Experimental study of gas entrainment from surface swirl

This work addresses the general topic of the gas entrainment from a free surface by a swirl issued from the pumping of fluids below the free surface. This phenomenon is investigated through an analytical experiment in water. A shear flow is generated between a horizontal flow and a stagnant flow. At the bottom of the test section, a vertical pumping is added to produce gas entrainment. One particularity of these experiments is the possibility to change the shape of the channel generating the inlet conditions by adding obstacles so as to trigger different conditions for the turbulent shear flow. Depending on the flow conditions, a surface swirl can be created with a sufficient strength to entrain gas below the free surface to the vertical pumping outlet. The frequency of gas entrainment occurrence is measured using visualizations of the flow. Two entrained regimes are identified for low and high Reynolds number flows: the surface swirl induced gas entrainment in its core reaching the suction nozzle and bubbles issued from the breaking of the gas core are pumped

PIV mapping of pressure and velocity fields in the plane magnetohydrodynamic Couette flow

We present the first simultaneous mapping of two-dimensional, time-dependent velocity and pressure fields in a plane Couette flow pervaded by a transverse magnetic field. While electromagnetic forces are strongest in fluids of high electric conductivity such as liquid metals, their opacity excludes optical optical measurement methods. We circumvent this difficulty using a transparent electrolyte (Sulfuric acid), whose weaker conductivity is offset by higher magnetic fields. We describe an experimental rig based on this idea, where the Couette flow is entrained by a tape immersed in sulfuric acid and positioned flush onto the bore of large superconducting magnet, so that most of the flow is pervaded by a sufficiently homogeneous transverse magnetic field. Velocity and pressure fields are obtained by means of a bespoke PIV system, capable of recording the fluid's acceleration as well as its velocity. Both fields are then fed into a finite difference solver that extracts the pressure field from the magnetohydrodynamic governing equations. This method constitutes the first implementation of the pressure PIV technique to an MHD flow. Thanks to it, we obtain the first experimental velocity and pressure profiles in an MHD Couette flows and show that the transitional regime between laminar and turbulent states is dominated by near-wall, isolated, anisotropic perturbations

Y-shaped jets driven by an ultrasonic beam reflecting on a wall

International audienceThis paper presents an original experimental and numerical investigation of acoustic streaming driven by an acoustic beam reflecting on a wall. The water experiment features a 2 MHz acoustic beam totally reflecting on one of the tank glass walls. The velocity field in the plane containing the incident and reflected beam axes is investigated using Particle Image Velocimetry (PIV). It exhibits an original y-shaped structure: the impinging jet driven by the incident beam is continued by a wall jet, and a second jet is driven by the reflected beam, making an angle with the impinging jet. The flow is also numerically modeled as that of an incompressible fluid undergoing a volumetric acoustic force. This is a classical approach, but the complexity of the acoustic field in the reflection zone, however, makes it difficult to derive an exact force field in this area. Several approximations are thus tested; we show that the observed velocity field only weakly depends on the approximation used in this small region. The numerical model results are in good agreement with the experimental results. The spreading of the jets around their impingement points and the creeping of the wall jets along the walls are observed to allow the interaction of the flow with a large wall surface, which can even extend to the corners of the tank; this could be an interesting feature for applications requiring efficient heat and mass transfer at the wall. More fundamentally, the velocity field is shown to have both similarities and differences with the velocity field in a classical centered acoustic streaming jet. In particular its magnitude exhibits a fairly good agreement with a formerly derived scaling law based on the balance of the acoustic forcing with the inertia due to the flow acceleration along the beam axis

Ecoulement généré par un faisceau d'ultrasons se réfléchissant sur une paroi

Le phénomène d'Acoustic Streaming permet de générer des écoulements permanents à l'aide d'ondes acoustiques. Notre équipe s'intéresse en particulier à la possibilité de créer un jet au sein d'un liquide à l'aide d'un faisceau d'ondes ultrasonores progressives. Nous avons en particulier publié plusieurs études expérimentales et théoriques montrant les ordres de grandeurs que l'on peut attendre dans ce type de jet, et la possibilité qu'ils offrent en termes de contrôle d'instabilité de convection. Nous nous intéressons ici au cas où le faisceau se réfléchi sur une paroi du réservoir contenant le liquide. En effet la plupart des applications envisagées sont en milieu relativement confiné et ce type de réflexion parait inévitable. Nous montrons comment prendre en compte cette réflexion du faisceau grâce à un modèle simple de propagation acoustique linéaire dont on déduit un champ de force générant l'écoulement. Nous montrons la bonne adéquation entre les champs de vitesses mesurés par PIV dans notre dispositif ASTRID (Acoustic STReaming Investigation Device) et les champs de vitesses obtenus numériquement à l'aide de notre modèle numérique sous STARCCM+

