VictorJuiz/thesis huggingface dataset:
A multi-dimensional flamelet model for ignition in multi-feed combustion systems [electronic resource]
Keywords: Aeronautics and Astronautics

This work develops a computational framework for modeling turbulent combustion in multi-feed systems that can be applied to internal combustion engines with multiple injections. In the first part of this work, the laminar flamelet equations are extended to two dimensions to enable the representation of a three-feed system that can be characterized by two mixture fractions. A coupling between the resulting equations and the turbulent flow field that enables the use of this method in unsteady simulations is then introduced. Models are developed to describe the scalar dissipation rates of each mixture fraction, which are the parameters that determine the influence of turbulent mixing on the flame structure. Furthermore, a new understanding of the function of the joint dissipation rate of both mixture fractions is discussed. Next, the extended flamelet equations are validated using Direct Numerical Simulations (DNS) of multi-stream ignition that employ detailed finite-rate chemistry. The results demonstrate that the ignition of the overall mixture is influenced by heat and mass transfer between the fuel streams and that this interaction is manifested as a front propagation in two-dimensional mixture fraction space. The flamelet model is shown to capture this behavior well and is therefore able to accurately describe the ignition process of each mixture. To provide closure between the flamelet chemistry and the turbulent flow field, information about the joint statistics of the two mixture fractions is required. An investigation of the joint probability density function (PDF) was carried out using DNS of two scalars mixing in stationary isotropic turbulence. It was found that available models for the joint PDF lack the ability to conserve all second-order moments necessary for an adequate description of the mixing field. A new five parameter bivariate beta distribution was therefore developed and shown to describe the joint PDF more accurately throughout the entire mixing time and for a wide range of initial conditions. Finally, the proposed model framework is applied in the simulation of a split-injection diesel engine and compared with experimental results. A range of operating points and different injection strategies are investigated. Comparisons with the experimental pressure traces show that the model is able to predict the ignition delay of each injection and the overall combustion process with good accuracy. These results indicate that the model is applicable to the range of regimes found in diesel combustion.
Multifunctional graphitic carbon nanomaterials for imaging, drug delivery and photothermal therapy [electronic resource]
Keywords: Chemistry

In recent years there has been a growing interest in utilizing nanomaterials for drug delivery and biomedical imaging applications. This work focuses on the development of two multifunctional graphitic-carbon based nanomaterials capable of acting as both drug delivery agents and as contrast agents for either magnetic resonance imaging or near-infrared fluorescence imaging. Both of these agents heat under near-infrared light and are capable of loading chemotherapy drugs making them multifunctional in nature. The first material discussed is a FeCo-graphitic carbon nanocrystal loaded with doxorubicin. Addition of near-infrared photothermal therapy significantly increases the cellular toxicity of these nanocrystals in vitro. Treatment of breast cancer tumors in mice using combined nanocrystal drug delivery and photothermal therapy resulted in complete tumor regression in 45% of mice. The imaging capability of these nanocrystals is demonstrated through high-resolution magnetic resonance imaging of microvessels in rabbits. The potential long-term biodistribution and safety of this material is evaluated. The second graphitic-carbon nanomaterial used in this work is single-walled carbon nanotubes. This material is developed as a deep-tissue fluorescent imaging agent due to their inherit photoluminescence beyond 1 micron. This light emission is demonstrated to be particularly useful for in vivo imaging by minimizing light scattering by tissues leading to crisp anatomical resolution.
Multiphase reference signal generation using coupled oscillators [electronic resource]
Keywords: Electrical Engineering

