La rencontre entre une couche limite laminaire se développant sur une plaque plane sous une surface libre et un obstacle rectangulaire émergent mène à l’apparition d’une zone de recirculation complexe appelée tourbillon en fer à cheval à l’amont de l’obstacle. Cette zone de recirculation est composée d’un certain nombre de vortex s’enroulant autour de l’obstacle et pouvant présenter des dynamiques variées. Le but de ce travail est de réaliser une étude expérimentale paramétrique du tourbillon en fer à cheval laminaire et faiblement turbulent. Les mesures sont réalisées par PIV et par trajectographies et des méthodes de détection basés sur la décomposition modale (POD) et la détection des points critiques sont utilisées afin de faciliter l’étude de la dynamique tourbillonnaire. Ce manuscrit se propose de : (i) Déterminer l’évolution des caractéristiques du tourbillon en fer à cheval en fonction des paramètres adimensionnels de l’écoulement. (ii) Mettre en place une typologie des différentes dynamiques exhibées. (iii) Déterminer les mécanismes à l’origine de l’apparition de ces différentes dynamiques à l’aide d’un modèle conceptuel. (iv) Et finalement, étudier l’influence de la longueur de l’obstacle sur le tourbillon en fer à cheval. / A laminar boundary layer developing under a free-surface and interacting with an emerging obstacle lead to the creation of a complex recirculation zone called horseshoe vortex upstream from the obstacle. This recirculation zone is composed of a given number of vortices wrapping around the obstacle with various kind of dynamics. The aim of this work is to perform an experimental parametric study of the laminar, and slightly turbulent horseshoe vortex. Measurements are obtained by PIV and trajectographies and detection method based on modal decomposition (POD) and critical points are used to ease the study of the vortex dynamics. This manuscript intends to : (i) Explain the horseshoe vortex main characteristics evolution with the non-dimensional parameters of the flow. (ii) Define a typology of the different observed vortex dynamics. (iii) Identify the physical mechanisms at the origin of those dynamics through the use of a conceptual model. (iv) And finally, show the influence of the obstacle length on the horseshoe vortex.

