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Hydrodynamic force coefficients of a vertical circular cylinder Mihelcic, Carolin Susan


The problem addressed in this thesis is that of the behaviour of large offshore structures subjected to ice and earthquake loading. The theoretical formulation of the fluid force and associated added mass and damping coefficients acting on an isolated vertical surface-piercing rigid circular cylinder which is excited by sinusoidal unidirectional ground motions is presented. The closed-form solution is first developed on the basis of potential flow theory for arbitrary values of excitation frequency and, in addition, its asymptotic form for high frequencies is considered. The latter is found to be accurate in predicting the high-frequency added mass only for high structure radius-to-water depth ratios and the high-frequency damping for all radius-to-depth ratios. A computer method for numerical evaluation of the force coefficients is devised and theoretical results for different values of radius-to-depth ratios are thereby generated. An experimental study has been conducted in the Earthquake Engineering Laboratory of the Department of Civil Engineering at the University of British Columbia to verify the theoretical results obtained for the vertical distribution of the force coefficients of a model cylinder which satisfies the large body regime of fluid-structure behaviour for which effects due to fluid viscosity are negligible. Owing to unanticipated technical problems, the current study is unsuccessful and data recorded in the sinusoidal tests are unrealistic, although the calculated coefficients appear to be independent of base displacement (an observation which indicates that viscous effects were insignificant during testing). Nevertheless, values of total force coefficients which were obtained experimentally for a similar model in a previous investigation are found to agree very well with the corresponding theoretical results for frequencies of up to 6 Hz. It is concluded that the theoretical formulation provided for the hydrodynamic force coefficients of a vertical surface-piercing circular cylinder subjected to horizontal sinusoidal base motions of arbitrary frequency may be used to accurately predict the total added mass and damping of real structures satisfying the conditions imposed by the theory.

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