<div>The growing need to improve the power density of compact thermal systems necessitates developing new techniques to modulate the convective heat transfer efficiently. In the present research, acoustic streaming is evaluated as a potential technology to achieve this objective. Numerical simulations using the linearized and fully non-linear Navier-Stokes equations are employed to characterize the physics underlying this process. The linearized Navier-Stokes equations accurately replicate the low-frequency flow unsteadiness, which is used to find the optimal control parameters. Local and global stability analysis tools were developed to identify the modes with a global and positive heat transfer effect.</div><div><br></div><div>High-fidelity numerical simulations are performed to evaluate the effect of the excitation at selected frequencies, directed by the linear stability analysis, on the heat and momentum transport in the flow. Results indicate that, under favorable conditions, superimposing an acoustic wave, traveling along with the flow, can <i>resonate</i> within the domain and lead to a significant heat transfer enhancement with minimal skin friction losses. Two main flow configurations are considered; at the fixed Reynolds number Re_b=3000, in the supersonic case, 10.1% heat transfer enhancement is achieved by an 8.4% skin friction increase; however, in the subsonic case, 10% enhancement in heat transfer only caused a 5.3% increase to the skin friction. The deviation between these two quantities suggests a violation of the Reynolds analogy. This study is extended to include a larger Reynolds number, namely Re_b=6000 at M_b=0.75 and a similar response is observed. The effect of excitation amplitude and frequency on the resonance, limit-cycle oscillations, heat transfer, and skin friction are also investigated here.</div><div><br></div><div>Applying acoustic waves normal to the flow in the spanwise direction disrupts the near-wall turbulent structures that are primarily responsible for heat and momentum transport near the solid boundary. Direct numerical simulations were employed to investigate this technique in a supersonic channel flow at M_b=1.5 and Re_b=3000. The external excitation is applied through a periodic body force in the spanwise direction, mimicking loudspeakers placed on both walls that are operating with a 180^o phase shift. By keeping the product of forcing amplitude A_f and pulsation period (<i>T</i>) constant, spanwise velocity perturbations are generated with a similar amplitude at different frequencies. Under this condition, spanwise pulsations at <i>T</i>=20 and <i>T</i>=10 show up to 8% reduction in Nusselt number as well as the skin friction coefficient. Excitation at higher or lower frequencies fails to achieve such high level of modulations in heat and momentum transport processes near the walls.<br> <br>In configurations involving a spatially-developing boundary layer, a computational setup that includes laminar, transitional, and turbulent regions inside the domain is considered and the impact of acoustic excitation on this flow configuration has been characterized. Large-eddy simulations with dynamic Smagorinsky sub-grid scale modeling has been implemented, due to the excessive computational cost of DNS calculations at high-Reynolds numbers. The optimal excitation frequency that resembles the mode chosen for the fully-developed case has been identified via global stability analysis. Fully non-linear simulations of the spatially-developing boundary layer subjected to the excitation at this frequency reveal an interaction between the <i>pulsations</i> and the perturbations originated from the tripping which creates a re-laminarization zone traveling downstream. Such technique can locally enhance or reduce the heat transfer along the walls.<br></div>

Doctor of Philosophy / Department of Electrical and Computer Engineering / Shelli K. Starrett / Monitoring and control of modern power systems have become very complex tasks due to the interconnection of power grids. These large-scale power grids confront system operators with a huge set of system inputs and control parameters. This work develops and compares intelligent systems-based algorithms which may be considered by power system operators or planners to help manage, process, and evaluate large amounts of data due to varying conditions within the system. The methods can be used to provide assistance in making operational control and planning decisions for the system in a timely manner. The effectiveness of the proposed algorithms is tested and validated on four different power systems. First, Artificial Neural Network (ANN) models are developed and compared for two different voltage collapse indices and utilizing two different-sized sets of inputs. The ANNs monitor and evaluate the voltage profile of a system and generate intelligent conclusions regarding the status of the system from a voltage stability perspective. A feature reduction technique, based on the analysis of generated data, is used to decrease the number of inputs fed to the ANN, decreasing the number of physical quantities that need to be measured. The major contribution of this work is the development of four different algorithms to control the VAR resources in a system. Four different objectives were also considered in this part of the work, namely: minimization of the number of control changes needed, minimization of the system power losses, minimization of the system's voltage deviations, and consideration of the computational time required. Each of the algorithms is iterative in nature and is designed to take advantage of a method of decoupling the load flow Jacobian matrix to decrease the time needed per iteration. The methods use sensitivity information derived from the load flow Jacobian and augmented with equations relating the desired control and dependent variables. The heuristic-sensitivity based method is compared to two GA-based methods using two different objective functions. In addition, a FL algorithm is added to the heuristic-sensitivity algorithm and compared to a PS-based algorithm. The last part of this dissertation presents the use of one of the GA-based algorithms to identify the size of shunt capacitor necessary to enhance the voltage profile of a system. A method is presented for utilizing contingency cases with this algorithm to determine required capacitor size.

