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[pt] A vibração excessiva da coluna de perfuração leva à perda da eficácia do processo de perfuração de poços de petróleo e causa danos prematuros ao equipamento. Portanto, entender a dinâmica do sistema é essencial. O comportamento complexo das colunas de perfuração estimulou um grande número de publicações sobre a dinâmica de perfuração. A vibração torsional está presente na maioria dos processos de perfuração, eventualmente atingindo o fenômeno de stick-slip. Essa vibração torsional resulta da interação não linear entre brocas e rochas. Apesar da complexidade da interação broca-rocha, a relação entre torque e velocidade de broca é, frequentemente, tratada como uma força de atrito seco em um sistema delgado. Um grande número de modelos de atrito é usado para descrever a interação entre brocas e rochas, embora um modelo adequado seja necessário para uma interpretação precisa de sistemas com atrito. Esta contribuição utiliza dados experimentais de uma bancada de testes, capaz de reproduzir o comportamento torsional de um sistema real, equipada com dispositivos de freio simples para introduzir atrito ao sistema, perturbando o movimento de rotação. A bancada é matematicamente modelada como um pêndulo de torção atuado. Esta dissertação propõe um modelo de atrito baseado em dados experimentais e analisa vários fenômenos observados em relação ao atrito, incluindo a histerese. Os resultados experimentais são usados para identificação dos parâmetros do modelo proposto. Por fim, simulações numéricas e resultados experimentais são comparados para validação do modelo de atrito proposto. / [en] Excessive drill string vibration leads to loss of the drilling process effectiveness and premature damage to the equipment. Therefore, understanding the system dynamics is essential. The complex behavior of drill strings attracted considerable attention in specialized literature. Due to the drill string slenderness, torsional vibration is present in most drilling routines, eventually reaching the stick-slip phenomenon. This torsional vibration results from the nonlinear interaction between drill-bits and rocks. Despite the complexity of the bit-rock interaction, researchers often treat the relationship between torque and bit velocity as a dry friction force in a slender system. A large number of dry friction models is used to describe the interaction between drill bits and rocks although a proper model is required for a precise interpretation of systems with friction. This contribution utilizes data from a test rig that is capable of reproducing full-scale system torsional behavior with simple brake devices to introduce friction into the system, disturbing the rotating motion. This test rig is modeled as an actuated torsional pendulum for the numerical investigations of the experimental friction. This dissertation proposes a friction model based on experimental data and analyzes various frictional phenomena observed, including hysteresis. The experimental results are used to identify the proposed model parameters. Lastly, it compares numerical simulations with the experimental results and validates the proposed friction model.

Tire, or in its primitive form, Wheel, an important invention for the transportation sector, has evolved from a circular block of hard and durable material to one of the most complex and influential components of an automobile. It is the only means of contact between the vehicle and the road and is responsible for generating forces and moments that impact vehicle performance, stability, and control. Tire tribology is the study of interacting surfaces in relative motion which includes friction and wear. Tire friction is an essential concept for estimating the tractive effort/ traction at the tire-road interface that further helps to determine the control and stability of the vehicle. In contrary, it also results in rolling resistance and wear. Tire and vehicle engineers are henceforth interested in a robust and efficient approach towards estimation of friction and wear. Past experimental observations using tread compound samples have revealed the different factors influencing the friction at the contacting interface. In addition, different mechanisms or components resulting in frictional losses, being Hysteretic, Adhesive and Viscous, and wear being abrasive, fatigue, adhesive and corrosive were also observed. Although experimental and empirical observations have provided us with an accurate estimation of friction and wear parameters, it is very tedious and expensive approach. Recent developments in the computational power encouraged researchers and engineers towards evolution of analytical and numerical models considering the underlying physical mechanisms at the contact interface. Past research studies developed multiscale techniques for estimation of friction coefficient due to hysteretic losses from internal damping of the rubber material because of oscillation from surface undulations. Later, contact mechanics models developed using Hertzian technique or stochastic approach were considered in conjunction with frictional losses to obtain the hysteretic component of friction to consider the effect of surface roughness. Previous studies at CenTiRe focused on surface characterization techniques and estimation of friction for dry surfaces using Persson and Klüppel's approach. Comparative studies unveiled the importance of considering pressure/ normal load towards friction estimation. In addition, it was found that effect of adhesion for estimation of contact mechanics parameters must be considered. The present work focusses on obtaining a conceptual framework to model a comprehensive friction model considering the effect of surface roughness, substrate condition and asperity interaction. A finite element simulation of rubber block sliding on a rough substrate is performed using a multiscale technique for estimation of friction and contact mechanics under dry condition. The estimated contact mechanics and friction is compared with analytical models and experimental measurements obtained using Linear sliding friction tester developed in collaboration with other members of the group. In addition, a FE model is developed to measure the wear properties of rubber material based on continuum damage mechanics and further obtain the wear profile of a rubber block sliding on a rough substrate. / Doctor of Philosophy / Tribology, a recent terminology for an age-old concept of friction, wear, and