Influence of the porosity pattern on the aerodynamics of a square plate

The evolution of the normal aerodynamic coefficient of 19 configurations of square plates with various porosity patterns, ranging from solid plate to homogeneous porous plate, is experimentally characterized. The variation of the porosity pattern is obtained by partially covering the holes of a commercial fly-swatter using adhesive tape. Evolution of the normal aerodynamic coefficient is assessed from the measurement of the angular position of the porous plate, placed as a freely rotating pendulum swept by a flow in a wind tunnel. These angular measurements are also supported by PIV measurements of the structure of the wake. We show that the porosity pattern determines whether or not an abrupt stall occurs. In particular, the details of the porosity pattern on the edges of the plate are decisive for the existence of abrupt stall

Path instabilities and drag in the settling of single spheres

The settling behavior of individual spheres in a quiescent fluid was studied experimentally. The dynamics of the spheres was analyzed in the parameter space of particle-to-fluid density ratio ($\Gamma$) and Galileo number ($\mathrm{Ga}$), with $\Gamma \in (1.1, 7.9)$ and $\mathrm{Ga} \in (100, 340)$. The experimental results showed for the first time that the mean trajectory angle with the vertical exhibits a complex behavior as $\mathrm{Ga}$ and $\Gamma$ are varied. Numerically predicted regimes such as Vertical Periodic and Planar Rotating were validated at high $\Gamma$ values. In particular, for the denser spheres, a clear transition from planar to non-planar trajectories was observed, accompanied by the emergence of semi-helical trajectories corresponding to the Planar Rotating Regime. The spectra of trajectory oscillations were also quantified as a function of $\mathrm{Ga}$, confirming the existence of oblique oscillating regimes at both low and high frequencies. The amplitudes of the perpendicular velocities in these regimes were also quantified and compared with numerical simulations in the literature. The terminal velocity and drag of the spheres were found to depend on the particle-to-fluid density ratio, and correlations between the drag coefficient and particle Reynolds number ($Re_p$) as a function of Ga were established, allowing for the estimation of drag and settling velocity using $\mathrm{Ga}$, a control parameter, rather than the response parameter $Re_p$

An iterative study of time independent induction effects in magnetohydrodynamics

International audienceWe introduce a new numerical approach to study magnetic induction in flows of an electrically conducting fluid submitted to an external applied field B-0. In our procedure the induction equation is solved iteratively in successive orders of the magnetic Reynolds number Rm. All electrical quantities such as potential, currents, and fields are computed explicitly with real boundary conditions. We validate our approach on the well known case of the expulsion of magnetic field lines from large scale eddies. We then apply our technique to the study of the induction mechanisms in the von Karman flows generated in the gap between coaxial rotating disks. We demonstrate how the omega and alpha effects develop in this flow, and how they could cooperate to generate a dynamo in this homogeneous geometry. We also discuss induction effects that specifically result from boundary conditions

Sédimentation de particules paramagnétiques soumises à la gravité et à un champ magnétique externe

