Global ETD Search

1	Acoustique des milieux bulleux : applications à la conception de métamatériaux et à la manipulation de bulles / Acoustics of bubbly media : Application to the design of metamaterials and to bubble manipulation Lanoy, Maxime 02 December 2016 (has links) Lorsqu’elle est excitée par une onde ultrasonore, une bulle d’air oscille. Les mouvementsl’interface eau-air entrainent un déplacement local du fluide hôte qui devient lesiège de la propagation d’une onde diffusée. Ce mécanisme très simple est fidèlementdécrit par le formalisme de Rayleigh-Plesset datant du début du siècle dernier. Abasse fréquence, au voisinage de la résonance de Minnaert, les oscillations de la bullepeuvent devenir particulièrement amples et la diffusion extrêmement efficace. Dansun environnement réaliste, les bulles sont présentes en grand nombre et la descriptionse complique sensiblement puisque la propagation résulte de l’interférence entre uneinfinité de séquences de diffusion.Au cours de cette thèse, nous introduirons les modèles de milieux effectifs quipermettent de prédire le comportement de fluides bulleux désordonnés. Après avoirété confrontées aux résultats issus de la simulation numérique, ces théories effectivesnous permettront de concevoir des matériaux désordonnés aux propriétés étonnantescomme la superfocalisation ou la réfraction négative.Nous envisagerons également l’étude d’arrangements cristallins et verrons que lapériodicité induit des modifications sensibles dans le comportement du milieu tout enconstituant un levier de contrôle efficace.Enfin, la bulle est susceptible de se mouvoir et, par conséquent, de ressentir desforces de pression de radiation. En particulier, Nous verrons comment la force deBjerknes secondaire, issue du champ diffusé par une bulle vers l’une de ses voisines,peut être exploitée afin de manipuler une ou plusieurs bulles mobiles au voisinaged’une ou plusieurs bulles piégées. / A bubble undergoing an acoustic wave is likely to oscillate. The displacement of theair-fluid interface generates a local compression of the outer fluid where a scatteredwave is thus created. This very simple mechanism has been successfully described bythe famous Rayleigh-Plesset formula derived at the beginning of the previous century.At low frequencies, the resonant behavior (Minnaert) is responsible for a strongenhancement of the oscillations of the bubble which thus becomes a very efficientscatterer. In a realistic media, bubbles are not isolated. The interferences occurringbetween an infinity of scattering sequences make the description of the propagation alot harder to achieve.In this thesis, we introduce the effective theories that are classically adopted todescribe random bubbly liquids. After being compared to our numerical results, thesetheories are used in order to design bubbly materials featuring some interesting propertiessuch as superfocusing or negative refraction.The case of periodic assembly is addressed as well for both a simple square latticebubble raft and a cubic centered faces bi-periodic crystal. We show that the periodicityinduces an explicit modification in the dispersion of the medium and offers asimple tunable parameter.At last, we focus on the possibility for the bubble to move inside the host fluid andwhich is thus likely to experience an acoustic radiation pressure called Bjerknes force.We show how the secondary Bjerknes force, resulting from the wave scattered by abubble toward its neighbors, can be exploited in order to manipulate one or severalfree bubble flowing near one or several trapped bubbles. Forces de Bjerknes Réfraction négative Super absorption Bjerknes force Negative refractive Absorption
2	Acoustically Enhanced Boiling Heat Transfer Douglas, Zachary W. 10 July 2007 (has links) An acoustic field is used to increase the critical heat flux of a copper boiling heat transfer surface. The increase is a result of the acoustic effects on the vapor bubbles. Experiments are being performed to explore the effects of an acoustic field on vapor bubbles in the vicinity of a rigid heated wall. Work includes the construction of a novel heater used to produce a single vapor bubble of a prescribed size and at a prescribed location on a flat boiling surface for better study of an individual vapor bubble s reaction to the acoustic field. Work also includes application of the results from the single bubble heater to a calibrated copper heater used for quantifying the improvements in critical heat flux. Enhanced boiling Pool boiling Contact line dynamics Bjerknes Separation distance
