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We report the results of an experimental investigation into the decay of turbulence in plane Couette-Poiseuille flow using 'quench' experiments where the flow laminarises after a sudden reduction in Reynolds number $Re$. Specifically, we study the velocity field in the streamwise-spanwise plane. We show that the spanwise velocity containing rolls, decays faster than the streamwise velocity, which displays elongated regions of higher or lower velocity called streaks. At final Reynolds numbers above 425, the decay of streaks displays two stages: first a slow decay when rolls are present and secondly a more rapid decay of streaks alone. The difference in behaviour results from the regeneration of streaks by rolls, called the lift-up effect. We define the turbulent fraction as the portion of the flow containing turbulence and this is estimated by thresholding the spanwise velocity component. It decreases linearly with time in the whole range of final $Re$. The corresponding decay slope increases linearly with final $Re$. The extrapolated value at which this decay slope vanishes is $Re_{a_z}\approx 656\pm10$, close to $Re_g\approx 670$ at which turbulence is self-sustained. The decay of the energy computed from the spanwise velocity component is found to be exponential. The corresponding decay rate increases linearly with $Re$, with an extrapolated vanishing value at $Re_{A_z}\approx 688\pm10$. This value is also close to the value at which the turbulence is self-sustained, showing that valuable information on the transition can be obtained over a wide range of $Re$., 22 pages, 15 figures

The hairpin instability of a jet in a crossflow (JICF) for a low jet-to-crossflow velocity ratio is investigated experimentally for a velocity ratio range of $R\in(0.14,0.75)$ and crossflow Reynolds numbers $Re_D\in(260,640)$. From spectral analysis we characterize the Strouhal number and amplitude of the hairpin instability as a function of $R$ and $Re_D$. We demonstrate that the dynamics of the hairpins is well described by the Landau model, and, hence, that the instability occurs through Hopf bifurcation, similarly to other hydrodynamical oscillators such as wake behind different bluff bodies. Using the Landau model, we determine the precise threshold values of hairpin shedding. We also study the spatial dependence of this hydrodynamical instability, which shows a global behaviour., Comment: 20 pages, 21 figures

The Stuart-Landau reduced order model is tested with experimental data from the wake of a cylinder wake forced by plasma actuators, finding an important role played by the changing mean flow.

We present in this article the work of Henri B\'enard (1874-1939), French physicist who began the systematic experimental study of two hydrodynamic systems: the thermal convection of fluids heated from below (the Rayleigh-B\'enard convection and the B\'enard-Marangoni convection) and the periodical vortex shedding behind a bluff body in a flow (the B\'enard-K\'arm\'an vortex street). Across his scientific biography, we review the interplay between experiments and theory in these two major subjects of fluid mechanics., Comment: 41 pages

By experimentally counterbalancing a pressure gradient with moving boundaries the subcritical transition to turbulence is studied in a shear flow with zero mean advection velocity. Nearly stationary turbulent structures result. Velocimetry is used to characterize the structures vs Reynolds number.

In this paper, we review some recent results concerning the physics of forcing in open flows. First, we recall some properties of global modes in wakes, showing their shapes and their dependence on the Reynolds number and underline the importance of the mean flow correction induced by the fluctuations. Second, we show how a local temporal forcing can affect these properties, always through the modification of the mean flow, until the wake reaches a critical transition even far from the threshold. At last, we address some conjectures about an extended model suitable for describing the dynamics of forced wakes.

