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Andreotti B.,University Paris Diderot | Claudin P.,University Paris Diderot | Pouliquen O.,CNRS IUSTI - University Institute of Thermodynamic Industrial Systems)
Geomorphology | Year: 2010

The wavelength at which a dune pattern emerges from a flat sand bed is controlled by the sediment transport saturation length, which is the length needed for the sand flux to adapt to a change in wind strength. The influence of the wind shear velocity on this saturation length and on the subsequent dune initial wavelength has remained controversial. In this paper, we present direct measurements of the saturation length performed in a wind tunnel experiment. Complementary to this, initial dune wavelengths are measured under different wind conditions - in particular after storms. Using the linear stability analysis of dune formation, it is then possible to deduce the saturation length from field data. Both direct and indirect measurements agree that the saturation length is almost independent of the wind strength. This result supports the idea that grain inertia is the dominant dynamical mechanism limiting sediment transport saturation on dunes. © 2010 Elsevier B.V.

Guazzelli E.,CNRS IUSTI - University Institute of Thermodynamic Industrial Systems) | Hinch J.,University of Cambridge
Annual Review of Fluid Mechanics | Year: 2011

This review concentrates on the fluctuations of the velocities of sedimenting spheres, and on the structural instability of a suspension of settling fibers. For many years, theoretical estimates and numerical simulations predicted the fluctuations of the velocities of spheres to increase with the size of the container, whereas experiments found no such variation. Two ideas have increased our understanding. First, the correlation length of the velocity fluctuations was found experimentally to be 20 interparticle separations. Second, in dilute suspensions, a vertical variation in the concentration due to the spreading of the front with the clear fluid can inhibit the velocity fluctuations. In a very dilute regime, a homogeneous suspension of fibers suffers a spontaneous instability in which fast descending fiber-rich columns are separated by rising fiber-sparse columns. In a semidilute regime, the settling is hindered, more so than for spheres. © 2011 by Annual Reviews. All rights reserved.

Tlili O.,University of Monastir | Mhiri H.,University of Monastir | Bournot P.,CNRS IUSTI - University Institute of Thermodynamic Industrial Systems)
International Journal of Thermal Sciences | Year: 2015

A computational study of fire induced airflow in an enclosure is presented. The fire is modeled by a volumetric heat source centrally located in a rectangular room with a door which provides natural ventilation. The first part of this work aims at evaluating the performance of three turbulence models to predict the airflow pattern inside the fire room. Numerical results are validated with available experimental data and it is concluded that the standard k-ε turbulence model coupled with an improved treatment of the wall functions namely the 'enhanced-wall treatment' gives the best compromise between accuracy of results and computation time. This latter is therefore applied in the second part of this work which includes an analysis of the impact of the roof shape on the hot gases evacuation process for several heat source locations inside the room. Two typical roof shapes are tested, a pyramidal roof and a domed one. Comparison of these results with those obtained with the basic model (flat roof) allowed the assessment of the major influence of the two critical parameters studied on the temperature and velocity distributions inside the fire room and the calculation of the design parameters such as the neutral plane height and mass flow rates at the opening for each fire scenario. © 2014 Elsevier Masson SAS. All rights reserved.

Druguet M.-C.,CNRS IUSTI - University Institute of Thermodynamic Industrial Systems)
Shock Waves | Year: 2010

This paper presents the methodology and the physical models implemented in our computational code to predict the flow field over a Mars Sample Return Orbiter entering a Mars-like atmosphere. Because no validation against flight data or experiments can be done at present, the results obtained here are compared to other computed results for verification purpose. The predicted flow fields show that the chemical species CO and CO2, that are highly radiative species, are present in a large amount and at high temperature in the shock layer and in the wake of the flow past the orbiter. © 2010 Springer-Verlag.

Forterre Y.,CNRS IUSTI - University Institute of Thermodynamic Industrial Systems)
Journal of Experimental Botany | Year: 2013

The ability of plants to move is central to many physiological processes from development to tropisms, from nutrition to reproduction. The movement of plants or plant parts occurs over a wide range of sizes and time scales. This review summarizes the main physical mechanisms plants use to achieve motility, highlighting recent work at the frontier of biology and physics on rapid movements. Emphasis is given to presenting in a single framework pioneering biological studies of water transport and growth with more recent physics research on poroelasticity and mechanical instabilities. First, the basic osmotic and hydration/dehydration motors are described that contribute to movement by growth and reversible swelling/shrinking of cells and tissues. The speeds of these water-driven movements are shown to be ultimately limited by the transport of water through the plant body. Some plant structures overcome this hydraulic limit to achieve much faster movement by using a mechanical instability. The principle is to impose an 'energy barrier' to the system, which can originate from geometrical constraint or matter cohesion, allowing elastic potential energy to be stored until the barrier is overcome, then rapidly transformed into kinetic energy. Three of these rapid motion mechanisms have been elucidated recently and are described here: the snapping traps of two carnivorous plants, the Venus flytrap and Utricularia, and the catapult of fern sporangia. Finally, movement mechanisms are reconsidered in the context of the timescale of important physiological processes at the cellular and molecular level. © The Author 2013.

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