1,767 research outputs found

    China and the United States\u27 Recovery from the Global Financial Crisis

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    Faced with the global financial crisis, which has a large impact on the world\u27s economy, China and the United Stated took different actions to pull the economy out of it, based on the fairly different financial, fiscal, and even political systems they have. This thesis focuses on the comparison of the financial and fiscal systems and trade structures between the two different countries, and how these have impact on their stimulus packages, thus influencing the economic recovery as a whole

    Numerical modeling of geological carbon sequestration : enhanced dissolution in randomly heterogeneous media

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    Geological Carbon Sequestration (GCS) plays a major role in mitigating the global warming due to the increasing concentration of CO2 in atmosphere. It is important to understand the GCS process to analyze and predict its influence on the CO2 reservoir site. This work firstly offers three finite difference programs for simulating the GCS process, secondly applies the program to perform quantitative analysis of the GCS, and finally gives several remarks related to the GCS. Firstly, we describe the three finite difference programs, which, to different extents, take into account the role of chemical reaction in the GCS process. The first program is a reactive two-phase flow model that only considers the interplay between brine and gas phases, which can be solved with explicit method. The second program is a simplified reactive three-phase flow model that also considers the heterogeneous reaction between mineral rock and the aqueous species, but all the chemical reactions are treated as equilibrium reactions and expressed with empirical formulas, to avoid performing grid-by-grid calculation of chemical reaction at each time step. The third program is a full reactive three-phase flow model that accounts for partial equilibrium reaction system (i.e., both equilibrium and kinetic reactions are included) as well as the pure equilibrium reaction system. In this third program, the partial equilibrium chemical system is solved with MAL, and chemical reactions and mass transport are alternately solved with Newton-Raphson method. All these programs have been approved through non-ideal benchmark test that only considers the transport issue. Secondly, the first program is employed to perform two qualitative analysis on the dissolution trapping. The first quantitative analysis focuses on the enhanced dissolution efficiency of overlying gaseous CO2 into underlying brine owing to gravity-driven convection (GDC) in the brine phase. The injected light CO2 will override the brine. Studies have shown that the dissolution is not only driven by molecular diffusion, but also enhanced by the GDC in the brine, because the dissolution of CO2 can increase the density of the brine in the upper portion. In literature, the GDC in homogeneous media has been well studied and researchers are attempting to investigate the GDC in realistic heterogeneous media. It is yet to find an efficient formula to predict the dissolution rate in heterogeneous media with anisotropic permeability distribution. This work conducts a large number of numerical simulations in various heterogeneous fields, analyzes the simulation results, and proposes two formulas that efficiently predict the dissolution rate based on geological and fluid parameters. The second quantitative analysis focuses on the enhanced dissolution trapping due to the layered permeability structure during the injection period. Results show that when buoyant forces are important, vertical segregation controls the overall behavior of CO2, diminishing the influence of small-scale heterogeneity on dissolution. However, when buoyant forces are relatively small compared to the degree of heterogeneity, CO2 migrates preferentially through high permeability layers and dissolution efficiency increases with heterogeneity due to the stretching of the CO2 plume that enhances mixing. As a result, in this situation, the upscaling of permeability leads to an underestimation of the dissolution efficiency. Additionally, we give in the appendix a parallel study on how to enhance NAPL removal and mixing with engineering chaotic advection.El secuestro geol贸gico de carbono (GCS) juega un papel importante en la mitigaci贸n del calentamiento global debido a la creciente concentraci贸n de CO2 en la atm贸sfera. Es importante entender el proceso de GCS para analizar y predecir su influencia en el sitio del dep贸sito de CO2. Este trabajo, en primer lugar, ofrece tres programas de diferencias finitas para simular el proceso GCS, en segundo lugar, aplica uno de los programas para realizar an谩lisis cuantitativos del GCS y, finalmente, ofrece varios comentarios relacionados con el GCS. Primero, describimos los tres programas de diferencias finitas, que, en diferentes grados, tienen en cuenta el papel de la reacci贸n qu铆mica en el proceso de GCS. El primer programa es un modelo de flujo reactivo de dos fases que solo considera la interacci贸n entre las fases de salmuera y gas, que se puede resolver con un m茅todo expl铆cito. El segundo programa es un modelo de flujo reactivo trif谩sico simplificado que tambi茅n considera la reacci贸n heterog茅nea entre la roca mineral y la especie acuosa, pero todas las reacciones qu铆micas se tratan como reacciones de equilibrio y se expresan con f贸rmulas emp铆ricas, para evitar realizar cuadr铆cula por cuadr铆cula el c谩lculo de la reacci贸n qu铆mica en cada paso de tiempo. El tercer programa es un modelo de flujo reactivo de tres fases completo que tiene en cuenta el sistema de reacci贸n de equilibrio parcial (es decir, se incluyen las reacciones de equilibrio y cin茅ticas), as铆 como el sistema de reacci贸n de equilibrio puro. En este tercer programa, el sistema qu铆mico de equilibrio parcial se resuelve con MAL, y las reacciones qu铆micas y el transporte de masa se resuelven alternativamente con el m茅todo de Newton-Raphson. Todos estos programas han sido aprobados a trav茅s de una prueba de referencia no ideal que solo considera el tema del transporte. En segundo lugar, el primer programa se emplea para realizar dos an谩lisis cualitativos sobre el trampeo por disoluci贸n. El primer an谩lisis cuantitativo se centra en la eficiencia de disoluci贸n mejorada del CO2 gaseoso superpuesto en la salmuera subyacente debido a la convecci贸n impulsada por gravedad (GDC) en la fase de salmuera. El CO2 ligero inyectado anular谩 la salmuera. Los estudios han demostrado que la disoluci贸n no solo es impulsada por la difusi贸n molecular, sino tambi茅n mejorada por el GDC en la salmuera, porque la disoluci贸n de CO2 puede aumentar la densidad de la salmuera en la parte superior. En la literatura, la GDC en medios homog茅neos ha sido bien estudiada y los investigadores est谩n intentando investigar la GDC en medios heterog茅neos realistas. A煤n est谩 por encontrar una f贸rmula eficiente para predecir la tasa de disoluci贸n en medios heterog茅neos con distribuci贸n de permeabilidad anisotr贸pica. Este trabajo realiza una gran cantidad de simulaciones num茅ricas en varios campos heterog茅neos, analiza los resultados de la simulaci贸n y propone dos f贸rmulas que predicen de manera eficiente la velocidad de disoluci贸n en funci贸n de par谩metros geol贸gicos y de fluidos. El segundo an谩lisis cuantitativo se centra en la captura de disoluci贸n mejorada debido a la estructura de permeabilidad en capas durante el per铆odo de inyecci贸n. Los resultados muestran que cuando las fuerzas de flotaci贸n son importantes, la segregaci贸n vertical controla el comportamiento general de CO2, lo que disminuye la influencia de la heterogeneidad a peque帽a escala en la disoluci贸n. Sin embargo, cuando las fuerzas de flotaci贸n son relativamente peque帽as en comparaci贸n con el grado de heterogeneidad, el CO2 migra preferentemente a trav茅s de capas de alta permeabilidad y la eficiencia de disoluci贸n aumenta con la heterogeneidad debido al estiramiento de la pluma de CO2 que mejora la mezcla. Como resultado, la ampliaci贸n de la permeabilidad conduce a una subestimaci贸n de la eficiencia. Adem谩s, proporcionamos en el ap茅ndice un estudio paralelo sobre c贸mo mejorar la eliminaci贸n y mezcla de NAPL con advecci贸n ca贸tica de ingenier铆a.Enginyeria del terren

