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The streaming plasma in the solar wind is a never ending source of energy, plasma, and momentum for planetary magnetospheres, and it continuously drives large-scale plasma convection systems in our magnetosphere and over our polar ionosphere. This coupling between the solar wind and the magnetosphere is primarily explained by two different processes: magnetic reconnection at high latitudes, which interconnects the interplanetary magnetic field (IMF) with the planetary dipole field, and low-latitude dynamos such as viscous interaction, where the streaming plasma in the solar wind may trigger waves and instabilities at the flanks of the magnetosphere, and thereby allow solar wind plasma to enter into the system.This work aims to further determine the nature and properties of these driving dynamos, both by statistical studies of their relative importance for ionospheric convection at Earth, and by assessment and analysis of the Kelvin-Helmholtz instability at Mercury, utilizing data from the MESSENGER spacecraft's first and third flyby of the planet.It is shown that the presence of the low-latitude dynamos is primarily dependent on the IMF direction: the driving is close to non-existent when the IMF is southward, but increases to the order of a third of the total ionospheric driving when the IMF turns northward (here, the magnitude of the driving is also shown to be dependent on the viscous parameters in the solar wind). The work also discusses the saturation of the reconnection generated potential, and shows that the terrestrial response follows a non-linear behavior for strong solar wind driving both when the IMF is southward and northward.Comparative studies of different magnetospheres provide an excellent path for increasing our understanding of space-related phenomena. Here, study of the Kelvin-Helmholtz instability at Mercury allows us to investigate how the different parameters of the system affect the mass, energy, and momentum transfer at the flanks of the magnetosphere. The large ion gyro radius expected is shown to develop a dawn-dusk asymmetry in the growth rates, with the dawn side as the more unstable of the two. This effect should be particularly visible when the planet is close to perihelion. Mercury's smaller scale size combined with the relatively high spacecraft velocity is also shown to provide excellent opportunities for studying the spatial structure of the waves, and a vortex reconstruction that can explain all the large-scale variations in the Kelvin-Helmholtz waves observed during MESSENGER's third Mercury flyby is presented. / QC 20110405

The effect of initial conditions in combined shear- and buoyancy- driven mixing was investigated through the use of an implicit large eddy simulation code under active development at Los Alamos National Laboratory and Texas A&M University. Alterations were done over several months both at Los Alamos National Laboratory and at the Texas A&M University campus, and include a transition from tilted rig to convective channel arrangement, introduction of an inertial reference frame, alteration of boundary conditions, etc. This work resulted in the development of a numerical framework with the capability to model various shear and Atwood number arrangements such as those seen in an inertial confinement fusion environment. In order to validate the code, it was compared to three published experiments, one with Atwood number 0.46 (White et al. 2010), one with high Atwood number 0.6 (Banerjee et al. 2010), and one with very low Atwood number 0.032 (Akula et al. 2012). Upon validating the code, pure Rayleigh-Taylor and pure Kelvin-Helmholtz instabilities were modeled along with five intermediate cases of increasing shear and constant density gradient. Plots of mixing width, Richardson number, growth parameter, and molecular mixing were compared in order to determine at what level of shear the minimum amount of mixing occurs. The results of height gradient and Reynolds number were to previous experiments and theory. The least amount of molecular mixing at the centerline was found to be when the system had a low Atwood number (0.032) and a multimode initial interface perturbation. While the increase in modes of the interface perturbation did not result in a significant change in the growth parameter, the level of molecular mixing at the centerline substantially decreased. As shear was increased in the system, the mixing width and molecular mixing subsequently increased. For this reason, the shear in the system should be eliminated, or at least minimized, if at possible so as to prevent any additional amalgamation in the system. Analysis of the Reynolds number revealed that with an increase in velocity difference between the fluid layers, the value consequently increased. This trend matches with theoretical results as the value is a function of the mixing width and velocity, thus further validating the code. Analysis of the transitional Richardson number revealed that it had a smaller value in the computational case over the experiment, but this fact can be attributed the difference in mixing width between the two methods. The development of the numerical framework with the capability to model various shear and Atwood number arrangements offers the platform for future study of hydrodynamic instabilities.

