Rivers are amongst the most dynamic ecosystems on earth. River ecosystems are highly disturbed environments, where riparian vegetation, water and sediments, are interconnected by positive and negative feedbacks, driven by a set of interactions. In the last two decades, it has been widely recognized that these eco-morphodynamic feedbacks play a crucial role in governing the equilibrium and dynamics of river ecosystem. However, the incomplete understanding and quantification of these feedbacks limit the comprehension of river behavior and the development of efficient predictive models. Thus, in this research, fundamental intrinsic feedbacks between riparian vegetation and hydro-morphodynamic disturbance are modeled, where the disturbance is generated bymthe vegetation itself. The aim is to investigate how these intrinsic feedbacks govern themequilibrium and dynamics of a simplified river ecosystem.mTo this end, numerical simulations were conducted using both a 0D model (non-spatial)mand a 1D model (spatial) coupling hydro-morphodynamics with vegetation dynamics. The case study is a straight channel where vegetation can grow only in the central patch, while upstream and downstream there are bare soil regions. The system is perturbed periodically by a succession of floods of constant amplitude. Vegetation growth occurs in between of two consecutive floods, during low flood periods. Vegetation consists of two components, the above-ground biomass (canopy) and below-ground biomass (root depth). In both models, the canopy increases the roughness, reducing flow velocity. Variations in the flow field and the reduction of bottom shear stress modify sediment transport, leading to a greater imbalance between the vegetated and bare areas and thus, inducing erosion. Erosion increases the probability of vegetation uprooting, and when scour reaches root depth, uprooting occurs. The overall feedback loop is negative: higher vegetation biomass causes greater sediment flux imbalance and more erosion, ultimately resulting in less vegetation. However, root growth may inhibit the negative feedback loop, promoting positive feedbacks. Indeed, this interplay between hydro-morphodynamic disturbance (erosion) and the vegetation resistance (root depth), governs the predominance of either a positive or a negative feedback overall balance. Model results demonstrate that when the positive feedback overall balance prevails, the system always reaches a stable configuration. Furthermore, the system can exhibit hysteresis, meaning that, depending on the initial condition, it can achieve a stable configuration in two alternative states, the fully vegetated condition or bare soil. In the presence of the vegetated patch, the system can also exhibit a more complex multi-stable behavior, with infinite equilibria between the two alternative states. This also implies that spatial interactions smooth out critical transitions and tipping points, by facilitating smoother shifts that occur gradually through multiple smaller intermediate steps. Indeed, the resilience of the system, which is the ability of the system to still maintain its fundamental structure and functions after being subject to the ecological disturbance, increases due to spatial interactions. In contrast, when the negative feedback overall balance prevails, the system never reaches a steady state but exhibits dynamic oscillations. The oscillations can be either (i) periodic or (ii) aperiodic, strongly dependent on initial conditions, and with a positive Maximum Lyapunov Exponent, indicating chaotic behavior. The study also reveals that the route to chaos is a period-doubling bifurcation, and the calculation of time scale of predictability shows that the system is predictable only for a few growth-flood cycles. These results suggest that altering the ratio between hydro-morphodynamic disturbance and vegetation resistance, such as through anthropogenic pressure and climate change, may shift the system from a positive to a negative feedback overall balance. This shift could lead from a stable state to periodic oscillations or unpredictable chaotic behavior, limiting long-term predictions of river trajectories. Additionally, understanding how positive and negative eco-morphodynamic feedbacks govern river dynamics can contribute to develop efficient predictive models. Models are essential tools for implementing efficient river management and facilitate effective communication with stakeholders.

As a growth industry, tremendous cost pressures are pushing the LED lighting market away from traditional power electronics converters and towards solutions that are more unconventional. Lower quality LED lights use simple low-cost converters, whereas high end product may add complexity in order to achieve a more dramatic energy savings. SIMO technology represents an opportunity in LED lighting to combine the low cost of single-stage converters with the energy saving capability of a two-stage, multiple string solution. This paper describes the modeling, analysis, design, and testing of a Multiple Independently Regulated Output Flyback (MIROF) converter, used in LED lighting for the purpose of multiple string control. This converter is based upon SIMO technology applied to a PFC Flyback converter. The result was the development of a novel control method and an operational demonstration unit. A cost comparison of the MIROF and a conventional two-stage driver shows a promising cost reduction benefit for the former, and comparative testing shows favorable performance of the MIROF converter compared to the two-stage approach. / Master of Science

