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).     

Condition d'explosion par impact de détonation

We study the induction of an explosion in a homogeneous substance solicited by a shock resulting from the impact of a planar autonomous detonation. We obtain the explosion time and a condition for initiation as a constraint on the length of the detonating charge.     

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.

     

Fissuration multiple par corrosion sous contrainte : Modélisation du comportement mécanique par une méthode d'homogénéisation

Stress Corrosion Cracking (SCC) phenomenon is characterised by initiation and propagation of surface cracks frequently multiple. In order to model the mechanical behaviour of such materials we propose, as for composites, the use of homogenization techniques. Two materials are considered, first one corresponding to the cracked external volume and the second to the internal safe material. The cracked volume is considered as a two phase material, i.e., elastic matrix containing elliptical voids. The overall behaviour of the equivalent material is obtained applying the usual homogenization rules. Comparison between simulations and experimental results is done.     

Mesure de la température de gouttelettes en combustion par fluorescence induite par laser à deux couleurs : résultats préliminaires et perspectives

Two color laser-induced fluorescence technique is able to measure mean temperature of flying droplets, in the evaporation regime without combustion. This paper presents preliminary results about the use of this technique in reactive media.     

Une analyse micromécanique 3-D de l'endommagement par mésofissuration

A micromechanical analysis of brittle damage is proposed. This analysis consists of a 3-D generalization of the study performed by Andrieux et al. In this approach, the macroscopic free energy for open microcracks and frictionless closed microcracks is built. The conditions for unilateral contact (opening/closure criterion, elastic moduli recovery) are also presented. The proposed construction ensures at the macroscopic level the symmetry of the elastic stiffness tensor and the continuity of stress at the damage deactivation.     

Modélisation thermique d'une interface solide–fluide par fonction de transfert

A thermal modelling of a solid–fluid interface is proposed. The developed procedure is based on a numerical identification, in a dimensionless space, of a unique transfer function, associated with a recurrent relation. The method is usable for direct or inverse problems and well fit for control–command applications. An inverse numerical simulation example is presented for a random thermal excitation.     

Approche statistique du tenseur des contraintes par le viriel généralisé

The classical virial theorem, applied to particles without internal structure, is used to make a direct statistical estimation of a fluid pressure avoiding a thermodynamical potential derivation. Generalising this theorem to structured media leads to a direct statistical estimation of the stress tensor.     

Une technique de suivi d'interfaces pour la modélisation de la cavitation par poche

A tracking method is presented for the modeling of partial and supercavitation. The velocity and pressure fields in the cavitating flow are computed by a Navier–Stokes solver using a pseudo-compressibility method. The cavity flow is computed from the velocity field by a tracking method based on a volume of fluid technique (VOF). The method is illustrated by several computations, two cases of partial cavitation on a hydrofoil and a case of a cavitating body emerging at a free surface.     

Les mouvements par nappes ou tranches planes et parallèles d'un fluide visqueux incompressible

Here we make use of the vocable ‘slices or layer-type’ (for lack of better appellation) whenever a pencil of plane parallel fluid sections is so conveyed by the flow that plane remain the sections. Pseudo stream functions and general equations governing the flow are proposed so as to enlarge the theory of plane and pseudo-plane flows. By taking the aspects of solid boundaries into account, we show that the layer-type flows can be sharply divided into three categories. Then by seeking polynomial solutions of the equations, we reveal two families of exact solutions of Navier–Stokes equations.     

Contrôle par jet pulsé de l'écoulement dans un divergent court à grand angle

This experimental study investigates the control of flow in a short diffuser with a 2×45° divergence angle, using wall synthetic jets. Measurements are made by particle image velocimetry. Velocity profiles, velocity fields, and vorticity maps show that the flow, initially separated as a free jet, undergoes a global excitation which creates periodic oscillating structures producing large fluid motions in the vicinity of the wall. This results in an increased mixing of the primary separated jet with the surrounding fluid.     

