Doctoral Dissertations

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A Pseudo-Binary Cascaded H-bridge Converter for Solid-State Transformer Applications and Modulation Techniques for the Minimization of the Common-Mode Voltage
Gutierrez Suarez, Bryan Ciro (Virginia Tech, 2024-11-20)
The trend in power electronics converters is to be highly efficient, compact, reliable, and cost-effective. Conventionally, power converters supply or consume power from the low-voltage utility, which impacts the size and efficiency of the system. For example, the recent proliferation of electric vehicles has demanded the rapid installation of dc fast chargers (DCFC) across the country. However, most of the commercial DCFCs operate at 480 V transferring hundreds of kilowatts, resulting in large line currents which could hinder the size, cost, and efficiency of conventional DCFCs. Conversely, modular multilevel power topologies can be directly tied to the medium-voltage (MV) grid, eliminating the line-frequency transformer and the bulky line cable requirements. Among these topologies, the cascaded H-bridge (CHB) has been extensively used in the industry for MV-high-power applications because of its cost and efficiency in this operation range. Thus, it is one of the prevalent topologies for MV solid-state transformers (SSTs) The asymmetrical hybrid binary CHB (HBCHB) allows increased output voltage levels at the expense of modularity. Based on the HBCHB, a converter new modular topology regarded as the pseudo-binary CHB (PBCHB) is proposed for the ac-dc front-end stage SSTs. To operate the PBCHB, a new hybrid modulator is developed to operate the three modular structures of the PBCHB with step-like sinusoidal waveforms at near-line-frequency commutations while an asymmetrical floating capacitor (FC) module operates at high-frequency PWM commutation. The FC module does not transfer active power but serves only as a power quality enhancer of the PBCHB. However, the modular structures symmetrically transfer all the power from the MV grid to the load. With the SST structure of the PBCHB, the dc-link voltages of the H-bridges are naturally balanced; yet the proposed hybrid modulator enables equal power transfer in the three modular structures. In addition, a controller for the FC voltage is designed, analyzed, and implemented in the proposed hybrid modulator The effectiveness of the proposed front-end SST with the proposed modulation and control technique is verified in a 1.2 kV/3 kW single-phase prototype, where each module was able to transfer 1kW each. Electromagnetic interference (EMI) also impacts the cost, size, and reliability of three-phase systems because they may require bulky EMI filters to avoid self-pollution and polluting the grid. The common-mode voltage (CMV) is one important factor of EMI emissions. Thus, reducing or eliminating it could improve the cost and size of the system. Space vector pulsewidth modulation (SVWPM) can directly design the CMV output and the switching sequence of three-phase converters. However, its implementation can become complex in converters with many levels such as MV grid-tied SST converters. This dissertation uses the digital gh coordinate and proposes a set of computations to easily retrieve the converter states with a reduced CMV and generate a symmetrical switching sequence with reduced number of commutations. To do this, a single vector among the nearest three vectors (NTVs) is sufficient to implement the switching sequence for the reduced commutation and reduced CMV SVPWM. Additionally, the dc bus can be fully utilized. Unlike conventional approaches, the developed technique is easily scalable because its computational complexity does not depend on the number of levels of the converter. The proposed reduced CMV technique was verified in a three-phase 15-level 311 V/600 W unit. Moving forward in the objective of CMV reduction, a new jk-coordinate system for multilevel converters is proposed for SVPWM with eliminated CMV. With the jk coordinates, the converter states that yield zero CMV (ZCMV) can be directly computed. In addition, a single jk vector is sufficient to generate the switching sequences of NTVs. Moreover, the switching sequences feature reduced losses for high-power-factor applications in the phase that naturally commutes twice during a sampling period. Similarly, the computation burden of the ZCMV SVPWM technique presented in this dissertation is not affected by the number of levels of the converter, thus, it is scalable. The three-phase 15-level 311 V/600 W prototype was utilized to verify this technique.
