796 research outputs found
Perspective: network-guided pattern formation of neural dynamics
The understanding of neural activity patterns is fundamentally linked to an
understanding of how the brain's network architecture shapes dynamical
processes. Established approaches rely mostly on deviations of a given network
from certain classes of random graphs. Hypotheses about the supposed role of
prominent topological features (for instance, the roles of modularity, network
motifs, or hierarchical network organization) are derived from these
deviations. An alternative strategy could be to study deviations of network
architectures from regular graphs (rings, lattices) and consider the
implications of such deviations for self-organized dynamic patterns on the
network. Following this strategy, we draw on the theory of spatiotemporal
pattern formation and propose a novel perspective for analyzing dynamics on
networks, by evaluating how the self-organized dynamics are confined by network
architecture to a small set of permissible collective states. In particular, we
discuss the role of prominent topological features of brain connectivity, such
as hubs, modules and hierarchy, in shaping activity patterns. We illustrate the
notion of network-guided pattern formation with numerical simulations and
outline how it can facilitate the understanding of neural dynamics
Spatialising Photographic Archives
Recent computer research allows previously unforeseen analysis of photographic archives. For multiple shots of a scene, new digital techniques permit (1) the recovery of the location and angle, in 3D, of the camera that took each shot and (2) the reconstruction, in 3D, of much of the geometry of the original location. While the underlying algorithms of this approach are open source, the only available front-end to them is closed and proprietary. In this project, we will develop an open-source platform for experimentation with archives not previously thought of as 3D data-sets. As a constructive proof, we will analyze and re-interpret a key historical event as documented by a photojournalist, tying the spatial and visual components of the resulting data-set to contemporaneous text sources (news articles, etc) to form a uniquely hybrid form of scholarship. We will express these results in forms available to the computer science, visual arts, and photo-journalistic communities
A new method for the determination of the growth rate from galaxy redshift surveys
Given a redshift survey of galaxies with measurements of apparent magnitudes,
we present a novel method for measuring the growth rate of
cosmological linear perturbations. We use the galaxy distribution within the
survey to solve for the peculiar velocity field which depends in linear
perturbation theory on , where is the bias factor of the
galaxy distribution. The recovered line-of-sight peculiar velocities are
subtracted from the redshifts to derive the distances, which thus allows an
estimate of the absolute magnitude of each galaxy. A constraint on is
then found by minimizing the spread of the estimated magnitudes from their
distribution function. We apply the method to the all sky Two-MASS
Redhsift Survey (2MRS) and derive at , remarkably
consistent with our previous estimate from the velocity-velocity comparison.
The method could easily be applied to subvolumes extracted from the SDSS survey
to derive the growth rate at . Further, it should also be
applicable to ongoing and future spectroscopic redshift surveys to trace the
evolution of to . Constraints obtained from this method are
entirely independent from those obtained from the two-dimensional distortion of
and provide an important check on , as alternative gravity
models predict observable differences.Comment: 9pages, 1figure Typos corrected. A slight change in the Discussion
and Acknowledgemen
An adaptive wavelet-based collocation method for solving multiscale problems in continuum mechanics
Computational multiscale methods are highly sophisticated numerical approaches to predict the constitutive response of heterogeneous materials from their underlying microstructures. However, the quality of the prediction intrinsically relies on an accurate representation of the microscale morphology and its individual constituents, which makes these formulations computationally demanding. Against this background, the applicability of an adaptive wavelet-based collocation approach is studied in this contribution. It is shown that the Hill–Mandel energy equivalence condition can naturally be accounted for in the wavelet basis, (discrete) wavelet-based scale-bridging relations are derived, and a wavelet-based mapping algorithm for internal variables is proposed. The characteristic properties of the formulation are then discussed by an in-depth analysis of elementary one-dimensional problems in multiscale mechanics. In particular, the microscale fields and their macroscopic analogues are studied for microstructures that feature material interfaces and material interphases. Analytical solutions are provided to assess the accuracy of the simulation results.</p
An adaptive wavelet-based collocation method for solving multiscale problems in continuum mechanics