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[pt] O trabalho propõe uma metodologia para elaboração de modelos geológicos e geomecânicos (3D) e realização de análises de estabilidade (2D)de taludes rochosos de minas a céu aberto, com base nos dados da mina de Morro da Mina, fornecidos pela empresa VALE, a qual esta pertence. A metodologia está dividida em duas etapas: modelagem e análises de estabilidade. Para a modelagem, foi utilizado o software Petrel 2004, que oferece ferramentas geoestatísticas, possibilitando a extrapolação das informações geotécnicas pontuais c`, fi, RQD, Q e RMR, obtidas por meio de testemunhos de sondagem, para o maciço inteiro. Utilizou-se a técnica de Krigagem Ordinária. O modelo gerado representou bem a distribuição destas propriedades no espaço. Na etapa de análise de estabilidade, foram utilizadas seções resultantes da modelagem geomecânica. Dois tipos de análises foram realizados: análises cinemáticas, com utilização do software Dips, da Rocscience, e análises por equilíbrio limite dos taludes globais e das bancadas, utilizando-se o software Slide 5.0, também da Rocscience. No primeiro tipo, realizado com dois conjuntos de mapeamentos diferentes, foi constatado que as bancadas devem receber bastante atenção nesta mina, e no segundo tipo, foi verificada a segurança quanto à ruptura circular das bancadas e taludes globais, porém recentemente ocorreu uma ruptura em um dos locais analisados. Os programas RocData 4.0 e RocProp, ambos da Rocscience, foram utilizados para estimar os parâmetros de resistência de Mohr-Coulomb, e os softwares AutoCAD 2004 e Microsoft Office Excel auxiliaram na preparação dos arquivos de entrada no Petrel 2004 e no Slide 5.0. / [en] This work presents a methodology to develop geological and geomechanic models (3D) and to carry out stability analyses (2D) of rock slopes of open pit mine, based on data of Morro da Mina mine, provided by the mining company VALE. The methodology is divided in two stages: modelling and stability analyses. For the modelling, the software Petrel 2004, which allows the use of geostatistical tools, was used, being possible the spatial distribution of geotechnical information, obtained from borehole cores, for the whole rock mass. The technique of Ordinary Kriging was used. The modeled properties were the following: c`, fi, RQD, Q e RMR. The generated model represented well the spatial distribution of these properties. The stability analyses were carried out using 2D sections and the necessary rock mass parameters were obtained from the geomechanical model. Two types of analyses were carried out: kinematic analyses, with use of the software Dips, from Rocscience, and limit equilibrium analyses of the global slopes and the benches, where the software Slide 5.0, also from Rocscience was used. The Kinematic analyses, carried out considering two sets of joint orientations, suggested that the benches have to receive enough attention in this mine, and the limit equilibrium analyses for circular failure of the benches and global slopes indicated high factors of safety. However, before this work initiating a failure already had happened in one of the sections analyzed. The programs RocData 4.0 and RocProp, both from Rocscience, were used to estimate the Mohr-Coulomb strength parameters, and the programs AutoCAD 2004 and Microsoft Office Excel helped at the development of the input files in the Petrel 2004 and Slide 5.0.

