L’étude des vortex trouve sa justification dans le rôle que ces derniers jouent dans la turbulence quantique. L’équation de Gross-Pitaevskii ne peut pas nous permettre de modéliser convenablement l’Hélium superfluide, mais on peut l’utiliser pour obtenir le paramètre d'ordre d’un superfluide modèle, ayant le maximum de propriétés en commun avec l’Hélium, notamment une courbe de dispersion identique, par la modification du terme d’interactions.En supposant que le minimum roton influence l’essentiel de la physique, on détermine la forme du paramètre d’ordre loin de la perturbation créée par le vortex rectilinéaire axisymétrique par deux approches différentes - il apparaît alors que seuls deux paramètres sont nécessaires pour caractériser entièrement le profil.Le modèle proposé par Pomeau-Rica, qui offre la possibilité d’étudier le superfluide près de la cristallisation, met en lumière l’impact de la profondeur du minimum roton sur l’amplitude des oscillations. Par comparaison avec les résultats obtenus ab initio par Reatto, les résultats donnés par le modèle de Berloff-Roberts exhibent un déphasage marqué, qui semble être une conséquence non-physique de la forme du spectre d’excitation. Les calculs énergétiques laissent à penser que les oscillations portent une faible fraction de l’énergie du vortex, l'énergie cinétique dominant.Le calcul du paramètre d’ordre est effectué pour un anneau de grande taille par rapport à la distance interatomique, à vitesse nulle et à vitesse non-nulle. La détermination des énergies potentielle et cinétique permet d’accéder à la vitesse maximale atteinte par l’anneau en fonction de son rayon et de la comparer à la vitesse critique de Landau. / Vortices study's justification lays in the fact that those former play an important part in quantum turbulence. The Gross-Pitaevskii equation can't be a proper model for superfluid helium, but we can still use it to determine the order parameter of a theoretical superfluid, which has then the maximum amount of properties in common with liquid helium, and in particular, the same dispersion relation, thus gained by modifying the interaction terms.We then make the assumption that all the physical properties of the superfluid are triggered by the existence of the roton minimum, which allows us to calculate the order parameter far from the perturbation created by an axisymmetric rectilinear vortex, using two different methods. At that point, it appears that only two parameters are needed to fully characterize vortex profil.Pomeau-Rica's model offers the possibility to study the superfluid near crystallization and reveals the influence of the roton minimum's shape and depth on oscillations' amplitude. Results are subsequently compared to those given by Reatto's ab initio calculations. In Berloff-Roberts' model, profil displays a strong phase shift, which seems to be a non-physical consequence of the dispersion relation's shape at high frequencies. Energies reckoning leads us to think that oscillations carry a small fraction of the total vortex' energy, meaning that the kinetic energy is dominant.The order parameter for a vortex ring, whose radius is much larger than the interatomic distance, is calculated at zero and nonzero speed. Potential and kinetic energies are estimated and help us obtain the maximal speed reached by such a ring, depending on its radius and finally discussed this speed in regard to the Landau critical speed.

This thesis will examine the interactions of a cloud of charged vortex rings (CVRs) in the low temperature limit in helium-II (0.2 < T < 0.8 K) in a cubic cell containing a quasi-uniform electric field. A model of geometric collisions between vortex rings is proposed to explain the observed critical density of CVRs given by nR^3 ~ 3 cm^{-1} R where n is the CVR number density and R is the average CVR radius. This model was simulated in a simplified situation where two perfectly circular parent CVRs collide geometrically to create two perfectly circular daughter CVRs, conserving momentum and charge and dissipating a random amount of energy. The simulations are in qualitative and quantitative agreement with experiment.For an intense injection of CVRs into a strong electric field the CVRs quickly reconnect with one another to form a tangle of charged vortex loops. These loops move as one quasi-connected unit, it was found that the charged tangle's response to forcing was given by a law of the form t3 ~ (QE)^{-1/3} where t3 is the charged tangle time of flight, Q is the charge of the tangle and E is the applied electric field. Simulations of the displacement current induced in two electrodes in the cell were run in order to glean some information as to the transverse distribution of charge in the tangle, which was found to be approximately constant with time of flight and injected charge.

No description available.