An Examination of High School Teacher Experiences with STEAM Curriculum Implementation
Mitchell, Anza Laquetta (Virginia Tech, 2024-11-19)
In the United States, there has been a growing focus on using science, technology, engineering, and mathematics to increase global competitiveness. Despite increased attention to these subjects and funding through the years, American students still fall behind students in other countries in math and science achievement. STEAM (science, technology, engineering, arts, and mathematics) education has been proposed as a reform effort to address this problem. Designed to prepare students for the interdisciplinary demands of the 21st-century workforce, STEAM education aims to incorporate art and design principles into science, technology, engineering, and mathematics subjects to foster creativity, critical thinking, and innovation. This study examines the experiences of high school teachers as they implement STEAM. Despite the potential benefits of STEAM to learning and STEM career interest, little is known about the teachers who are an integral part of its implementation. This study aims to fill this gap by examining how high school teachers' backgrounds and social interactions influence their understanding and implementation of STEAM curricula, the roles high school teachers play in developing and modifying said curricula, and the challenges high school teachers face when implementing it. This research was guided by a conceptual framework built upon three theories: social constructivism, teachers as curriculum makers, and barriers to technology integration. Social constructivism emphasizes the importance of social interactions and cultural context in shaping teachers' understanding of STEAM education. Teachers as curriculum makers highlights the active role teachers play in designing and adapting curricula to meet the needs of their students and educational environments. Barriers to technology integration provides a lens for understanding the challenges teachers face in implementing STEAM curricula. Using qualitative methods, including semi-structured interviews and thematic analysis, this exploratory study examined high school teachers' experiences implementing STEAM curricula. The findings reveal that high school teachers often lack formal STEAM pedagogical training, leading them to rely on prior content knowledge, informal learning experiences, and collaborative planning with colleagues to make decisions about implementation. Teachers face significant challenges in implementing STEAM, including a lack of funding, insufficient support from administrators, and difficulties with school timetabling. Despite these challenges, teachers hold positive attitudes toward STEAM and believe in its benefits for student career preparation, skill-building, and exposure to various subjects. The implications of this study highlight the need for targeted professional development programs that address the specific needs of high school teachers, emphasizing both content knowledge and interdisciplinary pedagogical strategies. They also highlight the importance of creating flexible and adaptable STEAM curricula that allow for teacher autonomy and creativity. Furthermore, the findings suggest that schools and districts should prioritize STEAM education by providing adequate funding, resources, and support structures to ensure its successful implementation in high schools. This research contributes to the growing body of literature on STEAM education by providing a detailed exploration of the experiences and challenges faced by high school teachers. It concludes with practical recommendations for educators, policymakers, and curriculum developers to support the effective implementation of STEAM education, aiming to prepare students with the skills and knowledge necessary for success in the 21st century.
Examining Elementary School Leaders' Perceptions of Induction Supports for Provisionally Licensed Teachers
Rios, Diana Frasier (Virginia Tech, 2024-11-18)
The purpose of this study was to examine elementary school leaders' perceptions of the induction programming they provided provisionally licensed teachers. The research question guiding this study was: What are the perceptions and experiences of elementary school leaders in relation to the induction programming and support they are responsible for providing provisionally licensed teachers? The sub-question was: How do elementary school leaders differentiate induction support for provisionally licensed teachers? This research contributes to the literature by presenting school leaders' perceptions of induction support for a growing number of provisionally licensed teachers entering the teaching profession. Qualitative study data were collected from 15 elementary school leaders across Virginia using a demographic survey and semi-structured, one-on-one interviews. A data analysis of leaders' perceptions provided insight on support structures, leaders' responsibilities, and additional support needed for provisionally licensed teachers. The summarized findings indicate that elementary school leaders view mentoring, coaching, and peer observations as crucial support structures for provisionally licensed teachers. The research highlighted the significance of communication, resource management, and workload management in fulfilling school leaders' responsibilities toward these teachers. Further findings emphasized the importance of career development for support staff, fostering a supportive school and community environment, the need for enhanced division-level support, and the necessity for targeted professional development aimed at provisionally licensed teachers. Implications included formalizing mentoring, coaching, and peer observation support, maintaining communication, allocating resources, and cultivating an inclusive school culture to support provisionally licensed teachers. The findings and implications may be helpful to school leaders planning induction support for provisionally licensed teachers.
