Doctoral Dissertations

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https://hdl.handle.net/10919/11041

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Bridging the gap: Biomechanics of vertical gap-crossing in snakes and cicadas
Pulliam, Joshua Nicholas (Virginia Tech, 2026-02-13)
The ability to locomote through arboreal environments is essential for the animals which make those environments their home. An animal's ability to move among branches can depend on both environmental features, such as branch diameter or discontinuities in branch support, as well as the animals' features such as claws, sticky toe pads, or the presence or absence of limbs. Periodical cicadas (Magicada septendecim) are an example of animals which must overcome both discontinuities and changes in branch diameter. After spending 13-17 years of their lives underground, periodical cicadas burrow their way to the surface, climb trees to molt into adults, and then climb to the edges of branches, mate, and oviposit their eggs. Both nymphs and adults must overcome the same surfaces, yet how the environmental factors of these surfaces interact with the age of the subject to influence climbing performance remains unknown. Additionally, the influence of arboreal surface structure on animal locomotion is not limited to insects, or even limbed animals. Limbless animals, such as arboreal snakes, must contend with these same issues. While perch diameter's effects on horizontal locomotor speed and balance have been well explored, its influence on limbless animal's abilities to cross vertical gaps between branches remains relatively understudied. Despite perch diameter's known influence on an animal's ability to prevent toppling, no study has recorded how this influence translates to its influence on vertical gap crossing ability for limbless animals. This dissertation seeks to answer how arboreal surface structures shape locomotor performance across diverse taxa. Chapter 2 presents a study asking how increasing perch diameter and varying surface friction influences adult periodical cicadas ability to navigate cylindrical surfaces, showing that, at smaller diameters, cicadas are able to cross distances which proved difficult to impossible at larger diameters. Chapter 3 expands on this, exploring gap size's relationships with successful climbs but also compares the performance of adult cicadas to those of nymphs. Additionally, a series of trials are introduced within this chapter to explore surface roughness's influence on climbing capability for both age groups, revealing relationships between surface roughness and success for both age groups, as well as age groups and how well they succeed, defined by how quickly they completed the cross. Finally, Chapter 4 explores diameter's influence on vertical gap crossing in arboreal snakes, exploring factors such as the heights they reach, the speed at which they reach these heights, how well they maintain balance in doing so, and the torque the portion of the bodies within the gap experience. My results have found that perch diameter does influence several performance variables in arboreal locomotion for cicadas and snakes. These variables include, but are not limited to, maximum size of gaps possible to cross for the former, and heights reached for the latter. Additionally, I found that age groups influence the performance of periodical cicadas on climbing structures, while the surface conditions of these structures influence their ability to successfully climb.
Exploring Science Teacher's Concerns and Self-Efficacy Toward Implementing Engineering Practices in Science Education
Morris, Angela Dawn Sanders (Virginia Tech, 2026-02-13)
Exploring Science Teacher's Concerns and Self-Efficacy Toward Implementing Engineering Practices in Science Education Angela Sanders Morris Scholarly Abstract The purpose of this study was to examine the levels of concern and self-efficacy held by Virginia secondary science teachers regarding the implementation of engineering practices as an instructional strategy to teach science. With the 2018 Virginia Science Standards of Learning requiring engineering integration, understanding teacher readiness for this shift is essential. Participants were high school science teachers from counties across Virginia's eight regions. Of the 1,536 teachers invited, 75 completed the STR4UEP survey. Using the Stages of Concern Questionnaire (SoCQ) and the Teaching Engineering Self-Efficacy Scale (TESS), this study measured teachers' concerns, confidence, and perceived readiness. Findings revealed high internal concerns across all demographic groups, particularly regarding awareness, information needs, personal ability, and classroom management related to engineering implementation. TESS results showed low-to-moderate confidence, with consistently modest scores for Engineering Pedagogical Content Knowledge (KS) and Outcome Expectancy (OE). An inverse relationship emerged between concerns and self-efficacy: teachers with higher concerns reported lower confidence in their ability to implement engineering practices effectively. Overall, the results indicate that Virginia secondary science teachers possess a relatively low level of readiness to integrate engineering practices into science instruction, underscoring the need for targeted professional development and sustained instructional support.
The Impact of Receiving Peer Help and a Manager's Reputation for On-The-Spot Rewards on Novice Auditors' Task Performance
Malone, Carissa Laura (Virginia Tech, 2026-02-13)
Auditors often receive help from peers, especially when performing novel tasks, such as data visualization, which facilitates timely audits and promotes teamwork. Psychology research identifies two types of help individuals can receive: empowering help (providing the full solution and tools to solve the problem on their own) and non-empowering help (providing the full solution without tools to solve the problem on their own). To promote this kind of helping behavior, firms have instituted on-the-spot rewards programs, though the extent to which individual supervisors use these rewards varies. I predict that the type of help provided (empowering or non-empowering) and supervisors' reputation for giving on-the-spot rewards (absence or presence) will jointly impact novice auditors' current and perceived future task performance. In an experiment, when performing data visualization, in the absence of a supervisor's reputation for providing on-the-spot rewards, auditors who receive empowering help, rather than non-empowering help, feel a greater sense of ownership. This leads to more accurate audit procedures, better client evidence requests, and greater perceived ability to perform future tasks. However, in the presence of a supervisor's reputation for rewards, auditors perceive that the help provider has ulterior motives, which diminishes the performance difference between empowering and non-empowering help. Therefore, certain audit firms' rewards programs can have unintended effects on firms' efforts to enhance team culture.
