Orientador: Geraldo de Freitas Maciel / Resumo: Esta pesquisa tem como principal objetivo verificar experimentalmente a presença de instabilidades na superfície livre de escoamentos lamosos, em canal com 10 metros de comprimento. Estes escoamentos, quando em condições favoráveis de vazão e inclinação, podem constituir um domínio favorável à geração de roll waves, instabilidades que evoluem adquirindo característica periódica e que se propagam com amplitudes, comprimentos e velocidade de propagação bem definidas. O fluido teste utilizado nos ensaios foi uma mistura de água+argila caulinítica (representativo do material lamoso em desastres naturais), confeccionada com uma concentração em volume de 25% e caracterizada reologicamente pelo modelo de Herschel Bulkley. Foram testados dois surfactantes (estabilizantes químicos), Tween 80 e goma xantana, para estabilizar os parâmetros reológicos da mistura, tendo a goma xantana garantido maior tempo de estabilização. A aferição da superfície livre foi realizada por meio de sistema não-intrusivo (transdutor ultrassônico), que se mostrou eficiente para aferição de ondas íngremes e de pequenas amplitudes. Os resultados de ensaios experimentais exploratórios foram confrontados com o modelo matemático 1D desenvolvido pelo grupo de pesquisa. / Abstract: This project has as main objective to experimentally verify the presence of instabilities in the free surface of muddy flows in a ten meter long channel. These flows, when situated under favorable flow and slope conditions may constitute a favorable domain for the generation of roll waves, instabilities that evolve acquiring periodic characteristics and propagate itself with well-defined amplitudes, lengths and velocity. The test fluid used in the experiment was a mixture between water + kaolinitic clay (representative of the muddy material in natural disasters), made with a 25% volume concentration and rheologically characterized by the Herschel Bulkley model. Two surfactants (chemical stabilizers), Tween 80 and xanthan gum, were tested to stabilize the rheological parameters of the mixture, with the xanthan gum ensuring longer stabilization time. The free surface measurement was performed using a non-intrusive system (ultrasonic transducer), which proved to be efficient for the measurement of steep waves with small amplitudes. The results of exploratory experimental tests were compared with the 1D mathematical model developed by the research group. / Mestre

Recent years have seen a rising interest in the use of computational fluid dynamics for investigating the hemodynamics in brain aneurysms. Hemodynamic parameters, such as wall shear stress or oscillatory shear index, are among the indicators used to detect higher risk of aneurysm rupture. This thesis describes the blood flow by generalized Navier-Stokes equations with the Navier slip boundary condition imposed on the impermeable rigid vessel wall, which is implemented by way of Nitsche's method. The influence of constitutive models and the Navier slip wall boundary conditions on the resulting blood flow and hemodynamic parameters is investigated. Five different non-Newtonian constitutive models are first compared on a 2D rectangular domain. The Carreau-Yasuda, modified Casson, and Quemada models are then employed to investigate the differences in one patient-specific aneurysm geometry obtained by CT scan. All non-Newtonian models are compared with the Newtonian model. Both steady and pulsatile simulations are performed to obtain the velocity field and hemodynamic quantities, such as wall shear stress, time-averaged wall shear stress, oscillatory shear index, and oscillatory velocity index. The results for this particular case showed that non-Newtonian models do not have a significant impact on the computed...

Une configuration en T-jonction a été étudiée pour sa simplicité en géométrie en comparant avec d’autres mélangeurs en industrie. Plus particulièrement, j’ai effectué des simulations numériques directes avec OpenFOAM des T-jonction convergent à section circulaire et rectangulaire. Les fluides Newtonien et non-Newtonien (modèle Bird-Carreau) ont été pris en compte. Dans un premier temps, j’ai comparé mes données avec le travail expérimental de Nguyen [1] sur le T-jonction circulaire en régime deflecting. J’arrive à valider la DNS avec les données expérimentales. L’organisation des structures cohérentes sont illustrées en régime laminaire et turbulent en Newtonien et en non-Newtonien. Dans un deuxième temps, j’ai simulé deux régimes (deflecting et impinging) dans un T-jonction rectangulaire en Newtonien et en non-Newtonien. J’ai montré l’existence de structures cohérentes (par example kidney vortex) qui servent de moteur au mélange du scalaire passif propre au non-Newtonien. L’efficacité de mélange est augmentée en régime impinging par rapport au régime deflecting. Le shifting du pic de turbulence est observé uniquement en régime impinging. / For the simplicity in geometry by comparing it with other mixers in the industry, flows in T-junction configuration have been studied. More specifically, Direct Numerical Simulations is carried out using OpenFOAM on a convergent T-junction configuration with circular and rectangular cross-section. Both Newtonian and non-Newtonian fluids (Bird-Carreau model) are taken into account. Firstly, DNS data is compared to Nguyen’s experimental work on the circular T-junction at regime deflecting [1]. Good agreement between simlation and experiment is achieved. The organization of coherent structures is illustrated in laminar and turbulent for both Newtonian and non-Newtonian cases. Secondly, two flow regimes (deflecting and impinging) are simulated in a rectangular T-junction for the same Newtonian and non-Newtonian fluids. The existence of non-Newtonian coherent structures (e.g. kidney vortex ) is shown. These structures are regarded as essential mixing mechanism of passive scalar mixing. The mixing efficiency is increased in regime impinging compared to regime deflecting. The shifting of the turbulence peak is only observed in regime impinging.

