Muitos componentes de máquinas, equipamentos e meios de transporte podem ser representados na forma de corpos rombudos cujos deslocamentos em meio fluido podem gerar ruído. Nesse contexto, a tese tem como proposta o estudo da previsão numérica do ruído gerado oriundo do escoamento em torno de corpos rombudos. Como exemplo representativo de um corpo rombudo, o som gerado do escoamento ao redor de um cilindro é estudado. O escoamento em torno de um cilindro é considerado com esteira tridimensional e turbulenta na condição do regime subcrítico, que é caracterizado pela separação da camada limite no regime laminar. O escoamento em torno de um cilindro é obtido através da simulação numérica não-estacionária considerando domínio computacional tridimensional. Para lidar com a turbulência e a tridimensionalidade, o escoamento é resolvido utilizando a metodologia de simulação de grandes escalas (LES). O domínio computacional é discretizado pelo método de volumes finitos. O ruído é calculado separadamente utilizando a analogia de Ffwocs Williams & Hawkings (FW-H), cuja equação de onda tem como termo fonte a solução do escoamento fornecida pela simulação numérica. Na analogia de FW-H, a flutuação de pressão acústica é obtida no campo afastado assumindo um meio quiescente entre a região das fontes sonoras (campo próximo) e o local considerado para o cálculo do ruído. Devido ao alto custo computacional da simulação tridimensional (3D), as simulações numéricas foram realizadas com comprimento de envergadura do cilindro limitado, possibilitando considerar parte dos efeitos tridimensionais da esteira. No cálculo final do ruído, os métodos de correção acústica de Kato et al. (1993) e Seo & Moon (2007) são empregados de forma a equiparar a geração sonora obtida com cilindro de menor comprimento de envergadura ao respectivo ruído obtido experimentalmente com cilindro de maior comprimento. Esta tese contribuiu com uma investigação numérica da metodologia de cálculo de ruído utilizando a analogia de Ffwocs Williams & Hawkings para um escoamento tridimensional em torno de um cilindro considerando número de Reynolds elevado de Re = 90.000 e Re = 22.000. Os resultados mostram que a metodologia é capaz de prever o som no campo afastado nos casos simulados, visto que os espectros sonoros obtidos concordam com os respectivos dados experimentais. Além disso, os métodos de correção acústica de Kato et al. (1993) e Seo & Moon (2007) mostraram ser adequados para o cálculo do ruído adicional, que considera um cilindro de comprimento maior, porém são dependentes da correta estimativa do comprimento de coerência do escoamento ao longo da envergadura do cilindro. Por fim, o espectro sonoro obtido no campo afastado é função do resultado fornecido pela simulação numérica do escoamento, principalmente quanto à coerência entre as flutuações de pressão ao longo do comprimento de envergadura do cilindro e à amplitude de flutuação de força na direção da sustentação exercida na parede do cilindro. / Many components of machines, equipments and means of transport can be represented as a bluff body whose motion through a fluid can generate noise. In this context, this thesis is focused on the study of numerical prediction of noise generated by the flow around bluff bodies. As an example of bluff body, the sound generated from flow around a circular cylinder is studied. The flow over a circular cylinder is investigated by considering the wake as tridimensional and turbulent in the subcritical regime, which is characterized by a laminar boundary layer separation. The flow over a circular cylinder is obtained by time-dependent numerical simulation considering three-dimensional computational domain. In order to cope with turbulence and three-dimensionality, the flow is solved using the Large Eddy Simulation (LES) methodology. The computational domain is discretized by the finite volume method. The noise is calculated separately using the Ffwocs Williams & Hawkings (FW-H) analogy, whose wave equation has as a source term the flow solution provided by the numerical simulation. With regard to the FW-H analogy, the acoustic pressure fluctuation is obtained in the far-field by assuming a quiescent medium between the sound sources region (near-field) and the location considered for acoustic computation. Due to the high computational cost of three-dimensional (3D) simulation, the numerical simulations were conducted with a cylinder span length limited in size, which allows taking into account part of the wake three-dimensionality. Regarding the final acoustic computation, the acoustic correction methods