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Ferrofluids are comprised of subdomain particles of magnetite or iron oxide material that can become magnetized in the presence of a magnetic field. These unique liquids are being incorporated into many new applications due to the ability to control them at a distance using magnetic fields. However, although our understanding of the dynamics of ferrofluids has evolved, many aspects of ferrohydrodynamics remain largely unexplored, especially experimentally. This study is the first to characterize the stability and internal dynamics of accumulating or dispersing ferrofluid aggregates spanning the stable, low Reynolds number behavior to unstable, higher Reynolds numbers. The dynamics of ferrofluid aggregates are governed by the interaction between the bulk flow shear stresses acting to wash away the aggregate and magnetic body forces acting to retain them at the magnet location. This interaction results in different aggregate dynamics, including the stretching and coagulation of the aggregate to Kelvin-Helmholtz shedding from the aggregate interface as identified by focused shadowgraphs. Using TRDPIV, the first time-resolved flow field measurements conducted in ferrofluids reveal the presence of a three-stage process by which the ferrofluid interacts with a pulsatile bulk flow. An expanded parametric study of the effect of Reynolds number, magnetic field strength, and flow unsteadiness reveals that the increased field results can result in the lifting and wash away of the aggregate by means of vortex strengthening. In pulsatile flow, different forms of the three-stage interaction occur based on magnetic field, flow rate, and Reynolds number. / Ph. D.

[pt] A estabilidade do escoamento estratificado de ar e água, sujeito a perturbações periódicas no nível de líquido, é investigada numericamente em um duto horizontal. Selecionou-se o Modelo de Dois Fluidos unidimensional para a simulação do escoamento. As equações de conservação de massa e de quantidade de movimento linear para as fases gás e líquido são discretizadas de acordo com o método dos Volumes Finitos. O acoplamento entre as equações é resolvido sequencialmente com uma versão modificada do método PRIME. Perturbações no nível de líquido foram introduzidas de maneira controlada na entrada da tubulação. A evolução dessas perturbações, ao longo da tubulação, é analisada e os resultados são comparados com as previsões fornecidas por um modelo baseado na teoria linear de Kelvin-Helmholtz. A velocidade de propagação, a frequência e o número de onda das perturbações apresentam excelente concordância entre a simulação e modelo, indicando que, de fato, ambas abordagens são capazes de prever características fundamentais dessas ondas. As taxas de crescimento previstas pelo modelo e as obtidas na simulação, também, foram comparadas apresentando razoável concordância. Os resultados mostram que a frequência da perturbação tem influência na taxa de amplificação e que ondas com frequências mais altas tendem a serem mais instáveis. Para tubulações longas, efeitos não lineares podem ser observados em regiões afastadas da entrada da tubulação. Nesse estágio é possível observar alterações no mecanismo de crescimento das perturbações. / [en] The stability of stratified air-water flow under the influence of periodic disturbances in the liquid level is investigated numerically for a horizontal pipe. One-dimensional two-fluid model is used for flow simulation. Conservation equations for mass and linear momentum are discretized for both phases using a finite volume method. Coupling between equations is achieved by sequentially solving a modified version of PRIME method. Controlled disturbances are introduced in the flow by oscillating the liquid level at the pipe inlet. The evolution of the generated disturbances along the pipe is analyzed and the results are compared with predictions given by a model based on linear theory of Kelvin-Helmholtz (KH). An excellent agreement is obtained for velocity, frequency and wave number of the perturbations. This indicates that both approaches are capable to predict the fundamental characteristics of the waves. Amplifications rates predicted by simulation and model have been also compared and the results show a reasonable agreement. It is found that the frequency of the perturbations plays a role in the amplification rate. For increasingly higher frequencies the disturbances tends to be more unstable. The analysis is extended for long pipes, in such cases the growth rates changes at locations far from the inlet. It is conjectured that non linear mechanisms are related to observations.