The continuing trend toward higher frequencies of operation poses formidable challenges in the design of multiphase reference signals at mm-wave frequencies and beyond. Conventional multiphase reference generation techniques face serious implementation or performance challenges when scaled to microwave and mm-wave frequencies. Ring oscillators suffer from poor phase noise, and hence fail to fulfill the stringent requirements of most wireless applications. Generating multiphase signals by dividing the output of an oscillator operating at multiples of the intended frequency of operation is impractical when frequencies approach the mm-wave range. Cross-coupled LC oscillators have been explored as a promising alternative for multiphase and, in particular, quadrature generation. However, the frequency ambiguity that results from multiple modes of operation, as well as the severe phase noise degradation due to their inherent off-resonance operation, has inhibited their utilization in practice. This work introduces a new topology for coupled oscillators that solves the frequency ambiguity issue and mitigates phase noise degradation in coupled oscillators by employing an array of LC oscillators that are coupled in a bidirectional fashion. The proposed bidirectional coupling enforces operation at the resonance frequency of the LC tanks of the oscillator in the loop, a property that proves to be key in solving both the aforementioned issues. A quadrature frequency doubling topology using bidirectionally-coupled oscillators is also presented. The proposed approach relaxes the linearity requirements on the mixers employed in the circuit, thus allowing the frequency doubler to use highly nonlinear mixers. An experimental prototype integrated in a 90-nm CMOS technology provides output phases in increments of 45 degrees and achieves a phase noise of −101 dBc/Hz at 1- MHz offset from a 19.6-GHz carrier. The quadrature 40-GHz signal generated on chip drives a single-sideband transmitter that achieves a sideband suppression of better than 45 dB.
Multiple systems support the neural representation of value over time [electronic resource]
Keywords: Psychology

Time, when prolonged to engender delays, has a dramatic impact on the way we value events and experiences in the world. Long delays make future rewards less attractive and make memories more difficult to retain. Thus, in the following series of studies, I investigate how time can be used as a manipulation to study the underlying neural processes that support valuation, learning, and decision making. Experiment 1 investigated how delays impact neural representations of value during intertemporal choice, where subjects chose between smaller, immediate rewards and larger, delayed rewards. Using FMRI, we dissociated neural representations of subjective value for delayed rewards according to reward magnitude and delay time. We found that mesolimbic dopamine regions correlated with the magnitude of future rewards, while dissociable activation in lateral cortical regions correlated with the delay of future rewards. The findings suggest that considering rewards far into the future recruits deliberative cognitive control processes in addition to the reward valuation processes of the mesolimbic system. Even when the timing of outcomes is not made explicit, the experience of delays during learning or decision making may affect how values are calculated and updated. Thus, Experiment 2 explored how the introduction of delays into an instrumental reward learning task affects which neural structures and learning strategies are recruited to support successful learning. We found that both mesolimbic (MPFC and NAcc) and declarative (MTL and lateral PFC) systems were recruited to support prediction error and value signals for learning over delay periods. Additionally, we provided evidence that value signals in the MTL were dependent on attention and working memory processes, as a secondary distraction task disrupted value signals in the MTL but not in the MPFC. The results demonstrate that reinforcement learning over delay periods requires shared activation between mesolimbic dopamine regions and declarative memory systems. Finally, Experiment 3 examined more directly how experiencing delays during learning diminishes explicit assessments of value. Participants learned the values associated with different visual cues by playing a repeated gambles game, experiencing reward outcomes that varied in reward magnitude, probability, and delay until delivery. We found that cues associated with longer delays were perceived as less valuable than cues with shorter delays, as indicated by subjects' explicit recall of reward probabilities. Delays also biased subjects' choices towards more risk-averse preferences. These finding suggest that the experience of delays diminishes assessments of value and biases choice behavior, even when people are not explicitly aware of delay times. Taken together, the present studies provide evidence that delays influence the neural processes underlying learning, valuation, and choice, and they suggest new applications for leveraging the timing of events to shift learning strategies and bias decision making heuristics.
Multiple tools reveal the reliance of mobile species on sensitive habitats: a case study of manta rays (Manta alfredi) and lagoons
Keywords: Stanford Biology Department
Palmyra Atoll
acoustic imaging
anthropogenic
habitat
isotopes
lagoon
Manta alfredi
mobile
photo identification
spatial ecology
tracking
Thesis

1.) Quantifying the ecological importance of individual habitats to highly mobile animals – and assessing the significance of their loss – is challenging because patterns of habitat reliance for wide-ranging animals are complex and difficult to observe directly.
2.) We investigated the importance of lagoons to the manta ray, Manta alfredi, a highly mobile and vulnerable species. Lagoons are highly vulnerable to anthropogenic disturbance and we hypothesized that they may provide habitat of particularly high ecological value for this species.
3.) To overcome the challenges associated with classifying patterns of habitat reliance for this mobile species we used a novel combination of research tools, each of which contributed unique insight into the dependency of manta rays on lagoons. High resolution tracking data provided information on how manta rays utilize space in lagoons; acoustic cameras logged patterns of animal entrance and departures from this habitat; stable isotope analysis provided the capacity to assay the habitat’s energetic importance; and photo identification/laser photogrammetry recorded key biological attributes about the population of habitat users.
4.) Cumulatively, these diverse methods provided a highly resolved understanding of manta ray reliance on lagoons. Manta rays spent long periods of residence within lagoons or frequently transited into them from other habitats and showed strong evidence of energetic dependence on lagoon resources. Within the lagoon habitat, they demonstrate affinities for key features in lagoons and show some temporal patterning in habitat utilization.
5.) This work demonstrates the value of using multiple, diverse research tools to properly understand the reliance of wide-ranging animals on focal habitats. This integrative approach can help visualize cryptic patterns of habitat reliance that will be critically important to guiding the strategic management and conservation of manta rays and other imperiled, highly mobile species.
Multiple unemployment equilibria under non-representative agent demand [electronic resource]
Keywords: Business.

