VTechWorks
VTechWorks provides global access to Virginia Tech scholarship, including journal articles, books, theses, dissertations, conference papers, slide presentations, technical reports, working papers, administrative documents, videos, images, and more by faculty, students, and staff. Faculty can deposit items to VTechWorks from Elements, including journal articles covered by the University open access policy. Email vtechworks@vt.edu for help.
Open Access Policy
Virginia Tech's open access policy enables researchers to deposit the accepted version of scholarly articles with no embargo.
Theses and Dissertations
Virginia Tech was first in the world to require ETDs in 1997, and continues to add scans of older theses and dissertations.
Open Textbooks
More than 40 freely available and openly licensed textbooks are among our most downloaded items.
Communities in VTechWorks
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Recent Submissions
Fundamental Investigations into the Metal-Organic Framework Redox-Hopping Charge Transport——Mechanisms and Improvement Strategies
Yan, Minliang (Virginia Tech, 2024-10-16)
Redox hopping is the dominant charge transport mechanism in many catalyst-modified metal-organic frameworks (MOFs). Previous studies have shown that ion diffusion is the rate-determining step of redox hopping, but the realization regarding to the fundamental mechanism of redox hopping in MOF is still infantile. In this dissertation, we will discuss the redox hopping process in MOFs from multiple perspectives, including how to use the Scholz model to analyze the coefficients in redox hopping, the influence of the type of carrier, the influence of electrolyte concentration, and the influence of temperature on redox hopping, so as to try to reveal the mechanism of the redox hopping process and make some constructive suggestions for the future design and application of MOF based on this topic.
Gender, Race, Marriage, and Health in Later Life
Geng, Jing (Virginia Tech, 2024-10-15)
Research on health in old age finds gender and racial differences in physical and mental health and points to several social factors that can influence health in later life, including marriage.
However, it remains unclear whether the health impacts of marriage differ between men and women or across racial groups in later life. Using the Health and Retirement Study, this project aims to explore the impact of both marital status and marital history on health outcomes, and how gender and race impact these effects. To capture a comprehensive view of health, this study incorporates a wide range of measures, addressing both physical and mental health. Using the gender relations framework, Chapter 2 examines the impact of marital status and history on gender differences in physical health outcomes, including self-rated health, functional limitations, and chronic conditions, of older Americans; Chapter 3 explores the influence of marital status and history on gender differences in mental health outcomes, including life satisfaction, positive affect, depression, and alcohol consumption, of older Americans. Along with the intersectionality framework, Chapter 4 investigates the effect of gender and race intersections on the relationship between marital status, marital history, and self-rated health of older Americans. The results highlight the critical need to consider both gender and race when evaluating the impact of marriage on health outcomes in later life.
Reduced Order Modeling for Efficient Stability Analysis in Structural Optimization
Sanmugadas, Varakini (Virginia Tech, 2024-10-15)
Design optimization involving complex structures can be a very resource-intensive task. Convex optimization problems could be solved using gradient-based approaches, whereas non-convex problems require heuristic methods. Over the past few decades, many optimization techniques have been presented in the literature to improve the efficiency of both these approaches. The present work focuses on the non-convex optimization problem involving eigenvalues that arises in structural design optimization. Parametric Model Order Reduction (PMOR) was identified as a potential tool for improving the efficiency of the optimization process. Its suitability was investigated by applying it to different eigenvalue optimization techniques. First, a truss topology optimization study was conducted that reformulated the weight minimization problem with a non-convex lower-bound constraint on the fundamental frequency into the standard convex optimization form of semidefinite programming. Applying PMOR to this, it was found the reduced system was able to converge to the correct final designs, given a reduced basis vector of suitable size was chosen. At the same time, it was shown that preserving the sparse nature of the mass and stiffness matrices was crucial to achieving reduced solution times. In addition, the reformulation to convex optimization form, while possible with the discretized form of vibrational governing equations, is not straightforward with the buckling problem. This is due to the non-linear dependence of the geometric stiffness matrix on the design variables. Hence, we turned to a metaheuristic approach as an alternative and explored the applicability of PMOR in improving its performance. A two-step optimization procedure was developed. In the first step, a set of projection vectors that can be used to project the solutions of the governing higher-order partial differential equations to a lower manifold was assembled. Invariant components of the system matrices that do not depend on the design variables were identified and reduced using the projection vectors. In the second (online) step, the buckling analysis problem was assembled and solved directly in the reduced form. This approach was applied to the design of variable angle tow (VAT) fiber composite structures. Affine matrix decompositions were derived for the linear and geometric stiffness matrices of VAT composites. The resulting optimization framework can rapidly assemble the reduced order matrices related to new designs encountered by the optimizer, perform the physics analysis efficiently in the reduced space, evaluate heuristics related to the objective function, and determine the search direction and convergence based on these evaluations. It was shown that the design space can be traversed efficiently by the developed PMOR-based approach by ensuring a uniform error distribution in objective values throughout the design space.
