Post-Wrinkling Behaviors in Layered Elastic Polymers

Surface instabilities include a variety of different modes such as wrinkles, folds, and creases. Such surface instabilities have been used in numerous contexts including changing the wetting, optical, and mechanical properties of different material surfaces for applications in flexible electronics, tissue engineering and biosensors. Interesting surface morphologies can arise due to differences in pre-strain, stiffness, and thickness between the film and substrate; however, the effects of these differences are not well understood. We have developed an experimental system, complemented by finite element simulations, to systematically vary the pre-stretch of the substrate, modulus contrast and thickness contrast to study the transitions of surface instabilities in compressed bi-layers as strain is increased. We find that for systems with large modulus contrast, but varying degrees of substrate pre-stretch, the pre-stretched substrates not only substantially shift the critical strain for post-wrinkling bifurcations, but also qualitatively change the post-wrinkling modes observed. Understanding that pre-stretching the substrate affects post-wrinkling behaviors, the remaining experiments were conducted with no substrate pre-stretch. Examining the effect of modulus contrast, we report that below a film to substrate elastic modulus ratio of approximately 2, the flat state transitions directly to the creased state. For slightly larger contrasts, wrinkling occurs first but there are two distinct types of post-wrinkling behaviors: (1) wrinkles that transition into creases without period-doubling; and (2) wrinkles to creases preceded by period doubling. Finally, considering the effect of thickness contrast between the film and substrate, we discover that the critical strain for post-wrinkling behavior increases as thickness contrast decreases leading to wrinkles with aspect ratios (defined as amplitude normalized by wavelength) as high as 0.6. Further increases in aspect ratio can be achieved by increasing the modulus contrast, leading first to the formation of localized ridges, and then uniform, high aspect ratio wrinkles. Understanding surface instabilities in such non-ideal bi-layer systems provides important insight into the behavior of natural and synthetic systems with varying substrate pre-strain and modulus and thickness contrast

Analyses multifractales et spatio-temporelles des précipitations du modèle Méso-NH et des données radar

International audienceDans le cadre des multifractals universels, il est possible de caractériser la variabilité spatio-temporelle de la pluie sur une grande gamme d'échelle à l'aide de trois paramètres invariants d'échelles. Dans cette étude, nous avons estimé ces paramètres multifractals sur des simulations numériques effectuées avec le modèle méso-échelle Méso-NH (développé par Météo - France et le Laboratoire d'Aérologie), et des images radar composites, couvrant le même événement pluvieux, à savoir un orage particulièrement violent, dit de type Cévenol, ayant eu lieu sur la partie sud de la France du 5 au 9 septembre 2005. La comparaison des résultats montre que les deux types de données présentent des domaines d'invariance d'échelle relativement similaires, et dont les propriétés sont en accord avec les modèles de précipitation spatio-temporels unifiés et scalant les plus simples. Néanmoins l'évaluation de leurs exposants conduit à des valeurs parfois fortement différentes

Performance of Green roof in stormwater management regarding high-resolution precipitation fields

International audienceAt the basin scale, green roofs can represent an efficient tool to manage stormwater in urbanized areas. Widely implemented and/or completed with additional sustainable urban drainage systems, they show a positive impact on urban runoff: decrease and slowdown of the peak discharge and decrease of runoff volume. To assess green roof performances at this scale, a specific module dedicated to simulate their hydrological behaviour has been developed in the Multi-Hydro rainfall-runoff model. Its distributed structure gives the opportunity to test the susceptibility of green roof response regarding spatial distributions of precipitation. Based on radar rainfall fields, an ensemble of 50 realistic downscaled rainfall fields with a resolution of 10 m in space has been generated by using multifractals downscaling technique. Simulations have been conducted on a small urban catchment close to Paris (France) where most of the buildings roofs are assumed to easily accept the implementation of green roof. Although green roof confirm their ability to reduce urban runoff, these results illustrate that peak discharge reduction seems to be clearly dependant of spatial distribution of precipitation. Implementation of green roof can also produce concomitance situations and higher peak discharges than those produced by impervious roofs

Bifurcations in the wake of a thick circular disk

Using DNS, we investigate the dynamics in the wake of a circular disk of aspect ratio χ = d/w = 3(where d is the diameter and w the thickness) embedded in a uniform flow of magnitude U0 perpendicular to its symmetry axis. As the Reynolds number Re = U0d/ν is increased, the flow is shown to experience an original series of bifurcations leading to chaos. The range Re ∈ [150, 218] is analysed in detail. In this range, five different non-axisymmetric regimes are successively encountered, including states similar to those previously identified in the flow past a sphere or an infinitely thin disk, as well as a new regime characterised by the presence of two distinct frequencies. A theoretical model based on the theory of mode interaction with symmetries, previously introduced to explain the bifurcations in the flow past a sphere or an infinitely thin disk (Fabre et al. in Phys Fluids 20:051702, 2008), is shown to explain correctly all these results. Higher values of the Reynolds number, up to 270, are also considered. Results indicate that the flow encounters at least four additional bifurcations before reaching a chaotic state

Green roof and storm water management policies: monitoring experiments on the ENPC Blue Green Wave

