The surface tension of liquid metals is an important and scientifically interesting parameter which affects many metallurgical processes such as casting, welding and melt spinning. Conventional methods for measuring surface tension are difficult to use for molten metals above temperatures of 1000 K. Containerless methods are can be used to measure the surface tension of molten metals above 1000 K. Oscillating drop method is one such method where a levitated droplet is allowed to undergo damped oscillations. Using the Rayleigh’s theory for the oscillation of force-free inviscid spherical droplets, surface tension and viscosity of the sample can be calculated from oscillation frequency and damping respectively. In this thesis, a numerical model is developed in ANSYS Fluent to simulate the oscillations of the molten metal droplet. The Volume of Fluid approach is used for multiphase modelling. The effect of numerical schemes, mesh size, and initialization boundary conditions on the frequency of oscillation and the surface tension of the liquid are studied. The single-phase model predicts the surface tension of zirconium within a range of 13% when compared to the experimental data. The validated single phase model is extended to predict the interfacial tension of a core-shell structured compound drop. We study the effect of the core and shell orientation at the time of flow initialization. The numerical model we developed predicts the interfacial tension between copper and cobalt within the range of 6.5% when compared to the experimental data. The multiphase model fails to provide any conclusive data for interfacial tension between molten iron and slag.

A method is presented for calculating the shape sensitivity of a wing aeroelastic response with respect to changes in geometric shape. Yates’ modified strip method is used in conjunction with Giles' equivalent plate analysis to predict the flutter speed, frequency, and reduced frequency of the wing. Three methods are used to calculate the sensitivity of the eigenvalue. The first method is purely a finite difference calculation of the eigenvalue derivative directly from the solution of the flutter problem corresponding to the two different values of the shape parameters. The second method uses an analytic expression for the eigenvalue sensitivities of a general complex matrix, where the derivatives of the aerodynamic, mass, and stiffness matrices are computed using a finite difference approximation. The third method also uses an analytic expression for the eigenvalue sensitivities but the aerodynamic matrix is computed analytically. All three methods are found to be in good agreement with each other. The sensitivities of the eigenvalues were used to predict flutter speed, frequency , and reduced frequency. These approximations were found to be in good agreement with those obtained using a complete reanalysis. However, it is recommended that higher order terms be used in the calculations in order to assure greater accuracy. / Master of Science / incomplete_metadata

Stimulated Electromagnetic Emissions SEEs may provide important diagnostic information about space plasma composition, energetics, and dynamics during active experiments in which ground-based high powered radio waves are transmitted into the ionosphere. The nonlinear plasma processes producing this secondary radiation are not well understood particularly for some recent observations where the transmitter (pump) frequency is near the second harmonic of the electron gyro-frequency. New, more comprehensive, experimental observations of spectral features within 1kHz of the pump wave frequency are reported here in order to begin more careful comparisons of the experimental observations and a possible theoretical underpinning which is also provided. The experimental observations typically show two distinct types of secondary radiation spectra which are a) discrete narrowband harmonic spectral structures ordered by the ion gyro-frequency, so-called stimulated ion Bernstein Scatter (SIBS) and, b) broadband spectral structure with center frequency near 500 Hz and similar spectral bandwidth named ion acoustic parametric decay (IAPD). A theoretical model is provided that interprets these spectral features as resulting from parametric decay instabilities in which the pump field ultimately decays into high frequency upper hybrid/electron Bernstein and low frequency neutralized ion Bernstein and/or obliquely propagating ion acoustic waves at the upper hybrid interaction altitude. Detailed calculations of the threshold level, growth rate, unstable wavenumber and frequency bandwidth of the instabilities are provided for comparisons with experimental observations. An assessment of the effect of the critical instability parameters are provided including pump electric field strength, proximity of the pump frequency to the electron gyro-frequency and pump electric field geometry. The model shows quite reasonable agreement with the experimental observations. Next, a two dimensional Particle-In-Cell Monte-Carlo Collision computational model (PIC-MCC) is employed in order to consider nonlinear aspects such as 1) electron acceleration through wave-particle heating, 2) more complex nonlinear wave-wave processes and 3) temporal evolution of electron irregularities through nonlinear saturation. The simulation results show that the IB associated parametric decay is primarily associated with electron acceleration perpendicular to the geomagnetic field. More gyro-harmonic lines are typically associated with more intense heating. Heating is reduced when the pump frequency is sufficiently close to 2fce. The IA associated parametric decay instability is primarily associated with electron tail heating along the magnetic field and heating is reduced when the pump frequency is sufficiently close to 2fce. Characteristics of caviton collapse behavior become prevalent in this case. Results are discussed within the context of some recent experimental observations. Further discussions are provided of connections with past observed SEE spectral features and potential new diagnostic information provided by these newly categorized spectra. / Ph. D.

