3,795 research outputs found
Dynamic and instability of submarine avalanches
We perform a laboratory-scale experiment of submarine avalanches on a rough
inclined plane. A sediment layer is prepared and thereafter tilted up to an
angle lower than the spontaneous avalanche angle. The sediment is scrapped
until an avalanche is triggered. Based on the stability diagram of the sediment
layer, we investigate different structures for the avalanche front dynamics.
First we see a straight front descending the slope, and then a transverse
instability occurs. Eventually, a fingering instability shows up similar to
rivulets appearing for a viscous fluid flowing down an incline. The mechanisms
leading to this new instability and the wavelength selection are discussed.Comment: 4 pages, 6 figures, to appear in the proceedings of Powders and
Grains 200
Bifurcation-based parameter tuning in a model of the GnRH pulse and surge generator
We investigate a model of the GnRH pulse and surge generator, with the
definite aim of constraining the model GnRH output with respect to a
physiologically relevant list of specifications. The alternating pulse and
surge pattern of secretion results from the interaction between a GnRH
secreting system and a regulating system exhibiting fast-slow dynamics. The
mechanisms underlying the behavior of the model are reminded from the study of
the Boundary-Layer System according to the "dissection method" principle. Using
singular perturbation theory, we describe the sequence of bifurcations
undergone by the regulating (FitzHugh-Nagumo) system, encompassing the rarely
investigated case of homoclinic connexion. Basing on pure dynamical
considerations, we restrict the space of parameter search for the regulating
system and describe a foliation of this restricted space, whose leaves define
constant duration ratios between the surge and the pulsatility phase in the
whole system. We propose an algorithm to fix the parameter values to also meet
the other prescribed ratios dealing with amplitude and frequency features of
the secretion signal. We finally apply these results to illustrate the dynamics
of GnRH secretion in the ovine species and the rhesus monkey
How Culture comes to Mind: From Social affordances to Cultural analogies
Until now, the naturalist attempts to account for cultural phenomena have tended to see them as representations that spread within the population thanks to the counterintuitive properties making them salient and easy to remember. As a supplement to this view, which postulates a kind of cognitive distance between individuals and culture, this paper proposes a naturalist model that takes into consideration the strong cognitive involvement and the participative rather than contemplative stance triggered by a good many cultural phenomena. Such a model tries to defend a «continuist view » of the link between nature and culture by calling partially into question the traditional emphasis of social sciences on the artificial, arbitrary dimension of social facts. For the authors, indeed, this emphasis does not account for the naturality and universality of a certain number of elementary social forms. Once the partial naturality of the social is asserted, the purpose is to describe the emergence of cultural phenomena. The hypothesis put forward here is that analogical capacities, also natural, which allow human minds to «draw » cultural forms from the world of nature, either physical or social, play a central role in the elaboration of a sphere of collective experience that is both cultural and intuitive.Comment la culture vient à l'esprit. Des affordances sociales aux analogies culturelles. Jusqu’à présent, les tentatives naturalistes visant à rendre compte des phénomènes culturels ont eu tendance à les appréhender comme des représentations qui se diffusent dans la population grâce à leurs propriétés contreintuitives, qui retiennent l’attention et facilitent la mémorisation individuelle. En complément à cette perspective, qui présuppose une forme de distanciation cognitive entre les individus et leur culture, cet article propose un modèle naturaliste qui prend acte de la forte implication cognitive et de la posture, non pas contemplative mais participative, que provoquent bon nombre de phénomènes culturels. Un tel modèle tente de défendre une «vision continuiste » du lien entre nature et culture en remettant partiellement en question la focalisation traditionnelle des sciences sociales sur la dimension artificielle et arbitraire des faits sociaux. Pour les auteurs, en effet, cette focalisation ne rend pas compte de la naturalité et de l’universalité d’un certain nombre de formes sociales élémentaires. Une fois posée la naturalité partielle du social, l’objectif est alors de rendre compte de l’émergence des phénomènes culturels. L’hypothèse défendue ici est que les capacités analogiques, elles aussi naturelles, qui permettent aux esprits humains de «dériver » les formes culturelles du monde de la nature, qu’il soit physique ou social, jouent un rôle central dans l’élaboration d’une sphère de l’expérience collective qui est tout à la fois culturelle et intuitive.Clément Fabrice, Kaufmann Laurence. How Culture Comes to Mind: From Social Affordances to Cultural Analogies. In: Intellectica. Revue de l'Association pour la Recherche Cognitive, n°46-47, 2007/2-3. Culture and Society: Some Viewpoints of Cognitive Scientists. pp. 221-250
Neuro-vestibular Examination During and Following Spaceflight
Adaptation to microgravity during spaceflight causes neurological disturbances that are either directly or indirectly mediated by the vestibular system. These disturbances could include space motion sickness, spatial disorientation, cognitive impairment, as well as changes in head-eye coordination, vestibulo-ocular reflex, and strategies for controlling posture and locomotion. It seems that otolith-mediated reflex gain adapts rapidly over time during spaceflight and after landing. However, animal studies have shown that structural modifications of the vestibular sensory apparatus develop during long-duration spaceflight. To date, no studies have characterized the severity of vestibular syndromes experienced by astronauts as a function of the duration of spaceflight, or whether the effects are caused by changes at the peripheral end organs, midbrain, cerebellum, or vestibular cortex
Transverse Instability of Avalanches in Granular Flows down Incline
Avalanche experiments on an erodible substrate are treated in the framework
of ``partial fluidization'' model of dense granular flows. The model identifies
a family of propagating soliton-like avalanches with shape and velocity
controlled by the inclination angle and the depth of substrate. At high
inclination angles the solitons display a transverse instability, followed by
coarsening and fingering similar to recent experimental observation. A primary
cause for the transverse instability is directly related to the dependence of
soliton velocity on the granular mass trapped in the avalanche.Comment: 3 figures, 4 pages, submitted to Phys Rev Let
Fast production of Bose-Einstein condensates of metastable Helium
We report on the Bose-Einstein condensation of metastable Helium-4 atoms
using a hybrid approach, consisting of a magnetic quadrupole and a crossed
optical dipole trap. In our setup we cross the phase transition with 2x10^6
atoms, and we obtain pure condensates of 5x10^5 atoms in the optical trap. This
novel approach to cooling Helium-4 provides enhanced cycle stability, large
optical access to the atoms and results in production of a condensate every 6
seconds - a factor 3 faster than the state-of-the-art. This speed-up will
dramatically reduce the data acquisition time needed for the measurement of
many particle correlations, made possible by the ability of metastable Helium
to be detected individually
Nonlocal effects in sand flows on an inclined plane
International audienceThe flow of sand on a rough inclined plane is investigated experimentally. We directly show that a jammed layer of grains spontaneously forms below the avalanche. Its properties and its relation with the rheology of the flowing layer of grains are presented and discussed. In a second part, we study the dynamics of erosion and deposition solitary waves in the domain where they are transversally stable. We characterize their shapes and velocity profiles. We relate their translational velocity to the stopping height and to the mass trapped in the avalanche. Finally, we use the velocity profile to get insight into the rheology very close to the jamming limit
- …