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