418 research outputs found
Dynamics of aeolian sand ripples
We analyze theoretically the dynamics of aeolian sand ripples. In order to
put the study in the context we first review existing models. We argue on the
local character of sand ripple formation. Using a hydrodynamical model we
derive a nonlinear equation for the sand profile. We show how the
hydrodynamical model may be modified to recover the missing terms that are
dictated by symmetries. The symmetry and conservation arguments are powerful in
that the form of the equation is model-independent. We then present an
extensive numerical and analytical analysis of the generic sand ripple
equation. We find that at the initial stage the wavelength of the ripple is
that corresponding to the linearly most dangerous mode. At later stages the
profile undergoes a coarsening process leading to a significant increase of the
wavelength. We find that including the next higher order nonlinear term in the
equation, leads naturally to a saturation of the local slope. We analyze both
analytically and numerically the coarsening stage, in terms of a dynamical
exponent for the mean wavelength increase. We discuss some future lines of
investigations.Comment: 22 pages and 10 postscript figure
A Model for the Threshold of the Rolling Transition
A dynamic transition has been identified recently by numerical simulations in the case of a granular flow along a rotating boundary (Rioual and Le Quiniou (2011)). We showed that this transition appears above a certain critical microscopic friction coefficient particle/boundary ”* from which the particles roll without any sliding. We propose here a model at the scale of the microstructure which aimes to be close to the physical process considered and predicts also a threshold for the apparition of this dynamic transition. We discuss the discrepancy on a quantitative level in terms of friction properties at the contact
Reconstruction of palaeoclimatic and palaeolimnological changes during the Last Interglacial from sedimentary diatom assemblages in the French Massif Central
The Study of Choice: Looking at Parent Surveys and Putting Them into Perspective
In the 1960s, interdistrict choice or âvoluntary desegregationâ became popular in a few cities across the country. While it is not widespread, it is seen as more effective than other school choice options. My research focuses on parentsâ perceptions of an interdistrict school choice program, Northeast Choice, which is located in a Northeastern city. Through analyzing surveys collected by Northeast Choice completed by the parents in the program, I examine how parents experience the actual program and the schools the children attend through the program. I find that the majority of the parents who responded have positive feedback and because of this, it can be interpreted that these parents are having good experiences with both the program and the schools. While interdistrict choice is not widespread, my findings are important because it shows that parents and students are benefiting from this program and that interdistrict choice should be an option in more cities across our country
Développement d'une nouvelle méthode de suivi de la corrosion par des capteurs sans fil basés sur la technologie RFID
International audienceUn capteur radiofrĂ©quence sans fil et totalement passif Ă©nergĂ©tiquement permettant la mesure des potentiels de corrosion a Ă©tĂ© dĂ©veloppĂ©. Il repose sur l'utilisation d'un rĂ©sonateur dont la frĂ©quence de rĂ©sonance dĂ©pend de la valeur du potentiel de corrosion. Deux exemples sont proposĂ©s dans ce rĂ©sumĂ©Â : le suivi de la passivation de l'acier et de la dĂ©gradation d'un revĂȘtement en Ă©poxy. Ces rĂ©sultats seront complĂ©tĂ©s Ă la confĂ©rence par des rĂ©sultats obtenus par un capteur basĂ© sur le mĂȘme principe et sensible Ă la variation de la permittivitĂ© diĂ©lectrique des polymĂšres
âAlors bien sĂ»r, ça va mal chez vous mais ça va trĂšs mal aussi chez moi. Et câest pas seulement ça quâon aurait Ă se dire.â
Honorable rassemblement des professionnels du spectacle vivant et des arts plastiques que celui qui eut lieu ce mercredi dans la grande salle de La Colline, Ă lâinitiative du Syndeac et de sa prĂ©sidente, Madeleine Louarn. StĂ©phane Braunschweig de souligner que nombre de personnes perdent en ce moment leur emploi dans le spectacle vivant et les centres dâart. BenoĂźt Lambert de rappeler la rĂ©alitĂ© du paradoxe bourdieusien des deux mains de lâĂtat : les services publics doivent contrecarrer lâef..