This dissertation analyzes the possibility that high unemployment can be be self-reinforcing through its effects on the distribution of income and, consequently, aggregate demand. The major results of my work demonstrate that these forces can lead to multiple macroeconomic equilibria which are indexed by the level of unemployment. By characterizing low output and employment states as equilibrium outcomes, this thesis provides a coherent theoretical explanation for periods of persistently high unemployment. The key property throughout is non-homotheticity of consumers' preferences, whereby the market demand for goods changes as income distribution changes, even if the total income of all consumers remains fixed. This implies that, unlike in many macroeconomic models, demand cannot be generated from a representative agent and, crucially, that changes in employment along the extensive, as opposed to intensive, margin can have more severe general equilibrium consequences.
Multi-terminal secrecy and source coding [electronic resource]
Keywords: Electrical Engineering

Network Information Theory is a branch of Information Theory that aims to address the fundamental limits of communications in a multiple user setting and when several sources of information are available. While a complete theory is still lacking, tools and techniques developed to address research problems in Network Information Theory have started to make an impact on other fields. In this thesis, we present results in two sub-areas in Network Information Theory. The first area, Information-Theoretic Secrecy, involves an application of Network Information Theory techniques to a secret communication setting, where a message is to be kept hidden from an eavesdropper. We consider two generalizations of the classical wiretap channel, where a sender wishes to communicate to a receiver over a noisy broadcast channel in the presence of an eavesdropper. The first generalization considers the setting where we have more than one receiver or more than one eavesdropper. The second setting considers the case where the transmission occurs over a broadcast channel with state - a channel model that can serve a base model for communicating over fast fading channels found in wireless communications. In the second part of this thesis, we turn our attention to the area of multi-terminal source coding. In particular, we investigate the Cascade Source Coding problem where a source node has a source sequence that it wishes to send to an intermediate node over a rate limited link, and also to an end node that the intermediate node can communicate with over a rate limited link. We investigate the optimum coding schemes under different assumptions on the side information available at the nodes, or operational restrictions on the reconstruction process.
Nanoelectromechanical relays for low power applications [electronic resource]
Keywords: Electrical Engineering

CMOS scaling has been very successful in generating small, fast, low cost electronics. However, in advanced CMOS nodes, the total power consumption is dominated by the static power dissipation, which is caused greatly by gate leakage, short channel effects, and finite subthreshold slope. Further scaling of CMOS only exacerbates these problems. Nanoelectromechanical (NEM) relays are promising devices for assisting CMOS systems by reducing the static power dissipation due to their zero leakage current, infinite subthreshold slope, and scalable actuation voltage. Electrostatically-actuated NEM relays are devices where the operation is based on the deformation of a flexible beam under the influence of electrostatic force in order to create a conducting path between two electrodes. This work studies the fabrication process development of sidewall-coated laterally-actuated NEM relays. The developed process enables decoupling of the mechanical and electrical properties of the relay, allowing independent optimization of each property and paving the path for creating a back-end-of-line (BEOL) compatible process. Furthermore, a major failure mechanism of NEM relays is beam-to-gate shorting after actuation. To ameliorate this problem, new designs with improved mechanical properties were simulated and tested. These designs utilize a stiff electrode and a compliant beam to eliminate undesired beam deformation near the gate electrode. These results in addition to variation studies, stress outcomes, and basic logic functionality of the NEM relays are shown.
Nanopillar structures for cellular interface [electronic resource]
Keywords: Materials Science and Engineering