The vortex streets produced by a flapping foil of span-to-chord aspect ratio of 4:1 are studied in a hydrodynamic tunnel experiment. In particular, the mechanisms giving rise to the symmetry breaking of the reverse B\'enard-von K\'arm\'an vortex street that characterizes fish-like swimming and forward flapping flight are examined. Two-dimensional particle image velocimetry measurements in the mid-plane perpendicular to the span axis of the foil are used to characterize the different flow regimes. The deflection angle of the mean jet flow with respect to the horizontal observed in the average velocity field is used as a measure of the asymmetry of the vortex street. Time series of the vorticity field are used to calculate the advection velocity of the vortices with respect to the free-stream, defined as the phase velocity $U_{phase}$, as well as the circulation $\Gamma$ of each vortex and the spacing $\xi$ between consecutive vortices in the near wake. The observation that the symmetry breaking results from the formation of a dipolar structure from each couple of counter-rotating vortices shed on each flapping period serves as starting point to build a model for the symmetry breaking threshold. A symmetry breaking criterion based on the relation between the phase velocity of the vortex street and an idealized self-advection velocity of two consecutive counter-rotating vortices in the near wake is established. The predicted threshold for symmetry breaking accounts well for the deflected wake regimes observed in the present experiments and may be useful to explain other experimental and numerical observations of similar deflected propulsive vortex streets reported in the literature., Comment: 10 pages

Thiria, Goujon-Durand & Wesfreid (J. Fluid Mech. vol. 560, 2006, p. 123), it was shown that vortex shedding from a rotationally oscillating cylinder at moderate Reynolds number can be characterized by the spatial coexistence of two distinct patterns, one of which is related to the forcing frequency in the near wake and the other to a frequency close to the natural one for the unforced case downstream of this locked region. The existence and the modification of these wake characteristics were found to be strongly affected by the frequency and the amplitude of the cylinder oscillation. In this paper, a linear stability analysis of these forced regimes is performed, and shows that the stability characteristics of such flows are governed by a strong mean flow correction which is a function of the oscillation parameters. We also present experiments on the spatial properties of the global mode and on the selection of the vortex shedding frequency as a function of the forcing conditions for Re = 150. Finally, we elucidate a diagram of locked and non-locked states, for a large range of frequencies and amplitudes of the oscillation.

The wake flow around a circular cylinder at $Re\approx100$ performing rotatory oscillations has been thoroughly discussed in the literature, mostly focusing on the modifications to the natural B\'enard-von K\'arm\'an vortex street that result from the forced shedding modes locked to the rotatory oscillation frequency. The usual experimental and theoretical frameworks at these Reynolds numbers are quasi-two-dimensional, since the secondary instabilities bringing a three-dimensional structure to the cylinder wake flow occur only at higher Reynolds numbers. In the present paper we show that a three-dimensional structure can appear below the usual three-dimensionalization threshold, when forcing with frequencies lower than the natural vortex shedding frequency, at high amplitudes, as a result of a previously unreported mechanism: a pulsed centrifugal instability of the oscillating Stokes layer at the wall of the cylinder. The present numerical investigation lets us in this way propose a physical explanation for the turbulence-like features reported in the recent experimental study of D'Adamo et al. (2011)., Comment: 18 pages, 13 figures. To appear in Proc. Roy. Soc. A. For supplementary video material, see http://vimeo.com/123152022

In this paper we are interested in identifying the physical mechanisms that accompany mean drag modifications in the cylinder wake flow subject to rotary control. We consider simple control laws where the obstacle rotates harmonically with frequencies varying from half to more than five natural frequencies. In our investigation we analyze the results of the numerical simulations at Re=150. All the simulations were performed using the vortex method, which in the paper is outlined and benchmarked. We confirm the earlier findings concerning mean drag reduction at higher forcing frequencies and show that for the considered values of Re this control technique is energetically inefficient. The main result is that changes of the mean drag are achieved by modifying the Reynolds stresses and the related mean flow correction. The controlled flows are carefully characterized in terms of these fields. Drag reduction is related to elongation of the recirculation bubble. It is argued that mean drag reduction is associa...

