56 research outputs found
Scalar transport in compressible flow
Transport of scalar fields in compressible flow is investigated. The
effective equations governing the transport at scales large compared to those
of the advecting flow are derived by using multi-scale techniques. Ballistic
transport generally takes place when both the solenoidal and the potential
components of the velocity do not vanish, despite of the fact that it has zero
average value. The calculation of the effective ballistic velocity is
reduced to the solution of one auxiliary equation. An analytic expression for
is derived in some special instances, i.e. flows depending on a single
coordinate, random with short correlation times and slightly compressible
cellular flow. The effective mean velocity vanishes for velocity fields
which are either incompressible or potential and time-independent. For generic
compressible flow, the most general conditions ensuring the absence of
ballistic transport are isotropy and/or parity invariance. When vanishes
(or in the frame of reference moving with velocity ), standard diffusive
transport takes place. It is known that diffusion is always enhanced by
incompressible flow. On the contrary, we show that diffusion is depleted in the
presence of time-independent potential flow. Trapping effects due to potential
wells are responsible for this depletion. For time-dependent potential flow or
generic compressible flow, transport rates are enhanced or depleted depending
on the detailed structure of the velocity field.Comment: 27 pages, submitted to Physica
Election turnout statistics in many countries: similarities, differences, and a diffusive field model for decision-making
We study in details the turnout rate statistics for 77 elections in 11
different countries. We show that the empirical results established in a
previous paper for French elections appear to hold much more generally. We find
in particular that the spatial correlation of turnout rates decay
logarithmically with distance in all cases. This result is quantitatively
reproduced by a decision model that assumes that each voter makes his mind as a
result of three influence terms: one totally idiosyncratic component, one
city-specific term with short-ranged fluctuations in space, and one long-ranged
correlated field which propagates diffusively in space. A detailed analysis
reveals several interesting features: for example, different countries have
different degrees of local heterogeneities and seem to be characterized by a
different propensity for individuals to conform to the cultural norm. We
furthermore find clear signs of herding (i.e. strongly correlated decisions at
the individual level) in some countries, but not in others.Comment: 15 pages, 9 figures, 7 table
Learning to coordinate in a complex and non-stationary world
We study analytically and by computer simulations a complex system of
adaptive agents with finite memory. Borrowing the framework of the Minority
Game and using the replica formalism we show the existence of an equilibrium
phase transition as a function of the ratio between the memory and
the learning rates of the agents. We show that, starting from a random
configuration, a dynamic phase transition also exists, which prevents the
system from reaching any Nash equilibria. Furthermore, in a non-stationary
environment, we show by numerical simulations that agents with infinite memory
play worst than others with less memory and that the dynamic transition
naturally arises independently from the initial conditions.Comment: 4 pages, 3 figure
Burgers velocity fields and dynamical transport processes
We explore a connection of the forced Burgers equation with the
Schr\"{o}dinger (diffusive) interpolating dynamics in the presence of
deterministic external forces. This entails an exploration of the consistency
conditions that allow to interpret dispersion of passive contaminants in the
Burgers flow as a Markovian diffusion process. In general, the usage of a
continuity equation , where
stands for the Burgers field and is the
density of transported matter, is at variance with the explicit diffusion
scenario. Under these circumstances, we give a complete characterisation of the
diffusive matter transport that is governed by Burgers velocity fields. The
result extends both to the approximate description of the transport driven by
an incompressible fluid and to motions in an infinitely compressible medium.Comment: Latex fil
Slow nonequilibrium dynamics: parallels between classical and quantum glasses and gently driven systems
We review an scenario for the non-equilibrium dynamics of glassy systems that
has been motivated by the exact solution of simple models. This approach allows
one to set on firmer grounds well-known phenomenological theories. The old
ideas of entropy crisis, fictive temperatures, free-volume... have clear
definitions within these models. Aging effects in the glass phase are also
captured. One of the salient features of the analytic solution, the breakdown
of the fluctuation-dissipation relations, provides a definition of a bonafide
{\it effective temperature} that is measurable by a thermometer, controls heat
flows, partial equilibrations, and the reaction to the external injection of
heat. The effective temperature is an extremely robust concept that appears in
non-equilibrium systems in the limit of small entropy production as, for
instance, sheared fluids, glasses at low temperatures when quantum fluctuations
are relevant, tapped or vibrated granular matter, etc. The emerging scenario is
one of partial equilibrations, in which glassy systems arrange their internal
degrees of freedom so that the slow ones select their own effective
temperatures. It has been proven to be consistent within any perturbative
resummation scheme (mode coupling, etc) and it can be challenged by
experimental and numerical tests, some of which it has already passed.Comment: 15 pages, 8 figure
Imported malaria among African immigrants: is there still a relationship between developed countries and their ex-colonies?
