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Real trailing wakes vortices persist for long durations, typically between one to three minutes. This duration is likely longer in the worst case scenario when crosswind and atmospherical turbulence, which are among the mechanisms for accelerated turbulent decay, are absent. In the vicinity of the ground this persistence is complicated by the rebound effect, which provokes the stagnation of the vortices above the runway area. The rebound is a result of the viscosity acting on the flow provoked by the vortices at the ground. In the absence of viscosity the vortices follow an hyperbolic motion due to the image effects of the ground.In real air trafic, the lengthy presence ofthe vortices constraints the take-off and landing frequencies of aircrafts. The way to circumvent rules and gain take-off and departure frequencies, without compromising safety, has been tackled in recent works mostly by gaining increased knowledge on vortex dynamics in ground effect, and by developping control strategies, for instance by selecting ideal configurations of obstacles pattern at the ground or by activating long-wave instabilities ideal wavelength by modulated ground altitude. Vortex control has been at the heart of this thesis, with the objective in mind to find new and especially optimal ways to reduce vortex lifetime in and out of ground proximity. We have explored several theoretical control strategies, based on optimal control and optimal perturbation techniques.We started in chapter [2] by describing the vortex dynamics in ground effect. Although largely discussed and presented in the literature before, we could verify and analyse the cause for vortex rebound and looping with precise two-dimensional direct numerical simulations. The flow was then employed to calculate an optimal control strategy based on blowing and suction at the ground. The motivation behind this theoretical setting was based on the a priori simplicity of such a system, although in the end an attempt to evaluate the cost of such a control showed the applicative difficulty. First we showed that the vortex rebound effect could be countered, with consequence that the controlled vortex is able to follow an inviscid like trajectory, that is, a hyperbolic line first down the ground and then parallel to it, outward. In effect the control strategy was able to double the lateral distance of the vortices. In the next chapter, chapter [3], we have explored the potential for vortex mitigation in ground effect through acting on the intrinsic stability of the vortices. Due to the peculiar kinematics of the vortices in ground proximity (described above : rebound and looping) we have employed a linear optimal perturbation strategy todescribe the linear stability of the vortices, first in a two-dimensaionl setting and then in three-dimensions. We concluded this linear analysis by conducting non-linear DNS simulations of the flow initialized by optimal perturbation of finite amplitudes. Initial amplitudes of 0.1 to 10% were considered. When applied with an amplitude of 1% of the total energy, the system is found to be completely mitigated, developping turbulence in a short amount of time. Comparison was taken against the uncontrolled system which, in the same time, remained coherent. This validated the strong response of the vortices to short wavelength initial perturbations.Eventually chapter [4] questions the localization of the optimal perturbations, such as those found in the previous chapter, however with a limited application to the case of a single vortex case, without ground effect,and for the flow past a wing, following some previous works of the literature. Usually the spatially wide distribution of the optimal perturbation structure is a difficulty when one searches for control devices to generate them. One way to relax the issue is to optimize more localized perturbations and this can be achieved using suboptimal optimization, like p−norm optimization., Les tourbillons de sillage réels persistent pendant de longues durées, généralement entre une et trois minutes. Cette durée est probablement plus longue dans le pire des cas, lorsque le vent de travers et la turbulence atmosphérique, qui font partie des mécanismes de désintégration turbulente accélérée, sont absents. Au voisinage du sol, cette persistance est compliquée par l'effet de rebond, qui provoque la stagnation des tourbillons au-dessus de la zone de la piste. Le rebond est le résultat de la viscosité agissant sur l'écoulement provoqué par les tourbillons au sol. En l'absence de viscosité, les tourbillons suivent un mouvement hyperbolique dû aux effets d'image du sol. Dans le trafic aérien réel, la présence prolongée des tourbillons limite les fréquences de décollage et d'atterrissage des avions. La manière de contourner les règles et de gagner des fréquences de décollage et d'atterrissage, sans compromettre la sécurité, a été abordée dans des travaux récents. Le contrôle des tourbillons a été au cœur de cette thèse, avec pour objectif de trouver de nouveaux moyens, particulièrement optimaux, pour réduire la durée de vie des tourbillons dans et hors de la proximité du sol. Nous avons exploré plusieurs stratégies théoriques de contrôle, basées sur des techniques de contrôle optimal et de perturbation optimale.Nous avons commencé dans le chapitre [2] par décrire la dynamique des tourbillons dans l'effet de sol.Cet écoulement a ensuite été utilisé pour calculer une stratégie de contrôle optimale basée sur le soufflage et la succion au sol. La motivation derrière ce cadre théorique était basée sur la simplicité a priori d'un tel système, bien que finalement une tentative d'évaluation du coût d'un tel contrôle ait montré la difficulté applicative. Nous avons d'abord montré que l'effet de rebond du vortex pouvait être contré, avec pour conséquence que le vortex contrôlé est capable de suivre une trajectoire de type inviscide, c'est-à-dire une ligne hyperbolique d'abord vers le bas du sol, puis parallèle à celui-ci, vers l'extérieur. En effet, la stratégie de contrôle a permis de doubler la distance latérale des tourbillons. Dans le chapitre suivant, le chapitre [3], nous avons exploré le potentiel d'atténuation des tourbillons dans l'effet de sol en agissant sur la stabilité intrinsèque des tourbillons. En raison de la cinématique particulière des tourbillons dans l'effet de sol (décrite ci-dessus : rebond et bouclage), nous avons employé une stratégie de perturbation optimale linéaire pour décrire la stabilité linéaire des tourbillons, d'abord dans un cadre bidimensionnel, puis en trois dimensions. Nous avons conclu cette analyse linéaire en effectuant des simulations DNS non linéaires de l'écoulement initialisé par une perturbation optimale d'amplitudes finies. Des amplitudes initiales de 0,1 à 10% ont été considérées. Lorsqu'elle est appliquée avec une amplitude de 1% de l'énergie totale, le système s'avère être complètement atténué, développant de la turbulenceen un court laps de temps. La comparaison a été faite avec le système non contrôlé qui, dans le même temps, est resté cohérent. Ceci valide la forte réponse des tourbillons aux perturbations initiales de courte longueur d'onde.Finalement, le chapitre [4] remet en question la localisation des perturbations optimales, telles que celles trouvées dans le chapitre précédent, cependant avec une application limitée au cas d'un seul tourbillon, sans effet de sol, et pour l'écoulement devant une aile, suivant certains travaux antérieurs de la littérature.Habituellement, la distribution spatialement large de la structure de perturbation optimale est une difficulté lorsqu'on cherche des dispositifs de contrôle pour les générer. Une façon de résoudre ce problème est d'optimiser des perturbations plus localisées, ce qui peut être réalisé en utilisant une optimisation sous-optimale, comme l'optimisation de la norme p.

