Study of Polydisperse Particulate Systems with a ‘Direct-Forcing/Fictitious Domain’ Method

Natural sediments responsible for the morphodynamic of the estuaries and coast are of different sizes and densities. Some are cohesive and some are non-cohesive. The transport in suspension and their sedimentation of such a polydisperse suspension are different than the ones for a monodisperse suspension. A fully resolved model based on the Direct-Forcing/Fictitious Domain method (DF/FD) was developed and applied to simulate settling of monodisperse particles in a water column. The behaviour of the suspension corresponds qualitatively to experimental results and average settling velocities follow a Richardson-Zaki type law. Then the model is applied to the sedimentation of suspension composed of particles of three diameters. The segregation of the bed is obtained naturally. The excess pore pressure is drawn and compared with the theory

LSST: from Science Drivers to Reference Design and Anticipated Data Products

(Abridged) We describe here the most ambitious survey currently planned in the optical, the Large Synoptic Survey Telescope (LSST). A vast array of science will be enabled by a single wide-deep-fast sky survey, and LSST will have unique survey capability in the faint time domain. The LSST design is driven by four main science themes: probing dark energy and dark matter, taking an inventory of the Solar System, exploring the transient optical sky, and mapping the Milky Way. LSST will be a wide-field ground-based system sited at Cerro Pach\'{o}n in northern Chile. The telescope will have an 8.4 m (6.5 m effective) primary mirror, a 9.6 deg$^2$ field of view, and a 3.2 Gigapixel camera. The standard observing sequence will consist of pairs of 15-second exposures in a given field, with two such visits in each pointing in a given night. With these repeats, the LSST system is capable of imaging about 10,000 square degrees of sky in a single filter in three nights. The typical 5$\sigma$ point-source depth in a single visit in $r$ will be $\sim 24.5$ (AB). The project is in the construction phase and will begin regular survey operations by 2022. The survey area will be contained within 30,000 deg$^2$ with $\delta<+34.5^\circ$, and will be imaged multiple times in six bands, $ugrizy$, covering the wavelength range 320--1050 nm. About 90\% of the observing time will be devoted to a deep-wide-fast survey mode which will uniformly observe a 18,000 deg$^2$ region about 800 times (summed over all six bands) during the anticipated 10 years of operations, and yield a coadded map to $r\sim27.5$. The remaining 10\% of the observing time will be allocated to projects such as a Very Deep and Fast time domain survey. The goal is to make LSST data products, including a relational database of about 32 trillion observations of 40 billion objects, available to the public and scientists around the world.Comment: 57 pages, 32 color figures, version with high-resolution figures available from https://www.lsst.org/overvie

Influx of nitrogen-rich material from the outer Solar System indicated by iron nitride in Ryugu samples

Large amounts of nitrogen compounds, such as ammonium salts, may be stored in icy bodies and comets, but the transport of these nitrogen-bearing solids into the near-Earth region is not well understood. Here, we report the discovery of iron nitride on magnetite grains from the surface of the near-Earth C-type carbonaceous asteroid Ryugu, suggesting inorganic nitrogen fixation. Micrometeoroid impacts and solar wind irradiation may have caused the selective loss of volatile species from major iron-bearing minerals to form the metallic iron. Iron nitride is a product of nitridation of the iron metal by impacts of micrometeoroids that have higher nitrogen contents than the CI chondrites. The impactors are probably primitive materials with origins in the nitrogen-rich reservoirs in the outer Solar System. Our observation implies that the amount of nitrogen available for planetary formation and prebiotic reactions in the inner Solar System is greater than previously recognized

Four‐dimensional‐STEM analysis of the phyllosilicate‐rich matrix of Ryugu samples

Ryugu asteroid grains brought back to the Earth by the Hayabusa2 space mission are pristine samples containing hydrated minerals and organic compounds. Here, we investigate the mineralogy of their phyllosilicate-rich matrix with four-dimensional scanning transmission electron microscopy (4D-STEM). We have identified and mapped the mineral phases at the nanometer scale (serpentine, smectite, pyrrhotite), observed the presence of Ni-bearing pyrrhotite, and identified the serpentine polymorph as lizardite, in agreement with the reported aqueous alteration history of Ryugu. Furthermore, we have mapped the d-spacings of smectite and observed a broad distribution of values, ranging from 1 to 2 nm, with an average d-spacing of 1.24 nm, indicating significant heterogeneity within the sample. Such d-spacing variability could be the result of either the presence of organic matter trapped in the interlayers or the influence of various geochemical conditions at the submicrometer scale, suggestive of a range of organic compounds and/or changes in smectite crystal chemistry

A dehydrated space-weathered skin cloaking the hydrated interior of Ryugu

Without a protective atmosphere, space-exposed surfaces of airless Solar System bodies gradually experience an alteration in composition, structure and optical properties through a collective process called space weathering. The return of samples from near-Earth asteroid (162173) Ryugu by Hayabusa2 provides the first opportunity for laboratory study of space-weathering signatures on the most abundant type of inner solar system body: a C-type asteroid, composed of materials largely unchanged since the formation of the Solar System. Weathered Ryugu grains show areas of surface amorphization and partial melting of phyllosilicates, in which reduction from Fe3+ to Fe2+ and dehydration developed. Space weathering probably contributed to dehydration by dehydroxylation of Ryugu surface phyllosilicates that had already lost interlayer water molecules and to weakening of the 2.7 µm hydroxyl (–OH) band in reflectance spectra. For C-type asteroids in general, this indicates that a weak 2.7 µm band can signify space-weathering-induced surface dehydration, rather than bulk volatile loss