© 2009 Millet et al; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution Licens
Background matching in the brown shrimp Crangon crangon : adaptive camouflage and behavioural-plasticity
A combination of burrowing behaviour and very efficient background matching makes the brown shrimp Crangon crangon almost invisible to potential predators and preys. This raises questions on how shrimp succeed in concealing themselves in the heterogeneous and dynamic estuarine habitats they inhabit and what type of environmental variables and behavioural factors affect their colour change abilities. Using a series of behavioural experiments, we show that the brown shrimp is capable of repeated fast colour adaptations (20% change in dark pigment cover within one hour) and that its background matching ability is mainly influenced by illumination and sediment colour. Novel insights are provided on the occurrence of non-adaptive (possibly stress) responses to background changes after long-time exposure to a constant background colour or during unfavourable conditions for burying. Shrimp showed high levels of intra- and inter-individual variation, demonstrating a complex balance between behavioural-plasticity and environmental adaptation. As such, the study of crustacean colour changes represents a valuable opportunity to investigate colour adaptations in dynamic habitats and can help us to identify the mayor environmental and behavioural factors influencing the evolution of animal background matching
Distributed Dynamical Computation in Neural Circuits with Propagating Coherent Activity Patterns
Activity in neural circuits is spatiotemporally organized. Its spatial organization consists of multiple, localized coherent patterns, or patchy clusters. These patterns propagate across the circuits over time. This type of collective behavior has ubiquitously been observed, both in spontaneous activity and evoked responses; its function, however, has remained unclear. We construct a spatially extended, spiking neural circuit that generates emergent spatiotemporal activity patterns, thereby capturing some of the complexities of the patterns observed empirically. We elucidate what kind of fundamental function these patterns can serve by showing how they process information. As self-sustained objects, localized coherent patterns can signal information by propagating across the neural circuit. Computational operations occur when these emergent patterns interact, or collide with each other. The ongoing behaviors of these patterns naturally embody both distributed, parallel computation and cascaded logical operations. Such distributed computations enable the system to work in an inherently flexible and efficient way. Our work leads us to propose that propagating coherent activity patterns are the underlying primitives with which neural circuits carry out distributed dynamical computation
COVID-19 symptoms at hospital admission vary with age and sex: results from the ISARIC prospective multinational observational study
Background:
The ISARIC prospective multinational observational study is the largest cohort of hospitalized patients with COVID-19. We present relationships of age, sex, and nationality to presenting symptoms.
Methods:
International, prospective observational study of 60â109 hospitalized symptomatic patients with laboratory-confirmed COVID-19 recruited from 43 countries between 30 January and 3 August 2020. Logistic regression was performed to evaluate relationships of age and sex to published COVID-19 case definitions and the most commonly reported symptoms.
Results:
âTypicalâ symptoms of fever (69%), cough (68%) and shortness of breath (66%) were the most commonly reported. 92% of patients experienced at least one of these. Prevalence of typical symptoms was greatest in 30- to 60-year-olds (respectively 80, 79, 69%; at least one 95%). They were reported less frequently in children (â€â18 years: 69, 48, 23; 85%), older adults (â„â70 years: 61, 62, 65; 90%), and women (66, 66, 64; 90%; vs. men 71, 70, 67; 93%, each Pâ<â0.001). The most common atypical presentations under 60 years of age were nausea and vomiting and abdominal pain, and over 60 years was confusion. Regression models showed significant differences in symptoms with sex, age and country.
Interpretation:
This international collaboration has allowed us to report reliable symptom data from the largest cohort of patients admitted to hospital with COVID-19. Adults over 60 and children admitted to hospital with COVID-19 are less likely to present with typical symptoms. Nausea and vomiting are common atypical presentations under 30 years. Confusion is a frequent atypical presentation of COVID-19 in adults over 60 years. Women are less likely to experience typical symptoms than men
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