Dynamique de défauts spatiotemporels dans un motif de spirales de Taylor

La vélocimétrie par imagerie de particules (PIV) a été mise en œuvre pour caractériser le comportement spatio-temporel des régimes de vortex spiral observé dans l'écoulement de Couette-Taylor. A partir des mesures de vitesses obtenues par PIV, les variations spatio-temporelles des composantes de vitesses radiale et axiale sont établies. Nous rapportons les principales caractéristiques (nombre d'onde, fréquences) ainsi que la déstabilisation de ces régimes vers des comportements désordonnés. Par ailleurs, nous décrivons le comportement observé dans les zones des défauts de spirales qui se manifestent dans l'écoulement. Une analyse fine autour de chaque défaut a été menée (profil d'amplitude, saut de phase, pinning, ...)

Formation dynamics of sand bedforms under solitons and bound states of solitons in a wave flume used in resonant mode

International audienceThis paper reports the results of an experimental study of the formation of sand bedforms under solitons and bound states of solitons in a wave flume used in resonant mode. A numerical modeling of Boussinesq equations is carried out to simulate the generated flow and a good agreement is obtained between the measured and calculated fluid velocities. An original method based on image processing is developed to obtain the morphology of the sandy bed over large areas. The size and the shape of ripples which form on the bed significantly change from one end of the flume to the other end. Accretion zones are observed beneath the nodes of the standing harmonic wave. These bed features result from the variation of the amplitude of the first harmonic of fluid velocity, and of the time interval between the passage of contra-propagative solitons and bound states of solitons, with the distance from the wave paddle along the flume. An equation obtained from a phenomenological analysis is proposed for the description of the temporal growth of the ripples

Segregation of sedimenting grains of different densities in an oscillating velocity field of strongly nonlinear surface waves

International audienceThis paper reports the results of an experimental and theoretical study of the segregation of heavy (sand) and light [polyvinyl chloride (PVC)] grains under the action of intense nonlinear water waves, solitons. The tests are carried out with initially carefully mixed grains at the bottom of a wave flume used in resonant mode. Ripples form on the bed and the segregation process is considered after the stopping of the wave paddle. The waves are damped and the PVC grains concentrate in a narrow region close to the ripple crest. A theoretical model explaining this grain density segregation is developed

Analyse de la performance hydraulique d'un nouveau type de bloc artificiel utilisé pour la protection côtière

Parmi l'ensemble des différentes parties composant une digue à talus maritime, la carapace représente l'élément clé de la stabilité de la structure. Elle se compose soit d'enrochements naturels soit de blocs artificiels en béton lorsque l'intensité de la houle requiert des poids d'éléments naturels que les carrières disponibles ne sont pas en mesure de fournir. Le développement des blocs artificiels performants a fait l'objet de multiples travaux de la part de chercheurs du monde entier. Il s'agissait principalement de proposer des formes optimisées conduisant à une grande stabilité hydraulique autorisant par là de faibles consommations de béton et donc des économies substantielles. L'objective principale de ces travaux de recherche est de mettre au point de nouveaux blocs artificiels de protection des digues à talus soumis à l'action des houles. Le travail contribuera à développer des méthodes d'optimisation des formes des blocs. Les enseignements recueillis à l'issue de la thèse doivent permettre de proposer des formes plus performances que celles aujourd'hui disponibles sur le marché. L'analyse de la performance de ce bloc sur modèle réduit nous apporte des résultats satisfaisants en termes de stabilité, malgré le franchissement restant à améliorer. Une étude approfondie, d'une part sur la cinématique de l'écoulement proche de la carapace et d'autre part les efforts subis par le bloc, nous permet de proposer une nouvelle formulation de la vitesse longitudinale liée au run-up. L'influence du caractère de l'onde (déferlement, claque) est déterminante pour les coefficients hydrodynamiquesAmong the whole various parts composing a rubble mound breakwater, armour layer represents the key component of the structure stability. It is composed either of quarry stones or of artificial concrete armour units when the design wave conditions require heavy armour elements that careers may not able to provide. The development of the concrete armour unit performance has been the subject of numerous studies. The main concern was to propose optimizations leading to a better hydraulic stability thereby allowing low consumption of concrete and thus significant savings. The main objective of this thesis is to develop a new artificial armor unit for rubble mound breakwaters. Work will contribute to develop methods for optimizing shapes of the armour units. The lesson collected at the end of the thesis will produce new forms more performing than those available today on the market. Based on hydraulic model tests, the results demonstrate the large hydraulic stability, in spite of overtopping remaining to be improved. In this study, other experimental tests were performed to investigate the velocity field close to the armor layer and also the wave -induced loadings acting on one instrumented block. A new formula is proposed concerning velocity longitudinal in relation to run-up. It is shown that the influence of the wave characteristics (breaking wave, impact) is a decisive parameter for the hydrodynamic coefficients.CAEN-BU Sciences et STAPS (141182103) / SudocSudocFranceF

