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 µ* 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

The Study of Choice: Looking at Parent Surveys and Putting Them into Perspective

In the 1960s, interdistrict choice or ‘voluntary desegregation’ 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 parents’ 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 radiofréquence sans fil et totalement passif énergétiquement permettant la mesure des potentiels de corrosion a été développé. Il repose sur l'utilisation d'un résonateur dont la fréquence de résonance dépend de la valeur du potentiel de corrosion. Deux exemples sont proposés dans ce résumé : le suivi de la passivation de l'acier et de la dégradation d'un revêtement en époxy. Ces résultats seront complétés à la conférence par des résultats obtenus par un capteur basé sur le même principe et sensible à la variation de la permittivité diélectrique des polymères

“Alors bien sûr, ça va mal chez vous mais ça va très mal aussi chez moi. Et c’est pas seulement ça qu’on aurait à se dire.”

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, à l’initiative du Syndeac et de sa présidente, Madeleine Louarn. Stéphane Braunschweig de souligner que nombre de personnes perdent en ce moment leur emploi dans le spectacle vivant et les centres d’art. Benoît Lambert de rappeler la réalité du paradoxe bourdieusien des deux mains de l’État : les services publics doivent contrecarrer l’ef..

An oxygen isotope record of lacustrine opal from a European Maar indicates climatic stability during the Last Interglacial

The penultimate temperate period, 127–110 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 ice‐volume. 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 high‐resolution record of oxygen isotopes in diatom opal of the Last Interglacial obtained from the Ribains Maar in France (44°50′09″N 3°49′16″E). 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 lake‐water related to the isotopic composition of precipitation and evaporation dominate the biogenic opal oxygen isotope record

Devant la folle réalité, la belle urgence de l’art

« Avez-vous pris le numéro de votre place ? » me demande-t-on alors que je présente un billet, comme à l’accoutumée, m’ayant tout l’air d’être valable... « Car il faut prendre le numéro de votre place », une place à bord ; nous ne sommes pas seulement dans un théâtre mais aussi dans le vaisseau démesuré du Fol Espoir. Tout y crée le voyage et l’événement : de la présence en province de la troupe du Théâtre du Soleil à ce palais des Sports de Lyon à la voûte maculée de projecteurs comme des ét..

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 (1999–2005). 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 “on–off” 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.