214 research outputs found
Numerical Study of the Effect of Freestream Turbulence on by-pass Transition in a Boundary Layer
No full textWe use direct numerical simulations in the presence of free-stream turbulence having different values of intensity, T u, and integral length scale, L, in order to determine which kind of structures are involved in the path to transition of a boundary-layer flow. The main aim is to determine under which conditions the path to transition involves structures similar to the linear or non-linear optimal perturbations. For high values of T u and L, we observe a large-amplitude path to transition characterized by localized vortical structures and patches of high- and low-momentum fluctuations. Such a scenario is found to correlate well with the L and hairpin structures resulting from the time evolution of non-linear optimal perturbations, whereas, for lower T u and L, a larger correlation is found with respect to linear optimal disturbances. This indicates that a large-amplitude path to transition exists, different from the one characterized by elongated streaks undergoing secondary instability. To distinguish between the two transition scenarios, a simple parameter linked to the streamwise localisation of high- and low-momentum zones is introduced. Finally, an accurate law to predict the transition location is provided, taking into account both T u and L, valid for both the transition scenarios
Stability of the flow over superhydrophobic micro roughnesses: The influence of the interface
No full textSuperhydrophobic surfaces are known for their drag reduction properties. However, the interface between the lubricant and the overlying flow may easily become unstable, leading to the depletion of the superhydrophobic layer and to a consequent drag increase. In this paper, we investigate the modal and non-modal instability of the flow over longitudinal trapezoidal superhydrophobic riblets, including, for the first time, the gas/liquid interface dynamics in the stability analyses. A two-dimensional stability problem, obtained with a domain transform technique and interface modelling using a linearised Young–Laplace equation, is coupled with the n−periodic stability framework introduced by Schmid et al. (2017). The latter technique, using a Bloch wave formalism, allows the computation of the stability of an array of n riblet units of given periodicity at a reasonable numerical cost. For small periodicities, the most unstable mode is a fundamental instability stemming from the three-dimensionalisation of a Tollmien–Schlichting wave. Conversely, in the case of large riblet periodicities, a subharmonic mode linked to capillarity effects becomes the most unstable. Nonmodal transient growth analysis shows that the superhydrophobic riblets have a weak effect on the overall growth. However, riblets having small periodicities induce a slight stabilisation of the flow, while large ones induce an increase of the energy growth on detuned perturbations. The resulting energy growth mechanism induces interface deformations encompassing more than one subunit
Les marqueurs de prolifération en oncologie vétérinaire : applications à l'étude pronostique du mastocytome cutané canin
No full textLe mastocytome cutané représente un type tumoral dominant de la cancérologie du chien mais demeure l'une des tumeurs canines au comportement clinique les plus imprévisibles. L'auteur évalue dans une étude rétrospective portant sur 120 chiens porteurs de mastocytome cutané, la valeur pronostique de la détection immunohistochimique des marqueurs de prolifération PCNA et Ki-67. Il montre que la détermination de l'expression de l'antigène Ki-67 permet de prédire avec une grande fiabilité l'évolution clinique post-chirurgicale de ce type tumoral
Un modèle rhéologique pour les argiles gonflantes
No full textAprès avoir rappelé les mécanismes physico-chimiques d< déformation des argiles saturées à l'échelle de la microstructure en condition isotherme, nous proposons un modèle élasto-plastique pour les argiles gonflantes. La formulation considère que pour les états de contrainte normalement consolidés, les déformations irréversibles proviennent essentiellement de la déformation des macro porosités, et sont traduites par la plasticité associative. En revanche pour les états de contrainte surconsolidés, des contraintes internes de répulsion non équilibrées prennent naissance au niveau des micro-porosités et provoquent la dilatance irréversible des macro-porosités. Le gonflement macroscopique induit par ce dernier mécanisme est formulé par un deuxième mécanisme plastique à écrouissage combiné, isotrope et cinématique. Le modèle rhéologique est formulé dans le cadre de la plasticité à écrouissage mécanique positif pour les états de contrainte normalement consolidés et à écrouissage « physico-chimique » pour les états de contrainte surconsolidés. Le modèle permet de représenter les caractéristiques fondamentales du comportement des argiles gonflantes saturées notamment l'existence d'une boucle d'hystérésis très marquée sur des chemins œdométriques cycliques et la présence d'une déformation volumique de contractance sur les chemins triaxiaux pour une argile gonflante surconsolidée. Le modèle comporte huit paramètres rhéologiques qui sont obtenus aisément i partir d'essais œdométriques et triaxaux. Enfin l'article présente des simulations sur différents chemins de sollicitation cycliques et les compare aux résultats expérimentaux réalisés sur l'argile de Boom. On constate que la prédiction du calcul est assez proche des résultats expérimentaux, ce qui confirme la possibilité de reproduire les caractéristiques les plus importantes du comportement des argiles gonflantes