The Impact of Online Professional Development on Science Teachers' Self-Efficacy and Outcome Expectancy Beliefs for Teaching Engineering Practices
Rushing, Patricia Jean (Virginia Tech, 2024-11-18)
With the growing emphasis on integrating engineering practices into science education, many science teachers are underprepared to meet these demands due to limited training and resources (Katehi et al., 2009; Brophy et al., 2008). Teachers often lack confidence in their ability to teach engineering concepts, making it difficult for them to incorporate these practices effectively (Hammack and Ivey, 2017; Christian et al., 2021). Self-efficacy, defined as a belief in one's ability to succeed in specific tasks, and outcome expectancy, the belief that certain actions will lead to desired results, both play crucial roles in professional development (Bandura, 2006). When teachers possess high self-efficacy and positive outcome expectancy, they are more likely to adopt new teaching practices and engage students in more meaningful ways (Tschannen-Moran and McMaster, 2009; Allinder, 1994). This study aimed to explore the impact of an online professional development module on science teachers' self-efficacy and outcome expectancy for teaching engineering practices. The intervention module was designed to provide mastery experiences, vicarious learning, social persuasions, and emotional/affective support to improve these beliefs. Quantitative results showed no statistically significant changes between the treatment and control groups, but statistically significant improvements in self-efficacy and outcome expectancy for teaching engineering practices in science curricula as a participant cohort overall. Qualitative data revealed that participants in the treatment group reflected more deeply on their teaching practices and reported increased confidence in integrating engineering practices into their curricula.
Spatial Ecology and Conservation Strategies for the Endangered Northern Long-Eared Bat (Myotis septentrionalis) in a Post-White-Nose Syndrome Landscape
De La Cruz, Jesse L. (Virginia Tech, 2024-11-18)
The endangered northern long-eared bat (Myotis septentrionalis) has experienced severe population declines since the 2007 introduction of Pseudogymnoascus destructans, the fungal pathogen that causes white-nose syndrome (WNS). Due to continued mortality, failed recruitment, and range fragmentation, effective species conservation would benefit from local and regional research related to habitat selection and suitability, seasonal activity, and large-scale estimates of continued occupancy across the post-WNS landscape. In this dissertation, I explored maternity colony home range size and habitat selection, both coastal and interior seasonal activity patterns, habitat suitability, and species occupancy trends in the Mid-Atlantic and Northeastern United States. Research from the Coastal Plain of North Carolina, based on diurnal roost locations, revealed that core and peripheral home range estimates were large but comparable to areas of contiguous forest cover in other regions of the species' range. Prior to juvenile volancy, female northern long-eared bats selected red maple (Acer rubrum), water ash (Fraxinus caroliniana), and loblolly pine (Pinus taeda) as day-roosts, but then used sweetgum (Liquidambar styraciflua), swamp bay (Persea palustris), and water tupelo (Nyssa aquatica) after juvenile volancy. At the second-order spatial scale (i.e., home range within a region), roosting home ranges were associated with woody wetlands farther from anthropogenic development and open water. However, within the third-order scale (i.e., core home range within a periphery), northern long-eared bats selected woody wetlands adjacent to intact upland forests, areas containing shorter trees and occurring proximal to open water. Research utilizing passive acoustic monitoring on the Coastal Plain of both Virginia and North Carolina found that northern long-eared bat relative activity was highest in areas containing greater proportions of woody wetlands and upland pine-dominated evergreen forests. Conversely, the likelihood to record northern long-eared bats was associated with smaller proportions of woody wetlands and open water resources, emphasizing the importance of upland forests adjacent to these features. I also observed a higher probability of recording northern long-eared bats during non-winter seasons and when temperatures were between 10 and 25 °C, potentially highlighting an optimal thermoneutral zone for the species regionally. Research using presence data from the Monongahela National Forest (MNF) in the Central Appalachians of West Virginia found that northern long-eared bat habitat, whether occupied or not, was abundant (43.1% of the MNF) and widely dispersed. Northern long-eared bat habitat on the MNF was often characterized as mature, interior mixed mesophytic forests. Research using passive acoustic surveys associated with hibernacula in western Virginia found that northern long-eared bats were most active near hibernacula during warmer weeks of the fall swarm and spring emergence, when rainfall was low. Similarly, the probability of northern long-eared bat activity was highest near hibernacula during the spring/summer season. However, unlike relative activity, the likelihood of recording northern long-eared bats was associated with more heterogeneous, interior forests. Finally, research using a combination of long-term repeated and single-season clearance survey data from New England found that northern long-eared bat occupancy was highest on steep hillsides in contiguous forested landscapes with minimal anthropogenic development. My results also indicated higher occupancy of northern long-eared bats in warmer regions, particularly along the New England coastline and offshore islands. These findings collectively stress the importance of managing areas of large core forest to promote sustainable roost formation and productive foraging areas, often associated with dynamic ecotones, to support the survival and recovery of northern long-eared bats in the post-WNS Mid-Atlantic and Northeast.
