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On Wake Interactions Between Thrust and Drag Producing Bodies
No full textWake interactions play a crucial role in modern aerodynamics and hydrodynamics. Thisdissertation focuses on the wake interactions that occur between two oscillatory propulsors as well as between a thrust producing jet wake and a drag producing separated wake. First, experiments are presented on the interaction between a pair of in-line, sinusoidally pitching hydrofoils with fixed Strouhal number of St = 0.25 and Reynolds number of Re = 10, 000. The spatial locations of the hydrofoils relative to one another in the streamwise and spanwise directions is varied. Streamwise spacings range from 0.625 chords of spacing to 1.125 chords of spacing. Spanwise spacings ranged from fully aligned (no spacing) up to 0.5 spans of spacing. The phase delay, or offset, between the oscillations of the hydrofoils is also varied ranging from 0� to 360�. The hydrodynamic forces acting on both hydrofoils is recorded to investigate changes in time-averaged thrust, lift, spanwise force, power consumption, and propulsive efficiency. Additionally, a novel method to mitigate confounding factors from connecting rod drag is employed using streamlined fairings. The effects of spanwise spacing on the interaction of the hydrofoils were found to depend on the spanwise contraction of the leader hydrofoil�s wake as it advected downstream before impinging on the span of the follower hydrofoil. The rate of spanwise contraction of the leader�s wake is combined with force measurements to define small and large spanwise spacings. Small spanwise spacings are those which do not reduce the portion of the follower hydrofoil�s span where the incoming wake shed by the leader is directly impinging. Small spanwise spacings were found to only minimally change the thrust, power, and efficiency of the follower hydrofoil. Large spanwise spacings are those which do reduce the portion of the follower hydrofoil�s span directly interacting with the wake of the leader hydrofoil, linearly weakening the wake interaction between the hydrofoils. This results in the hydrodynamic performance of the follower hydrofoil being driven towards its isolated value. The delineation between small and large spanwise spacings was also dependent on the streamwise spacing between the hydrofoils. Smaller streamwise spacings are found to narrow the range of spanwise spacings that would be considered small. Moreover, negligible spanwise forces are measured for all of the spanwise spacings tested, which shows that independently swimming foils would be neutrally stable to spanwise perturbations. In addition the performance of the follower hydrofoil showed a strong dependence on its streamwise positioning relative to the leader hydrofoil. Near streamwise spacing (0.625 to 0.875 chords) and far streamwise spacing (0.875 to 1.125 chords) regions were identified, with the delineation between the two occurring at 0.875 chords of streamwise spacing. Distinct trends in the performance of the follower hydrofoil were identified in each streamwise region. In the far region moving the hydrofoils closer together in the streamwise direction resulted in an increase in the maximum time-averaged power consumption of the follower hydrofoil while this trend reversed in the near region. Using these experimental results, an empirical model for the average power consumption of the follower hydrofoil is developed. Separately, an experimental study on the interaction of a small Uncrewed Aerial System (sUAS) with a fast-moving ground vehicle in a wind tunnel setting is presented. In contrast to the first study, which considered the interaction of two thrust producing wakes of equal magnitude, the goal of this study is to investigate the interactions of thrust and drag producing wakes of different sizes relative to one another. The study records the aerodynamic forces experienced by the sUAS and the resulting flow fields for a variety of relative spacings between the models. The flow around each of the models is characterized using both time averaged and high speed particle image velocimetry (PIV). In order to better understand the wakes of the models, efforts are made to bound the wakes using readily observable flow features in the time average PIV data. The extents of the separated wake structure of the ground vehicle was found to be easily bounded by contours of Utotal = 0.95U?. The jet wake of the sUAS was found to be bound by contours of URMS = 0.10U?. Interaction with the wake of the RGV was found to cause significant changes to the aerodynamic performance characteristics of the sUAS. In particular, close proximity to the RGV was found to increase the unsteady lift and thrust forces acting on the sUAS as well as the time average thrust force. Analysis of the interaction flow fields at a variety of relative positions between the models revealed several locations where the interaction flow field appeared to be a simple