La collaboration scientifique franco-argentine est très vivante aujourd’hui, tant à l’échelle de grands projets bi- ou multinationaux qu’à celle d’échanges de quelques chercheurs. Nous rappellerons d’abord la place privilégiée occupée de tout temps par les relations scientifiques franco-argentines, s’appuyant sur une forte influence culturelle française en Argentine. Nous présenterons ensuite, à titre d’exemple, une coopération développée depuis une trentaine d’années jusqu’à la formation d’un Laboratoire international associé. Enfin, nous examinerons les perspectives et les modalités ouvertes aujourd’hui pour les coopérations scientifiques avec ce pays. Fil: Wesfreid, José Eduardo. Laboratoire PMMH; Francia Fil: Ippolito, Irene Paula. Universidad de Buenos Aires. Facultad de Ingeniería. Departamento de Física. Grupo de Medios Porosos; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Rosen, Marta. Universidad de Buenos Aires. Facultad de Ingeniería. Departamento de Física. Grupo de Medios Porosos; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Hulin, Jean Pierre. Laboratoire FAST; Francia

We present the results of experimental study of a Couette–Taylor system with superimposed axial flow and an upper free surface, in the high Taylor number regime. At large Taylor numbers, when the rotational speed of the inner cylinder increases, bubbles created near the free surface are distributed throughout the test section and permit the study of the spatial and temporal properties of turbulent flows using visualization techniques. In addition to classic travelling Taylor vortices, intermittent pulses of vortices with higher phase velocities are also observed. These patterns are described in terms of the rotational speed and the intensity of the throughflow.

When a ferrofluid layer is subjected to a uniform and vertically oriented magnetic field, an interfacial instability occurs, above a critical value of the magnetic field, giving rise to a hexagonal array of peaks. On increasing the magnetic field, a smooth morphological transition from the hexagonal array to a square array was observed above a second threshold. The hexagon–square transition phenomenology, in addition to the role of penta–hepta defects initially present in the hexagonal pattern, was investigated. Furthermore, the pattern and wavenumber selection was studied by two different procedures: first by imposing jumps in field intensity and second by varying the magnetic field in a quasi-static way. The results obtained were very different for the two procedures. They indicated that the square pattern was a metastable state induced by the compression of the hexagonal pattern on increasing the control parameter. This hypothesis was confirmed by performing an additional experiment where the pattern was isotropically compressed. In this experiment, the transition was induced at a constant magnetic field lower than the transition onset value. However, the theoretical values for stability domains of hexagons and squares proposed in the literature were found to not agree with the experimental values.

Resume On presente des resultats sur la presence de bulles d'air comme traceurs dans un ecoulement hautement turbulent de Couette-Taylor a surface libre avec ou sans debit axial. Les travaux experimentaux ainsi qu'un calcul numerique nous ont permis de deduire que ces bulles d'air se logeaient pres du cylindre interieur tournant, a une separatrice sur deux, celles correspondant aux lignes de courant sortantes.

Experimental measurements performed under conditions which reproduce most of the dynamical characteristics of the natural evolution of vorticity filaments in turbulent flows are presented here. Strong deviations from the Burgers vortex (which is a non-confined stretched vortex model) are observed and analyzed.

The phase dynamics of a one-dimensional cellular pattern is studied on a regular array of liquid columns formed below an overflowing horizontal cylinder. When a uniform wavelength dilation is imposed by static boundaries, an oscillatory secondary bifurcation is observed ~‘‘optical mode’’!. The case of a moving boundary ~sinusoidal motion! allows us to observe three dynamical states: phase diffusion, phase diffusion coupled with the oscillatory state, and propagation of dilation waves. The dependence of the phase diffusion coefficient D upon the pattern wavelength is investigated for different flow rates: D is nearly constant until the appearance of the oscillations and jumps to a larger value when the optical mode is excited. This unusual behavior is recovered by an analytical treatment of Coullet-Ioss equations. @S1063-651X~98!12002-0#

In this paper we present experimental evidence that the scaling laws for the velocity structure functions ${S}_{n}(r)=〈[V(x+r)\ensuremath{-}V(x){]}^{n}〉$ $n=2,4,6,8$ hold in various parts of the flow domain. The exponents that characterize the scaling are, however, a function of the position in the wake that is the local strength and ubiquity of coherent structures. This variation is shown to be systematic and considerably exceeds the inaccuracy involved in the determination of the exponents. This is an objective indication of the influence that the organized flow structures and inhomogeneity may have on intermittency. In the analysis we invoke the concept of the extended self-similarity (ESS).