of Kato et al. (1993) and Seo & Moon (2007) are used in order to match the sound obtained by the short cylinder span to the correspondent sound obtained experimentally for a long cylinder span. This thesis contributed to investigate numerically the computational method of applying the Ffwocs Williams & Hawkings analogy for solving the noise generated from a threedimensional flow over a circular cylinder with high Reynolds number, particularly, at Re = 90,000 and Re = 22,000. The results show that this computational method is able to predict the far-field sound for the simulated cases, since the noise spectra obtained are found to be in agreement with the corresponding experimental data. In addition, the acoustic correction method of Kato et al. (1993) and Seo & Moon (2007) provided good predictions with regard to the adding noise computation, however, its results are dependent on accurate estimation of the spanwise coherence length of the flow. Finally, the sound spectrum obtained in the far-field is tied to the flow behavior provided by the numerical simulation, especially regarding the coherence between the pressure fluctuations over the spanwise length of the cylinder and the lift force fluctuation amplitude exerted on the cylinder wall.

Dissertação apresentada para obtenção do grau de Doutor em Engenharia Mecânica na Faculdade de Engenharia da Universidade do Porto, sob a orientação do Prof. Dr. César de Sá

The main contribution of the present work is the proposition of a framework for analysis of active suspensions using the Caenorhabditis elegans nematode as the living model. To do so, five different perspectives are used: kinematics, macro-reological, numerical, theoretical and micro-reological. First, a theoretical and experimental analysis of the kinematic motion of the nematodes suspended in a biological fluid is presented. Two different populations are examined: starving and well fed nematodes. We show that the relationship between the length of an individual nematode and the wavelength of its movement is linear and can be adjusted by a theoretical prediction proposed in this work. A deep discussion on propulsive mechanics based on a scale analysis that identifies three major forces acting on an individual nematode is made. In addition, we investigated the shear viscosity of Caenorhabditis elegans suspensions. The oscillatory shear experiments revealed an anomalous viscosity behavior with the variation of the volumetric fraction of suspension, ?. The effective viscosity of the suspension decreased with increasing nematode volumetric fraction at low concentrations. Based on the experimental data, a phenomenological equation for the effective viscosity of the suspension as a function of the volumetric fraction of particles is proposed. The collective behavior of the nematodes is also observed in linear regime through the difference of normal stresses. Finally, step strain tests are conducted to obtain the relaxation times. The presence of a negative active stress due to the nematoid driving behavior persists for a period of time, leading to a negative undershoot and an oscillatory behavior in the relaxation function. In order to propose a rheological model, simplifications are made in the model and immersed boundary method simulations are conducted in a flexible filament, varying the type of movement that it performs. It is observed that the presence of asymmetries in its undulating movement generates drastic changes on its kinematic responses. A rheological model as a function of filament orientation is proposed and validated with experimental data in linear regime. After validation of the proposed constitutive equation, the model is observed under the nonlinear regime of oscillatory shear, in which the rheological characterizations are made based on existing frameworks using Lissajous-Bowditch curves and Pipkin diagrams. Finally, a protocol for analysis of suspensions in a microrheometer is presented. Particles are added and tracked as unidirectional oscillatory shear (pulsatile flow) is applied. The velocity and shear rate profiles are obtained, as well as the rheological signals equivalent to the strain rate and stress. Signal analysis tools are used and an artificial intelligence system is proposed to remove the component added to the signal by unidirectional shear, aiming to reconstruct the signal with null temporal average and allowing the application of well known rheological theories, such as the decomposition of stresses in coefficients of Chebyshev, for the calculation of viscommetric quantities of compliances and fluidities. The