3	Changes in the equatorial mode of the Tropical Atlantic in different oceanic reanalyses / Mudanças no modo equatorial do Atlântico Tropical em diferentes reanálises oceânicas Júnior, Paulo Sergio da Silva 19 March 2019 (has links) In the Tropical Atlantic Ocean, the main mode of SST variability is the Atlantic Equatorial Mode or Atlantic Niño, which is strongly associated with rainfall patterns in northeastern Brazil and the West Africa Monsoon. The region of largest interannual variability, where the Atlantic Cold Tongue forms, is also a region of consistent biases in climate models. In this study, we investigate the interannual variability of the Tropical Atlantic and its changes in the recent decades in terms of the Bjerknes Feedback Index in a set of seven ocean reanalyses for the periods 1980-1999 and 2000-2010 and for an XX century ocean reanalysis for 1950-2010. Warming trends are observed in SSTs in the cold tongue region, as well as a decrease interannual variability. These in turn are associated with a weakening in the Bjerknes Feedback in the early XXI century, resulting from a stronger thermal damping and weaker thermocline feedback, associated with a weaker response of equatorial zonal thermocline slope to equatorial zonal wind stress. However, the spread among the reanalysis products is large, which makes necessary the use of multiple products and an ensemble analysis to minimize errors and obtain more robust results. This is further reinforced as no significant shifts in the Bjerknes Feedback Index were found for the period previous to 1980, since only one reanalysis product covers this period and its individual errors are large. / No Atlântico Tropical, o principal modo de variabilidade da temperatura da superfície do mar é o modo equatorial, ou El Niño do Atlântico, que está fortemente associado aos padrões de precipitação no Nordeste do Brasil e à Monção Oeste-Africana. A região de maior variabilidade interanual, onde se forma a Língua Fria do Atlântico, é também uma região de consistente discordância entre modelos climáticos. Neste estudo, são investigadas a variabilidade interanual do Atlântico Tropical e suas mudanças nas últimas décadas por meio do Índice do Feedback de Bjerknes considerando um grupo de sete reanálises oceânicas para os períodos de 1980-1999 e 2000-2010 e uma reanálise do século XX para 1950-2010. Um aquecimento é observado na região da língua fria, assim como uma diminuição na variabilidade interanual. Essas mudanças estão ligadas a um enfraquecimento do Feedback de Bjerknes no início do século XXI, como resultado de um amortecimento térmico mais intenso e um enfraquecimento do feedback da termoclina, associado a uma resposta mais fraca do gradiente zonal da termoclina equatorial à tensão de cisalhamento do vento. Contudo, a dispersão entre as reanálises é alta, o que torna necessária a análise comparativa de múltiplos produtos, visando obter resultados mais robustos. Da mesma forma, não foi possível tirar conclusões sobre mudanças no Feedback de Bjerknes no período anterior a 1980, uma vez que somente uma reanálise cobria este período e os erros individuais são grandes. Atlantic Cold Tongue Atlantic Niño Atlântico Tropical Bjerknes Feedback Index Índice do Feedback de Bjerknes Interannual variability Língua Fria do Atlântico Modo Equatorial do Atlântico Tropical Atlantic Variabilidade Interanual
4	On the role of wind driven ocean dynamics in tropical Atlantic variability Da Silva, Meyre Pereira 16 August 2006 (has links) The response of the tropical Atlantic Ocean to wind stress forcing on seasonal and interannual time scales is examined using an ocean data assimilation product from the Geophysical Fluid Dynamics Laboratory (GFDL), and an ocean general circulation model which incorporates a three dimensional flux correction technique to correct biases of the mean state of the ocean. On a seasonal time scale, we investigated the impact of the annual migration of the ITCZ on the exchange pathways of the northern tropical Atlantic. The results indicate that seasonal variation of the zonal slope of the thermal ridge along the boundary between the north equatorial countercurrent and north equatorial current in response to changes in the ITCZ controls, to a large extent, the amount of water participating in the equatorial circulation. These