Master of Science (MS), Bowling Green State University, 2017, Geology

The length of a day on Earth (abbreviated LOD) is not exactly 24 hours. There is a small excess LOD that varies on timescales ranging from a few days to thousands of years, generally on the order of milliseconds. One characteristic of LOD variations is a sinusoidal component with a period of ~6 years. The cause of the ~6-year signal is unknown, but is generally suspected to be exchanges of angular momentum between the mantle and the core. This study aimed to test the hypothesis that the ~6-year LOD signal is due to coupling between the mantle and fluid outer core. The flow of the core's fluid deforms the base of the mantle, leading to redistribution of Earth's mass (causing changes in the gravitational field) and deformation of the overlying crust. Surface deformation data from a global network of high-precision Global Positioning System (GPS) stations was analyzed, and the component that acts on the ~6-year timescale was isolated and inverted for the core's flow. Resulting angular momentum changes were computed for the outer core and compared to the LOD signal to search for evidence of core-mantle coupling. Outer core angular momentum changes obtained from GPS deformation data exhibit evidence of the suspected core-mantle coupling, but this result is sensitive to inversion parameters. Changes in the gravitational field were also modeled and found to be smaller than the errors in the currently available data.

Committee: Yuning Fu PhD (Advisor); Richard Gross PhD (Committee Member); Marco Nardone PhD (Committee Member); Margaret Yacobucci PhD (Committee Member) Subjects: Geology; Geophysics

Doctor of Philosophy, The Ohio State University, 2014, Electrical and Computer Engineering

This work presents the development of an integrated flight controller for a generic model of a hypersonic air-breathing vehicle. The flight control architecture comprises a guidance and trajectory planning module and a nonlinear inner-loop adaptive controller. The emphasis of the controller design is on achieving stable tracking of suitable reference trajectories in the presence of a specific engine fault (inlet unstart), in which sudden and drastic changes in the vehicle aerodynamics and engine performance occur. First, the equations of motion of the vehicle for a rigid body model, taking the rotation of the Earth into account, is provided. Aerodynamic forces and moments and engine data are provided in lookup-table format. This comprehensive model is used for simulations and verification of the control strategies. Then, a simplified control-oriented model is developed for the purpose of control design and stability analysis. The design of the guidance and nonlinear adaptive control algorithms is first carried out on a longitudinal version of the vehicle dynamics. The design is verified in a simulation study aiming at testing the robustness of the inner-loop controller under significant model uncertainty and engine failures. At the same time, the guidance system provides reference trajectories to maximize the vehicle's endurance, which is cast as an optimal control problem. The design is then extended to tackle the significantly more challenging case of the 6-degree-of-freedom (6-DOF) vehicle dynamics. For the full 6-DOF case, the adaptive nonlinear flight controller is tested on more challenging maneuvers, where values of the flight path and bank angles exceed the nominal range defined for the vehicle. Simulation studies show stable operation of the closed-loop system in nominal operating conditions, unstart conditions, and during transition from sustained scramjet propulsion to engine failure mode.

Committee: Andrea Serrani Prof. (Advisor); Umit Ozguner Prof. (Committee Member); Zhang Wei Prof. (Committee Member) Subjects: Aerospace Engineering; Computer Engineering; Electrical Engineering; Engineering

Master of Science (MS), Ohio University, 2008, Physics and Astronomy (Arts and Sciences)

Rotation periods or lower limits for 34 Near-Earth Asteroids (NEAs) were obtained through optical light curves. Two codes were developed in order to obtain the true fraction of Fast-Rotating Asteroids (FRAs), F, using Fortran 95 and IDL. The first code models the shape of an asteroid and simulates its light curve. The second code, uses the results obtained from the observational program and the simulated light curves to obtain the probability density of F, P(F). The observational and statistical analysis indicates that the population of asteroids with D<150m is almost equally divided between fast and slow rotators, and that the majority of the population of asteroids with D>150m consists of slow-rotators. These results also indicate that selection effects have significantly influenced the currently known distribution of rotation periods of NEAs and therefore that it is not representative of the real population of NEAs.

Committee: Thomas S. Statler PhD (Advisor); Alexander Nieman PhD (Committee Chair); Joseph C. Shields PhD (Committee Member) Subjects: Astrophysics