Polymer and surface roughness effects on the drag crisis for falling spheres

8 pagesWe make time resolved velocity measurements of steel spheres in free fall through liquid using a continuous ultrasound technique. We explore two different ways to induce large changes in drag on the spheres: 1) a small quantity of viscoelastic polymer added to water and 2) altering the surface of the sphere. Low concentration polymer solutions and/or a pattern of grooves in the sphere surface induce an early drag crisis, which may reduce drag by more than 50 percent compared to smooth spheres in pure water. On the other hand, random surface roughness and/or high concentration polymer solutions reduce drag progressively and suppress the drag crisis. We also present a qualititative argument which ties the drag reduction observed in low concentration polymer solutions to the Weissenberg number and normal stress difference

Deuterium adsorption on (and desorption from) SiC(0001)-(3×3), (√3×√3)R30°, (6√3×6√3)R30° and quasi-free standing graphene obtained by hydrogen intercalation

International audienceWe present a comparative high-resolution electron energy-loss spectroscopy study on the interaction of atomic hydrogen and deuterium with various reconstructions of SiC(0 0 0 1). We first show that on both the (3 × 3) and reconstructions, deuterium atoms only bind to silicon atoms, thereby confirming the silicon-rich appellation of these reconstructions. Deuterium passivation of the (3 × 3) is only reversible when exposed to atomic deuterium at a surface temperature of 700 K since tri- and dideuterides, necessary precursors for silicon etching, are not stable. On the other hand, we show that the deuteration of the is always reversible because precursors to silicon etching are scarce on the surface. Then, we demonstrate that hydrogen (deuterium) adsorption at 300 K on both the (buffer-layer) and the quasi-free-standing graphene occurs on carbon atoms justifying their carbon-rich appellation. Comparison of the deuterium binding in the intercalation layer of quasi-free-standing graphene with the deuterated surface provides some indication on the bonding structure at the substrate intercalation layer. Finally, by measuring C-H (C-D) vibrational frequencies and hydrogen (deuterium) desorption temperatures we suggest that partial sp2-to-sp3 rehybridization occurs for the carbon atoms of the buffer-layer because of the corrugation related to covalent bonding to the SiC substrate. In contrast, on quasi-free-standing graphene hydrogen (deuterium) atoms adsorb similarly to what is observed on graphite, i.e. without preferential sticking related to the underlying SiC substrate

Expert judgement methodology for failure anticipation in nuclear power plants

Risk analysis is a tool for investigating and reducing incertainty related to outcomes of future activities. We are interested here in failure anticipation in nuclear power plant. This involves very specific systems with little or no existing historical failures. In such cases, both engineering judgement and historical data are used to quantify uncertainty related to the predictions, like probabilities and failure rates. This paper is focussed on this aspect. The purpose is to provide an expert judgment elicitation methodology for anticipating the failures of a component, up to the end of its design life cycle period, including eventually an extension period

Modelling mechanical properties of spruce and Douglas fir timber by means of X-ray and grain angle measurements for strength grading purpose

This study proposes a model using data from a scanner (X-ray and grain angle measurements) to perform strength grading. The research also includes global measurements of modulus of elasticity (obtained by vibrations and ultrasound methods), static bending stiffness and bending strength of 805 boards of Douglas fir and 437 boards of spruce. This model can be used in an industrial context since it requires low computational time. The results of this study show that the developed model gives better results than the global non-destructive measurements of the elastic modulus commonly used in the industry. It also shows that this improvement is particularly higher in the case of Douglas fir than for spruce. The comparison has been made on both the quality of the mechanical properties assessment and on the improvement of the grading process according to the European standards by using different index.ANR ClaMe

Le numérique à l’école : évolution ou révolution pédagogique ?

