No description available.

No description available.

No description available.

[pt] Após a década de 80 tornou-se frequente a utilização de poços direcionais, tanto poços horizontais quanto os de longo alcance, nas operações de desenvolvimentos de campos petrolíferos, os quais permitiram grande avanço na exploração. Este cenário, porém, traz grandes desafios para operações de perfuração relacionadas à limpeza de poços. A limpeza de poços consiste na remoção dos cascalhos de dentro do anular do poço através da circulação do fluido de perfuração. Devido à inclinação do poço, os cascalhos sofrem a ação da gravidade sendo empurrados para o fundo do canal, já que possuem densidade maior que a da fase líquida, assim, transportados a uma velocidade menor que a velocidade do escoamento. Ao se depositarem no fundo do canal, formam um leito estacionário, obstruindo parte do anular e diminuindo a vazão, o que gera problemas, como por exemplo, redução da taxa de penetração, desgaste prematuro da broca, elevação do torque e arraste, aprisionamento da coluna, perda de circulação, dentre outros, podendo gerar a perda do poço. Desta forma, o perfeito entendimento do processo de sedimentação e transporte de partículas sólidas suspensas em fluido é fundamental para a otimização do processo de perfuração de poços. Este trabalho analisa o escoamento laminar bidimensional de suspensões de partículas sólidas devido a um gradiente de pressão entre duas placas paralelas, representando uma descrição simplificada do escoamento que ocorre em um anular de poço durante o processo de perfuração. O perfil do leito de partículas ao longo do canal e a relação vazão-diferença de pressão para diferentes condições de escoamento são determinados pela solução numérica das equações que descrevem o problema. A formulação matemática leva a um sistema acoplado de três equações diferenciais: conservação de massa e de quantidade de movimento e a equação de transporte, que engloba os efeitos de difusão de partículas devido à frequência de interação entre as partículas, ao gradiente de viscosidade e à diferença de densidade entre o líquido e as partículas. O sistema é resolvido pela técnica de Elementos Finitos, através do método de Galerkin. Os resultados obtidos serão de extrema importância no desenvolvimento de modelos mais precisos que descrevam o processo de transporte de sólidos em anulares de poços. / [en] After the 80 s, the use of directional wells, both horizontal and long range wells, became frequent in the development of oil fields, which allowed great progress in exploration. This scenario, however, brings great challenges to operations related to wellbore cleaning. Wellbore cleaning consists in the removal of cuttings from within the annular through the circulation of drilling fluid. Due to the inclination of the well, the cuttings undergo the action of gravity and more to the bottom of the channel, as they have higher density than the liquid phase. They may be transported with a speed less than the speed of the liquid flow. When deposited on the channel, the cuttings form a stationary bed, blocking part of the annular and decreasing the flow rate that causes problems, such as reducing the rate of penetration, premature wear of the bit, high torque and drag, trapping column, loss of circulation and others, which may cause the loss of the well. Therefore, the fundamental understanding of particle sedimentation and transport in a suspending flowing liquid is necessary for drilling operation design and optimization. This research studies the laminar two-dimensional flow of solid particles suspended in a liquid due to a pressure gradient between two parallel plates, representing a simplified description of the flow that occurs in an annular during the drilling process. The profile of the particle bed along the channel and the flow rate pressure difference relationship for different flow conditions are determined. The mathematical formulation leads to a coupled system of three differential equations: mass and momentum conservation and transport equation, which includes the effects of particle diffusion due to the frequency of interaction between the particles, the gradient of viscosity and the difference density between the liquid and the particles. The system is solved by the finite element method Galerkin. The results will be of extreme importance in the development of more accurate models that describe the solids transport process in annular space.

No description available.

No description available.

