Thesis (M.Ed.)--University of Hong Kong, 1994. / Includes bibliographical references (leaves 70-75). Also available in print.

Thesis (M. Soc. Sc.)--University of Hong Kong, 1984.

Thesis (M.Soc.Sc.)--University of Hong Kong, 1984. / Also available in print.

Since their invention in the early 1900s, Tuned Vibration Absorbers (TVAs) have shown to be effective in suppressing vibrations of machines and structures. A vibration absorber is a vibratory subsystem attached to a primary system. It normally consists of a mass, a spring, and a damper. Mounted to the primary system, a TVA counteracts the motions of the primary system, "absorbing" the primary structure's vibrations. A conventional passive TVA, however, is only effective when it is tuned properly, hence, the name "tuned" vibration absorber. In many practical applications, inevitable off-tuning (or mistuning) of a TVA occurs because of the system's operating conditions or parameter changes over time. For example, the mass in a building floor could change by moving furnishings, people gathering, etc., which can "off-tune" TVAs. When TVAs are off-tuned, their effectiveness is sharply reduced. Moreover, the off-tuned TVAs can excessively amplify the vibration levels of the primary structures; therefore, not only rendering the TVA useless but also possibly causing damage to the structures. Off-tuning is one of the major problems of conventional passive TVAs. This study proposes a novel semiactive TVA, which strives to combine the best features of passive and active TVA systems. The semiactive TVA in this study includes a Magneto-Rheological (MR) damper that is used as a controllable damping element, for providing the real-time adjustability that is needed for improving the TVA performance. This study is conducted in two phases. The first phase provides a numerical investigation on a two-degree-of-freedom (2-DOF) numerical model in which the primary structure is coupled with a TVA. The numerical investigation considers four semiactive control methods for the MR TVAs, in addition to an equivalent passive TVA. These numerical models are optimally tuned using numerical optimization techniques to compare each TVA system. These tuned systems then serve as the basis for numerical parametric studies for further evaluation of their dynamic performance. The parametric study covers the effects of damping, as well as system parameter variations (off-tuning). The results indicates that semiactive TVAs are more effective in reducing the maximum vibrations of the primary structure and are more robust when subjected to off-tuning. Additionally, the numerical study identifies the "On-off Displacement-Based Groundhook control (on-off DBG)" as the most suitable control method for the semiactive TVA among control methods considered in this study. For the second phase of this study, an experimental study is performed on a test setup, which represents a 2-DOF structure model coupled with an MR TVA. Using this setup, a series of tests are conducted in the same manner as the numerical study to evaluate the performance of the semiactive TVA. The primary purposes of the experiment are to further evaluate the most promising semiactive control methods and to serve as a "proof-of-concept" of the effectiveness of this MR TVA for floor vibration applications. The results indicate that the semiactive TVA with displacement-based groundhook control outperforms the equivalent passive TVA in reducing the maximum vibrations of the primary structure. This confirms the numerical result that identifies on-off DBG control method as the "best" control method for the MR TVA among four semiactive control schemes considered. The experimental robustness study is also conducted, focusing on the dynamic performance of both the passive and the semiactive TVAs when the mass of the primary system changes (mass off-tuning). The mass of the primary system varied from -23 % to +23 % of its nominal value by adding and removing external masses. The experimental results show that the semiactive TVA is more robust to changes in the primary mass than the passive TVA. These results justify the benefits of the use of semiactive MR TVAs in structures, such as building floor systems. The off-tuning analysis further suggests that, in practice, semiactive TVAs should be tuned slightly less than their optimum in order to compensate for any added masses to the structure. Additionally, the lessons learned from the experimental study have paved the way for implementing the semiactive MR TVA on a test floor, which is currently in progress under a separate study. / Ph. D.

Between 1960 and 1968 the United States conducted intensive psychological operations (PSYOP) in Vietnam. To date, no comprehensive study of the psychological war there has been conducted. This dissertation fills that void, describing the development of American PSYOP forces and their employment in Vietnam. By looking at the complex interplay of American, North Vietnamese, National Liberation Front (NLF) and South Vietnamese propaganda programs, a deeper understanding of these activities and the larger war emerges. The time period covered is important because it comprises the initial introduction of American PSYOP advisory forces and the transition to active participation in the war. It also allows enough time to determine the long-term effects of both the North Vietnamese/NLF and American/South Vietnamese programs. Ending with the 1968 Tet Offensive is fitting because it marks both a major change in the war and the establishment of the 4th Psychological Operations Group to manage the American PSYOP effort. This dissertation challenges the argument that the Northern/Viet Cong program was much more effective that the opposing one. Contrary to common perceptions, the North Vietnamese propaganda increasingly fell on deaf ears in the south by 1968. This study also provides support for understanding the Tet Offensive as a desperate gamble born out of knowledge the tide of war favored the Allies by mid-1967. The trend was solidly towards the government and the NLF increasingly depended on violence to maintain control. The American PSYOP forces went to Vietnam with little knowledge of the history and culture of Vietnam or experience conducting psychological operations in a counterinsurgency. As this dissertation demonstrates, despite these drawbacks, they had considerable success in the period covered. Although facing an experienced enemy in the psychological war, the U.S. forces made great strides in advising, innovating techniques, and developing equipment. I rely extensively on untapped sources such as the Foreign Broadcast Information Service transcripts, Captured Document Exploitation Center files, and access to the U.S. Army Special Operations Command Archives. Additionally, I have digitized databases such as the Hamlet Evaluation System and Terrorist Incident Reporting System for Geographic Information System software analysis. The maps provide examples of the possibilities available to the historian using these datasets.

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