A. James Clark School of Engineering
Permanent URI for this communityhttp://hdl.handle.net/1903/1654
The collections in this community comprise faculty research works, as well as graduate theses and dissertations.
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Item Examination of US Transportation Public-Private Partnership Experience: Performance and Market(2024) Zhang, Kunqi; Cui, Qingbin; Civil Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Worldwide, public-private partnership (P3) project performance and benefits accrued to market participants are understudied. Focusing on the US, this dissertation examines the country’s transportation P3 experience through three empirical studies comparing P3 to design-bid-build (DBB), the traditional delivery method. Throughout, the Information Source for Major Projects database, built by a University of Maryland team in which the author led the data collection effort, served as the data source. In the first study, the researchers examined P3 cost and time performance using piecewise linear growth curve modeling, recognizing that past cross-sectional studies had produced mixed results. With 133 major transportation projects, the longitudinal analysis confirmed P3’s time performance advantage and efficiency diffusion effecting cost savings in DBB, where efficiency diffusion was a new term describing the spillover and internalization of technical and managerial innovations inducing an efficient outcome. The second study used social network analysis to investigate collaboration patterns among different types of players in the P3 market (i.e., public sponsors, special purpose vehicles, investors, lenders, advisors, contractors, and professional service firms). With 135 projects and 1009 organizations, data found that both P3 and DBB networks are small worlds. Exponential random graph modeling revealed that practicing in the DBB market helps firms participate in P3 projects and that large firms (vis-à-vis small/medium-sized firms) are not privileged. The third study, further exploring the P3 market, focused on the Disadvantaged Business Enterprise (DBE) program. Administered by the US Department of Transportation, the program promotes the participation of small, disadvantaged firms in federal-aid projects. Linear regressions on 134 contracts showed that P3 associates with higher DBE goals in terms of percentage of dollars to be awarded to DBEs, whereas the delivery method does not affect the actual attainment. Overall, the findings justify continued policy support towards P3 implementation.Item EFFECTS OF AUGMENTED REALITY BASED OBJECT ILLUMINATION ON HUMAN PERFORMANCE(2020) Stone, Matthew; Akin, David L; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Extravehicular Activities (EVAs) in space are generally considered to be high-risk, costly activities, due to the nature of the working environment and the limitations imposed on astronaut mobility and dexterity. Procedures are scheduled out and rehearsed far in advance, with time being considered a precious commodity during missions. Providing artificial task guidance to astronauts could potentially improve their efficiency, enabling for shorter duration EVAs and/or a larger quantity of tasks completed. This research quantitatively measured the effects of virtually illuminating or “cueing” objects of interest on a user’s ability to complete a predefined task, through the use of augmented reality (AR) “active display” symbology. This was achieved through the implementation of a Microsoft HoloLens™ head mounted display. It was demonstrated that, after controlling for a variety of factors, virtual illumination techniques improved task completion speed by approximately 100% and reduced perceived mental workload, with no adverse effects on accuracy.Item RISK ASSESSMENT AND MITIGATION OF TELECOM EQUIPMENT UNDER FREE AIR COOLING CONDITIONS(2012) Dai, Jun; Pecht, Michael G; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In recent years, about 40% of the total energy is devoted to the cooling infrastructures in data centers. One way to save energy is free air cooling (FAC), which utilizes the outside air as the primary cooling medium, instead of air conditioning, to reduce the energy consumption to cool the data centers. Despite the energy saving, the implementation of free air cooling will change the operating environment, which may adversely affect the performance and reliability of telecom equipment. This thesis reviews the challenges and risks posed by free air cooling. The increased temperature, uncontrolled humidity, and possible contamination may cause some failure mechanisms, e.g., Conductive anodic filament (CAF) and corrosion, to be more active. If the local temperatures of some hot spots go beyond their recommended operating conditions (RoC), the performances of the equipment may be affected. In this thesis, a methodology is proposed to identify the impact of free air cooling on telecom equipment performance. It uses the performance variations under traditional air condition (A/C) to create a baseline, and compares the performance variation under variable temperature and humidity representing FAC with the baseline. This method can help data centers determine an appropriate operating environment based on the service requirements, when FAC is implemented. In addition, a statics-based approach is also developed to identify the appropriate metric for the performance variations comparison. It is the first study focusing on the impact of