This thesis deals with the development and application of Monte Carlo simulations to study electron transport in bulk GaN in the wurtzite crystal structure and the properties of field effect transistors made from the material. There is a particular emphasis on transport in the high electric field regime and transistors operating at high voltages. The simulation model includes five sets of non-parabolic conduction band valleys which can be occupied by electrons during high field transport. The effects on electron transport of impurities and the relevant phonon scattering mechanisms have been considered. Results for electron transport at both low and high electric field are presented and compared with the properties of GaN in the zincblende structure, of other group-III nitride semiconductors, and of GaAs. The dependence of the transport properties on the material parameters is discussed and also with regard to the temperature, donor concentration and electric field magnitude and direction. The transport properties of electrons in wurtzite GaN n+-i(n)-n+ diodes are also explored, including the effect of the upper valleys and the temperature on hot electron transport. Simulations have also been carried out to model the steady-state and transient properties of GaN MESFETs that have recently been the subject of experimental study. It has been suggested that traps have a substantial effect on the performance of GaN field effect transistors and we have developed a model of a device with traps to investigate this suggestion. The model includes the simulation of the capture and release of electrons by traps whose charge has a direct effect on the current flowing through the transistor terminals. The influence of temperature and light on the occupancy of the traps and the /- V characteristics are considered. It is concluded that traps are likely to play a substantial role in the behaviour of GaN field effect transistors. Further simulations were performed to model electron transport in AlGaN/GaN hetero-junction FETs. So called HFET structures with a 78 nm Alo.2Gao.8N pseudomorphically strained layer have been simulated, with the inclusion of spontaneous and piezoelectric polarization effects in the strained layer. The polarization effects are shown to not only increase the current density, but also improve the electron transport by inducing a higher electron density close to the positive charge sheet that occurs in the channel.

