Game Theoretic Formation of a Centrality Based Network

We model the formation of networks as a game where players aspire to maximize their own centrality by increasing the number of other players to which they are path-wise connected, while simultaneously incurring a cost for each added adjacent edge. We simulate the interactions between players using an algorithm that factors in rational strategic behavior based on a common objective function. The resulting networks exhibit pairwise stability, from which we derive necessary stable conditions for specific graph topologies. We then expand the model to simulate non-trivial games with large numbers of players. We show that using conditions necessary for the stability of star topologies we can induce the formation of hub players that positively impact the total welfare of the network.Comment: Submitted to 2012 ASE Social Informatics Conferenc

High Performance Microchannel Heat Exchanger Design

Novel manufacturing methods allows for miniaturization of microchannel dimensions with the potential to enhance heat exchanger performance by increasing surface area per unit mass. In this thesis, microchannel heat exchanger design is analyzed to determine factors limiting effectiveness and power density. Ideal heat exchanger performance is characterized for one-pass counterflow heat exchangers with varying channel dimensions, flow regimes, and thermal conductivities. Smaller channel sizes lead to increased pressure drop and temperature gradients within the channel, necessitating low flow rates. Axial conduction is found to drive down effectiveness at low flow rates, with this effect more pronounced with decreased channel dimensions. Reducing the heat exchanger’s thermal conductivity to 1W/(m∙K) is found to reduce axial conduction. However, pressure drop still limits performance for existing designs, so optimized designs are necessary to create high performance microchannel heat exchangers. A multi-pass design consisting of stacked one-pass layers to keep pressure drop at an acceptable level is tested at various channel dimensions and flow regimes. The multi-pass design achieved an 11.2 times increase in power density and 1.08 times increase in maximum achievable effectiveness over leading commercial heat exchanger designs

Tool Condition Monitoring and Replacement for Tubesheet Drilling

Tool Condition Monitoring (TCM) methods have shown significant potential to automatically detect worn tools without intervention in the machining process, thus decreasing machine downtime and improving reliability and part quality. Previous research on TCM systems have used a wide variety of time-domain and frequency-domain features extracted from cutting force related parameters as well as mechanical and acoustical vibrations to infer the wear state of tools. This project concerns the process of drilling thousands of tight-tolerance holes on tubesheets and baffles of heat exchangers using large diameter indexable insert drills on a horizontal boring machine. To address the issues involved in the process, the aim of this research is to develop a non-intrusive, indirect, online TCM system on the horizontal boring machine to monitor the drill wear and hole quality while drilling. The specific objectives are to establish an indirect TCM system for the drilling process, to develop models to predict tool wear and the machining accuracy of the drilled holes, and to develop an optimum tool replacement strategy. The TCM system developed used two cutting-force related signals on the horizontal boring machine, namely the spindle motor current and the axial feed motor current. Features extracted from these data streams, as well as the machining parameters, the cutting speed and the feed rate, and the number of holes drilled with the current inserts, are the inputs to a series of models to predict the tool wear state and the hole diameter. The first model is an autoregressive model that allows the prediction of the extracted features for the next hole before it is drilled. As each hole is drilled, this model is updated with the most recent data to improve the accuracy of the prediction. The predicted values for the features are then used as inputs to the second and third models which are surface response models, one to estimate the tool wear state and one to estimate the hole diameter. A tool replacement strategy based on applying limits to the predicted hole diameter was also developed. Adjusting these limits allows the strategy to be tuned for either hole accuracy or tool life depending on the requirements of a specific application. Tuning the replacement strategy for tool life resulted in a significant 44% increase in tool life and a non-trivial reduction in machine down time due to fewer tool changes while holding a hole diameter tolerance of ±0.1mm. The TCM system ensured that not a single over tolerance hole would have been drilled which is critically important since over tolerance holes can result in a scrapped workpiece. The proposed 3-model TCM system shows promise in being able to significantly reduce the risk of drilling out of tolerance holes while at the same time increasing tool life and correspondingly decreasing tool change time. The models are able to accurately predict the insert flank wear and as well as the actual hole diameter within acceptable error. The TCM system could be implemented in an industrial settingwith minimal revision and since it is an indirect system there would be no intrusion into the manufacturing operation. One limitation of the TCM system as proposed is that it is only capable of detecting gradual tool wear and not catastrophic tool failure, a limitation that was known from the outset but was not investigated as it was beyond the scope of this project. The proposed TCM system would allow the integration of additional functionality to instantaneously detect catastrophic tool failure. Finally, for use in a production environment, the developed models need to be implemented on a standalone device that requires essentially no operator input to monitor continuous drilling operations for tubesheet and baffle applications. This implementation could include automatic detection of the machining parameters using frequency analysis of the motor signals

Spinor dynamics in an antiferromagnetic spin-1 thermal Bose gas

We present experimental observations of coherent spin-population oscillations in a cold thermal, Bose gas of spin-1 sodium-23 atoms. The population oscillations in a multi-spatial-mode thermal gas have the same behavior as those observed in a single-spatial-mode antiferromagnetic spinor Bose Einstein condensate. We demonstrate this by showing that the two situations are described by the same dynamical equations, with a factor of two change in the spin-dependent interaction coefficient, which results from the change to particles with distinguishable momentum states in the thermal gas. We compare this theory to the measured spin population evolution after times up to a few hundreds of ms, finding quantitative agreement with the amplitude and period. We also measure the damping time of the oscillations as a function of magnetic field.Comment: 5 pages, 3 figure