Ce débat entre experts et praticiens fait apparaître des lignes de fracture et de force : fracture entre le temps long de l’école et l’immédiateté numérique de la jeunesse, tensions sur les mésusages technologiques, mais aussi travail d’équipe, développement de projets, enrichissement de la culture humaniste. Le numérique bouscule l’école, ses certitudes, son organisation, ses instruments. Son enseignement ne saurait, selon les participants, se réduire à celui d’une discipline, et requiert une approche transversale de la littératie numérique, associant usage responsable des instruments, connaissance des grand concepts du numérique, et développement d’une réflexion scientifique, éthique et philosophique.This debate between experts and practitioners reveals the main areas of discussion and divergence: the gap between the long timeframe of school and the digital immediacy of youth, tensions surrounding the misuse of technologies, but also teamwork, project development and an enriched culture of humanism. Digital technologies are disrupting school’s certainties, organisation and -equipment. According to the speakers, the teaching of digital technologies cannot be reduced to a single discipline and requires a multisectoral approach to digital literacy – one that combines the responsible use of equipment, knowledge of major digital concepts and the development of scientific, ethical and philosophical reflections.Este debate entre expertos y practicantes hace aparecer unas líneas de fractura : fractura entre el tiempo largo de la escuela y la inmediatez digital de la juventud, tensiones entre los malos usos tecnológicos, pero también trabajo colectivo, desarrollo de proyectos, enriquecimiento de la cultura humanista. Lo digital empuja a la escuela, sus certidumbres, su organización, sus instrumentos. Según los participantes, su enseñanza no podría reducirse al de una asignatura, y requiere una aproximación transversal de la alfabetización digital, que asocie uso responsable de los instrumentos, conocimiento de los grandes conceptos de lo digital y desarrollo de una reflexión científica, ética y filosófica

MRI investigation of granular interface rheology using a new cylinder shear apparatus

The rheology of granular materials near an interface is investigated through proton magnetic resonance imaging. A new cylinder shear apparatus has been inserted in the MRI device, which allows the control of the radial confining pressure exerted by the outer wall on the grains and the measurement of the torque on the inner shearing cylinder. A multi-layer velocimetry sequence has been developed for the simultaneous measurement of velocity profiles in different sample zones, while the measurement of the solid fraction profile is based on static imaging of the sample. This study describes the influence of the roughness of the shearing interface and of the transverse confining walls on the granular interface rheology

Rheophysics of dense granular materials : Discrete simulation of plane shear flows

We study the steady plane shear flow of a dense assembly of frictional, inelastic disks using discrete simulation and prescribing the pressure and the shear rate. We show that, in the limit of rigid grains, the shear state is determined by a single dimensionless number, called inertial number I, which describes the ratio of inertial to pressure forces. Small values of I correspond to the quasi-static regime of soil mechanics, while large values of I correspond to the collisional regime of the kinetic theory. Those shear states are homogeneous, and become intermittent in the quasi-static regime. When I increases in the intermediate regime, we measure an approximately linear decrease of the solid fraction from the maximum packing value, and an approximately linear increase of the effective friction coefficient from the static internal friction value. From those dilatancy and friction laws, we deduce the constitutive law for dense granular flows, with a plastic Coulomb term and a viscous Bagnold term. We also show that the relative velocity fluctuations follow a scaling law as a function of I. The mechanical characteristics of the grains (restitution, friction and elasticity) have a very small influence in this intermediate regime. Then, we explain how the friction law is related to the angular distribution of contact forces, and why the local frictional forces have a small contribution to the macroscopic friction. At the end, as an example of heterogeneous stress distribution, we describe the shear localization when gravity is added.Comment: 24 pages, 19 figure

Mechanical behaviour of pre-stressed spruce timber–timber 2.5-mm-step grooved connections under shearing tests

A smart shear connection system was tested in order to be used in manufactured elements of a lattice of wooden slats and a cross offset. This type of floor element can present advantages both in terms of weight and suitable insulating filler (Adalberth et al. 2001; Kawasaki and Kawai 2006; Kermani and Hairstans 2006; Dodoo et al. 2014). This connection is made to provide a substitute for a glue connection. The shear connection proposed is made by double-sided grooving timber interfaces to form a cross section. A specifically designed test assembly was constructed to measure and guarantee homogeneous contact pressure in interface test specimens. Shear test results are presented and compared in terms of capacity and stiffness with glue connections. The principal conclusions are as follows: the allowable shearing resistance of the grooved timber–timber joints can approximately reach the resistance of glued connections. The stiffness of the joints is improved by increasing the pre-stress applied for holding contact in the assembly. The grooved timber-to-timber joints exhibit non-linear behaviour which dominates the behaviour of the whole system. Therefore, the behaviour of a unit connection must be determined to obtain the mean load-carrying capacity and stiffness of a system with grooved connections

Recent Developments of an Opto-Electronic THz Spectrometer for High-Resolution Spectroscopy

A review is provided of sources and detectors that can be employed in the THz range before the description of an opto-electronic source of monochromatic THz radiation. The realized spectrometer has been applied to gas phase spectroscopy. Air-broadening coefficients of HCN are determined and the insensitivity of this technique to aerosols is demonstrated by the analysis of cigarette smoke. A multiple pass sample cell has been used to obtain a sensitivity improvement allowing transitions of the volatile organic compounds to be observed. A solution to the frequency metrology is presented and promises to yield accurate molecular line center measurements