Ce manuscrit présente une étude couplée, numérique et théorique, portant sur les écoulements tournants transitionnels et turbulents. L'accent y est mis sur la formation de structures macroscopiques cohérentes au sein de l'écoulement, générées par des procédés rendus fortement tri-dimensionnels par la présence des couches limites sur les disques et le long des parois cylindriques extérieure (carter) et/ou intérieure (moyeu). La complexité de ces écoulements pose de véritables difficultés en recherche fondamentale mais les résultats de ces travaux ont aussi une importance non négligeable pour les machines industrielles tournantes, depuis les disque-durs jusqu'aux turbopompes spatiales, la conception de ces dernières étant la motivation première pour ces travaux de thèse. Ce travail peut être divisé en deux sous-parties. Dans un premier temps, les cavités industrielles sont modélisées par de simples cavités rotor/stator lisses pour y étudier la dynamique de l'écoulement. Comme les campagnes expérimentales sur les machines industrielles ont révélé de dangereux phénomènes instationnaires en leur sein, l'accent est mis sur l'obtention et l'étude des fluctuations de pression dans les écoulements modèles. Ensuite, les SGE de trois configurations de turbine industrielle réelle sont réalisées pour étudier les fluctuations de pression in situ et appliquer les diagnostiques éprouvés sur les géométries modèles. / This thesis provides a numerical and theoretical investigation of transitional and turbulent enclosed rotating flows, with a focus on the formation of macroscopic coherent flow structures. The underlying processes are strongly threedimensional due to the presence of boundary layers on the discs and on the walls of the outer (resp. inner) cylindrical shroud (resp. shaft). The complexity of these flows poses a great challenge in fundamental research however the present work is also of importance for industrial rotating machinery, from hard-drives to space engines turbopumps - the design issues of the latter being behind the motivation for this thesis. The present work consists of two major investigations. First, industrial cavities are modeled by smooth rotor/stator cavities and therein the dominant flow dynamics is investigated. For the experimental campaigns on industrial machinery revealed dangerous unsteady phenomena within the cavities, the emphasis is put on the reproduction and monitoring of unsteady pressure fluctuations within the smooth cavities. Then, the LES of three configurations of real industrial turbines are conducted to study in situ the pressure fluctuations and apply the diagnostics already vetted on academic problems.

An overview of unsteady incompressible jet flows is presented, with the primary interest being radially developing jets in cylindrical polar coordinates. The radial free jet emanates from some orifice, being axisymmetric about the transverse (z) axis and possessing reflectional symmetry across its z=0 centreline. The radial wall jet is also axisymmetric about the transverse axis, however in this case impermeability and no-slip conditions are imposed at the wall, which is situated at z=0. The numerical solution of a linear perturbation superposed on the free jet, whose temporal form is assumed to be driven by a periodic source pulsation, gives rise to a wave-like disturbance whose amplitude grows downstream as its local wavelength decreases. An asymptotic analysis of this linear perturbation, which applies to the wall jet as well with some minor changes, captures the exact nature of the exponential spatial growth, and also algebraic attenuation of the growth. The linear theory is only valid for a small amplitude pulsation (|ε| << 1, where ε is the perturbation amplitude). When a nonlinear pulsation (ε = O(1)) is applied to the radial free jet, any linear theory must be dropped. Solving the full nonlinear system of equations reveals singular behaviour at a critical downstream location, which corresponds to the presence of an infinitely steep downstream gradient. The replacement of molecular diffusivity with a larger-scale eddy viscosity does little to affect the qualitative growth of the linear perturbation. In order for an experimental study to reproduce any of the discussed boundary-layer results, we must consider the behaviour of jet-type flows at finite Reynolds number. This involves solving the full Navier-Stokes equations numerically, to determine the Reynolds number at which we should expect to qualitatively recover boundary-layer behaviour. The steady solution for the radial free jet and its linear pulsation are studied in this way, as is the linear pulsatile planar free jet. We may enhance the streamwise velocity of a radial jet by applying swirl around the z axis. Modulating this swirl is looked at as a possible mechanism to induce the previously discussed pulsation, which then motivates the introduction of a finite spinning disk problem. In this case the system may be completely confined within an enclosed cylinder, making a hypothetical experimental approach somewhat more approachable.