Recent studies about the Lamb vector have led to the development of the vortex-force theory, a formulation able to predict the aerodynamic force in compressible flows and decompose it into lift, lift-induced drag and profile drag. Here, a revised formulation of the vortex-force theory developed at ONERA in collaboration with the University of Naples is presented and applied to steady transonic flows. In the mathematical developments, special care is given to the presence of shock wave discontinuities within the flow field. The equivalence between the new definition, the Kutta-Joukowski lift theorem, Maskell’s lift-induced drag formula and Betz’s profile drag formula is emphasized. The revised formulation also presents several practical advantages: the decomposition is naturally invariant to the reference point chosen for the calculation of moment transformations, and the computation can be performed in a better refined part of the grid. The formulation is finally tested on the NASA Common Research Model (CRM) wing-fuselage configuration in cruise flight conditions.; Des études récentes sur le vecteur Lamb ont conduit au développement de la théorie de la force tourbillonnaire, une formulation capable de prévoir la force aérodynamique en écoulement compressible et de la décomposer en portance, traînée induite et traînée de profil. On présente ici une formulation révisée de la théorie de la force tourbillonnaire, développée à l'ONERA en collaboration avec l'Université de Naples, afin de l'appliquer en écoulement transsonique permanent. Dans les développements mathématiques, une attention particulière est accordée à la présence de discontinuités d'ondes de choc dans l'écoulement. On souligne par ailleurs l'équivalence entre la nouvelle définition, le théorème de portance de Kutta-Joukowski, la formule de traînée induite de Maskell et la formule de traînée de profil de Betz. La formulation révisée présente également plusieurs avantages pratiques : la décomposition est naturellement invariante au point de référence choisi pour le calcul des moments, et les calculs peuvent être effectués dans une partie mieux raffinée du maillage. La formulation est finalement testée sur la configuration aile-fuselage du Common Research Model (CRM) de la NASA pour des conditions de vol en croisière.

In the Arabian Sea, eddies have a greater impact on the circulation than the large-scale circulation.In this manuscript, we present the three-dimensional structure of mesoscale eddies in the Arabian Sea, through the joint use of altimetry and in situ measurements. We then describe the lifecycle of these eddies using numerical simulations, under the assumption that they are isolated from the rest of the dynamics. For these simulations, a composite vortex extracted from in situ data is used as an initial condition. We discuss the impact of vortex stability on the vortex lifetime and the generation of submesoscale structures at the surface.We also discuss the interaction between eddies and the western boundaries of ocean basins, and relate these results to the measurement of a cyclonic submesoscale vortex performed during the PHYSINDIEN 2019 experiment.Because mesoscale eddies are actually not isolated from the ambient eddy field, we finally discuss one part of the eddy interactions that occur in the Arabian Sea: mergers. One of the main results of this study is that the comparison between the distance for which two eddies in the ocean merge, and the critical merging distance obtained from idealized studies of two eddies isolated on an f-plane omits the main parameters involved in merging: the ambient turbulent field and the β-effect., Dans des régions particulières du globe, les tourbillons ont un impact sur la circulation plus grand que la circulation grande échelle elle-même. C'est le cas en mer d'Arabie ou le transport des propriétés de la colonne d'eau est dominé par le transport induit par les tourbillons. Dans ce manuscrit de thèse, nous présentons tout d’abord la structure tridimensionnelle des tourbillons de méso-échelle en mer d'Arabie, grâce à l'utilisation conjointe de mesures altimétriques et de mesures in situ. Nous décrivons ensuite le cycle de vie de ces tourbillons à l’aide de simulations numériques, sous l'hypothèse qu’ils sont isolés du reste de la dynamique. Pour ces simulations, un tourbillon composite extrait des données in situ est utilisé comme condition initiale. Cela permet de réaliser des simulations très idéalisées, mais pour la première fois pertinentes et représentatives de la dynamique tourbillonnaire d'une région particulière. Nous discutons de l’impact de la stabilité des tourbillons sur leur durée de vie et sur la génération de structures de sousméso-échelle à la surface. Nous discutons également l’interaction entre les tourbillons et les frontières ouest des bassins océaniques, et mettons en relation ces résultats avec la mesure d'un tourbillon cyclonique de sousméso-échelle effectuée durant la campagne PHYSINDIEN 2019. Comme les tourbillons de méso-échelle ne sont en réalité pas isolés du champ tourbillonnaire ambiant, nous discutons finalement d'une partie des interactions entre tourbillons qui se produisent en mer d’Arabie : les fusions. Un des résultats principaux de cette étude est que la comparaison entre la distance pour laquelle deux tourbillons dans l'océan fusionnent, et la distance critique de fusion obtenue à partir d'études idéalisées de fusion de deux tourbillons isolés sur un plan f omet les paramètres principaux impliqués dans la fusion : le champ turbulent ambiant et l'effet β.

C’est dans l’œuvre de Poe, à laquelle Isidore Ducasse a plusieurs fois fait référence, qu’il put trouver la figure inspirante du maelstrom, ce tourbillon aquatique qui, par ses mouvements centrifuges et centripètes, devait inspirer la structure même des Chants de Maldoror. Ce n’est pas là néanmoins le seul emprunt de Ducasse à l’œuvre du novéliste américain qu’il aura vraisemblablement découvert à travers Baudelaire., It was in Poe’s work, which Isidore Ducasse makes reference to several times, that he found the inspiring figure of the maelstrom, that tornado in the water which, with its centrifugal and centripetal movements, was to inspire the very structure of the Chants de Maldoror. Nevertheless, this is not Ducasse’s only borrowing from the work of the American short-story writer, whom he probably discovered by way of Baudelaire.

International audience; Experimental analysis and modelling of the losses in the tip leakage flow of an isolated, non-rotating blade setup B. Deveaux · C. Fournis · V. Brion · J. Marty · A. Dazin Abstract Improving pressure rise capabilities of axial compressors requires an in-depth understanding of the losses produced in the tip leakage region. Here, a generic setup that magnifies the tip region of an isolated, non-rotating blade is used with the objectives of describing the main flow components and evaluating the related sources of loss. The flow at the tip is structured by the jet flow out of the gap which, under the effect of the main stream, rolls-up into a tip-leakage vortex. The current setup is characterized by the tip gap height and the thickness of the incoming boundary layer at the casing, here a flat plate, for a given incidence of the blade. Measurements are performed using LDV and a multi-port pressure probe. Variations in the tip-leakage flow are found to be mainly driven by gap height. A small, intermediate and large gap regimes are more specifically found, with threshold around 4% and 8% of gap to chord ratio for the present setting. The incoming boundary layer thickness is shown to provoke a notable effect on the vortex lateral position and total pressure losses. The local entropy creation rate is computed from LDV data and used to identify the sources of loss in the flow. A decomposition into wake and vortex losses is further proposed, allowing to relate the contributions of the various flow components to the overall losses. An empirical model of the formation of the tip vortex is developed to account for the increased losses as a function of gap height. The model provides B. Deveaux · C. Fournis · V. Brion · J a useful mean for the practical approximation of the gap sensitivity of pressure losses.; L'amélioration des capacités de montée en pression des compresseurs axiaux nécessite une compréhension approfondie des pertes produites dans la région de fuite de la pointe. Ici, une configuration générique qui agrandit la région de la pointe d'une lame isolée et non tournante est utilisée dans le but de décrire les principaux composants du flux et d'évaluer les sources de perte associées. Le débit à la pointe est structuré par l'écoulement du jet hors de l'espace qui, sous l'effet du courant principal, s'enroule dans un vortex de fuite de pointe. La configuration actuelle est caractérisée par la hauteur de l'entrefer et l'épaisseur de la couche limite entrante au niveau du boîtier, ici une plaque, pour une incidence donnée de la lame. Les mesures sont effectuées à l'aide de LDV et d'une sonde de pression à ports multiples. On constate que les variations du débit de fuite de pointe sont principalement dues à la hauteur de l'espace. On trouve plus spécifiquement des régimes à petit, intermédiaire et grand écart, avec un seuil d'environ 4% et 8% du rapport entre l'écart et l'accord pour le cadre actuel. Il est démontré que l'épaisseur de la couche limite entrante provoque un effet notable sur la position latérale du vortex et les pertes de charge totales. Le taux de création d'entropie locale est calculé à partir des données LDV et utilisé pour identifier les sources de perte dans le flux. Une décomposition en sillage et pertes tourbillonnaires est en outre proposée, permettant de relier les contributions des différentes composantes de l'écoulement aux pertes globales. Un modèle empirique de la formation du vortex de la pointe est développé pour tenir compte des pertes accrues en fonction de la hauteur de l'espace. Le modèle fournit une moyenne utile pour l'approximation pratique de la sensibilité à l'écart des pertes de charge.

