Experiments on vortex-induced vibration of a long pipe model (L/D = 400) were conducted in linearly sheared flows. The top flow speed was varied from 0.2m/sec to 2.0m/sec in step of 0.1m/sec. A fiber glass pipe was instrumented with FBG (Fiber Bragg Grating) sensors to measure the strains in VIV responses. The number of strain measurement stations was 15 for each fiber with spacing of 1m. Initial pipe tension was varied as 80kgf, 90kgf and 100kgf. The modal analysis of strain measurement data was conducted. The peak VIV frequency coincides with the Strouhal frequency corresponding to 70% of the maximum flow speed. The mode number of the maximum amplitude vibration is determined by the integer ratio of the peak vibration frequency to the natural frequency of the model. The main vibration mode is caused by the VIV mode corresponding to the flow speed at the mid region of the model. There exist traveling waves in high flow speed region and standing waves in low speed region. The VIV response in linearly sheared flow is multi-mode.
Vortex-induced vibration of slender marine structures such as marine risers, pipelines and cables is the critical factor determining the structures fatigue life. Vortex-induced vibration (VIV) is caused by alternating shedding of vortices from either sides of the structure. The dissipated kinetic energy of the flow due to vortex shedding provokes lift forces and structure vibration in vortex shedding frequency (fv) determined by Strouhal relation. The vibration component of fv occurs in the cross-flow (CF) direction. The CF vibration is coupled with the in-line (IL) vibration, whose frequency is doubled to be 2fv. Depending on phase angle between CF and IL displacements, the vibration trajectory of a point in structure results in crescent shape or figure eight.