major contribution of the study concerns the observation, characterization, modeling and simulation of a microsized animal that moves in different fashion, depending on the environment, and the surrounding fluid. The rheological properties analyzed, simuations performed and model proposed can be used for both production of artifitial microorganisms and control of living organisms. Moreover, this combination of analyses and techniques can be used to study any type of passive and active suspension providing new and conclusive results regarding the rheological characterization and the physical behavior of the particles. / A principal contribuição do presente trabalho é a proposição de um framework de análise de suspensões ativas utilizando como modelo vivo o nematoide Caenorhabditis elegans. Para tanto, cinco perspectivas diferentes são utilizadas: cinemática, macrorreológica, numérica, teórica e microrreológica. Primeiramente, uma análise teórica e experimental do movimento cinemático das partículas ativas suspensas em um fluido biológico é apresentada. Duas populações diferentes são examinadas: na ausência de alimento e com nematoides bem alimentados. Mostramos que a relação entre o comprimento de um nematoide individual e o comprimento de onda de seu movimento é linear e pode ser ajustada por uma previsão teórica proposta neste trabalho. Uma profunda discussão sobre a mecânica de propulsão com base em uma análise de escala que identifica três forças principais que atuam em um nematoide individual é feita. Além disso, investigamos a viscosidade de cisalhamento das suspensões de Caenorhabditis elegans. Os experimentos em cisalhamento oscilatório revelaram um comportamento anômalo da viscosidade com a variação da fração volumétrica de suspensão, ?. A viscosidade efetiva da suspensão diminuiu com o aumento da fração volumétrica do nematoide para pequenas concentrações. Baseando-se nos dados experimentais, uma equação fenomenológica para a viscosidade efetiva da suspensão em função da fração volumétrica de partículas é proposta. O comportamento coletivo dos nematoides é também observado, em regime linear, pela diferença de tensões normais. Finalmente, o teste de step strain é conduzido para obter os tempos de relaxação. A presença de uma tensão ativa negativa devido ao comportamento impulsor do nematoide persiste por um certo período, levando a um undershoot negativo e a um comportamento oscilatório na função de relaxação. A fim de propor um modelo reológico, simplificações são efetuadas no modelo e simulações usando o método de fronteira imersa são conduzidas em um filamento flexível, variando o tipo de movimento que este realiza. Observa-se que a presença de assimetrias em seu movimento ondulatório gera drásticas mudanças em suas respostas cinemáticas. Um modelo reológico em função da orientação do filamento é proposto e validado com os dados experimentais em regime linear. Após a validação da equação constitutiva proposta, o modelo é observado sob o regime não-linear do cisalhamento oscilatório, no qual as caracterizações reológicas são feitas com base nos frameworks existentes, utilizando curvas de Lissajous-Bowditch e diagramas de Pipkin. Por fim, é apresentado um protocolo de análise de suspensões em um microrreômetro. Partículas são adicionadas e rastreadas à medida que um cisalhamento unidirecional (escoamento pulsátil) é aplicado. Os perfis de velocidade e taxa de cisalhamento são obtidos, assim como os sinais reológicos equivalentes à taxa de deformação e tensão. Ferramentas de análise de sinais são utilizadas e um sistema de inteligência artificial é proposto para remoção da componente constante do sinal adicionada pelo cisalhamento unidirecional, visando reconstruir o sinal com média temporal nula e possibilitando a aplicação de teorias reológicas já conhecidas, como a decomposição de tensões em coeficientes de Chebyshev para o cálculo das quantidades viscométricas de conformidade e fluidez. A principal contribuição do estudo diz respeito à observação, caracterização, modelagem e simulação de um animal microscópico que se movimenta de maneira diferente dependendo do ambiente e do fluido circundante. As propriedades reológicas analisadas, as simulações realizadas e o modelo proposto podem ser utilizados tanto para a produção de microorganismos artificiais quanto para o controle de organismos vivos. Além disso, essa combinação de análises e técnicas pode ser usada para estudo de qualquer tipo de suspensão ativa e passiva, fornecendo resultados novos e conclusivos em relação à caracterização reológica e ao comportamento físico das partículas.