Engineered models of the lymphatic stroma to study cell and fluid transport
Hammel, Jennifer H. (Virginia Tech, 2024-11-18)
The lymphatic system plays essential roles in regulating fluid balance and immunosurveillance. Across the body, local lymphatic vessels collect waste in the form of lymph and deliver it to nearby lymph nodes (LNs) to be filtered and screened for pathogens. With broad implications in adaptive immunity, cancer metastasis, and cancer treatment, developing novel in vitro models will provide new platforms to explore lymphatic function in health and disease. This dissertation sought to develop tissue-specific engineered models of the LN stroma and the meningeal lymphatics to examine the transport of cells and fluid. Within the LN, fibroblastic reticular cells (FRCs) maintain a network of extracellular matrix conduits that guide varying rates of interstitial fluid flow (IFF) based on inflammatory state. Eventually, that flow exits the LN through the afferent lymphatics, consisting of lymphatic endothelial cells (LECs). We first developed a spatially organized model of the LN stroma consisting of a monolayer of LECs on the underside of a tissue culture insert and an FRC-laden hydrogel within. We demonstrate that high magnitude IFF (3.0 µm/s) had positive impacts on FRCs but disrupted the integrity of the LEC barrier, and these effects were accompanied by increased secretion of a variety of inflammatory chemokines. We also show that IFF of any magnitude decreased T cell egress from the model. Next, we sought to apply the LN stroma model toward understanding metastasis. LN metastasis is the most important prognostic factor in breast cancer, with size of metastasis informing treatment plan. Metastasis greatly alters the structure of the LN, which in turn alters transport. However, the impact of altered transport on cancer progression is not well understood. We added different numbers of breast cancer cells to our LN stroma model to examine tumor burden. We found that tumor cells invaded the LEC barrier at similar numbers regardless of initial burden. Additionally, at the highest tumor burden, diffusivity in the stroma was significantly decreased. Most excitingly, flow velocity was positively correlated with FRC spread in the hydrogel, demonstrating the contributions of FRCs to transport. Finally, we looked to the central nervous system (CNS). The meningeal lymphatics are responsible for draining cerebrospinal fluid to the cervical lymph nodes for CNS immunosurveillance. We developed a simple model of a meningeal lymphatic vessel lumen consisting of a monolayer of LECs on the underside of a tissue culture insert and a monolayer of meningeal fibroblasts within. This is, to our knowledge, the very first in vitro model of the meningeal lymphatics. We demonstrate that our model has barrier function and is capable of immune cell transmigration and egress. We examined how systemic chemotherapy for breast cancer could cause off-target disruption of the meningeal lymphatics and found that docetaxel was significantly deleterious. We further began to explore leukemia cell behavior in our LN stroma and meningeal lymphatics model. Throughout this dissertation, we emphasize the importance of incorporating fluid and cell transport into engineered models of immunity. These models represent a step toward building up the complexity of in vitro lymphatic models to improve pre-clinical screening and understand pathophysiology.