O Estado de São Paulo possui uma extensa malha rodoviária que atravessa diferentes compartimentos geológicos e geomorfológicos. Os taludes presentes nestas rodovias podem apresentar diferentes processos de instabilização a depender destes condicionantes. Na busca do entendimento da deflagração destes processos em taludes de corte, a presente pesquisa é baseada no estudo da influência da forma de escoamento superficial, da geometria dos taludes e dos tipos de perfis de alteração dos solos. Para este estudo foi escolhida uma área de 13,6 km² localizada na Rodovia Luís Augusto de Oliveira (SP-215), entre os quilômetros 170 e 183, nos municípios de São Carlos, Ibaté e Ribeirão Bonito em SP. A pesquisa foi realizada em duas escalas, de semi-detalhe e de detalhe. Na primeira foram definidas quatro classes de potencial de instabilização para os setores no entorno dos taludes utilizando como base os condicionantes críticos cartografados nos mapas temáticos (declividades maiores que 15%, perfil de encosta côncavo, direção de escoamento superficial coincidente com o mergulho do talude e de uso e ocupação de cultura), além da altura dos setores. A definição do potencial de instabilização foi realizada considerando os condicionantes de forma isolada e de forma conjunta (fazendo uso do somatório ponderado). Os potenciais de instabilização definidos pelos dois métodos foram validados em relação a disposição das cicatrizes de escorregamento em campo. Os potenciais de instabilização alto e muito alto, definidas a partir dos condicionantes críticos considerados de forma conjunta, coincidiram com os locais com escorregamento. Na segunda escala de análise foi feita análise da estabilidade de seções geológico-geotécnicas dos taludes de corte, utilizando do método determinístico de Bishop Simplificado. Uma segunda validação do potencial de instabilização foi feito comparando as classes com os valores de fatores de segurança (FS) obtidos das análises de estabilidade (feitas com os parâmetros de resistência dos solos da retroanálise e os mínimos obtidos nas referências). Os FS mostraram-se compatíveis com as classes definidas, principalmente os FS obtidos das análises de estabilidade feitas com os parâmetros de resistência obtidas nas retroanálises. A metodologia de investigação proposta de potencial de instabilização apresentou resultados relevantes no estudo de estabilidade de taludes de corte em condições de relevo suave, como ocorre na área de estudo. / São Paulo State has an extensive road network that crosses different geological and geomorphological compartments. The slopes present in these highways can have different instability processes depending on their conditions. In the search for better understanding the deflagration processes in cutting slopes, the present research is based on the study of the influence of the surface runoff, the slope geometry, and the soil alteration profile. For this study, an area of 13.6 km² was selected, which is located on the Luís Augusto de Oliveira Highway (SP-215), between the kilometers 170 and 183, in São Carlos, Ibaté and Ribeirão Bonito cities in SP. The research was carried out in two scales (semi-detail and detail). In the semi-detail scale, four classes of instability potential were defined for the sectors around the slopes. The index was defined using as a reference the critical conditioners outlined in the thematic maps (such as slope bigger than 15%, concave slope profile, runoff direction coincident with slope dip and land use and occupation), as well as the height of the sectors. The definition of the instability potential was performed considering the conditioners by themselves and in a joint manner (using the weighted summation). The instability potentials defined by the two methods were validated in relation to the landslides scars disposition in the field. The high and very high instability potentials classes, which were defined considering the conditioners together, coincided with the landslides scars. The detail scale of analysis corresponded to stability analysis of geological-geotechnical sections of cut slopes using the Simplified Bishop\'s deterministic method. A second validation of the instability potential was made comparing the classes with the safety factor (FS) values obtained in the stability analyzes (made with the soil resistance parameters from retroanalysis and with the minimums parameters obtained in the references). The FS were compatible with the defined classes, mainly the FS from the stability analysis made with the resistance parameters obtained in the retroanalysis. The proposed research methodology of instability potential presented relevant results in the study of stability of cut slopes under conditions of gentle relief, as occurs in the study area.

In this thesis we modify an existing model for the spread of resistant bacteria in a hospital. The existing model does not account for some of the trends seen in the data found in literature. The new model takes some of these trends into account. For the new model, we examine issues relating to identifiability, sensitivity analysis, parameter estimation, uncertainty analysis, and equilibrium stability.