Ploucquet, Gottfried
No full textGottried Plouqcuet is known as a precursor of symbolic logic and as a philosopher connected to occasionalism. Born in Stuttgart on 25 August 1716, he studied there at the Ducal Gymnasium. In 1732 he was granted a scholarship to study at the theological seminary of the University of Tübingen. While studying Wolff’s mathematical writings Ploucquet found his way to philosophy. During his whole life, he never separated philosophy from mathematics. Having come to philosophy through the foyer of mathematics, he suffered when the sober form of a system became the victim of a frivolous cover, and he feared that in this way the limits of genuine philosophy were displaced. Israel Gottlieb Canz (1690-1753) instructed him in philosophy and Christoph Matthäus Pfaff (1686-1760) in theology. In 1734 he became a magister of philosophy and in 1740 doctor of theology. In 1747 he won the prize-question on the monads of the Royal Prussian Academy of Sciences in Berlin and in 1749 he was voted external member of that academy, after which came the call to the chair of logic and metaphysics at the University of Tübingen, a position he kept for the rest of his life, with the exception of the Summer Term of 1778, in which he taught at the Military Academy in Stuttgart, where he had among his students also the young Friedrich Schiller. Ploucquet was well acquainted with ancient philosophers, whom he read in the original texts and whose positions he compared with those of the modern, whom he knew just as well. He often referred to older systems, such as those by Leibniz and Wolff, and took much from Descartes, Malebranche, and Locke, while taking critical stances against Robinet, Helvetius, and Kant. When Kant in his Einzigen möglichen Beweisgrund für das Daseyn Gottes of 1763 dealt with the notion of an absolute existence, Ploucquet checked Kant’s work for fallacies and defended against Kant’s criticism the proof based on the idea of a contingent world. Ploucquet died in Tübingen on 13 September 1790, after an ictus he received in 1781 had left him paralyzed. He kept nonetheless absolving at least one of his professorial duties until the end: he dictated the logical and metaphysical theses submitted to the students that took the Magister exam in philosophy at the University of Tübingen. This detail is not without relevance, for when Friedrich Wilhelm Hölderlin and Georg Friedrich Wilhelm Hegel were examined in Tübingen in 1790, they answered questions that had been formulated by Ploucquet. Moreover, it has been shown that Johann Friedrich Flatt and Christoph Bardili, who both taught logic and metaphysics to Hölderlin and Hegel (Flatt as an extraordinary professor at the university, Bardili as a Repetent in the Stift), used for their courses the last edition of Ploucquet’s logic and metaphysics textbook
Numerical simulation of transonic buffet over an airfoil
No full textA zonal detached eddy simulation (DES) method is presented that predicts the buffet phenomenon on a supercritical airfoil at conditions very near shock buffet onset. Some issues concerning grid generation, as well as the use of DES for thin-layer separation, are discussed. The periodic motion of the shock is well reproduced by averaged Navier–Stokes equations (URANS) and zonal DES, but the URANS calculation has needed to increase the angle of attack compared to the experimental value and the standard DES failed to reproduce the self-sustained motion in the present calculation. The main features, including spectral analysis, compare favorably with experimental measurements (Jacquin, L., Molton, P., Deck, S., Maury, B., and Soulevant, D., “An Experimental Study of Shock Oscillation over a Transonic Supercritical Profile,” AIAA Paper 2005-4902, June 2005). A very simple model based on propagation velocities yields the main frequency of the motion. As suggested by Lee (Lee, B. H. K., “Transonic Buffet on a Supercritical Airfoil,” Aeronautical Journal, May 1990, pp. 143–152), this calculation highlights that upstream propagating waves are generated by the impingment of large-scale structures on the upper surface of the airfoil in the vicinity of the trailing edge. These upstream propagating waves can regenerate an instability leading to a feedback mechanism
Diffuse transport in clay media: µm to nm scale characterization of pore space and mineral spatial organization