Analysis of Acceleration Techniques and Fast Nonlinear Solvers
Wan, Ning (Virginia Tech, 2026-02-13)
This dissertation focuses on the analysis and development of acceleration techniques and fast solvers for nonlinear systems of equations. Building upon the fixed-point and extrapolation frameworks introduced in the early chapters, we explore structural connections between residual-based acceleration methods and Krylov subspace techniques. The first main contribution is a unified algebraic framework establishing the equivalence between the Anderson Acceleration method and the CROP (Conjugate Residual with Optimal Trial Vector) algorithm. By formulating both methods within a common affine subspace representation, we show that their full, untruncated forms produce identical iterates, motivating new hybrid variants such as CROP-Anderson and real-residual CROP (rCROP) methods. The second contribution is a perturbation analysis of Anderson-type variants, examining the effects of deterministic and stochastic errors on convergence. Numerical experiments confirm that acceleration efficiency depends critically on both the choice of update strategy and the nature of perturbations. The third contribution extends this unified perspective to nonlinear Krylov subspace methods. Nonlinear extensions of GMRESR, GCRO, and LGMRES are derived, forming the nlKrylov family of algorithms, and analyzed in the context of inexact Newton solvers, with convergence results established under relaxed conditions on residual and Jacobian approximations.
Product User Well-being in a Circular Economy
Svensson, Sahra Helen (Virginia Tech, 2026-02-13)
Alternative economic models, such as the Doughnut Economy and Circular Economy (CE), are often introduced in response to increasing concerns regarding the environmental degradation and social inequality linked to the current growth-oriented economic paradigm. These economic models are often designed to foster a sustainable development, allowing for the satisfaction of the needs of today's generations without harming the opportunities of future generations to meet their needs. It requires balancing resource use with changes to how human needs are fulfilled - ensuring that social, economic, and environmental dimensions all work together. This dissertation explores how alternative economic models can be designed to fully consider individuals' well-being, hence maximizing their quality of life whilst minimizing their environmental footprint. This entails clarifying what these models may look like when fully implemented and examining how such economies could influence various aspects of well-being, including happiness, social belonging, and meaning. A CE constitutes a pathway towards sustainability; in a CE, waste and resource extractions - hence, negative environmental impact - are largely eliminated, accomplished through the continuous recirculation and valorization of goods and materials within economic systems. The commitment to a CE is increasing worldwide and policymakers, business leaders, and organizations are currently making consequential decisions about whether and how CE principles are interpreted and implemented, thus shaping the specific forms that a CE may take. This has far-reaching implications for individuals in their role as "product users" in a CE, namely how they are expected to engage in so-called "CE behaviors", e.g., reuse, recycling and sharing, and how this impacts their quality of life. Such outcomes are important to consider as an economic system that is not feasible or viable for its participants can be expected to fail. As such, for the success of a CE and the people living in such an economy, it is imperative that CE behaviors are structured to not only promote sustainability, but to also enhance product users' well-being. However, the everyday impacts of a fully realized CE on individual product users - and the associated well-being outcomes from normalized engagement in CE behaviors - remain poorly understood. This gap risks undermining both human flourishing and sustainability, potentially causing unintended consequences for quality of life and social stability. To address this gap, this dissertation elucidates the connections between a CE and product user well-being by offering both substantive guidance and methodological tools for examining how a CE may transform consumption practices and, by extension, the opportunities, challenges and trade-offs for product user well-being within such a future. This work clarifies: (1) terminology and understanding of the movement of materials and the roles of product users in a CE; (2) the product user's experience of CE behaviors; (3) the dimensions of well-being as they relate to product users; (4) the specific conditions of CE behaviors with the greatest impact on well-being; (5) the system actors that influence these salient conditions; and (6) the implications of different CE versions for the quality of life of product users - using integrated systems models. These findings contribute to designing economic systems that jointly advance sustainability and enhance quality of life.
Analysis of Trace Gas Heterogeneity using In-Situ and Remote Sensing Measurement Techniques
Earley, Jeffrey Davis (Virginia Tech, 2026-02-11)
Differential Optical Absorption Spectroscopy (DOAS) is a remote sensing spectroscopic technique capable of retrieving vertical column densities of trace gases in the atmosphere. While the technique can be used to retrieve vertical profiles of trace gases this retrieval is not applicable to Mobile-DOAS measurements due to the lengthy measurement times. In addition, the requirement for rapid measurements for Mobile-DOAS application prevents a single instrument from retrieving information about the horizontal trace gas distributions around the instrument, in addition to the lack of vertical distribution information. However, with the addition of collocated in-situ measurements of surface concentration, as well as the use of Multi Axis DOAS (MAX-DOAS) measurements taken from multiple azimuth viewing directions, it is possible to estimate these distributions using a minimal amount of measurements, allowing this gap in mobile DOAS measurements to be filled. The relationship between the vertical column density measured by Mobile Zenith-DOAS and in-situ surface concentration is first explored using measurements made during the first TRACER-AQ field campaign. This relationship, expressed as the column to surface ratio, is then used to identify trace gas plumes and other transport patterns that can be separated from local, vehicular emission based on this ratio during two case days, in order to determine the origin of the high Ozone event that was observed on both days. In addition, the effect of the trace gas vertical distribution on satellite-based DOAS measurements is also determined, where it is found that the column to surface ratio is proportional to worsening agreement between Mobile Zenith-DOAS and TROPOMI Satellite-Based DOAS, however the relationship is only somewhat correlated (r2=0.3), indicating that other factors are a greater influence on the ability of Ground-Based DOAS and Satellite-Based DOAS to agree. During the second TRACER-AQ field campaign, the column to surface ratio was combined with analysis of MAX-DOAS measurements in order to determine both the vertical and horizontal distributions of trace gas around the instruments. This analysis of the horizontal distribution is expressed with the Horizontal Heterogeneity Index, which is a comparison between two MAX-DOAS azimuth viewing directions, behind and to the right of the moving vehicle, with the Zenith DOAS measurements each MAX-DOAS instrument is making. The analysis of both distributions is then used to pinpoint the potential sources giving rise to two more high Ozone events during two more case days during the second field campaign. In addition, the horizontal trace gas distribution was also compared to TROPOMI validation, and was found to correlate more strongly than the vertical distribution (r2=0.69), showing that while there are many issues with satellite validation, the horizontal distribution is most likely responsible for deviations between Ground-Based and Satellite-Based DOAS measurements. Finally, analysis of trace gas distributions requires low uncertainty retrievals of Zenith DOAS VCD's. This is typically performed using radiative transfer simulations, but which require information of the aerosol scattering properties that are not retrievable from a mobile platform. In order to perform these simulations, a novel technique of averaging the aerosol conditions as measured by a network of six AERONET aerosol measuring instruments was investigated through a series of sensitivity studies. These studies analyzed the effect of potential errors in the aerosol properties on the resulting Air Mass Factor estimations, in order to determine how these errors effect Air Mass Factor uncertainty. The studies showed that the total error introduced into the model through potential errors in aerosol properties, as well as through errors in the assumption of an a priori trace gas profile used to retrieve the Air Mass Factors, was less than 20% for AMF retrievals in Visible light and 25% for UV retrievals, approximately the same level of uncertainty used for Zenith AMF retrievals in other studies, demonstrating that this averaging technique is capable of retrieving sufficiently low uncertainty AMF estimations.
