For high speed vessels and offshore structures, wave impact, a main source of environmental loads, causes high local stresses and structural failure. However, the prediction of wave impact loads presents numerous challenges due to the complex nature of the instant structure-fluid interaction. The purpose of the present study is to develop an effective wave impact model to investigate the dynamic behaviors of specific shaped elements as they impact waves. To achieve this objective, a wave impact model with a body swinging on a pendulum system is developed. The body on the pendulum goes through a wave free surface driven by gravity at the pendulum's natural frequency. The system's motion and impact force during the entire oscillation time beginning from the instant of impact are of interest. The impact force is calculated by applying von Karman's method, which is based on momentum considerations. The usual wave forces are presented in the Morison's equation and incorporated into dynamic systems with other wave forces. For each body shape, the dynamic system is described by a strongly nonlinear ordinary differential equation and then solved by a Runge-Kutta differential equation solver. The dynamic response behavior and the impact force time history are obtained numerically and the numerical results show support the selection of a pendulum model as an efficient approach to study slamming loads. The numerical prediction of this model is compared to previous experiments and classification society codes. Moreover, a basic design of wave impact experiments using this pendulum model is proposed to provide a more accurate comparison between numerical results and experimental data for this model. This design will also serve as a first look at the experimental application of the pendulum model for the purpose of forecasting slamming force.

Understanding why fish move the way in which they do has applications far outside of biology. Biological propulsion has undergone millions of years of refinement, far outpacing the capabilities of anything created by man. Research in the areas of unsteady/biological propulsion has been increasing in the last several decades with advances in technology. Researchers are currently conducting experiments using pitching and heaving airfoils, mechanized fish, and numerical fish. However, the surrogate propulsors that are being used in experiments are driven analytically, whereas in this study, a method has been developed to exactly follow the motion of swimming fish. The research described in this thesis couples the image analysis of swimming fish with computational fluid dynamics to accurately simulate a virtual fish. Videos of two separate fish swimming modes were analyzed. The two swimming modes are termed `free-stream swimming' and the `Kármán gait'. Free-stream swimming is how fish swim in a section of water that is free of disturbances, while Kármán gait swimming is how fish swim in the presence of a vortex street. Each swimming mode was paired with two simulation configurations, one that is free of obstructions, and one that contains a vortex street generating D-section cylinder. Data about the efficiency of swimming, power output, and thrust production were calculated during the simulations. The results showed that the most efficient mode of swimming was the Kármán gait in the presence of a Kármán vortex street. Evidence corroborating this has been found in the literature. The second most efficient means of swimming was found to be free-stream swimming in the absence of obstructions. The other two configurations, which are not observed in experiments, performed very poorly in regard to swimming efficiency.

Ce travail de thèse s’articule autour de plusieurs questions relatives à l’instabilité dynamo dans des écoulements turbulents en métaux liquides. Cette instabilité de conversion d’énergie cinétique en énergie magnétique dans les fluides électriquement conducteurs est à l’origine, par exemple, des champs magnétiques terrestre et solaire. En particulier, nous abordons l’estimation du seuil de l’instabilité, l’influence de l’écoulement et des conditions aux limites ainsi que les mécanismes de saturation du champ magnétique. Ces travaux expérimentaux s’appuient sur deux écoulements turbulents de type von Kármán : en sodium liquide à Cadarache (collaboration VKS) et en gallium liquide à l’ENS de Lyon.Dans un premier temps, l’étude est consacrée à l’analyse de critères permettant d’estimer la distance au seuil de l’instabilité dynamo, à travers la mesure de la réponse magnétique du système à une excitation pour la dynamo auto-entretenue VKS. Ces critères ont été validés dans les configurations dynamos de l’expérience puis appliquées aux configurations non-dynamo.Ensuite, nous illustrons l’influence de l’écoulement sur le champ dynamo à travers l’étude de bifurcations globales. Une bistabilité hydrodynamique, pilotant deux branches dynamos d’amplitude différentes, est décrite ainsi que les liens entre les états magnétiques et hydrodynamiques.Nous portons notre attention sur l’étude des mécanismes de saturation à travers la dynamo semi- synthétique de Bullard-von Karman mettant en jeu un mécanisme d’induction turbulente et un mécanisme de bouclage artificiel permettant l’observation d’une dynamo à faible nombre de Reynolds magnétique. L’instabilité démarre à travers un régime intermittent et sature par la rétroaction des forces de Lorentz sur l’écoulement. Nous donnons les lois d’échelle et le bilan de puissance de ce régime. Un régime d’instabilité sous-critique est aussi introduit et caractérisé.Nous détaillons dans une dernière partie, les techniques de mesure spécifiques aux métaux liquides utilisées et développées au cours de la thèse. / This PhD thesis deals with several problems relative to the dynamo instability in liquid metals turbulent flows. This instability converts kinetic energy into magnetic one in electrically conductive flows. It is the root of the magnetic field of the Earth and the Sun.We address the estimation of threshold of the instability, the influence of the flow configuration and of the electromagnetic boundary conditions as well as the saturation mechanism of the magnetic field. This experimental work rely on two turbulent flows of von Kármán type: in liquid sodium located in Cadarache (VKS collaboration) and in liquid gallium in ENS de Lyon.First we analyze several criteria about the estimation of the distance to threshold of the dynamo instability with the magnetic response of the system to a magnetic excitation for the self sustained dynamo in the VKS experiment. These method have been checked for dynamo configurations and then applied for non-dynamo configurations. Then, we study the influence of the flow on the dynamo field under the action of global hydrodynamic bifurcations. We describe a bistability of the flow which triggers two dynamo branches of different amplitude and the dynamics of the transitions between both hydrodynamic and magnetic states.We then focus on the saturation mechanism with the semi-synthetic Bullard-von Karman dynamo, involving a turbulent induction mechanism and an artificial electronic feedback. This setup allows to observe dynamo action for very low magnetic Reynolds number, far below the natural threshold of the instability.We observe an intermittent regime close to threshold and a fluid saturation by Lorentz force feedback on the flow. We specify the scaling laws and a power budget estimation of this regime. A sub-critical regime is also introduced and characterized.In the last section we detailed several measurement techniques in liquid metals developed and used during the PhD.