International audience; The dynamics and control of a vortex pair in ground effect are investigated ina planar, incompressible and laminar setting. The evolution of the vortices obtained numerically shows vortex rebound as a consequence of the separation of the boundary layer induced at the wall by the vortices. An optimal control approach is developed and employed for vortex Reynolds numbers of 200 and 1000 in order to identify the optimal Dirichlet boundary condition at the wall to counteract this rebound and allow for an increased lateral displacement of the vortex, similarly to the inviscid evolution of the flow, which features hyperbolic trajectories. The work is primarily a conceptual approach to deal with aircraft separation distances in airport airspace by moving the vortices laterally, away from the runway but may also apply to the control of coherent structures in wall bounded turbulence. The most efficient control is able to double the lateral position and yields mostly vertical in and outflow at the wall. An optimal horizon time is found, equal to 5 characteristic time units of the vortex system, beyond which control is not able to further displace the vortices. The control is shown to delay the separation of the boundary layer at the origin of vortex rebound by applying suction ahead of the vortex, and to generate a vorticity flux at the wall, leading to a pusher vortex of sign opposite to that of the primary vortex, that attenuates the effect of the no-slip boundary condition at the wall by pushing the vortex outward.; La dynamique et le contrôle d'une paire de tourbillons en effet de sol sont étudiés dans un cadre plan, incompressible et laminaire. L'évolution des tourbillons obtenus montre le rebond du tourbillon à la suite de la séparation de la couche induite au sol par ces tourbillons. Une approche de contrôle optimale est développée et utilisée pour des nombres de Reynolds entre 200 et 1000 afin d'identifier la condition limite optimale au sol pour contrer ce rebond et permettre un déplacement latéral accru du tourbillon, de la même manière que l'évolution inviscide de l'écoulement, qui présente des trajectoires hyperboliques. Le travail est principalement une approche conceptuelle pour gérer les distances de séparation des aéronefs dans l'espace aérien de l'aéroport en déplaçant les tourbillons latéralement, loin de la piste, mais peut également s'appliquer à la contrôle des structures cohérentes dans une turbulence de paroi. Le contrôle le plus efficace est en mesure de doubler la position latérale et donne principalement des vitesses au sol verticales. On trouve un temps d'horizon optimal égal à 5 unités de temps caractéristiques du système tourbillonnaire, au-delà duquel le contrôle n'est pas en mesure de déplacer davantage les tourbillons. Il est démontré que le contrôle retarde la séparation de la couche limite à l'origine du tourbillon rebond en appliquant une succion en avant du tourbillon, et qui génère un flux de vorticité à la paroi, menant à un tourbillon "repoussoir" de signe opposé à celui du tourbillon primaire. Ce dernier atténue l'effet de la condition aux limites d'adhérence à la paroi en "poussant" le tourbillon vers l'extérieur.

International audience; The dynamical significance of vortical structures in the near field of round jets, viz. the vortex rings and the stream-wise vortices, has been known for several decades (Ho & Huerre, 1984). A fundamental understanding of their interactions , especially in turbulent jets at high Reynolds number (Re), is expected to provide an impetus to devising control strategies; for instance in improving combustion efficiency and reducing jet noise. One such possible interaction was recently observed in Davoust et al. (2012) that involved production of additional streamwise vorticity, as a result of an influence of the fluctuations of streamwise vorticity on those of azimuthal vorticity in high Re jets. This additional vorticity showed up in streamwise vorticity correlations in the form of radial organization as opposed to its azimuthal organization classically found. In further exploring this possibility, we have observed in our experiments, that strengthening the vortex rings relative to the streamwise vortices, renders them less susceptible to an influence from the streamwise vortices. This paper presents the results for a Re = 1.5 × 10 5 and low Mach number round jet in which the most energetic vortex rings are acoustically excited at different excitation amplitudes. A comparison of the relative strengths of the two structures with increasing excitation levels, corroborates the interaction proposed in Davoust et al. (2012).

International audience; High-fidelity simulation performed with the CFD code elsA has been used to improve a fast prediction code, named ARHIS, dedicated to the prediction of the aerodynamic response of a blade in interaction with a vortex. To do so, comparisons have been made on a well documented blade-vortex interaction configuration. After validation of the computation methodology against available measurements, discrepancies between elsA and ARHIS solutions have been pointed out at the blade tip, due to the bidimensional approach used in the latter. Consequently, a new model of tip loss factor has been developed in ARHIS providing a better agreement between fast and high fidelity simulations. The maximal error in terms of amplitude of the lift jump due to the blade vortex interaction goes from 20,1% to 5,2%. This new model has been applied to several cases to assess the impact of the blade shape by considering modification of the aspect ratio, sweep angles, parabolic tip and tapered blade. Even if not designed specifically to account for modification of the blade shape, it is clear that the new tip loss factor model still improves the agreement between AHRIS and elsA results. Finally, the improved model has been tested for the evaluation of the noise emission of complete rotors in descent flight including a straight and a double swept rotor blade. In all cases, the new model induces a reduction of the predicted noise level of about 0,5 to 1 dB. This noise reduction appears to be an improvement in one case but a deterioration in the other one maybe because of a compensation of errors in the entire computational chain. This new modeling needs consequently to be applied on a larger database to conclude on his efficiency in terms of noise predictions.; Des simulations hautes fidélités réalisées avec le code CFD elsA ont été utilisées pour améliorer un code de prédiction rapide appelé ARHIS et dédié à la prédiction de la réponse aérodynamique d'un profil en interaction avec un tourbillon. Pour ce faire, des comparaisons ont été effectuées sur une configuration d'interaction pale-tourbillon bien documentée dans la littérature. Après validation de la méthodologie de calcul par rapport aux mesures disponibles, des divergences entre les solutions elsA et ARHIS ont été signalées en tête de pale. En conséquence, un nouveau modèle de facteur de perte en bout de pale a été développé dans ARHIS, offrant un meilleur accord entre les simulations rapides et hautes fidélités. L'erreur maximale en termes d'amplitude du saut de portance due à l'interaction pale-tourbillon est réduite de 20,1% à 5,2%. Ce nouveau modèle a été appliqué à plusieurs cas pour évaluer l’impact de la forme de la pale en considérant notamment une modification du rapport d'aspect, l'introduction d'angles de flèche ou l’utilisation d'un saumon parabolique. Même s'il n'est pas conçu spécifiquement pour tenir compte de la modification de la forme de la pale, il est clair que le nouveau modèle de facteur de perte améliore la concordance entre les résultats AHRIS et elsA dans tous les cas. Enfin, le modèle amélioré a été testé pour évaluer l’émission sonore de rotors complets en vol de descente sur les cas HARTII (pales droites) et ERATO (pales à double flèche). Dans tous les cas, le nouveau modèle induit une réduction du niveau de bruit prévu d'environ 0,5 à 1 dB. Cette réduction du bruit semble être une amélioration dans un cas, mais une détérioration dans l'autre qui peut être due à une compensation d'erreurs dans l'ensemble de la chaîne de calcul. Cette nouvelle modélisation doit donc être appliquée à une base de données plus vaste pour conclure sur son efficacité en termes de prédiction de bruit.

