Vitesse de chute d'une particule lourde isolée dans un écoulement turbulent

The settling velocity of isolated spherical particles, in grid generated turbulence, points out the role of the vortex structures in sediment process. The analysis of mean and turbulent velocities fields shows that the ratios between turbulence scales and characteristics of the particles are of great importance on the settling velocity evolution.     

Visualisation de l'écoulement discontinu gaz-solide dans un changement de direction

With the use of 25 images per s camera, the process of solid plugs of a gas-solid plug flow in a pipe bend with constant curvature (R_b/D=0) can be highlighted. This process can be seen on frame sequences 1, 2, 3 and 4. This study is realised in horizontal-vertical pipes, for solid-gas mass flow rates (q=q_s/q_g) equal to 12.7 and 17.9. The visualisations show that the flow deceleration and propulsion stages of the solid plug are not correlated because at the end of the deceleration stage, the plug velocity is zero. This work has allowed us to validate the different hypotheses necessary for the stress model between solid plug and the outer face of a pipe bend.     

Étude numérique de l'écoulement de fluides non newtoniens à travers un élargissement brusque

We investigate numerically the problem of laminar flow of incompressible nonNewtonian pseudo-plastic fluids across an abrupt enlargement. The effects of different parameters on the evolution of streamlines are studied. The key point is to propose a correlation allowing efficient calculation of the re-establishment length.     

Analyse expérimentale de l'écoulement diphasique eau—air dans un réseau de fractures

We present the first experimental results of air-water two-phase flow in an artificial fracture network. They are interpreted by using the Kr-relative permeability concept. It appears that, contrary to two-phase flows in a single fracture, the sum of Kr is of the order of 1.     

Écoulement dans un convergent axisymétrique: calcul de la contrainte de cisaillement pariétale maximale

The maximal wall shear stress (MWSS) in the convergent part of a stenosis is computed as a function of characteristic geometrical parameters (stenosis degree and length), of the entry velocity profile and of flow Reynolds number, by means of the Interactive Boundary-Layer (IBL) theory, in the simplified case where the flow is axisymmetric and stationary. The independence of the MWSS on the entry velocity profile is showed. A heuristic analysis, followed by regression analysis of the numerical results, allows drawing out the simple dependence of the MWSS on the other parameters, all measurable in clinical practice. The so obtained relationship extends the results found in the literature, achieved by resolution of Navier-Stokes equations for particular geometrical characteristics.     

Ondes variqueuses absolument instables dans un canal élastique

Conditions for absolute instability of the elastic walls of a plane channel with potential flow are investigated. The absolute instability threshold is given in closed form in the thin film approximation for walls made of plates or membranes. The more general case is solved numerically and is compared with more classical results for a single wall in the presence of a semi-infinite fluid domain. It is found that absolute instability is strongly influenced by the membrane or plate nature of the wall stiffness.     

Étude d'un procédé de brassage électromagnétique en écoulement de Stokes

This note deals with a process allowing electromagnetic mixing of a conductor fluid in Stokes flow. The flow, which is generated by two inductor wires (arranged around a cylindrical vat), undergoes a rotation. In order to solve analytically this electromagnetic flow, we use a magnetostatic pattern and we replace the dynamics in the electromagnetic layer by a tangential velocity. We superimpose then a periodic rotation on the flow (triangular signal). The investigated parameters are an a dimensional number (τ), which compares the velocities of the rotation and the flow, the inversion's periodicity of the rotation (k) and the distance of the wires from the vat (d). In studying the stretching and scattering of a drop of the fluid, an optimal choice of these parameters is subsequently suggested. Finally, prospects are considered of the possibilities of stationary three-dimensional and mixing.     

Réduction électrochimique de porphyrines portant un groupement électroactif pyridinium

The electrochemical reduction of meso-tetraphenylporphyrins β-substituted by a pyridinium group was investigated in N,N-dimethylformamide. The measurements showed the presence of two distinct electroactive sites – the pyridinium cation and the porphyrin ligand – involved in three successive one-electron charge transfers. The pyridinium cation is reduced before the porphyrin ligand, leading to the formation of a neutral radical species. The reduction of the porphyrin ligand takes place at more cathodic potentials that are close to those of the corresponding unsubstituted porphyrin. This result was expected taking into account the neutral state of the pyridinium group that follows its reduction. The decrease of the reduction potentials of the pyridinium cation fits well with the evolution of the electronic densities of the porphyrin ligand in the series H₂ < Cu < Zn. The depicted evolution clearly demonstrates the presence of the mutual electronic interactions between the two different electroactive sites in these molecules and the important electrodonating effect of the porphyrin ligand. The reduction of the pyridinium group, which is reversible for the free base, becomes irreversible for the metalloporphyrin. A possible dimerisation of the molecules via the reduced pyridinium group is proposed. These results are discussed on the basis of the d–π interactions existing in the metalloporphyrins.     