Bandwidth enhancement of low-profile omnidirectional, electrically-small antennas has evolved from the design and construction of AM transmitter towers eighty years ago to current market demand for battery-powered personal communication devices. Electrically-small antenna theory developed with well-known approximations for characterizing radiation properties of antenna structures that are fractions of the radiansphere. Current state-of-the-art wideband small antennas near kaH1 have achieved multiple-octave impedance bandwidths when utilizing volume-efficient designs. Significant advances in both the power and miniaturization of microelectronics have created a second possible approach to enhance bandwidth. Frequency agility, via switch tuning of reconfigurable structures, offers the possibility of the direct integration of high-speed electronics to the antenna structure. The potential result would provide a means to translate a narrow instantaneous bandwidth across a wider operating bandwidth. One objective of the research was to create a direct comparison of the passive- multi-resonant and active-reconfigurable approaches to enhance bandwidth. Typically, volume-efficient, wideband antennas are unattractive candidates for low-profile applications and conversely, active electronics integrated directly antenna elements continue to introduce problematic loss mechanisms at the proof-of-concept level The dissertation presents an analysis method for wide bandwidth self-resonant antennas that exist in the 0.5dkad1.0 range. The combined approach utilizes the quality factor extracted directly from impedance response data in addition to near-and-far field modal analyses. Examples from several classes of antennas investigated are presented with practical boundary conditions. The resultant radiation properties of these antenna-finite ground plane systems are characterized by an appreciable percentage of radiated power outside the lowest-order mode. Volume-efficient structures and non-omnidirectional radiation characteristics are generally not viable for portable devices. Several examples of passive structures, representing different antenna classes are investigated. A PIN diode, switch-tuned low-profile antenna prototype was also developed for the comparison which demonstrated excessive loss in the physical prototype. Lastly, a passive, low-profile multi-resonant antenna element with monopole radiation is introduced. The structure is an extension of the planar inverted-F antenna with the addition of a capacitance-coupled parasitic to enhance reliable operation in unknown environments. / Ph. D.

The objective of the study was to investigate multi-modality effects (visual and auditory) on the perception of urgency in elderly individuals addressing issues of safety and comfort. Twenty individuals (10 young, 10 elderly) from Montgomery County of Virginia participated in the laboratory study. In the pre-experimental audiometric tests, as expected elderly individuals had higher hearing thresholds (dBHL) than young across all the frequencies (250, 1000, 2000 and 4000 Hz) of pure tones. The difference was more pronounced at 4000 Hz. In visual acuity tests administered, elderly individuals had a lower Snellen VA than young at both near and far distances. The participants also were administered Bausch and Lomb color test; all participants were able to differentiate red color. Method of adjustment was used to collect empirical data. Rating test was administered after every treatment to objectively assess the participants' feeling of perceived urgency associated with the auditory warning signal. All the participants rated the experimental sound as urgent. Repeated measures was conducted to remove variations due to time and treatment conditions. A mixed-factorial design was used to investigate the main and interaction effects. The age effect was found to be marginally statistically significant. Modality and trial effects were not found to be significant. The reanalysis conducted after removal of outliers resulted in marginal statistical significance for age and modality main effects. Although not significant, a trend was seen in the pulse intensity levels (dBL) across modalities in the elderly individuals. Future research should be conducted to investigate modality effects in elderly individuals in a naturalistic driving environment which would give a better insight of the effects of dual modalities. / Master of Science

As the use of unmanned vehicles continues to grow, so does the need for systems to safely test and operate these vehicles. While there are safety systems designed for this purpose, they are often developed for a specific vehicle platform. The Multi-Level Emergency Stop (MLES) system provides three user-defined emergency response contingencies that can be adapted to a wide variety of unmanned vehicles. The Multi-Level Emergency Stop system is designed to be an ad-on safety system that can be integrated into ground, air, or surface unmanned vehicles. A complete MLES system consists of a hand held transmitter and a vehicle mounted receiver. The three levels of contingencies are controlled by three switches on the transmitter. These switches engage and disengage contacts located in the receiver via a wireless link. The function of these contacts is determined by the user for each unique application. Presented in this thesis is the detailed hardware design and software layout of the Multi-Level Emergency Stop system. Also included are the performance results and operational tests. / Master of Science