changes can be explained in terms of a simple dynamical model where local Ekman pumping dominates thermocline variation in the western part of the basin, and Rossby wave adjustment comes into play in the eastern basin. On an interannual time scale, we examined the upper heat budget of the equatorial Atlantic in order to identify the key mechanisms by which wind-driven ocean dynamics control SST variability during the onset and peak phases of the Atlantic zonal mode. It is found that, in contrast with Pacific ENSO, both Bjerknes and Ekman feedbacks act together to force the zonal mode, although their relative importance and dominance depend on season and location. tropical Atlantic ITCZ sea surface temperature wind stress Bjerknes and Ekman feedbacks
5	Bubble pulsation and translation near a soft tissue interface Tengelsen, Daniel R. (Daniel Ross), 1983- 25 June 2014 (has links) A Lagrangian formalism presented by Hay, Ilinskii, Zabolotskaya, and Hamilton [J. Acoust. Soc. Am. 132, 124--137 (2012)] to calculate the pulsation of a spherical bubble, immersed in liquid and near one or two viscoelastic layers, is extended here to include bubble translation. The method presented here is simplified from that given by Hay et al. in that only a single interface between a liquid and a viscoelastic half-space is considered. In the present approach the force on the bubble due to the presence of the liquid-solid interface is calculated using a Green's function that takes into account elastic waves and viscosity in the layer, and the viscous boundary layer within the liquid near the interface. Previous models and experiments have shown that the direction of bubble translation near a viscoelastic layer is correlated with the direction of a liquid jet often produced by the bubble during collapse. In this dissertation an attempt is made to model the pulsation and translation of a spherical bubble near a liquid-solid interface to infer the direction of bubble translation in reference to material parameters of the liquid and viscoelastic medium, and the standoff distance of the bubble from the interface. The analysis is simplified by demonstrating that the direction of bubble translation can be inferred from the phase of the component of the Green's function associated with the reverberant pressure gradient. For linear bubble pulsation it is shown that the domain of material properties of the viscoelastic medium which generally corresponds to bubble translation away from the interface occurs when the effective stiffness of the viscoelastic medium is greater than the effective damping for both itself and the liquid. The analysis is performed assuming the viscoelastic medium is similar to soft tissue, and its dynamics are described by a Voigt, Kelvin, or Maxwell model. The simulations are compared with existing experimental data. Effects of high-amplitude bubble pulsation are explored in terms of how the simulations differ as the pulsation amplitude increases. At higher pulsation amplitudes, it is shown that bubble translation is still described qualitatively by analyzing the phase of the reverberant pressure gradient. / text Bubbles Nonlinear dynamics Translation Contrast agents Microbubbles Rayleigh-Plesset Greens function Bjerknes
6	Manipulation of biomimetic objects in acoustic levitation Castro, Angelica 18 December 2013 (has links) (PDF) La lévitation acoustique par des ondes stationnaires ultrasonores (USW), permettent la manipulation des objets micrométriques. L'objectif principal de cette thèse est d'explorer les possibilités offertes par la lévitation acoustique pour manipuler des particules, des cellules et même des bactéries. Nous avons conçu et construit tous les résonateurs et nous avons développé les méthodologies que nous allons montrer dans ce travail expérimental. Selon la nature des particules, leur déplacement est donné par son interaction avec la force acoustique primaire. La position où les particules se déplacent est le point dont les forces acoustique et gravitationnel sont équilibrées. Dans le plan de lévitation, les interactions connues comme force secondaire de Bjerknes est la première étape du processus d'agrégation. Nous présentons une méthodologie pour mesurer cette force. Nous avons mesuré cette force en conditions de micropesanteur. Dans nous résonateurs, nous travaillons avec un grand nombre des particules dont les agrégats sont 3D. Nous introduisons le mode acoustique pulsé