Vortex-induced oscillations are considered using a semi-empirical modeling approach. Based on the fluid momentum theorem of Iwan-Blevins (Iwan, W. D. and Blevins, R. D., "A Model of Vortex Induced Oscillation of Structures," J. Applied Mechanics, Vol. 41, No. 3, Sep. 1974.), the model involves a highly simplified version of the complex flow field and major assumptions concerning the nature of coupling between the fluid and the oscillating structure. Three physical problems are considered: spring-mounted cylinders, harmonically forced cylinders and elastic cables. In each problem only oscillations transverse to the flow are considered. The structure is assumed to be a circular cross-section and placed in a uniform cross-flow. The problem of modal interaction for elastic cables is given particular attention. The model produces a set of non-linear ordinary differential equations describing the coupled fluid/structure oscillations. Steady-state monofrequency (single frequency of oscillation), approximate solutions of these equations are found analytically and are examined for stability purposes. Finally the model's predictions for elastic cables are used to give a qualitative picture of modal interaction.
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