On the existence and evolution of a spanwise vortex in laminar shallow water dipoles

The present work investigates the existence and evolution of a spanwise vortex at the front of shallow dipolar vortices. The vortex dipoles are experimentally generated using a double flap apparatus. Particle image velocimetry measurements are performed in a horizontal plane and in the vertical symmetry plane of the flow. The dynamics of such vortical structures is investigated through a parametric study in which both the Reynolds number Re=U0D0/ν∈[90,470] and the aspect ratio α = h/D0∈[0.075,0.7],associated with the shallowness of the flow, are varied, where U0 is the initial velocity of the vortex dipole, D0 is the initial diameter, h is the water depth, and v is the kinematic viscosity of the fluid. The present experiments confirm the numerical results obtained in a companion paper by Duran-Matute et al. [Phys. Fluids 22, 116606 (2010)], namely that the flow remains quasi parallel with negligible vertical motions below a critical value of the parameter α2Re. By contrast, for large values of α2Re and α≲0.6, a three-dimensional regime is observed in the shape of an intense spanwise vortex generated at the front of the dipole. The present study reveals that the early-time motion and dynamics of the spanwise vortex do not scale on the unique parameter α2Re but is strongly influenced by both the aspect ratio and the Reynolds number. A mechanism for the generation of the spanwise vortex is proposed. For α≳0.6, a third regime is observed, where the spanwise vortex is replaced by a vorticity tongu

Dynamique tridimensionnelle de dipôles tourbillonnaires en eau peu profonde

Les dipôles tourbillonnaires représentent des structures récurrentes dans les écoulements en eau peu profonde. Ils résultent souvent d'une bidimensionnalisation de la turbulence qui est liée à l'effet du confinement vertical. Pourtant, dans plusieurs études récentes, une structure tourbillonnaire secondaire à axe horizontal a été observée sur le front de dipôles expérimentaux en eau peu profonde, remettant ainsi en question l'hypothèse de bidimensionnalité de l'écoulement. Dans le cadre de ces travaux de thèse, nous nous sommes intéressés à ce dernier point en étudiant la génération et la dynamique d'une structure secondaire sur le front du dipôle. Les dipôles tourbillonnaires sont générés expérimentalement par la fermeture d'une paire de pales dans une couche d'eau de faible épaisseur initialement au repos afin d'assurer une structure bidimensionnelle initiale. Les paramètres sans dimension pilotant l'écoulement sont le nombre de Reynolds Re = U0D0/_ (ou U0 et D0 représentent respectivement la vitesse de propagation et le diamètre initial du dipôle et _ est la viscosité cinématique de l'eau) et le rapport de forme des dipôles (ou nombre de confinement) C = h/D0 (ou h est la hauteur d'eau). Une étude paramétrique dans laquelle le nombre de Reynolds Re et le rapport de forme du dipôle C varient a été menée (C 2 [0.075, 0.7] et Re 2 [90, 470]). Dans le champ des paramètres étudiés, cinq différentes structures de l'écoulement sont observées. Une cartographie des différents régimes d'écoulement a été obtenue et a permis de montrer que la tridimensionnalisation des dipôles tourbillonnaires en eau peu profonde était contrôlée par un unique paramètre, C2Re. Lorsque une structure secondaire est observée sur le front du dipôle, la dynamique aux temps courts de celle-ci dépend du nombre de Reynolds alors que sa dynamique aux temps longs dépend du rapport de forme du dipôle. Afin de caractériser la structure dans son ensemble et en particulier d'accéder à chacune des étapes de sa tridimensionnalisation, une nouvelle technique de PIV 3D-3C scannée a été mise en place. Elle permet une mesure expérimentale du champ de pression et révéle que les branches du tourbillon transverse généré sur le front du dipôle montent en direction de la surface libre de part et d'autre des tourbillons primaires composant le dipôle.Vortex dipoles are recurrent coherent structures in shallow water flows. Usually they result from the collapse of patches of turbulence due to vertical confinement. Yet, recent studies have shown the presence of a secondary vortical structure at the front of shallow dipolar vortices with horizontal vorticity, contrasting with the usual concept of two-dimensional flows in shallow water configurations. This possible threedimensionalization has been investigated in the present study, in particular, the criteria for the existence of the spanwise vortex, its generation process and its dynamics. To this end, and to assure initially two-dimensional vortices, vortex dipoles are generated by a flap apparatus in a shallow water layer which is initially at rest. The non-dimensional parameters governing the flow are the Reynolds number Re = U0D0/_ (where U0 and D0 represent the initial propagation velocity of the dipole and the initial diameter of the dipole, respectively, and _ is the kinematic viscosity) and the aspect ratio of the dipole (or confinement number) C = h/D0 (where h is the water depth). To investigate the conditions of the three-dimensionalization, a parametric study in which Re and C are both varied has been performed (C 2 [0.075, 0.7] and Re 2 [90, 470]). In the range of parameters investigated, five different structures of the flow have been observed. A cartography of the different topologies in the parameters space and a criterion for the three-dimensionalization of the flow are given, in particular, the three-dimensionalization of shallow dipoles is shown to depend on a single parameter, C2Re. When the spanwise vortex exists, its early-time dynamics depends on the Reynolds number, whereas its late-time dynamics depends on the dipole aspect ratio. The vortex dipole having been shown to be intrinsically three-dimensional, a new 3D-3C scanning PIV measurement technique has been set-up and used to investigate the flow structure. It allows an experimental measure of the pressure field and reveals that the branches of the spanwise vortex generated on the dipole front ascend to the free surface on each side of the primary vortices composing the dipole