International audience; We investigate optimal perturbation in the flow past a finite aspect ratio (AR) wing. The optimization is carried out in the regime where the fully developed flow is steady. Parametric study over time horizon (T), Reynolds number (Re), AR, angle of attack and geometry of the wing cross-section (flat plate and NACA0012 airfoil) shows that the general shape of linear optimal perturbation remains the same over the explored parameter space. Optimal perturbation is located near the surface of the wing in the form of chord-wise periodic structures whose strength decreases from the root towards the tip. Direct time integration of the disturbance equations, with and without nonlinear terms, is carried out with linear optimal perturbation as initial condition. In both cases, the optimal perturbation evolves as a ownstream travelling wavepacket whose speed is nearly the same as that of the free stream. The energy of the wavepacket increases in the near wake region, and is found to remain nearly constant beyond the vortex roll-up distance in nonlinear simulations. The nonlinear wavepacket results in displacement of the tip vortex. In this situation, the motion of the tip vortex resembles that observed during vortex meandering/wandering in wind tunnel experiments. Results from computation carried out at higher Re suggest that, even beyond the steady flow regime, a perturbation wavepacket originating near the wing might cause meandering of tip vortices.

This thesis investigates, theoretically and numerically, the vertical decorrelation of an initially vertical vortex by an ambient sinusoidal shear flow in a stratified fluid. It has been conjectured that such process should trigger the shear instability and, as such, contribute to the generation of small scales in strongly stratified turbulence. This type of turbulence is encountered in the atmosphere and the oceans in an intermediate range of scaleswhere Coriolis effects are negligible.The first part analyses the evolutions of the total energy and enstrophy of the vortex in Direct Numerical Simulations (DNS) as functions of the control parameters. This study reveals that the dynamics differs from freely decaying flows: because of the presence of the ambient shear flow, the balance between stretching and dissipation terms in the global enstrophy budget implies that the maximum enstrophy of the vortex scales as $Re^{2/3}$, where $Re$ is the Reynolds number, instead of simply $Re$. However, such simplified balance does not account for the observed effect of the stratification.In order to overcome this difficulty, the local dynamics of the vortex has been investigated by means of two asymptotic analyses, presented in the second part. A short-time analysis first proves that the initial response of the vortex is non-hydrostatic regardless of the stratification. A long-wavelength analysis provides governing equations for the evolution of the angular velocity of the vortex and the deformations of its axis. Internal waves are excited at the start-up of the motion, explaining the initial non-hydrostatic regime. The vortex is mostly advected in the direction of the shear flow but also perpendicularly owing to the self-induced motion. Its angular velocity decays because of dynamic and viscous effects. The former effect is due to the squeezing of the isopycnals in the vortex core which implies a decrease of the vertical vorticity to conserve potential vorticity.In a third part, the DNS show that the shear instability is only triggered if the Froude number is moderate and the wavelength of the shear small enough. The numerical results are compared to the asymptotic predictions. In particular, the evolutions of the vertical shear of horizontal velocity and of the vertical buoyancy gradient for small Froude number are comprehensively and finely captured by the long-wavelength asymptotic predictions. The minimum value of the asymptotic Richardson number almost never goes below the critical threshold $1/4$ necessary for the development of the shear instability. The saturation of the vertical shear is due to the decay of the vortex in the regions of high ambient shear. These results suggest that the shear instability is not easilytriggered by decorrelation processes in strongly stratified flows in contradiction with previous conjectures.; Cette thèse étudie, théoriquement et numériquement, la décorrélation verticale d’un tourbillon initialement vertical par un écoulement externe cisaillé sinusoïdalement dans un fluide stratifié. Il a été proposé qu’un tel mécanisme devrait déclencher des instabilités de cisaillement et ainsi contribuer à la production de petites échelles en turbulence fortement stratifiée, que l’on rencontre dans l’atmosphère et les océans dans une gamme d’échelles où la force de Coriolis est négligeable.La première partie de la thèse étudie les évolutions des énergies et enstrophie totales du tourbillon calculées au moyen de Simulations Numériques Directes (DNS), en fonction des paramètres de contrôle. Cette analyse montre que la dynamique est différente de celle des écoulements libres en déclin : du fait de la présence du cisaillement ambiant, la condition de saturation entre les termes d’étirement et de dissipation dans le bilan d’enstrophie globale implique que l’enstrophie maximale du tourbillon sature proportionnellement à $Re^{2/3}$, où $Re$ est le nombre de Reynolds, au lieu de $Re$. Néanmoins, cette condition de saturation ne rend pas compte de l’effet observé de la stratification.Afin de l’expliquer, la dynamique locale du tourbillon a été étudiée au travers de deux analyses asymptotiques présentées dans une deuxième partie. Une étude pour temps courts prouve que la réponse initiale du tourbillon est non-hydrostatique quelle que soit la stratification. Une autre analyse pour grande longueur d’onde fournit les équations qui décrivent l’évolution de la vitesse angulaire du tourbillon et des déformations de son axe. L’excitation d’ondes internes au début de l’évolution est à l’origine du régime non-hydrostatique initial. Le tourbillon est principalement advecté dans la direction du cisaillement ambiant mais aussi perpendiculairement du fait de son auto-induction. Sa vitesse angulaire décroît à cause d’effets dynamique et visqueux. La décroissance dynamique est dûe à un resserrement des lignes isopycnes dans le cœur du tourbillon, qui implique une atténuation de la vorticité verticale afin que la vorticité potentielle se conserve.Dans une troisième partie, des DNS révèlent que l’instabilité de cisaillement se développe seulement lorsque la stratification est modérée et la longueur d’onde du cisaillement ambiant suffisamment petite. Les résultats numériques sont comparés aux prédictions asymptotiques. En particulier, les évolutions du cisaillement vertical de vitesse horizontale et du gradient vertical de flottabilité sont prédites de manière fine et exhaustive par l’analyse asymptotique pour grande longueur d’onde lorsque le nombre de Froude est petit. Le nombre de Richardson asymptotique admet un minimum qui n’est pratiquement jamais inférieur au seuil critique $1/4$ nécessaire au déclenchement de l’instabilité de cisaillement. La saturation du cisaillement vertical est dûe au déclin du tourbillon dans les régions où le cisaillement ambiant est maximal. Ces résultats suggèrent que l’instabilité de cisaillement est difficilement déclenchée par des processus de décorrélation dans les écoulements fortement stratifiés, contredisant ainsi les conjectures précédemment formulées dans la littérature.

International audience; The aeroacoustic response of the orthogonal interaction of a rotating blade with an isolated Batchelor vortex is studied by means of numerical simulation. The relative influence of the vortex tangential and axial velocity on the blade aerodynamics and on the acoustics radiated in the far field is analyzed by comparing the interaction with a Batchelor vortex to the interactions considering the vortex tangential or axial velocity components. Analyses show that the vortex tangential velocity contributes mostly to the noise level at low frequencies, whereas the vortex axial velocity is responsible for the contribution at high frequencies. For the range of frequencies in between, the interaction noise results from constructive interferences of the noise radiated separately by each velocity component of the vortex.