An oxygen isotope record of lacustrine opal from a European Maar indicates climatic stability during the Last Interglacial
The penultimate temperate period, 127â110 ka before present (BP), bracketed by abrupt shifts of the global climate system initiating and terminating it, is considered as an analogue of the Holocene because of a similar low global iceâvolume. Ice core records as well as continental and marine records exhibit conflicting evidence concerning the climate variability within this period, the Last Interglacial. We present, for the first time, a highâresolution record of oxygen isotopes in diatom opal of the Last Interglacial obtained from the Ribains Maar in France (44°50âČ09âłN 3°49âČ16âłE). Our results indicate that the Last Interglacial in southwestern Europe was generally a period of climatic stability. The record shows that the temperate period was initiated by an abrupt warm event followed midway by a minor climatic transition to a colder climate. An abrupt isotopic depletion that occurs simultaneously with abrupt changes in pollen and diatom assemblages marks the end of the temperate period, and is correlative with the Melisey I stadial. Variations in the isotopic composition of lakeâwater related to the isotopic composition of precipitation and evaporation dominate the biogenic opal oxygen isotope record
Devant la folle rĂ©alitĂ©, la belle urgence de lâart
« Avez-vous pris le numĂ©ro de votre place ? » me demande-t-on alors que je prĂ©sente un billet, comme Ă lâaccoutumĂ©e, mâayant tout lâair dâĂȘtre valable... « Car il faut prendre le numĂ©ro de votre place », une place Ă bord ; nous ne sommes pas seulement dans un thĂ©Ăątre mais aussi dans le vaisseau dĂ©mesurĂ© du Fol Espoir. Tout y crĂ©e le voyage et lâĂ©vĂ©nement : de la prĂ©sence en province de la troupe du ThĂ©Ăątre du Soleil Ă ce palais des Sports de Lyon Ă la voĂ»te maculĂ©e de projecteurs comme des Ă©t..
Mission possible: diatoms can be used to infer past duckweed (lemnoid Araceae) dominance in ponds
Compared to larger lakes, ponds have rarely been the focus of palaeoecological studies. A common feature of ponds, especially those subject to eutrophication, is mass surface coverings of lemnoid Araceae (duckweed) which have severe implications for ecological processes in small waterbodies, in particular lowered oxygen content. To help understand the implications of duckweed dominance for the long-term ecology of ponds, and to determine the potential for palaeoecological studies in ponds more generally, we develop a new diatom-based Lemna-indicator metric. Recent studies of diatom host-plant relationships have shown significant associations between duckweed and the epiphytes Lemnicola hungarica and Sellaphora saugerresii (formally known as Sellaphora seminulum). To determine the potential of these species as palaeo-indicators of long-term duckweed dynamics in ponds, we investigated the diatom composition of surface sediment assemblages in sets of duckweed and non-duckweed-dominated ponds in Norfolk, eastern England. In addition, we undertook diatom analysis of two cores from a small farmland pond (Bodham Rail Pit) subject to a known duckweed dominance event (1999â2005). Both L. hungarica and S. saugerresii were significant predictors of past Lemna dominance in the surface sediments. Further, in the core study, both diatom species accurately and closely tracked the documented âonâoffâ duckweed cycle. Our study suggests huge potential for using ponds in palaeoecological studies and for diatom-based investigations of floating plant histories
An analytical analysis of vesicle tumbling under a shear flow
Vesicles under a shear flow exhibit a tank-treading motion of their membrane,
while their long axis points with an angle < 45 degrees with respect to the
shear stress if the viscosity contrast between the interior and the exterior is
not large enough. Above a certain viscosity contrast, the vesicle undergoes a
tumbling bifurcation, a bifurcation which is known for red blood cells. We have
recently presented the full numerical analysis of this transition. In this
paper, we introduce an analytical model that has the advantage of being both
simple enough and capturing the essential features found numerically. The model
is based on general considerations and does not resort to the explicit
computation of the full hydrodynamic field inside and outside the vesicle.Comment: 19 pages, 9 figures, to be published in Phys. Rev.
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