lubrication. the study of interacting surfaces in relative motion which includes friction and wear. Friction is the resisting force at the contact interface leading to heat build-up and material loss at the contact interface which is known as flash temperature and wear respectively. Tire is one of the most complex and influential components of a vehicle that helps in optimizing its performance for better stability and control. Knowledge of tire friction and wear is important for tire engineering and vehicle dynamics engineers as it helps in characterizing the handling characteristics of the vehicle, characterizing the tire material compounds to understand the tire durability. Rubber is a viscoelastic material, the friction and wear in rubber is intricate as opposed to other elastic materials. Based on experimental observations in the past, friction and wear are influenced by factors like material properties, normal load/ pressure, sliding velocity, temperature, surface characteristics, and environmental conditions. In addition, the frictional losses at the contact interface are considered to compose of adhesion, hysteresis and viscous components and wear is categorized as – adhesive, abrasive, fatigue, corrosive and erosive. Recent developments in computational power encouraged researchers and engineers in developing analytical and computational models that consider the physical mechanisms occurring at the contact interface. The present research focuses on obtaining a comprehensive friction and contact mechanics model considering the effect of surface roughness at different length scales, surface condition (dry/ wet) and asperity interaction. In addition, the developed model in conjunction with a brush model is considered for estimating the tire traction characteristics such as the forces and moments. A finite element simulation of rubber block sliding on a rough substrate is performed using a multiscale technique for estimation of friction, contact mechanics and abrasion parameters under dry condition. The results thus obtained are compared with the analytical model that is developed for wet conditions. Experimental validation of the friction estimated using the analytical and numerical methods will be performed using a linear sliding friction tester developed in collaboration with other members of the group.

In this work, we develop an interface-preserving level-set method in the finite-element framework for interfacial flows with moving contact lines. In our method, the contact line is advected naturally by the flow field. Contact angle hysteresis can be easily implemented without explicit calculation of the contact angle or the contact line velocity, and meshindependent results can be obtained following a simple computational strategy. We have implemented the method in three dimensions and provide numerical studies that compare well with analytical solutions to verify our algorithm. We first develop a high-order numerical method for interface-preserving level-set reinitialization. Within the interface cells, the gradient of the level set function is determined by a weighted local projection scheme and the missing additive constant is determined such that the position of the zero level set is preserved. For the non-interface cells, we compute the gradient of the level set function by solving a Hamilton-Jacobi equation as a conservation law system using the discontinuous Galerkin method. This follows the work by Hu and Shu [SIAM J. Sci. Comput. 21 (1999) 660-690]. The missing constant for these cells is recovered using the continuity of the level set function while taking into account the characteristics. To treat highly distorted initial conditions, we develop a hybrid numerical flux that combines the Lax-Friedrichs flux and a penalty flux. Our method is accurate for non-trivial test cases and handles singularities away from the interface very well. When derivative singularities are present on the interface, a second-derivative limiter is designed to suppress the oscillations. At least (N + 1)th order accuracy in the interface cells and Nth order accuracy in the whole domain are observed for smooth solutions when Nth degree polynomials are used. Two dimensional test cases are presented to demonstrate superior properties such as accuracy, long-term stability, interface-preserving capability, and easy treatment of contact lines. We then develop a level-set method in the finite-element framework. The contact line singularity is removed by the slip boundary condition proposed by Ren and E [Phys. Fluids, vol. 19, p. 022101, 2007], which has two friction coefficients: βN that controls the slip between the bulk fluids and the solid wall and βCL that controls the deviation of the microscopic dynamic contact angle from the static one. The predicted contact line dynamics from our method matches the Cox theory very well. We further find that the same slip length in the Cox theory can be reproduced by different combinations of (βN; βCL). This combination leads to a computational strategy for mesh-independent results that can match the experiments. There is no need to impose the contact angle condition geometrically, and the dynamic contact angle automatically emerges as part of the numerical solution. With a little modification, our method can also be used to compute contact angle hysteresis, where the tendency of contact line motion is readily available from the level-set function. Different test cases, including code validation and mesh-convergence study, are provided to demonstrate the efficiency and capability of our method. Lastly, we extend our method to three-dimensional simulations, where an extension equation is solved on the wall boundary to obtain the boundary condition for level-set reinitializaiton with contact lines. Reinitialization of ellipsoidal interfaces is presented to show the accuracy and stability of our method. In addition, simulations of a drop on an inclined wall are presented that are in agreement with theoretical results. / Doctor of Philosophy / When a liquid droplet is sliding along a solid surface, a moving contact line is formed at the intersection of the three phases: liquid, air and solid. This work develops a numerical method to study problems with moving contact lines. The partial differential equations describing the problem are solved by finite element methods. Our numerical method is validated against experiments and theories. Furthermore, we have implemented our method in three-dimensional problems.