We investigate perturbations that maximize the gain of disturbance energy in a two-dimensional isolated vortex and a counter-rotating vortex pair. The optimization is carried out using the method of Lagrange multipliers. For low initial energy of the perturbation ( $E(0)$ ), the nonlinear optimal perturbation/gain is found to be the same as the linear optimal perturbation/gain. Beyond a certain threshold $E(0)$ , the optimal perturbation/gain obtained from linear and nonlinear computations are different. There exists a range of $E(0)$ for which the nonlinear optimal gain is higher than the linear optimal gain. For an isolated vortex, the higher value of nonlinear optimal gain is attributed to interaction among different azimuthal components, which is otherwise absent in a linearized system. Spiral dislocations are found in the nonlinear optimal perturbation at the radial location where the most dominant wavenumber changes. Long-time nonlinear evolution of linear and nonlinear optimal perturbations is studied. The evolution shows that, after the initial increment of perturbation energy, the vortex attains a quasi-steady state where the mean perturbation energy decreases on a slow time scale. The quasi-steady vortex state is non-axisymmetric and its shape depends on the initial perturbation. It is observed that the lifetime of a quasi-steady vortex state obtained using the nonlinear optimal perturbation is longer than that obtained using the linear optimal perturbation. For a counter-rotating vortex pair, the mechanism that maximizes the energy gain is found to be similar to that of the isolated vortex. Within the linear framework, the optimal perturbation for a vortex pair can be either symmetric or antisymmetric, whereas the structure of the nonlinear optimal perturbation, beyond the threshold $E(0)$ , is always asymmetric. No quasi-steady state for a counter-rotating vortex pair is observed.

Introduction par Florent Perrier Fourier est parfois contraint de ramasser sa pensée en de brèves notices comme en témoignent ses Manuscrits. La rue-galerie ne devient alors presque plus qu’un réceptacle pour salles de concert, opéras, bals ou fêtes, un lieu de parades et de banquets, de spectacles vivants qui convergent pour finir vers l’orgie de Musée, intrication permanente entre relations publiques et relations esthétiques. Pour “une population exercée toute entière aux arts comme les har...

Introduction par Florent Perrier Fourier est parfois contraint de ramasser sa pensée en de brèves notices comme en témoignent ses Manuscrits. La rue-galerie ne devient alors presque plus qu’un réceptacle pour salles de concert, opéras, bals ou fêtes, un lieu de parades et de banquets, de spectacles vivants qui convergent pour finir vers l’orgie de Musée, intrication permanente entre relations publiques et relations esthétiques. Pour “une population exercée toute entière aux arts comme les har...

Avalanches, landslides, and debris flows are geophysical hazards, which involve rapid mass movement of granular solids, water, and air as a multi-phase system. In order to describe the mechanism of immersed granular flows, it is important to consider both the dynamics of the solid phase and the role of the ambient fluid. In the present study, the collapse of a granular column in fluid is studied using 2D LBM - DEM. The flow kinematics are compared with the dry and buoyant granular collapse to understand the influence of hydrodynamic forces and lubrication on the run-out. In the case of tall columns, the amount of material destabilised above the failure plane is larger than that of short columns. Therefore, the surface area of the mobilised mass that interacts with the surrounding fluid in tall columns is significantly higher than the short columns. This increase in the area of soil - fluid interaction results in an increase in the formation of turbulent vortices thereby altering the deposit morphology. It is observed that the vortices result in the formation of heaps that significantly affects the distribution of mass in the flow. In order to understand the behaviour of tall columns, the run-out behaviour of a dense granular column with an initial aspect ratio of 6 is studied. The collapse behaviour is analysed for different slope angles: 0°, 2.5°, 5° and 7.5°.

International audience; The influence of the length-to-depth aspect ratio and of wall asymmetry on the main vortical flow structures evolving in rectangular two-dimensional deep cavities is studied experimentally using wall-pressure and particle image velocimetry (PIV) measurements. Wall-pressure and cavity flow statistics have been analyzed and shown that the flow features are strongly affected especially by the asymmetry. An emphasis is given concerning the behavior of the shear layer oscillations that are compared to the analytical deep-cavity model prediction proposed by P.J.W. Block (NASA Tech. Note. 1976). The results show good agreement with Block’s model if the value of the convection velocity is properly adjusted. Stochastic estimation of the cavity flows demonstrates that convective structures are involved downstream of the cavity along the wall and highlights the physical nature of the pressure-producing flow structures.; En utilisant des mesures de la pression du mur et de PIV, nous étudions expérimentalement l’influence du rapport de forme longueur versus profondeur et de l’asymétrie du mur sur la structure d’écoulement tourbillonnant principal évoluant dans des cavités profondes à deux dimensions rectangulaires. La pression du mur et la statistique d’écoulement dans la cavité sont analysées et montrent que les caractéristiques d’écoulement sont fortement affectées surtout par l’asymétrie. Nous mettons l’emphase sur le comportement des oscillations de la couche cisaillée, qui sont alors comparées aux prédictions du modèle analytique pour cavité profonde proposé par P.J.W. Block (NASA Tech. Note. 1976). Les résultats montrent un bon accord avec le modèle de Block, si la valeur de la vitesse de convexion est ajustée de façon appropriée. L’estimé stochastique des écoulements dans la cavité montre que les structures convectives existent en aval de la cavité le long du mur et souligne la nature physique des structures d’écoulement produisant de la pression. [Traduit par la Rédaction]

[1] A 16 year record of expendable bathythermograph transects across Drake Passage is used to examine variability in upper-ocean heat content that is not associated with the annual cycle. Links between upper-ocean heat content and anomalous heat fluxes, winds, two large-scale climate indices, and mesoscale eddies and meanders are examined. Results suggest that interannual variations in surface heat fluxes explain ∼5 to 10% of the variance in upper-ocean heat content. Anomalous surface heat fluxes are linked to meridional wind anomalies upstream of Drake Passage, which in turn are linked to forcing by El Nino/Southern Oscillation (ENSO) and the Southern Annular Mode (SAM). ENSO and SAM are correlated with upper-ocean heat content at near-zero lags, and statistically significant correlations occur at longer time lags as well. The impact of mesoscale eddies and meanders on upper-ocean heat content is explored with the use of a tracked eddy database. An empirical relationship is constructed relating upper-ocean heat content anomalies to eddy length scales and amplitudes. Eddies and meanders are estimated to account for more than one third of the nonannual cycle variance in Drake Passage upper-ocean heat content.

The traditional point of view is that in the ocean, the meridional transport of heat is achieved by the wind-driven and meridional overturning circulations. Here we point out the fundamental role played by ocean mixing processes. We argue that mixing (i.e., water mass conversion) associated with eddies, especially in the surface mixed layer, can play an important role in closing the ocean heat budget. Our results argue that the lateral mixing applied at the surface of ocean/ climate models should be playing an important role in the heat balance of these models, indicating the need for physically-based parameterizations to represent this mixing. Copyright 2007 by the American Geophysical Union.

The movement and pinning of neutron vortices has been studied with reference to the superfluid component whose relaxation gives the large discontinuities in angular velocity time-derivative observed for PSR 0355 + 54 and 0833 {minus} 45. At high temperatures, early in the life of a pulsar, the vortices are not pinned but move in almost exact corotation with the superfluid. The balance of evidence indicates that corotation of vortices and superfluid, which has consequences for the interpretation of timing noise and the estimation of internal temperatures, persists throughout the life of a pulsar. 18 refs.

