Vortex-induced vibrations of structures

Vortex-induced oscillations. often of concern when a bluff structure is exposed to fluid cross-flow, are considered herein using a semi-empirical modeling approach. Based an the fluid momentum theorem, the model involves a highly simplified abstraction of the complex flow field. and major assumptions concerning the nature of the coupling between the fluid and the oscillating structure. Three prototype problems are studied. including harmonically forced cylinders, spring-mounted cylinders. and taut elastic cables; in each case the structure is assumed to be of circular cross-section and situated in a uniform cross-flow. Only oscillations transverse to the flow are considered. The problem of modal interaction for elastic cables, typically of interest when the fluid flow excites high-mode-number resonances is given particular attention. The model produces a set of nonlinear, ordinary differential equations describing the coupled fluid/structure oscillations. Steady-state oscillatory solutions to these equations are found analytically and are examined for stability. Using various regression techniques, the steady-state solutions are then fit to experimental data for forced and spring-mounted cylinders. Finally, the model's predictions for elastic cables are used to postulate a qualitative picture of modal interaction, certain features of which have been observed experimentally