Morphology and dynamics of inflated subaqueous basaltic lava flows

International audienceDuring eruptions onto low slopes, basaltic Pahoehoe lava can form thin lobes that progressively coalesce and inflate to many times their original thickness, due to a steady injection of magma beneath brittle and viscoelastic layers of cooled lava that develop sufficient strength to retain the flow. Inflated lava flows forming tumuli and pressure ridges have been reported in different kinds of environments, such as at contemporary subaerial Hawaiian-type volcanoes in Hawaii, La Réunion and Iceland, in continental environments (states of Oregon, Idaho, Washington), and in the deep sea at Juan de Fuca Ridge, the Galapagos spreading center, and at the East Pacific Rise (this study). These lava have all undergone inflation processes, yet they display highly contrasting morphologies that correlate with their depositional environment, the most striking difference being the presence of water. Lava that have inflated in subaerial environments display inflation structures with morphologies that significantly differ from subaqueous lava emplaced in the deep sea, lakes, and rivers. Their height is 2-3 times smaller and their length being 10-15 times shorter. Based on heat diffusion equation, we demonstrate that more efficient cooling of a lava flow in water leads to the rapid development of thicker (by 25%) cooled layer at the flow surface, which has greater yield strength to counteract its internal hydrostatic pressure than in subaerial environments, thus limiting lava breakouts to form new lobes, hence promoting inflation. Buoyancy also increases the ability of a lava to inflate by 60%. Together, these differences can account for the observed variations in the thickness and extent of subaerial and subaqueous inflated lava flows

Prédiction de trajectoires d'objets immergés par couplage entre modèles d'écoulement et équations d'Euler-Newton

Numerical instabilities due to fluid inertia appear when solving the free motion of a solid submerged within a heavy fluid such as water. In the present thesis, a numerical scheme is proposed to overcome this problem. Three-dimensional simulations using Computational Fluid Dynamics and Euler-Newton equations use too much computing resources for a reasonable investigation of the general case. It was therefore decided to design and build a two-dimensional hydrodynamic tunnel in order to validate the numerical tool. First, a static two-dimensional tank has been built to verify the feasibility of such an apparatus. It reveals the chaotic aspect of the trajectories of light objects when viscous forces are highly unsteady. It is observed in the hydrodynamic tunnel that an income flow stabilizes the translations. The evolution of the angle is still controlled by the wake. In the case of a parallelepipedic object, presenting sharp corners, boundary layer separations occur and induce instabilities. The prediction of the angle is then difficult. This method is then used to simulate biomimetic propulsion using a porpoising foil. The hydrodynamic solver is a potential flow code. To understand the influence of each parameter on the performances, all degrees of freedom are fixed. Our results for the thrust loading coefficient are in conformity with the Theodorsen theory over the whole range of parameters. The parametric study confirms that the Strouhal number is playing the same role for the oscillating wing, the advance parameter is playing for the propeller. The two propulsion devices are found to be comparable and a general guidance for comparison between the two propulsion systems is developed.When a change of pace is required, the variable pitch propeller is more efficient than a variation of the pitch amplitude during the foil motion. Results in free motion demonstrate the robustness of the method.Des instabilités numériques dues à l'inertie du fluide apparaissent lorsque l'on résout les équations du mouvement pour un solide immergé dans un fluide dense tel que l'eau. Dans cette thèse, un schéma numérique stable dans ce cas est proposé. Les simulations tridimensionnelles de mouvements libres d'un objet couplé avec les équations résolvant l'écoulement utilisent trop de ressources informatiques pour étudier un grand nombre de cas. Il fut donc décidé de concevoir et de construire une veine hydrodynamique 2D pour valider le code numérique. Un dispositif en fluide statique est premièrement mis en place pour vérifier la faisabilité de trajectoires 2D correctes. L'aspect chaotique de certaines trajectoires est mis en évidence. Ce comportement est dû aux fortes instabilités du sillage. On observe dans la veine hydrodynamique que l'écoulement stabilise les translations, qui sont correctement prédites. La rotation est, quant à elle, toujours soumises aux instabilités du sillage. D'autant plus que l'objet utilisé est un rectangle qui, de par ses arêtes vives, présente des décollements de sa couche limite au cours de sa trajectoire. Ceci implique de fortes instabilités empêchant une prédiction correcte de l'angle au cours des essais. Cette méthode est également utilisée pour simuler la propulsion biomimétique grâce à un aileron oscillant. Le code hydrodynamique est alors un code potentiel utilisant la méthode des éléments frontières. Afin de comprendre l'influence des différents paramètres sur les performances du mouvement, tous les degrés de liberté sont fixés. Nos résultats pour le coefficient de poussée sont en accord avec la théorie de Theodorsen. L'étude paramétrique confirme que le nombre de Strouhal joue le même rôle pour l'aileron oscillant que le paramètre d'avance joue pour l'hélice. Les rendements propulsifs obtenus pour ces deux moyens de propulsion sont comparables. Une procédure de comparaison générale entre les moyens de propulsion est développée. Cependant, lorsqu'un changement de rythme est nécessaire, une hélice à pas variable donne une meilleure efficacité qu'un aileron changeant d'amplitude de tangage, même si l'amplitude de tangage a le même effet que le pas. Les résultats en mouvements libres mettent en évidence la rapidité du couplage et sa robustesse