Heat transfer augmentation is of paramount importance in energy transfer and storage systems and the idea of using the inherent vibrations in a system to enhance heat transfer needs to be thoroughly researched upon. The current study numerically investigates an infinitesimally thin plate-fin undergoing forced oscillations over a range of amplitudes and frequencies in the presence of an approach flow. Reduced frequencies of 0.25 ≤ k ≤16 and plunge amplitudes of 0.03125 ≤ h ≤ 8 are investigated at Re=100 and Pr = 0.71. It is shown that the combined effect of frequency and amplitude on heat transfer enhancement can be accounted for as a single parameter “plunge velocity” (0.25 ≤ kh ≤ 4) instead of the individual frequency and amplitude values. For kh > 0.5 a significant increase in Nusselt number ( is observed compared to a stationary plate. With increasing kh or more vigorous oscillations, the increase in becomes more prominent and similar trends and comparable magnitudes were observed for a constant value. Unlike the hydrodynamic counterpart of the study, both Leading Edge Vortices (LEVs) and Trailing Edge Vortices (TEVs) are found to act positively to induce enhanced heat transfer on the plate. Finally, the dependence of heat transfer augmentation on the frequency and amplitude of vibration is quantified with a simple parameterization for a plate-fin in a fluid medium. / M.S. / Heat transfer enhancement is of paramount importance in energy transfer and storage systems. The idea of using the inherent mechanical vibrations in a heat producing system to enhance transfer of unwanted heat from that system needs to be thoroughly researched upon. To investigate this idea, we numerically study an infinitesimally thin plate-fin undergoing forced oscillations over a range of amplitudes and frequencies in the presence of an incoming air flow. It is shown that the combined effect of frequency and amplitude on heat transfer enhancement can be accounted for as a single parameter called “plunge velocity” instead of the individual frequency and amplitude values. For a significant plunge velocity, a significant increase in Nusselt number ( is observed compared to a stationary plate representing an increase in the extent of heat transferred. With more vigorous oscillations, the increase in becomes more prominent and similar trends and comparable magnitudes were observed for a constant value. Finally, the dependence of heat transfer augmentation on the frequency and amplitude of vibration is quantified with a simple parameterization for a plate-fin in a fluid medium.

<p>Neuronal current MRI (ncMRI) is a field of study to directly map electrical activity in the brain using MRI, which has many benefits over functional MRI. One potential ncMRI method, Lorentz effect imaging (LEI), has shown promise but needs a better theoretical understanding to improve its use.</p><p>We develop three computational models to simulate the LEI experiments of an electrolyte filled phantom subject to a current dipole based on: ion flow, particle drift, and electromagnetohydrodynamics (EMHD). With comparative experimental results, we use the EMHD model to better understand the Lorentz effect over a range of current strengths. We also quantify the LEI experimental images and assess ways to measure the underlying current strength, which would greatly benefit comparative brain mapping.</p><p>EMHD is a good predictor of LEI signal loss. We can measure the underlying current strength and polarity in the phantom using LEI images. We can also use trends from the EMHD model results to predict the required current density for signal detection in future LEI experiments. We can also infer the electric field strength, flow velocity, displacement, and pressure from the predicted current magnitude in an LEI experiment.</p><p>The EMHD model provides information that greatly improves the utility and understanding of LEI. Future study with our EMHD model should be performed using shorter dipole lengths, higher density and lower strength of current sources, and varying current source frequencies to understand LEI in the setting of mapping brain activity.</p> / Dissertation