This study, explores the use of an iterative solution approach for the linearized Euler equations formulated in the frequency domain for fan tone noise propagation and radiation through bypass jets. The aim is to be able to simulate high frequency propagation and radiation phenomena with this code, without excessive computational resources. All computations are performed in parallel using MPI library routines on a computer cluster. The linearized Euler equations support the Kelvin-Helmholtz type convective physical instabilities in jet shear flows. If these equations are solved directly in frequency domain, the unstable modes may be filtered out for the frequencies of interest. However, direct solutions are memory intensive and the reachable frequency is limited. Results provided shown that iterative solution of LEE is more efficient when considered memory requirement and might solve a wider scope of frequencies, if the instabilities are controlled.

Ce travail expérimental analyse le mélange de deux fluides miscibles associé à un écoulement induit par gravité dans la géométrie confinée d'un tube incliné. L'étude de la vitesse du front en fonction des paramètres de contrôle (contraste de densité entre les fluides \Delta\rho/\rho <

L'interaction entre le vent solaire (VS) et la magnétosphère (MSP) terrestre se fait par l'intermédiaire de la magnétopause (MP). Le VS éjecté du Soleil, voyage transportant avec lui le champ magnétique interplanétaire (CMI). Ce dernier interagit avec le champ géomagnétique provoquant le phénomène de reconnexion magnétique (RM). La RM permet l'entrée d'une grande quantité de particules du VS dans la MSP. Si le CMI est dirigé vers le nord, la RM peut avoir lieu à haute latitude, mais n'est pas assez efficace pour justifier la quantité de plasma typique du VS, observée par les satellites à l'intérieur de la MSP. En outre, dans les cas où le CMI est dirigé vers le nord, la formation d'une couche de mélange est observée à basse latitude. Les tourbillons de Kelvin-Hemholtz (KH) fournissent un mécanisme efficace pour la formation d'une couche de mélange à la MP. Les simulations numériques montrent que l'évolution temporelle de l'instabilité de KH dépend fortement des profils initiales à grande échelle. La comparaison des données spatiales et des simulations numériques est donc d'une importance fondamentale dans ce contexte. Les principaux résultats obtenus au cours de ce travail sont la caractérisation de la turbulence à l'intérieur des tourbillons de KH, ainsi que des événements de RM à petite échelle; la sélection d'un événement où nous avons une combinaison des données des satellites avant et après KHI se développe; l'observation d'un décalage entre les profils de densité et de vitesse et constat que ce décalage initial entraîne une évolution différente de la simulations numériques qui est en accord avec les observations satellites. / Solar Wind (SW) and the Earth's magnetosphere interaction is mediated by the magnetopause. The SW carries with it the Interplanetary Magnetic Field (IMF) which interacts with northwards geomagnetic field lines causing magnetic reconnection (MR) events that make SW particles to be tranferred into the Earth's magnetosphere. If the IMF is directed northward, MR takes place at high latitude, but it is not efficient enough to justify the amount of SW plasma observed by satellites inside the magnetosphere. During northwards conditions one observe the formation of a wide boundary layer (BL) at the low latitude. This BL is thought to be driven by the the Kelvin-Helmholtz instability (KHI) , originating from the velocity shear between SW and the almost static near-Earth plasma. Numerical simulations (NS) have shown that the long time evolution of the KHI depends strongly on the initial large scale field profiles used as initial conditions. In order to make a further step towards the comprehension of this complex system, it is imperative to combine satellite data and NS. The idea here is to initialize NS by using in-situ observations of the main field profiles since only a correct initialization can reproduce the correct dynamics. The main results achieved in this work are: characterize the turbulence inside KH vortices and the small scale MR; select one event where there is a combination of a satellite measurements before and after KH develops, find that density and velocity profiles are shifted by a distance comparable to their shear lengths and that this initial shift cause a different evolution of the KHI leading to a final state in agreement with satellites observations.

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