Origin of Sediment Fluxes in Inner Shelf Zone via Acoustic Measurements, Grand-Popo, Bénin

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Utilisation de caméra pour l'observation des plages de récifs frangeants microtidales : Leçon d'une année d'observation à La Réunion

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Eddy Formation and Shedding in a Separating Boundary Current.

International audienceThis study deals with the separation of western boundary currents within a reduced-gravity framework, and it analyzes the formation of eddies in the separation region and the conditions of their shedding into the open ocean. It shows that the separation point of the current oscillates along the coast so that the retroflected eastward current develops meanders. These meanders grow, drift westward under the influence of β, and finally hit the coastal current, which leads to the periodic formation of eddies. This study also highlights the impact by the geometrical configurations of the flow and coastline upon the existence or lack of a subsequent shedding of these eddies: a shedding occurs when no obstacle hinders the β-induced westward drift of the eddies. This happens when either (i) the current retroflects far enough beyond the tip of the coast so that, because of β, the eddies can propagate westward without being blocked, or (ii) the tilt of the coast is small enough so that the alongshore component of the β-induced velocity is enhanced and the eddies can escape from the retroflection region

Suspended sediment fluxes in a shallow macrotidal estuary

International audienceResidual suspended sediment flux in estuaries is dependent on water level, velocity, and suspendedsediment concentration (SSC), but complex interactions between these variables and other forcingmechanisms can lead to drastic differences in the magnitude and direction of sediment flux. The goal ofthis study was to quantify residual suspended sediment flux in a shallow, macrotidal estuary, and todetermine its most important forcing mechanisms, using the Dyer flux decomposition equation and asimplified analytical model. Water level, velocity, and acoustic backscatter were measured in the AulneRiver estuary in Brittany, France, and acoustic backscatter converted to SSC. The vertical tide was slightlyflood dominant near the mouth, but strongly flood dominant upstream. Velocity was ebb dominantthroughout the estuary. The magnitude and direction of total residual suspended sediment flux changedwith position in the estuary and seasonally. The Eulerian flux was dominant at the mouth, but the tidalpumping and Stokes drift components increased in importance landward. Residual suspended sedimentflux in the Aulne is dependent on several processes in addition to those included in the simplified model.The strong spring-neap control and tidal resuspension of sediments in the Aulne and the presence ofhigher-order tidal velocity terms contribute in a non-negligible way to residual suspended sediment flux.Finally, all of the first five components of the Dyer flux decomposition equation are needed to accuratelyrepresent residual suspended sediment flux in the Aulne

Utilisation de caméra pour l'observation des plages de récifs frangeants microtidales : Leçon d'une année d'observation à La Réunion

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Contribution de l'analyse acoustique au suivi d'endommagement de capacite de stockage sous haute pression

SIGLECNRS-CDST / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc

Empirical parameterization of wave runup and dune erosion during storm conditions on a natural macrotidal beach

International audienceAn experimental study based on field measurements for runup parameterization was conducted on a high energy macrotidal beach located on North Brittany (Vougot Beach). The approach was based on morphological and hydrodynamic high frequency monitoring collected between 2008 and 2013. The aim was to quantify in-situ environmental conditions and dimensional swash parameters for the best calibration of Battjes (1971) runup formula. In addition, an empirical equation based on observed tidal water level and offshore wave height was produced to estimate extreme water levels defined as the sum of (i) astronomic tide, (ii) storm surge, and (iii) vertical wave runup. A good correlation between this empirical equation (1.01Hmoξo) and field runup measurements (Rmax) was obtained (R2 85%). The goodness of fit given by the RMSE was about 0.29 m. Extreme water levels were then used to explain dune erosion processes that occured during the winter storms 2013-2014. A good relationship was noted between dune erosion and high water levels when they exceed the dune foot elevation