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ItemCharacterizing Single Ventricle Patient-Specific Anatomy Using Segmentation of MRI and 3D Reconstruction to Aid Surgical Planning(Georgia Institute of Technology, 2008-08-01) Jayaprakash, GopinathSingle ventricle congenital heart defects occur 2 per every 1000 live births in the USA. In these cases, cyanosis occurs due to the mixing of venous deoxygenated blood and oxygenated blood from the lungs. These defects are surgically treated by the total cavo-pulmonary connection (TCPC), where the superior and inferior vena cavae are connected to the pulmonary arteries routing the systemic venous return directly to the lungs. However, this Fontan repair results in high energy losses and therefore the optimization of this connection prior to the surgery could significantly improve post-operative performance. In this paper, the in-house segmentation and 3D reconstruction scheme is used in the following studies. First, 3D geometrical analysis of the TCPCs is used to determine the advantages and disadvantages of two commonly performed TCPC palliations intra-atrial and extra-cardiac configurations. Then, a surgical planning outline is proposed with segmentation of pre and post surgical Magnetic Resonance Imaging (MRI) data followed by the 3D reconstruction with emphasis on extracting surrounding vessels and structures. A pediatric surgeon performs a virtual surgery on the reconstruction of the patient s pre-Fontan anatomy prior to the actual surgery. A segmentation of the heart, aorta and surrounding vessels superimposed with the Glenn, when used with the SURGEM® tool, simulates the actual Fontan operation. This outline allows the surgeon to envision numerous scenarios of possible surgical options, and accordingly to predict the post operative procedures. The segmentation tool is improved upon to increase the accuracy and efficiency of the process and enhance the quality of the anatomical reconstructions.
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ItemCylindrical beam volume holograms recorded in reflection geometry for diffuse source spectroscopy(Georgia Institute of Technology, 2008-05-06) Jolly, SundeepMultimodal multiplex spectroscopy (MMS) has been demonstrated to increase the optical throughput of a spectrometer as opposed to that of conventional optical spectrometers and has been implemented using three-dimensional photonic crystals and spherical-beam volume holograms recorded in the transmission geometry as spectral diversity filters. While such efforts have resulted in compact and sensitive Fourier-transform holographic spectrometers, there still remains much room for performance improvements. Previous studies [6,7,9] have proven the utility of spherical-beam volume holograms recorded in the transmission geometry as spectral diversity filters for spectrometers. The role of the recording geometry in the performance of cylindrical-beam volume holograms as spectral diversity filters is investigated here. The transmission recording geometry is compared to the reflection recording geometry on the basis of the spectral operating range of the resultant spectral diversity filters.
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ItemDesign and development of a novel compact soft-surface structure for performance improvement and size reduction of a microstrip Yagi array antenna(Georgia Institute of Technology, 2008-05-05) Thai, Trang ThuyAn new antenna structure based on a microstrip Yagi array antenna and a soft surface (SS) ring is designed and developed, which enables a highly directional gain in addition to an improved front-to-back (F/B) ratio of more than 20 dB. The SS ring is shown to be capable of greatly improving the performance while miniaturizing the design s size by half. The implementation of the SS ring to a microstrip Yagi array antenna is demonstrated for different ground sizes to verify its functionality in suppressing surface waves, showing that an improvement of at least 3 dB in the F/B ratio can be obtained. The design is investigated at the center frequency of 5.8 GHz, however, the structure can be easily scaled to other frequency ranges. A parametric analysis is performed to give insight into the operational mechanism of the SS ring and on the critical dimensions that affect the SS structure surrounding the antenna array. In addition, measurements are presented to validate the results obtained via simulation. The principles established in this paper are applicable to other planar antenna designs.
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ItemImage Analysis of Acoustically Excited Bluff Body Flames(Georgia Institute of Technology, 2007-12-17) Plaks, Dmitriy VitalThis thesis analyzes the effects of various bluff bodies on the downstream flow field. Bluff bodies, for example, those typically found in jet engine augmentors, are objects designed to impede the flow in order to stabilize a flame. The effects of different bluff body shapes (cylindrical and triangular), size (6.35 mm, 9.53 mm, 12.7 mm, and 19.1 mm) and heat release are examined with respect to their influence on downstream vorticity strength, vortex separation distance, and vorticity divergence angle. Particle Image Velocimetry (PIV) is used to obtain the velocity field data from which the vorticity field is calculated. The mean flow velocity, U∞ is 2.7 m/s, and the flow is acoustically excited at 300 Hz with a normalized acoustic velocity of u'/U∞ = 0.8. The vorticity divergence angle increases with increasing bluff body size, is not affected by bluff body shape, and has a non-linear correlation with heat release. Downstream vorticity strength is affected by all three parameters (bluff body shape, size and heat release) in a non-linear manner. Vortex separation distance is a function primarily of bluff body size, increasing for larger bodies; however, the separation distance decreases with increasing heat release. Bluff body shape also has an effect on vortex separation distance as the cylindrical bluff body creates a larger separation distance between vortices.