Hystérésis et réduction de sons auto-entretenus commandées par effet Coanda

An air flow crossing two baffles in tandem inside a duct, generates strong self-sustained tones. Four radial slots created on the upstream baffle, leads to a large hysteresis loop of the sound pressure level when velocity varies, the second branch of which corresponds with a reduction of the self-sustained tones. Measurements of velocity show that a Coanda effect happens on the jets issued from the slots, which induces this flow behaviour.     

Endommagement d'un composite polypropylène renforcé par des fibres de verre courtes : approche expérimentale

To obtain good mechanical performance of the composite material, it is important to optimise the fibres ratio as well as the fibre/matrix interface quality which have influence on the damage. The main objective of this study is to determine the structural parameters influence on damage evolution concerning two types of polypropylene glass fibres composites. With a classical approach of damage mechanical theory which consists of load–unload tensile tests, acoustic emission permits to detect and follow damage mechanisms during loading. Fractographic analysis highlights the different assumptions and conclusions for this study.     

Étude expérimentale des courants de dérive induits par la houle au-dessus d'un fond ridé

Wave-induced current measurements have been carried out above rippled beds in a channel, using laser Doppler anemometry. Very simple formulas for the drift currents just outside the boundary layer above rippled and very rough beds in turbulent regime are proposed. The vertical profiles of drift currents in the bottom boundary layer are compared to the results of the recent model presented by Davies and Villaret.     

Caractérisation de la gélification d'une résine thermodurcissable par méthode rhéologique

Résumé Une nouvelle façon de repérer la gélification d'une résine thermodurcissable est proposée, à partir de mesures rhéologiques. Le point de gel est relié à une diminution de la vitesse de croissance du module visqueux observée sur les courbes expérimentales en cours de cinétique à température constante. Les temps de gel obtenus sont du même ordre que ceux donnés par les autres méthodes rhéologiques, mais font cependant apparaître des différences sensibles. Le temps de gel obéit à une loi d'Arrhénius en fonction de la température de cuisson.Le module visqueux au point de gel et, par conséquent la viscosité en ce point, varient avec la température. Il en est de même pour le facteur de perte tan . Par contre, le module élastique au point de gel se conserve quelle que soit la température. Ces résultats ont été obtenus sur deux formulations de résine: DGEBA (n = 0) – mPDA et DGEBA (n = 0) – DDM à la stoechiométrie.

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A new method of characterizing gelation of a thermosetting resin from rheological measurements is proposed. Gelation is associated with a reduction in the speed of growth of the viscous modulus which is observed on the experimental curves during isothermal kinetics. Times of gelation obtained in this way are of the same order of magnitude as those found with other rheological methods, however distinct differences are observed. These times follow the Arrhenius' law as a function of the processing temperature.The viscous modulus at the gelation point and, as a consequence, the viscosity at this point, vary with temperature. This comes also true for the loss factor tan . In contrast, the elastic modulus at the gelation point does not change with the temperature. These results were obtained with two resin formulas: DGEBA (n = 0) – mPDA and DGEBA (n = 0) – DDM in stoechiometric concentration.

     

Modélisation du comportement thermique d'un outil de fraisage : approche par identification de système non entier

In this work we present an experimental apparatus devoted to the thermal characterisation of a milling tool. The experimental device used thermistors, one for each insert. Each thermistor is located at a point in the tool close to the tip of the insert. The heat flux in each insert is expressed according to the temperature at the sensor from a non-integer model. The parameters of the model are identified from transient evolutions measurements of the temperature on the sensor and on the cutting edge. An application shows the difference in the behaviour of each insert during machining from the estimated heat fluxes. To cite this article: J.-L. Battaglia et al., C. R. Mecanique 330 (2002) 857–864.     

Calcul numérique des ondes de surface par une méthode de projection avec un maillage eulérien adaptatif

A non hydrostatic vertical two-dimensional numerical model is proposed to calculate free-surface flows. This model is based on resolving the full Navier–Stokes equations by a finite-difference method coupled with Chorin's projection method. An adaptative-Eulerian grid in the sigma-coordinate system is used. The model permits the calculation of surface-waves in estuarine and coastal zones. A benchmark test relative to the soliton propagation is realised to validate the model.