O objetivo deste projeto de mestrado é estudar o escoamento bidimensional ao redor de um cilindro isolado oscilando forçadamente e apoiado em base elástica através de simulações computacionais utilizando o Método de Elementos Espectrais. Este tópico suscita grande interesse no meio tecnológico, pois esta configuração aparece com bastante freqüência em estruturas marítimas, como os \"risers\" de produção e umbilicais de plataformas de exploração de petróleo, e também no meio acadêmico, uma vez que se trata do estudo de fenômenos complexos originados do escoamento ao redor de uma geometria simples: cilindro circular. A pesquisa tem seu principal enfoque na avaliação da utilização do método espectral de elementos finitos para solução do escoamento bidimensional no limite do regime de esteira laminar, Re < ou = 200 , ao redor de um cilindro oscilando forçadamente e apoiado em base elástica. As simulações do escoamento ao redor de um cilindro oscilando forçadamente são realizadas com duas amplitudes de oscilação do cilindro: 0.15D e 0.40D . Para cada amplitude são utilizadas dez freqüências de oscilação: 0.8 s f , 0.85 s f , 0.9 s f , 0.95 s f , 0.975 s f , 0.9875 s f , 1.025 s f , 1.05 s f , 1.075 s f e 1.1 s f ; onde D é o diâmetro do cilindro e s f é a freqüência de desprendimento de vórtices para cilindro fixo. O objetivo destas simulações é a tentativa de observação do fenômeno da mudança brusca do ângulo de fase entre a força transversal à direção da corrente e o deslocamento do cilindro, \"phase-jump\", obtendo, deste modo, uma base para comparação com trabalhos já existentes sobre o mesmo tema, avaliando o método numérico utilizado para o caso do escoamento ao redor de um cilindro oscilando. As simulações do escoamento ao redor de um cilindro apoiado em base elástica (livre para oscilar) foram feitas com apenas um grau de liberdade: a direção transversal ao escoamento incidente (eixo y no caso estudado). O resultado mais importante deste tipo de configuração, do ponto de vista prático (projetos de \"risers\", e outras estruturas cilíndricas), é a curva max A / D versus r V , isto, devido à dependência da vida útil destas estruturas à máxima amplitude de oscilação. Para reproduzir a curva max A / D versus r V , foram escolhidos os seguintes valores de velocidade reduzida: 1.0, 2.0, 3.0, 3.5, 4.0, 4.5, 5.0, 6.0, 7.0, 8.0, 9.0, e 10.0. / The goal of this work is to study through numerical simulations using the Spectral Elements Method the two-dimensional the flow past a single circular cylinder that is either in simple harmonic cross-flow oscillation or elastically mounted. This is a very important topic for the technological environment because this configuration is identical to offshore structures, like risers of oil production platform and is also important to scientific environment due to the study of complex phenomena originated from the flow past a very simple geometry: a circular cylinder. The focus of this research is to evaluate the use of Spectral Element Method for solution of two-dimensional flow past a circular cylinder that is either in simple harmonic cross-flow oscillation or elastically mounted, at the laminar wake limit, Re < or = 200 . Two oscillation amplitudes were employed for the forced oscillations simulations: 0.15D e 0.40D . And, for each amplitude, were selected ten frequencies: 0.8 s f , 0.85 s f , 0.9 s f , 0.95 s f , 0.975 s f , 0.9875 s f , 1.025 s f , 1.05 s f , 1.075 s f e 1.1 s f ; where D is the cylinder diameter and s f is the vortex shedding frequency. The phase jump phenomenon, e.g., the fast change of phase angle, is observed and compared with existents works evaluating the Spectral Elements Method for forced oscillations cases. In the elastically mounted simulations the most important result, for the practical point of view (risers design), is the max A / D x r V curve, e.g., maximum amplitude versus velocity ratio, due to the relation of risers lifetime with maximum oscillation amplitude. To build the max A / D x r V curve were selected the follow values of velocity ratio: 1.0, 2.0, 3.0, 3.5, 4.0, 4.5, 5.0, 6.0, 7.0, 8.0, 9.0, e 10.0.

