Browsing by Subject "Instability"
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Item From growth-based to people-centered : how Chinese leaders have modified their governing strategies to sustain legitimacy in the reform era(2013-12) Zhang, Wenjie, active 2013; Galbraith, James K.This dissertation analyzes changes in the ruling strategies of the Chinese Communist Party (CCP) in the context of economic reforms, beginning in 1978. By employing both quantitative and qualitative methods, this dissertation investigates how Chinese leaders have utilized legitimating strategies, while modifying their governing strategies, in order to a) solidify the population, b) consolidate ruling authority and c) maintain political and social stability. Specifically, this dissertation looks at how Chinese policymakers have developed effective public policies in response to rapidly rising wage inequality, one of the most pressing problems undermining the CCP’s ruling authority. By providing an original estimate of China’s wage inequality and analyzing the government’s response to it, this dissertation provides a unique look at how the CCP has transformed government functions from growth-based to people-centered to meet various social, political and economic challenges. A comparative statistical analysis helps illustrate the philosophical roots and sources of the CCP’s political legitimacy. The technique of Theil Statistics is applied to measure China’s wage inequality during the reform period. A multivariate hierarchical regression analysis is employed to measure the impact of rising inequality on Chinese society. Two models on social welfare system reform are studied in order to understand Hu Jintao and Wen Jiabao’s people-centered governing philosophy and the rationale for constructing a service-oriented government.Item The macro- and micro-instabilities in the pedestal region of the Tokamak(2015-05) Ma, Jingfei; Morrison, Philip J.; Horton, C. W. (Claude Wendell), 1942-; Berk, Herbert; Fitzpatrick, Richard; Hallock, GaryIn this paper, we present the theoretical and numerical studies of the linear characteristics and nonlinear transport features of the instabilities driven by the steep profile gradient and edge current in the pedestal region of the tokamak. Two important instabilities, the peeling-ballooning (P-B) modes (macro-instability) and the drift-Alfven modes (micro-instability), are studied using the fluid analysis and the BOUT++ codes. In particular, the edge-localized modes (ELMs), which appear to be the energy burst in the nonlinear stage of the peeling-ballooning mode, are numerically studied and the results are compared with the experimental measurement. In addition, the features of the impurity transport in the edge region of the tokamak are theoretically analyzed. Firstly, we explore the fundamental characteristics of the P-B modes and the ELM bursts numerically using the three-field reduced MHD model under the BOUT++ framework, in the shifted-circular geometry, i.e. the limiter tokamak geometry. In the linear simulations, the growth rate and real frequency and the mode structure versus the toroidal mode number (n) are shown. The features of the ELM bursts are shown in the nonlinear simulations, including the time evolution of the relative energy loss (ELM size) and the pedestal profile. Secondly, two original research projects related to the P-B modes and the ELM burst are described. One is the study of the scaling law between the relative energy loss of ELMs and the edge collisionality. We generate a sequence of shifted-circular equilibria with different edge collisionality varying over four orders of magnitude using EFIT. The simulation results are in good agreement with the multi-tokamak experimental data. Another is the study of the differences of the linear behaviors of the P-B modes between the standard and snowflake divertor configurations. Using DIII-D H-mode ElMing equilibria, we found that the differences are due to the local magnetic shear change at the outboard midplane, which is the result of the realization of the snowflake configuration. Finally, the micro-instability, the drift-Alfven instability in the pedestal region of the DIII-D tokamak is studied. A modified six-field Landau fluid model under BOUT++ framework is used to study the linear characteristics and transport features of the drift-Alfven modes. Based on the DIII-D H-mode discharge, a sequence of divertor tokamak equilibria with different pedestal height is generated by the ’VARYPED’ tool for our studies. Qualitative agreement is obtained between theoretical analysis and the simulation results in the linear regime. Moreover, the heat transport induced by the drift-Alfven turbulence is explored and the convection level is estimated for both ions and electrons.Item Stability and turbulence characteristics of a spiraling vortex filament using proper orthogonal decomposition(2015-05) Mula, Swathi Mahalaxmi; Tinney, Charles Edmund, 1975-The stability and turbulence characteristics of a vortex filament emanating from a single-bladed rotor in hover are investigated using proper orthogonal decomposition. The rotor is operated at a tip chord Reynolds number and a tip Mach number of 218,000 and 0.22, respectively, and with a blade loading of CT /σ = 0.066. In-plane components of the velocity field (normal to the axis of the vortex filament) are captured by way of 2D particle image velocimetry with corrections for vortex wander being performed using the Γ1 method. Using the classical form of POD, the first POD mode alone is found to encompass nearly 75% of the energy for all vortex ages studied and is determined using a grid of sufficient resolution as to avoid numerical integration errors in the decomposition. The findings reveal an equal balance between the axisymmetric and helical modes during vortex roll-up which immediately transitions to helical mode dominance at all other vortex ages. This helical mode is one of the modes of the elliptic instability. While the snapshot POD is shown to reveal similar features of the first few energetic modes, the classical POD is employed here owing to the easier interpretation of the Fourier-azimuthal modes. The spatial eigenfunctions of the first few Fourier-azimuthal modes associated with the most energetic POD mode are shown to be sensitive to the choice of the wander correction technique used. Higher Fourier-azimuthal modes