A study of student affairs educators' and practitioners' perceptions of alternative futures for student affairs programs
Speier, Ronald Paul (Virginia Polytechnic Institute, 1979)
Rare Earth Elements (REEs) Recovery and Hydrochar Production from Hyperaccumulators
Li, Shiyu (Virginia Tech, 2024-11-14)
Phytomining is a promising method for metal recovery, but rare studies have been devoted to metal recovery from hyperaccumulator biomass. The objective of this study was to propose efficient and sustainable methods for treating REE hyperaccumulators, aimed at enhancing REE recovery and obtaining value-added byproducts. Firstly, grass seeds fed with a solution containing Y, La, Ce, and Dy, were found to have the capacity to accumulate around 510 mg/kg (dry basis) of total rare earth elements (TREEs) in grass leaves. With the use of conventional hydrometallurgy, around 95% of Y, La, Ce, and Dy were extracted from the GL using 0.5 mol/L H2SO4 at a solid concentration of 5 wt.%. Subsequently, microwave-assisted hydrothermal carbonization (MHTC) was used to convert the leaching residue into hydrochar to achieve a comprehensive utilization of GL biomass. Scanning electron microscopy (SEM) analysis revealed that the original structure of GL was destructed at 180 °C during MHTC, producing numerous microspheres and pores. As the reaction temperature increased, there was a concurrent increase in carbon content, HHV, and energy densification, coupled with a decrease in hydrogen and oxygen contents of hydrochar. The results showed that the waste biomass of the GL after REE extraction can be effectively converted into energy-rich solid fuel and low-cost adsorbent via MHTC. In addition to utilizing conventional hydrometallurgy for REE recovery and employing MHTC to convert leaching residue into hydrochar, MHTC was also applied to directly recover REEs and produce hydrochar from the GL as a more efficient approach. The effects of acid type and acid concentration on REE extraction from GL using MHTC were investigated. The utilization of 0.2 mol/L H2SO4 led to the extraction of nearly 100% of REEs from the GL into the resulting biocrudes. Concurrently, the acid-mediated MHTC system also caused the degradation of amorphous hemicellulose and crystalline cellulose present in the GL, thereby enhancing the thermal stability of the resulting hydrochar. The physiochemical properties of the hydrochar were also influenced by acid type and acid concentration. Using 0.2 mol/L H2SO4 as the reaction medium, MHTC resulted in a yield of 28% hydrochar with enhanced high heating value and energy densification. These results suggest that MHTC in the presence of an appropriate concentration of H2SO4 is an effective way to extract REEs and produce hydrochar from the GL. A process that combines solvent extraction and struvite precipitation was developed for the treatment of biocrudes containing REEs and other elements. In the extraction step, 95.6% of REEs were extracted using 0.05 mol/L di(2-ethylhexyl)phosphoric acid (D2EHPA) with an aqueous to organic (A/O) ratio of 1:1 at pH 3.0. However, other impurity metals were co-extracted into the organic phase with the REEs. To solve this issue, a subsequent scrubbing step using deionized water was applied, with the removal of over 98% of these impurities, while incurring negligible loss of REEs. After the scrubbing step, over 97% of REEs were ultimately stripped out from the organic phase as REE oxalates using 0.01 mol/L oxalic acid. Furthermore, phosphorous (P) was found to be retained in the raffinate after the solvent extraction process. 94.4% of the P was recovered by forming struvite precipitate at pH 9.0 and a Mg/P molar ratio of 1.5. In general, high purity and value-added REE products and struvite precipitate were eventually achieved from biocrudes in environmentally friendly and economically viable ways. In summary, this study contributes a sustainable and efficient framework for REE hyperaccumulator treatment that integrates acid leaching, MHTC, solvent extraction, and struvite precipitation. This work supports a circular economy, minimizing waste and promoting resource reuse.