The Influence of AR Head-Mounted Displays on Spatial Perception and Worker Response in Construction Training
Withers, Jeremy W. (Virginia Tech, 2026-02-10)
Construction job sites present significant risks that extend beyond physical hazards to include psychophysiological factors that shape workers' perceptions, attention, and decision-making. The mobility and influence of Augmented Reality Head-Mounted Displays (AR-HMDs) directly interact with these factors by altering how workers perceive their environment, process information, and manage sensory input. While AR-HMDs offer new opportunities for immersive, adaptive training in real-world construction scenarios, they also introduce cognitive and sensory risks that have been insufficiently explored in construction research. Now that AR-HMDs are deployed on construction sites, it's crucial to understand not only their technical performance but also the human-centered impacts on safety, perception, and decision-making. Existing studies have largely overlooked the psychophysiological constraints that influence safety outcomes; yet, understanding these responses is essential for both immediate task performance and long-term learning and risk-taking behaviors. To address these gaps, we develop theoretical constructs and framework variables to evaluate and predict spatiotemporal psychophysiological responses associated with AR-HMD use in construction training, specifically asking: What are the risks associated with AR-HMDs' spatial influence on perception in construction environments? The framework is developed and tested through four objectives, progressing from a detailed scoping literature review and evaluation of AR-HMD impact on humans to psychophysiological prediction and median severity-of-impact classification using EEG location matrices. Objective 1 develops a conceptual model that classifies AR-HMD and Human-Computer Interaction (HCI) risks and standardizes the domain language for evaluating these technologies, highlighting underexplored cognitive, sensory, and physical human-factor risks. Additionally, objective 1 identifies and classifies spatial perception variables; develops a mental model of how workers perceive and spatially analyze immersive AR-HMD environments; examines embodiment, presence, and spatial presence; and finalizes the theoretical framework for empirical testing. Objective 2 tests the framework in a controlled environment using a full-scale passive and active haptic frame. A within-subjects design captures both psychological (survey-based) and physiological (EEG, heart rate) responses, which are analyzed using Power Spectral Density (PSD), Independent Component Analysis, and regression modeling to identify hemispheric differences and misalignments between perceived safety and actual psychophysiological responses. Objective 3 advances the framework into predictive modeling, using the same haptic-frame environment, deep learning models, including 2D CNN-LSTM sequence modeling and 3D CNN-LSTM architectures that are applied to predict temporal and cognitive state changes from 4D EEG input (frequency, amplitude, time, channels), extending the framework from measurement to prediction. Together, these three objectives show how conceptual modeling, spatial perception analysis, experimental validation, and predictive analytics can be systematically connected to evaluate AR-HMD situational risks. The outcomes reveal the extent of spatial and behavioral influences across key variables, supporting the development of likelihood and severity matrices for academia and industry, as outlined in objective 4.
GaN Device Characterization, Converter Optimization and Development for Enhanced Aviation Systems
Zhao, Tianyu (Virginia Tech, 2026-02-10)
GaN device is one of the most promising candidates in high-efficiency and high-density power conversion applications. Due to the low ON-Resistance GaN devices are widely adopted in various soft-switching converters. Based on superior Baliga's Figure of Merit, GaN devices also have advantages over SiC and Si devices in hard switching conditions. However, the existence of dynamic $R_{DS(on)}$ weakens the advantage on the low conduction loss. The small footprint limits the heat dissipation ability hence the maximum switching frequency.Further improvement of power density may have constraints from passive components, which requires systematic optimization of GaN-based high-density converters.\\ By designing a multi-purpose testing platform, the dynamic $R_{DS(on)}$ of one GaN device is characterized under both DPT and soft-switching continuous tests, with different junction temperatures . Normalized $R_{DS(on)}$ is quantified and compared. The data can then be used to estimate extra loss from the dynamic $R_{DS(on)}$ under realistic power converter operating conditions.\\ The advantage of lower $R_{DS(on)}$ is especially prominent on low-Voltage rated GaN devices, which makes multi-level topology more preferable for GaN-based converters. A three-level Totem pole PFC converter for aircraft in-seat power supply is designed and optimized. The three-level topology enables the selection of the 200 V GaN device, leading to smaller conduction loss and a 98.4\% peak efficiency. With the help of the PR compensator and input voltage feedforward, the phase-leading problem caused by the digital delay is greatly reduced and proven in 800Hz line frequency. The THD can meet the standard by doubling the sampling frequency and improving sensing. The EMI performance also meets the requirement with a one-stage filter at ac side and a common-mode inductor on the DC bus.\\ For inverter design of unmanned aerial vehicle applications where there's a lower voltage DC-link, two-level gains advantage over three level after systematic optimization and evaluation on the weight and loss. Paralleling up to four GaN devices effectively balanced the switching and conduction loss. By designing the power loop inductance to 0.11 nH, 100V GaN devices were safely operated under a 70 V DC bus with only a maximum 9.8 V overshoot. To prevent the inverter from short-circuit faults, the short-circuit protection based on the measurement of the voltage on the power loop inductor with a low-pass filter is successfully applied with only 115 ns protection time. The designed prototype shows expected thermal performance under rated 3.3 kVA power and survives a two-second 5.1 kVA transient power.