The erosion and transport of fertile topsoil is a serious problem in the U.S., Australia, China and throughout Europe. It results in extensive environmental damage, reduces soil fertility and productivity, and causes significant environmental loss. It is as big a threat to the future sustainability of global populations as climate change, but receives far less attention. With both chemicals (fertilizers, pesticides, herbicides) and biological pathogens (bacteria, viruses) preferentially sorbing to silt and clay sized soil particles, estimating contaminant fluxes in eroded soil also requires predicting the transported soils particle size distribution. The Hairsine-Rose (HR) erosion model is considered in this thesis as it is one of the very few that is specifically designed to incorporate the effect of particle size distribution, and differentiates between non-cohesive previously eroded soil compared with cohesive un-eroded soil. This thesis develops a new extended erosion model that couples the HR approach with the one-dimensional St Venant equations, and an Exner bed evolution equation to allow for feedback effects from changes in the local bed slope on surface hydraulics and erosion rates to be included. The resulting system of 2I +3 (where I = number of particle size classes) nonlinear hyperbolic partial differential equations is then solved numerically using a Liska-Wendroff predictor corrector finite difference scheme. Approximate analytical solutions and series expansions are derived to overcome singularities in the numerical solutions arising from either boundary or initial conditions corresponding to a zero flow depth. Three separate practical applications of the extended HR model are then considered in this thesis, (i) flow through vegetative buffer strips, (ii) modelling discharge hysteresis loops and (iii) the growth of antidunes, transportational cyclic steps and travelling wave solutions. It is shown by comparison against published experimental flume data that predictions from the extended model are able to closely match measurements of deposited sediment distribution both upstream and within the vegetative buffer strip. The experiments were conducted with supercritical inflow to the flume which due to the increased drag from the vegetative strip, resulted in a hydraulic jump just upstream of the vegetation. As suspended sediment deposited at the jump, this resulted in the jump slowly migrating upstream. The numerical solutions were also able to predict the position and hydraulic jump and the flow depth throughout the flume, including within the vegetative strip, very well. In the second application, it is found that the extended HR model is the first one that can produce all known types of measured hysteresis loops in sediment discharge outlet data. Five main loop types occur (a) clockwise, (b) counter-clockwise, (c,d) figure 8 of both flow orientations and (e) single curve. It is clearly shown that complicated temporal rainfall patterns or bed geometry are not required to developed complicated hysteresis loops, but it is the spatial distribution of previously eroded sediment that remains for the start of a new erosion event, which primarily governs the form of the hysteresis loop. The role of the evolution of the sediment distribution in the deposited layer therefore controls loop shape and behavior. Erosion models that are based solely on suspended sediment are therefore unable to reproduce these hysteretic loops without a priori imposing a hysteretic relationship on the parameterisations of the erosion source terms. The rather surprising result that the loop shape is also dominated by the suspended concentration of the smallest particle size is shown and discussed. In the third application, a linear stability analysis shows that instabilities, antidunes, will grow and propagate upstream under supercritical flow conditions. Numerical simulations are carried out that confirm the stability analysis and show the development and movement of antidunes. For various initial parameter configurations a series of travelling antidunes, or transportational cyclic steps, separated by hydraulic jumps are shown to develop and evolve to a steady form and wave speed. Two different forms arise whereby (a) the deposited layer completely shields the underlying original cohesive soil so that the cohesive layer plays no role in the speed or shape of the wave profile or (b) the cohesive soil is exposed along the back of the wave such that both the non-cohesive and cohesive layers affect the wave profile. Under (a) the solutions are obtained up to an additive constant as the actual location of the boundary of the cohesive soil is not required, whereas for (b) this constant must be determined in order to find the location on the antidune from where the cohesive soil becomes accessible. For single size class soils the leading order travelling wave equations are fairly straightforward to obtain for both cases (a) and (b). However for multi-size class soils, this becomes much more demanding as up to 2I + 3 parameters must be found iteratively to define the solution as each size class has its own wave profile in suspension and in the antidune.

