12023 research outputs found
Sort by
Analysis of Flapping Propulsion: Comparison, Characterization, and Optimization
Get PDFIn recent decades, the development of autonomous underwater vehicles (AUVs) has rapidly increased and inspiration for novel designs has recently come from nature, primarily based on the fast, efficient, and maneuverable flapping motion of fish. Due to its potential, flapping propulsion is investigated through three studies.
The first study involves the comparison between swimming by flapping and by periodic contractions. A direct comparison is made between the two propulsion mechanisms by simplifying the motions, utilizing a machine that can operate in either mode of propulsion, and evaluating the average thrust generated and the average input power required per cycle between the two mechanisms when the overall kinematics are identical. The two propulsion mechanisms are tested using a variety of overall kinematics, flexible plates, and modified duty cycles, all of which suggest that flapping propulsion is the more efficient; however, periodic contractions with a modified duty cycle are shown to generate more thrust per cycle.
The second study involves the characterization of the impact of chord-wise curvature on the hydrodynamic forces and torques, motivated by the dorso-ventral bending of a fish's caudal fin during locomotion. The impact of curvature is shown to depend on the planform area of the flapping plate. Plates with a smaller or an identical planform area compared with a baseline rigid flat rectangular plate either decrease or increase the generated thrust, respectively. These phenomena are utilized to develop an actuated plate for velocity modulation and a snap-buckling plate to provide a greater thrust and efficiency compared with a rigid plate propulsor.
The third study involves the development and demonstration of a method to experimentally optimize an arbitrary three-dimensional trajectory for a flapping propulsor. The trajectory is parameterized by variables inspired by birds and fish, executed by a mechanism that can actuate an arbitrary motion in a hemisphere, and optimized using an adaptive evolutionary strategy. The trajectories are scored based upon their difference from a desired force set-point and their efficiency. All trajectory searches demonstrate good convergence properties and match the desired force set-point almost immediately. Additional generations primarily improve the efficiency. This novel approach finds optimal trajectories for generating side-forces, similar to how a fish's pectoral fin or a bird's wing functions, and for generating thrust, similar to how a fish's caudal fin operates.</p
Theoretical Foundations for Quantum Measurement in a General Relativistic Framework
Get PDFIn this work, we develop theoretical formulations to analyze experimentally relevant quantum measurement schemes in a general relativistic framework, and discuss their implications versus the Newtonian or non-relativistic viewpoints. Specifically, we address (i) matter waves in simple free fall, (ii) the Mach-Zehdner atom interferometer with light-matter interaction and (iii) optomechanical systems. The motivation is to explore the regime of physics where gravity and relativistic effects become pertinent for quantum experiments due to the increase in system size and complexity. Such experiments may illuminate a way forward to reconcile the independently successful but apparently paradoxical theories of gravity and quantum mechanics, where sound theoretical foundations are necessary to help guide the search for new physics at their interface
Computational and Neural Mechanisms Underlying Decision-Making in Humans
Get PDFHow do we make economic decisions in everyday life? How do we make decisions in the face of uncertainty regarding the statistics of the environment? These are the questions that played a pivotal role in the formation of the field "Decision Neuroscience". In each chapter of this thesis, we investigated the computational and neural mechanisms to tackle these questions using behavioral and neural data acquired through fMRI experiments.
In the first chapter, we investigated the computational and neural basis of economic decision-making in a binary choice task between two food items. We analyzed behavioral and neural data in a task where participants conducted a sequence of binary choices under the manipulation of fixation-based attention. We developed and calibrated a computational model based on evidence sampling and accumulation to show that the model not only accurately captured basic properties such as choice and reaction time (RT) but also the effect of attentional manipulation in participants’ behavior. We found that the evidence accumulation process predicted by the model to drive a decision was implemented in the areas of frontoparietal network including dmPFC and IPS. These regions also exhibited increased functional connectivity with the activity in vmPFC during choice period where sampled evidence was represented. Our results suggest the involvement of these areas in value-based binary choice.
In the second chapter, we examined the computations involved in the decision making under uncertainty. In particular, we aimed to pin down the computations related to temporal change detection. Temporal change detection is the capacity to detect change in the statistics that govern the timing of occurrence of events. We analyzed behavioral data from a novel task where participants observed a sequence of images presented at irregular timings and tasked to detect a change in the frequency of image presentations. We developed and compared computational models from Bayesian to heuristic models and found that all the models captured quantitative aspects of participants’ behavior equally well despite the difference in their computational complexity. Thus, we could not distinguish computations involved in temporal change detection solely from the behavioral data.
