Revisiting the role of friction coefficients in granular collapses: confrontation of 3-D non-smooth simulations with experiments

In this paper, transient granular flows are examined both numerically and experimentally. Simulations are performed using the continuous 3D granular model introduced in Daviet & Bertails-Descoubes (2016), which represents the granular medium as an inelastic and dilatable continuum subject to the Drucker-Prager yield criterion in the dense regime. One notable feature of this numerical model is to resolve such a non-smooth rheology without any regularisation. We show that this non-smooth model, which relies on a constant friction coefficient, is able to reproduce with high fidelity various experimental granular collapses over inclined erodible beds, provided the friction coefficient is set to the avalanche angle - and not to the stop angle, as generally done. In order to better characterise the range of validity of the fully plastic rheology in the context of transient frictional flows, we further revisit scaling laws relating the shape of the final collapse deposit to the initial column aspect ratio, and accurately recover established power-law dependences up to aspect ratios in the order of 10. The influence of sidewall friction is then examined through experimental and simulated collapses with varying channel widths. The effective flow thickness is estimated in relation to the channel width, thereby challenging previously held assumptions on its estimation. Finally, we discuss the potential extension of the constant coefficient model with a hysteretic model to refine predictions of early-stage collapse dynamics, illustrating the impact of such phenomenology on transient flows and paving the way to more elaborate analysis.Comment: 25 figures and 6 movie

Spectral-phase interferometry for direct electric-field reconstruction applied to seeded extreme-ultraviolet free-electron lasers

We present a setup for complete characterization of femtosecond pulses generated by seeded free-electron lasers (FEL's) in the extreme-ultraviolet spectral region. Two delayed and spectrally shifted replicas are produced and used for spectral phase interferometry for direct electric field reconstruction (SPIDER). We show that it can be achieved by a simple arrangement of the seed laser. Temporal shape and phase obtained in FEL simulations are well retrieved by the SPIDER reconstruction, allowing to foresee the implementation of this diagnostic on existing and future sources. This will be a significant step towards an experimental investigation and control of FEL spectral phase

Turbulent Flows over Rough Permeable Beds in Mountain Rivers: Experimental Insights and Modeling

Steep mountain streams exhibit shallow waters with roughness elements such as stones and pebbles that are comparable in size to flow depth. Owing to the difficulty in measuring fluid velocities at the interface, i.e., from the rough permeable bed to the free surface, experimental results are rare although they are essential to improve models. Using a novel experimental procedure, this thesis attempts to improve predictions of the vertical structure of turbulent flows over rough permeable beds. To explore flows at the bed interface, I devised an experimental set-up where a fluid flowed over glass spheres (8 mm < dp < 14 mm) in a narrow flume (W = 6 cm) with slopes varying from 0.5 % to 8 %. The Refractive Index Matching (RIM) technique has been employed. This involves matching the refractive index of the fluid with that of the glass spheres, thereby allowing the interior of the medium to be examined and velocities to be measured by Particle Image Velocimetry (PIV). Vertical profiles are retrieved by employing the spatiotemporal double averaging method. In the course of this manuscript, flow processes are studied at the mesoscopic scale, i.e., by averaging quantities over distances ranging from 5 to 10 grain diameters. For open-channel flows over rough permeable beds, the spatial averaging procedure yields a continuous porosity profile. When applied to the Navier-Stokes equations, it produces a momentum equation with several terms including drag forces and three stresses: the turbulent, dispersive, and viscous stresses. The momentum equation was employed to devise a one dimensional (1D) model describing the vertical structure of unidirectional turbulent flow. A turbulent boundary layer over the rough bed was observed while experiments were performed at intermediate Reynolds numbers, i.e., Re = O (1000). In such conditions, viscosity plays a critical role through the van Driest damping effect. To model vertical profiles, the Darcy-Ergün equation is well suited to the prediction of friction forces in the permeable bed, i.e., in roughness and subsurface layers. Based on the \textit{Prandtl mixing length theory}, turbulent stress is predicted from a mixing length distribution that considers dispersive effects and assumes a continuous porosity profile. This alternative contrasts with most existing boundary layer models which postulate a discontinuous porosity profile for permeable or impermeable walls. Finally, hydraulic conditions collected by an Unmanned Aerial Vehicle (UAV) and classical flow resistance equations (Chézy, Keulegan, ...) were compared with profile simulations and demonstrate a good agreement between predictions and observations. It reveals the crucial role of fluid depth definition in equations in small submergence conditions. Furthermore, incipient sediment motion conditions have been estimated and compared to empirical results showing the importance of turbulence and lift force for grain entrainment. With regard to fluid dynamics, mountain streams are a case study of the larger scientific family of turbulent flows interacting with porous structures. Insights and developments acquired in the course of this thesis are likely to be transferable to other domains working with these phenomena such as flows over buildings, vegetal canopies or rough wings

High-resolution monitoring of bedload transport rates: a benchmark of two approaches (accelerometers and image processing).