Cette thèse étudie la formation et l'évolution des jets et des vortexdans les atmosphères planétaires turbulentes, à l'aide d'une doubleapproche de simulations numériques et d'expériences delaboratoire. Pour l'approche numérique, un modèle en fluidesshallow-water quasi-géostrophique à deux couches dans le plan betaavec des conditions canal a été utilisé. Comme dans Panetta (1988), onimplémente un cisaillement vertical pour représenter le gradientlatitudinal de température moyenné spatialement, qui est partiellementmaintenu par un forçage thermique. Les instabilités baroclinesaffaiblissent le gradient de température, alors que le forçagethermique le restaure, ce qui crée une dynamique non-linéaire trèsriche.Tout d'abord, nous avons considéré l'écoulement sur un fond plat, etavons modélisé les mouvements convectifs par des paires decyclones/anticyclones ou `hetons' comme dans Thomson (2016). Nousobtenons ainsi des jets principalement baroclines, oscillants entredes phases calmes et des phases turbulentes, où l'écoulement perd sazonalité. Des vortex se forment à partir des jets méandreux etl'énergie zonale diminue alors que l'énergie tourbillonnaireaugmente. Ces phases turbulentes durent typiquement pendant unepériode de relaxation du forçage thermique. On étudie les effets ducisaillement vertical, du forçage thermique et des hetons, enregardant les transferts d'énergie entre les énergies cinétiques etpotentielles, leurs composantes barotropes et baroclines ainsi queleurs composantes zonales et tourbillonnaires. Ceci nous amène àrepenser le paradigme classique des transferts d'énergie présenté dansSalmon (1982). De plus, nous étudions comment une analyse de stabilitélinéaire de l'écoulement zonal instantané est reliée aux phases calmeset turbulentes.Ensuite, nous considérons l'effet d'une topographie de grande échelle,comme une première approche pour comprendre le rôle de la topographiedans la formation des jets et des vortex. Nous utilisons le mêmemodèle que dans la première étude mais nous ajoutons un fondtopographique linéaire méridionalement, qui a l'avantage de dépendred'un seul paramètre, la pente. Une pente négative approfondit lacouche inférieure par rapport à un fond plat, ce qui augmente lepotentiel des instabilités baroclines, alors qu'une pente positive aun effet stabilisateur. Nous supprimons le forçage par les hetons etperturbons l'écoulement grâce à une zone de Rossby de faibleamplitude dans la couche inférieure à l'instant initial. L'effetprincipal du forçage par les hetons est d'agir comme une sorted'amortissement : les fluctuations de l'énergie sont constamment plusextrêmes que sans forçage. Une analyse de stabilité linéaire esteffectuée afin de déterminer les zones de stabilité etd'instabilité.Pour l'étude expérimentale, nous utilisons une cuve tournanteremplie par deux couches de fluides avec une stratification au sel etun couvercle rigide en rotation différentielle. Nous étudions unfront barocliniquement instable dans le régime des vacillationsd'amplitude, qui est caractérisé par l'émergence et ladisparition de vortex de grande échelle. L'analyse de deuxexpériences à la limite de la géostrophie, avec des nombres deRossby de Ro=0.4 et Ro=0.6, montre des comportement trèsdifférents. Pour un faible nombre de Rossby, nous observons desdipôles baroclines alors que pour un large nombre de Rossby nousobtenons des vortex barotropes. Nous examinons l'activité des ondesde petite échelle par différentes méthodes qui révèlent laprésence d'ondes d'inertie gravité comme précurseures del'émergence des vortex.Afin de poursuivre nos recherches sur les fronts à l'interface entredeux couches de fluides immiscibles, nous avons développé unenouvelle méthode de détection de la hauteur et de la pente baséesur les lois optiques de la réfraction. Les équations théoriquesassociées sont résolues numériquement et validées à l'aidede plusieurs situations idéalisées. / This thesis investigates the formation and evolution of jets andvortices in turbulent planetary atmospheres using a dual approach ofhigh-resolution numerical simulations and novel laboratoryexperiments. A two-layer quasi-geostrophic beta-channel shallow watermodel is used for the numerical study. As in Panetta (1988), avertical shear is implemented to represent a spatially-meanlatitudinal temperature gradient, which is partially maintained bythermal damping. Baroclinic instabilities work to erode thetemperature gradient, while thermal damping acts to restore it. Asthe basic state vertical shear is unstable, the thermal damping cannotlead to a full recovery, thus modifying subsequent instabilities andleading to rich nonlinear dynamical behaviour.First, we consider flow over a flat bottom, and model convectivemotions like those thought to occur on Jupiter by pairs ofcyclones/anti-cyclones or `hetons' as in Thomson (2016). We therebyobtain predominantly baroclinic jets, oscillating between quiescentphases, when jets are zonal and the energy is nearly stationary, andturbulent phases, when the flow loses its zonality, vortices pinch offfrom the meandering jets, and zonal energy components drop while eddyenergy components increase. These turbulent phases typically last fora thermal damping relaxation period. The impacts of vertical shear(baroclinicity), thermal damping and heton forcing are comprehensivelyinvestigated by considering the energy transfers occurring betweenkinetic and potential energy, their barotropic and baroclinic parts aswell as their zonal and eddy parts. This leads to a rethinking of theclassic paradigm of energy transfer presented by Salmon (1982), asthis paradigm is too simplistic to explain the results found.Then, we consider the effect of large-scale bottom topography, as afirst approach to understanding the role of topography in jet andvortex formation. We use the same model as in the first study butinclude a linearly sloping topography which has the advantage of beingcharacterised by a single parameter, the slope. We omit the hetonforcing and instead perturb the flow with a small amplitude Rossbywave initially. The main effect of heton forcing is actually to act asa kind of damping: energy fluctuations are consistently less extremethan when no forcing is used. A linear stability analysis is carriedout to motivate a series of nonlinear simulations investigating theeffect of topography, in particular, differences from the flat bottomcase previously examined. We find that destabilising topography makesthe jets more dynamic.In the experimental part, a two-layer salt-stratified fluid is used ina rotating tank with a differentially rotating lid to generate theshear across the interface. We consider a baroclinically unstablefront in the regime of amplitude vacillation, which is found to becharacterised by the sequential emergence and disappearance of alarge-scale vortex. Analysing two similar experiments at the limit ofgeostrophy, with different Rossby numbers Ro=0.4 and Ro=0.6, showssurprisingly different behaviours, with a baroclinic dipole for small,and a barotropic vortex for the large Rossby number. The small-scalewave activity is explored using different methods, and the resultssuggest small, spontaneously-arising inertia-gravity waves precedingthe emergence of the vortex which stirs the interface, thus having animpact on the mixing between the two layers. The recovery period ofthe amplitude vacillation, as well as the intensity of the vortex,increases with the Rossby number.For further research on fronts at two-layer immiscible interfaces, avery accurate novel optical method has been developed to detect theheight and slope, based on the refractive laws of optics. Theassociated theoretical equations are solved numerically and validatedin various idealised situations.