The difference between mass and momentum transport in longitudinal vortical structures is shown in this paper. The patterns observed by visualization using laser induced fluorescence are the signature of the mass transport, while the flow structures revealed by anemometry show the momentum transport. Experiments and numerical simulations are performed in order to compare the velocity field of the Gortler instability with the mass distribution of a passive scalar. The typical scales involved in this problem are discussed with relation to the Schmidt number, in order to compare the size of the observed “mushrooms” with the size of the longitudinal Gortler vortices. It is found that the nonlinearities which strongly influence the velocity perturbation do not modify the shape and the size of the scalar structures.

We perform experiments and numerical simulations on the mean streamwise velocity field of an unstable cylinder wake. We show that the mean velocity exhibits two weak secondary minima as the consequence of nonlinear interactions, resulting in a strong mean flow correction of the unstable mode. This correction, dominating the basic flow, governs the decrease in the length of the recirculation region $\ensuremath{\Delta}{L}_{r}$ in the supercritical regime. This explains the early classical observations of M. Nishioka and H. Sato [J. Fluid Mech. 89, 49 (1978)].

The destabilization of a thin film by gravity is of central importance in many industrial devices, especially those involved in Thermics (film boiling, heat exchangers). In the simplest case of a liquid film hanging below a solid ceiling, spatial structures are forced by the destabilizing action of gravity (Rayleigh-Taylor instability): one observes regular lattices of pendant drops, that in turn become instable by coalescences and falling of the drops. When the film is supplied with liquid at a constant rate, other flow regimes are observed: periodic emission of drops, formation of regular arrays of liquid columns, liquid sheet. Liquid columns exhibit a collective dynamics that is typical of non-linear systems: modulations of their spatial periodicity, diffusion of the perturbations, self-sustained oscillations, propagation of solitary dilation waves, coalescences and nucleations of the columns, etc.

We have calculated the general dispersion relationship for surface waves on a ferrofluid layer of any thickness and viscosity, under the influence of a uniform vertical magnetic field. The amplification of these waves can induce an instability called peaks instability (Rosensweig instability). The expression of the dispersion relationship requires that the critical magnetic field and the critical wavenumber of the instability depend on the thickness of the ferrofluid layer. The dispersion relationship has been simplified into four asymptotic regimes: thick or thin layer and viscous or inertial behaviour. The corresponding critical values are presented. We show that a typical parameter of the ferrofluid enables one to know in which regime, viscous or inertial, the ferrofluid will be near the onset of instability., 21 pages, 6 eps figures, Latex, to be published in Journal de Physique II

This paper contains an analysis of the scaling properties of two-dimensional (2D) turbulence obtained by means of numerical simulation using the {ital vortex blob} method. The flow under consideration is the turbulent wake behind a bluff body with a developed enstrophy cascade and reduced inverse energy cascade. The concept of {ital extended self-similarity} (ESS) and the associated {ital relative} scaling exponents {bar {zeta}}{sub m,n}={zeta}{sub n}/{zeta}{sub m} are invoked within the framework of 2D turbulence. The scaling exponents in the enstrophy range are found to systematically vary with the downstream distance from the obstacle, thus revealing their nonuniqueness. In terms of the {ital relative} exponents, the present results quantitatively agree with recent laboratory experiments of Gaudin {ital et al.} [PMMH-ESPCI Report No. A 96/57, 1996 (unpublished)]. Error bars and the accuracy of the ESS scaling are carefully checked. {copyright} {ital 1997} {ital The American Physical Society}

We present new experimental results on the development of turbulent spots in channel flow. The internal structure of a turbulent spot is measured, with Time Resolved Stereoscopic Particle Image Velocimetry. We report the observation of travelling-wave-like structures at the trailing edge of the turbulent spot. Special attention is paid to the large-scale flow surrounding the spot. We show that this large-scale flow is an asymmetric quadrupole centred on the spot. We measure the time evolution of the turbulent fluctuations and the mean flow distortions and compare these with the predictions of a nonlinear reduced order model predicting the main features of subcritical transition to turbulence.