We show that dipolar interactions have dramatic effects on the groundstates of rotating atomic Bose gases in the weak interaction limit. With increasing dipolar interaction (relative to the net contact interaction), the mean-field, or high filling fraction, groundstate undergoes a series of transitions between vortex lattices of different symmetries: triangular, square, ``stripe'', and ``bubble'' phases. We also study the effects of dipolar interactions on the quantum fluids at low filling fractions. We show that the incompressible Laughlin state at filling fraction $\nu=1/2$ is replaced by compressible stripe and bubble phases., Comment: 4 pages, 2 figures

Modeling the particle separation efficiency has been a topic of interest since the air cyclones was introduced for gas-particle separation in the fields of environmental science and chemical engineering. In this work, a new simple time-of-flight model is theoretically developed to predict the particle separation efficiency in cyclones. In this model, the equivalent volume method is employed to geometrically modify the cylindrical-conical type cyclone as a right cylindrical cyclone in order to overcome the nonuniform effect on the particle separation distance. Based on the analysis of the gas flow pattern and the particle dynamics in the cyclone separator, the differential equation for the time-of-flight model is established according to the principle of particle mass balance. The model can be finally expressed as a simple explicit function including the main cyclone dimensions and operating parameters, without the need for solving complex equation of mathematical physics. The influences of the short-circuit flow near the bottom of cyclone outlet duct and the exchange flow between outer and inner vortex flow are comprehensively considered to revise the effective residence time of gas flow, a key parameter in the present model. By comparisons with experimental data as well as other classical separation models for the cyclones with different geometrical configurations and operating conditions, the results show that the present model has a relatively high predicted accuracy with the mean squared error of 0.0158. It is demonstrated that the present model has considerable availability for predicting the particle separation efficiency for cyclone separators.

The existence of fire whirls in some ship engine room is studied by using the basic elements of the fire whirls formation. The temperature characteristic of fire whirls are used to study the path shift of fire whirls and the result is nearly circular. The center shift angular velocity of fire whirls is analyzed with the least square fitting in the conditions of the constant wind velocity and the variable wind velocity from four sides. In the situation of the constant wind velocity, the result shows that the center shift angular velocity of fire whirls comply with the change regularity of a cube polynomial with different wind speed. When the wind velocity is more than 6.58 m/s, it strongly affects the change rate of the center shift angular velocity. However, in the situation of variable wind velocity, when the change rule of wind velocity follow increasing or descending regularity, the change law of the center shift angular velocity is proportional to the size of the wind velocity, but when the wind velocity is random variation, the change fluctuation of center shift angular velocity is comparatively small. Computational results have important significance for the structure fireproofing, fire evaluation and shipping design.

The near-ground flow structure of tornadoes is of utmost interest because it determines how and to what extent civil structures could get damaged in tornado events. We simulated tornado-like vortex flow at the swirl ratios of S = 0.03–0.3 (vane angle θv = 15°–60°), using a laboratory tornado simulator and investigated the near-ground-vortex structure by particle imaging velocimetry. Complicated near-ground flow was measured in two orthogonal views: horizontal planes at various elevations (z = 11, 26 and 53 mm above the ground) and the meridian plane. We observed two distinct vortex structures: a single-celled vortex at the lowest swirl ratio (S = 0.03, θv = 15°) and multiple suction vortices rotating around the primary vortex (two-celled vortex) at higher swirl ratios (S = 0.1–0.3, θv = 30°–60°). We quantified the effects of vortex wandering on the mean flow and found that vortex wandering was important and should be taken into account in the low swirl ratio case. The tangential velocity, as the dominant velocity component, has the peak value about three times that of the maximum radial velocity regardless of the swirl ratio. The maximum velocity variance is about twice at the high swirl ratio (θv = 45°) that at the low swirl ratio (θv = 15°), which is contributed significantly by the multiple small-scale secondary vortices. Here, the results show that not only the intensified mean flow but greatly enhanced turbulence occurs near the surface in the tornado-like vortex flow. The intensified mean flow and enhanced turbulence at the ground level, correlated with the ground-vortex interaction, may cause dramatic damage of the civil structures in tornadoes. This work provides detailed characterization of the tornado-like vortex structure, which has not been fully revealed in previous field studies and laboratory simulations. It would be helpful in improving the understanding of the interaction between the tornado-like vortex structure and the ground surface, ultimately leading to better predictions of tornado-induced wind loads on civil structures.

In this study, performances of Ranque–Hilsh Vortex Tubes (RHVTs) were experimentally investigated under three different situations based on inlet pressure and the ratio of mass flow rate of the cold stream to the mass flow rate of the inlet stream (ξ). 1st situation is the conventional RHVT. 2nd situation is the threefold cascade type RHVT. Here three RHVTs were used. 3rd situation deals with the six cascade type RHVT. In this case, six RHVTs were used. When all the test results under Pi = 360 kPa and Pi = 400 kPa pressure conditions are compared, it is found that the best performance occurs at the 3rd situation. Moreover, the performance of the 3rd situation at Pi = 400 kPa pressure is higher than that at Pi = 360 kPa. Maximum values of ΔT and ξ are obtained as 67.6 °C and 0.9 for 3rd situation.

Despite its long history, detailed studies on the dynamics of mechanical watch movement are still lacking. In this paper, extending our previous mechanics model, the dynamical wave motion of the hairspring—balance wheel assembly, the core of the timekeeping mechanism called the escapement, is analyzed by partial differential equations. It is found that the corresponding eigenfunctions may be formulated into a Fourier series, through which the relative amplitudes of high order harmonics can be obtained. The relative error incurred owing to the high order harmonics is estimated to be around 10 −3 , or 10 2 s/day, much higher than the needed accuracy. This is why manual adjustment is needed in practice. Moreover, addition of the pricy Tourbillon mechanism, which is designed to counteract the gravity effect, is found to trigger complexity in the motion and hence, may not be worth much.

The dynamics of a two-dimensional vortex system with strong periodic square columnar pins is investigated. For the case vortex number matching pinning number, we find that the vortex liquid is frozen into square lattice via a continuous transition, and the freezing (melting) temperature Tm is the same as the thermal depinning temperature of vortices, which are different from the first-order phase transition at weak pinning. The zero-temperature critical depinning force Fc0 is exactly the same as the maximum pinning force, and the depinning property at T = 0 can be expressed by scaling v ∼ (F − Fc0)β with the exponent β close to 0.5. The v–F curves at temperatures below Tm show that vortices are pinned at small driving force.

The supersonic swirling separator is a new apparatus for offshore and subsea natural gas separation, due to its lightweight and the viability of unmanned operation. A new supersonic swirling separator was designed for the numerical calculation, in which a central body was inserted based on the principle of conservation of angular momentum. Axial and radial distribution of the main parameters of natural gas flow was investigated with RNG K-ɛ turbulence model. The effects of the shock waves on the natural gas flow fields were analyzed in the supersonic separator. The results show that water and heavy hydrocarbons can be separated from natural gas due to the low temperature and high centrifugal field. The non-uniformity of radial distribution of the gas dynamic parameters significantly affects the gas/liquid separation. The position of the shock wave determines the distribution of the temperature, which has a great influence on the re-evaporation of liquid droplets.