Etude expérimentale et théorique d'un écoulement entraîné par des ultrasons (acoustic streaming)

Il s'agit d'une étude théorique et expérimentale d'écoulements stationnaires entraînés par des ultrasons dans des liquides (en anglais : acoustic streaming). Nous développons des modèles basés sur les équations de Navier-Stokes incompressibles dans lesquelles est introduit un terme de force due aux ondes acoustiques. Un dispositif permet de mesurer les champs de pression acoustique, le champ de vitesse stationnaire par PIV ou LDA. L'objectif étant d'établir un benchmark. On présentera aussi des possibilités de contrôle d'instabilité ou d'amélioration du mélange grâce à ces écoulements

Experimental and theoretical caracterization of acoustic streaming. Prospect of an use for photovoltaic Silicon solidification.

La présente étude s'intéresse à un écoulement d'acoustic streaming, c'est-à-dire un écoulement généré par la propagation d'une onde acoustique dans un fluide. Le travail consiste à comparer deux approches: expérimentale et numérique. Les ultrasons sont émis à 2MHz par un transducteur piézo-électrique de 28.5mm de diamètre. Ce dernier est plongé dans une cuve d'eau équipée de deux parois absorbantes: l'une sert à séparer le champ proche du champ lointain et l'autre est placée à l'extrémité du domaine fluide afin d'éviter toutes réflexions. On réalise ainsi une étude en champ proche et une étude en champ lointain. Les mesures sont de deux types: champ de pression acoustique (hydrophone) et champ de vitesse (PIV). En parallèle, on effectue des simulations numériques directes avec le logiciel StarCCM+TM. Il s'agit de résoudre les équations de Navier-Stokes en fluide incompressible complétées d'un terme source de force acoustique. L'expression de ce dernier est obtenue par séparation des échelles de temps, ce qui consiste à négliger à l'échelle de temps acoustique les variations temporelles lentes, de l'écoulement généré. La démarche est ensuite analogue à celle utilisé en turbulence pour le calcul des tenseurs de Reynolds. On obtient finalement un bon accord entre les résultats expérimentaux et ceux de la modélisation numérique.Acoustic streaming, i.e. the flow induced by a propagating acoustic wave, is investigated here with both experiental and numerical approaches. The ultrasound source is a 2MHz transducer with a 29mm diameter. The transducer is introduced inside a water tank with two absorbing walls. An intermediate absorbing wall is used to separate the near field from the far field. An other absorbing wall is placed in the opposite side to teh source to avoid reflective waves. Both near field and far field are studied. The measurements concern the acoustic pressure field (hydrophone) and the velocity field (PIV). Numerical simulations are also performed with the software STARCCM+TM. They solve the incompressible Navier-Stokes equations with an acoustic force source term. Ths term is obtained by time scale separation: the slow variations of the flow are neglected on an acoustic time scale with regard to the fast variations of the acoustic field. The procedure is then similar to that used in turbulence for Reynolds stress calculation. A good agreement is eventually obtained between the experimental and numerical results

Caractérisation expérimentale et théorique des écoulements entraînés par ultrasons. Perspectives d'utilisation dans les procédés de solidification du Silicium Photovoltaïque

