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Brull S.,Bordeaux Polytechnic Institute | Pavan V.,CNRS IUSTI – University Institute of Thermodynamic Industrial Systems) | Schneider J.,University of Toulon
European Journal of Mechanics, B/Fluids | Year: 2012

The aim of this article is to construct a BGK operator for gas mixtures starting from the true NavierStokes equations. That is the ones with transport coefficients given by the hydrodynamic limit of the Boltzmann equation(s). Here the same hydrodynamic limit is obtained by introducing relaxation coefficients on certain moments of the distribution functions. Next the whole model is set by using entropy minimization under moment constraints as in Brull and Schneider (2008, 2009) [23,24]. In our case the BGK operator allows to recover the exact Fick and Newton laws and satisfy the classical properties of the Boltzmann equations for inert gas mixtures. © 2012 Elsevier Masson SAS. All rights reserved.

Magnin M.,French Institute of Health and Medical Research | Magnin M.,niversite Lyon 1 | Rey M.,Center Hospitalier University Timone | Bastuji H.,French Institute of Health and Medical Research | And 5 more authors.
Proceedings of the National Academy of Sciences of the United States of America | Year: 2010

Thalamic and cortical activities are assumed to be time-locked throughout all vigilance states. Using simultaneous intracortical and intrathalamic recordings, we demonstrate here that the thalamic deactivation occurring at sleep onsetmost often precedes that of the cortex by several minutes, whereas reactivation of both structures during awakening is synchronized. Delays between thalamus and cortex deactivations can vary from one subject to another when a similar cortical region is considered. In addition, heterogeneity in activity levels throughout the cortical mantle is larger than previously thought during the descent into sleep. Thus, asynchronous thalamocortical deactivation while falling asleep probably explains the production of hypnagogic hallucinations by a still-activated cortex and the common self-overestimation of the time needed to fall asleep.

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.

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.

Nott P.R.,Indian Institute of Science | Guazzelli E.,CNRS IUSTI – University Institute of Thermodynamic Industrial Systems) | Pouliquen O.,CNRS IUSTI – University Institute of Thermodynamic Industrial Systems)
Physics of Fluids | Year: 2011

This paper addresses a fundamental discrepancy between the suspension balance model and other two-phase flow formulations. The former was proposed to capture the shear-induced migration of particles in Stokesian suspensions, and hinges on the presence of a particle phase stress to drive particle migration. This stress is taken to be the "particle stress," defined as the particle contribution to the suspension stress. On the other hand, the two-phase flow equations derived in several studies show only a force acting on the particle phase, but no stress. We show that the identification of the particle phase stress with the particle contribution to the suspension stress in the suspension balance model is incorrect, but there exists a well-defined particle phase stress. Following the rigorous method of volume averaging, we show that the force on the particle phase may be written as the sum of an interphase drag and the divergence of the particle phase stress. We derive exact micromechanical relations for these quantities. We also comment on the interpretations and results of previous studies that are based on the identification of the particle phase stress with the particle contribution to the suspension stress. © 2011 American Institute of Physics.

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.

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.

Souverein L.J.,CNRS IUSTI – University Institute of Thermodynamic Industrial Systems) | Debieve J.-F.,CNRS IUSTI – University Institute of Thermodynamic Industrial Systems)
Experiments in Fluids | Year: 2010

The effect of upstream injection by means of continuous air jet vortex generators (AJVGs) on a shock wave turbulent boundary layer interaction is experimentally investigated. The baseline interaction is of the impinging type, with a flow deflection angle of 9.5° and a Mach number M e = 2.3. Considered are the effects of the AJVGs on the upstream boundary layer flow topology and on the spatial and dynamical characteristics of the interaction. To this aim, Stereoscopic Particle Image Velocimetry has been employed, in addition to hot-wire anemometry (HWA) for the investigation of the unsteady characteristics of the reflected shock. The AJVGs cause a reduction of the separation bubble length and height. In addition, the energetic frequency range of the reflected shock is increased by approximately 50%, which is in qualitative agreement with the smaller separation bubble size. © 2010 The Author(s).

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.

Pavan V.,CNRS IUSTI – University Institute of Thermodynamic Industrial Systems)
Journal of Statistical Physics | Year: 2011

In this paper we extend the general construction of entropic approximation for kinetic operators modelling canonical systems. More precisely, this paper aims at pursuing to thermalized systems the works of Levermore, Schneider and Junk on moments problems relying on entropy minimization in order to construct BGK approximations and moments based equations. © 2011 Springer Science+Business Media, LLC.

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