Computational multiscale methods are highly sophisticated numerical approaches to predict the constitutive response of heterogeneous materials from their underlying microstructures. However, the quality of the prediction intrinsically relies on an accurate representation of the microscale morphology and its individual constituents, which makes these formulations computationally demanding. Against this background, the applicability of an adaptive wavelet-based collocation approach is studied in this contribution. It is shown that the Hill–Mandel energy equivalence condition can naturally be accounted for in the wavelet basis, (discrete) wavelet-based scale-bridging relations are derived, and a wavelet-based mapping algorithm for internal variables is proposed. The characteristic properties of the formulation are then discussed by an in-depth analysis of elementary one-dimensional problems in multiscale mechanics. In particular, the microscale fields and their macroscopic analogues are studied for microstructures that feature material interfaces and material interphases. Analytical solutions are provided to assess the accuracy of the simulation results.</p
Dark-Matter Decays and Self-Gravitating Halos
We consider models in which a dark-matter particle decays to a slightly less
massive daughter particle and a noninteracting massless particle. The decay
gives the daughter particle a small velocity kick. Self-gravitating dark-matter
halos that have a virial velocity smaller than this velocity kick may be
disrupted by these particle decays, while those with larger virial velocities
will be heated. We use numerical simulations to follow the detailed evolution
of the total mass and density profile of self-gravitating systems composed of
particles that undergo such velocity kicks as a function of the kick speed
(relative to the virial velocity) and the decay time (relative to the dynamical
time). We show how these decays will affect the halo mass-concentration
relation and mass function. Using measurements of the halo mass-concentration
relation and galaxy-cluster mass function to constrain the
lifetime--kick-velocity parameter space for decaying dark matter, we find
roughly that the observations rule out the combination of kick velocities
greater than 100 km/s and decay times less than a few times the age of the
Universe.Comment: 17 pages, 10 figures, replaced with published versio
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Cluster-based reduced-order modelling of a mixing layer
We propose a novel cluster-based reduced-order modelling (CROM) strategy of
unsteady flows. CROM combines the cluster analysis pioneered in Gunzburger's
group (Burkardt et al. 2006) and and transition matrix models introduced in
fluid dynamics in Eckhardt's group (Schneider et al. 2007). CROM constitutes a
potential alternative to POD models and generalises the Ulam-Galerkin method
classically used in dynamical systems to determine a finite-rank approximation
of the Perron-Frobenius operator. The proposed strategy processes a
time-resolved sequence of flow snapshots in two steps. First, the snapshot data
are clustered into a small number of representative states, called centroids,
in the state space. These centroids partition the state space in complementary
non-overlapping regions (centroidal Voronoi cells). Departing from the standard
algorithm, the probabilities of the clusters are determined, and the states are
sorted by analysis of the transition matrix. Secondly, the transitions between
the states are dynamically modelled using a Markov process. Physical mechanisms
are then distilled by a refined analysis of the Markov process, e.g. using
finite-time Lyapunov exponent and entropic methods. This CROM framework is
applied to the Lorenz attractor (as illustrative example), to velocity fields
of the spatially evolving incompressible mixing layer and the three-dimensional
turbulent wake of a bluff body. For these examples, CROM is shown to identify
non-trivial quasi-attractors and transition processes in an unsupervised
manner. CROM has numerous potential applications for the systematic
identification of physical mechanisms of complex dynamics, for comparison of
flow evolution models, for the identification of precursors to desirable and
undesirable events, and for flow control applications exploiting nonlinear
actuation dynamics.Comment: 48 pages, 30 figures. Revised version with additional material.
Accepted for publication in Journal of Fluid Mechanic
Issues in higher education policy : an update on higher education policy issues in 2004 in 11 Western countries
Higher education is a dynamic field. It is, however, also a field where changes don¿t take place overnight. This 2004 update report covers a period of 1.5 years, a period in which some earlier policy initiatives have been implemented and new ones have emerged. It is therefore not surprising to observe that many of the policy issues on the agenda in the previous Update Report (April, 2003) still are a topic of debate today.\ud
The main part of the report are the descriptions of the current (2004) higher education debates and policy initiatives for each of the eleven IHEM countries, arranged in four themes educational and research infrastructure, finance, governance and quality. In conclusion, four `overarching¿ policy issues in contemporary European (and Australian) higher education are discussed. These issues are:\ud
* The Bologna process and changing degree structures\ud
* The changing organisation of research\ud
* Financial accountability and responsibility\ud
* Interactive governanc
« On n’écoute que des clips ! » Penser la mise en tension médiatique de la musique à l'image
Les clips semblent aujourd’hui omniprésents. Ils sont diffusés par une multitude de canaux généralistes ou dédiés, selon divers formats et thématiques, dans quantité de pays. Ils sont non seulement visibles à la télévision et dans de nombreux espaces publics (parfois même sans son), mais également sur d’innombrables écrans via internet. Les clips occupent une place prépondérante dans la médiatisation d’événements musicaux, notamment les festivals, les concerts ou les séances studio. Ils parti..
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