Étude du contrôle du décollement sur un profil d'aile par mesures PIV et analyse PODStudy of separation control on a wing profile using PIV measurements and POD analysis

The purpose of this study is the active control of flow separation on an ONERA D airfoil, using continuous blowing microjets. Particle Image Velocimetry (PIV) is used to measure velocity fields at midspan around the airfoil in both controlled and uncontrolled cases. Post-processing based on Proper Orthogonal Decomposition (POD) is developed to exhibit the main energetic features of the flow. Therefore, the detailed study of the POD modes for separated and stalled cases leads to a better understanding of the control mechanisms. Moreover, the POD basis computed for uncontrolled and controlled cases, opens the way to optimisation of control strategies. To cite this article: J. Favier, A. Kourta, C. R. Mecanique 334 (2006).     

Étude d'une ligne de jets impactant une paroi concave par PIV stéréoscopiqueStudy of a row of jets impinging a concave wall by stereoscopic PIV

The aerodynamic study of a row of axisymmetric jets impinging a concave wall is carried out from velocity measurements obtained by the standard and stereoscopic Particle Image Velocimetry. The principle and the specific aspects of the stereoscopic PIV set up, a recent technique of three-dimensional velocimetry, are explained. After a statistical data processing, the three-dimensional structure and the characteristics of multiple jets impinging a concave wall are described with the mean velocity fields and the turbulent values in several planes of the flow. To cite this article: V. Gilard, L.-E. Brizzi, C. R. Mecanique 334 (2006).     

Une comparaison des schémas d'intégration temporelle explicites de Chung–Lee et Tchamwa–WielgoszA comparison between the Tchamwa–Wielgosz and the Chung–Lee explicit time integration algorithms.

Numerous effort during the past ten years have focused on developing explicit time integration algorithms for structural dynamics that include controllable numerical dissipation of the spurious oscillations. Stability and consistency of the Tchamwa–Wielgosz algorithm are analysed for the linear damped/undamped cases. Furthermore, numerical results are compared for this algorithm and the Chung–Lee explicit method. To cite this article: V. Grolleau et al., C. R. Mecanique 332 (2004).     

Étude par la PIV des structures à la périphérie d'une cavité rotor–stator aspiréeStudy of structures on the periphery of a rotor–stator cavity with suction by means of PIV

We experimentally study the three-dimensional structure of the spiral instability observed on the periphery of a opened rotor–stator cavity with suction. We use the stereoscopic Particles Images Velocimetry technique which gives sequences of maps of the three velocity components of fluid particles located in a meridian plane. This sequence is acquired with a low frequency during the running of a great number of structures. The originality of this study consists in an optimized temporal reclassifying of the maps on a single period that allows a three-dimensional reconstitution of the instability. To cite this article: D. Rémy et al., C. R. Mecanique 332 (2004).     

On some localized waves described by the extended KdV equation

The influence of higher-order nonlinear terms on the shape of solitary waves is studied for mechanical systems governed by a generalization of the 5th order Korteweg–de Vries equation. New localized travelling wave with intrinsic oscillations (not breathers) is shown to arise from arbitrary initial pulse thanks only to the higher-order quadratic nonlinearity, while cubic nonlinearity is responsible for the formation of so-called ‘fat’ solitary wave. To cite this article: A.V. Porubov et al., C. R. Mecanique 333 (2005).     

Amortissement visqueux d'un résonateur mécanique en environnement gazeuxViscous damping of a mechanical resonator in a gaseous environment

We study dissipation phenomena due to the presence of a gaseous environment, leading to the damping of the oscillations in vibrating systems such as mechanical resonators. In the so-called “viscous” pressure region (p ranging between 10^?3 mbar and 1 bar), we suggest a simple model allowing an order of magnitude analysis of the dissipation mechanisms. This model, based on the classical form of the energy conservation equation in fluid mechanics, leads to a p^1/2 variation scale for the dissipation. In addition, we present experimental results that are found to be in good agreement with the predictions of the model. To cite this article: D. Perret et al., C. R. Mecanique 331 (2003).     

