Explosions at the water surface

International audienceWe study the shape and dynamics of cavities created by the explosion of firecrackers at the surface of a large pool of water. Without confinement, the explosion generates a hemispherical air cavity which grows, reaches a maximum size and collapses in a generic w-shape to form a final central jet. When a rigid open tube confines the firecracker, the explosion produces a cylindrical cavity that expands without ever escaping the free end of the tube. We discuss a potential flow model, which captures most of these feature

Wave drag on a submerged sphere

International audienceWe measure the wave drag acting on fully submerged spheres as a function of their depth and velocity, with an apparatus that measures only the component of the drag due to the proximity of the free surface. We observe that close to the surface the wave drag is of the order of the hydrodynamic drag. In our range of study, the measured force is more than one order smaller than predictions based on linear response. In order to investigate this discrepancy, we measure the amplitude of the waves at the origin of the wave drag, comparing the measurement with a theoretical model. The model captures the measurements at "large depth" but the wave's amplitude saturates at "small depth," an effect that partially accounts for the difference between the predicted and measured wave drag. (C) 2015 AIP Publishing LLC

Indiscrétions aux interfaces

We investigate several phenomena at the surface of water, that are either traces of an object, or of a former event. First, we study the cavities formed by explosions at the surface of water. In a large bath, the cavities grow with a hemispherical shape, and then collapse anisotropically, what creates a vertical jet. This evolution results of an energy balance between kinetic and potential energy. Confining the cavities in vertical opened tubes we observe that the cavities never escapes. In a second part, we study the impact of vortex rings on surfaces. We observe that the vortex diameter is doubled in the interaction we the free surface of water, while it expands a lot more in ethanol. This difference comes from the fact that the free surface of water is equivalent to a rigid surface for the ring, what enhances the formation of secondary vortices. The third part deals with the wave drag of submerged spheres. We observe that there is a large discrepancy between the predicted and measured force, in the range where it is large. We measure the waves amplitude and show that taking into account their saturation can correct the theoretical estimation.Un corps qui se déplace dans l'eau y laisse des empreintes qui deviennent visibles s'il se rapproche de la surface. Dans cette thèse, nous étudions expérimentalement et théoriquement trois types différents d'empreintes, les "explosives", les "tourbillonnantes" et les "ondulantes". Les premières sont créées par des explosions à la surface de l'eau et engendrent des cavités dont on étudie la taille maximale et à la dynamique. Contrairement aux cavités d'impact, ces cavités explosives se caractérisent par une ouverture isotrope (cavité hémisphérique) et une fermeture anisotrope qui donne lieu à la formation d'un jet central. Si l'explosion est confinée par un cylindre ouvert de "petit" diamètre, on montre que la cavité se développe dans le cylindre mais n'en sort jamais, quelle que soit l'énergie de l'explosion. Pour les empreintes "tourbillonnantes", on étudie l'impact d'un tourbillon toroïdal sur une surface libre avec une attention particulière pour la taille maximale de la région perturbée et la durée de l'interaction. On montre que la nature chimique de l'interface joue un rôle crucial sur l'interaction: dans l'eau, celle-ci engendre des tourbillons secondaires qui limitent à deux diamètres la taille de la zone perturbée. Ces tourbillons sont supprimés dans l'éthanol et l'interaction s'y poursuit sur des dizaines de diamètres. La troisième partie "ondulante" porte sur la trainée de vague de sphères immergées. Expérimentalement, nous observons un fort écart entre les modèles théoriques classiques et la mesure, dans la gamme de profondeur et de vitesse pour laquelle celle-ci est importante. En étudiant le sillage de surface formé par les sphères, on montre que la saturation en amplitude de celui-ci est à l'origine de cet écart

Wave drag on a submerged sphere

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The anatomy of a crease, from folding to ironing

International audienceIn this paper we explore experimentally the folding and unfolding of creases of thin sheets of polymer materials and corresponding weaved fabric with a multiscale structure. A method to form reproducible folds is first proposed, which allows extraction of the fold's properties - in particular its maximum curvature - versus the material and sheet properties, in full agreement with an elasto-plastic model of the folding process. Then we explore the free unfolding of a fold and demonstrate a time-dependent logarithmic reopening, interpreted in terms of a plastic creep of the material under the stored elastic stress. Finally we consider the effect of humidity on this unfolding process. We demonstrate that - in strong contrast to a bulk sheet - the weaved fabric made of the same material exhibits a strong humidity dependence, with humidity speeding up the logarithmic unfolding process. This phenomenon, which exhibits a close analogy to the previously observed humidity induced ageing of granular material properties, points to the observed key role of hygrometry in the unfolding of multiscale materials, as observed during ironing. © The Royal Society of Chemistry 2012

Classification of 101 BRCA1 and BRCA2 variants of uncertain significance by cosegregation study: A powerful approach

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Classification of 101 BRCA1 and BRCA2 variants of uncertain significance by cosegregation study: A powerful approach

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