Wormlike micelles are viscoelastic fluids that present an intermediate behavior between solids and ordinary liquids since they are elastic at short time scales but flow easily at large time scales. In opposition to Newtonian fluids, which have constant viscosity, these fluids usually exhibit a non-Newtonian response with a rate-dependent shear viscosity. Wall-bounded oscillatory flows of Newtonian and complex fluids are found in many practical situations. Oscillatory pipe flows are especially important in physiology in connection with the circulatory and respiratory systems of human beings, as well as in industrial processes such as fluid pumping, secondary oil recovery or filtration, and in acoustics. Pulsating flows are of particular interest also in the rheological characterization of complex fluids. We analyze the laminar oscillatory flow of viscoelastic fluids using the Maxwell and Oldroyd-B models. We have shown that in wall-bounded oscillatory flows of viscoelastic fluids the two characteristic lengths of the Ferry waves, the damping length and wavelength, together with the characteristic separation of the walls, define all the flow properties for fluid models with a linear shear-stress equation in unidirectional flow. In wall-bounded settings there exists the possibility that shear waves generated at different locations superpose themselves before decaying so that the shear waves interfere, giving rise to a resonant flow at well defined frequencies of driving. The theoretical predictions obtained for the laminar velocity profiles are validated by carrying out time-resolved Particle Image Velocimetry (PIV) experiments in a vertical pipe at small driving amplitudes. The oscillatory pipe flow has been investigated in the whole range of experimentally accessible driving frequencies and amplitudes, and classified in three main flow regimes: laminar, vortical, and non-axisymmetric vortical. By ramping up and down the driving amplitude at constant frequency we have been able to characterize the transition from laminar to more complex flows, under controlled driving conditions. The first hydrodynamic instability occurs when the laminar base flow becomes unstable against the formation of axisymmetric toroidal vortices that appear distributed along the cylinder. The calculation of root-mean-square fluctuations in the vertical direction, of the vertical and radial components of the velocity (averaged in time or over the tube diameter) has allowed to determine the critical amplitude at which the instability sets in with high accuracy. In the vortical flow an abrupt increase of the fluctuations is observed, that accounts for the loss of the vertical translational symmetry and the formation of vortices in the flow. This transition exhibits hysteresis when the driving amplitude is ramped up and down, which makes us presume that the bifurcation from the laminar flow has a subcritical nature. A second hydrodynamic instability occurs when the vortical flow loses the axial symmetry. In this flow regime the vortices are heavily distorted and no longer axisymmetric. The velocity and vorticity maps of the vortical flow measured in a meridional plane of the tube appear periodic in time, on time scales comparable to the driving period. Interestingly, the vortex formation is favored in the acceleration phases of the piston oscillation. Besides, we have uncovered a spatio-temporal dynamics on long time scales (much larger than the relaxation time of the fluid) that substantially modifies the flow organization. This slow dynamics is more effective in the bottom half of the cylinder, specially next to the driving piston. A global inspection of the vortical flow along the tube length reveals that the instability takes place earlier in the bottom part of the tube, in the vicinity of the driving piston. At increasing the driving amplitude the boundary between laminar and vortical flow progressively raises towards the top regions. And above a critical driving amplitude the entire fluid flow is vortical. The mechanism triggering the hydrodynamic instability from the laminar to the axisymmetric vortical flow is not yet clear. / L'objectiu d'aquesta Tesi és estudiar el flux oscil•latori vertical en fluids micel•lars. Els fluids micel•lars són fluids complexos amb propietats viscoelàstiques, de manera que mostren un comportament intermedi entre els sòlids i els líquids: són elàstics a escales de temps curtes però flueixen a escales de temps més llargues. En contraposició als fluids Newtonians, que tenen una viscositat constant, els fluids complexos mostren un comportament no-Newtonià, amb una viscositat que depèn del ritme de deformació. El fluxos oscil•latoris de fluids Newtonians o complexos en geometries confinades són especialment importants en fisiologia, en relació amb el sistema circulatori i respiratori d'éssers