School Leader Perceptions of the Effect of Social Media Use by Middle School Students' School Experiences
London, Antoine Thomas (Virginia Tech, 2026-02-09)
The purpose of this qualitative study was to examine middle school educational leaders' perceptions of the impact of social media on students' academic performance and social experiences, as well as the implications for digital citizenship education and professional development. Social media has become a pervasive element of adolescent life, influencing both formal and informal learning environments. Despite its prevalence, limited research has explored how school leaders interpret these influences and respond to emerging challenges. Guided by three research questions, this study employed semi-structured interviews with twelve middle school administrators. Data were analyzed using thematic coding, which revealed several emergent themes. Findings suggest there are several opportunities to continue refining processes currently used to respond to contemporary social media challenges for school leaders. This includes, but is not limited to, a heightened emphasis on intentional professional development for educational stakeholders, specifically focused on strategic planning for the integration and management of social media within the middle school context. The study underscores the need for targeted professional development for school leaders by school divisions. Recommendations include the establishment of division-level teams to manage social media–related issues and the integration of structured training programs to equip administrators with strategies for addressing these challenges. These findings contribute to the growing body of literature on social media's role in education and provide actionable insights for policy development and leadership practice.
Bioaerosol Dispersal Across Scales: Regional Patterns, Field Study, and Model Evaluation
Nimmala, Bhargavi (Virginia Tech, 2026-02-09)
Bioaerosols--including seeds, pollen, fungal spores, bacteria, and viruses--are fundamental agents connecting atmospheric processes to agriculture, ecosystem function, and human and animal health. This dissertation uses Lagrangian stochastic (LS) models to simulate how these particles travel and deposit across scales relevant for cross-pollination, with applications to many types of biological aerosols. First, we map seasonal and regional patterns of windborne hemp pollen across the United States by running LS models with weather data to simulate day- and night-time dispersal from summer through fall. These simulations identify areas more susceptible to cross-pollination and show how patterns shift across seasons and between day and night. We find regions more vulnerable to cross-pollination, with seasonal and diurnal shifting patterns in dispersal. Next, we work to detect and model genetically modified switchgrass pollen released from a small field in low-wind conditions during three sampling campaigns with a suite of novel samplers. We find that only our highest-volume samplers were able to detect pollen and that reducing the averaging window in the simulations substantially improved emission-rate estimates. Finally, we evaluate the 3D LS models used in this dissertation by comparing them to a high-fidelity model driven by large-eddy simulation (LES) in seven daytime convective boundary layer conditions. The LS models show good accuracy in strongly convective conditions, but they fail in near-neutral conditions due to issues in how they are parameterized rather than in their underlying equations. Together, these results clarify when LS models can effectively substitute for more computationally intensive LES, reveal how sampler design and averaging choices shape what can be extracted from field measurements, and demonstrate the value of weather-aware modeling for cross-pollination risk assessment and broader questions of bioaerosol transport. Collectively, this work strengthens the scientific foundation needed to predict, manage, and mitigate the movement of biological aerosols in an increasingly variable atmosphere.
Recommended Syllabus Components: What Trauma-Informed Components Do Higher Education Instructors Include in their Syllabi?
Walters, Amanda Derringer (Virginia Tech, 2026-02-05)
A majority of students entering college have experienced at least one potentially traumatizing event, and up to half of college students experience trauma during their time in college. Trauma can impact a student's engagement, behavior, and cognitive functioning, affecting their learning and memory. Trauma-informed pedagogy is a pedagogical approach rooted in SAMHSA's trauma-informed framework wherein instructors can support the resilience and academic success of all students, particularly those who have experienced trauma. While trauma-informed pedagogy is an emerging field, and research ties trauma-informed pedagogical principles to positive student learning outcomes, few studies have investigated what trauma-informed pedagogical practices are currently being implemented at the college level. This quantitative study investigated three questions: (1) What trauma-informed syllabus components do higher education instructors include in their syllabi? (2) What differences exist in the inclusion of trauma-informed syllabus components across content area domains? (3) What differences exist in the inclusion of trauma-informed syllabus components across academic levels? A sample of 1,000 syllabi across 86 institutions were evaluated for the inclusion of 16 trauma-informed syllabus components. Findings reveal that while some trauma-informed practices are being widely implemented in college syllabi (up to 92% of syllabi), others remain less common (0.5% of syllabi). Significant differences were found across content areas in some components, while academic levels demonstrated generally similar adoption rates. These results illustrate the current landscape of trauma-informed pedagogical adoption across college classrooms.
Urban Agricultural Education: Exploring Good Practices for Recruiting and Retaining Underrepresented Youth Into the Agricultural Industry.
Robinson, Quintin Orlando (Virginia Tech, 2026-02-05)
The agricultural industry continues to face a shortage of qualified workers in high-skill, high-wage positions, while underrepresented youth remain largely excluded from agricultural education and careers. This study examines successful urban School-Based Agricultural Education (SBAE) programs that have effectively recruited, retained, and prepared underrepresented youth for agricultural professions. Guided by the Theory of Planned Behavior (TPB), this qualitative multiple-case study explores how external factors, school practices, and teacher influences shape students' attitudes, perceived behavioral control, and subjective norms toward agricultural careers. Data were collected through interviews with teachers, administrators, and alumni from four urban high school SBAE programs recognized for engaging diverse student populations. Findings indicate that family perceptions, financial constraints, program accessibility, and exposure significantly influence students' intentions to pursue agricultural careers. Self-efficacy, mentorship, inclusive curricula, and strong community-school partnerships were identified as critical to student persistence and engagement. Early exposure and culturally relevant learning experiences were key in transforming negative perceptions of agriculture into sustained career interest. Recommendations include expanding urban agricultural programs, strengthening mentorship and community partnerships, and developing inclusive curricula that reflect the cultural and historical contributions of underrepresented groups in agriculture. These findings can guide educators, policymakers, and community leaders in creating equitable and sustainable pathways that increase the participation of underrepresented youth in the agricultural workforce.