We exhibit multiple periodic, collision-based orbits of the Newtonian n-body problem. Many of these orbits feature regularizable collisions between the masses. We demonstrate existence of the periodic orbits after performing the appropriate regularization. Stability, including linear stability, for the orbits is then computed using a technique due to Roberts. We point out other interesting features of the orbits as appropriate. When applicable, the results are extended to a broader family of orbits with similar behavior.

[pt] O deslocamento de um líquido por outro em espaços anulares é comumente encontrado na indústria do petróleo, e a maioria deles envolve materiais não newtonianos. O espaço anular muitas vezes apresenta irregularidades causadas pela erosão, onde quantidades consideráveis de fluido de perfuração podem ser deixadas para trás durante o processo de deslocamento, comprometendo a qualidade da operação de cimentação. Motivados por esse processo industrial, testes de deslocamento entre líquidos a vazão constante foram realizados em espaços anulares cuja parede externa possui, em uma determinada posição axial, um aumento repentino de diâmetro seguido de uma diminuição repentina de diâmetro mais a jusante. O objetivo dos experimentos era determinar a eficiência do deslocamento em função da vazão, reologia dos fluidos e geometria da cavidade. Os resultados revelaram forte influência desses parâmetros na eficiência de deslocamento. Ao mesmo tempo, um estudo numérico foi desenvolvido. Simulações numéricas das equações de Navier-Stokes em geometria axissimétrica para fluidos incompressíveis foram acopladas ao método Level-Set para captura da interface. Fluidos com viscosidade constante e o modelo newtoniano generalizado com função viscosidade de Carreau-Yasuda foram utilizados. Isso permitiu simular deslocamentos entre dois fluidos newtonianos e entre um fluido newtoniano e outro não-newtoniano. Este foi utilizado tanto como fluido deslocador quanto como deslocado. Foram realizadas simulações para várias razões de diâmetros, viscosidades, tempos de relaxação, e números de capilaridade e de Reynolds. Identificamos quando a aproximação do espaço anular por duas placas paralelas pode ser aplicada e calculamos como a forma da interface depende dos parâmetros investigados. / [en] The displacement of a fluid caused by another one, inside annular spaces, is commonly found in the oil industry and most of these rearrangements involve non-Newtonian materials. The annular space often shows irregularities caused by erosion, in which considerable amounts of drilling fluid can be left behind during the displacement process, compromising the cementing operation efficiency. Motivated by that industrial process, fluid-fluid displacement tests at constant flow rate were performed in annular spaces in which their exterior walls displayed - in a determined axial position - an abrupt expansion followed by an abrupt contraction. The purpose of the tests were to determine the displacement efficiency as a function of flow rate, rheological properties and geometric cavity. The results revealed a strong influence of these parameters on the displacement efficiency. At the same time, a numerical research was developed. Numerical simulations of the Navier-Stokes equations in axisymmetric geometry for incompressible fluids were coupled to the Level-Set method to capture the interface. Fluids with constant viscosity and the generalized Newtonian model with viscosity function of Carreau-Yasuda were used. That allowed to simulate displacements between two Newtonian fluids and a Newtonian and a non-Newtonian fluid. This was used both as a displacer and as a displaced fluid. Simulations were performed for several diameters and viscosities ratios, relaxatation time, capilar and Reynolds numbers. We identified when the approximation of the annular space by two parallel plates can be applied and calculated how the shape of the interface depends on the investigated parameters.

