Coupled cross-flow and in-line vortex-induced vibration of elastic cable systems.

摘要

Flow-induced vibrations caused by vortex shedding from structures have been observed in diverse engineering applications including ocean engineering structures. The focus of many of prior studies has been on understanding the vortex-induced vibrations (VIV) solely in the lift direction (one-dimensional vibrations). The objective of this research is to extend our understanding to two-dimensional VIV by simultaneously considering the in-line and cross-flow responses of elastic cable systems. Coupled cross-flow and in-line vibrations are analyzed for two different cable applications: (1) cable suspensions, and (2) cable-buoy systems. Three coupling mechanisms are identified and include: (1) cable geometric nonlinearities, (2) coupled unsteady lift and drag, and (3) amplitude dependent mean drag.; Coupled cross-flow and in-line VIV is first investigated by focusing on two-dimensional motions local to a cable equilibrium. Prior experiments on spring supported cylinders confirm that coupled cross-flow/in-line responses exist by virtue of the simultaneous excitation in these two directions. Coupled cross-flow and in-line VIV is modeled by incorporating cable structural nonlinearities, and coupled fluid lift and drag. Inclusion of cable geometric coupling alone leads to coupled periodic responses that differ qualitatively (i.e., in number and stability of periodic motions) when compared to those of the decoupled models. Inclusion of direct fluid coupling produces non-planar figure eight motions of the cable cross section that exhibit similar characteristics to those previously measured for spring supported cylinders.; Another coupled response results from the amplified mean drag exerted on the cable during lock-in. As the drag increases during lock-in, the cable will slowly drift downstream. This slow drifting motion may appreciably alter the cable geometry and tension, the natural frequencies of the cable, the flow velocity relative to the cable. If these changes are significant, the drift may alter, and even disrupt, the resonance condition for lock-in. If lock-in is disrupted, the mean drag is reduced and the cable may slowly drift upstream returning to its original state. The whole process may then repeat. Theoretical models for both suspended cables and cable-buoy systems are derived that describe the nonlinear response of the cable to fluid excitation (VIV) and increased drag. These cable/fluid models capture both fast (small amplitude) vibrations due to periodic vortex shedding and slow (large amplitude) drifting motion due to increased drag.