Experiences of Male Learning Community Participants at a Private Historically Black University

Many U.S. colleges and universities have created programs to improve retention and graduation rates of Black male students. The purpose of this qualitative case study was to examine the lived experiences of Black male learning community participants and discover what faculty and staff perceive to be major influences on the retention of this population. The conceptual framework was rooted in Tinto\u27s student integration model and Swail\u27s geometric model of student persistence and achievement. The research questions within this study examined what faculty/staff perceive to be a major influence on the retention of minority male learning community participants. The questions also examined the perceptions of black male learning community participants regarding the major influence on their decision to remain at the selected institution after their first year. Data collection included semistructured interviews with 2 faculty, 2 staff, and 6 student learning community participants, and examination of national and local records. Data coding and analysis revealed 5 themes: college selection, college perception, learning community experience, Black male retention, and Black male mentorship. Findings indicated that most learning community participants returned to the institution based on faculty/staff support and a brotherhood bond with their peers. Faculty and staff reported that the institution could retain more minority male students if faculty/staff engagement with students and male mentorship beyond the learning community were increased. The project included a professional development program on effective minority male mentoring strategies for faculty/staff at the selected institution. This project may bring a new perspective on the idea of minority male retention while providing university officials with qualitative data to increase retention

Variable impedance energy dissipation on the micro-scale : field responsive fluids in novel geometries

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2006.Includes bibliographical references (leaves 187-189).The aim of this thesis was to further characterize the effectiveness of field responsive fluids (FRFs) in geometries pertinent to the soldier and to examine the effects of specific geometric and kinematic parameters, including patterned surface geometry, electrode gap distance, and normal force on the performance of homogeneous ERF composites. Field responsive fluid composites designed for variable impedance energy absorption incorporated electrorheological fluid (ERF) and shear-thickening fluid (STF) in novel geometries to absorb compressive and tensile/shear forces. ER and ST fluids change their apparent viscosity in the presence of elevated electric and shear fields, respectively, and the magnitude of this effect can be adjusted using the magnitude of the input field, allowing variable impedance operation. Several test fixtures were developed to test these novel FRF composites. A compression apparatus was designed and constructed to test STF-filled foam over a range of strain rates not previously examined in the literature. Silicon-based microchannel devices with etched features on the order of 100 pm and etch depths of 7-90 pm were fabricated to test homogeneous ER fluids in small electrode gaps.(cont.) Tests using these silicon devices allowed creation of 5 kV/mm (5 V/pm) electric fields across electrode gaps as small as 20 pm, with increases of measured shear force as high as 350% from no electric field to full 5 kV/mm operation. Production of these devices in bulk using established silicon processing techniques was demonstrated, and factors affecting the manufacture of these devices were investigated.by Ryan A. Griffin.S.M

Human Powered Vehicle Team Challenge

Design and build a Human Powered Vehicle in a team of five. Compete at the ASME HPVC competition in March of 2022. Re-establish the Human Powered Vehicle Team at the University of Akron

SN 2016iet: The Pulsational or Pair Instability Explosion of a Low Metallicity Massive CO Core Embedded in a Dense Hydrogen-Poor Circumstellar Medium

We present optical photometry and spectroscopy of SN 2016iet, an unprecedented Type I supernova (SN) at $z=0.0676$ with no obvious analog in the existing literature. The peculiar light curve has two roughly equal brightness peaks ($\approx -19$ mag) separated by 100 days, and a subsequent slow decline by 5 mag in 650 rest-frame days. The spectra are dominated by emission lines of calcium and oxygen, with a width of only $3400$ km s$^{-1}$, superposed on a strong blue continuum in the first year, and with a large ratio of $L_{\rm [Ca\,II]}/L_{\rm [O\,I]}\approx 4$ at late times. There is no clear evidence for hydrogen or helium associated with the SN at any phase. We model the light curves with several potential energy sources: radioactive decay, central engine, and circumstellar medium (CSM) interaction. Regardless of the model, the inferred progenitor mass near the end of its life (i.e., CO core mass) is $\gtrsim 55$ M$_\odot$ and up to $120$ M$_\odot$, placing the event in the regime of pulsational pair instability supernovae (PPISNe) or pair instability supernovae (PISNe). The models of CSM interaction provide the most consistent explanation for the light curves and spectra, and require a CSM mass of $\approx 35$ M$_\odot$ ejected in the final decade before explosion. We further find that SN 2016iet is located at an unusually large offset ($16.5$ kpc) from its low metallicity dwarf host galaxy ($Z\approx 0.1$ Z$_\odot$, $M\approx 10^{8.5}$ M$_\odot$), supporting the PPISN/PISN interpretation. In the final spectrum, we detect narrow H$\alpha$ emission at the SN location, likely due to a dim underlying galaxy host or an H II region. Despite the overall consistency of the SN and its unusual environment with PPISNe and PISNe, we find that the inferred properties of SN\,2016iet challenge existing models of such events.Comment: 26 Pages, 17 Figures, Submitted to Ap