagram in narrow and wide corridors are performed. The results of
the proposed model show good agreement with empirical data obtained in
controlled experiments.Comment: 10 pages, 14 figures, accepted for publication as a Regular Article
in Physical Review E. This version contains minor change
Entire solutions of hydrodynamical equations with exponential dissipation
We consider a modification of the three-dimensional Navier--Stokes equations
and other hydrodynamical evolution equations with space-periodic initial
conditions in which the usual Laplacian of the dissipation operator is replaced
by an operator whose Fourier symbol grows exponentially as \ue ^{|k|/\kd} at
high wavenumbers . Using estimates in suitable classes of analytic
functions, we show that the solutions with initially finite energy become
immediately entire in the space variables and that the Fourier coefficients
decay faster than \ue ^{-C(k/\kd) \ln (|k|/\kd)} for any . The
same result holds for the one-dimensional Burgers equation with exponential
dissipation but can be improved: heuristic arguments and very precise
simulations, analyzed by the method of asymptotic extrapolation of van der
Hoeven, indicate that the leading-order asymptotics is precisely of the above
form with . The same behavior with a universal constant
is conjectured for the Navier--Stokes equations with exponential
dissipation in any space dimension. This universality prevents the strong
growth of intermittency in the far dissipation range which is obtained for
ordinary Navier--Stokes turbulence. Possible applications to improved spectral
simulations are briefly discussed.Comment: 29 pages, 3 figures, Comm. Math. Phys., in pres
Multitransitional observations of the CS core of L673
A multitransitional study with the BIMA interferometric array was carried out
toward the starless core found in the L673 region, in order to study the
small-size structure of the cores detected with previous single--dish
observations, which provides us with a test of the predictions of the chemical
model of Taylor et al. (1996; 1998). We detected emission in the CS (2-1), N2H+
(1-0), and HCO+ (1-0) lines. Several clumps of size ~0.08 pc were found for
each line distributed all over the region where previous single-dish emission
was found (Morata et al. 1997). Each molecular transition traces differently
the clump distribution, although in some cases the detected clumps are
coincident. The distribution of the N2H+ emission and the single-dish NH3
emission are coincident and compatible with an origin in the same gas. The
large fraction of missing flux measured for the CS (2-1) transition can be
explained if the cloud is formed by a clumpy and heterogeneous medium. Four
positions were selected to derive the abundance ratios [N2H+/CS] and [HCO+/CS]
from the molecular column density determinations, and to compare them with the
values predicted by the chemical model. The model was able to explain the
interferometric observations, and, in particular, the chemical differentiation
of the detected clumps and the coincidence of the NH3 and N2H+ emissions. The
lack of HCO+ towards the two selected positions that trace the more evolved
clumps cannot be accounted for by the model, but it is possibly due to strong
self-absorption. We propose a classification of the studied clumps according to
the stage of chemical evolution indicated by the molecular abundances.Comment: 10 pages, 9 figures, accepted for publication in A&
A self-calibration approach for optical long baseline interferometry imaging
Current optical interferometers are affected by unknown turbulent phases on
each telescope. In the field of radio-interferometry, the self-calibration
technique is a powerful tool to process interferometric data with missing phase
information. This paper intends to revisit the application of self-calibration
to Optical Long Baseline Interferometry (OLBI). We cast rigorously the OLBI
data processing problem into the self-calibration framework and demonstrate the
efficiency of the method on real astronomical OLBI dataset
Efecto de utilización de mezclas bituminosas con polvo de neumático sobre las características funcionales de un pavimento
Estudios experimentales realizados en todo el mundo avalan la posibilidad de utilizar,
en circunstancias delimitadas, polvo de neumático procedente de la trituración de
neumáticos fuera de uso en las mezclas bituminosas.
El objetivo principal de la presente tesina es analizar el efecto que tiene la adición de
polvo de neumático en un microaglomerado sobre sus características funcionales,
especialmente textura, resistencia al deslizamiento y sonoridad, a partir de las
medidas realizadas sobre diferentes tramos experimentales a escala real
Flexibility-assisted heat removal in thin crystalline silicon solar cells
Thin crystalline silicon solar photovoltaics holds great potential for
reducing the module price by material saving and increasing the efficiency by
reduced bulk recombination loss. However, the module efficiency decreases
rather sensitively as the module temperature rises under sunlight. Effective,
inexpensive approach to cooling modules would accelerate large-scale market
adoption of thin crystalline silicon photovoltaics. For effective cooling, we
exploit high flexibility of single-crystalline thin silicon films to create
wavy solar cells. These wavy cells possess larger surface area than
conventional flat cells, while occupying the same projected area. We
experimentally demonstrate that the temperature of thin wavy crystalline
silicon solar cells under the sunlight can be significantly reduced by
increased convective cooling due to their large surface area. The substantial
efficiency gain, achieved by the effective heat removal, points to
high-performance thin crystalline silicon photovoltaic systems that are
radically different in configuration from conventional systems.Comment: 33 pages, 17 figure
Efecto de utilización de mezclas bituminosas con polvo de neumático sobre las características funcionales de un pavimento
Estudios experimentales realizados en todo el mundo avalan la posibilidad de utilizar,
en circunstancias delimitadas, polvo de neumático procedente de la trituración de
neumáticos fuera de uso en las mezclas bituminosas.
El objetivo principal de la presente tesina es analizar el efecto que tiene la adición de
polvo de neumático en un microaglomerado sobre sus características funcionales,
especialmente textura, resistencia al deslizamiento y sonoridad, a partir de las
medidas realizadas sobre diferentes tramos experimentales a escala real
Constraining Disk Parameters of Be Stars using Narrowband H-alpha Interferometry with the NPOI
Interferometric observations of two well-known Be stars, gamma Cas and phi
Per, were collected and analyzed to determine the spatial characteristics of
their circumstellar regions. The observations were obtained using the Navy
Prototype Optical Interferometer equipped with custom-made narrowband filters.
The filters isolate the H-alpha emission line from the nearby continuum
radiation, which results in an increased contrast between the interferometric
signature due to the H-alpha-emitting circumstellar region and the central
star. Because the narrowband filters do not significantly attenuate the
continuum radiation at wavelengths 50 nm or more away from the line, the
interferometric signal in the H-alpha channel is calibrated with respect to the
continuum channels. The observations used in this study represent the highest
spatial resolution measurements of the H-alpha-emitting regions of Be stars
obtained to date. These observations allow us to demonstrate for the first time
that the intensity distribution in the circumstellar region of a Be star cannot
be represented by uniform disk or ring-like structures, whereas a Gaussian
intensity distribution appears to be fully consistent with our observations.Comment: 23 pages, 14 figures, accepted for publication in A
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