Essays on the economics of bee-friendly beef
Chishimba, Elizabeth (Virginia Tech, 2024-11-12)
The decline in bee populations, driven by biodiversity loss, poses a major threat to food production systems, as many wild plants and agricultural crops rely on bee pollination services. Addressing this issue requires coordinated efforts from producers, retailers, and consumers to restore pollinator habitats. This study explores two key aspects of promoting bee-friendly beef as a potential solution to support pollinator conservation. First, the study examines beef cattle producers' willingness to adopt native warm-season grass and wildflower (NWSG-WF) pastures through conservation programs. A discrete choice experiment revealed that producers require cost-share payments to convert pastureland into NWSG-WF systems. For a 10% conversion, producers would accept an average of $134.58 per acre (51% of establishment costs), while increasing conversions to 20% or 30% would raise the required payments to $154.55 (59%) and $199.06 (76%), respectively. Factors influencing producer participation include education level and interest in pollinator restoration, with those having thin forage coverage being more likely to adopt NWSG-WF. Second, a nationwide choice experiment survey of 2,136 U.S. beef consumers reveals a clear preference for bee-friendly beef over conventional options. On average, consumers are willing to pay $0.87 more per pound for beef produced using pollinator-friendly practices. Certain consumer segments, such as those who volunteer or donate to environmental organizations ($1.74/lb) and those knowledgeable about pollinator decline ($1.39/lb), exhibit an even higher willingness to pay. This suggests that raising public awareness about pollinator conservation could drive greater support for eco-labeled products like bee-friendly beef. Overall, the findings highlight opportunities to promote bee-friendly beef through consumer awareness campaigns and targeted conservation incentives for producers. By aligning consumer preferences with producer incentives, bee-friendly beef production can contribute to both pollinator conservation and sustainable cattle farming.
The effect of ACTH and progesterone on semen characteristics, blood hormones and testicular histology in the bull
O'Connor, Michael L. (Virginia Polytechnic Institute, 1979)
Development of a short test for the identification of aggregates causing poor concrete performance when frozen
Hsieh, Tai-Chou (Virginia Polytechnic Institute, 1967)
Paradigms of social stratification: the contemporary power elite debate
Kerbo, Harold R. (Virginia Polytechnic Institute, 1975)
Some dynamic mechanical properties of apple fruits and their use in the solution of an impacting contact problem of spherical fruit
Hamann, Donald D. (Donald Dale) (Virginia Polytechnic Institute, 1967)
Millenarian movements and the politics of liberation: the Rastafarians of Jamaica
De Albuquerque, Klaus (Virginia Polytechnic Institute, 1976)
Intracellular distribution of nitrogen during synchronous growth of Chlorella pyrenoidosa
Hare, Theodore Arthur (Virginia Polytechnic Institute, 1967)
Cultivating Sustainability: Analyzing Soil Health Dynamics and Economics of Cover Crops in the Mid-Atlantic
Haymaker, Joseph R. (Virginia Tech, 2024-11-11)
This research investigated the long-term effects of transitioning from intensive tillage to no-till (NT) practices with cover crop (CC) incorporation on soil quality, agronomic performance, and economic returns in Virginia's Coastal Plain. Nine years after integrating NT practices and CCs, improvements in soil physical and chemical properties were observed, including a 22% to 65% increase in soil organic matter (SOM) in the top 5 cm, a 4% reduction in bulk density, and enhanced soil moisture retention in corn production. Timing of CC termination played a crucial role in optimizing biomass production and nutrient accumulation. Overall accumulation rates were 44.4 kg dry biomass ha-1 d-1, 1.22 kg N ha-1 d-1, 0.16 kg P ha-1 d-1, 1.36 kg K ha-1 d-1, and 0.08 kg S ha-1 d-1 of delayed termination between March 15 and April 30. Each additional day of cover crop growth contributed to a fertilizer value of $3.91 ha-1, highlighting the economic advantage of extending CC growth during this critical period. In 2023, CC effects on corn N fertilizer demand and yields were assessed by applying variable N rates of 0, 56, 112, and 168 kg N ha-1 at sidedressing. Greatest corn yields at each N rate were observed following hairy vetch and a vetch-dominant CC mix, which had low C:N ratios (≤12:1) and accumulated 134 to 186 kg N ha-1 in their aboveground biomass. Corn yields after these CCs were 8.5 to 9.3 Mg ha-1 at the zero N sidedressing rate, increasing to 10.8 to 11.3 Mg ha-1 at the 168 kg N ha-1 rate. However, increasing the N rate yielded minimal economic benefits for these treatments. Vetch treatments produced the highest net returns, with greater returns at lower N rates, as vetch generated an additional US$1,012 ha-1 at the zero N sidedressing rate compared to the no CC control. Conversely, cereal rye produced a negative net return across all N rates, with positive returns achievable only with state cost-share payments. The findings underscore the importance of adaptive N management strategies and policy adjustments to support environmentally and economically sustainable cover crop practices in corn production.