We consider a multi-task learning problem, where an agent is presented a number of N reinforcement learning tasks. To solve this problem, we are interested in studying the gradient approach, which iteratively updates an estimate of the optimal policy using the gradients of the value functions. The classic policy gradient method, however, may be expensive to implement in the multi-task settings as it requires access to the gradients of all the tasks at every iteration. To circumvent this issue, in this paper we propose to study an incremental policy gradient method, where the agent only uses the gradient of only one task at each iteration. Our main contribution is to provide theoretical results to characterize the performance of the proposed method. In particular, we show that incremental policy gradient methods converge to the optimal value of the multi-task reinforcement learning objectives at a sublinear rate O(1/√k), where k is the number of iterations. To illustrate its performance, we apply the proposed method to solve a simple multi-task variant of GridWorld problems, where an agent seeks to find an policy to navigate effectively in different environments. / Master of Science / First, we introduce a popular machine learning technique called Reinforcement Learning (RL), where an agent, such as a robot, uses a policy to choose an action, like moving forward, based on observations from sensors like cameras. The agent receives a reward that helps judge if the policy is good or bad. The objective of the agent is to find a policy that maximizes the cumulative reward it receives by repeating the above process. RL has many applications, including Cruise autonomous cars, Google industry automation, training ChatGPT language models, and Walmart inventory management. However, RL suffers from task sensitivity and requires a lot of training data. For example, if the task changes slightly, the agent needs to train the policy from the beginning. This motivates the technique called Multi-Task Reinforcement Learning (MTRL), where different tasks give different rewards and the agent maximizes the sum of cumulative rewards of all the tasks. We focus on the incremental setting where the agent can only access the tasks one by one randomly. In this case, we only need one agent and it is not required to know which task it is performing. We show that the incremental policy gradient methods we proposed converge to the optimal value of the MTRL objectives at a sublinear rate O(1/ √ k), where k is the number of iterations. To illustrate its performance, we apply the proposed method to solve a simple multi-task variant of GridWorld problems, where an agent seeks to find an policy to navigate effectively in different environments.

The purpose of this study is to examine the relationship among student civic knowledge scores and several different variables each at the student, classroom/school, and national levels using the IEA CIVED study international data set collected in 1999 from 27 countries. The student level predictors included two elements of socioeconomic status (a student's parental education, their home literacy level measured by the number of books at home), student's perception of an open classroom climate, student aspiration of obtaining higher education, and other variables that were identified as relevant to the dependent variable in the literature. The classroom/school level predictors included teacher's degree in civics, in-service training, teaching confidence, and school safety in addition to the compositional variable created as the classroom/school averages by aggregating the student level variables. Then I investigated whether instructional methods focusing on the student activities the teacher employed in the classroom and an open classroom climate were associated after accounting for the above student and school level background variables. National level variables such as GNP, GINI index, democratic system, public education expenditure, and etc. as well as compositional variables obtained by aggregating the classroom/school variables were also added to the model to investigate if they were associated with students' civic knowledge scores and whether they could explain between nations variability. The study used a three-level hierarchical linear model to analyze the data, with number of students, N=56,579, number of classrooms/schools, J=3443, and number of countries, K=27. Some of the key findings was that there were significant variations of civics knowledge among nations, and significant variations of civic knowledge scores between school and within nations, no statistically significant association between teacher's practice and civics knowledge scores, however the student perception of an open classroom climate was significant at all 3 levels. These findings were interpreted in terms of policies and practices that could be implemented to improve students' civic knowledge. / Ph. D.

The limited available budget along with old aging infrastructure in nation magnifies the role of strategic decision making for maintenance of infrastructure. The challenging objective is to maintain the infrastructure asset systems in a state of good repair and to improve the efficiency and performance of the infrastructure systems while protecting and enhancing the natural environment. Decision makers are in need of a decision support system to consider these multiple objectives and criteria to effectively allocate funding and achieve the highest possible return on investment on their infrastructure. The research proposes and validates a framework for such decisions. The proposed model aims at finding optimal techniques for maintenance of multiple roadway asset items while taking into account time, cost, level of service and environmental impacts. Therefore, the goal is to answer what are the optimal combinations of maintenance techniques for roadway assets while more than one objective is being optimized. In other words, the main objective is to develop a decision support system for selecting and prioritizing necessary actions for MRandR (Maintenance, Repair and Rehabilitation) of multiple asset items in order for a roadway to function within an acceptable level of service, budget, and time while considering environmental impacts. To achieve these desirable outcomes, this model creates a two-stage framework for a sustainable infrastructure asset management. First a multi-objective problem based on the multi colony ant colony optimization is analyzed. The objectives of the problem are: (i) Minimizing maintenance costs, (ii) Minimizing maintenance time, (iii) Minimizing environmental impacts and (iv) Maximizing level of service improvement. In the second stage, the results of the multi objective optimization will be prioritized using a Multi Criteria Decision Making (MCDM) process. The proposed approach will simultaneously optimize four conflicting objectives along with using a multi criteria decision-making technique for ranking the resulted non-dominated solutions of multi objective optimization. The results of implementation of the proposed model on a section of I-64 highway are presented for a sub-set of asset items. Moreover, the proposed model is validated using a scalable test problem as well as comparison with existing examples. Results reveal the capability of the model in generation of optimal solutions for the selection of maintenance strategies. The model optimizes decision making process and benefits decision makers by providing them with solutions for infrastructure asset management while meeting national goals towards sustainability and performance-based approach. In addition, provides a tool to run sensitivity analysis to evaluate annual budget effects and environmental impacts of different resource allocation scenarios. Application of the proposed approach is implemented on roadway asset items but it is not limited to roadways and is applicable to other infrastructure assets. / Ph. D.