Vortex formation at intakes can cause damage, clogging, reduced flow efficiency, and even loss of life. For practical prediction of vortex risk, engineers often compare expected design parameters with published data by using parameter proximity to evaluate the relative risk of vortex formation. Unfortunately, this procedure is ill-defined, and the resulting risk estimates are highly subjective. In response, a formal equivalent of the data proximity procedure was developed by implementing the nearest neighbors algorithm on available experimental and field data. This database was partitioned and the machine learning parameters adjusted to obtain a stochastic model with maximum predictive accuracy. Unlike the flow parameters and submergence, the approach geometry was not found to be a significant factor in the model, although this may be attributable to data noise and range of tested values. The final model, which excluded the channel approach geometry, fit all vertical intake vortex formation data to within 0.1% error and perfectly fit the horizontal intake data. Probability charts generated from the model show regions of vortex formation and problems more numerous and larger on average than regions of low vortex probability, thus validating consideration of potential vortex formation risk for conservative intake design.

Hydrocyclones for separating light dispersed phase are applicable to various processes in the separation of light dispersion. The relationship of the structure parameters and flow field is in the frontier of the research and not clear so far. In this paper, Phase Doppler Particle Analyzer (PDPA) was used for the first time to systematically study the influence of the change of structure and size of vortex finder on the zero axial velocity wave zone (ZAVWZ), the tangential velocity gradient, the centrifugal separation factor and the size distribution of dispersed phase. The hydrocyclone was designed with diameter of Φ 35 mm, hollow glass beads ( d 50 = 30.61 μm, ρ = 0.82 g/cm 3 ) were used as the dispersed phase and water was used as the continuous phase. The test results were used as the evaluation index to optimize the depth of vortex finder at 0.143 L s and its diameter at 0.071 D . The performance of the hydrocyclone was also examined under the same material and flow condition as the PDPA test for flow field, which showed the separation performance of the optimized hydrocyclone reached up to 92.3%, while the inlet flow rate was 1.5 m 3 /h and the split ratio was 8%. At last, the reliability for taking the test results, the length of zero axial velocity wave zone (ZAVWZ), the centrifugal separation factor and the tangential velocity gradient as the evaluation index were also verified.

The first large-scale quantum mechanical simulations of pulsar glitches are presented, using a Gross-Pitaevskii model of the crust-superfluid system in the presence of pinning. Power-law distributions of simulated glitch sizes are obtained, in accord with astronomical observations, with exponents ranging from -0.55 to -1.26. Examples of large-scale simulations, containing ~200 vortices, reveal that these statistics persist in the many-vortex limit. Waiting-time distributions are also constructed. These and other statistics support the hypothesis that catastrophic unpinning mediated by collective vortex motion produces glitches; indeed, such collective events are seen in time-lapse movies of superfluid density. Three principal trends are observed. (1) The glitch rate scales proportional to the electromagnetic spin-down torque. (2) A strong positive correlation is found between the strength of vortex pinning and mean glitch size. (3) The spin-down dynamics depend less on the pinning site abundance once the latter exceeds one site per vortex, suggesting that unpinned vortices travel a distance comparable to the intervortex spacing before repinning. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.

A recently proposed reduced-gravity model of the warm-water branch of the middepth meridional overturning circulation in a rectangular basin with a circumpolar connection is extended to include time dependence. The model describes the balance between gain of warm water through northward Ekman advection across the circumpolar current, loss of warm water through eddy fluxes southward across the current, net gain or loss of warm water through diabatic processes north of the current, and changes in the thickness of the warm-water layer. The steady solutions are the same as those found previously, when the previous parameterization of diabatic fluxes is used. Time-dependent solutions are considered for the approach of the solution to a new equilibrium when the forcing or parameters are abruptly changed and then held fixed. An initial adjustment occurs through a combination of boundary and equatorial adjustment, followed by planetary wave propagation. The longer-term adjustment to equilibrium consists primarily of the slow change in eastern boundary thickness of the warm layer, which controls the mean depth of the entire layer. An approximate analytical solution of the time-dependent equations yields an explicit expression for the intrinsic time scale of the long-term adjustment, which depends on the eddy and diabatic flux parameters and on the equilibrium solution toward which the time-dependent solution adjusts. Numerical solutions are also considered with a second, advective–diffusive diabatic flux parameterization. These solutions differ quantitatively but not qualitatively from those with the original parameterization. For the range of parameter values considered, the adjustment time scale has dimensional values of several decades to more than a century, but the meridional flux of warm water may respond to changes in external parameters or forcing much more rapidly than this time scale for equilibration of the eastern boundary thickness and thermocline structure.

Horizontal, vertical and temporal distribution of a cyclonic (counterclockwise) eddy, where biological productivity is high, downstream of the Tsushima Islands in the eastern channel of the Tsushima Straits in November 2007 was revealed using conductivity–temperature–depth and acoustic Doppler current profiler data. The eddy had a horizontal scale of approximately 40–60 km, and the accompanying baroclinic current was more than 15 cm s−1 at the edge of the eddy. The island-induced cyclonic eddy moved east-northeastward at about 10 km day−1 (∼10 cm s−1) along the Tsushima Warm Current and was intensified by the barotropic instability in the current shear. The cyclonic eddy with high surface chlorophyll a concentrations intensified in the vicinity of the Tsushima Islands and was advected by the Tsushima Warm Current towards the southwestern Japan Sea.

The interaction between non-uniform near-surface currents and long surface waves is shown to produce large-scale secondary circulations. The circulations are caused by the Craik–Leibovich vortex force imposed on the existing non-uniform current by the surface waves. The current could be produced by different types of sources, such as by ship wakes or by river and sewer outflows. In this paper the circulations are considered for three representative types of currents: a near-surface jet, a shear current, and an underwater jet. A model similar to the model of Langmuir circulations is formulated and studied numerically. The general model takes into account the effect of viscosity on the main current as well as the effect of the circulation-related advection on the main current and secondary flow itself. A simplified model that describes the initial stage of the development of circulations was used in order to demonstrate the strength of the phenomenon and its dependence on some parameters of the problem. At this initial stage, the effect of viscosity on the main current as well as the effect of advection caused by the circulations was neglected (under assumption that the perturbation velocity is small). The effect of the viscosity on the circulations was included in the solution, and it was shown that initial development of the circulations is practically independent of the viscosity. This fact simplifies the solution of the problem and removes the uncertainty related to the value of the turbulent viscosity at the initial stage of the circulations. The results obtained demonstrate that strong circulations are generated under very realistic assumptions regarding the parameters of the current and the surface waves. The maximum velocity at the surface produced by such circulations can easily reach several centimeters per second. A circulatory flow with this magnitude of velocity at the surface can significantly affect short surface waves and, correspondingly, radar and optical signatures produced by the initial currents on the sea surface. Some important conclusions about the nature of these signatures are made based on numerical results and simple qualitative arguments. Theoretical predictions include, for example, the asymmetry of centerline ship wakes and the difference in the width and length between images of two wakes of similar ships moving in opposite directions when ambient surface waves are present.

The efficiency of horizontal perforated and solid plates, installed on top of an intake for prevention of surface vortex formation, was experimentally investigated. The experiments were conducted on a horizontal intake of a reservoir. By changing the angle of the approach flow, submergence of the intake and the discharge, 36 air-core surface vortices with different strengths were generated. The strengths of these vortices were evaluated by measuring tangential velocities using an acoustic Doppler velocimeter. A relationship between the strength of the surface vortex and the necessary percentage of uniform opening of the perforated plate is presented. It was found that a perforated plate of width 1D and 1·5D length normal to the intake (D is the tunnel diameter) with 50% uniform opening and a solid plate of 1·5D width and 2D length eliminated all surface vortices at the intake. Pressure was also measured in a section immediately downstream of the intake by means of four transducers. By using the measured pressure values, the effect of anti-vortex plates on local loss at the intake was determined.