summation of the isolated flow fields. From this information an additive wake hypothesis was formulated and investigated through the further investigation of the isolated flow fields as well as through an energy spectrum analysis of the wake flow fields. This study categorizes the observed wake interactions as: (1) Interactions where the sUAS did not modify the wake of the ground vehicle model; (2) Interactions where the sUAS altered the RGV wake, the sUAS jet wake was visible in the interaction case, did not modify the upstream flow, and some similarities were observed between the interaction case and a combination of isolated flow fields; (3) Interactions where the two wakes interacted in a complex manner, which occurred at locations where the sUAS was positioned near the heavily decelerated regions of flow in the wake of the ground vehicle, modified the upstream wake of the ground vehicle, or was close enough for ground effect. This third interaction region demonstrated that an sUAS, which is considerably smaller than a commercial ground vehicle, has the capability to significantly alter the flow field around a ground vehicle and thus significantly impact the type of wake interaction that it may encounter while operating near the ground vehicle. These studies emphasize the importance of relative positioning between objects in flow and highlight the effects of relative spacing on the types of wake interactions that will take place in a variety of flow conditions
The Influence of Changing Reward Sequences on Cognitive Flexibility
No full textCognitive control, or the way in which the mind carries out tasks, can be divided into two control modes: stable and flexible. Cognitive stability allows goal maintenance, reduced distractibility, and more difficulty in adjusting or updating thoughts and behaviors. Conversely, cognitive flexibility encourages divergent thinking, eases shifting between mental representations, and makes processing more susceptible to distraction. A number of factors can modulate the balance between flexible and stable cognitive control modes, which may benefit performance in a variety of contexts. Recent task-switching studies have demonstrated an increase in cognitive flexibility associated with increases in reward prospect. The following experiments expanded upon these findings, investigating whether increasing reward sequence can produce more flexibility within two separate tasks outside the task-switching domain: problem solving and semantic retrieval. In Experiment 1, I explored the relationship between reward sequences and cognitive flexibility in a modified water jug task, expanding on my preliminary study showing that increased reward prospect led participants to break mental set more frequently. In Experiment 2, I explored the effect of reward sequence on flexibility within the verbal fluency task, where switching between semantic subcategories served as a measure of cognitive flexibility. Because different reward sequences produce different levels of flexibility in a task-switching environment, I expected a similar effect in other tasks that measure flexibility as well. In Experiment 1, reward sequence did not influence participants� likelihood of breaking mental set, nor did it influence the speed with which mental set was broken. In Experiment 2, reward sequence did not affect the number of clusters, size of clusters, nor speed with which individuals switched between clusters of items from different subcategories. Results from both experiments suggest that changing reward sequences do not seem to significantly influence flexible behavior in contexts outside of task-switching. Consequently, it may be necessary to reevaluate the way we define cognitive flexibility to avoid labeling different cognitive mechanisms as the same concept
Caregiving for Older Adults During Emerging Adulthood: The Relationship Between Caregiver Identity and Caregiver Distress
No full textAs the older adult (age 65+) population grows in the United States, the demographics of caregivers are expanding to include more emerging adults (ages 18-25) who are participating in caregiving tasks. Minimal research thus far has focused on this subgroup of caregivers to older adults, especially with regards to understanding factors that may influence their levels of caregiver distress. Guided by caregiver identity theory, the current study examined 1) the relationship between caregiver identity and caregiver distress, 2) formal support service utilization as a mediating factor between caregiver identity and caregiver distress, and 3) the inclusion of ageist attitudes within caregiver identity models. A sample of 259 emerging adult caregivers were recruited through Amazon Mechanical Turk (Amazon MTurk) and asked to respond to survey measures online. A structural equation model estimated the relationships between the latent constructs of caregiver identity discrepancy, caregiver identity salience, support service utilization, ageist attitudes, and caregiver