humans, i també en processos industrials com el bombejat de fluids, l'extracció de petroli, i en particular són interessants en la caracterització reològica de fluids complexos. Primer estudiem el flux oscil•latori des d'una perspectiva teòrica i analitzem el flux laminar de fluids viscoelàstics utilitzant els models de Maxwell i Oldroyd-B en un tub vertical. Hem mostrat que en fluxos confinats existeix la possibilitat que les ones de cisalla generades a les diferents parets se sobreposin abans d'esmorteir-se i que eventualment donin lloc a un fenomen de ressonància. Les prediccions teòriques obtingudes pel flux laminar són validades duent a terme experiments de Velocimetria d'Imatges de Partícules (PIV) en un tub vertical, per amplituds petites del forçament oscil•latori. Quan s'incrementa l'amplitud de l'oscil•lació el flux laminar evoluciona cap a fluxos que presenten una dependència espai-temporal més complexa. Fent rampes d'amplitud creixent a una freqüència fixada hem pogut caracteritzar experimentalment la transició del flux laminar a aquests fluxos més complexos, sota condicions de forçament ben controlades. La primera inestabilitat apareix quan el flux laminar esdevé inestable amb la corresponent formació d'anells de vorticitat apilats al llarg del tub. Es manifesta una segona inestabilitat per amplituds del forçament més grans, per la qual el flux vortical perd la simetria axial. En aquest nou règim els vòrtex estan fortament distorsionats i no són axisimètrics. Fent rampes d’amplitud creixent i decreixent hem observat que aquestes dues transicions presenten histèresi, i que per tant són de caràcter subcrític.

Traditionally, the sea has been considered as the ultimate sink of most pollutions, due to its huge volume and assimilating capacities, and the coastal waters, because of their intermediate position between deep seas and human activities, has been acting as a buffer zone. However, during the past years, the interest in environmental issues in general, and in the conservation of the water quality in coastal regions, in particular, has steadily increased. For coastal waters, this growing interest can be related to the increase in population and industrial activity along rivers and on coastal areas, which poses an important threat on coastal ecosystems, and may limit the use of coastal waters for human activity. This problem is drastically increased during the tourist season, when a large number of tourists tend to favour coastal resorts. Although at different scales, both the discharge of wastewater from a marine outfall or the discharge of nutrient- or sediment-laden freshwater from a river play an important role in many physical, chemical and biological processes, which in turn may affect the human use (recreational, economical, etc), of coastal waters. It is important, therefore, to be able to model these processes in order to optimise the multiple uses of coastal waters, accommodating the economical interests to marine production and recreation. The present thesis describes the development, validation and application to real cases of a three-dimensional numerical model for substance (pollution) transport in coastal waters, which can be applied to a wide range of situations, as marine outfall discharges, river freshwater discharges or sediment transport. The three-dimensional advection-diffusion equation is solved using a random walk Lagrangian particle formulation, in which the substance to be transported is divided into discrete "packets" that are moved around due to the effects of the transport mechanisms relevant in coastal waters. Transport induced by currents, waves, and turbulent diffusion is included in the model calculations, together with other mechanisms such as sediment settling and resuspension, buoyancy effects due to density differences, and microbiological inactivation. Specific formulations have been developed to account for the transport due to the initial discharge velocity, for buoyant transport in the frame of particle models in waterbodies, for sediment resuspension, and to increase the efficiency of the model by uncoupling the nearfield modelling from that of the farfield. Concentration distributions are obtained at the end of the simulation by mapping the position of the resulting set of discrete particles onto the nodes of a grid, that may be deformable, employing different optional methods. This transport model has been validated using several test cases of known analytical solution, and has been applied to a series of real cases for which numerical results could be compared to observational evidence (such as experimental data, video recordings and satellite images). The comparison yields a good agreement between observational evidence and modelled data, and reveals that the model can be applied to a wide range of transport problems in various environments.