Swirling flows abound in nature and numerous engineering applications. Under conditions which are not completely understood, the swirling cores could undergo a sudden enlargement of their vortex core, leading to a ’vortex breakdown’. The physics of vortex breakdown and strategies to control it have been active areas of research for nearly half a century. There are many competing theories of vortex breakdown in the literature; broadly, these are surmised on similarities to flow separation, hydrodynamic instability or transition from a supercritical to a subcritical state. However, a rational criterion for vortex breakdown continues to be elusive. One of the most well known criteria in the literature is the one due to Brown and Lopez (1990) based on an inviscid vortex dynamics model which suggests that the helix angle of the velocity vector should enclose the helix angle of the vorticity vector. However it appears that this only suggests that the stream surface would diverge and not necessarily constitute a condition for breakdown. In this work, we propose a new criterion based on helicity (scalar product of velocity and vorticity vectors) for characterizing breakdown since it has fundamental topological interpretations relating to change in linkages of vortex lines. In particular, it is suggested that the breakdown location corresponds to the location where helicity becomes zero. We study the problem of vortex breakdown in a cylindrical container with a rotating top lid in order to clarify and elucidate our hypothesis. We present results from Direct Numerical Simulation of this problem for three different Reynolds numbers and evaluate the utility of our proposed helicity criterion. Our studies indicate that helicity is indeed a better choice for characterizing vortex breakdown.

Fluid simulations for computer graphics applications have attracted the attention of many researchers and practitioners due to the enhanced realism that natural phenomena simulation adds to graphical applications. Vortex methods are receiving increasing attention from the computer graphics community for simple and direct modeling of complex flow phenomena such as turbulence. However, vortex methods have not been developed yet to the level of other techniques for fluid simulation in computer graphics. In this work we present a novel simulation framework to model inviscid flows using Lagrangian vortex particle methods. We introduce novel stable methods to solve the vorticity flow equations that produce highly detailed visual fluid simulations. We incorporate the full interplay of solids and fluids in our framework. The coupling between free-form solids, represented by arbitrary surface meshes and fluids simulated with vortex methods, leads to visually rich simulations. Previous vortex simulators only focus on modeling the solid as a boundary for the flow. We model solid boundaries using an extended potential flow at the solid surface coupled with a boundary layer simulation. This allows the accurate simulation of two processes of visual interest. The first is the introduction of surface vorticity in the main flow as turbulence (vortex shedding). The second is the motion of the solid induced by fluid forces, which is calculated from the dynamics of vorticity in the flow and the rate of vorticity creation at solid surfaces. We demonstrate high quality results of our methods simulating flows around solid objects and solid object propulsion due to flows. This work ameliorates one of the important omissions in the development of vortex methods for computer graphics, which is the simulation of two-way coupling of solids and fluids.

Beams of light carrying orbital angular momentum, such as vortex beams, have many applications in imaging and micromanipulation. We focus on applications in communication, in particular for quantum key distribution, where the security of communication channels is enhanced with the laws of quantum mechanics. However, this procedure requires superpositions of vortex beams. We want to generate such beams using integrated optics components due to their small size and their advantages in scalability and stability. Angular gratings, which are ring resonators with an embedded Bragg grating, are integrated structures known to generate vortex beams. We propose that a ring resonator with two embedded Bragg gratings, each on the inner and outer sidewalls, would generate a superposition of vortex beams. We verify this claim through analytical models, simulations and experiments.

$aCe manuscrit traite de la caractérisation numérique et expérimentale du comportement d’hydroliennes. D’un point de vue expérimental, des essais ont été réalisés au bassin à houle et courant de l’IFREMER de Boulogne-Sur-Mer, sur des maquettes d’hydroliennes tri-pales à axe horizontal. Des configurations comprenant une seule hydrolienne, d’une part, et deux hydroliennes alignées avec l’écoulement, d’autre part, ont été considérées pour une large gamme de TSR et, le cas échéant, de distances interhydroliennes. Le comportement des hydroliennes est analysé à la fois en termes de performances (coefficients de puissance et de traînée) et de développement du sillage. Les effets du taux de turbulence ambiante sont également examinés. Par ailleurs, des simulations numériques, obtenues à l’aide d’un code tridimensionnel instationnaire, fondé sur la méthode Vortex particulaire et développé au LOMC (UMR 6294, CNRS –Université du Havre) en partenariat avec l’IFREMER, sont présentées. Le code de calcul permet également d’étudier les performances et le sillage d’une hydrolienne. Ce dernier a été complètement réécrit dans le cadre de cette thèse et le support théorique et technique des différents aspects du code est fourni dans ce manuscrit, où la méthode Vortex telle qu’elle est utilisée dans le code est exposée en détail. / This manuscript deals with the numerical and experimental characterisation of the behaviour of marine current turbines. In the experimental part, trials were run at IFREMER’s wave and current flume tank in Boulogne-Sur-Mer, on prototypes of three-bladed horizontal axis turbines. Configurations with one single turbine on the one hand, and two turbines aligned with the incoming flow on the other hand, were considered for a large range of TSRs and, when relevant, of inter-device distances. The behaviour of the turbines is analysed in terms of performances (power and thrust coefficients) and development of the wake. The effects of the ambient turbulence intensity rate are also considered. Besides, numerical computations, obtained from a tridimensional unsteady software, based on the Vortex particle method and developed at the LOMC (UMR 6294, CNRS – University of Le Havre) in partnership with IFREMER, are presented. The numerical tool also enables to study the performances and the wake of a turbine. It was rewritten during this PhD and its technical and theoretical support is available in the manuscript, where the Vortex method, as it is used in the software, is described in details.