distress. After iterations of hypothesized models were tested using theoretically justifiable modifications, both original and alternative models produced acceptable fit to the data. The findings of the current study indicate that caregiver identity within emerging adults is a unique, multidimensional process consisting of both identity discrepancy and identity salience latent constructs. Results showed caregiver identity discrepancy theory may not be applicable to emerging adults, possibly due to identity flexibility during this developmental period. High caregiver identity salience was protective against caregiver distress for emerging adult caregivers, highlighting the importance of meaning-making within stressful caregiving contexts. Ageist attitudes represent an important area of future research within studies of caregiving identity development. Implications for formal support services to best meet the needs of emerging adult caregivers are discussed
Learning Planning Knowledge in Structured Domains with Numeric Fluents
No full textAutomated planning is the process of using domain knowledge to discover a sequence of actions that, when enacted upon the world state, transition it from some initial state to a target goal state. Planners require at minimum a specification of what actions can be taken in the world, under what (pre-) conditions, and producing what effects. Descriptions of actions can be generalized to operators, which are applicable to many states. In Hierarchical Task Network (HTN) planning, the planner accepts an additional type of knowledge called task-decomposition methods (or simply methods). Methods describe how to perform a complex task as a (possibly ordered) collection of simpler tasks and are used to decompose complex tasks into a collection of simpler tasks. The methods define a hierarchical network of task-subtask relationships that guides the planner�s search. Of course, defining the methods increases the knowledge-engineering effort and domain expertise required of the domain author, but a well-written HTN planning domain can provide significant speed-up over a classical planner. Poorly written domain descriptions can cause an HTN planner to erroneously declare a problem unsolvable, deliver a sub-optimal plan, or spend more time solving a problem than would a classical planner. Automation of the design process can reduce the human workload and may reduce the errors in the domain. Even the generation of incomplete domain descriptions are beneficial, as the time taken to complete or validate a partial domain is still less than the time to write a domain from scratch.In this work, we describe a novel automated process for learning numeric domain descriptions for use in HTN planning, from plan traces. This includes two automated learners. The first is an automated action learner which can learn linear numeric preconditions and effects and can learn more complex numeric equations using user-provided domain information. The second is an automated method learner that can learn arbitrarily complex numeric preconditions. There are two versions, one which learns numeric subgoals and one which learns numeric subtasks. The system takes advantage of an extension of planning landmarks, called domain landmarks, to construct landmark methods that decompose complex, high-level domain tasks into landmark tasks based on the domain landmarks, which are then themselves decomposed via various strategies. We test the correctness of the learned operators learned under different hyperparameters by solving randomized test problems and validating the resulting plans on a domain simulator. We test the methods learned by our procedure by solving planning problems and comparing their performance against the performance of methods learned by other method-learning algorithms. We find that our operator learning technique learns usable operators for simple numeric domains under some hyperparameters settings. We find that in method learning, the use of landmark methods is either competitive with or improves performance over purely-right recursive methods. We also discuss trade-offs in the structure used to decompose the landmark tasks, showing that more planning-time efficient structures sacrifice plan-efficiency and vice-versa
School Climate and Student Motivation: Considering Student Adjustment as a Mediator
No full textPrevious research identifies numerous effects that a student�s motivation has on their educational experience. Studies have shown that the lack of motivation is connected to poorer academic performance and attendance rates and increased behavioral concerns and dropout rates. Therefore, understanding what factors influence students� motivation is crucial in helping students succeed. School climate has been found to have a positive relationship with student motivation. However, less is known about how individual student adjustment may influence the relationship between school climate and student motivation. This study assessed the latent variable of school climate and a mediational effect of student adjustment on the association between school climate and motivation, defined as academic self-efficacy, in a sample of adolescent Latinx students. Results showed the subscales of teacher caring, office disciplinary referrals, and school problems created the variable of school climate. Additionally, student adjustment was a significant mediator between the association of school climate and academic self-efficacy. Results are discussed in the context of the current literature and suggest practical implications for school psychologists and educators