are observed in the outer portions of the vortex and appeared not to be affected by the choice of the wander correction technique used.Item Wettability & coalescence modulation of water droplets through surface engineering, surfactants and electrowetting(2022-04-11) Lokanathan, Manojkumar; Bahadur, Vaibhav; Bogard, David; Mohanty, Kishore; Wang, Yaguo; Hajimirza, ShimaFluidic separation of two or more immiscible fluids is a key process in several applications. While oil-water separation has been extensively studied, there remain significant avenues for further improvement in the effectiveness, energy consumption and speed of separation. This dissertation includes multiple fundamental studies on the influence of surface engineering (texture and chemistry), surfactants and electric fields towards enhancing separation by controlling wettability of droplets and droplet coalescence. The first task (Chapter 3) details a study of wettability of water (in oil) and oil (in water) on sub-millimeter/micro/nano textured surfaces fabricated on a variety of substrates (metals, polymers, elastomers). Importantly, all the fabrication processes employed involved non-cleanroom-based scalable techniques. Textured metal surfaces coated with Teflon AF were superhydrophobic (in oil) with very low roll-off angles (4°–7°). Uncoated textured metal surfaces were superoleophobic (in water) with roll-off angles of 3°–9°. Secondly, textured polymer and elastomer surfaces exhibited ultrahydrophobicity (in oil); however not all textured elastomers exhibited superoleophobicity (in water). Thirdly, droplet roll-off was not observed on any textured elastomer and polymer surface, despite very favorable contact angles, indicating that high contact angles do not always translate to superhydrophobicity/oleophobicity. Chapter 4 analyzes and quantifies the extent of wettability alteration of water droplets on a hydrophobic surface (in air) via the use of surfactants and electrowetting (EW). Nine surfactants were chosen from the categories of anionic, cationic and zwitterionic surfactants. EW further enhanced wettability of surfactant solutions, and further reduced the contact angle (CA) by as much as 35°. Interestingly, it was seen that the influence of EW in enabling CA reduction was reduced by the addition of surfactants at pre-CMC (critical micelle concentration) levels. Conversely, surfactants strengthened the influence of EW at higher concentrations. Finally, it was seen that at post CMC concentrations, the saturation contact angles were independent of surfactant concentrations. Chapter 5 analyses dielectrophoretic (DEP) control of a water droplet at the interface of two other immiscible liquids. An analytical model was developed which balances gravity, buoyancy, capillary, and dielectrophoretic forces to predict the change in the position of the droplet and the immersion angle. Experiments and analysis were conducted for Bond numbers ranging from 0.1 to 1.7, the latter being the critical size at which a droplet will ‘sink’ due to its weight. The predicted immersion angles and threshold voltage showed a good match with the measurements. Chapter 6 studies the influence of surfactant concentration, applied voltage, frequency and electrode geometry (spacing) on surface electrocoalescence for micron-scale water droplets in hydrocarbon media. Phase maps were developed for various electrocoalescence possibilities to identify the parameter space for significant coalescence using three dimensionless parameters: i) modified electric capillary number (Ca [superscript asterisk over subscript e], ii) frequency (τ), and iii) surfactant concentration (C*). Electrocoalescence effectiveness was quantified using the parameter (δ/α): δ is the droplet density/area and α is the fraction of surface not covered by droplets. Strong coalescence (no surfactant) corresponded to δ/α < 10 droplets/mm², with best-case δ/α = 1.6 droplets/mm², with no droplets < 20 µm diameter and electrocoalesced droplets as large as 750 µm. With surfactant, electrocoalescence weakened; parameter space for strong electrocoalescence progressively reduced with concentration. Nonetheless, electrocoalescence at all concentrations resulted in substantial radius enhancement (after/before electrocoalescence); measured ratio ranged from 3.1-6.3 in the parameter space of Ca [superscript asterisk over subscript e]: 3.3-4.9, and τ ≤ 1.25 ∗ 10⁻². This study also characterized droplet generation (via satellite droplet ejection (SDE)) of 2-10 µm radii droplets. SDE was seen to scale with voltage, frequency and concentration, and inversely with electrode spacing. Overall, it was shown that water droplets can be coalesced or generated using the same microfluidic device; the parametric space to enable fluidic separation and droplet generation was identified. Chapter 7 models the microfluidic system discussed in Chapter 6 using machine learning (ML) algorithms, such as artificial neural network (ANN), eXtreme gradient boosting (XGBOOST) and polynomial regression. Features such as voltage, frequency, electrode spacing, concentration and initial droplet density normalized with uncovered area ratio (δᵢ/αᵢ), were utilized to predict nine targets: uncovered area ratio (α [subscript f]), final droplet density normalized with uncovered area (δ [subscript f]/α [subscript f]), and seven droplet density distribution (radius) bins ranging up to 500 µm. The ANN was the most accurate and consistent among the three ML models with R² of 0.89. The model accurately predicted the droplet distribution bins for three distinct test cases consisting of good coalescence, poor coalescence and satellite droplet ejection (droplet generation). SHAP (Shapley Additive exPlanations) dependence plots highlighted the parametric influence of the features for each output. Overall, this dissertation has led to significant contributions in the field of droplet coalescence and generation. This multidisciplinary work has involved experiments, analytical modeling, numerical simulations and statistical modeling. The results show that surface engineering, surfactants and EW, in conjunction, offer powerful approaches to enhance droplet wettability and coalescence. This research impacts applications in energy (oil-water separation, enhanced oil recovery), pharmaceutical (droplet emulsion generation) and infrastructure (municipal and industrial water treatment, oil spills) areas.