Invasive aquatic species such as Hydrilla verticillata (hydrilla) have become a pervasive and nearly ineradicable part of the waterways of the American south. Hydrilla is an aggressive colonizer; grows rapidly and rapidly blocks flow areas, which greatly reduces the capacity of water supply canals. Hydrilla grows up through the water column and is present throughout flow zones that are typically assumed to be free flowing and without resistance, other than that transmitted via the mechanics of a Newtonian fluid. Hydrilla is highly flexible and its morphology in the flow field is dependent on many parameters, including flow, growth stage, cross-section geometry and substrate. Traditional methods of calculating canal flow capacities assume that resistance to flow originates at the boundary of the channel. These methods typically attempt to account for vegetation by increasing resistance coefficients, which are associated with the boundary of the canal. A combination of field studies and experimentation in three separate laboratory channels was used to characterize the behavior of hydrilla and its impacts on open-channel flow. This work developed relationships for energy losses of flow within the vegetation, as well as velocity gradients within the vegetation and through the vegetation water interface to the open water. The information developed in this investigation was used to develop a model of the cross-section of flow with vegetation growing in the center of the channel. The model is based on the Prandtlvon Kármán universal-velocity-distribution law; and uses modifications to the method of calculating the hydraulic radius, to account for the increased frictional elements and reduced flow areas in the canal cross-section. A simple function was developed to estimate the remaining flow capacity in a canal as a function of the remaining unblocked area. The Prandtl-von Kármán universal-velocity-distribution law, together with modifications to the method for calculating the hydraulic radius, can improve estimates of the flow in channels impacted by submerged aquatic vegetation. The effects of a broad range of parameters can thus be represented by a relatively simple function, which was developed in this project.