Experimental and field studies have shown how intermittent bedload transport can be at low flow rates. The development and validation of bedload-transport equations require high-resolution records over long periods of time. Two technologies are considered in the present investigation: image processing and accelerometers mounted on impact plates. The former has been successfully applied to monitor bedload transport in many flume experiments, and the latter has shown encouraging results at different field sites. Calibration is a major issue in both cases, and it is often difficult to assess the precision of the data collected. In our talk, we show how to calibrate and compare the performances of accelerometer and image-processing based techniques in laboratory conditions. The accelerometer is fixed on a perforated steel plate, which is placed vertically at the lower end of the flume. The vibrations due to the particles impacting the plate are recorded with a sampling frequency of 10 kHz. The proxy for bedload transport rate is chosen as the number of peaks above a fixed threshold of the recorded signal. Note that impact plates are usually set in flush with the bed, and to our knowledge, the vertical configuration presented here has not yet been documented. The experimental setup for image processing involves a video-camera that takes top-view images of the particles moving over a white board mounted at the outlet of the flume. Data storage poses an issue, which can be got round by pre-processing the images in real time. The bedload transport rate is estimated based on the number of particles that are identified on the images. The two technologies have the advantages of being cost-effective and demanding limited effort for implementation. They provide high-resolution bedload transport rates over several hours. Estimates of bedload discharge were found to be robust and accurate for low sediment transport rates. At higher rates, the sensors may saturate due the arrival of particle clusters. This effect is described and, to a certain degree, taken into account in the calibration relations

Vélocimétrie par scan 3D d'écoulements turbulents : application à un écoulement à surface libre sur fond poreux

International audienceCet article présente une méthode de mesure utilisant deux techniques laser connues : le scan par adaptation d’indice de réfraction (RIMS) et la vélocimétrie par images de particules (PIV). Le couplage de ces deux techniques permet de mesurer finement les vitesses locales du fluide dans un milieu granulaire saturé. Dans notre cas, nous étudions l’écoulement d’un fluide à surface libre sur un milieu granulaire. Le fluide iso-indice utilisé a des propriétés proches de celles de l’eau et le lit granulaire est formé de billes de verre. C’est une configuration qui s’approche de l’objet d’étude dans notre laboratoire : les torrents de montagne. La spécificité de cette technique est la mesure en continu des vitesses du fluide tandis que la nappe laser se déplace dans la direction perpendiculaire à l’écoulement. Nous utilisons la méthode des doubles moyennes pour déduire les grandeurs d’intérêt telles que la contrainte turbulente ou la contrainte induite par la forme du lit. Nos résultats expérimentaux montrent la robustesse et la répétabilité de cette technique pour étudier le mélange dans les trois dimensions spatiales

Du jeu des identités à la transformation de réalités partagées : un programme d’ateliers d’expression théâtrale pour adolescents immigrants et réfugiés