An empirical wake model accounting for velocity deficit and turbulence intensity in a simple tidal park

International audienceThis study presents a low computational model developed to estimate the velocity deficit and turbulence intensity in the wake of full-scale tidal turbine. The proposed model is applied in an ideal channel with hydrodynamics similar to the Alderney Race consisting of 10 turbines with realistic configuration. The model provides reasonable results compared to the numerical data, especially in the far wake region. However, the appearing variationin the velocity deficit model may be attributed to the use of ambient turbulence on the farm.Cette étude présente le développement d’un modèle à bas coût de calcul pour estimer le déficit de vélocité et l’intensité de la turbulence dans le sillage d’une turbine hydrolienne en grandeur réelle. Le modèle proposé est appliqué dans un canal idéal dont les conditions hydrodynamiques sont similaires à celle du Raz Blanchard. Une configuration de ferme pilote de 10 turbines est considérée. Le modèle fournit des résultats raisonnables comparés aux données obtenues avec un modèle numériques plus complet, en particulier dans la région du sillage lointain. Cependant, une variation apparaît dans le résultat du modèle en déficit de vitesse qui peut être attribuée à l’utilisation de la turbulence ambiante de la ferme

Numerical investigation of the local impact of hydrokinetic turbine on sediment transport—Comparison between two actuator models

International audienceHydrokinetic turbines interact with the dynamics of the sedimentary bottom at small and large scale. Despite the interest that the study of these interactions deserves, little research has been published in the field. In this paper, we investigate by numerical simulation the interaction between modeled hydrokinetic turbines and bed load under clear-water scour conditions. The mixture of water and sediment is accounted for by an Eulerian multiphase model developed in the open source platform OpenFOAM. The turbine blades are parameterized by two models: the Blade Element Method (BEM) and the Actuator Disc Theory (AD). A good agreement with measurements is obtained in the near wake. The effects of the two different turbine models on the bedload sediment transport and on the wake characteristics are then examined. The local impact of the turbine modeled by BEM is more pronounced on the bed than the one modeled by the AD. So, the BEM modeling is preferable to the AD one for local studies of the impact of a turbine on the bed morphology. However, combining BEM with the two-phase fluid-sediment is time consuming and make difficult the use of this method to study a farm of several stream turbines. In such a case the use of AD is still remains recommended for now. Moreover, more studies must be done at real scale as the turbine rotational speed is relatively low and could induced less impact of the swirl on the bottom shear stress

On the Usefulness of the Proper Orthogonal Decomposition on the Description of the Highly Concentrated Sediment Release Phenomena Resulting from a Two-Phase Solid-Fluid Simulation: Effect of the Ambient Current

Proper orthogonal decomposition (POD) is used to examine the release of highly concentrated water–sediment mixture in water, with or without ambient current. This technique allows us to extract the dominant features in spatio-temporal data sets and the POD total energies associated to the base parameter of the decomposition. Both one-component and two-component POD techniques are, respectively, applied on data relative to the solid volume fraction and on the solid volume fraction velocities. The analysis is based on an experiment in the literature and data sets provided by a two-phase flow solid-fluid numerical simulation. For release phenomenon without ambient current, the analysis of the POD results highlights that the impact of the particle diameter on the solid phase dynamics and the particle dispersion is modest during the falling time, but that it becomes preponderant during the formation of a turbidity current. Aided by POD, the impact of the ambient current is studied for a given particle diameter. As the ambient current becomes strong, we can observe the effect of the resistance of the bottom against the water–sediment mixture transport. According to the strength of the ambient current, the POD results show that the dynamics of the release phenomenon have two different regimes on either side of a clearly identified threshold value

A Laboratory Study of the Shallow Flow Field in a Vegetated Compound Channel

The significant defensive role of vegetation in general and mangroves in particular for coastal and estuarine regions has been increasingly recognized. However, understanding the shallow flow field in and around the region of vegetation is still limited. In order to gain more insight, a laboratory experiment of a vegetated compound channel was conducted. The emerged circular cylinders are a representative model for the emergent mangrove forest. Scenarios of different widths and densities of vegetation were considered. Data acquired from Acoustic Doppler Velocimetry (ADV) and force sensors have been analysed. The influences of vegetation on the shallow flow field were clarified by comparing different scenarios with and without vegetation. Furthermore, the results confirm the presence of the large horizontal coherent structures (LHCSs). The large coherent structures formed in the mixing layer at the vegetation interface emerge, promoting the transverse momentum exchange toward the forests. The LHCSs has not only boosted flow penetration into the vegetated floodplain area but also strongly disturbed the flow inside the forest. As the LHCSs move, they cause the fluctuation of the force on the cylinders. The fluctuations are largest at the vegetation edge. Negative values of stream wise forces were also recorded.</p