Contrôle par rotation ou par aspiration de l'écoulement autour d'un cylindre calculé par Simulation des Grandes Échelles

Control of the flow around a circular cylinder is studied using Large Eddy Simulation. The influence of control by rotation and suction on the flow characteristics is considered for several Reynolds numbers. Comparisons with experiments were conducted at $\mathit{Re}={10}^5$ for the flow with and without control. A drag reduction up to 30% is obtained for an usual suction intensity. To cite this article: G. Fournier et al., C. R. Mecanique 333 (2005).     

Changements de structure provoqués par l'écoulement

Résumé De même que la structure conditionne les propriétés rhéologiques, l'état d'écoulement d'un système peut être un facteur déterminant de sa structure par suite des réarrangements moléculaires qu'il provoque. On étudie le cas d'un liquide pur à fortes interactions intermoléculaires et celui des suspensions colloïdales et solutions macromoléculaires. Les calculs sont développés dans des cas types où l'écoulement entraîne des changements de phase ou des variations de l'état d'agrégation. L'étude rhéologique ou rhéooptique de tels systèmes permet la détermination expérimentale des énergies d'interaction intermoléculaires. On en donne des exemples.

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Zusammenfassung Ebenso wie die Struktur die Theologischen Eigenschaften beeinflußt, kann der Fließzustand eines Systems ein maßgebender Faktor seiner Struktur als Folge der molekularen Umordnung, die er erzeugt, sein. Es wird der Fall einer reinen Flüssigkeit mit starken intermolekularen Wechselwirkungen sowie der kolloiden Suspensionen und makromolekularen Lösungen untersucht. Die Rechnungen sind für typische Fälle durchgeführt, bei denen das Fließen Phasenänderungen und Änderungen des Aggregatzustandes hervorruft. Die rheologische und rheooptische Untersuchungen solcher Systeme erlaubt eine experimentelle Bestimmung der zwischenmolekularen Wechselwirkungsenergien, wie an Beispielen gezeigt wird.

     

Modélisation de l'effet de portance dans un écoulement à bulles

We propose here a lift coefficient modeling in a turbulent bubbly boundary layer under the assumption of bidimensional stationary and vertical upward fully developed flow. The basic frame of this approach is to consider that the volumetric flux of bubbles is composed of the convective flux by the liquid, the contribution by the gravitational field and those of the lift force and the bubble dispersion terms. A discussion of results completes this study.     

Contrôle passif d'écoulements incompressibles autour d'obstacles à l'aide de milieux poreux

Passive control of the flow behind a bluff-body is obtained by integrating porous area on the body. The penalisation method is used to modelize the flow in three different media. In fact each medium can be considered as a porous medium. The fluid is identified as a porous medium of infinite permeability and the solid is identified as a porous medium of zero permeability. This way, it is easy to compute the flow in each medium using the same parameter. Some benefical effects are due to the porous interface: the flow is smoothed, and the enstrophy and drag are significantly reduced.     

Approximation d'un contrôle optimal distribué par un circuit électronique réparti: application au contrôle de vibrations

This paper focuses on the design of distributed control related to distributed mechanical systems. Sensors and actuators are assumed to be numerous and periodically distributed. The problem adressed in this paper is: “Can we find a way to approximate an optimal control law with a distributed electronic circuit?” A solution to this problem is proposed in the framework of vibration control of a thin plate using piezoelectric actuators and sensors.     

Modélisation du frottement pariétal dans un écoulement à bulles

In the present study, we propose a wall friction modeling in a vertical upward bubbly flow under the assumptions of bidimensional stationary and fully developed flow. This approach is based on the two-fluid model and on the hypothesis that the averaged flow is parallel to the wall. The turbulence modeling is an essential part in this model. Comparison with experimental data (presently available) exhibits satisfactory agreement. Other experimental studies, especially at a high void fraction, are desirable to examine further the model validity.     

Écoulement turbulent dans une cavité rotor–stator fermée de grand rapport d'aspect

A direct numerical simulation is combined with laboratory study to describe the turbulent flow in an annular high speed rotor–stator cavity. Comparisons are made for a turbulent flow characterized by a Reynolds number $\mathit{Re}=\Omega R_2^2/\nu =9.5\times {10}^4$ in a shrouded cavity of large aspect ratio $G=(R_2?R_1)/h=18.32$, where $R_1$ and $R_2$ are the inner and outer radii of the rotating disk, and h is the inter-disk space. A close agreement is found between the computed results and the experimental data for the mean and turbulent fields.? To cite this article: S. Poncet, A. Randriamampianina, C. R. Mecanique 333 (2005).     

Inversion d'un écoulement plan de Stokes – application à l'écoulement à débit moyen nul entre deux plans parallèles

We present the interest and some characteristics of the inverse transformation of a 2D Stokes flow. This method is applied to the cellular flow between two parallel plates induced by a rotating cylinder to obtain the flow around two circular cylinders in contact placed in the centre of a rotating circular cylinder.     

Un opérateur de diffusion spatialement sélectif pour le transport d'un traceur passif ou actif par un écoulement de grande échelle

In a fluid flow, fields are measurable up to a cut-off scale at which they are regularized. We show that, for a smooth velocity field, this regularization adds to the advection equation a diffusive term proportional to the strain tensor. We study in two dimensions its effect on the dynamics of velocity and vorticity, and on the conservation of quadratic invariants. Vorticity and energy are still conserved, while enstrophy and tracer variance are dissipated depending on the flow topology. These properties (conservation, dissipation, spatial selectivity) suggest the use of this selective strain–diffusion operator for numerical simulations of inhomogeneous flows in the quasi-two-dimensional approximation.     