Several series of experiments in stratified and in rotating/stratified decaying flows after a grid is used to stir the two layer stable fluid brine and fresh water set up. We measure by comparing the gained potential energy with the available kinetic energy AKE, the relative efficiency of mixing. The experiments in stratified rotating flows with grid driven turbulence were both periodic (quasi stationary) and non-monotonic (decaying) forcing. This thesis compares experimental, numerical and field observations on the structure and Topology of the Stratified Rotating Flows as well as their decay, the horizontal spectra changes appreciable with slopes from 1.1 to 5, but vorticity and local circulation, and also the initial topology and forcing of the flow. A detailed study of the vorticity decay and vortex and energy structure has been performed, the new results show that neither stratified nor rotating flows exhibit pure 2D structures. The work parameterizes the role of the Richardson number and the Rossby number, both in the experiments and in the ocean visualizations is very important. The conditions of vortex decay show the effects of the internal waves in the decay turbulent conditions both for stratified and rotating flows. The parameter space (Re,Ri,Ro) has been used to interpret many previously disconnected explanations of the 2D-3D turbulent behaviour. The comparison of numerical simulations with experiments has allowed implementing new theoretical aspects of the interaction between waves and vortices finding the surprising and very interesting result that these interactions depend on the level of enstrophy. This also leads to new ways of using multifractal analysis ad intermittency in ocean environmental observations. A large collection of SAR images obtained from three European coastal areas were used for routine satellite analysis by SAR and other sensors, which seem very important to build seasonal databases of the dynamic conditions of ocean mixing. The topology of the basic flow is very important and in particular the topology of the vortices and their decay which depends on ambient factors such as wave activity, wind and currents. We find more realistic estimates of the spatial/temporal non-homogeneities (and intermittency obtained as spatial correlations of the turbulent dissipation); these values are used to parameterize the sea surface turbulence, as well as a laboratory experiments at a variety of scales. Using multi-fractal geometry as well, we can establish now a theoretical pattern for the turbulence behaviour that is reflected in the different descriptors. Vorticity evolution is smoother and different than that of scalar or tracer density. The correlation between the local Ri and the fractal dimension detected from energy or entropy is good. Using multi-fractal geometry we can also establish certain regions of higher local activity used to establish the geometry of the turbulence mixing that needs to be studied in detail when interpreting the complex balance between the direct 3D Kolmogorov type cascade and the Inverse 2D Kraichnan type cascade.

El objetivo principal de este trabajo es predecir la dispersión de un flujo bifásico en el lecho de un reactor de hidrodesulfuración. Se consideran propiedades físicas constantes, sin incluir reacciones químicas ni procesos de transferencia de calor ni materia. Dos herramientas de simulación diferentes se han utilizado en este estudio. Una de ellas es el software comercial Fluent, ampliamente utilizado en el campo de la Computación en Dinámica de Fluidos, en el que se han introducido nuevos modelos de arrastre y capilaridad no incluidos en la versión comercial del software. La segunda herramienta utilizada es el código doméstico Multiphase Flow Solver, escrito en lenguaje de programación Fortran, que ha sido desarrollado durante esta tesis doctoral, constituyendo un importante objetivo de la misma. Este código incorpora los modelos necesarios para describir los procesos físicos que describen el comportamiento de este tipo de flujos multifásicos y su dispersión en medios porosos. / The main objective of this work is to predict the two-phase flow dispersion in the bed of a hydrodesulfurization reactor. Physical properties are considered to remain constant, without including chemical reactions nor heat and mass transfer processes. Two different simulation tools have been used in this study. One of them is the commercial software Fluent, widely used in the field of Computational Fluid Dynamics, in which new drag and capillarity models not included in the commercial version of the software have been implemented. The second tool used is the domestic code Multiphase Flow Solver, written in Fortran programming language, which has been developed during this doctoral thesis, being one important goal of it. This code includes the models needed to describe the physical processes which describe the behavior of this type of multiphase flows and their dispersion in porous media.