Optimizing Hydroponic Lettuce Yield: Evaluation of Potassium Source and Concentration with Plant Imaging
Doss, Mitchell C. (Virginia Tech, 2024-10-15)
Controlled Environment Agriculture (CEA) is an expanding industry in the United States. Lettuce (Lactuca sativa) possesses considerable economic importance as one of the foremost leafy vegetables worldwide in terms of financial value for production in CEA. Rapid growth rate, high yield, and potential return further enhance its suitability for cultivation within CEA. Pursuing optimal and sustainable nutrient concentrations for crops in CEA is a continuous requirement for an expanding industry. This study is set to investigate the impact of different potassium (K) sources and concentrations on 'Green Oakleaf' lettuce growth. The K concentration within the Virginia Tech base hydroponic lettuce fertilizer was increased,1x, 2x, and 3x the normal rates of K ppm, through the addition of potassium chloride (KCl) or potassium sulfate (K2SO4). To account for rising electrical conductivity (EC), due to the increase of K, KCl treatments were recreated with standardized EC through the addition of Sodium Chloride (NaCl). The Spatially and Mechanically Accurate Robotic Table (SMART) in an indoor space was utilized to cultivate and image plant growth over time through pixel count. Growth index and chlorophyll content were recorded weekly, while the fresh weight and dry weight of shoots were recorded at harvest. The KCl treatment with the highest concentration,4x K ppm, as well as those with standardized EC, led to reduced yield when compared to other treatments. All K2SO4 treatments showed no difference in growth when compared to the base solution. K source and concentration, as well as fertilizer EC, were all identified as factors influencing the impact of fertilizers on the optimization of lettuce yield.
Helping Developers Migrate their Code across Programming Languages
Elarnaoty, Mohammed Elsayed (Virginia Tech, 2024-10-15)
Migrating source code from one programming language to another is a common task in software development.
This migration can be done by completely rewriting the code in the target language, or it can be facilitated through code-reuse or automation techniques.
This thesis explores both approaches.
For code-reuse, two new cross-language code search techniques are proposed that enable developers to search for code in one language using code from another.
These techniques address the limitations of existing methods in the context of code migration.
The first technique leverages a Siamese network combined with Word2Vec embeddings, while the second employs transformers.
For code automation, the concept of Translation Types is introduced to categorize code translations.
An empirical study was conducted to analyze the differences between human-translated and machine-translated code.
Based on these findings, two multi-output code translation techniques were developed that produce multiple translations aligned with the different styles that developers use when translating their code.
The first tool employs a denoising autoencoder and a blueprint-guided beam search algorithm to generate translations of specific types.
This algorithm mimics the translation operations that developers apply in similar software projects.
The second tool utilizes GPT-4 with a specialized prompt to generate translations tailored to the requested types.
In the evaluation, these approaches produced automated code translations that better aligned with developer preferences while maintaining correctness compared to existing methods.