714 research outputs found
Does communication enhance pedestrians transport in the dark?
We study the motion of pedestrians through an obscure tunnel where the lack
of visibility hides the exits. Using a lattice model, we explore the effects of
communication on the effective transport properties of the crowd of
pedestrians. More precisely, we study the effect of two thresholds on the
structure of the effective nonlinear diffusion coefficient. One threshold
models pedestrians's communication efficiency in the dark, while the other one
describes the tunnel capacity. Essentially, we note that if the evacuees show a
maximum trust (leading to a fast communication), they tend to quickly find the
exit and hence the collective action tends to prevent the occurrence of
disasters
The SiC problem: astronomical and meteoritic evidence
Pre-solar grains of silicon carbide found in meteorites and interpreted as
having had an origin around carbon stars from their isotopic composition, have
all been found to be of the beta-SiC polytype. Yet to date fits to the 11.3
microns SiC emission band of carbon stars had been obtained only for alpha-SiC
grains. We present thin film infrared (IR) absorption spectra measured in a
diamond anvil cell for both the alpha- and beta- polymorphs of synthetic SiC
and compare the results with previously published spectra taken using the KBr
matrix method. We find that our thin film spectra have positions nearly
identical to those obtained previously from finely ground samples in KBr.
Hence, we show that this discrepancy has arisen from inappropriate `KBr
corrections' having been made to laboratory spectra of SiC particles dispersed
in KBr matrices. We re-fit a sample of carbon star mid-IR spectra, using
laboratory data with no KBr correction applied, and show that beta-SiC grains
fit the observations, while alpha-SiC grains do not. The discrepancy between
meteoritic and astronomical identifications of the SiC-type is therefore
removed. This work shows that the diamond anvil cell thin film method can be
used to produce mineral spectra applicable to cosmic environments without
further manipulation.Comment: to be published in Astrophysical Journal Letter 4 pages, 3 figure
IR emission from circumstellar envelopes of C-rich stars
The reliability of a theoretical model that solves the radiative transfer equation in dust clouds surrounding a central star is checked. In particular, it is found that both classical scattering by dust and the back-heating effects are negligible in the radiative transfer when envelopes similar to IRC+10216 are taken into consideration. In addition, new fits of IRC+10216 spectra are presented which were obtained, when the source is in different luminosity phases, under the assumption that amorphous carbon grains are in the circumstellar envelope. The same model is currently used to simulate the emission from carbon-rich sources showing the silicon carbide feature at 11.3 microns
Experimental evidence for amorphous carbon grains in comets
Amorphous carbon grains similar to those produced in the laboratory, but with a higher hydrogen content, appear to be good candidates to simulate both the IR continuum emission and the 3.4 micron band measured for P/Halley. The comparison of the cometary features with those detected in the laboratory for carbon grains characterized by various sp(exp 2)/sp(exp 3) ratios seems to indicate that a prevalent diamond-like (sp(exp 3)) structure should be present in cometary particles. These kinds of solid particles seem also suitable to explain the daily and monthly variations of the 3.4 micron band intensity, relative to the continuum, and, at the same time,- to fulfill the abundance constraints. The same grains appear to be able to reproduce the absorption bands detected in the IR galactic source IRS 7. This result may be considered as a first experimental evidence of a relation existing between interstellar dust and cometary materials
Model reduction of Brownian oscillators: quantification of errors and long-time behaviour
A procedure for model reduction of stochastic ordinary differential equations
with additive noise was recently introduced in [Colangeli-Duong-Muntean,
Journal of Physics A: Mathematical and Theoretical, 2022], based on the
Invariant Manifold method and on the Fluctuation-Dissipation relation. A
general question thus arises as to whether one can rigorously quantify the
error entailed by the use of the reduced dynamics in place of the original one.
In this work we provide explicit formulae and estimates of the error in terms
of the Wasserstein distance, both in the presence or in the absence of a sharp
time-scale separation between the variables to be retained or eliminated from
the description, as well as in the long-time behaviour.
Keywords: Model reduction, Wasserstein distance, error estimates, coupled
Brownian oscillators, invariant manifold, Fluctuation-Dissipation relation
Raman properties of various carbonaceous materials and their astrophysical implications
It is well known that a large number of celestial objects exhibit, in the range 3 to 12 micron, a family of emission features called unidentified infrared bands (UIR). They usually appear together and are associated with UV sources. Recently various authors have suggested that these features could be attributed to solid carbonaceous materials. Following this interest, a systematic analysis was performed of various types of amorphous carbon grains and polycyclic aromatic hydrocarbons (PAH), produced in lab. Updating results of Raman measurements performed on several carbonaceous materials, chosen according to their astrophysical interest, are presented. The measurements were made by means of a Jobin-Yvon monochromator HG2S and standard DC electronic. The line at 5145 A of an Ar+ laser was used as excitation source
Planning For Age-Friendly Cities: Towards a New Model
This dissertation examines the potential for professional/community planning to respond pro-actively and strategically to the impending demographic changes which will be brought about by the aging of the baby boom generation. This multi-phased investigation was designed to explore whether professional planning could uncover models and concepts which can be used to make cities and communities more age-friendly. Several conclusions can be drawn from the study. It was found that planners are not ready for demographic change nor are they prepared for helping create age-friendly cities. This is due to several reasons, including a lack of resources forcing them to concentrate on short-term, immediate issues; lack of power and credibility; and a perception that the elderly are a lower priority in society. For planners to become proactive and strategic in planning for age-friendly cities, they will need to re-examine their tendency to focus mainly on land use planning; focus on the long-term agenda; establish credibility with politicians; develop visionary skills; and become educators and facilitators, engaging key stakeholders and community groups.
The data indicated that planners have a limited knowledge of gerontology. However, a high level of congruence was found between the newer planning models and key research dimensions in gerontology. If these two fields were to work closer together, each would become better equipped to produce knowledge which will help society deal with aging demographics. Evidence also showed congruence between the newer planning models, building age-friendly cities and the environmental agenda. Common factors such as urban intensification (particularly in the core), building more compact urban form and increasing multi-modal transportation options (including pedestrianism) help reduce sprawl, congestion and pollution and concomitantly help create an environment which is healthier and friendly to all ages, including seniors. Findings from this research are used to develop a hybrid “Wise Growth” planning model to encourage the development of age-friendly cities
Transport and nonequilibrium phase transitions in polygonal urn models
We study the deterministic dynamics of N point particles moving at a constant speed in a 2D table made of two polygonal urns connected by an active rectangular channel, which applies a feedback control on the particles, inverting the horizontal component of their velocities when their number in the channel exceeds a fixed threshold. Such a bounce-back mechanism is non-dissipative: it preserves volumes in phase space. An additional passive channel closes the billiard table forming a circuit in which a stationary current may flow. Under specific constraints on the geometry and on the initial conditions, the large N limit allows nonequilibrium phase transitions between homogeneous and inhomogeneous phases. The role of ergodicity in making a probabilistic theory applicable is discussed for both rational and irrational urns. The theoretical predictions are compared with the numerical simulation results. Connections with the dynamics of feedback-controlled biological systems are highlighted
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