Acoustic streaming, i.e. the flow induced by a propagating acoustic wave, is investigated here with both experiental and numerical approaches. The ultrasound source is a 2MHz transducer with a 29mm diameter. The transducer is introduced inside a water tank with two absorbing walls. An intermediate absorbing wall is used to separate the near field from the far field. An other absorbing wall is placed in the opposite side to teh source to avoid reflective waves. Both near field and far field are studied. The measurements concern the acoustic pressure field (hydrophone) and the velocity field (PIV). Numerical simulations are also performed with the software STARCCM+TM. They solve the incompressible Navier-Stokes equations with an acoustic force source term. Ths term is obtained by time scale separation: the slow variations of the flow are neglected on an acoustic time scale with regard to the fast variations of the acoustic field. The procedure is then similar to that used in turbulence for Reynolds stress calculation. A good agreement is eventually obtained between the experimental and numerical results.La présente étude s'intéresse à un écoulement d'acoustic streaming, c'est-à-dire un écoulement généré par la propagation d'une onde acoustique dans un fluide. Le travail consiste à comparer deux approches: expérimentale et numérique. Les ultrasons sont émis à 2MHz par un transducteur piézo-électrique de 28.5mm de diamètre. Ce dernier est plongé dans une cuve d'eau équipée de deux parois absorbantes: l'une sert à séparer le champ proche du champ lointain et l'autre est placée à l'extrémité du domaine fluide afin d'éviter toutes réflexions. On réalise ainsi une étude en champ proche et une étude en champ lointain. Les mesures sont de deux types: champ de pression acoustique (hydrophone) et champ de vitesse (PIV). En parallèle, on effectue des simulations numériques directes avec le logiciel StarCCM+TM. Il s'agit de résoudre les équations de Navier-Stokes en fluide incompressible complétées d'un terme source de force acoustique. L'expression de ce dernier est obtenue par séparation des échelles de temps, ce qui consiste à négliger à l'échelle de temps acoustique les variations temporelles lentes, de l'écoulement généré. La démarche est ensuite analogue à celle utilisé en turbulence pour le calcul des tenseurs de Reynolds. On obtient finalement un bon accord entre les résultats expérimentaux et ceux de la modélisation numérique

From flying wheel to square flow: Dynamics of a flow driven by acoustic forcing

International audienceAcoustic streaming designates the ability to drive quasisteady flows by acoustic propagation in dissipative fluids and results from an acoustohydrodynamics coupling. It is a noninvasive way of putting a fluid into motion using the volumetric acoustic force and can be used for different applications such as mixing purposes. We present an experimental investigation of a kind of square flow driven by acoustic streaming, with the use of beam reflections, in a water tank. Time-resolved experiments using particle image velocimetry have been performed to investigate the velocity field in the reference plane of the experiments for six powers: 0.5, 1, 2, 4, 6, and 8 W. The evolution of the flow regime from almost steady to strongly unsteady states is characterized using different tools: the plot of time-averaged and instantaneous velocity fields, the calculation of presence density maps for vortex positions and for the maximal velocity and vorticity crest lines, and the use of spatiotemporal maps of the waving observed on the jets created by acoustic streaming. A transition is observed between two regimes at moderate and high acoustic forcing

Instabilities in the Rayleigh-Benard-Eckart problem

International audienceThis study is a linear stability analysis of the flows induced by ultrasound acoustic waves (Eckart streaming) within an infinite horizontal fluid layer heated from below. We first investigate the dependence of the instability threshold on the normalized acoustic beam width Hb for an isothermal fluid layer. The critical curve, given by the critical values of the acoustic streaming parameter, Ac, has a minimum for a beam width Hb≈0.32. This curve, which corresponds to the onset of oscillatory instabilities, compares well with that obtained for a two-dimensional cavity of large aspect ratio [Ax=(length/height)=10]. For a fluid layer heated from below subject to acoustic waves (the Rayleigh-Bénard-Eckart problem), the influence of the acoustic streaming parameter A on the stability threshold is investigated for various values of the beam width Hb and different Prandtl numbers Pr. It is shown that, for not too small values of the Prandtl number (Pr>Prl), the acoustic streaming delays the appearance of the instabilities in some range of the acoustic streaming parameter A. The critical curves display two behaviors. For small or moderate values of A, the critical Rayleigh number Rac increases with A up to a maximum. Then, when A is further increased, Rac undergoes a decrease and eventually goes to 0 at A=Ac, i.e., at the critical value of the isothermal case. Large beam widths and large Prandtl numbers give a better stabilizing effect. In contrast, for Prandtl numbers below the limiting value Prl (which depends on Hb), stabilization cannot be obtained. The instabilities in the Rayleigh-Bénard-Eckart problem are oscillatory and correspond to right- or left-traveling waves, depending on the parameter values. Finally, energy analyses of the instabilities at threshold have indicated that the change of the thresholds can be connected to the modifications induced by the streaming flow on the critical perturbations