Estudos prévios têm indicado que o aumento da demanda de estabilidade manual induz maior estabilidade do equilíbrio corporal no controle das posturas quieta e perturbada. O presente estudo teve por objetivo avaliar o efeito da demanda de estabilidade imposta por uma tarefa manual sobre a regulação postural dinâmica em função da frequência de oscilação da base de suporte. Participaram desse estudo 20 estudantes universitários de ambos os sexos. A tarefa consistiu em manter um cilindro o mais estável possível sobre uma bandeja enquanto o equilíbrio em postura ereta era mantido em translações sinusoidais contínuas da plataforma móvel por 1 min., nas frequências de 0,4 Hz (baixa) ou 1 Hz (alta). A demanda de estabilidade manual foi manipulada por meio da disposição do cilindro. Na condição chamada de alta restrição (AR), o cilindro foi apoiado sobre sua face circular; na condição chamada de baixa restrição (BR), o cilindro foi apoiado sobre sua face plana. Os resultados mostraram que AR induziu menores amplitudes de oscilações de cabeça, centro de massa e bandeja na frequência de 0,4 Hz, porém não na frequência de 1 Hz. A análise das variáveis de coordenação revelou que na frequência de 0,4 Hz, AR levou a maiores valores de fase relativa entre as rotações cíclicas de tornozelo-quadril e tornozelo-ombro, como também na coordenação entre CM-plataforma e CP-plataforma. As análises mostraram que a condição de alta restrição também afetou o controle postural e manual na frequência de oscilação de 1 Hz, com aumento da amplitude de oscilação de CP e de rotações do ombro. Estes resultados sugerem que o controle postural dinâmico é regulado em diferentes parâmetros de maneira integrada e flexível, com interação recíproca entre os componentes da tarefa manual e postural, para atender à demanda de estabilidade imposta por uma tarefa posturomanual / Previous studies have indicated that increased steadiness constraints given by a manual task leads to gains of postural stability in quiet and perturbed standing. The aim of the present study was to evaluate the effects of manual task constraints on the regulation of dynamic postural control according to the frequency of sinusoidal translations of the base of support. Twenty university students performed a dual posturomanual task consisting of maintaining a cylinder on a handheld tray as stable as possible while maintaining the dynamic upright balance on a platform oscillating in sinusoidal translations at 0.4 Hz (low) or 1 Hz (high) frequencies during intervals of 1 min. The effects of manual task were studied through the manipulation of task constraint conditions set by positioning the cylinder supported on its round or flat side, corresponding respectively to high (HC) and low (LC) manual task constraints. Results showed that HC led to reduced oscillation amplitudes of the head, center of mass, and tray at 0.4 Hz, but not at 1 Hz. The coordination variables indicated that at 0.4 Hz, HC led to increased relative phase values between ankle-shoulder and hip-shoulder rotations, as also between center of mass-feet and center of pressure-feet oscillations in the slow oscillation frequency. Further analyses showed that the high manual task constraint also affected variables related to both postural and manual task components at 1 Hz, with increased center of pressure and shoulder rotation amplitudes of oscillation. These results suggest that dynamic postural control is regulated through different parameters in a flexible and integrated way in order to attend the stability demands imposed by a posturomanual task

Dans cette thèse, nous résolvons la conjecture de Fuglede sur le corps des nombres p-adiques, et étudions certaines propriétés aléatoires des suites liées à la conjecture de Sarnak, ainsi que leur propriétés oscillantes. Dans la première partie, nous prouvons d'abord la conjecture de Fuglede pour des ensembles ouverts compacts dans Q_p. Celle-ci indique qu'un ensemble ouvert compact dans Q_p est un ensemble spectral si et seulement s'il pave Q_p par translation. Il est également prouvé qu'un ensemble ouvert compact est un ensemble spectral (ou une tuile) si et seulement s'il est p-homogène. Nous caractérisons les ensembles spectraux dans Z / p^n Z ( p>1 premier, n>0 entier) par la propriété de pavage et aussi par leur homogénéité. Finalement, nous montrons la conjecture de Fuglede dans Q_p sans la restriction d'être ouvert compact en montrant que tout ensemble spectral ou toute tuile doivent être ouvert et compact à un ensemble de mesure nulle près. Dans la seconde partie, nous donnons d'abord plusieurs définitions équivalentes d'une suite oscillante en termes de disjonction de différents systèmes dynamiques sur des tores. Ensuite, nous définissons la propriété de Chowla et la propriété de Sarnak pour des suites numériques prenant des valeurs 0 ou des nombres complexes de module 1. Nous prouvons que la propriété de Chowla implique la propriété de Sarnak. Il est également prouvé que pour Lebesgue presque tout b> 1, la suite (e^{2 pi b^n})_{n in N} partage la propriété de Chowla et est par conséquent orthogonale à tout système dynamique topologique d'entropie nulle. Nous discutons également si les échantillons d'une suite aléatoire donnée ont presque sûrement la propriété de Chowla. Nous construisons certaines suites aléatoires dépendantes ayant presque sûrement la propriété de Chowla / In this thesis, we solve Fuglede's conjecture on the field of p-adic numbers, and study some randomness and the oscillating properties of sequences related to Sarnak's conjecture. In the first part, we first prove Fuglede's conjecture for compact open sets in the field Q_p which states that a compact open set in Q_p is a spectral set if and only if it tiles Q_p by translation. It is also proved that a compact open set is a spectral set (or a tile) if and only if it is p-homogeneous. We characterize spectral sets in Z/p^n Z (p>1 prime, n>0 integer) by tiling property and also by homogeneity. Finally, we prove Fuglede's conjecture in Q_p without the assumption of compact open sets and also show that the spectral sets (or tiles) are the sets which differ by null sets from compact open sets. In the second part, we first give several equivalent definitions of oscillating sequences in terms of their disjointness from different dynamical systems on tori. Then we define Chowla property and Sarnak property for numerical sequences taking values 0 or complex numbers of modulus 1. We prove that Chowla property implies Sarnak property. It is also proved that for Lebesgue almost every b>1, the sequence (e^{2 pi b^n})_{n in N} shares Chowla property and consequently is orthogonal to all topological dynamical systems of zero entropy. We also discuss whether the samples of a given random sequence have almost surely Chowla property. Some dependent random sequences having almost surely Chowla property are constructed