Unconventional superconductors are those which are not described by the BCS theory, and for which no known theoretical description currently exists. The careful study of the behaviour of superconducting vortices in such systems yields crucial insights into the underlying physics of these exciting materials. This thesis describes a series of magnetometry experiments conducted on three different unconventional superconductors: Sr2RuO4, MgB2 and Bi2Sr2CaCu2O8+; utilising two techniques: Scanning Hall probe microscopy and magneto-optical imaging. An exotic p-wave chiral order parameter is thought to exist in Sr2RuO4 and is expected to produce several identifiable magnetic signatures. A search for these signals via scanning Hall probe measurements failed to detect any such signatures, but did capture a structural transition of the vortex lattice that is consistent with the proposed chiral order parameter. Studies of several samples also suggest that the vortex behaviour is strongly modified with even tiny amounts of disorder, a conclusion that has important consequences for interpretations of vortex patterns in Sr2RuO4. Several recent experiments have reported vortex configurations consistent with a competing short-range repulsion and long-range attraction in the intervortex interaction in MgB2 single crystals. We observe the spontaneous formation of vortex chains and labyrinths in a 160nm MgB2 thin-film that are suggestive of a non-monotonic vortex interaction, but perhaps more indicative of an intermediate-range attraction in harness with short and long-range repulsions. The suitability of seven potential mechanisms of vortex attraction in MgB2 are reviewed in light of the unusually short electronic mean-free path of our sample. Finally, magneto-optical imaging has been used to study the penetration of flux into regular polygon-shaped Bi2Sr2CaCu2O8+ platelets with various geometries. The variation of HP with geometry qualitatively contradicts conventional estimates of demagnetisation factors based on equivalent ellipsoids using inscribed circles. This work has important implications for the estimation of appropriate effective demagnetisation factors in arbitrarily shaped superconducting bodies.

Thesis (Sc.D.)--Massachusetts Institute of Technology, Dept. of Chemical Engineering, 1979. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND SCIENCE. / Vita. / Bibliography: leaves 381-382. / Sc.D.

Thesis (Chem.E)--Massachusetts Institute of Technology, Dept. of Chemical Engineering, 1980. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND SCIENCE. / Includes bibliographies. / by Kenneth George Hellyar. / Chem.E

Onshore and offshore structures are subject of strict safety regulations, and acceptable design implies requirements for accurate models of potentially dangerous phenomena. The phenomenon of Vortex-Induced Vibrations (VIVs) develops when a slender structure interacts with a fluid flow. Vortices grow in the disturbed boundary layer and spread behind the structure, resulting in fluctuations of the fluid forces acting on the body. Slender structures are present almost everywhere in the form of tall buildings and skyscrapers, cranes, antennas, power lines, suspension bridges, umbilicals, risers and free spans of pipelines which deliver water, oil and gas. The deeper in the water and higher in the sky these structures are, more likely they can experience VIVs and the lock-in state due to the exposure to various flow profiles. The wake oscillator method allows to model fluid variables during VIV lock-in using self-excited and self-limited oscillators of Van der Pol or Rayleigh type. In this research, the capabilities of alternative nonlinear oscillators as fluid equations are considered for modelling elastically supported rigid structures with one and two degrees-of-freedom in uniform flow. For modelling two-dimensional flexible structures in uniform and sheared flows, new wake oscillator models are developed in this work and applied with alternative damping terms. The dynamics of the uniform flow model of flexible structure is investigated in detail with the focus on coexisting solutions of the displacement amplitudes. Empirical coefficients for wake oscillator models are calibrated in this study using constrained nonlinear minimization and experimental data available in the literature. The validation performed confirms the most successful results for the suite of models of 2DOF rigid structure for low mass ratio, where agreement with both in-line and cross-flow displacement records was obtained.

Vortices interacting with the solid surface of aerodynamic bodies are prevalent across a broad range of geometries and applications, such as dynamic stall on wind turbine and helicopter rotors, the separated flows over flapping wings of insects, birds, formation of the vortex wakes of bluff bodies, and the lift-producing vortices formed by aircraft leading-edge extensions and delta wings. This study provides fundamental insights into the formation and evolution of such vortices by considering the leading-edge vortices formed in variations of a canonical flapping wing problem. Specifically, the vorticity transport for three distinct maneuvers are examined, a purely rolling wing, a purely pitching wing and a rolling and pitching wing, of aspect-ratio two. Once the maneuvers are characterized, a passive bleed hole will be introduced to a purely rolling wing, to alter flow topology and vorticity transport governing the circulation on the wing. Three-dimensional representations of the velocity and vorticity fields were obtained via plenoptic particle image velocimetry (PPIV) measurements are used to perform a vorticity flux analysis that serves to identify the sources and sinks of vorticity within the flow. Time-resolved pressure measurements were obtained from the surface of the airfoil, and used to characterize the flux of vorticity diffusing from the solid surface. Upon characterizing all of the sources and sinks of vorticity, the circulation budget was found to be fully accounted for. Interpretation of the individual vorticity balance contributions demonstrated the Coriolis acceleration did not contribute to vorticity generation and was a correction term for the apparent vorticity. The transport characteristics varied among the three cases that were investigated. The spanwise convective contribution was signification over various spanwise locations for the pure roll case. For the pure pitch the shear layer contribution and the diffusive contribution. The circulation was dependent the pitch rate, which was evident only at the beginning of the motion, and circulation growth at later times depended only on the pitch angle.The combined pitch roll cases, the transport behavior strongly resembled that of pitch, with little evidence of roll influence, despite that the flow structure and circulation distribution on the inboard part of the wing exhibited roll-like behaviors. In the final case where the wing is pitching and rolling , the shear layer contribution was balanced by the diffusive contribution, similar to that of the pure pitch case. By adding a passive bleed hole to the purely rolling cases, it was found to alter the both the flow topology and vorticity transport.