que nous permet générer des agrégats 2D. Lorsque les particules deviennent plus petites de 1µm, sa manipulation est difficile en raison de l'influence de l'acoustic streaming qui modifie le comportement des particules. Le mode acoustique pulsé permet de réduire ou de contrôler l'acoustic streaming que nous permet manipuler des particules de taille submicronique, des bactéries et des micro-cylindres catalytiques. Une séparation a été faite par un mélange des particules de 7-12µm dans le dispositif s-SPLITT. Néanmoins la combinaison de forces hydrodynamique et acoustique (HACS) a permis améliorer la séparation. Résonateur acoustique Ondes ultrasonores stationnaires Force de Bjerknes Séparation Mode pulsé Acoustic streaming Particle manipulation Lévitation acoustique
7	Changes in Cross-Equatorial Ocean Heat Transport Impact Regional Climate and Precipitation Sensitivity Oghenechovwen, Oghenekevwe C. 01 December 2022 (has links) Do changes in how cross-equatorial energy transport is partitioned between the ocean and atmosphere impact the hemispheric climate response to forcing? To find out, we alter the cross-equatorial ocean heat transport in a state-of-the-art GCM and ascertain how changes in energy transport and its partitioning impact hemispheric climate and precipitation sensitivity following abrupt CO2-doubling. We further evaluate the applicability our results in CMIP6-class ESMs, where AMOC facilitates the northward cross-equatorial ocean heat transport. In our experiments, changes in ocean cross-equatorial energy transport trigger compensating changes in atmospheric energy transport through changes in the Hadley cells and a shift in the Intertropical Convergence Zone. However, the climate sensitivity in each hemisphere is linearly related to the ocean heat transport convergence, not atmospheric energy transport convergence, due to the impact of ocean heating on evaporation and atmospheric specific humidity. Similarly, we also find that ocean heat transport convergence controls the hemispheric precipitation sensitivity through the impact of ocean heating on surface evaporation. This relationship is also evident in CMIP6 models, where we find differences in hemispheric precipitation sensitivity to be related to the Atlantic Meridional Overturning Circulation (AMOC). Changes in the AMOC control hemispheric differences in upper ocean heat content, which then affect how the hydrologic cycle responds to CO2 forcing in each hemisphere. These results suggest that ocean dynamics impact the hemispheric climate response to CO2 forcing, particularly how much regional precipitation changes with warming. / Graduate CMIP6 Energy Transport Climate impacts Climate change Regional climate Earth System Models Community Earth System Model Radiative Feedbacks Cross-Equatorial Ocean Heat Transport Precipitation Sensitivity Hydrologic Sensitivity Hydrologic Cycle Intertropical Convergence Zone Hadley Cells Forcing Atmosphere-ocean interaction Coupled Climate Dynamics Hemispheric climate response Bjerknes compensation Energy Transport Partitioning
8	Partikelmodellierung der Strukturbildung akustischer Kavitationsblasen in Wechselwirkung mit dem Schalldruckfeld / Particle modeling of acoustic cavitation bubble structure formation and interaction with the acoustic pressure field Koch, Philipp 29 August 2006 (has links) No description available. 530 Physik Mathematics and Computer Science akustische Kavitation Strukturbildung akustische Lichtenbergfigur nichtlineare radiale Blasendynamik Keller-Miksis-Modell Oberflächenstabilität Gasdiffusion Partikelmodellierung primäre und sekundäre Bjerkneskraft Reibungskraft virtuelle Masse Ultraschallfeld Helmholtz-Gleichung stehende Schallfeldmode Resonanzverschiebung Blasen-Flüssigkeits-Gemisch van Wijngaarden-Wellengleichung Einzelblase Vielblasensysteme Hochgeschwindigkeitsaufnahme acoustic cavitation structure formation acoustic Lichtenberg figur nonlinear radial bubble oscillation Keller-Miksis model surface stability gas diffusion particel modeling primary and sekundary Bjerknes force drag force added mass ultrasonic field Helmholtz equation standing wave field resonance shift bubble liquid mixture van Wijngaarden wave equation single bubble multi bubble systems high-speed video observations 33.12 30.20 RHK 000: Ultraschall {Physik: Akustik} RHH 150: Schallausbreitung -reflexion in Flüssigkeiten {Physik: Akustik}

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