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Turbomáquinas são máquinas operacionais que transferem energia mecânica entre um rotor e um fluido. Estas máquinas têm muitas aplicações industriais. Um dos componentes de uma turbomáquina responsável pela transferência da energia, ou receber a rotação do eixo e transformar em energia de fluido em caso de bomba ou transferir a energia do fluido para o eixo em caso de uma turbina, é o impelidor ou rotor. O fenómeno da cavitação envolve escoamento bifásico: o líquido a ser bombeado e as bolhas de vapor que são formadas durante o processo de bombeamento. O processo de formação dessas bolhas é complexo, mas ocorre principalmente devido a presença de regiões de pressões muito baixas. O colapso dessas bolhas pode muitas vezes levar a deterioração do material, dependendo da intensidade ou da velocidade de colapso das bolhas. O principal objetivo deste trabalho foi estudar o comportamento hidrodinâmico do escoamento nos canais do impelidor de uma turbomáquina do tipo radial usando recursos de fluidodinâmica computacional (CFD). Uma abordagem Euler-Lagrange acoplada com o modelo da equação de Langevin foi empregada para estimar a trajetória das bolhas. Resultados das simulações mostram as particularidades de um escoamento líquido-bolha de vapor passando em um canal de geometria curva, fornecendo assim informações que podem nos ajudar na prevenção da cavitação nessas máquinas. / Turbomachines are operational machines that transfer mechanical energy between a rotor and a fluid. This type of machinery is employed in many industries. One of the main components of a turbomachine responsible for the energy transference, either receiving the rotation of the shaft and transforming it into fluid energy in the case of a pump or transferring energy from the fluid to the shaft in the case of a turbine, is the impeller or rotor. The cavitation phenomenon involves two-phase flow: the liquid to be pumped and the vapor bubbles which are formed during pumping. The formation process of these bubbles is complex, but occurs mainly due to the presence of regions of very low pressure. The collapse of the bubbles can often lead to a deterioration of the material, depending on the intensity or speed of bubbles collapse. The main objectives of this work was to study the hydrodynamic behavior of the flow in the impeller channels of a turbomachine (radial flow turbopump) using computational fluid dynamics (CFD resources). An Euler-Lagrange approach coupled with the Langevin equation model, was employed to estimate the bubbles tracking trajectory. Results of the simulations show the details of liquid-vapor bubble flow in a curved channel, providing insights that help us in the cavitation prevent of this machines.

Made available in DSpace on 2014-06-11T19:27:13Z (GMT). No. of bitstreams: 0 Previous issue date: 2013-08-23Bitstream added on 2014-06-13T20:35:21Z : No. of bitstreams: 1 barrios_ans_me_ilha.pdf: 1240045 bytes, checksum: a46aee2a6cd45b28e7f169c070d0d6bb (MD5) / A usinagem dos metais não é um processo completamente compreendido devido à sua natureza altamente não linear e ao complexo acoplamento entre deformação e o campo de temperatura. O processo de deformação é altamente concentrado em uma zona pequena e as temperaturas geradas nas zonas de deformação afetam ambas a ferramenta e a peça. Conhecendo o fluxo de calor gerado pelo processo de usinagem e, assim, o campo de temperatura, podem-se determinar as melhores condições de corte e também projetar novos produtos com melhor desempenho. Dada a grande importância do estudo térmico para a usinagem, muitas pesquisas continuam hoje sendo desenvolvidos. Este trabalho objetivou estudar a transferência de calor no processo de fresamento de topo do aço comercial VP100, empregado na fabricação de moldes e matrizes. Propôs-se, para isso, um modelo térmico tridimensional resolvido por um método numérico para estimar dois parâmetros: o fluxo líquido de calor e coeficiente de transferência de calor convectivo. Os parâmetros foram estimados através da implementação de um rotina computacional