In the third chapter, we aimed to elucidate the computations involved in temporal change detection from the perspective of neural implementation using fMRI data by leveraging the computational models examined in the previous chapter. We found that the key variable to guide a decision derived from a computationally frugal heuristic model correlated with the activity of the frontalparietal network including dlPFC and IPS, while similar variables derived from more computationally taxing Bayesian models did not show significant correlation with any of the brain regions. Our results suggest that humans might be relying on a simple heuristic model to implement temporal change detection.</p
An Exploration of Letter-Writing in Jane Austen's Work
No full textAs a medium of communication, written letters stand apart not only in the intimacy of the connection they spark between writer and recipient but also in the very real, tangible nature of their existence. Jane Austen's use of written letters as key literary devices, especially in Pride and Prejudice, Sense and Sensibility, and Persuasion, reflects her dexterity at exploring and developing the intricate web of social connection among her characters, particularly in the courtship plot. These letters seek to express what cannot be said aloud, overstepping whatever walls that have been put up by social convention or emotion in an attempt to reach directly to a person’s mind or heart. Almost as illuminating as the contents of the letters themselves is their sheer materiality – as readers, we are allowed to observe the process of their writing, seeing as they are handled, passed from person to person, read and re-read. In painting such a detailed, viscerally relatable picture of the exchange of these letters, Austen elevates them from words inked on a paper to something sacred, adding dimension to the novel by binding together the
thoughts and emotions of characters in ways that dialogue or gestures alone cannot
Clumped and Intramolecular Isotopic Perspectives on the Behavior of Organic and Inorganic Carbon in the Shallow Crust and Deep Biosphere
Get PDFAlthough the upper crust is the most accessible swath of the subsurface, the geochemical processes therein are challenging to observe. Here, I use site-specific and ‘clumped’ isotope analyses (where multiple rare, heavy isotopes are bound in single molecules) of carbonates and organic solids to explore petrology, brittle tectonics, diagenesis, and biodegradation in the shallow crust. In carbonates, I employ contact aureoles, regional metamorphic terranes, thermal and geochemical models, and high-pressure experiments, to extract nuanced thermal and chemical histories. In the organic realm, I develop new methods to measure the clumped 13C – 2H composition of methoxyl groups (R – O – CH3) from kerogen, lignin, and other complex organic substrates. This work expands the field of clumped isotope geochemistry into the realm of solid compounds, and enables new, site-specific isotopic perspectives on the shallow crustal transformations of organic carbon substrates and the microbial assemblages that live on them.</p
Performance and Stability Optimization of Solar Fuel Devices
Get PDFFossil fuels enabled the Industrial Revolution, and have been the most important power for promoting the world's economic growth ever since. However, burning fossil fuels have also been causing severe air pollution, and global warming is also related to excessive use of fossil fuels. Solar energy is considered to be the largest renewable clean energy resource. The principal problems of solar energy are low energy concentration and intermittency. Storing solar energy in chemical bonds, similar to photosynthesis in nature, is a possible way to overcome these two problems. Carbon-free chemicals, like hydrogen gas produced by solar-driven water splitting, or carbon-neutral chemicals, like methane, ethylene, formic acid, carbon monoxide, etc. produced from solar-driven CO2 reduction, are all promising clean fuels for solar storage, as they feature high energy/power intensity, are easy and cheap to store and transport, and have direct interface with existing infrastructures.
In this thesis, we focus on improving the efficiency and stability of the solar-driven fuel generation devices, which consist of (photo-)anode and (photo-)cathode. For the anode part, cobalt oxide Co3O4 ultrathin (2 nm) films by atomic layer deposition (ALD) were deposited onto silicon photoanode prior to deposition of thick nickel oxide (NiOx) layers. The photovoltage of the photoanode increased from 200 mV to 580 mV after including the interfacial Co3O4 layer, and the anode was stable in 1.0 M KOH(aq) for 1700 hours, which was equivalent to one year of operation in the field at a maximum photocurrent density of 30 mA/cm2 assuming a 20% solar capacity factor. Furthermore, the non-uniform sputtered (NiOx) layer of the n-Si/SiOx/Co3O4/NiOx photoanode was removed, and the 2 nm Co3O4 film was thickened to 50 nm, and the stability of n-Si/SiOx/50 nm-Co3O4 was improved to 2500 hours with lower efficiency decay rate. For the cathode part, an optimized Pd/C nanoparticle coated Ti mesh cathode exhibited < 100 mV overpotential at 8.5 mA/cm2 current density, and > 94% Faradaic efficiency for the reduction of 1 atm of CO2(g) to formate in 2.8 M KHCO3. A solar-driven CO2 reduction (CO2R) cell was constructed with this cathode, showing 10% solar-to-fuels conversion efficiency.