La résonance de plasma Josephson (JPR) nous permet d'évaluer la distance moyenne rw des excursions thermiques relatives de deux vortex bidimensionnels consécutifs sur la même ligne de flux, dans le supraconducteur lamellaire Bi2Sr2CaCu2O8+d (BSCCO). Ici, on a déterminé rw dans le solide de vortex dans des monocristaux de BSCCO sous-dopés, à l'aide de mesures expérimentales de la JPR en utilisant la perturbation de cavité résonante et la méthode bolométrique. Dans les échantillons vierges, sous faible champ magnétique, la dépendance en température T et en champ B de rw s'explique par le rôle dominant de la tension de ligne et sa renormalisation par les fluctuations thermiques. Celles-ci provoquent son amollissement, conduisant à la transition de phase du premier ordre entre le solide et le liquide de vortex. On observe une dépendance rw(T,B) identique dans les cristaux irradiés aux ions lourds, pour B << Bf , validant une description en terme de 'supraconducteur discret' du matériau.

We present an experimental study on the behavior of bubbles captured in a Taylor vortex. The gap between a rotating inner cylinder and a stationary outer cylinder is filled with a Newtonian mineral oil. Beyond a critical rotation speed (ω[subscript c]), Taylor vortices appear in this system. Small air bubbles are introduced into the gap through a needle connected to a syringe pump. These are then captured in the cores of the vortices (core bubble) and in the outflow regions along the inner cylinder (wall bubble). The flow field is measured with a two-dimensional particle imaging velocimetry (PIV) system. The motion of the bubbles is monitored by using a high speed video camera. It has been found that, if the core bubbles are all of the same size, a bubble ring forms at the center of the vortex such that bubbles are azimuthally uniformly distributed. There is a saturation number (N[subscript s]) of bubbles in the ring, such that the addition of one more bubble leads eventually to a coalescence and a subsequent complicated evolution. Ns increases with increasing rotation speed and decreasing bubble size. For bubbles of non-uniform size, small bubbles and large bubbles in nearly the same orbit can be observed to cross due to their different circulating speeds. The wall bubbles, however, do not become uniformly distributed, but instead form short bubble-chains which might eventually evolve into large bubbles. The motion of droplets and particles in a Taylor vortex was also investigated. As with bubbles, droplets and particles align into a ring structure at low rotation speeds, but the saturation number is much smaller. Moreover, at high rotation speeds, droplets and particles exhibit a characteristic periodic oscillation in the axial, radial and tangential directions due to their inertia. In addition, experiments with non-spherical particles show that they behave rather similarly. This study provides a better understanding of particulate behavior in vortex flow structures. / Singapore-MIT Alliance (SMA)

In an attempt to generalize previous models of the bidirectional vortex mean flow, a new solution is presented that can cope with arbitrary injections and outlet conditions. In the process, the steady, inviscid and axisymmetric equations of motions are reduced to one partial differential equation for the stream function, known as the Bragg-Hawthorne equation, which is solved exactly. The solution is shown to be highly dependent on the imposed boundary conditions: the mean flow changes according to the manner by which the fluid is injected or extracted from the vortex chamber. From the stream function, the velocity is obtained along with the vorticity and pressure distributions which are carefully derived and analyzed. The results are then compared to several inviscid models found in the literature. After determining an exact inviscid solution to the problem, viscous effects at the core are added to overcome the known singularity that arises at the centerline. The governing equations are hence revisited while keeping the viscous diffusion term in the tangential momentum equation. The core region, where viscous effects lead to the onset of a forced vortex, is rescaled using appropriate transformations. An asymptotic approximation is then applied to linearize and solve the resulting ODE for the tangential vi velocity. The inner viscous solution is then matched to the outer inviscid result using Prandtl’s Matching Principle. Finally, the viscous correction is passed onto the vorticity and pressure formulations.