Using a large-time-resolved particle image velocimetry field of view, a developing turbulent spot is followed in space and time in a rectangular channel flow for more than 100 advective time units. We show that the flow can be decomposed into a large-scale motion consisting of an asymmetric quadrupole centred on the spot and a small-scale part consisting of streamwise streaks. From the temporal evolution of the energy of the streamwise and spanwise velocity perturbations, it is suggested that a self-sustaining process can occur in a turbulent spot above a given Reynolds number.

We characterize experimentally and numerically the two-dimensional structure of the global mode of the longitudinal component of the velocity in flow behind obstacles of different geometrical shapes. Measurements in a low velocity water tunnel and numerical simulations of the longitudinal component V x of the oscillating velocity are carried out in the near wake region of the flow where vortex shedding is regular. We find that in the range of Reynolds numbers studied, the position (X max , Y max ) of the maximum of the global mode of V x shifts very little away from the central line in the y direction. The amplitude A max of the maximum of the global mode scales as (R-R c ) 1/2 in a range of R-R c larger then in previous studies. We also confirm that X max of the global mode scales as (R-R c ) -1/2 in the same range of Reynolds numbers. An analysis of the initial slope of the global mode is presented.

Nous nous interessons a la croissance spatiale de vortex longitudinaux qui apparaissent sous l'effet de la force centrifuge dans une couche limite se developpant sur une paroi concave. Cette instabilite induit des perturbations du champs de vitesse perpendiculaires a la direction de l'ecoulement moyen. Les profils de vitesse correspondant sont mesures par Velocimetrie Doppler Ultrasonore (UDV). Dans le cas de rouleaux stationnaires nous obtenons une evolution spatiale des composantes radiale et transversale de la vitesse dans les tourbillons, caracterisee par une zone de croissance exponentielle suivie d'une zone de saturation due aux nonlinearites. Dans le cas instationnaire, nous identifions le mode sinueux de l'instabilite secondaire et nous observons une dependance spatiale de la frequence et de l'amplitude des oscillations des vortex. Nous illustrons ensuite le role moteur de ces instabilites secondaires dans le mecanisme de transition vers le regime turbulent.

We are interested in identifying the mean-flow correction in the development of an instability. In the presented configuration (a 2D jet confined in a rectangular cavity that presents an oscillating instability), numerical simulations allow the identification of this mean-flow correction. We show that this distortion contributes to the non-linear saturation of the instability and that it is itself generated by the instability development: the action of the Reynolds tensor due to oscillating terms produces the mean-flow change.

The linear stability of a time‐periodic flow is considered. The fluid motion is taking place in a Hele–Shaw cell made of two vertical rectangular parallel plates separated by a gap of small extent compared to the dimensions of the plates. The flow is generated by oscillating the cell about its vertical symmetry axis. Our stability analysis was motivated by the experimental results reported some years ago by Bolton and Maurer [Bull. Am. Phys. Soc. 32, 2097 (1987)] who observed the onset of longitudinal rolls in this configuration. The inviscid stability criterion for steady flow subjected to Coriolis force is applied at different times to assess the instability mechanism in the two opposite regimes of respectively low and high frequency of oscillation. For moderate values of the frequency, implementation of Floquet theory is used to find the critical values of the instability parameters. Finally a connection is established between the present results and those we obtained recently for a pulsed flow in a Ta...

Numerical simulations of wake flow behind an equilateral triangular obstacle are presented. The form of global modes and their dependence on the Reynolds number found in this study are in accordance with recent experimental results of Goujon–Durand et al. [Phys. Rev. E 50, 308 (1994)]. A scaling law of the amplitude oscillating with the fundamental frequency corresponding to the maximum of the global mode is found to agree with the Landau model in a range of Reynolds Re numbers larger than in previous studies. The position of the maximum amplitude of the fundamental modes scales as (Re−Rec)−1/2. The amplitude of the second harmonic of the longitudinal component of the velocity as well as the correction to the mean flow have different critical behavior than the velocity components oscillating with fundamental frequency. During linear growth the position of the maximum of the global modes is constant and moves only in the nonlinear regime. The effects of the blockage and the boundary conditions on the side ...