La migration à l’adolescence est particulièrement délicate à cause du fardeau conjugué que représente à ce stade de la vie l’intégration des multiples pertes associées à la migration et l’adaptation au statut de jeune adulte. Le programme d’ateliers d’expression théâtrale vise à faciliter l’adaptation des adolescents immigrants et réfugiés à leur nouvel environnement à partir d’un travail créatif autour des enjeux identitaires liés à la migration et à un statut de minorité. Ces ateliers conjuguent une approche inspirée du théâtre playback qui permet une mise en scène du vécu personnel et le théâtre forum de Boal qui met l’accent sur la transformation collective de l’expérience. Les résultats d’une évaluation qualitative des ateliers d’expression théâtrale suggèrent que ceux-ci constituent un lieu d’expression ou les participants se sentent en sécurité et soutenus par l’équipe ainsi que par le caractère rituel du jeu théâtral. Les ateliers permettent de représenter la multiplicité des valeurs et des références internes et externes de l’adolescent et de les renégocier sans dichotomiser le « eux » et le « nous », en s’adressant aux questions de justice sociale qui se posent à la collectivité. Ils favorisent aussi l’élaboration des transitions de l’adolescence en permettant l’évocation des pertes de la migration et le passage vers une identité hybride.Migration during adolescence represents a challenge for the youth who need to simultaneously work through the multiple losses associated with the migratory journey and adapt to a young adult status. The drama workshop program described here was designed to facilitate the adjustment of newly arrived immigrant teens. The aim of the program is to make it easier for adolescents to adjust to their new environment through creative group work around identity issues. The program also seeks to improve intergroup relations in multiethnic schools. The workshops are inspired both from playback theater and from Boal’s form theater which emphasizes the collective transformation of the singular experience. The qualitative assessment of the program effects on the adolescents suggests that the workshops constitute a safe space of expression, in which the team and the ritual nature of the play hold the participants. The workshops facilitate the representation of the multiplicity of values in the adolescent world and invite them to reconsider the way in which they interact, with their environment, without splitting between “us” and “them,” but rather creating solidarities around issues of social justice. The workshops also address the life transformation associated both with adolescence and migration and help the elaboration of the losses linked to the migratory journey and the construction of a hybrid identity.La migración en la adolescencia es particularmente delicada a causa de la carga conjunta que representa en esta etapa de la vida la integración de las múltiples pérdidas asociadas a la migración y la adaptación a un estatus de joven adulto. El programa de talleres de expresión teatral busca facilitar la adaptación de los adolescentes inmigrantes y refugiados a su nuevo entorno, a partir de un trabajo creativo relacionado con las cuestiones de identidad ligadas a la migración y a un estatus de minoría. Estos talleres conjugan un enfoque inspirado del teatro playback, que permite una puesta en escena de una experiencia personal, y el teatro forum de Boal, que pone el acento en la transformación colectiva de la experiencia. Los resultados de una evaluación cualitativa de los talleres de expresión teatral sugieren que éstos constituyen un lugar de expresión en el que los participantes se sienten seguros y apoyados por el equipo, así como por el carácter ritual de la representación teatral. Los talleres permiten representar la multiplicidad de los valores y referencias internas y externas del adolescente, y renegociarlos sin crear una dicotomía entre el “ellos” y “nosotros” al tratar cuestiones de justicia social que se plantean a la colectividad. Favorecen también la elaboración de las transiciones de la adolescencia al permitir la evocación de las pérdidas de la migración y el paso hacia una identidad híbrida.A imigração na adolescência é especialmente delicada, por causa do fardo conjugado que representa, neste ponto da vida, a integração das múltiplas perdas associadas à imigração e à adaptação ao estatuto de jovem adulto. O programa de ateliês de expressão teatral visa facilitar a adaptação dos adolescentes imigrantes e refugiados ao seu novo ambiente, a partir de um trabalho criativo a respeito das preocupações identitárias relacionadas à imigração e a um estatuto de minoria. Estes ateliês conjugam uma abordagem inspirada no teatro playback, que permite uma representação da vivência pessoal, e no teatro fórum de Boal, que ressalta a transformação coletiva da experiência. Os resultados de uma avaliação qualitativa dos ateliês de expressão teatral sugerem que estes continuam sendo um lugar de expressão onde os participantes sentem-se em segurança e apoiados pela equipe e pelo caráter ritual da representação teatral. Os ateliês permitem representar a multiplicidade dos valores e das referências internas e externas do adolescente e renegociá-los sem dicotomizar o “eles” e o “nós”, falando sobre questões de justiça social que são colocadas à coletividade. Os ateliês favorecem também a elaboração das transições da adolescência permitindo a evocação das perdas da imigração e a passagem para uma identidade híbrida

European Loess Records

Considering their distribution, thickness, and complexity around the margins of the Quaternary ice-sheets in the Northern Hemisphere, loess sequences can be considered as one of the best records of global environmental changes on the continents. European loess sequences have been intensively studied for many decades, but increasingly higher stratigraphic resolution and the availability of a growing range of climate proxy indicators has resulted in some notable advances in recent years. Climatic variability has been analyzed at high resolution, based on different proxies

The PrePack optimization problem

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