Nous nous intéressons à la dynamique et aux effets collectifs de particules en transport turbulent. Les écoulements chargés en particules sont omniprésents dans la nature (écoulements pyroclastiques, transport de polluants ...) et dans de nombreux procédés industriels (mélangeurs diphasique ...). Dans ces exemples, l'écoulement est généralement turbulent. Les interactions entre les particules et l'écoulement sont complexes de part la nature multi-échelles des structures de la turbulence et les diverses caractéristiques des particules. Depuis de nombreuses décennies, la communauté scientifique s'intéresse à ces écoulements, cherchant notamment à établir des modèles prédictifs fiables. Les théories les plus abouties sont les théories inertielles. Celles-ci considèrent les particules comme ponctuelles [1]. La démocratisation des techniques d'imagerie rapide et des moyens numériques ont ouvert la voie à diverses études ayant permis de montrer les limitations prédictives de ces approches idéalisées pour décrire le transport turbulent de particules matérielles réelles et de taille finie. Ceci montre la nécessité de mieux comprendre les couplages entre particules et entre les particules et l'écoulement porteur afin de prendre en compte la multitude des processus physiques sous-jacents (dispersion, effets de l'inertie, rôle de la turbulence, effet de la gravité, formation d'amas et effets collectifs, ...) [2] [3]. Dans un système réel, il est généralement difficile de démêler les effets spécifiques de ces processus. Pour cela, il est nécessaire de développer des études expérimentales permettant d'agir sur l'un ou l'autre de ces effets de couplage. Pour aborder ces problèmes, le travail que nous présentons propose une étude expérimentale préliminaire du rôle des interactions inter-particulaires (s'affranchissant dans un premier temps des effets de la turbulence) : la sédimentation de particules paramagnétiques soumises à la gravité et à un champ magnétique externe dans un fluide initialement au repos.   Le dispositif expérimental est constitué d'une cellule de Hele-Shaw contenant une suspension (eau) de sphères paramagnétiques de 250 um de diamètre et de densité 1,6 placées entre deux bobines de Helmholtz produisant, au choix, un champ homogène ou inhomogène dont l'amplitude peut-être ajustée. L'ensemble des particules forme initialement un paquet dense au fond de la cellule. Celle-ci pivote rapidement induisant la sédimentation des particules.   Nous suivons la déstabilisation du front de particules (analogue à l'instabilité de Rayleigh-Taylor fluide (RT)) par imagerie rapide pour différentes valeurs de l'amplitude du champ externe. Nos premières mesures montrent un ralentissement de la décompaction à mesure que le champ augmente, suggérant un possible effet cohésif résultant de la formation de chaînes de forces entre les particules (par interactions entre dipôles induits). Ceci est compatible avec l'observation qualitative de l'atténuation de la digitation de RT de l'interface, pouvant s'interpréter comme une augmentation de la tension de surface effective du front de particules, induite par la plus forte interaction entre les particules.   Dans des études à venir nous considérons le cas d'un champ magnétique appliqué inhomogène afin d'étudier non seulement les interactions entre particules mais également la possibilité d'agir avec ou contre la gravité. Ceci pourrait ouvrir une voie intéressante pour des études ultérieures visant à démêler les effets de l'inertie et de la gravité dans les écoulements turbulents chargés en particules. Références : [1] M. R. Maxey and J. J. Riley. Equation of motion for a small rigid sphere in a nonuniform flow. Phys. Fluids 26, 883 (1983) [2] Mickaël Bourgoin and Haitao Xu. Focus on dynamics of particles in turbulence. New J. Phys. 16, 085010 (2014) [3] Nauman M. Qureshi, Mickaël Bourgoin, Christophe Baudet, Alain Cartellier, and Yves Gagne. Turbulent transport of material particles : An experimental study of finite size effects. Phys. Rev. Lett. 99, 184502 (2007)

3D Acoustic Lagrangian Velocimetry

International audienceWe report Lagrangian measurements obtained with an acoustic Doppler velocimetry technique. From the Doppler frequency shift of acoustic waves scattered by tracer particles in a turbulent flow, we are able to measure the full three-component velocity of the particles. As a first application, we have studied velocity statistics of Lagrangian tracers in a turbulent air jet at Rλ∼320 and at various distances from the nozzle. The choice of an air jet is motivated by the fact that jets produce a well characterized high level tubulence and open air flows are well suited to simultaneaously achieve classical hot wire Eulerian measurements. Therefore, we are also able to explicitly address the question of the differences between Eulerian and Lagrangian statistics. As Lagrangian tracers we use soap bubbles inflated with Helium which are neutrally buoyant in air and can be assimilated to fluid particles. Velocity statistics are analysed. We show that the Lagrangian autocorrelation decays faster in time than its Eulerian counterpart. Finally we present Lagrangian time velocity increments statistics which, as already reported by previous work, exhibits stronger intermittency than Eulerian velocity increments