<p>We present an investigation of the magnetic field geometries and inhomogeneous distribution of chemical elements in the atmospheres of peculiar A and B stars. Our study combines high-quality spectroscopic and spectropolarimetric stellar observations with the development and application of novel techniques for theoretical interpretation of the shapes and variability of stellar line profiles. In particular, we extend the method of Doppler imaging to the analysis of spectra in the four Stokes parameters, making it possible to derive detailed and reliable stellar magnetic maps simultaneously with the imaging chemical inhomogeneities.</p><p>The magnetic Doppler imaging is applied to study of magnetic topologies and distributions of chemical elements in the peculiar stars α² CVn and 53 Cam. We found that the magnetic field geometry of 53 Cam is considerably more complex than a low-order multipolar topology, commonly assumed for magnetic A and B stars. Our Doppler imaging analysis also led to a discovery and study of spots of enhanced mercury abundance in the atmosphere of α And, a star where the presence of a global magnetic field is unlikely.</p><p>The ESO 3.6-m telescope is used to collect unique, very high spectral- and time-resolution observations of rapidly oscillating peculiar A (roAp) stars and to reveal line profile variations due to stellar pulsations. We present a detailed characterization of the spectroscopic pulsational behaviour and demonstrate a remarkable diversity of pulsations in different spectral lines. The outstanding variability of the lines of rare-earth elements is used to study propagation of pulsation waves through the stellar atmospheres and identify pulsation modes. This analysis led to a discovery of a non-axisymmetric character of pulsations in roAp stars.</p><p>Our study of chemical stratification in the atmosphere of the roAp star γ Equ provides a compelling evidence for significant variation of the chemical composition with depth. We find a combined effect of extreme chemical anomalies and a growth of pulsation amplitude in the outermost atmospheric layers to be the most likely origin of the high-amplitude pulsational variations of the lines of rare-earth elements.</p><p>Observations of cool magnetic CP stars are obtained with the ESO Very Large Telescope and are used for empirical investigation of the anomalies in the atmospheric temperature structure. We show that the core-wing anomaly of the hydrogen Balmer lines observed in some cool CP stars can be attributed to a hot layer at an intermediate atmospheric depth.</p>

<p>This thesis is based on work done in two different experimental projects. </p><p>The first project, the Tau RICH, is a previously proposed τ-neutrino appearance experiment for the CERN neutrino beam at the Gran Sasso laboratory in Italy. The proposed experimental concept is based on the use of focusing RICH detectors with liquid radiator (C₆F₁₄). Simulations made with a Geant4 code show that in the proposed experimental set-up, Cherenkov light from delta electrons will constitute a severe background that in practice would render the experiment unfeasible.</p><p>The second project, ATLAS, is a general purpose detector at the CERN 14 TeV proton-proton collider LHC which will start operation in 2007. To make the reconstruction and selection of the events in ATLAS more accurate, complete and up-to-date information on the interaction of the produced particles with the detector is needed. A service program code, the Material Integration Service (MIS), has been developed which makes use of the detector descriptions already available in a Geant4 code and which uses a novel algorithm, based on line integrals evaluated within small volume elements that build up the detector. This code is demonstrated to constitute a practically useful tool of satisfactory performance and accuracy.</p><p>The charged Higgs boson production in the gluon-bottom quark mode, gb → tH^±, followed by charged Higgs decays into a chargino and a neutralino, is studied for a specific choice of values for the Minimal Supersymmetric Standard Model (MSSM) parameters. It is shown, using a Monte Carlo code to simulate the ATLAS detector and the assumed MSSM physics model, that for an integrated luminosity of 300 fb^-1, in the intermediate region 4 < tanβ < 10 where H^± decays to SM particles cannot be used for H^± discovery, charged Higgs decays to non-SM particles can be used for Higgs discovery at the 5 σ significance level.</p>