Evaluation of Seed Impact Mills for Harvest Weed Seed Control in Soybean and Wheat in the Eastern United States
Russell, Eli Carnley (Virginia Tech, 2024-11-11)
Harvest Weed Seed Control (HWSC) concentrates, removes, or destroys weed seeds as they pass through the combine. Seed impact mills are modifications that are mounted directly to the back of a combine and are one way to implement HWSC. Seed impact mills kill weed seeds during harvest, preventing seeds from being added to the soil seedbank. Mills like the Redekop Seed Control Unit (SCU) and the integrated Harrington Seed Destructor (iHSD) could be used in soybean and wheat production in the eastern United States. Understanding the effectiveness and limitations of these mills is important for grower adoption. So, the aim of this research was to evaluate the efficacy of two seed impact mills, the Redekop SCU and the iHSD, in soybean and wheat. The first objective tested general seed kill of problematic species in soybean and wheat and seed kill in adverse conditions, such as high chaff flow rate into the mill and high chaff moisture. Results from objective one indicate that both the Redekop SCU and iHSD killed >98% and >91% of problematic weed seeds in soybean and wheat, respectively. Increases in chaff flow rate and chaff moisture resulted in a decrease in seed kill for specific species depending on the mill. But even at high chaff flow rates, seed kill remained >98% and >77% in soybean and wheat, respectively. At high chaff moisture, seed kill remained >98% and >74% in soybean and wheat, respectively. The second objective evaluated the percentage of weed seeds that bypassed the seed impact mill by exiting the combine in the straw fraction and the percentage of weed seeds that were killed when they entered the seed impact mill during harvest with a commercial combine. Results at field scale indicated that <5% of weed seeds bypassed the seed impact mill by exiting the combine in the straw fraction during harvest in soybean and wheat. Additionally, during a commercial harvest, the seed impact mills killed >99% and >89% of seeds in soybean and wheat, respectively. The third objective monitored population density changes for common ragweed (Ambrosia artemisiifolia) in soybean and Italian ryegrass (Lolium perenne ssp. multiflorum) in wheat following a harvest with a seed impact mill. Results from objective three indicated that in the growing season following a harvest with a seed impact mill, common ragweed density was reduced by 26% and 77% in the spring and fall, respectively, in soybean, and Italian ryegrass density was reduced by 48% in wheat. The fourth objective evaluated Palmer amaranth (Amaranthus palmeri) and its ability to shift its flowering timing in response to HWSC. If weeds flower earlier, they could shatter seeds earlier, and those seeds would bypass HWSC. Through selective breeding, two populations of Palmer amaranth experienced a shift in flowering timing such that the third generations flowered 54.7 and 41.0 days sooner in the greenhouse than the initial generations. In a common garden experiment, the second generations flowered 5.5 and 8.9 days sooner than the initial generations. These results indicate that seed impact mills, like the Redekop SCU and iHSD, can deliver high seed kill rates to a range of weed species at commercial scale in both soybean and wheat. Even in adverse conditions, the mills still killed >74% of seed from tested species. However, weed species can adapt to HWSC selection pressures, resulting in a loss of HWSC efficacy. Overall, this research indicates that seed impact mills are a good tool that growers can implement to reduce the number of weed seeds being returned to the soil seedbank.