Splashing or not

The splashing of a droplet when impacting a solid surface is common to our everyday experience as well as to industrial applications that require controlled deposition of liquid mass. Still the mechanism for splashing is not well understood. A recent study showed that a decrease in the ambient pressure inhibits splashing, motivating a hypothesis on the existence of a thin film of air trapped between the drop and the surface. The early dynamics of splashing could occur while the drop is still spreading on an air film. To gain insight into this early dynamics, we supplement the side view with a synchronized bottom view, obtained using a novel Total Internal Reflection technique. I will discuss the existence of a transition regime between spreading and splashing. This regime appears by changing the impact velocity or the ambient pressure, while keeping the other fixed

A three-dimensional experimental investigation of the structure of the spanwise vortex formed by a shallow vortex dipole

The three-dimensional dynamics of shallow vortex dipoles is investigated by means of an innovative 3D-3C (three dimensions, three components) scanning PIV technique. In particular, the three-dimensional structure of a frontal spanwise vortex is characterized. The technique also allows the computation of the pressure field, which is not available using standard 2D PIV measurement. The influence of such complex vortex structures on the mass transport is discussed in light of the available pressure field

3D measurements of inclined vortex rings interacting with a density stratification

Vortex rings are coherent vortical structures that dominate the dynamics of numerous flows as they are generated each time an impulsive jet occurs in a homogeneous fluid. They are also considered as elementary bricks of turbulence. Their faculty to propagate along their revolution axis by self-induction confers to such structures interesting transport properties, namely, transport of momentum, mass and heat. They are therefore often qualified as good candidates for mixing. From this perspective, the present study addresses the interaction of a vortex ring with a density stratification in order to get a better understanding of the subsequent mixing mechanisms. A new 3D time-resolved technique is used and gives a highlight at short timescale on the 3D vorticity reorganization and at larger timescale on the 3D patterns of internal gravity waves forced by the impacting/penetrating vortex. The influence of the Reynolds number of the vortex ring and its angle of attack relative to isopycnals will be detailed

A three-dimensional experimental investigation of the structure of the spanwise vortex generated by a shallow vortex dipole

The three-dimensional dynamics of shallow vortex dipoles is investigated by means of an innovative three-dimensional, three-component (3D-3C) scanning PIV technique. In particular, the three-dimensional structure of a frontal spanwise vortex is characterized. The technique allows the computation of the three-dimensional pressure field and the planar (x, y) distribution of the wall shear stress, which are not available using standard 2D PIV measurements. The influence of such a complex vortex structure on mass transport is discussed in the context of the available pressure and wall shear stress fields