Στην παρούσα διατριβή αναπτύσσεται η γραμμικοποιημένη μέθοδος των πεπερασμένων διαφορών. Η μέθοδος αυτή αποτελεί νέα αριθμητική τεχνική επιλύσεως των διαφορικών εξισώσεων δευτέρας τάξεως, κανονικών και με μερικές παραγώγους και εφαρμόζεται σε εσωτερικές και εξωτερικές ροές. Συγκεκριμένα, στο πρώτο κεφάλαιο γίνεται εισαγωγή στις μεθόδους διακριτοποιήσεως μιας διαφορικής εξισώσεως και στον τρόπο επίλυσης ενός γραμμικού συστήματος αλγεβρικών εξισώσεων. Στο δεύτερο κεφάλαιο εκτίθεται η μέθοδος των Allen και Southwell για τη λύση της εξισώσεως της στροβιλότητας προκειμένου περί δισδιάστατης ροής και η τροποποίηση και βελτίωση της μεθόδου από τον Dennis. Εν συνεχεία περιγράφεται η περαιτέρω βελτίωση της μεθόδου, διά της οποίας ο πίνακας του συστήματος των συντελεστών των αγνώστων καθίσταται διαγωνίως υπέρτερος. Στο τρίτο κεφάλαιο η βελτιωμένη μέθοδος επεκτείνεται στις τρεις διαστάσεις με μεταβλητούς συντελεστές και εφαρμόζεται στην λύση αντιστοίχου προβλήματος ροής ιδανικού ρευστού εντός σωλήνα. Στο τέταρτο κεφάλαιο η παρούσα μέθοδος γενικεύεται μεταβλητούς συντελεστές. Στο πέμπτο κεφάλαιο επιλύεται η εξίσωση της στροβιλότητας στις τρεις διαστάσεις σε σύστημα κυλινδρικών συντεταγμένων και ο αλγόριθμος εφαρμόζεται στο πρόβλημα του von Kármán. Τέλος, στο έκτο κεφάλαιο εξετάζεται η περίπτωση της αριθμητικής εξομοίωσης στροβίλων Taylor προκειμένου περί ροής ιξώδους ρευστού, το οποίο περιέχεται εντός σφαιρικού κελύφους, η δε κίνηση του ρευστού προκαλείται από την διαφορική περιστροφή των δύο σφαιρών γύρω από την κοινή διάμετρό τους. / In the present dissertation a numerical technique is developed on the solution of ordinary and/or partial differential equations of second order with variable coefficients. In particular, in the first chapter we make a general introduction to the discretisation methods and to the methods of solution of a linear system of algebraic equations. In the second chapter the method of Allen and Southwell on the solution of the vorticity equation in two dimensional flow is presented as well as its modification and improvement by Dennis. The contribution of the present work is on the further improvement of the previous method, such that the matrix of the coefficients of the unknowns becomes diagonally dominant and its extension to three dimensional problems. The consecutive steps of the method (Linearized Finite Difference Method, LFDM) are worked out on the ordinary differential equation with variable coefficients. Next, the method is applied to the solution of two problems from Fluid Mechanics. In the third chapter the improved method is extended to three dimensions with variable coefficients. Accordingly the method is applied to the solution of the equations of motion of a perfect fluid moving in a straight tube. In the fourth chapter the present method is generalized with variable coefficients. In the fifth chapter the three dimensional equation of the vorticity expressed in cylindrical coordinates is solved and the results are applied to the von Kármán problem. Finally in the sixth chapter we use this method to simulate Taylor vortices in spherical annular flow in the presence of heat, the motion of the fluid being induced by the differencial rotation of two spheres about their common diameter.

Les éléments de construction en béton renforcé par plaque constituent une technologie de construction émergente dont l’utilisation commence à se développer. Ces éléments sont constitués de deux plaques en acier en parement d’un béton de remplissage. Les plaques, connectées entre elles par des tiges métalliques, travaillent de façon composite pour octroyer à la structure ses propriétés de résistance mécanique. En cas de voilement local, les transferts d’action acier-béton ne sont plus développés entraînant une perte de résistance globale de la structure. Le présent document est consacré au risque voilement local induit par une élévation de la température. Deux aspects seront particulier étudiés : définir un domaine de flambement tenant compte de la plasticité local et obtenir une estimation de la déformée à une température donnée. Les résultats seront confrontés aux solutions issues de la simulation numérique et de l'expérimentation. / Steel concrete structures are a new building technology and its use starts to spread in civil engineering. these structures are made of two steel plates connected by tiebars and a concrete core. In order to provide resistance, steel and concrete must develop composite actions. In case of buckling of the steel plate, the transfer of the actions between steel and concrete is lost. This paper focus on preventing buckling failure’s mode of the structure in case of thermal loading. Two main issues are studied : defining an analytic operating for steel concrete structures regarding thermal buckling aspect including yielding coupling and getting an analytic estimation of the deflection. The results are finally confronted with numerical simulation and experimentation.

Les travaux présentés dans cette synthèse en vue d'une Habilitation à Diriger des Recherches concernent des problèmes aux limites non linéaires mixtes elliptiques hyperboliques auxquels on adjoint une contrainte inégalité. Ici nous présentons le cas significatif présenté par le problème aux limites résultant du modèle de Karman et Guderley avec condition d'entropie. Ce problème a l'avantage de se présenter simplement tout en présentant un terme non linéaire conduisant aux difficultés fondamentales des problèmes mixtes non linéaires. Ce problème aux limites est mal posé : il n'existe pas de cadre fonctionnel assurant l'existence de solutions. Notre propos est de proposer une méthode d'analyse assurant la cohérence entre les résultats fonctionnels et numériques. On commence par traiter le problème aux limites sans contrainte. L'utilisation d'une formulation variationnelle et de la formule de Green généralisée ramènent le problème à montrer qu'une projection adaptée s'annule. L'introduction d'une norme adaptée conduit à minimiser une fonctionnelle. On utilise des solutions généralisées à epsilon près correspondant aux quasi minima d'Ekeland. On considère ensuite le problème aux limites avec contrainte. Le paramètre d'entropie est recherché dans un intervalle réel suffisamment grand a priori. La contrainte est également transformée par la formule de Green généralisée ce qui conduit à une contrainte égalité. Une fonctionnelle pénalisée est minimisée.