Abstract The small scale of nano-materials makes them one of the best man-made candidates to interact with biological systems at subcellular or even molecular level. It has been the focal point of the research interests to interfacing live cells with one dimensional nanostructures, such as nanowires and nanopillars. In my Phd research, I have utilized nanopillar based structures and devices to interface biological cells electrically, optically and mechanically. 1. We achieve improved electric interface between biological cells and solid state device by using arrays of vertically aligned nanopillar electrodes. Their tight attachment to the cell membrane allows us to acquire intracellular-like action potential signals non-destructively from cultured cardiomyocytes, which is responsible for various important cellular functions. 2. We demonstrate below-the-diffraction-limit observation volume in vitro and inside live cells by using vertically aligned silicon dioxide nanopillars. With a diameter much smaller than the wavelength of visible light, a transparent silicon dioxide nanopillar embedded in a nontransparent substrate restricts the propagation of light and affords evanescence wave excitation along its vertical surface. This effect creates highly-confined illumination volume that selectively excites fluorescence molecules in the vicinity of the nanopillar. We show that this nanopillar illumination can be used for in vitro single molecule detection with high fluorescence background. In addition, we demonstrate that vertical nanopillars interface tightly with live cells and function as highly localized light sources inside the cell. Furthermore, chemical modification of the nanopillar surface provides a unique way to locally recruit proteins of interest and simultaneously observe their behavior within the complex, crowded environment of the cell. 3. We engineer and fabricate vertically nanopillar arrays, and culture various types of cells atop. We study the cell growth pattern in presence of nanopillar arrays, including attachment, migration, etc. We also design patterned nanopillar arrays and utilized them to guide and control cell growth via cell-nanopillar interaction.
Nanoscale germanium crystal growth and epitaxy control for advanced electronics and solar cells [electronic resource]
Keywords: Materials Science and Engineering

Semiconductor crystal growth at the nanoscale and integration of different materials systems are central themes of materials research. They enable novel materials processes and device applications, and may shape the landscape of future technologies. A major challenge is growth of high-quality single crystal semiconductors (e.g. Ge) on large-mismatch (e.g. Si) and non-crystalline (e.g. glass) substrates, while managing the thermal constraints of the underlying substrates. As-grown vertical semiconductor nanowires have been demonstrated as sensors, and nanoelectronic and nanophotonic devices. However, little attention has been paid to their unique structural properties: vertical Ge nanowires can be epitaxially grown on (111)-oriented Ge and Si substrates. In my thesis, I will focus on nanowire-seeded crystallization and metal-induced crystallization to realize three-dimensional integration and nanostructured solar cells. I have also studied the thermal stability of core-shell nanowire heterostructures, which represent a new type of epitaxy control in a nanoscale coaxial geometry. Fundamental aspects of crystal growth at the nanoscale will be discussed.
Natural variation and evolved trade-offs in yeast carbon metabolism [electronic resource]
Keywords: Genetics

The processes by which the budding yeast Saccharomyces cerevisiae metabolizes carbon sources by both fermentation and respiration have been studied for more than a century. Yeast metabolism has been used both industrially, for the production of important molecules such as ethanol, and as a model for basic scientific research. Applied scientists have studied yeast metabolism to create and optimize novel metabolic phenotypes not naturally found in Saccharomyces yeasts. In parallel, basic scientists have used yeast as a model to understand fundamental processes such as evolutionary adaptation, as well as the pathways of carbon metabolism themselves. There are many unanswered questions in both of these fields, some of which I have addressed in this work. With respect to the industrial importance of yeast, I asked whether there are naturally existing Saccharomyces yeasts that can metabolize the five-carbon sugars important for lignocellulosic ethanol production (such as xylose), and, if so, what is the genetic basis for their phenotypes? Having characterized natural genetic variation in xylose metabolism, I also wanted to understand something more fundamental about how carbon metabolism can adapt, including the molecular nature of adaptations to selection on a limiting carbon source. Specifically, I asked what is the niche breadth of, and are there genetic trade-offs in, yeast that have been evolved under glucose-limitation? I have used a combination of classical genetics, physiology, and high-throughput genomics to answer these two questions. I have discovered novel xylose-utilizing Saccharomyces yeasts and have shed considerable light on the genetic basis for their phenotypes. In addition, I have discovered at least one trade-off for adaptation to limiting glucose, namely that amplification of the hexose-transporter genes HXT6 and HXT7 causes reduced fitness in carbon-rich environments. These two projects highlight two major spheres of Saccharomyces research, and they provide key answers to outstanding questions in both fields.