When experiencing a building's interior or exterior conditions, one may be inclined to 'feel-out' its spatial and volumetric proportions, judge their appropriateness, its quality of formal conditions, its power, its clearness of the structure, and get a sense for the way its architecture was placed onto the site. It is said that, 'knowledge is key', and knowing how a building is soundly and structurally assembled and seated onto the earth -- is key. This thought brings to the table an important question, why do we build beautifully sound and monolithic (at times) structural systems then choose to cover them up entirely? In the context of the Washington, D.C.'s current building climate -- why must we build a dense grove of slender wood posts atop concrete plinths only to cover them up in clothing with certain ephemeral stylistic ideas? Obvious reasons such as insulation and weatherproofing are valid, but thermal barrier technology now allows for exposing the raw architectural elements without sacrificing thermal qualities. Can we use this technology to our advantage, and if so, how would one begin to conceive of a structural system which celebrates the bearing members in an architectural manner? Are there ways to interact more directly with the structure itself? In what manner will the site specific and environmental constraints play a role in making creative architectural decisions? I believe the research conducted in the past year resulted in a truthful approach toward form finding, space making, and respecting the chosen site and its unique constraints. / Master of Architecture

Distributed sensing has been an attractive alternative to the traditional single-point sensing technology when measurement at multiple locations is required. Traditional distributed sensing methods based on silica optical fiber and electric coaxial cables have some limitations for specific applications, such as in smart textiles and wearable sensors. By adopting the fiber thermal drawing technique, we have designed and fabricated multi-material electrode-embedded polymer fibers with distributed sensing capabilities. Polymers sensitive to temperature and pressure have been incorporated into the fiber structure, and thin metal electrodes placed inside fiber by convergence drawing have enabled detection of local impedance change with electrical reflectometry. We have demonstrated that these fibers can detect temperature and pressure change with high spatial resolution. We have also explored the possibility of using polymer optical fiber in a Raman scattering based distributed temperature sensing system. Stokes and Anti-Stokes signals of a PMMA fiber illuminated by a 532 nm pulsed laser was recorded, and the ratio was used to indicate local temperature change. We have also developed a unique way to fabricate porous polymer by thermal drawing polymer materials with controlled water content in the polymer. The porous fibers were loaded with a fluorescent dye, and its release in tissue phantoms and murine tumors was observed. The work has broadened the scope of multi-material, multi-functional fiber and may shed light on the development of novel smart textile devices. / PHD / In recent years smart textiles and wearable gadgets have already changed the way we live. There has been increasing industrial interest to develop novel flexible, stretchable devices that can interact with human and the environment. Thermal drawing technique originally invented for manufacturing telecommunication optical fiber has been used by researchers to fabricate fibers with more functionality. In this work, we report the progress made on the fabrication of multi-material fiber. Soft polymer fibers capable of measuring temperature and pressure were designed and made by the thermal drawing technique. Submillimeter fibers with thin copper electrodes have shown potential to be readily embedded in a smart fabric to provide 1D information in one direction or woven into a 2D pattern for area monitoring. We have also explored another temperature measurement scheme using polymer optical fibers with a pulsed laser. Compared with the electronic fibers, it is less susceptible to electrical noise and more robust. Lastly, we have shown a unique way to generate porosity in thermally drawn polymer fibers. The elongated pores in the fibers come from water escaping the fiber during the fabrication process. The three aspects of the project expand the scope of multi-material, multi-functional fiber and can shed light on the future development of electronic textile devices.