International audienceCurrently widespread in new urban projects, green roofs have shown a positive impact on urban runoff at the building/parcel scale. Nevertheless, there is no specific policy promoting their implementation neither in Europe nor in France. Moreover they are not taken into account (and usually considered as an impervious area) in the sizing of a retention basin for instance. An interesting example is located in the heart of the Paris-East Cluster for Science and Technology (Champs-sur-Marne, France). Since 2013 a large (1 ha) wavy-form vegetated roof (called bleu green wave) is implemented. Green roof area and impervious areas are connected to a large retention basin, which has been oversized. The blue green wave represents a pioneering site where an initially amenity (decorative) design project has been transformed into a research oriented one. Several measurement campaigns have been conducted to investigate and better understand the hydrological behaviour of such a structure. Rainfall, humidity, wind velocity, water content and temperature have been particularly studied. The data collected are used for several purposes: (i) characterize the spatio-temporal variability of the green roof response, (ii) calibrate and validate a specific model simulating its hydrological behavior. Based on monitoring and modeling results, green roof performances will be quantified. It will be possible to estimate how they can reduce stormwater runoff and how these performances can vary in space and in time depending on green roof configuration, rainfall event characteristics and antecedent conditions. These quantified impacts will be related to regulation rules established by stormwater managers in order to connect the parcel to the sewer network. In the particular case of the building of a retention basin, the integration of green roof in the sizing of the basin will be studied

Quantifying the impact of small scale unmeasured rainfall variability on urban runoff through multifractal downscaling: A case study

International audienceThis paper aims at quantifying the uncertainty on urban runoff associated with the unmeasured small scale rainfall variability, i.e. at a resolution finer than 1. km. ×. 1. km. ×. 5. min which is usually available with C-band radar networks. A case study is done on the 900. ha urban catchment of Cranbrook (London). A frontal and a convective rainfall event are analysed. An ensemble prediction approach is implemented, that is to say an ensemble of realistic downscaled rainfall fields is generated with the help of universal multifractals, and the corresponding ensemble of hydrographs is simulated. It appears that the uncertainty on the simulated peak flow is significant, reaching for some conduits 25% and 40% respectively for the frontal and the convective events. The flow corresponding the 90% quantile, the one simulated with radar distributed rainfall, and the spatial resolution are power law related. © 2012 Elsevier B.V

Impact de la variabilité non-mesurée des précipitations sur les débits en hydrologie urbaine : un cas d'étude dans le cadre multifractal

National audienceCet article utilise les propriétés multifractales d'un évènement pluvieux dans la région de Londres le 9 février 2009, pour mieux comprendre et quantifier 'incertitude associée à la variabilité spatio-temporelle des précipitations non mesurées par les radars météorologiques en bande C, dont la résolution estimée est de 1 km*1 km*5min, et comment elle se transfère aux prévisions des débits en réseaux d'assainissement. Le cas d'étude hydrologique est celui du bassin versant urbain de 900 hectares de Cranbrook (Londres). Les propriétés multifractales sont en accord avec le modèle spatio-temporel le plus simple, reposant sur un exposant d'anisotropie entre l'espace et le temps. Ceci permet de désagréger le champ de précipitation à l'aide de cascades multifractales spatio-temporelles. Un ensemble de champs de précipitations désagrégés réalistes est alors généré à l'aide des multifractals universels, puis l'ensemble des hydrogrammes correspondants est obtenu par un modèle urbain pluie-débit semi-distribué. Il apparait que les queues de probabilité issues de l'analyse de 100 échantillons de précipitation présentent un comportement en loi de puissance, qui est retrouvé sur les débits de pointe mais avec des exposants différent

Simulation of Green Roof Impact at Basin Scale by Using a Distributed Rainfall-Runoff Model

International audienceCurrently widespread in new urban projects, green roofs have shown a positive impact on urban runoff at the building scale, that is, decreased and slow peak discharge and decreases runoff volume. The aim was to study the possible impact of green roof at the catchment scale, more compatible with stormwater management issues. For this purpose, a distributed rainfall-runoff model (Multi-Hydro) devoted to urban environment and able to simulate the hydrological behaviour of green roof has been used to assess the green roof impact at such a scale. It has been applied on an urban catchment (Loup basin located in the Seine-Saint-Denis county, East of Paris, France) where most of the building roofs are flat and assumed to easily accept the implementation of green roof. Catchment responses to several rainfall events covering a wide range of meteorological situation have been simulated. The simulation results show that green roof can significantly reduce runoff volume and the magnitude of peak discharge (up to 80%) depending on the rainfall event and the initial saturation of the substrate

First Results of the Phase II SIMPLE Dark Matter Search

We report results of a 14.1 kgd measurement with 15 superheated droplet detectors of total active mass 0.208 kg, comprising the first stage of a 30 kgd Phase II experiment. In combination with the results of the neutron-spin sensitive XENON10 experiment, these results yield a limit of |a_p| < 0.32 for M_W = 50 GeV/c2 on the spin-dependent sector of weakly interacting massive particle-nucleus interactions with a 50% reduction in the previously allowed region of the phase space formerly defined by XENON, KIMS and PICASSO. In the spin-independent sector, a limit of 2.3x10-5 pb at M_W = 45 GeV/c2 is obtained.Comment: 4 pages, 4 figures; PRL-accepted version with corrected SI contour (Fig. 4