Physical modeling of resonance phenomena in the long wave dynamics

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Physical Modeling of Resonance Phenomena in the Long Wave Dynamics

We have prepared two sets of experiments in wave flume to demonstrate resonance phenomena in laboratory conditions and to model effects occurring in natural conditions. The first set was performed to investigate nonlinear wave run-up on the beach caused by harmonic wave maker located at some distance from the shore line. It is revealed that under certain wave excitation frequencies a significant increase in run-up amplification is observed [ 1]. It is found that this amplification is due to the excitation of resonant mode in the region between the shoreline and wave maker. Frequency and magnitude of the maximum amplification are in good correlation with the numerical calculation results represented in the recently published paper [ 2]. The second set of experiments was performed to study resonance effects due to parametric excitation of edge waves. It is known that surface waves propagating toward the shore can excite edge waves propagating along the shore line. Although the edges wave amplitude decreases in an offshore direction they may contain enough energy to be responsible for erosion of the shore and generate so-called cusps [ 3]. We investigate parametric mechanism of such generation when plane surface wave with frequency Ω excite edge wave with frequency Ω/ 2. It is shown that parametric generation of edge waves can amplify run-up up to two times.Abcha Nizar, Ezersky Alexander, Pelinovsky Efim. Physical Modeling of Resonance Phenomena in the Long Wave Dynamics. In: SimHydro 2014. New Trends in Simulation. 11-13 June 2014 Ecole Polytech’ Nice (France) 2014

Frequency lock-in and phase synchronization of vortex shedding behind circular cylinder due to surface waves

(IF 1.17; Q3)International audienceThe influence of harmonic surface wave on non-regular Karman Vortex Street is investigated. In our experiments, Karman Street arises behind a vertical circular cylinder in a water flow and harmonic surface waves propagating upstream. It is found that surface waves can modify regimes of shedding in Karman Street: frequency lock-in and synchronization of vortex shedding can arise. Intensive surface waves can excite symmetric vortex street instead of chess-like street, and completely suppress shedding behind the cylinder. It is shown experimentally that such effects occur if frequency of harmonic surface wave is approximately twice higher than the frequency of vortex shedding. Region of frequency lock-in is found on the plane amplitude–frequency of surface wave

Hydrodynamique d’un écoulement induit par un rideau de bulles d’air : étude expérimentale

National audienceNous présentons les résultats expérimentaux, de l'influence d'un rideau de bulles d'air sur le déferlement des vagues, ce dernier est de type plongeant. Les expériences ont été réalisées en canal à houle à une profondeur d'eau constante. Une pompe à air a été utilisée pour générer les bulles d'air et contrôler leur concentration. Une faible concentration de bulles d'air provoque une dissipation d'énergie négligeable des vagues, cependant elle influence significativement la position du point de déferlement. A partir de cela, nous avons mené une étude expérimentale pour appréhender d'avantage les mécanismes physiques pilotant l’interaction bulles d’air/déferlement. La vitesse des bulles d'air a été mesurée à l'aide d'une sonde conductive à double pointe tandis que les champs de vorticité d'écoulement ont été caractérisés au moyen d'un vélocimètre ADV

Experimental study of wave breaking in the presence of air bubbles curtain

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