Mechanical Engineering - Ph.D. / Sc.D.
http://hdl.handle.net/1721.1/7848
2015-03-05T04:10:36ZThe effects of secondary flows on the heat transfer to turbine nozzle endwall and rotor shroud.
http://hdl.handle.net/1721.1/95535
The effects of secondary flows on the heat transfer to turbine nozzle endwall and rotor shroud.
Nebo, Anthony Chibuzo
Thesis (Sc.D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 1979.; MICROFICHE COPY AVAILABLE IN ARCHIVES AND ENGINEERING.; Vita.; Includes bibliographical references.
1979-01-01T00:00:00ZAsymptotically optimal path planning and surface reconstruction for inspection
http://hdl.handle.net/1721.1/93868
Asymptotically optimal path planning and surface reconstruction for inspection
Papadopoulos, Georgios
Motivated by inspection applications for marine structures, this thesis develops algorithms to enable their autonomous inspection. Two essential parts of the inspection problem are (1) path planning and (2) surface reconstruction. On the first problem, we develop a novel analysis of asymptotic optimality of control-space sampling path planning algorithms. This analysis demonstrated that asymptotically optimal path planning for any Lipschitz continuous dynamical system can be achieved by sampling the control space directly. We also determine theoretical convergence rates for this class of algorithms. These two contributions were also illustrated numerically via extensive simulation. Based on the above analysis, we developed a new inspection planning algorithm, called Random Inspection Tree Algorithm (RITA). Given a perfect model of a structure, sensor specifications, robot dynamics, and an initial configuration of a robot, RITA computes the optimal inspection trajectory that observes all surface points on the structure. This algorithm uses of control-space sampling techniques to find admissible trajectories with decreasing cost. As the number of iterations increases, RITA converges to optimal control trajectories. A rich set of simulation results, motivated by inspection problems for marine structures, illustrate our methods. Data gathered from all different views of the structure are assembled to reconstruct a 3D model of the external surfaces of the structure of interest. Our work also involved field experimentation. We use off-the-shelf sensors and a robotic platform to scan marine structures above and below the waterline. Using such scanned data points, we reconstruct triangulated polyhedral surface models of marine structures based on Poisson techniques. We have tested our system extensively in field experiments at sea. We present results on construction of various 3D surface models of marine structures, such as stationary jetties and slowly moving structures (floating platforms and boats). This work contributes to the autonomous inspection problem for structures and to the optimal path, inspection and task planning problems.
Thesis: Ph. D., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2014.; Cataloged from PDF version of thesis.; Includes bibliographical references (pages 163-179).
2014-01-01T00:00:00ZAligning stakeholder interests : from complex systems to emerging markets
http://hdl.handle.net/1721.1/93867
Aligning stakeholder interests : from complex systems to emerging markets
Austin-Breneman, Jesse
Design often requires balancing competing objectives from a variety of stakeholders. From the design of large-scale complex engineering systems to the design of end-user products for emerging markets, managing the trade-offs between different objectives from a systems-level perspective is a key challenge for design teams. This thesis investigates differences between how formal strategies can be used to balance trade-offs and how practitioners currently perform this task. Through the use of interviews, case studies, and field and laboratory experiments, this thesis seeks to examine how real-world designers approach these problems. The work investigates practitioner strategies and analyzes them to gain a better understanding of how human design teams operate. These insights are then used to inform proposed guidelines for performing design tasks in these contexts. First, observations of practitioners in space system design lead to a new way of modeling interactions between sub-systems. Then, interviews with designers working on products for emerging markets are used to formulate a new methodology, Design for Micro-Enterprise, that focuses on the needs of small-scale entrepreneurs. Results from the analysis suggest that focusing on a micro-entrepreneur's business strategy may be a successful approach to balancing both the end-user and supply chain requirements in these markets.
Thesis: Ph. D., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2014.; Cataloged from PDF version of thesis.; Includes bibliographical references (pages 87-94).
2014-01-01T00:00:00ZDescribing functions for information channels subject to packet loss and quantization
http://hdl.handle.net/1721.1/93866
Describing functions for information channels subject to packet loss and quantization
Gilbertson, Eric (Eric W.)
underwater environments. Examples include autonomous underwater vehicle (AUV) coordination and navigation, Wide Area Measurement System (WAMS) control of power grids, and automobile networked subsystems control. As one would expect, demand on throughput grows to fill the available channel capacity; in a clean short-range RF setting, entire images may be transferred in each cycle, as part of a vision-based control system, whereas in the ocean, acoustic channels with perhaps twenty bits per second allow only the most basic sensor and command information to be shared regularly. Packet-based wireless communication systems like these that operate near their limits are necessarily quantized, and often prone to loss. These properties directly impact the overall system performance, and thus methodologies for understanding and designing feedback systems with quantization and packet loss are valuable. This thesis makes several contributions to feedback control of systems subject to quantization and stochastic packet loss in the sensor feedback channel. First, we derive and verify describing functions (DFs) for information channels subject to quantization and packet loss. The DFs represent the loss and quantization effects by frequency- and amplitude-dependent gains and phases, similar to transfer functions. These DFs are unique because, unlike most other DFs that describe hardware and physical elements, these describe stochastic information channels. DFs are presented for a general codec algorithm, and for four commonly-used sensor-feedback codecs: Zero-Output, Hold- Output, Linear Filter, and Modified Information Filter. These are each given as closed-form mathematical expressions of the provably optimal gains and phases for each case, with each decoder a specific case of the general codec algorithm. Gains and phases predicted by the models are verified by simulation for open-loop stable, open-loop unstable, minimum phase and non-minimum phase example systems. Second, we show how the DFs can be used as analysis tools to predict limit cycles in dynamic feedback control systems. Computation times using the DFs are shown to be orders of magnitude faster than those from simulation for these calculations. Third, we propose a synthesis method to use the DFs to design a codec for the sensor feedback channel that decreases limit cycle amplitudes induced by quantization and packet loss for a large class of systems. Up to three-fold reductions in limit cycle amplitudes are shown, with the tradeoff being slightly higher system sensitivity to disturbances and slightly higher steady state errors to step inputs. The designed codec is of the special and simple form of a constant times the sent signal if the signal is received and a different constant times the previous decoded signal if the sent signal is lost. This is the equivalent structure and computation complexity to both Zero-Output and Hold-Output decoders. A DF for this decoder allows the constants to be solved for as functions of target limit cycle amplitudes. The constants reduce to solutions of cubic equations, which are guaranteed to have a real root, and thus the codec is physically realizable. The codec allows for multiple limit cycle frequency solutions for the same amplitude solution. The analysis and synthesis tools are verified both by numerical examples, and by a physical experiment controlling heading of a small robotic raft where the designed decoder results in smaller limit cycles than does a linear-filter-based decoder.
Thesis: Ph. D., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2014.; Cataloged from PDF version of thesis.; Includes bibliographical references (pages 205-213).
2014-01-01T00:00:00Z