Chaotic mixing in effective compressible flows

International audienceWe study numerically joint mixing of salt and colloids by chaotic advection and how salt inhomogeneities accelerate or delay colloid mixing by inducing a velocity drift V dp between colloids and fluid particles as proposed in recent experiments [J. Deseigne et al., Soft Matter 10, 4795 (2014)]. We demonstrate that because the drift velocity is no longer divergence free, small variations to the total velocity field drastically affect the evolution of colloid variance σ ^2 = − ^2. A consequence is that mixing strongly depends on the mutual coherence between colloid and salt concentration fields, the short time evolution of scalar variance being governed by a new variance production term P = − /2 when scalar gradients are not developed yet so that dissipation is weak. Depending on initial conditions, mixing is then delayed or enhanced, and it is possible to find examples for which the two regimes (fast mixing followed by slow mixing) are observed consecutively when the variance source term reverses its sign. This is indeed the case for localized patches modeled as Gaussian concentration profiles

Turbulent transport of material particles: An experimental study of finite size effects

We use an acoustic Lagrangian tracking technique, particularly adapted to measurements in open flows, and a versatile material particles generator (in the form of soap bubbles with adjustable size and density) to characterize Lagrangian statistics of finite sized, neutrally bouyant, particles transported in an isotropic turbulent flow of air. We vary the size of the particles in a range corresponding to turbulent inertial scales and explore how the turbulent forcing experienced by the particles depends on their size. We show that, while the global shape of the intermittent acceleration probability density function does not depend significantly on particle size, the acceleration variance of the particles decreases as they become larger in agreement with the classical scaling for the spectrum of Eulerian pressure fluctuations in the carrier flow

Columnar structure formation of a dilute suspension of settling spherical particles in a quiescent fluid

The settling of heavy spherical particles in a column of quiescent fluid is investigated. The performed experiments cover a range of Galileo numbers ($110 \leq \text{Ga} \leq 310$) for a fixed density ratio of $\Gamma = \rho_p/\rho_f = 2.5$. In this regime the particles are known (M. Jenny, J. Du\v{s}ek and G. Bouchet, Journal of Fluid Mechanics 508, 201 (2004).) to show a variety of motions. It is known that the wake undergoes several transitions for increasing $\text{Ga}$ resulting in particle motions that are successively: vertical, oblique, oblique oscillating, and finally chaotic. Not only does this change the trajectory of single, isolated, settling particles, but it also changes the dynamics of a swarm of particles as collective effects become important even for dilute suspensions, with volume fraction up to $\Phi_V = \mathcal{O}\left(10^{-3}\right)$, which are investigated in this work. Multi-camera recordings of settling particles are recorded and tracked over time in 3 dimensions. A variety of analysis are performed and show a strong clustering behavior. The distribution of the cell areas of the Vorono\"i tessellation in the horizontal plane are compared to that of a random distribution of particles and shows clear clustering. Moreover, a negative correlation was found between the Vorono\"i area and the particle velocity; clustered particles fall faster. In addition, the angle between two adjacent particles and the vertical is calculated and compared to a homogeneous distribution of particles, clear evidence of vertical alignment of particles is found. The experimental findings are compared to simulations.Comment: 8 pages, 6 figure

Large-scale fluctuations and dynamics of the Bullard-von Karman dynamo

International audienceA synthetic fluid dynamo built in the spirit of the Bullard device (Bullard, The stability of a homopolar dynamo, Proc. Camb. Phil. Soc. 1955, 51, 744) is investigated. It is a two-step dynamo in which one process stems from the fluid turbulence, while the other part is achieved by a linear amplification of currents in external coils (Bourgoin et al., A bullard-von Karman dynamo, New J. Phys., 2006, 8, 329). Modifications in the forcing are introduced in order to change the dynamics of the flow, and hence the dynamo behavior. Some features, such as on-off intermittency at onset of dynamo action, are very robust. Large-scale fluctuations have a significant impact on the resulting dynamo, in particular in the observation of magnetic field reversals