No full textIn the framework of radioactive waste repository, clayrock formations are foreseen as barrier materials due to their diffusion properties. In clay materials, the dominant transport mode is diffusive and depends mainly on various parameters such as the mobility of the species in water, the accessible porosity, the pore space geometry and the retardation as a result of reactions such as sorption or ion exchange (Tournassat and Appelo, 2011). In this way, the European CATCLAY project (EURATOM FP7), in the context with research on transport in porous materials, was proposed to describe the cation migration processes in natural clayrocks. The project is structured along 3 RTD workpackages, combining modeling and experimental studies from a simpler, analogous system (monophasic compacted clay system) to clayrocks (Callovo-Oxfordian argillites, Opalinus Clay and Boom Clay). Part of this experimental studies focuses on small scale structure (µm - nm) property of rocks in order to determine how the spatial distribution of mineral and pores at small scales can influence diffusion driven transport of sorbing cations. The present study focuses on compacted illite properties (simpler analogous system) in hopes to extent this study to the natural clayrock formation. Illite was chosen by the way that is the main constituent of clayrock. Compacted illite material represents thus an analogy with the clay matrix constituting clay-rocks. Our approach is mainly based on imaging the small scale structural organization of compacted illite material and analyzing the obtained images in order to extract information on pore space and mineral spatial distribution. Techniques for imaging the texture of illite material like water saturated, in compacted state, were first developed. The first step was to improve classic resin impregnation method in order to preserve the texture without losing the clay confinement and modifying the pore space geometry. This has been done by taking into account the molecule size of the monomer, the low viscosity, the dipole moment (adapted for the clayrock with swelling clay content) and the controlled time polymerization. MMA monomer proved to be the most suitable resin in our study. The small scale structure of impregnated sample was then imaged in 2D using Transmission Electron Microscopy (TEM) and in 3D using Focused Ion Beam coupled to Scanning Electron Microscopy (FIB/SEM). For TEM observations, a set of ultra-thin serial sections (50 - 100 nm) were cut using a microtome. A set of 2D images were then acquired using a resolution ranged between 100 nm and 10 Å. TEM images clearly show us the multi-scale organization of clay materials (Figure 1 and 2); we observe the 10 Å spacing sheets constituting the illite particles, nanometer size illite clay particles more or less aggregated and the surrounding pores having a size ranging from few hundred nanometers to nanometer. FIB/SEM analysis is currently in progress. From FIB/SEM, a set of serial images can be acquired using the "slice and view" method (Keller et al., 2011). Then, 2D FIB/SEM images need to be aligned to reconstruct a 3D volume. Image resolution is limited to 10-20 nm. Both methodologies (FIB-tomography and TEM techniques) are thus complementary method for the up-scaling characterization of the structural organization of compacted clayey materials. TEM images analysis allow to scale down the resolution size since only a part of the pore space could thus be imaged with FIB/SEM method (Keller et al., 2011). Viewing and performing a qualitative description of images constitute a major result and can help us to better understand how the transfer pathways and retention sites are organized in the porous media. Thanks to image analysis method, pores and minerals can be thresholded from grey level TEM and FIB/SEM images. Quantitative parameters can be then computed based from segmented images. In this objective, we currently focus our analysis in order to determine the size and the morphology of pores, the main geometrical features of clay particles (number of layers, size, shape...), the spatial distribution of clay particles (individual/aggregates, type of contact between the clay particles, orientation...) and the pores connectivity. Quantitative parameters are expected to be used in various transfer modeling approaches. This will be done in the framework of SIMISOL project which is focused on the modeling cation diffusion from atomic to nanometer scales
Characterizing soil macroporosity by X-ray microfocus computed tomography and quantification of the coring damages.
No full textX-ray Computed Tomography (X-ray µCT) was employed to characterize vertical
variations of structural porosity of a soil profile (pore dimension higher than
5.103 µm3). Three distinct horizons of a Cambisol have
been studied for a total depth of 75 cm: L, S1/S2 and
S2/SFe horizons. Samples have been cored in situ by
driving in PVC tubes (inner diameter 10 cm). From reconstructed and filtered
volumes, pores segmentation allows to study variations of structural porosity
within the profile. Two kinds of porosity were identified: biological pores
(tube-like) and physical pores (fracture-like). Structural porosity content
varies strongly according to the horizons: from 5.48% in the L horizon to 6.48%
in the S1/S2 horizon. The 3D connectivity of both of these
pore types was also assessed. During sampling, soil shearing induced damages
around the cores. Identification and quantification of the damaged zone was
performed from the calculation of porosity profile from core surface to core
heart. In average, the damaged zone reaches a depth of 1 cm. Porosity loss
(compaction) or porosity increase (fracturing) was observed according to the
studied profile
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