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ItemLow Static Power and High Throughput Wave-Pipelined Global Interconnect Circuits(Georgia Institute of Technology, 2007-12-17) Youngblood, Mark WilliamThis research project will explore a low-power, high-throughput design using high threshold voltage transistors in combination with wave-pipelining techniques across global interconnect circuits.
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ItemNon-Deterministic Design of Utility Scale Wind Energy Systems(Georgia Institute of Technology, 2007-05) Scott, RobertThe wind is an increasingly significant source of energy with the rising price of non-renewable fuels. The purpose of this project is to determine the specific intensity and frequency of wind speed required to sustain a large-scale wind farm with power output on the order of hundreds of megawatts. To this end, a non-deterministic methodology will be developed to analyze the viability of wind energy systems. A deterministic analysis method considers the majority of design parameters to be known or fixed and may only perform trade studies on a few parameters at a time to optimize performance. In the case of the energy market though, this is not an advantageous strategy since several factors related to economic viability such as energy prices, interest rates, government incentives, acquisition costs and maintenance are highly variable and cannot be assumed to be known. A non-deterministic, statistical approach to wind turbine design has the advantage of predicting with corresponding levels of certainty the power output and economic viability of an energy system. The primary goal of this project is to define the envelope of operating conditions for a large-scale wind project while considering variables of both engineering and economic significance. The National Renewable Energy Laboratory’s (NREL) Hybrid Optimization Model for Electric Renewables (HOMER) will be incorporated into the previous analysis using YawDyn and PROPID to determine the economic returns on investment in hypothetical financing cases. Cost factors will now be assigned a mean value along with a probability distribution. Monte Carlo simulations will be run for a large number of variations in the assumed economic and engineering variables to develop an accurate estimate of the price per kilowatt-hour of energy produced from the simulated wind project for a variety of site conditions with the goal of finding the most suitable environment for sustainable wind development.
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ItemOhmic Contacts for Wide Bandgap Devices(Georgia Institute of Technology, 2007-05) Mehra, ZenBipolar devices based on GaN and SiC have gained tremendous popularity as an alternative to Si based devices, primarily due to the ability to sustain high temperature and high voltage operations that can be attributed to their high breakdown field and saturation velocity. Any bipolar device based on wide bandgap technology requires high performing ohmic contacts that have low specific contact resistivity and exhibit linear I-V behavior, as opposed to Schottky characteristics. A range of high work function metals like Ni, Au, Pd, Pt, Al can be used to realize these contacts. Further, annealing under specific conditions is required to ensure extremely high doping in the near surface layer. In this research, the Ni/Au stack for p-type contacts, and the Ti/Al/Ti/Au layer for n-type contacts have been specifically investigated over a range of annealing time periods and temperatures. Control wafers with GaN:Mg (p-type) or GaN:Si and GaN/AlGaN (n-type) formed the basis of this study. The Transmission Line Model (TLM) technique was used to conduct measurements, and obtain the specific contact resistance. Specific contact resistances as low as 9.44*10-3 Ω-cm2 for n-type, and 5.73*10-3 Ω-cm2 for p-type were achieved. These shall form the basis for ultimate fabrication of an InGaN/GaN HBT with a high current gain, breakdown voltage (VBR) and current density (J). The quality of resulting contact is seen to depend on the initial doping/bulk resistance, determination of a unique time-temperature window, and careful process control. A further investigation is conducted into non-linear behavior exhibited by p-type contacts for low separation.
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ItemDirect Conversion for Space Solar Power(Georgia Institute of Technology, 2007-05) Boechler, Nicholas SebastianSpace Solar Power (SSP) is a powerful yet nearly untapped resource with revolutionary potential. SSP systems currently have several roadblocks to their implementation. With the technology in use today, converting solar power to useable energy is inefficient, the required converters have a large mass per unit power, and launching those converters is expensive. More fundamentally, in all current SSP systems, energy is generated in the form of a direct current before being converted again into whatever form is necessary. In addition to the large mass per unit energy of this conversion equipment, such conversion involves significant efficiency losses, further resulting in the prohibitive cost of launching these converters into space. If techniques could be discovered for converting broadband sunlight directly to a useable narrowband application dependent frequency, many fundamental breakthroughs in aerospace endeavors can be achieved. This project studied a large number of options that might lead to direct conversion. Those technology options were analyzed according to which would warrant further exploration from the point of view of aerospace systems applications and possible power per unit mass. Based on these technologies, several advanced concepts were considered. It is also important to make an estimate of the possible power per unit mass that could be achieved with each concept, so that architecture developers can proceed with the development of applications enabled by direct conversion technology. Accordingly, estimates of the possible power per unit mass of potential direct conversion systems were made, and future applications that would benefit from those direct conversion systems were identified. Three possible concepts were developed. These concepts include: a shocked photonic crystal system; a solar pumped maser based on naturally occurring astronomical masers; and an optical antenna array with central signal processing. The optical antenna array and the solar pumped maser were estimated to have a specific power approximately 15.0 and 10.8 times greater, respectively, than conventional photovoltaic systems. Additionally, several applications were identified that would benefit from direct conversion systems, including a SSP grid and electric propulsion.