On perspective errors in endoscopic PIVSur les erreurs de perspective dans PIV endoscopique

Results are presented from a single and dual lens endoscopic PIV imaging system with a view to application of PIV where optical access is restricted such as internal flows. The dual lens PIV images were processed using cross-correlation, a cubic mapping function and standard stereoscopic relationships. For the single lens system, the images were processed using cross-correlation and a quadratic distortion-mapping function. Results have shown the single lens system to have in-plane errors an order of magnitude greater than the stereoscopic dual lens endoscopic PIV system. These errors are generated by perspective effects. Use of the stereo arrangement is therefore recommended wherever quantitative 3D velocimetry data is acquired using an endoscopic system. To cite this article: M. Reeves, N.J. Lawson, C. R. Mecanique 332 (2004).     

Mouvements propres d'un système à 1 D.D.L. avec amortissement viscoélastique

In this note, one considers a 1-d.o.f. oscillator consisting of a mass and a viscoelastic spring, the rheology of which is represented by a generalized so-called Biot's model with $2n+1$ parameters. It is proved that its $n+2$ independent eigenmotions are of the following nature: two eigenmotions damped with or without oscillations (as in the case of classical viscous damping) and n eigenmotions damped without oscillations which are induced by the viscoelastic rheology. To cite this article: P. Muller, C. R. Mecanique 333 (2005).     

Parametric excitation of waves on a free boundary of a horizontal fluid layer

We consider the dynamical stability of horizontal fluid layer, performing harmonic oscillations in vertical direction. The continued fractions approach allowed us to avoid the conventional restriction to the case of small viscosity and almost-resonant frequencies. Our numerical results cover a wide range of the parameters (viscosity, amplitude and frequency of the oscillation, and depth of the layer). To cite this article: V.I. Yudovich et al., C. R. Mecanique 332 (2004).     

A novel implicit method for coastal hydrodynamics modeling: application to the Arcachon lagoon

The present Note reports on numerical modeling of shallow flows in coastal areas. Successful numerical simulations of such flows should be able to cope with strong irregularities of the bathymetry and to reproduce the covering/uncovering (wetting/drying) of tidal flats due to the tidal oscillations of the free surface. Also, adoption of large time steps is necessary to simulate phenomena which last actually several days or months. In the present study, a new numerical model based on an implicit resolution of the shallow water equations is proposed. A penalty method has been employed for numerical treatment of dry zones emerging during the wetting and drying processes. The capability of the present model has been verified by comparison with standard test cases. Further applications and comparisons have been also carried out to simulate the tidal propagation in the Arcachon lagoon. To cite this article: A. Le Dissez et al., C. R. Mecanique 333 (2005).     

Use of turbulent flow statistical properties for correcting erroneous velocity vectors in PIVUtilisation de l'Estimation Stochastique pour corriger les vecteurs vitesse PIV erronés

For particular turbulent flows, PIV measurements technique provides more than 10% of spurious velocity vectors at each time step. To correct these vectors, we propose to use the Linear Stochastic Estimation (LSE) dealing with the spatial correlation tensor of the velocity. If this tensor cannot be determined in some measurement zones, Proper Orthogonal Decomposition is used to model this tensor. Both reconstruction methodologies are tested from PIV measurements performed in a Spark Ignition engine flow. We show that for coherent structures analysis, the LSE reconstruction method provides better results than classical mathematical interpolation methods. To cite this article: Ph. Druault, Ph. Guibert, C. R. Mecanique 332 (2004).     

Stimulation de l'élargissement d'un jet plan par contrôle fluidique alternéAlternate fluid control for stimulating plane-jet expansion

The present experimental study reports on the efficiency of an alternate use of two actuators placed on either side of a plane jet, to increase lateral jet expansion and to enhance mixing. The investigation is carried out using particle image velocimetry (PIV). Compared to a continuous sinusoidal excitation on both sides of the plane jet, the jet widens considerably when the actuators are switched on alternately with a well chosen timing. An increase of the widening by a factor of two is achieved. The iso-intensity contours of the fluctuating velocity indicate that the high-fluctuation zone greatly spreads out. To cite this article: M. Ben Chiekh et al., C. R. Mecanique 331 (2003).     

Instabilités de l'écoulement produisant le bruit de fenteFlow instabilities producing the slot-tone

The nature of the instability governing the self-sustained tones produced by a low Mach number plane jet impinging on a slotted plate, known as slot-tone, is identified experimentally. For a given Reynolds number, the natural shear-layer and the jet column mode frequencies of the free jet delimit the values of the measured slot-tone operating frequencies. The oscillations at lower frequencies are the result of the amplification of the jet column mode, and those at higher frequencies correspond to the shear layer instabilities. To cite this article: A. Billon et al., C. R. Mecanique 332 (2004).     