Doctor of Philosophy / Department of Biological and Agricultural Engineering / Ronaldo G. Maghirang / Emission of particulate matter (PM) and various gases from open-lot beef cattle feedlots is becoming a concern because of the adverse effects on human health and the environment; however, scientific information on feedlot emissions is limited. This research was conducted to estimate emission rates of PM[subscript]10 from large cattle feedlots. Specific objectives were to: (1) determine feedlot PM[subscript]10 emission rates by reverse dispersion modeling using AERMOD; (2) compare AERMOD and WindTrax in terms of their predicted concentrations and back-calculated PM[subscript]10 emission rates; (3) examine the sensitivity of both AERMOD and WindTrax to changes in meteorological parameters, source location, and receptor location; (4) determine feedlot PM[subscript]10 emission rates using the flux-gradient technique; and (5) compare AERMOD and computational fluid dynamics (CFD) in simulating particulate dispersion from an area source. PM[subscript]10 emission rates from two cattle feedlots in Kansas were determined by reverse dispersion modeling with AERMOD using PM[subscript]10 concentration and meteorological measurements over a 2-yr period. PM[subscript]10 emission rates for these feedlots varied seasonally, with overall medians of 1.60 and 1.10 g /m[superscript]2 -day. Warm and prolonged dry periods had significantly higher PM emissions compared to cold periods. Results also showed that the PM[subscript]10 emissions had a diurnal trend; highest PM[subscript]10 emission rates were observed during the afternoon and early evening periods. Using particulate concentration and meteorological measurements from a third cattle feedlot, PM[subscript]10 emission rates were back-calculated with AERMOD and WindTrax. Higher PM[subscript]10 emission rates were calculated by AERMOD, but their resulting PM[subscript]10 emission rates were highly linear (R[superscript]2 > 0.88). As such, development of conversion factors between these two models is feasible. AERMOD and WindTrax were also compared based on their sensitivity to changes in meteorological parameters and source locations. In general, AERMOD calculated lower concentrations than WindTrax; however, the two models responded similarly to changes in wind speed, surface roughness, atmospheric stability, and source and receptor locations. The flux-gradient technique also estimated PM[subscript]10 emission rates at the third cattle feedlot. Analyses of PM[subscript]10 emission rates and meteorological parameters indicated that PM[subscript]10 emissions at the feedlot were influenced by friction velocity, sensible heat flux, temperature, and surface roughness. Based on pen surface water content measurements, a water content of at least 20% (wet basis) significantly lowered PM[subscript]10 emissions at the feedlot. The dispersion of particulate from a simulated feedlot pen was predicted using CFD turbulence model ([kappa]-[epsilon] model) and AERMOD. Compared to CFD, AERMOD responded differently to wind speed setting, and was not able to provide detailed vertical concentration profiles such that the vertical concentration gradients at the first few meters from the ground were negligible. This demonstrates some limitations of AERMOD in simulating dispersion for area sources such as cattle feedlots and suggests the need to further evaluate its performance for area source modeling.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / O esquema iterativo de fonte de espalhamento (SI) é tradicionalmente aplicado para a convergência da solução numérica de malha fina para problemas de transporte de nêutrons monoenergéticos na formulação de ordenadas discretas com fonte fixa. O esquema SI é muito simples de se implementar sob o ponto de vista computacional; porém, o esquema SI pode apresentar taxa de convergência muito lenta, principalmente para meios difusivos (baixa absorção) com vários livres caminhos médios de extensão. Nesta dissertação descrevemos uma técnica de aceleração baseada na melhoria da estimativa inicial para a distribuição da fonte de espalhamento no interior do domínio de solução. Em outras palavras, usamos como estimativa inicial para o fluxo escalar médio na grade de discretização de malha fina, presentes nos termos da fonte de espalhamento das equações discretizadas SN usadas nas varreduras de transporte, a solução numérica da equação da difusão de nêutrons em grade espacial de malha grossa com condições de contorno especiais, que aproximam as condições de contorno prescritas que são clássicas em cálculos SN, incluindo condições de contorno do tipo vácuo. Para aplicarmos esta solução gerada pela equação da difusão em grade de discretização de malha grossa nas equações discretizadas SN de transporte na grade de discretização de malha fina, primeiro implementamos uma reconstrução espacial dentro de cada nodo de discretização, e então determinamos o fluxo escalar médio em grade de discretização de malha fina para usá-lo nos termos da fonte de espalhamento. Consideramos um número de experimentos numéricos para ilustrar a eficiência oferecida pela presente técnica (DSA) de aceleração sintética de difusão. / The scattering source iterative (SI) scheme is traditionally applied to converge finemesh numerical solutions to fixed-source discrete ordinates neutron transport problems. The SI scheme is very simple to implement under a computational viewpoint. However, the SI scheme may show very slow convergence rate, mainly for diffusive media (low absorption) with several mean free paths in extent. In this work we describe an acceleration technique based on an improved initial guess for the scattering source distribution within the slab. In other words, we use as initial guess for the fine-mesh average scalar flux in the scattering source terms of the SN discretized equations used in the transport sweeps, the coarse-mesh solution of the neutron diffusion equation with special boundary conditions to account for the classical SN prescribed boundary conditions, including vacuum boundary conditions. To apply this coarse-mesh diffusion solution into the fine-mesh SN transport sweep discretized equations, we first perform within-node spatial reconstruction, and then we determine the fine-mesh average scalar flux for use in the scattering source terms. We consider a number of numerical experiments to illustrate the efficiency of the offered diffusion synthetic acceleration (DSA) technique.

Particle-laden turbulent flow through a straight square duct at Reτ = 300 is studied using direct numerical simulation (DNS) and Lagrangian particle tracking. A parallelized 3-D particle tracking direct numerical simulation code has been developed to perform the large-scale turbulent particle transport computations reported in this thesis. The DNS code is validated after demonstrating good agreement with the published DNS results for the same flow and Reynolds number. Lagrangian particle transport computations are carried out using a large ensemble of passive tracers and finite-inertia particles and the assumption of one-way fluid-particle coupling. Using four different types of initial particle distributions, Lagrangian particle dispersion, concentration and deposition are studied in the turbulent straight square duct. Particles are released in a uniform distribution on a cross-sectional plane at the duct inlet, released as particle pairs in the core region of the duct, distributed randomly in the domain or distributed uniformly in planes at certain heights above the walls. One- and two-particle dispersion statistics are computed and discussed for the low Reynolds number inhomogeneous turbulence present in a straight square duct. New detailed statistics on particle number concentration and deposition are also obtained and discussed.