    Tolerant, broadband tunable 2鈥壝椻2 coupler circuit

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    We propose a circuit design for a broadband tunable 2 x 2 waveguide coupler, consisting of a two-stage Mach-Zehnder interferometer with electro-optic phase shifters in each stage. We demonstrate that such design can be configured as a tunable coupler with arbitrary coupling ratio and with a uniform response over 50-nm spectral range around 1550 nm. The design is also tolerant to fabrication variations that affect the coupling ratios of the directional couplers

    Distribution-Sensitive Bounds on Relative Approximations of Geometric Ranges

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    A family R of ranges and a set X of points, all in R^d, together define a range space (X, R|_X), where R|_X = {X cap h | h in R}. We want to find a structure to estimate the quantity |X cap h|/|X| for any range h in R with the (rho, epsilon)-guarantee: (i) if |X cap h|/|X| > rho, the estimate must have a relative error epsilon; (ii) otherwise, the estimate must have an absolute error rho epsilon. The objective is to minimize the size of the structure. Currently, the dominant solution is to compute a relative (rho, epsilon)-approximation, which is a subset of X with O~(lambda/(rho epsilon^2)) points, where lambda is the VC-dimension of (X, R|_X), and O~ hides polylog factors. This paper shows a more general bound sensitive to the content of X. We give a structure that stores O(log (1/rho)) integers plus O~(theta * (lambda/epsilon^2)) points of X, where theta - called the disagreement coefficient - measures how much the ranges differ from each other in their intersections with X. The value of theta is between 1 and 1/rho, such that our space bound is never worse than that of relative (rho, epsilon)-approximations, but we improve the latter\u27s 1/rho term whenever theta = o(1/(rho log (1/rho))). We also prove that, in the worst case, summaries with the (rho, 1/2)-guarantee must consume Omega(theta) words even for d = 2 and lambda <=3. We then constrain R to be the set of halfspaces in R^d for a constant d, and prove the existence of structures with o(1/(rho epsilon^2)) size offering (rho,epsilon)-guarantees, when X is generated from various stochastic distributions. This is the first formal justification on why the term 1/rho is not compulsory for "realistic" inputs
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