A dissipation-based control method for the multi-scale modelling of quasi-brittle materials

Multi-scale models based on computational homogenisation are nowadays developed for the simulation of complex material behaviour. The use of homogenisation techniques on finite-sized representative volume elements in the presence of quasi-brittle damage may lead to the presence of snap-backs in the macroscopic material response. A methodology to simulate this type of response in the multi-scale technique is proposed, based on the control of the dissipation at the mesoscopic scale. To cite this article: T.J. Massart et al., C. R. Mecanique 333 (2005).     

Self-excited stick–slip oscillations of drill bitsOscillations stick–slip auto-entretenues d'un outil de forage

This Note studies the self-excited stick–slip oscillations of a rotary drilling system with a drag bit, using a discrete model which takes into consideration the axial and torsional vibration modes of the bit. Coupling between these two vibration modes takes place through a bit-rock interaction law which accounts for both frictional contact and cutting processes at the bit-rock interface. The cutting process introduces a delay in the equations of motion which is ultimately responsible for the existence of self-excited vibrations, exhibiting stick–slip oscillations under certain conditions. To cite this article: T. Richard et al., C. R. Mecanique 332 (2004).     

‘Les fleurs du mal’ – an adaptive wavelet method of arbitrary lines I: convection–diffusion problems« Les fleurs du MAL » – une méthode d'ondelettes adaptive de lignes arbitraires I : problèmes de convection–diffusion

Baudelaire's ‘les fleurs du mal’ refers to various new developments (‘les fleurs’) of the method ofarbitrarylines (mal), since it was first published (in C. R. Acad. Sci. Paris, Sér. I, in 1991). Here we revisit the basic mal (semi-discretization) methodology for stationary convection–diffusion problems and develop an adaptive, wavelet-based solver that is capable of capturing the thin layers that arise in such problems. We show the efficacy and high accuracy of the wavelet-mal solver by applying it to a challenging 2D problem involving both boundary and interior layers. To cite this article: X. Ren, L.S. Xanthis, C. R. Mecanique 332 (2004).     

An example of entropy balance in natural convection,Part 2: the thermodynamic Boussinesq equations

Numerical simulations of natural convection performed with the usual Boussinesq equations result in unbalanced irreversibility budget. The thermodynamic Boussinesq equations solve this problem, especially because they simulate production of kinetic energy within the fluid through its expansion and contraction. These fluid volume changes, without which natural convection would not occur, also induce heat transfer by piston effect. The piston effect, which appears then as an intrinsic component of buoyancy-induced natural convection, introduces the non-dimensional adiabatic temperature gradient as a control parameter of natural convection. To cite this article: M. Pons, P. Le Quéré, C. R. Mecanique 333 (2005).     

Control of three-dimensional wakes using evolution strategies

We investigate three-dimensional cylinder wakes of incompressible fully developed flows at $\mathit{Re}=300$, resulting from control induced by tangential motions of the cylinder surface. The motion of the cylinder surface, in two dimensions, is optimized using evolution strategies, resulting in significant drag reduction and drastic modification of the wake as compared to the uncontrolled flow. The quasi-optimal velocity profile obtained in 2D is modified by spanwise harmonics and applied to 3D flows. The results indicate important differences in the flow physics induced by two and three dimensional control strategies. To cite this article: P. Poncet et al., C. R. Mecanique 333 (2005).     

Homogénéisation de coques minces piézoélectriques perforées

In this Note we establish the limit constitutive law of a piezoelectric material with periodically perforated microstructures and whose reference configuration is a thin shell with fixed thickness. The justification of the new associated model (we show that the limit global and local problems are more complicated than the intial one) is obtained using the periodic unfolding method introduced by Cioranescu, Damlamian and Griso. To cite this article: M. Ghergu et al., C. R. Mecanique 333 (2005).     

On the effect of small fluctuations in the volume fraction of constituents on the effective properties of composites

This study deals with three-scale composite materials comprised of nonlinear constituents. At the meso scale the composite can be considered as locally homogeneous with a macroscopic spatial variation of the constituents volume fraction. When these variations about a mean value are small, a Taylor expansion to second-order of the effective properties of the composite with respect to the fluctuations is given. This expansion can be used to discuss the beneficial or deleterious effects of clusters of inhomogeneities. It can also be used to derive new upper and lower bounds for the effective properties of nonlinear composites from dilute results. To cite this article: P. Suquet, C. R. Mecanique 333 (2005).