Application of Co-Design Principles for Design of Series Elastic Joints
Pressgrove, Isaac James (Virginia Tech, 2026-02-04)
Compliant joints enhance the performance of dynamic legged robots by enabling more robust, efficient, and resilient locomotion. A widely adopted approach for introducing compliance into robotic joints is the use of Series Elastic Actuators (SEAs). Designing SEAs, however, requires balancing the stiffness of the elastic element with the structure and gains of the control system, as both strongly influence actuator bandwidth, disturbance rejection, and overall efficiency. Prior work across many domains has demonstrated that co-design methodologies, those that optimize mechanical and control parameters simultaneously, can produce high-performance, robust systems. This dissertation advances the capabilities of dynamic legged robots through the development of a comprehensive co-design strategy for SEAs. The proposed framework addresses key limitations of traditional SEA design, particularly their difficulty in balancing the trade-off between high bandwidth achievable by stiff actuators and the disturbance rejection afforded by increased compliance. By jointly optimizing the gains of a simple, easily implemented PID–feedforward controller alongside the stiffness of the elastic element, the approach presented here improves both controllable bandwidth and transient response without requiring complex control architectures. A systematic method for identifying cost functions that are broadly applicable, implementation-friendly, and reliably indicative of system performance is presented. These cost functions are then used within a co-design optimization applied to several SEA configurations, demonstrating both generality and performance improvements over conventional sequential design approaches. In addition, this work investigates how infill density influences the flexural rigidity of fused deposition modeling (FDM) printed PLA beams. These experiments support the use of FDM-printed components as compliant elements within SEAs. Using static three-point bending tests, regression models are developed to predict part flexural rigidity as a function of print infill. These models are integrated into the co-design framework, replacing direct selection of elastic stiffness with the specification of beam geometry and infill percentage. The resulting co-designed hardware is validated on an SEA knee-joint test bench, and experimental results are compared with simulations to evaluate sim-to-real fidelity. This work makes three key contributions: (1) the development of a broadly applicable co-design methodology for SEAs, (2) the creation of predictive regression models for the mechanical properties of FDM-printed PLA beams, and (3) the integration of these results into a unified co-design strategy enabling SEAs that leverage additively manufactured compliant elements.
UAV-Enabled Wireless Communications: Deployment, Optimization, and Analysis
SabzehAli Touranposhti, Javad (Virginia Tech, 2026-02-04)
Unmanned aerial vehicles (UAVs), known as drones, are a promising solution, as aerial base stations (BSs) or relays, in wireless communications systems. Due to their high likelihood of line-of-sight (LoS) links and ease of deployment, they play a crucial role in providing faster and better wireless network access and service where extra network resources are needed short term, like sports events, or to those emerging services that require high-capacity communications. Moreover, they can help extend wireless coverage to locations deprived of end-to-end wireless communications services in remote/rural areas due to natural disasters or being distant from the conventional terrestrial BSs. However, utilizing this new technology comes with its own novel challenges. In this dissertation, we focus on unprecedented challenges in UAV communications and networks, considering some unique features of UAV networks, such as their optimal placement and wireless backhaul links. First, we focus on provisioning wireless coverage to those emerging services, like extended reality, demanding high-capacity communications. High frequencies, i.e., millimeter-wave (mmWave) and, terahertz frequency bands offer the substantial bandwidth required for such services. These high-frequency communications, however, depend critically on maintaining LoS connections to user terminals. In practical scenarios, users distributed in three-dimensional space often experience severely limited visibility due to environmental obstructions like buildings and foliage. We study the problem of finding an optimal 3D placement and antenna orientation for mmWave-equipped UAVs to minimize the number of required UAVs while maximizing the signal-to-noise ratios (SNRs) to all users. Our approach formulates this as an integer linear programming (ILP) optimization problem, establishes its computational intractability (NP-hardness), and develops a computationally efficient geometric algorithm that consistently achieves near-complete LoS coverage across diverse simulation scenarios. Our second research thrust targets wireless connectivity in remote rural environments—such as agricultural Internet of Things (IoT) deployments—where conventional terrestrial infrastructure is limited or absent. A fundamental challenge in such UAV-assisted networks is determining the minimal UAV deployment that simultaneously achieves two objectives: complete ground user coverage and reliable wireless backhaul connectivity linking all UAVs to terrestrial BSs. We formulate this joint optimization—termed the Backhaul-and-coverage-aware Drone Deployment (BoaRD) problem—as an ILP problem and prove its NP-hardness. Our solution approach employs a graph-theoretic algorithm that efficiently solves the problem with provable performance bounds. Comparative analysis using ILP solvers demonstrates that our algorithm achieves near-optimal performance for smaller problem instances. For large-scale scenarios with extensive coverage areas and numerous users, comprehensive simulations show our algorithm substantially outperforms baseline algorithms while guaranteeing complete user coverage and end-to-end connectivity. Finally, building upon these deployment optimization contributions, our third research thrust develops a comprehensive analytical framework for multi-hop UAV-assisted cellular networks. While the previous work provides deterministic algorithms for specific deployments, understanding system-wide performance requires statistical modeling of networks with random spatial distributions. We develop a comprehensive stochastic geometry framework for analyzing multi-hop UAV-assisted cellular networks that addresses fundamental gaps in existing analytical approaches. Traditional stochastic geometry techniques for terrestrial networks are insufficient for characterizing the complex 3D spatial relationships, interference patterns, and unique propagation characteristics inherent in multi-hop UAV deployments. We extend existing mathematical frameworks to accommodate the distinctive features of aerial networks, including realistic 3D spatial distributions of UAVs across multiple operational altitudes, probabilistic air-to-ground channel models that distinguish between LoS and NLoS conditions, and the intricate interference correlations that arise in multi-hop communication paths. Our framework derives novel mathematical constructs and probability distributions that enable precise characterization of multi-hop network behavior under random spatial deployments in the 3D space. We provide comprehensive closed-form expressions for coverage probability analysis covering both amplify-and-forward (AF) and decode-and-forward (DF) relaying protocols, accounting for the hybrid communication scheme where UEs can connect either directly to serving BSs or through the multi-hop UAV network based on received signal quality. Additionally, we introduce optimal relay selection strategies that maximize end-to-end SINR by jointly considering all link qualities in the formed multi-UAV network and accounting for the complex interdependencies between sequential links in the presence of interference. Through extensive theoretical analysis and simulation validation, our results demonstrate that well-designed multi-hop UAV networks can significantly enhance coverage probability and network reliability compared to single-hop architectures, particularly in challenging environments where direct links between UAVs and terrestrial BSs are weak or unavailable due to distance or environmental obstructions.