Vortex rings in non-Newtonian viscoelastic fluids play yo-yo

Vortex rings are coherent vortical structures widely presents in geophysical flows and engineering applications. Numerous applications imply industrial processes including food processing, or petrol industry. Those applications are very often confronted with non-Newtonian fluids. Nevertheless, to the best of our knowledge, only few studies dealing with vortex dynamics in non-Newtonian shear-thinning fluids exist, and none with viscoelastic ones. The aim for the present study is to characterize experimentally the dynamics of vortex rings generated thanks to a piston-cylinder apparatus in various viscoelastic fluids as a function of the generalized Reynolds number, the piston stroke and the final piston position relative to the cylinder exit. In particular, the elastic property of the fluid will be highlighted by the furling-unfurling of vortex rings

Dynamics of vortex rings in viscoelastic fluids

Vortex rings are coherent vortical structures widely encountered in geophysical flows and engineering applications. They are found in industrial systems, for instance during injection processes or in the flow in the vicinity of blades in mixing systems. Numerous studies are concerned by vortex rings. But, to the best of our knowledge, only few of them address vortex dynamics in non-Newtonian shear-thinning fluids, and none in viscoelastic ones while such fluids are widely involved in industrial processes. The purpose is here to study the dynamics of vortex rings in viscoelastic fluids. In the present experiments these structures are generated through a piston-cylinder system and the mechanical parameters for injection are the piston velocity and stroke, Vp and L. Three different viscoelastic fluids are used: two aqueous solutions of Zetag 7587 (0.04% and 0.1%) and a solution of PAM (0.1%). The Newtonian reference fluid is water. A fluorescent dye visualization technique is used and images are recorded using a HD camera (2160x2560 pixels, 50Hz). In addition to the video sequences obtained, image processing is used to determine the two main characteristics of the vortex ring: its motion and its size. As expected vortex ring in Newtonian fluid furls, progresses downstream by auto-induced effects and diffuses (as materialized by its diameter increase). The behaviour strongly differs for Non-Newtonian viscoelastic fluids: first, the rolling up phase is delayed and occurs further downstream. Then the vortex ring actually furls and expends during its way downstream. Contrarily to non-elastic fluids and unexpectedly, the viscoelastic ring afterwards stops, unfurls and goes back, inducing a contraction in the radial direction. This new dynamics is studied through time evolution of vortex position and diameter for different fluids and flow configurations. The competitive influences of the fluid nature and elasticity and of inertia are emphasized

Dynamics of an inclined Vortex Ring interacting with a density stratification

Vortex Rings are coherent vortical structures that dominate the dynamics of numerous flows as they are generated each time an impulsive jet occurs in a homogeneous fluid (for instance, plumes can be considered as Vortex Rings). Such structures have the faculty to self-propagate along their revolution axis, conferring them capacities of transport and mixing that could be exploited. Among applications, one can mention nuclear safety and the need to mix fluids of different density to prevent explosion hazard. The scope of the present study is to identify and evaluate the mixing mechanisms associated with a Vortex Ring interacting with a density stratification, in particular, the reorganization of the flow and the generation of internal waves. The influence of the Vortex Ring propagation speed and propagation angle relative to the density gradient on its dynamics and mixing power are studied thanks to 2D and 3D time-resolved TOMO-PIV

Splashing or not

The splashing of a droplet when impacting a solid surface is common to our everyday experience as well as to industrial applications that require controlled deposition of liquid mass. Still the mechanism for splashing is not well understood. A recent study showed that a decrease in the ambient pressure inhibits splashing, motivating a hypothesis on the existence of a thin film of air trapped between the drop and the surface. The early dynamics of splashing could occur while the drop is still spreading on an air film. To gain insight into this early dynamics, we supplement the side view with a synchronized bottom view, obtained using a novel Total Internal Reflection technique. I will discuss the existence of a transition regime between spreading and splashing. This regime appears by changing the impact velocity or the ambient pressure, while keeping the other fixed