Quantitative velocity flow field measurements of the vortex system for a small scale model intake have been taken using a three component PIV system along with fan face total pressure distortion measurements. The effect of velocity ratio, non-dimensional height and approaching boundary layer thickness have been assessed under quiescent and headwind conditions. A range of inlet vortex flow modes are identified which depend on the intake velocity ratio and are characterised by the primary vorticity source. As the headwind velocity increases, and the velocity ratio reduces, the vortex strength initially increases until a local maximum is reached. Further reductions in the velocity ratio leads to a strong reduction in the vortex strength until a ‘blow-away’ condition is achieved at which the vortex no longer forms. A clear link is established between the external vortical flow-field and the intake internal flow-field. Consequently, these characteristics are also observed for the total pressure distortion coefficient within the intake. As the ground clearance is increased the peak vortex strength reduces and the corresponding velocity ratio increases. At intermediate velocity ratios, the vortex strength is greater at higher ground clearances. At the selected external vortex measurement plane, the vortex strength is generally insensitive to the approaching boundary layer thickness. The intake total pressure distortion increases with boundary layer thickness due to increased total pressure loss for a given ingested mass-flow. The vortex strength and distortion distributions are shown to be self-similar when non-dimensionalised by the peak values and associated velocity ratio. This dataset establishes a quantitative map for headwind ground vortices which presents a new formation criterion as well as a measure of the vortex strength as a function of ground clearance and intake velocity ratio.

A study on the trajectory of flying debris in a tornado-like vortex was carried out by numerical simulation. A numerical tornado simulator was developed, which has the equivalent configuration as that of the laboratory experiment. Numerical calculations were done by Large Eddy Simulation and a series of unsteady flow fields of a tornado-like vortex was generated. Wind characteristics in the vortex were examined. Finally, three dimensional trajectories of flying debris were computed and the statistical distributions of the maximum horizontal speed of debris were obtained.

We study local density of states (LDOS) of vortex state in mesoscopic square superconductors with Bogoliubov-de Gennes (BdG) equation. We develop one effective numerical method based on the finite element method to self-consistently solve the BdG equation. The LDOS for various vortex states is obtained. Our results about the single vortex show that the LDOS has the particle-hole asymmetry and the results for one- and two-vortex state agree very well with the experimental observation. Besides, we predict the LDOS of multi-vortex states, which is crucial for the further STM/STS experimental study of vortex state in mesoscopic superconducting system.

New insights into the Lagrangian and eddy dynamical processes within the East Australian Current (EAC) and the Tasman Sea are presented. We briefly discuss the past campaigns undertaken to observe the EAC and the Tasman Sea eddies as well as the motivation to renew the deployment of drifting buoys into the EAC and the Tasman Sea. The specific features discussed are motivated by the recent observing campaigns using drifting buoys and the availability of high spatial- and temporal-resolution estimates of the ocean state and circulation from eddy resolving models. The interpretation of these features is also aided by other components of the ocean observing system. The dynamics presented includes: (a) transient EAC separation through a vortex dipole, (b) stratified vortex mergers and secondary circulation of EAC eddies, (c) eddy networks in the Tasman Sea and (d) formation and propagation of the EAC separation point. The importance of these dynamical features to the EAC and the Tasman Sea and their implications for the observing system and modelling are discussed.

Results on the tangential velocity in a fluidized bed measured by a laser Doppler velocimeter and by a mechanical propeller are presented. The reasons for the difference in the bed rotation velocities obtained by these measurement techniques are analyzed on the basis of numerical calculations of the multiphase flow dynamics.

We observed particles of different density ratio α = ρ p /ρ f in thermocapillary liquid bridges with steady and with time-dependent flow under normal- and under microgravity. Particle accumulation structures (PAS) visualize some features of the hydrothermal wave in the liquid bridge. Relatively fast formation of PAS from particles which are considerably less dense than the fluid (α = 0.42) in oscillatory thermocapillary flow of top-heated liquid bridges was observed and explained by an additional buoyancy-assisted mechanism which brings the particles into the surface flow. This PAS from particles with α = 0.42 will persist under normal gravity for infinite time. In contrast to these less dense particles the heavier particles with α > 1 settle down under normal gravity on the lower end face of the liquid bridge after some time and are no longer in suspension and PAS will fade out. On the other hand, particles with α = 0.42 will be less suited for experiments under microgravity than particles with α > 1 because most of them will be trapped in the vortex centre of the thermocapillary flow. The sedimented particles with α > 1 are a means to visualize some features of standing hydrothermal waves which are visualzed and discussed for the first time.

We study the Landau–Lifshitz–Gilbert equation for the dynamics of a magnetic vortex system. We include the spin-torque effects of an applied spin current, and we rigorously derive an equation of motion (“Thiele equation”) for vortices if the current is not too large. Our method of proof strongly utilizes the geometry of the problem in order to obtain the necessary energy estimates.

The mixing behavior at different Reynolds numbers is analyzed for different T-shaped micro-channels with a rectangular cross-section. The focus is on analyzing the effect of aspect ratio on the critical Reynolds number at which engulfment occurs. Soleymani et al. (2008) have recently proposed an empirical equation based on extensive numerical simulations to determine this critical Reynolds number. This is valid only when the depth of the channels (C) is lower than the widths of the mixing (A) and inlet (B) channels i.e. when A/C and B/C are both greater than unity. We find that when they are both less than unity there is a delay or even absence of engulfment even when Re is increased. The predictions of these simulations are validated experimentally for two geometries.

To assess accurately the effect of tidal mixing in the Kuril Straits on the formation of the North Pacific Intermediate Water (NPIW), the spatial distribution of diapycnal diffusivity recently obtained by the present authors is incorporated into an eddy-permitting OGCM. It is shown that the NPIW is successfully reproduced, although the diapycnal diffusivity averaged over the entire Kuril Straits is an order of magnitude less than has previously been assumed as a tuning parameter to reproduce the NPIW in low-resolution OGCMs. This strongly suggests that the effect of tidal mixing in the Kuril Straits on the formation of the NPIW is relatively minor and that the physical processes omitted by the low-resolution OGCMs, such as isopycnal mixing along the Kuroshio Extension region, are much more important. This suggestion gives warning of the danger that some misleading conclusions might be derived from OGCMs that employ diapycnal diffusivity just as a tuning parameter to reproduce the observed features.

An experimental simulation of tornado-like vortices is conducted in a small tornado vortex simulator in order to study the effect of swirl ratio on flow characteristics. Particle Image Velocimetry (PIV) method is employed to quantitatively determine the tornado-vortex velocity field for swirl ratios ranging from 0.08 to 1. The radial and tangential components of velocity as well as the core radius of the tornado increase with increase in swirl ratio. The location of the maximum radial and tangential velocities is adjacent to the ground where the tornado vortex interacts with the surface. The values of normal and shear turbulent stresses indicate the existence of a laminar core for small swirl. As expected the shear stresses increase with swirl ratio as the vortex becomes turbulent. The highest turbulent production corresponds to the critical case of vortex touchdown.