A Multi-Method Analysis of Hospital Readmissions and Medicare Penalties
Mauti, Eddah Kwamboka (Virginia Tech, 2026-02-04)
Hospital readmissions can worsen patient outcomes and contribute to substantial healthcare costs. Because many readmissions are considered preventable, reducing them has become a national priority in the United States, with Medicare penalizing hospitals whose readmission rates exceed benchmarks. This research examines factors associated with readmissions, identifies the types of hospitals most likely to receive penalties, and evaluates how hospitals respond to these penalties. The work is presented in three essays. The first essay provides a comprehensive literature review of factors associated with 30-day, 90-day, and 1-year stroke readmissions, along with a meta-analysis of U.S. studies on 30-day stroke readmissions. The findings show that readmissions increase with advanced age and a high comorbidity burden, and decrease with effective discharge planning, early follow-up with a primary care physician, and receipt of thrombolytic therapy. The second essay investigates hospital characteristics associated with Medicare readmission penalties, with a particular focus on affiliation with a healthcare system. Using Poisson regression, the analysis shows that hospitals affiliated with a healthcare system receive more penalties than non-affiliated hospitals, and that smaller hospitals are less likely to be penalized compared to larger hospitals. The third essay evaluates whether hospitals are more motivated to reduce readmissions by reputational pressure related to high readmission rates or by the financial losses associated with penalties. Using system dynamics modeling of Massachusetts hospitals, reputational pressure emerges as the stronger motivator. The analysis also identifies variation in sensitivity: specialty hospitals compared to community hospitals and hospitals with lower net incomes compared to those with higher net incomes are more sensitive to reducing readmissions, whereas urban hospitals are less sensitive than rural hospitals. Overall, this dissertation offers valuable insights into the factors driving hospital readmissions and identifies strategies to effectively reduce them.
Shorebirds in a changing landscape: Assessing the top-down and bottom-up drivers of American oystercatcher (Haematopus palliatus) reproductive success in the Virginia barrier islands
Call, Mikayla Nicole (Virginia Tech, 2026-02-04)
Understanding the factors that affect population growth is a vital component of the conservation of imperiled species. On the United States (U.S.) Atlantic and Gulf coasts, the American oystercatcher (Haematopus palliatus) is largely managed within a conservation framework that aims to identify and manipulate factors that limit reproductive success. Coordinated research and monitoring across the species' range have identified several key threats to the survival of nests and chicks, including predation, habitat loss and degradation, and human disturbance. However, the relative importance of those threats can change, and new threats may emerge, as coastal ecosystems dynamically respond to global climate change. In the Virginia barrier islands, where American oystercatcher reproductive success has been historically high due to the widespread availability of undeveloped coastal habitat, declines in the average annual productivity rate of American oystercatchers since 2016 may signal evolving threats to reproduction. To provide insight into changing threats to American oystercatcher productivity in Virginia, and to inform adaptive management, we investigated the drivers of American oystercatcher reproductive success on the Virginia barrier islands. We considered three components of American oystercatcher conservation and management: (1) interactions between American oystercatchers, their conspecifics, and a complex predator community; (2) altered availability of nesting habitat due to climate-driven changes within barrier island ecosystem; and finally, (3) community support for widespread protection of the barrier island landscape and participation in conservation initiatives designed to protect breeding American oystercatchers, such as adhering to island access restrictions and recreation guidelines. We first investigated nest and chick survival on Metompkin Island and Fisherman Island—two islands that represent a range of geomorphological and ecological conditions present within the Virginia barrier island system—in 2021–2023 using a combination of field-based surveys and automated radio telemetry methods. Next, we used remotely sensed data to explore how climate-driven ecosystem state change on the Virginia barrier islands may be driving changes in the abundance and distribution of American oystercatcher nesting habitat across the barrier island system from 2004–2021. Finally, we evaluated the short-term outcomes of a field-based high school environmental education program using pre- and post-program surveys of students to assess how environmental education may promote changes in knowledge and public attitudes related to shorebird conservation initiatives in Virginia. Through routine surveys of nests and radio-marked chicks, we found that the cumulative probability of a nest surviving the incubation stage was greater than 90%, while the cumulative probability of a chick surviving from hatch to fledging was approximately 51%. Thus, chick survival, rather than nest survival, may be limiting reproductive success in the Virginia barrier islands. Flooding was a primary cause of nest loss (32% of failed nests), and storm-driven overwash in late April and early May forced peak nesting to shift later in the breeding season as breeding pairs renested, particularly in 2022. Despite ongoing management of mammalian predators through lethal removal, reproductive success still appears to be limited through the top-down effects of predation on both stages of American oystercatcher reproduction, as both nests (22% of failed nests) and chicks (46.4% of marked chicks) were lost from non-mammalian predators, including raptors (e.g., peregrine falcons [Falco peregrinus] and great horned owls [Bubo virginianus]) and Atlantic ghost crabs (Ocypode quadrata). Remotely sensed data provided an opportunity to investigate factors for which we lacked data collected in situ. Using aerial orthoimagery of the barrier islands collected by the National Agriculture Imagery Program (U.S. Department of Agriculture), we found that habitat abundance increased by 228.26 ha from 2004–2016, and then decreased by 124.38 ha from 2016–2021. Across the 2004-2021 timespan, abundance of American oystercatcher nesting habitat (defined as sites where the relative probability of nesting is ≥ 90%) was both temporally and spatially variable within the Virginia barrier island systems, with changes on only a few islands (e.g., Metompkin Island, Cedar Island, and Cobb Island) driving most of observed system-wide trends. We suggest that temporal variability may be a result of patterns of storminess along the Virginia coast 2004–2021, while spatial variability is likely due to localized differences in the geomorphological