Mise en évidence expérimentale de la sensibilité de l'écoulement de type rotor-stator aux effets de bord

Experimental investigation of the external peripheral geometry effects in a rotor-stator system has brought to light the strong influence on the flow structure of a small difference between the disc radii: two distinct regimes can be observed. At given values of the Reynolds number and aspect ratio the flow field is in agreement with the infinite dises solution as soon as the ratio of the stator and the rotor radii becomes sufficiently large.     

Effets de parois sur la distribution de taux de vide dans les écoulements à bulles

On the basis of numerical simulations of bubbly flows, we evaluated the role of turbulence and interfacial forces in the void fraction distribution near the wall. The simulations of pipe flow in micro-gravity situations show that the turbulent contributions issued from the added mass force play an important role in the phase distribution phenomenon. In normal gravity, the simulations of the boundary layer and pipe bubbly flows allow us to adjust the lift force model in which we take into account the wall interaction effect.     

Étude numérique de la stabilité des écoulements de convection naturelle dans des espaces annulaires horizontaux de faible épaisseur

The mechanisms driving the multiplicity of multicellular natural convective flows found at the top of narrow horizontal air-filled concentric annuli (the inner cylinder temperature exceeding the outer) are considered. For radius ratio less than R = 1.20, the base flow undergoes an imperfect bifurcation. The two stable branches are characterised.     

Contrôle des jets par écoulement à contre-courant

The expansion of axisymmetric jets and their initial turbulence level are considerably enhanced by using a suction collar of sufficient length around the nozzle and by applying a counterflow near the jet exit. The observed phenomena seem to be due to recirculations generated by the suction in the collar chamber.     

Identification et Commande Multidimensionnelle d'un Echangeur Thermique à Lit de Bulles

This article presents the study, identification and control of a heat exchanger made of a horizontal tube bundle and an air bubble dispersion in water. A tendency model (partial derivative equation system) allows us to predict the length and to choose the stationary working point of the exchanger. Around this point the process identification is made by means of three methods (model method, least-squares generalized method, maximum likelihood method). Using the theory of optimal quadratic proportional integral control, the multivariable control of the system is operated on a minicomputer. The introduction into the process state equations of a model of measurable disturbances noticeably improves the regulation performances.     

Effet de l'accélération d'un écoulement sur le transfert de masse ou de chaleur à la surface d'une bulle sphérique

Numerical results concerning mass or heat transfer on a spherical bubble embedded in axisymmetrical accelerated flows are presented. The Reynolds number is varied between 0.1 and 500. Two different situations are considered, namely an unsteady flow with a constant acceleration, and a pure straining flow where the homogeneous strain induces a spatial acceleration. In the latter case, the acceleration is seen to have little effect on the transfer. In contrast in the unsteady case, computations reveal that dramatic history effects may happen. A comparison of characteristic times allows us to explain the various evolutions shown by the computations. These numerical results are confirmed by analytical solutions obtained in the corresponding situations by assuming a potential flow.     

Les sels de bismuth(III), catalyseurs dans la réaction de clivage des éthers

Bismuth(III) salts as catalysts in the ether cleavage reaction. Bi (OTf)₃ and BiCl₃ react in catalytic quantity to cleaved the tertiobutyl–oxygen bound of the ethers 1, 3, 5.     

Modélisation de la durée des sons dans un système de reconnaissance automatique de la parole

A major deficiency of standard Hidden Markov Models (HMM) is that both the spectral and the prosodic features are uniformly processed. To combine more efficiently the prosodic cues with the acoustic ones, a segmental Two Level HMM has been recently studied by Suaudeau. In this paper, we present an adapted version in which the segmental processing is replaced by the classical centisecond one, and the prosodic feature is the sound duration. Experiments on a French database show that such an explicit introduction of prosodic parameters reduces recognition error rates.     

Flux d'énergie d'un écoulement de Poiseuille vers la paroi d'un tube élastique,a. Modes d'instabilité axisymétrique

Ne compute the energy flux from a fluid flowing to an elastic hollow cylindrical tube generated by unstable modes. The basic fluid velocity flow is the parabolic Hagen-Poiseuille flow. We show that the energy flux from the fluid flowing to the elastic wall is positive when the mode is unstable, negative when the mode is stable, and null when the mode is neutral. Moreover the energy flux from the fluid flowing to the elastic solid is generated by the component of the force perpendicular to the wall at the interface for high Reynolds numbers, and essentially by the streamwise component of the force at interface for low Reynolds numbers.     

Étude de la résonance du mode géostrophique dans un tore

We show that when (ΩR/c₀) ≪ 1, and (a/R) ≪ 1, the air motion in a torus is governed by the problem of inertial oscillations in this torus excited by a perturbation on its surface, which moves with constant angular velocity Ω and gets recurrently at the same place with period 2π/Ω. This excitation makes one of the inertial oscillation modes enter in resonance, the geostrophic mode, which increases proportionally to time when t → ∞.