desenvolvida, que teve como dados de entrada as curvas de temperatura experimental do processo de usinagem para um fluido de corte sintético, semissintético e usinagem a seco, utilizando duas velocidades de corte: 200 e 450 m/min. Ademais, foram analisados o perfil de microdureza e a caracterização microestrutural dos corpos de prova, além do desgaste de flanco máximo das ferramentas após a usinagem. Constatou-se que o fluido de corte e a velocidade de corte influenciaram de forma significativa o resfriamento do corpo de prova. Além disso, a usinagem não afetou a microdureza abaixo da superfície fresada da peça, mas a microestrutura até 10 μm de profundidade deformou-se na direção do... / Machining of metals is a process not completely understood because of the highly nonlinear nature and the complex interaction between deformation and temperature field. The process of deformation is highly concentrated in a small zone and the temperatures generated in the deformation zones affect both tool and workpiece. Knowing the heat flux generated by the machining process and thus the temperature field, best conditions for cutting and also for design of new products with improved performance can be determined. Because of the great importance of the thermal effect for machining, several researches have been developed nowadays. This research aimed to study the heat transfer end milling process in commercial VP100 steel, used for producing molds and dies. Thus, a tridimensional thermal model solved by a numerical method was proposed to estimate two parameters: net heat flux and coefficient of convective heat transfer. The parameters were estimated by using computational programming where the input data were temperature curves of the end milling process when using synthetic and semi-synthetic cutting fluid, and dry machining at 200 and 450 m/min cutting speeds. In addition, microhardness and microstructure of workpiece as well as tool wear were analyzed after machining. Cutting fluid and cutting speed significantly influenced on workpiece cooling. Besides, milling process did not affect the part microhardness beneath machined surface, but microstructure up to 10 μm depth was deformed in feed direction of tool, which presented an initial maximum flank wear similar for all cutting conditions (VBBmax ≅ 0.1 mm). Synthetic and semi-synthetic cutting fluids did not presented different cooling capabilities, generating... (Complete abstract click electronic access below)

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / O presente trabalho propõe analisar metodologias para o cálculo do gradiente em malhas não-estruturadas do tipo Voronoi que são utilizadas no método de Volumes Finitos. Quatro metodologias para o cálculo do gradiente são testadas e comparadas com soluções analíticas. As técnicas utilizadas são: Método do Balanço de Forças, Método do Mínimo Resíduo Quadrático, Método da Média dos Gradientes Projetos e Método da Média dos Gradientes Projetados Corrigidos. Uma análise por série de Taylor também foi feita, e as equações analíticas comparadas com resultados numéricos. Os testes são realizados em malhas cartesianas e malhas triangulares, que em um trabalho anterior apresentaram alguns resultados inconsistentes. A influência do ponto gerador e do ângulo de rotação é analisada. É verificado que a posição do ponto gerador e a metodologia utilizada em cada malha influencia no cálculo do gradiente. Dependendo da malha e da metodologia utilizada, as equações analíticas indicaram que existem erros associados, que prejudicam o cálculo do gradiente. / Presente work propose examine methodologies for calculate gradient using a non-structured mesh of Voronois type, used on finite volume method. Four methodologies for calculate gradient are tested and compared with analytical solutions. The techniques used are: the Forces Balance Method, Minimum Quadratic Residue Method, Average Projected Gradient Method and the Revised Average Projected Method. An analysis using Taylor series was also made, and the analytical equations compared with numerical results. Tests are performed over Cartesian and triangular meshes, second one which in a previous work showed some inconsistent results. The influence of the gerator point displacement and rotation angle is analyzed. It was found that the position of the generator point and the methodology used influences gradient value. Accordingly to the mesh and the methodology used, analytical equations indicates that there are associated errors, which affect gradient value.