and ecological processes that control habitat availability, including sediment dynamics (e.g., sand erosion versus accretion), dune-building processes, and vegetative succession. Finally, we used an evaluation of The Nature Conservancy's field-based environmental education program for high school students as a case study to demonstrate the value of incorporating environmental education into a multidisciplinary conservation program. Using surveys that assessed components of environmental literacy—including knowledge of ecological concepts, feelings of connection and stewardship toward the local ecosystem, and behavioral intentions—we examined short-term educational outcomes for students who participated in a formal field trip to one of the Virginia barrier islands. By comparing responses on surveys delivered immediately before and immediately after the field trip, we found that participation in the field trip increased students' knowledge about barrier island ecosystems and sense of place attachment. Additionally, students self-reported positive environmental literacy outcomes on the post-trip assessment, including increased interest in scientific learning, increased likelihood of participating in stewardship behaviors, and increased intentions to participate in conservation behaviors. Given the pace at which ecosystems are changing in response to climate change and anthropogenic drivers, it is expected that the factors limiting American oystercatcher population growth will also continuously change. To protect against population declines, shorebird managers will need to continuously monitor for change. They will also need to consider the use of creative management approaches, including ecosystem-based management to address complex predation and climate threats, and environmental education and outreach programs to promote broad community support for conservation initiatives.
Automated Implementation of Advanced Electronic Structure Methods
Gaudel, Bimal (Virginia Tech, 2026-02-04)
The continuous demand for higher accuracy in computational chemistry necessitates the development of advanced many-body electronic structure methods. However, the derivation and efficient implementation of these theories constitute a significant bottleneck. As the rank of the associated tensors increases, the governing equations explode in complexity, rendering manual implementation labor-intensive, error-prone, and difficult to optimize for modern hardware. To address this challenge, this dissertation presents SeQuant, a comprehensive framework for the automated derivation and parallel implementation of many-body quantum chemistry methods. Built upon a robust symbolic algebra engine, SeQuant allows for the expression of theories in the natural language of second quantization. It automates the transformation of high-level theoretical ansatzes into explicit tensor contraction expressions and subsequently generates optimized, high-performance C++ code. A central innovation of this work is the extension of automated implementation to reduced-scaling methods, which exploit the sparsity inherent in electronic correlation. We introduce a novel "tensor-of-tensors" data structure designed to manage the irregular sparsity patterns of Pair Natural Orbital (PNO) formulations. This development enables the first fully automated implementation of PNO-Coupled Cluster (PNO-CC) methods, bridging the gap between symbolic abstraction and the runtime requirements of sparse tensor algebra. The results demonstrate that SeQuant not only reproduces established dense methods (such as CCSD and CCSDT) with high fidelity but also effectively handles the complexity of sparse, local correlation approaches. By decoupling the complexity of the physics from the details of the implementation, this framework establishes a new paradigm for method development, dramatically accelerating the translation of theoretical insights into computational reality.
Modeling and Stability Analysis for Multiphase Constant On-Time Control With Phase Overlapping
Sridhar, Sundaramoorthy (Virginia Tech, 2026-02-03)
Multiphase buck voltage regulators (VRs) powering today's high-performance processor loads must regulate output voltage within a small tolerance window and handle a large steady-state current. For output voltage regulation, multiphase current-mode and V² constant on-time (COT) control techniques are widely used because these control methods can help increase control bandwidth, minimize output impedance, and achieve fast transient response with less output capacitance. To supply large current demands, the phase count of buck VRs continues to increase. With increasing phase count, steady-state phase overlapping becomes inevitable in the practical duty range. However, currently, there is no good small-signal model to analyze stability and design high-bandwidth control loops for multiphase COT buck converters operating under phase-overlapping conditions. To address this issue, this dissertation presents a systematic small-signal modeling approach for multiphase COT buck converters operating with phase overlap, utilizing the describing function method. Prior works extended the small-signal models of single-phase COT control to design multiphase COT buck converters. These extensions rely on the assumption that multiphase and single-phase COT buck converters are equivalent from a small-signal perspective. This equivalence applies when there is no phase overlapping or when the duty cycle D < 1/N for an N-phase COT buck converter. However, this extension does not work when the duty cycle D>1/N (with phase overlap). In multiphase COT buck converters, total current ripple is used to modulate each phase duty cycle and achieve automatic phase interleaving. When the duty cycle D < 1/N, the total current ripple exhibits COT modulation after introducing a perturbation, so single-phase and multiphase COT control have identical small-signal models. However, if the duty cycle D > 1/N, then the total current ripple will exhibit variable on-time and off-time modulation after introducing a perturbation. Due to this difference in modulation principle, single-phase COT control models cannot be used for multiphase COT control design when the duty cycle D > 1/N. In this dissertation, we first developed a general describing function model for multiphase current-mode COT buck converters. As for single-phase current-mode control, the total current feedback in these converters makes the whole system dynamically non-linear. Hence, we treated the entire current loop and power stage as one entity. Unlike prior works, we derived a continuous-time frequency-domain model for multiphase current-mode COT control by performing a Fourier analysis directly on its perturbed waveform. In this way, we account for the change in total current modulation principle between the no-phase and phase-overlapping regions. The proposed model is quite general and applies to arbitrary phase numbers and phase overlapping numbers. We have also extended this model to multiphase current-mode COT buck converters with coupled inductors. Next, the proposed model for multiphase current-mode COT control was used to study the