CNT-Filled Conductive Adhesive for Surface Mount for Polymer Thick Film Circuits
No full textDue to the low melting temperature of polymer substrates, the use of molten metal solderfor component mounting on polymer thick film printed circuits is not a viable option. Therefore, electrically conductive adhesives, primarily based on epoxy and silicone chemistries filled with conductive metal fillers, are used for the surface mounting of components. For epoxy chemistries, high achievable filler loadings make them excellent for applications requiring high electrical conductivity. However, the high required filler loading necessary to achieve good electrical conductivity often leads to epoxy conductive adhesives being highly brittle. Therefore, a solution that would allow for improved ductility while maintaining the necessary electrical conductivity would be highly beneficial. In this work, carbon nanotubes (CNTs) were proposed as a potential epoxy conductive adhesive filler that would allow for these properties based on an extensive literature proving their ability to improve the mechanical and electrical properties of epoxy resins. Therefore, a series of experiments were performed utilizing a commercially available epoxy-multi-wall carbon nanotube blend (MWCNT-D.E.R. 383) to reduce the silver loading in a series of epoxy conductive adhesives to study the effect on viscosity, cure profile, volume resistivity, and flexibility. Batches with successively reduced silver loading, 80,76, 72, 60, and 68 wt. % respectively, were fabricated and subsequently characterized. The results demonstrated that CNTs successfully allowed for the reduction of required silver loading, thereby improving flexibility while maintaining good electrical conductivity. Based on these results, this work provides a novel fabrication process for novel Ag-CNT-Epoxy conductive adhesive formulations with tailorable trade-offs between electrical and mechanical properties
Three essays in microeconomics
No full textChapter 1: Consumer Behavior and Product Return PoliciesThis paper studies the relationship between consumer behavior and firm product return policies. To characterize possible types of consumers with regard to their tendencies to return products, we construct a model with three essential factors related to product returns: hassle costs experienced by consumers from returning a product, the extent to which a consumer can use a product before returning it, and a firm\u27s value of salvaging a returned product. Consumer types fall into three categories: consumers who always return products and (partially) use them before doing so, consumers who never return products, and consumers whose return decisions are sensitive to a firm\u27s return policy. In characterizing optimal firm return policies and prices, we derive the following results from the types of consumers we analyze. Of the three consumer types, the firm can feasibly not serve the never-return type consumers and has a profit motive in some scenarios not to do so. Additionally, because a firm\u27s optimal profit is strictly decreasing in the hassle cost of the return policy sensitive consumer type, the firm has an incentive to make product returns easier for these consumers.Chapter 2: Start or Quit: the Effect of E-Cigarette Innovations on Adolescents Smoking BehaviorsThis paper utilizes National Youth Tobacco Survey data from 2011 to 2020 to construct an event study examining the impact of temporal e-cigarette innovations on tobacco cigarettes and e-cigarette smoking rates among adolescents. Specifically, I evaluate the cross-product effect of e-cigarettes on tobacco cigarettes. The empirical evidence indicates that the temporal innovations of e-cigarettes have limited success in encouraging adolescents to quit smoking tobacco cigarettes while simultaneously leading more non-smokers to start using e-cigarettes. Furthermore, this paper presents evidence that peer effects and the appeal of e-cigarette flavors primarily drive adolescents\u27 e-cigarette usage. Our analysis suggests that, when regulating the e-cigarette market, policymakers should consider the influence of e-cigarette innovation on adolescent usage of e-cigarettes and tobacco cigarettes.Chapter 3: Price Discrimination in the Two-Sided Food DeliveryThis paper investigates a two-sided market in the food delivery industry where the platform interacts