The objective of this study is to investigate and characterize the mechanism of the island wake behind an island called Liu Chiu Yu off the southwestern Taiwan coast based on the in-situ data of Sb-ADCP, CTD and satellite images. The findings suggest that a counter-clockwise eddy and a clockwise eddy both are with 0.01 S vortice appears in the wake of Liu Chiu Yu when the background flows are toward the northwest. The system of two eddies with opposite rotation and a central return flow develops an unsteady eddy shedding. On the other hand, when the background flows are toward the southeast, island wake generated in the lee of Liu Chiu Yu is attached system of two eddies with opposite rotation and a central return flow. The Sb-ADCP data shows that the flow pattern in the northeastern coast of Liu Chiu Yu is mainly semidiurnal. The major axis of the ellipse of the semidiurnal current is parallel to the orientation of the coast line (northeast to southwest) and the shape of the ellipse is quite long and narrow. The amplitude of the semidiurnal current is approximately two times that of the diurnal current. Generally, the currents are stronger and the occurring probability of the island wake is higher during spring tide. The CTD data shows that the eddy center appears to be divergent and upwelling occurs in the areas under the influence of island wakes. The upwelling pumps deep seawater to the surface and results in low temperature, high salinity, high oxygen concentration and low chlorophyll concentration. On the other hand, in the eddy edges, downwelling occur causing high temperature, low salinity, low oxygen concentration and high chlorophyll concentration. Strong shear was formed at the depth of 60m inside the island wake which generates thermocline so that the mixing phenomenon is quite obvious there. Moreover, in the regions without the influence of island wakes, the stratifying effect is clear and the horizontal variation of temperature, salinity and oxygen concentration is small. Therefore, neither upwelling nor downwelling occurs there. Furthermore, along the edge between blocking and free-stream areas, the shear stress increases and the mixing phenomenon arises to a certain degree. The satellite images show that an island wake appears in the southeastern Liu Chiu Yu during spring tide. The island wake develops a phenomenon called von Karman vortex street. At the same time, a counter-clockwise eddy with heavy suspensions appears in the northern Liu Chiu Yu. The radius of the eddy is around 4 Km. The area of the lowest chlorophyll concentration is located at the center of the eddy. By analyzing all these data, it is concluded that the island wake in Liu Chiu Yu usually appears during spring tide.

Ce travail de thèse s'articule autour de plusieurs questions relatives à l'instabilité dynamo dans des écoulements turbulents en métaux liquides. Cette instabilité de conversion d'énergie cinétique en énergie magnétique dans les fluides électriquement conducteurs est à l'origine, par exemple, des champs magnétiques terrestre et solaire. En particulier, nous abordons l'estimation du seuil de l'instabilité, l'influence de l'écoulement et des conditions aux limites ainsi que les mécanismes de saturation du champ magnétique. Ces travaux expérimentaux s'appuient sur deux écoulements turbulents de type von Kármán : en sodium liquide à Cadarache (collaboration VKS) et en gallium liquide à l'ENS de Lyon.Dans un premier temps, l'étude est consacrée à l'analyse de critères permettant d'estimer la distance au seuil de l'instabilité dynamo, à travers la mesure de la réponse magnétique du système à une excitation pour la dynamo auto-entretenue VKS. Ces critères ont été validés dans les configurations dynamos de l'expérience puis appliquées aux configurations non-dynamo.Ensuite, nous illustrons l'influence de l'écoulement sur le champ dynamo à travers l'étude de bifurcations globales. Une bistabilité hydrodynamique, pilotant deux branches dynamos d'amplitude différentes, est décrite ainsi que les liens entre les états magnétiques et hydrodynamiques.Nous portons notre attention sur l'étude des mécanismes de saturation à travers la dynamo semi- synthétique de Bullard-von Karman mettant en jeu un mécanisme d'induction turbulente et un mécanisme de bouclage artificiel permettant l'observation d'une dynamo à faible nombre de Reynolds magnétique. L'instabilité démarre à travers un régime intermittent et sature par la rétroaction des forces de Lorentz sur l'écoulement. Nous donnons les lois d'échelle et le bilan de puissance de ce régime. Un régime d'instabilité sous-critique est aussi introduit et caractérisé.Nous détaillons dans une dernière partie, les techniques de mesure spécifiques aux métaux liquides utilisées et développées au cours de la thèse.

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