Experimental study on vortex-induced vibration of a long flexible pipe in sheared flows

Abstract

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. Copyright ？ 2011 by The International Society of Offshore and Polar Engineers (ISOPE).