FAC on the telecom equipment performance. This thesis also proposes a multi-stage (design, test, and operation) approach to mitigate the reliability risks of telecom equipment under free air cooling conditions. Specifically, a prognostics-based approach is proposed to mitigate the reliability risks at operation stage, and a case study is presented to show the implementation process. This approach needn't interrupt data center services and doesn't consume additional useful life of telecom equipment. It allows the implementation of FAC in data centers which were not originally designed for this cooling method.Item Experimental Investigation of Shrouded Rotor Micro Air Vehicle in Hover and in Edgewise Gusts(2011) Hrishikeshavan, Vikram; Chopra, Inderjit; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Due to the hover capability of rotary wing Micro Air Vehicles (MAVs), it is of interest to improve their aerodynamic performance, and hence hover endurance (or payload capability). In this research, a shrouded rotor conguration is studied and implemented, that has the potential to oer two key operational benets: enhanced system thrust for a given input power, and improved structural rigidity and crashworthiness of an MAV platform. The main challenges involved in realising such a system for a lightweight craft are: design of a lightweight and stiff shroud, and increased sensitivity to external flow disturbances that can affect flight stability. These key aspects are addressed and studied in order to assess the capability of the shrouded rotor as a platform of choice for MAV applications. A fully functional shrouded rotor vehicle (disk loading 60 N/m2<\super>) was designed and constructed with key shroud design variables derived from previous studies on micro shrouded rotors. The vehicle weighed about 280 g (244 mm rotor diameter). The shrouded rotor had a 30% increase in power loading in hover compared to an unshrouded rotor. Due to the stiff, lightweight shroud construction, a net payload benefit of 20-30 g was achieved. The different components such as the rotor, stabilizer bar, yaw control vanes and the shroud were systematically studied for system efficiency and overall aerodynamic improvements. Analysis of the data showed that the chosen shroud dimensions was close to optimum for a design payload of 250 g. Risk reduction prototypes were built to sequentially arrive at the nal conguration. In order to prevent periodic oscillations in flight, a hingeless rotor was incorporated in the shroud. The vehicle was successfully flight tested in hover with a proportional-integral-derivative feedback controller. A flybarless rotor was incorporated for efficiency and control moment improvements. Time domain system identification of the attitude dynamics of the flybar and flybarless rotor vehicle was conducted about hover. Controllability metrics were extracted based on controllability gramian treatment for the flybar and flybarless rotor. In edgewise gusts, the shrouded rotor generated up to 3 times greater pitching moment and 80% greater drag than an equivalent unshrouded rotor. In order to improve gust tolerance and control moments, rotor design optimizations were made by varying solidity, collective, operating RPM and planform. A rectangular planform rotor at a collective of 18 deg was seen to offer the highest control moments. The shrouded rotor produced 100% higher control moments due to pressure asymmetry arising from cyclic control of the rotor. It was seen that the control margin of the shrouded rotor increased as the disk loading increased, which is however deleterious in terms of hover performance. This is an important trade-off that needs to be considered. The flight performance of the vehicle in terms of edgewise gust disturbance rejection was tested in a series of bench top and free flight tests. A standard table fan and an open jet wind tunnel setup was used for bench top setup. The shrouded rotor had an edgewise gust tolerance of about 3 m/s while the unshrouded rotor could tolerate edgewise gusts greater than 5 m/s. Free flight tests on the vehicle, using VICON for position feedback control, indicated the capability of the vehicle to recover from gust impulse inputs from a pedestal fan at low gust values (up to 3 m/s).Item Performance Measurement, Simulation, and Analysis of the Cox Tee Dee 0.010, the World's Smallest Production Internal Combustion Engine(2006-12-15) Sookdeo, Troy; Cadou, Christopher; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The Cox Tee Dee 0.010 is a two-stroke 0.010 cubic inch model engine designed to power small propeller-based hobby aircraft. First manufactured in 1961, it remains the smallest working piston engine ever mass-produced, but no scientific measurements of its performance are available in the open literature. These measurements are important because they could facilitate the development of small unmanned air vehicles. This thesis reports measurements of power output and efficiency using a specialized dynamometer. An unsuccessful attempt is made to correlate the measurements with simulations based on Stanford University's Engine Simulation Program (ESP). Instead, the results are compared to the predictions of a simple zero-dimensional thermodynamic MATLAB simulation of an engine cycle developed at the University of Maryland. Differences and correlations are discussed and the engine performance is analyzed in the context of propulsion systems for small UAVs and for compact power generation.