We present the dynamics of the liquid viscosity and of the liquid bath depth: it can be shown that the dominant wavelength is smaller with higher fields but differs if the liquid film is thick or thin, inviscid or viscous. The development of the pattern involves nonlinear interactions. While a stable deformed interface has been observed on the analogous case of a magnetic fluid under magnetic field, experimentally we only found a stable non-plane interface for a confined geometry where the volume conservation makes forming the destabilisation more difficult: indeed the critical field for obtaining of peaks is then greater than the value of the horizontally infinite surface. Otherwise, some peaks emit at their apex, killing the field: thus the peaks fall, do not emit any more and the electric field is installed again; this gives an oscillatory phenomenon. For a sufficiently large liquid bath different wavelengths can be seen, depending on the electric field and the way this field was reached. For confined geometries one or two peaks grow on the surface and the critical field can be significantly higher. >

The wake of a flexible foil undergoing pitching oscillations in a low-speed hydrodynamic tunnel is used to examine the effect of chord-wise foil flexibility in the dynamical features of flapping-based propulsion. We compare the regime transitions in the wake with respect to the case of a rigid foil and show that foil flexibility inhibits the symmetry breaking of the reverse B\'enard-von K\'arm\'an wake reported in the literature. A momentum balance calculation shows the average thrust to be up to three times greater for the flexible foil than for the rigid foil. We explain both of these observations by analyzing the vortex dynamics in the very near wake., Comment: 10 pages

Fluidic oscillator : application to single-phase flows flowmetry The authors have experimentally and numerically analysed self-sustained oscillations in a cavity confined stream. They have studied the dynamics of the oscillator which is at the origin of flowmeters fluidics prototypes. They determined how to choose the frequency when the fluid velocity of the flow configuration and the geometrical characteristics of the cavity are known. They also analysed the linear and the nonlinear stability.

The wake of a circular cylinder performing rotary oscillations is studied using hydrodynamic tunnel experiments at $Re=100$. Two-dimensional particle image velocimetry on the mid-plane perpendicular to the axis of cylinder is used to characterize the spatial development of the flow and its stability properties. The lock-in phenomenon that determines the boundaries between regions of the forcing parameter space were the wake is globally unstable or convectively unstable is scrutinized using the experimental data. A novel method based on the analysis of power density spectra of the flow allows us to give a detailed description of the forced wake, shedding light on the energy distribution in the different frequency components and in particular on a cascade-like mechanism evidenced for a high amplitude of the forcing oscillation. In addition, a calculation of the drag from the velocity field is performed, allowing us to relate the resulting force on the body to the wake properties., 8 pages, 8 figures

It is well known that an electric field that is applied normally to the free surface of a conducting fluid has a destabilizing effect. Here we study the linear growth of electro-capillary instabiEties in the very generil case where the viscosity of the fluid and its thickness are of any value. We derive the asymptotic behavio~lr in various regimes and give analytical dispersion equations in the case of thin or thick, inviscid or viscous films and compare with previous results. Then we present a diagram of the corresponding simplifications of the dispersion relation and as we derive them directly from the general equation, we are able to derive their conditions of validity explicitly. We also show the simflarities and the differences with the case of a fiq~lid falling from a solid flat plane (Rayleigh-Taylor instabilities) without electric field and with ferrofl~lid instabilities.