Cette thèse décrit l'étude d'une chambre de combustion innovante de type Trapped Vortex Combustor (TVC): ce concept utilise des cavités de gaz chauds pour stabiliser des flammes prémélangées pauvres. A partir d'une étude globale d'un point de fonctionnement instable, l'approche scientifique vise à différencier l'impact des différents mécanismes physiques. La structure de l'écoulement inerte est étudiée finement avant de mener une étude spatio-Temporelle sur un point de fonctionnement instable, en conditions réactives. L'analyse permet de comprendre les interactions entre la structure de la flamme, la topologie de l'écoulement et l'acoustique du brûleur. Différents mécanismes pilotant l'apparition d'instabilités de combustion sont mis en évidence, et des recommandations sont fournies afin de les supprimer. Un vérification a posteriori permet de valider l'importance de ces mécanismes, notamment grâce à la détermination de diagrammes de stabilité de flamme. / This thesis describes the investigation of an innovative trapped vortex combustor (TVC): this concept uses recirculating hot gas flow trapped in cavities to stabilize lean main flames. Based on a global investigation of an unstable operating condition, the scientific strategy aims to treat separately the different physical mechanisms. The inert flow structure is analyzed prior to leading a spatio-Temporal study on an unstable mode. This investigation aims to understand the flaine-Flow-Acoustic interactions in the combustor. Several mechanisms piloting combustion instabilities are highlighted, and recommandations are provided in order to suppress them. An a posteriori check validate the preponderance of these mechanisms, in particular with the determination of stability flaine diagrams.

Experimental studies have been conducted on the topic of the interaction of compressiblevortex rings on stationary surfaces. Throughout the campaign experimentswere carried out at pressure ratios of ! 4, 8, and 12. In the classical set up of airas both the driver and driven gas, these corresponded to theoretical incident Machnumbers Ms of 1.34, 1.54, and 1.61.Experiments were conducted on vortex rings impinging on a stationary surfacelocated at three (increasing) distances (1.66, 3.33, and 5.00 inner diameters) fromthe shock tube exit and on a stationary surface at a set distance but at three anglesinclinations (75, 60, and 45deg at 3.33 inner diameters). Results of the impingementof a vortex ring on a stationary solid surface perpendicular to the flow showed asymmetrical impingement process. A boundary layer is generated over the surfacewith an associated increase in pressure. An increase in velocity due to the radialexpansion causes the pressure over the surface to decrease. This expansion leads tothe development of azimuthal wave instabilities along the core. Pressure was seen toincrease with an increase in incident Mach number value. The variation in distanceresulted in an increase in pressure with an increase in distance. This counter-intuitiveresult can be explained by the higher translational velocity at impingement, alongwith the absence of the initial radial expansion of the counter-rotating vortex rings. The variation in surface angle inclination introduced several degrees of asymmetry. One core of the vortex ring impinges first on the surface due to its closerproximity to it, while the other core is still free to propagate. This process generatesan asymmetric boundary layer over the surface, and a higher rate of stretching ofthe lower core, resulting in its dissipation. At higher incident Mach numbers, theembedded rearward facing shock is reflected and propagates perpendicularly to thesurface. At the inclination angles of 60 and 45deg, the counter-rotating vortex ringsare fully deflected upwards and orbit around the main vortex. This phenomenonresult in a significant difference in pressure distribution between the upper and lowersections of the surface.

This research will investigate various swept wing models, designing the mechanism for their pitching motion and control, designing wind tunnel implementation, and performing data measurements and analysis using particle image velocimetry. A NACA0012 section with an aspect ratio of AR = 4, free stream velocity of U∞=34 m/s, and Reynolds Number is Rec=2x105. Swept airfoils of Λ=0°, 15°, and 30° will be pitched sinusoidally between an AoA of 4°and 22°, at a reduced frequency of k=πfc/U∞=0.2. Higher sweep angles developing arch-type vortices interact with wing tip flow and abrupt tip stall is observed. Lower sweep angles possessed defined leading edge vortices persist near the tip after lift has collapsed at mid span. Stall angle was delayed during dynamic motion of the wing as well as the presence of arch and ring type vortices increased with sweep angle and contributed to flow reattachment along the top surface of the wing.