Flow-induced vibration of long structures

When a body is exposed to a flowing fluid, oscillations can occur due to one or more of several different mechanisms. The resulting large amplitudes of motion and fatigue are potential sources of structural failure. Furthermore, the drag force on a structure can be increased due to the effectively larger cross-sectional area presented to the flow from the oscillation. A complete understanding of the nature of such vibration is essential in the design of many civil and mechanical engineering systems. Previous solutions to the vortex-induced vibration problem were primarily based on modal analysis, using a one- or two-mode approximation. Use of modal analysis implies a "locked-in" condition: the vortex shedding frequency and a natural frequency of the system are coincident. Observations made on long cable systems indicate that the amplitude of response is smaller than is predicted by a conventional modal analysis. The drag forces on such structures are therefore overestimated by current design approaches. In very long structures, typical of those found in ocean applications, modes are closely spaced, and it is not reasonable to assume total spanwise correlation in the fluid forces or response. The approach used herein attempts to avoid the limitations associated with the modal solution of such problems by implementing a solution based on traveling waves. The technique draws on earlier theoretical and empirical models for the complex vortex-shedding phenomenon, and incorporates these into a new method for analyzing the structural response problem. The traveling wave approach can be used to model effectively spanwise variable flow environments by summing the calculated responses of adjacent active sections of cable. Until this method was developed, there was no suitable method available for modeling flow characteristics of this type. Modal analysis is effectively limited to systems with uniform flow over all or part of the system