In this study, a passive flow control experiment on a 3D bluff-body using vortex generators (VGs) is presented. The bluff-body is a modified Ahmed body (Ahmed in J Fluids Eng 105:429–434 1983) with a curved rear part, instead of a slanted one, so that the location of the flow separation is no longer forced by the geometry. The influence of a line of non-conventional trapezoidal VGs on the aerodynamic forces (drag and lift) induced on the bluff-body is investigated. The high sensitivity to many geometric (angle between the trapezoidal element and the wall, spanwise spacing between the VGs, longitudinal location on the curved surface) and physical (freestream velocity) parameters is clearly demonstrated. The maximum drag reduction is −12%, while the maximum global lift reduction can reach more than −60%, with a strong dependency on the freestream velocity. For some configurations, the lift on the rear axle of the model can be inverted (−104%). It is also shown that the VGs are still efficient even downstream of the natural separation line. Finally, a dynamic parameter is chosen and a new set-up with motorized vortex generators is proposed. Thanks to this active device. The optimal configurations depending on two parameters are found more easily, and a significant drag and lift reduction (up to −14% drag reduction) can be reached for different freestream velocities. These results are then analyzed through wall pressure and velocity measurements in the near-wake of the bluff-body with and without control. It appears that the largest drag and lift reduction is clearly associated to a strong increase of the size of the recirculation bubble over the rear slant. Investigation of the velocity field in a cross-section downstream the model reveals that, in the same time, the intensity of the longitudinal trailing vortices is strongly reduced, suggesting that the drag reduction is due to the breakdown of the balance between the separation bubble and the longitudinal vortices. It demonstrates that for low aspect ratio 3D bluff-bodies, like road vehicles, the flow control strategy is much different from the one used on airfoils: an early separation of the boundary layer can lead to a significant drag reduction if the circulation of the trailing vortices is reduced.

We present recent results obtained in the study of the instabilities induced by gravity on liquid layers hanging below solid surfaces, in situation of perfect wetting (without contact lines). Two cases are discussed: two-dimensional films of constant volume, and films continuously supplied with “fresh” liquid by an external source. In the first case, we show experimental evidences suggesting that the formation of a two-dimensional structure within the film arises by means of front propagation mechanisms. In the simplest situation, propagation of a one-dimensional structure (“roll” formation), the measured front speed is close to that deduced from the linear marginal stability theory. In the second case, the liquid film is hanging below an horizontal overflowing half-cylinder. Depending on the rate of supply, different regimes are observed (dripping, arrays of parallel jets, triangular sheets). The arrays of drops and jets exhibit interesting spatio-temporal phase dynamics: oscillations, pairing or nucleation of cells, forced tilt waves.

We study experimentally and theoretically the evolution of two-dimensional patterns in the Rayleigh—Taylor instability of a thin layer of viscous fluid spread on a solid surface. Various kinds of patterns of different symmetries are observed, with possible transition between patterns, the preferred symmetries being the axial and hexagonal ones. Starting from the lubrication hypothesis, we derive the nonlinear evolution equation of the interface, and the amplitude equation of its Fourier components. The evolution laws of the different patterns are calculated at order two or three, the preferred symmetries being related to the non-invariance of the system by amplitude reflection. We also discuss qualitatively the dripping at final stage of the instability.

The existence of a self-sustaining process between streamwise vortices and streaks has been suggested at moderate Reynolds numbers. Such a mechanism has never been demonstrated experimentally. Using small cylinders as vortex generators to create streamwise counterrotating vortices, we show, through the characterization of the spatial transient growth of the energy of the longitudinal and spanwise velocity perturbations, that such a mechanism exists above a given Reynolds number. From instantaneous particle image velocimetry fields in a horizontal plane, we show that the self-sustaining process can also be associated with the longitudinal destabilization of streamwise streaks.

We study experimentally the vortex streets produced by a flapping foil in a hydrodynamic tunnel, using 2D Particle Image Velocimetry (PIV). An analysis in terms of a flapping frequency-amplitude phase space allows to identify: 1) the transition from the well-known B\'enard-von K\'arm\'an (BvK) wake to the reverse BvK vortex street that characterizes propulsive wakes, and 2) the symmetry breaking of this reverse BvK pattern giving rise to an asymmetric wake. We also show that the transition from a BvK wake to a reverse BvK wake precedes the actual drag-thrust transition and we discuss the significance of the present results in the analysis of flapping systems in nature., Comment: Revised version includes minor changes in the discussion of the drag coefficient calculation. The 4 figures are unchanged