Vérification expérimentale de la relation de réciprocité d'Onsager pour l'électro-osmose et l'électro-filtration dans une argile naturelle

This Note is devoted to the experimental verification of the Onsager's reciprocal relations in the particular case of electro-osmosis and electro-filtration. A special set up has been designed to carry out the measurements of both the electro-osmotic permeability and the streaming potential. This has been performed by using a natural material i.e., saturated kaolinite. To cite this article: K. Beddiar et al., C. R. Mecanique 330 (2002) 893–898.     

Sur la nature de la transition à l'instationnaire d'un écoulement de convection naturelle en cavité différentiellement chauffée à grands écarts de température

Natural convection of air inside a rectangular cavity, differentially heated under large temperature gradients, is considered. The low Mach approximation equations are those obtained by Paolucci allowing for filtering of sound waves. Transition to unsteadiness is studied with numerical simulation, with a finite volume code based on a fractional time step method derived from projection methods used for incompressible flows. When the fluid physical properties are prescribed constants, transition to unsteadiness follows the classical scheme of a Hopf bifurcation. The transition is quite different when viscosity obeys Sutherland's law while the Prandtl number is kept constant. There is evidence of hysteresis, therefore the transition seems to be subcritical. In the vicinity of the transition, on the large amplitude branch, an intermittent solution is observed, with periodic bursts separating quasi-steady states.     

Impact non-pénétrant sur le thorax : une contribution à l'étude de la propagation des ondes

The stress wave generated by a nonpenetrating impact on the thorax is likely to cause severe injury to the lung. Theoretical studies are necessary to help the understanding of injury mechanisms. Within the framework of elastodynamics, we study the transmission of a wave at the interface between two weakly coupled semi-infinite media representing the thoracic wall and the lung. By using an appropriate method, we describe the distribution of energy carried by the shear and pressure waves in the medium representing the lung. These results should contribute to a better interpretation of the experimental results.     

Extension du modèle d'échelles mixtes à la diffusivité de sous-maille

In the large-eddy simulation frame for non-isothermal turbulent flow, the Mixed Scale Model is extended to the subgrid diffusivity, in order to dissociate the computation of subgrid viscosity and diffusivity. The identification of the subgrid thermal dissipation term in the subgrid flux transport equation leads to an algebraic expression of the subgrid diffusivity. This diffusive model, as the Smagorinsky one, is weighted by a model based on scale similarity. This expression leads to satisfactory results when applied to a buoyant turbulent flow in a differentially heated cavity.     

Homogénéisation d'équations et d'inéquations à paramètres oscillants. Application à l'étude d'écoulement de faible épaisseur en régime transitoire

Double scale homogenization is used to average stationary equation or inequalities in which both highly oscillating variables and parameters appear. We demonstrate how the limit is obtained using a two-stage procedure , firstly by carrying out a classical homogenization process by freezing the oscillating parameter, then by averaging the result with respect to this parameter. These results allow us to average the pressure for a newtonian fluid in a narrow gap between two rough unstationnary surfaces.     

Stabilité de l'écoulement de Hartmann chauffé par le basStability of the Hartmann flow heated from below

A numerical study is conducted in order to determine the influence of a vertical magnetic field, the Reynolds number and a temperature stratification on the instabilities occurring in the Hartmann flow heated from below. For $\mathit{Pr}=0.001$ and $\mathit{Ha}?2.5$, the results show that the vertical magnetic field has a stabilizing effect on both transverse oscillatory travelling waves $(T)$ and longitudinal stationary rolls $(L)$. The temperature stratification is responsible of a destabilization of the transverse $(T)$ modes and the appearance of longitudinal $(L)$ modes non-existent for the isothermal Hartmann flow. Moreover, the extent of the domains of Re where the transverse modes $(T)$ prevail is found to narrow when Ha increases and to widen when Ra increases for a given value of Ha. On the other hand, for the $(L)$ modes, the extent of the domains of Re where they prevail increases when Ha grows. To cite this article: W. Fakhfakh et al., C. R. Mecanique 334 (2006).     

Calcul approché de la trainée d'une bulle sphérique de gaz mise brusquement en ascension dans un liquide

An analysis is made for a gas bubble impulsively started to rise with a constant velocity in a quiescent liquid of infinite extent. Satisfactory results for the total drag force are obtained for spherical air bubbles in water, provided that the method developed here is applied at early times, the flow separation is negligible and the Reynolds number R_e is sufficiently large.     

Naissance de la convection dans une cavité poreuse 3D soumise à des vibrations de hautes fréquences

Analytical techniques are used to study the onset of convection in three-dimensional boxes of fluid saturated porous material heated from below and subjected to vertical high-frequency vibration. Increasing the vibration amplitude delays the onset of convection and may even create subcritical solutions so as generate different structures of flows.     

Effet d'une modulation magnétique sur le seuil d'instabilité convectif au sein d'une couche de liquide magnétique chauffée par le haut

The effect of a time-periodic magnetic field on the onset of convection in a horizontal magnetic fluid layer heated from above and bounded by isothermal nonmagnetic boundaries is investigated. We consider the case where the magnetic field obeys a periodic rectangular pulse. A first-order Galerkin method is performed to reduce the governing linear system to a parametric differential equation. Therefore, the Floquet theory is used to determine the convective threshold for the rigid–rigid and free–free cases. With an appropriate choice of the ratio of the magnetic and gravitational forces, we show the possibility to produce a competition between the harmonic and subharmonic modes at the onset of convection.