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 $Re_p$

Periodic formation and propagation of double layers in the expanding chamber of an inductive discharge operating in Ar/ SF 6 mixtures

International audienceIt has previously been shown ͓Tuszewski et al., Plasma Sources Sci. Technol. 12, 396 ͑2003͔͒ that inductive discharges in electronegative gases are subject to two types of instability: the source instability related to the E to H transition and a transport instability, occurring downstream when an expanding chamber is present. These two types of instability are observed in our " helicon " reactor operated without a static magnetic field in low-pressure Ar/ SF 6 mixtures. Temporally and spatially resolved measurements show that, in our experiment, the downstream instability is a periodic formation and propagation of a double layer. The double layer is born at the end of the source tube and propagates slowly to the end of the expansion region with a velocity of 150 m s −1

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)

Transition from hydrodynamic turbulence to magnetohydrodynamic turbulence in von Kármán flows

International audienceThe influence of an externally applied magnetic field on flow turbulence is investigated in liquid-gallium von-Ka ́rma ́n (VK) swirling flows. Time-resolved measurements of global variables (such as the flow power consumption) and local recordings of the induced magnetic field are made. From these measurements, an effective Reynolds number is introduced as Rmeff = Rm(1−α√N), so as to take into account the influence of the interaction parameter N. This effective magnetic Reynolds number leads to unified scalings for both global variables and the locally induced magnetic field. In addition, when the flow rotation axis is perpendicular to the direction of the applied magnetic field, significant flow and induced magnetic field fluctuations are observed at low interaction parameter values, but corresponding to an Alfve'n speed vA of the order of the fluid velocity fluctuations urms. This strong increase in the flow fluctuations is attributed to chaotic changes between hydrodynamic and magnetohydrodynamic velocity profiles

Flow dynamics and magnetic induction in the von-Karman plasma experiment

The von-Karman plasma experiment is a novel versatile experimental device designed to explore the dynamics of basic magnetic induction processes and the dynamics of flows driven in weakly magnetized plasmas. A high-density plasma column (10^16 - 10^19 particles.m^-3) is created by two radio-frequency plasma sources located at each end of a 1 m long linear device. Flows are driven through JxB azimuthal torques created from independently controlled emissive cathodes. The device has been designed such that magnetic induction processes and turbulent plasma dynamics can be studied from a variety of time-averaged axisymmetric flows in a cylinder. MHD simulations implementing volume-penalization support the experimental development to design the most efficient flow-driving schemes and understand the flow dynamics. Preliminary experimental results show that a rotating motion of up to nearly 1 km/s is controlled by the JxB azimuthal torque

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

Long-term memory in experiments and numerical simulations of hydrodynamic and magnetohydrodynamic turbulence

International audienceWe analyze time series stemming from experiments and direct numerical simulations of hydrodynamic and magnetohydrodynamic turbulence. Simulations are done in periodic boxes, but with a volumetric forcing chosen to mimic the geometry of the flow in the experiments, the von Kármán swirling flow between two counterrotating impellers. Parameters in the simulations are chosen to (within computational limitations) allow comparisons between the experiments and the numerical results. Conducting fluids are considered in all cases. Two different configurations are considered: a case with a weak externally imposed magnetic field and a case with self-sustained magnetic fields. Evidence of long-term memory and 1/f noise is observed in experiments and simulations, in the case with weak magnetic field associated with the hydrodynamic behavior of the shear layer in the von Kármán flow, and in the dynamo case associated with slow magnetohydrodynamic behavior of the large-scale magnetic field