This thesis can be divided into three parts. The first part discusses importance of solar fuels, as well as gives an introduction of solar-fuel generators. The second part includes Chapter II and Chapter III, which deal with performance improvement of silicon photoanode with ALD Co3O4 thin films. The third part is Chapter IV, in which we study the cathode for CO2 reduction to formate, and demonstrate a 10% efficiency solar-driven CO2 reduction cell with the cathode.</p
The Quest for Electrocatalytic Nitrogen Fixation with a Molecular Catalyst and What We Learned Along the Way
Get PDFThis report details research into the mechanism and operating principles underlying the nitrogen fixation efficacy of a tris(phosphine)borane iron complex (P3BFe). The data presented provide what is to our knowledge the first unambiguous demonstration of electrocatalytic nitrogen fixation by a molecular catalyst and contribute to a growing body of evidence that metallocenes may play multiple roles during reductive catalysis.</p
Search for SUSY with Delayed Photons at the Compact Muon Solenoid
Get PDFThe Compact Muon Solenoid (CMS) experiment records data from proton-proton (pp) and heavy ion (Pb-Pb and Pb-p) collisions at the Large Hadron Collider (LHC) to search for physics beyond the Standard Model, test theories of supersymmetry (SUSY), and measure properties of known particles with higher precision. In 2025, the LHC will be upgraded to the High Luminosity LHC (HL-LHC), where the luminosity will be increased by a factor of 10. This will increase the number of pile-up collisions to 140-200 events per proton-proton bunch crossing, compared to the current 40 events per crossing (where each bunch crossing occurs every 25 ns). In order to fully exploit the sensitivity of the CMS experiment, the current detectors must be upgraded to mitigate the effects of the large number of pileup interactions expected in collisions at the HL-LHC. New capabilities, such as precision timing measurements in calorimetric devices and minimum ionizing detectors, have been shown to effectively mitigate the effects due to pileup, and are expected to benefit the overall physics reach of the experiment. In addition to mitigating pileup and increasing the detector capabilities, precision timing is beneficial in the search for particles beyond the Standard Model. A simulation of a benchmark long lived neutralino SUSY search is presented, and it is shown that the generator particle flight times can be faithfully reconstructed using the detector-level information. Identification algorithms for the SUSY model have been significantly improved with the use of a Boosted Decision Tree, and it is demonstrated that this algorithm has many benefits as compared to cut based IDs. With use of the BDT for the long lived neutralino SUSY model, the background rejection is increased significantly, with constant signal acceptance of 53.6%. This is an improvement in the significance of the signal selection by a factor of 2.38. Further improvement is seen with the inclusion of detector timing information in the BDT – with this contributing ≈25% of the information used in signal event identification. We thus demonstrate that with the BDT, the SUSY neutralino search can be performed with increased signal identification significance, and the searches’ sensitivity is expected to improve with the time resolution attained by the upgraded CMS calorimeter
Examination of Selenium Incorporation and Product Formation in the Nitrogenase FeMo-Cofactor
Get PDFNitrogenase is the only known enzyme to convert the triply bonded atmospheric dinitrogen (N2) to bioavailable ammonia (NH3) in an ambient environment, breaking one of the strongest chemical bond in nature in the process. Industrially, the Haber-Bosch process is also capable of reducing dinitrogen to ammonia, and is essential for worldwide food production 1,2. Due to the high temperatures and pressures required for the Haber-Bosch process (between 300-550ºC and 15-25 MPa) and its requirement for molecular hydrogen, it has become paramount to scientifically investigate the biological processes of nitrogen fixation to ultimately develop more efficient methods to produce bioavailable ammonia. Nitrogenase utilizes two component proteins, the Fe-protein and the MoFe-protein, to reduce ammonia in an ATP-hydrolysis dependent and electron-intensive reaction. Besides the canonical dinitrogen reduction reaction, nitrogenase can reduce a variety of other substrates including: acetylene (C2H2), carbon dioxide (CO2), carbon monoxide (CO), carbonyl sulfide (COS), nitrous oxide (N2O), diazene (N2H2), and more 3-11. CO has long been of interest to the study of the mechanism of nitrogenase, owing to its isoelectronic identity to N2, and its potent inhibitor properties at well as its ability to serve as a weak substrate 12,13. Like CO, cyanide compounds (X-CN) are also of interest to the study of nitrogenase due to the isoelectronic nature of CN- to N2. However, cyanide compounds serve as particularly interesting spectroscopic and crystallographic tools, because X in X-CN can be substituted for more significant sulfur or selenium (Se). In this study, we investigate the substrate properties of SeCN-, with Se-incorporation into the active site FeMo-cofactor and concurrent reduction of SeCN- to methane (CH4). This study serves as yet another link between substrate reduction in nitrogenase. Part of this work describes the incorporation of Se into the cofactor as a vehicle for high-resolution study of nitrogenase under turnover using spectroscopy and crystallography, while another part describes a proposal for future work on the trapping of enzyme intermediates by fast-growing crystallography.</p
On Hodge-Newton Reducible Local Shimura Data of Hodge Type
Get PDFRapoport-Zink spaces are formal moduli spaces of p-divisible groups which give rise to local analogues of certain Shimura varieties. In particular, one can construct them from purely group theoretic data called local Shimura data.
The primary purpose of this dissertation is to study Rapoport-Zink spaces whose underlying local Shimura datum is of Hodge type and Hodge-Newton reducible. Our study consists of two main parts: the study of the l-adic cohomology of Rapoport-Zink spaces in relation to the local Langlands correspondence and the study of deformation spaces of p-divisible groups via the local geometry of Rapoport-Zink spaces.
The main result of the first part is a proof of the Harris-Viehmann conjecture in our setting; in particular, we prove that the l-adic cohomology of Rapoport-Zink spaces contains no supercuspidal representations under our assumptions. In the second part, we obtain a generalization of Serre-Tate deformation theory for Shimura varieties of Hodge type.</p