Colloque avec actes et comité de lecture. Internationale.; International audience; Nous étudions les allées tourbillonnaires produites par un aileron oscillant dans un canal hydrodynamique, à l'aide de la vélocimétrie par images de particules (PIV). Une étude paramétrique en fonction de la fréquence adimensionnelle (le nombre de Strouhal) et l'amplitude du battement de l'aileron a permis d'identifier : 1) la transition entre une allée type Bénard-von Karman (BvK) et l'allée inversée typique des sillages propulsifs; et 2) la brisure de symétrie de cette allée inversé donnant lieu à un sillage asymétrique. Une estimation de la force de traînée moyenne sur l'aileron est obtenue à partir d'un bilan de quantité de mouvement utilisant les mesures PIV de champ de vitesse moyenne en aval de l'aileron. Nous montrons que la transition observée dans la structure du sillage entre une allée de BvK et une allée inversée se fait avant la transition traînée--propulsion.

International audience; We study experimentally the vortex streets produced by a high-aspect-ratio pitching foil placed in a hydrodynamic tunnel. PIV measurements give access to the spatio-temporal characteristics of the vorticity field in the wake and allow for a calculation of the spatial distribution of velocity fluctuations as well as an indirect estimate of the forces on the foil. A parametric study in terms of the non-dimensional frequency (the Strouhal number) and amplitude of the flapping motion allows to identify: 1) the transition from the well-known Bénard-von Kármán (BvK) wake to the reverse BvK vortex street that characterizes propulsive wakes, and 2) the symmetry breaking of this reverse BvK wake. The latter constitutes a spontaneous mechanism giving rise to a mean vortex-induced lift force on the flapping foil.

An actuator is used to control the drag exerted on a bluff-body at large Reynolds number (typically Re = 20 000 ). The geometry is similar to a backward-facing step whose separation point is modified using a rotating cylinder at the edge. The slow fluctuations of the total drag are directly measured by means of strain gauges. The size of the low-pressure region behind the body is decreased and the drag reduced: the faster the rotation of the cylinder the lower the drag. The goal of the control is for the system to find itself an optimal set point, defined as the lowest cost of global energy consumption of the system (drag reduction versus energy used by the actuator). For this purpose an extremum-seeking control method is applied in order to deal with the large background noise due to turbulence. It consists in a synchronous detection of the response measured in the drag measurements to a modulation of the actuator. This experiment attests from the real efficiency for local active control to reduce autonomously the global energy consumption of a turbulent separated flow, taking into account drag and actuator's energy cost.

A classical actuator is used to control the drag exerted on a bluff body at large Reynolds number (Re=20000). The geometry is similar to a backward-facing step whose separation point is modified using a rotating cylinder at the edge. The slow fluctuations of the total drag are directly measured by means of strain gauges. As shown by visualizations, the actuator delays the separation point. The size of the low-pressure region behind the body is decreased and the drag reduced. It is found that the faster the rotation of the cylinder, the lower the drag. In a first study, the goal of the control is for the system to reach a drag consign predetermined by the experimentalist. The control loop is closed with a proportional integral correction. This adaptive method is shown to be efficient and robust in spite of the large fluctuations of the drag. In the second method, the system finds itself its optimal set point. It is defined as the lowest cost of global energy consumption of the system (drag reduction versus energy used by the actuator). For this purpose, an extremum seeking control method is applied in order to deal with the large background noise due to turbulence. It consists in a synchronous detection of the response measured in the drag measurements to a modulation of the actuator. The phase shift and amplitude of the modulation estimate the local gradient of the total energy function. With this gradient estimation, the system goes to the minimum of global power consumption by itself. The system is found to be also robust and reacts successfully to changes of the external mean flow. This experiment attests to the real efficiency of local active control in reducing autonomously the global energy consumption of a system under turbulent flow.

We describe the spatial inhomogeneities in hydrodynamic patterns, in the cases of confined systems such as the Rayleigh-Benard convection and open systems such as as the Benard-von Karman instability of vortex shedding. From experimental results, we define the typical correlation lengths and their scaling law in each situation.