with restaurants first, and then consumers place orders based on a two-part pricing mechanism and the number of restaurants. Unlike the existing literature in which platform(s) simultaneously set prices for the two sides of the market, we model the market using this sequential setup. We develop a model based on Rochet and Tirole (2003), in which a monopoly platform uses a two-part pricing mechanism for the buyer side of the market. Our results show that the platform subsidizes the seller\u27s side to increase the number of restaurants in the market, surprisingly, if and only if the level of the network externality is below a threshold. Additionally, the platform\u27s optimal transaction fee for buyers is decreasing, and the membership fee is increasing in buyers\u27 valuation of restaurant diversity, which also reflects the level of network externality
Enhancement of Dry Cooling Systems through the Integration of CaCl2·6H2O-based Latent Heat Cold Storage
No full textThis research reports work to enhance dry cooling systems by integrating a CaCl?·6H?O-based latent heat cold storage technique, underpinned by in-depth molecular dynamics (MD) simulations and extensive experimental work. The MD simulations first examined the hydration behavior of CaCl? solutions, illustrating how changes in salt concentration impact the behavior of the sal at the molecular level. These simulations predicted radial distribution functions and coordination numbers, providing a detailed understanding of the interaction between ions and water molecules. At a critical concentration of 10 wt.% CaCl?, hydration structures reached a threshold beyond which ion-pairs became noticeable. This molecular insight is pivotal for the system\u27s thermal performance as it influences the freezing point depression and the effectiveness of the phase change material (PCM) in thermal storage applications.Furthermore, the MD studies investigated the impact of scaled ionic charges on the freezing point depression of aqueous CaCl? solutions. The results highlighted the superiority of the Madrid-2019 force field, which employs scaled ionic charges over the traditional OPLS-AA force field with full ionic charges. This advanced modeling provided a more consistent prediction of experimental data, especially at higher concentrations, enhancing the accuracy of the system\u27s thermophysical property predictions.Subsequently, the experimental work, built on the theoretical insights, focused on the development of the PCM. A CaCl?·6H?O-based PCM was synthesized, with careful consideration of the material\u27s thermal properties, such as latent heat and supercooling characteristics. The experimental work involved incorporating nucleation agents like SrCl?·6H?O to minimize supercooling, enhancing the freeze-thaw stability of the PCM, which proved crucial for its effective use in thermal storage applications. Additionally, the inclusion of 2 wt% KCl was also investigated, which successfully shifted the salt peritectic composition, thus further stabilizing the PCM during phase transitions.Based on the developed composition, a cost-effective PCM was fabricated and integrated into pervious concrete through innovative micro- and macro-encapsulation techniques, creating a series of latent heat storage modules. These modules underwent rigorous thermal performance testing, revealing an increase in thermal capacities up to 49.2 kWh/m³ for charging and 43.6 kWh/m³ for discharging, in the temperature range between 15 and 35°C. The most effective module design was capable of consistently reducing the temperature of the airflow by an average of 2°C over three hours, showcasing its potential to act as a pre-cooler in dry cooling systems. This module utilizes a combination of micro-encapsulated PCM within pervious concrete and a metal tube array filled with PCM, which together provide an extended surface area for heat exchange and enhanced thermal conductivity.Building on this foundation, the work further investigated the experimental validation of two prototype-scale latent heat storage modules: PC-Ma-Mi-PCM and Ma-PCM. The PC-Ma-Mi-PCM module demonstrated the ability to lower air temperature by 4°C for 4 hours by stacking three of the modules together, while the Ma-PCM module, with eight times larger volume and higher energy storage capacity, was capable of reducing inlet air temperature by 4°C for up to 6 hours. These results were achieved through tests conducted in a specialized wind tunnel facility, equipped with a temperature measurement system and a large-scale PCM production setup. The findings from these prototype-scale tests would significantly contribute to the field of latent heat storage systems, offering novel solutions for energy management and conservation.Conclusively, the synthesis of simulation and experimental results in this thesis articulates a novel narrative of innovation in dry cooling technologies, with broad implications on energy efficiency and sustainable water conservation for power plants