total current loop stability issue under phase overlapping conditions. Also, the minimum external ramp required for stability with different number of phase overlapping is derived. With two overlapping phases, the critical external ramp slope equals half of the sensed total current rising slope. However, the critical ramp slope with three overlapping phases equals the sensed total current rising slope and increases further with an increase in the number of overlapping phases. After that, we developed a general small-signal model for multiphase V² COT control, including the capacitance voltage ripple and phase overlapping effects. Using this model, it is demonstrated that direct output voltage feedback is inherently unstable in phase overlapping regions. In other words, unlike single-phase V² COT buck converters, even large ESR OSCON capacitors cannot ensure stability with phase overlapping. Hence, external ramp compensation is required to ensure stability when phase overlapping occurs. However, an external ramp can ensure stability only if the output voltage ripple has sufficient current strength. Hence, a physical current loop is required to ensure stability with low ESR ceramic capacitors. The critical limits derived were verified using SIMPLIS simulation and experimental results. In sum, this dissertation provided a mathematical framework to derive small-signal models for multiphase COT control methods based on phase manager under phase overlapping conditions. Small-signal models were derived for the popular multiphase current-mode and V² COT control under phase overlapping conditions. Using this model, we identified that total current loop of multiphase current-mode COT control becomes unstable without sufficient external ramp under phase overlapping conditions. Also, the stability boundaries with different number of phase overlapping are provided. The method proposed here can also be extended to other variations of multiphase COT control using phase manager.
Analysis of a Nonlinear vibration absorber for vibration control in a hand-held impact machine
Alabi, Oreoluwa Adekolade (Virginia Tech, 2026-02-03)
Hand-held impact machines (HIMs), such as jackhammers and chipping hammers, operate through the repetitive impacts of a percussive mechanism. Due to their widespread use, it is essential that these tools are designed for safe daily operation. This need is underscored by the fact that approximately 20% of operators risk developing vibration-related hand injuries, which can be career-ending. As a step toward improving the safety of these tools, this dissertation focuses on modeling their dynamic behavior to evaluate the effectiveness of vibration control strategies. The novelty of this work lies in the use of nonlinear mass– spring–damper models to describe tool dynamics, coupled with lumped-mass models of the hand–arm system. Traditionally, linear models have been employed for such evaluations; by contrast, this study introduces nonlinear modeling to capture the more realistic dynamics of HIMs. Furthermore, the role of a cubic nonlinear absorber in attenuating vibrations transmitted to the hand is systematically investigated through this framework. Key findings include the observation of nonlinear phenomena such as unstable periodic solutions, quasi- periodicity, chaos, and grazing. Frequency response analyses demonstrate the superiority of the cubic absorber over its linear counterpart, with notable improvements in performance when combined with an inerter. Parametric studies further reveal how the absorber can be tuned to enhance vibration attenuation across different nonlinear HIM models.
"You Have to Focus on School…You Can't Focus on Getting Paid": A Multi-Study Exploration of Perspectives and Experiences of Engineering Students Who Work
Johnson, Taylor Yvette (Virginia Tech, 2026-02-03)
Working while attending college is a prevalent practice among undergraduate students in the United States. However, adapting engineering programs to this reality and promoting the retention of undergraduate students remains a challenge. This research on working engineering students is significant as it sheds light on the need for such adaptation and its potential impact on engineering education. In this multi-study dissertation, I highlight the realities of working engineering students across multiple contexts, including community colleges and four-year universities, from various perspectives, such as those of students and advisors, in three manuscripts. The first manuscript is a phenomenologically informed qualitative study that focuses on the experiences of community college students and their understanding of computing identity as it relates to their jobs and coursework in introductory artificial intelligence (AI) courses. Upon analyzing their interviews, I found that working while enrolled in introductory computing courses provides participants with opportunities to demonstrate their computing knowledge in the workplace, gain recognition as a computing professional, and develop their interest in computing. The second manuscript is a qualitative study that focuses on the experiences of ten undergraduate engineering students who work while enrolled at an R1, 4-year university and how they navigate undergraduate engineering education while working. I studied the interactions between work and school in the experiences of engineering students who work, as well as the strategies they used to navigate their engineering education. I found that when participants encountered scenarios in which engineering education affected their work, they modified their work responsibilities during the academic year. When they faced situations where work affected their engineering education, they often avoided out-of-class engagement opportunities or modified their course schedules or routines (e.g., dropped classes, took semesters off). In Manuscript 3, I examined the elements of engineering education at universities that most frequently negatively impact engineering students who work, and how these elements affect them. For this study, academic advisors from nine R1 universities responded to a questionnaire that inquired about aspects of engineering education, drawing on the literature on students who work. Using descriptive statistics and descriptive qualitative coding, I analyzed the questionnaire responses. I found that the elements that most frequently negatively affected engineering students who work included time-intensive assignments, course scheduling, and teamwork expectations. These elements impacted students by increasing their time to graduation, reducing their engagement in extracurricular activities, and negatively affecting their academic performance. This research on working engineering students not only highlights the complexities in their situations but also offers practical implications for addressing these challenges. The solution is not necessarily to advise students to stop working, but to find ways to reconcile the tensions between their roles as students and workers. Implementing such strategies could benefit both students and institutions.

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