Using Screenshots as a Medium to Support Knowledge Workers' Productivity
Hu, Donghan (Virginia Tech, 2024-11-08)
As computer users increasingly rely on digital tools for daily tasks, the complexity of their working environments continues to grow. Modern knowledge workers must navigate a diverse array of digital resources, including documents, websites, applications, and other information. This complexity presents challenges in managing multiple activities to maintain productivity, such as handling interruptions, resuming tasks, curating resources, recalling context, retrieving previously closed digital resources, and fostering self-reflection. Despite these challenges, there has been limited research on leveraging visual cues to help users reconstruct their previous mental contexts, retrieve digital resources, and enhance self-reflection for behavioral change. Therefore, this Ph.D. dissertation addresses these gaps by focusing on: (1) investigating the existing challenges users face in curating digital resources, (2) designing and implementing supportive applications for task resumption, (3) developing methods that utilize screenshots and metadata for reconstructing mental context and retrieving resources, and (4) enhancing the processes of self-reflection and behavioral change to improve overall productivity.
Embeddings for Disjunctive Programs with Applications to Political Districting and Rectangle Packing
Fravel III, William James (Virginia Tech, 2024-11-08)
This dissertations represents a composite of three papers which have been submitted for publication: The first chapter deals with a non-convex knapsack which is inspired by a simplified political districting problem. We present and derive a constant time solution to the problem via a reduced-dimensional reformulation, the Karash-Kuhn-Tucker optimality conditions, and gradient descent. The second chapter covers a more complete form of the political districting problem. We attempt to overcome the non-convex objective function and combinatorially massive solution space through a variety of linearization techniques and cutting planes. Our focus on dual bounds is novel in the space. The final chapter develops a framework for identifying ideal mixed binary linear programs and applies it to several rectangle packing formulations. These include both existing and novel formulations for the underlying disjunctive program. Additionally, we investigate the poor performance of branch-and-cut on the example problems.
Triple Helix Relations in Local and International Scientific Collaborations: A Case Study of Thailand, the United States, and China
Petri, Bunyakiat (Virginia Tech, 2024-11-04)
Local and international scientific collaborations are crucial for innovation and sustainable development. However, there is a gap in understanding how these collaborations affect national innovation ecosystems. This study examines the dynamics of Triple Helix Relations, focusing on collaborations within Thailand and its international partnerships with the United States and China from 2006 to 2022. I use Shannon's mutual information, enhanced by Loet Leydesdorff, to analyze the synergy among various local sectors and conduct interviews with eighteen researchers and policymakers, utilizing Latour and Woolgar's cycle of credibility. The study delves deeply into the complexities of collaboration dynamics and motivations. The analysis reveals nuanced patterns of collaboration, spanning both within Thailand and across international boundaries. I distinguish collaborations based on the nationality of partners (Thai-China vs. Thai-U.S.) and subject areas (engineering, medicine, agricultural and biological sciences). The findings show significant variation in collaboration patterns depending on these factors. Universities are the main contributors to scientific publications, while the Thai government is more active in medicine and collaborations with the United States. Industry is more engaged in agricultural, biological sciences, and engineering, especially with China. One recurring theme that emerges from our interviews is the importance of relationship networks as significant assets in collaborative endeavors. Different credibility resources and networks yield varying levels of negotiation power and influence dynamics in different collaborative settings. Understanding these dynamics could assist smaller countries like Thailand in devising strategies to maximize the benefits of international collaborations. Despite the opportunities globalization presents, I observe a decline in local collaborative synergy among Thailand's three sectors, university, government, and industry. Local collaborations are mostly bilateral, indicating a need for greater involvement from the third sector to foster sustainable growth and development. This study demonstrates the use of STS concepts and various analytical tools, such as co-authored publications and Shannon's mutual information, to showcase collaboration trends and synergy among local sectors in Thailand. The study also includes case studies from diverse countries to consider different conditions affecting collaboration dynamics. Emphasizing recent data, the study aims to capture the evolving landscape of international and local scientific collaborations comprehensively.

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