We develop a viscoelastic version of the volume of fluid algorithm for tracking deformable interfaces. The code uses the piecewise linear interface calculation method to reconstruct the interface, the continuous surface force formulation to model interfacial tension forces and utilizes the semi-implicit Stokes solver (enabling computations at low Reynolds numbers). The algorithm is primarily designed to simulate the flow of superposed fluids and the drop in a flow problem in 2D shear flows of viscoelastic and/or Newtonian fluids. The code is validated against linear stability theory for the two-layer flow case and against experimental and other documented numerical investigations for the droplet-matrix case. / Ph. D.

Yes / In this paper, an Euler-Lagrange model for computational fluid dynamics was used to model a full-scale gas-mixed anaerobic digester. The flow profiles, local values of non-Newtonian viscosity and average shear rate were analysed. Recommendations to enhance the effectiveness of mixing were given. In particular, the gas mixing input power can be reduced without appreciable detrimental effects on the mixing effectiveness.

Drilling is crucial to many industries, including hydrocarbon extraction, CO2 sequestration, geothermal energy, and others. During penetrating the subsurface rocks, drilling fluid (mud) is used for drilling bit cooling, lubrication, removing rock cuttings, and providing wellbore mechanical stability. Significant mud loss from the wellbore into the surrounding formation causes fluid lost-circulation incidents. This phenomenon leads to cost overrun, environmental pollution, delays project time and causes safety issues. Although lost-circulation exacerbates wellbore conditions, prediction of the characteristics of subsurface formations can be obtained. Generally, four formation types cause lost-circulation: natural fractures, and induced fractures, vugs and caves, and porous/permeable medium. The focus in this work is on naturally fractured formations, which is the most common cause of lost circulation. In this work, a novel prediction tool is developed based on analytical solutions and type-curves (TC). Type-curves are derived from the Cauchy equation of motion and mass conservation for non-Newtonian fluid model, corresponding to Herschel-Bulkley model (HB). Experimental setup from literature mimicking a deformed fracture supports the establishment of the tool. Upscaling the model of a natural fracture at subsurface conditions is implemented into the equations to achieve a group of mud type-curves (MTC) alongside another set of derivative-based mud type-curves (DMTC). The developed approach is verified with numerical simulations. Further, verification is performed with other analytical solutions. This proposed tool serves various functionalities; It predicts the volume loss as a function of time, based on wellbore operating conditions. The time-dependent fluid loss penetration from the wellbore into the surrounding formation can be computed. Additionally, the hydraulic aperture of the fracture in the surrounding formation can be estimated. Due to the non-Newtonian behavior of the drilling mud, the tool can be used to assess the fluid loss stopping time. Validation of the tool is performed by using actual field datasets and published experimental measurements. Machine-Learning is finally investigated as a complementary approach to determine the flow behavior of mud loss and the corresponding fracture properties.

Rheological properties of idealised models which exhibit all the non-Newtonian flow phenomenology commonly seen in dense suspensions are investigated by particulate-dynamics computer-simulations. The objectives of these investigations are: (i) to establish the origins of various aspects of dense suspension rheology such as shear-thinning, shear thickening and dilatancy; (ii) to elucidate the different regions of a typical dense suspension rheogram by examining underlying structures and shear induced anisotropies in kinetic energy, diffusivity and pressure; (iii) to investigate the scaling of the simplest idealised model suspension; i.e. the hard-sphere model in Newtonian media and its relationship to the isokinetic flow curves obtained through non-equilibrium molecular dynamics (NEMD) simulations; (iv) to preliminarily determine the effect of perturbations present in all real colloidal suspensions, namely particle size polydispersity and a slight 'softness' of the interparticle potential. Non-equilibrium isokinetic simulations have been performed upon ;systems of particles interacting through the classical hard-sphere potential and a perturbation thereof, in which the hard-core is surrounded by a 'slightly soft' repulsive skin. The decision to base the present work upon isokinetic studies was made in order to obtain a better under- standing of suspension rheology by making a direct connection with previous NEMD studies of thermal systemst(93). These studies have shown that the non-linear behaviour exhibited by these systems under shear is atttributable to a shear-induced perturbation of the equilibrium phase behaviour. The present study shows this behaviour to correspond to the high shear region of the generalised suspension flow curve. / Science and Engineering Research Council and Unilever Research

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