Philosophiae Doctor - PhD / Numerical approximations of multiscale problems of important applications in ecology are investigated. One of the class of models considered in this work are singularly perturbed (slow-fast) predator-prey systems which are characterized by the presence of a very small positive parameter representing the separation of time-scales between the fast and slow dynamics. Solution of such problems involve multiple scale phenomenon characterized by repeated switching of slow and fast motions, referred to as relaxationoscillations, which are typically challenging to approximate numerically. Granted with a priori knowledge, various time-stepping methods are developed within the framework of partitioning the full problem into fast and slow components, and then numerically treating each component differently according to their time-scales. Nonlinearities that arise as a result of the application of the implicit parts of such schemes are treated by using iterative algorithms, which are known for their superlinear convergence, such as the Jacobian-Free Newton-Krylov (JFNK) and the Anderson’s Acceleration (AA) fixed point methods.

Cette thèse a pour but l’étude de la rotation de l’écoulement des grandes échelles dans deux configurations. La première configuration se concentre sur l’effet de la rotation de l’écoulement sur une flamme laminaire. Elle est stabilisée dans le sillage d’un cylindre. La rotation de l’écoulement est introduite en faisant tourner le cylindre autour de son axe. La simulation numérique directe (Direct Numerical Simulation (DNS)) montre que la rotation du cylindre rompt la symétrie des deux branches de la flamme. La fonction de transfert de flamme (Flame Transfer Function (FTF)), obtenue grâce à l’inversion de Wiener-Hopf, indique qu’un faible taux de rotation réduit le gain de la FTF et donc la flamme devient presque insensible aux perturbations acoustiques à une fréquence donnée. De plus, il est démontré que cette diminution du gain est due à une interférence destructive des fluctuations de chaleur produites par les deux branches de la flamme. La fréquence à laquelle le gain de la FTF devient presque nul est ajustable par la vitesse de rotation du cylindre. Cette étude suggère que le contrôle de la symétrie de la flamme pourrait être un outil de contrôle en boucle ouverte des instabilités thermoacoustiques. Dans le cas de la deuxième configuration, la rotation de l’écoulement est induite par une instabilité hydrodynamique, aussi nommée Precessing Vortex Core (PVC) dans un système d’injection de carburant industriel. Des expériences et des simulations aux grandes échelles (Large Eddy Simulation (LES)) montrent que l’écoulement non-réactif dans l’injecteur pri- maire peut être décomposé en une contribution moyenne et un PVC. Cette instabilité hydro- dynamique est étudiée par l’analyse de stabilité linéaire (Linear Stability Analysis (LSA)) en utilisant deux approches différentes (locale et BiGlobale). Les résultats de l’expérience, de la LES et de la LSA démontrent que le montage d’une tige centrale à l’intérieur de l’injecteur stabilise le PVC. De plus, le même injecteur industriel est étudié dans le cas d’un écoule- ment réactif par LES. Les résultats démontrent que la flamme stabilise le PVC. L’analyse de stabilité BiGlobal montre que le gradient de densité dans le front moyen de la flamme a un effet important sur l’amortissement du PVC. Enfin, l’impact de la tige centrale est également étudié pour le cas réactif. La tige centrale impacte marginalement la forme globale de la flamme, mais a un effet positif sur l’accrochage de la flamme dans la zone de combustion pauvre. En comparant deux cas par LSA, celui avec la tige et celui sans la tige, les résultats suggèrent que la tige augmente considérablement l’amortissement du PVC. Cela pourrait causer une diminution de la turbulence dans l’écoulement et empêcher l’extinction de la flamme et donc expliquer l’influence bénéfique de la tige sur la stabilisation de la flamme. / This thesis investigates large scale flow rotation in two configurations. In the first, the effect of flow rotation on a laminar flame is investigated. The flame is anchored in the wake of a cylindrical bluff body. The flow rotation is introduced by turning the cylinder along its axis. It is shown by Direct Numerical Simulation (DNS), that the cylinder rotation breaks the symmetry of both flame branches. Flame Transfer Function (FTF) measurements performed by the Wiener-Hopf Inversion suggest, that low rotation rates lead to deep gaps in the gain and the flame becomes almost insensitive to acoustic perturbation at a specific frequency. It furthermore is demonstrated that this decrease in gain of the FTF is due to destructive interference of the heat release signals caused by the two flame branches. The frequency at which the gain becomes almost zero can be adjusted by tuning the cylinder rotation rate. The study suggests that controlling the symmetry of the flame could be a tool of open-loop control of thermoacoustic instabilities.