Hysteresis current control has been widely used in power electronics with the advantages of fast dynamic response under parameter, line and load variation and ensured stability. However, a main disadvantage of hysteresis current control is the uncertain and varying switching frequency which makes it difficult to form an average-value model. The changing switching frequency and unspecified switching duty cycle make conventional average-value models based on PWM control difficult to apply directly to converters that are controlled by hysteresis current control. In this work, a new method for average-value modeling of hysteresis current control in boost converters, three-phase inverters, and brushless dc motor drives is proposed. It incorporates a slew-rate limitation on the inductor current that occurs naturally in the circuit during large system transients. This new method is compared with existing methods in terms of simulation run time and rms error. The performance is evaluated based on a variety of scenarios, and the simulation results are compared with the results of detailed models. The simulation results show that the proposed model represents the detailed model well and is faster and more accurate than existing methods. The slew-rate limitation model of hysteresis current control accurately captures the salient detail of converter performance while maintaining the computational efficiency of average-value models. Validations in hardware are also presented.

本論文擬探討中國大陸金融全球化戰略的形成，以及中國大陸金融全球化過程所產生的問題與現象。本論文擬以新制度主義論作為理論的切入點，探討中國大陸金融全球化戰略形成過程中所產生的現象。 本論文的理論發現在於下列諸項：（1）中國大陸在金融全球化的戰略形成有明顯的「路徑依賴」現象。這表現在中國歷史中的儒家文化及共產主義思想影響中國大陸政府及官僚和經理人在執行金融制度改革中的行為模式。儒家文化對中國大陸金融全球化的影響在於政府控制經濟體的交易模式及金融運作、注重經濟體內民眾的福祉及提供一個和諧發展的全球經濟新願景。共產主義對中國大陸金融全球化的影響在於經濟改革過程中政府及官僚及經理人仍以集體經濟的行為模式在金融制度改革過程中影響制度的運行。（2）中國大陸的金融全球化過程中有明顯的制度內捲化現象（institutional involution），造成金融制度在變革過程中受到原有非正式制度諸如行為模式及文化的影響，致使制度變革無法達到預期的結果，而形成一種發展遲滯的現象。

This thesis demonstrates a method for transformer core diagnostics. The method uses the no-load current of the transformer as an indicator, and gives different characteristic signatures for different types of faults or defects. Using the no-load current for the diagnostic gives high sensitivity. The method is therefore able to detect defects that are too small to have an impact on the losses. In addition to different types of fault, the method can in some cases also distinguish between faults in different locations within the core. Both single-phase and three-phase transformers can be diagnosed using this method, and the measurements can be easily performed at any facility capable of measuring the no-load loss. There are, however, some phenomena that occur in large transformers, and in transformers with high rated voltages. Examples include capacitive resonance and magnetic remanence. This thesis proposes and demonstrates techniques for compensating for these phenomena. With these compensating techniques, the repeatability of the measurements is high. It is shown that units with the same core steel tend to have very similar no-load behavior. The diagnostics can then be performed either by comparing the transformer to another unit, or to simulations. The thesis presents one possible simulation method, and demonstrates the agreement with measurements. This topological simulation method includes both the electric circuit and an accurate model of the magnetic hysteresis. It is therefore also suitable for other, related, studies in addition to core diagnostics. Possible subjects include ferroresonance, inrush, DC magnetization of transformers, and transformer core optimization. The thesis also demonstrates that, for three-phase transformers, it is possible to compare the phases to each other. This technique makes it possible to diagnose a transformer even without a previous measurement to compare to, and without the data required to make a simulation. / <p>QC 20170607</p>