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Investigations of the velocity and vorticity fields in the wake of a flexible riser with a length to diameter ratio of 181 were conducted in a towing tank at moderate Reynolds numbers in the range of 9400–47,000. Wake velocity measurements were made with the riser freely vibrating in both in-line and cross-flow directions. The motion and wake field of the riser, undergoing free vibration, were simultaneously measured by accelerometers installed inside the riser and by using a digital particle image velocimetry (DPIV) system. The vortex-induced vibration (VIV) results show that the riser freely oscillated at multiple vibration frequencies and amplitudes at each Reynolds number. Mixed vortex modes, ‘2S’, ‘2P’ and ‘P+S’, were observed in the near wake of the riser at different instants of time. The occurrence of these vortex modes depended on the Reynolds number, dominant frequency and mean amplitude. At lower Reynolds number, the single stable mode ‘2S’ dominated the wake. With the increase of Reynolds number, the percentage of the ‘2S’ modes decreased while the percentage of ‘2P’ modes increased steadily except at Reynolds numbers of 14,100 and 47,000. The ‘P+S’ modes occurred mostly at a Reynolds number of 14,100 accompanied by more ‘2P’ modes and less ‘2S’ modes. At this Reynolds number, the frequency of the VIV was very close to the natural frequency of 0.72 Hz, which was obtained from a riser decay test in steady water and the average amplitude to diameter ratio reached 0.95, the highest found in these tests.

Item Type:	Refereed Article
Keywords:	Long flexible cylinder; Vortex-induced vibration; Wake vortex pattern; Instant vector field
Research Division:	Engineering
Research Group:	Maritime engineering
Research Field:	Naval architecture
Objective Division:	Expanding Knowledge
Objective Group:	Expanding knowledge
Objective Field:	Expanding knowledge in engineering
UTAS Author:	Bose, N (Professor Neil Bose)
ID Code:	59343
Year Published:	2009
Web of Science® Times Cited:	14
Deposited By:	NC Maritime Engineering and Hydrodynamics
Deposited On:	2009-11-28
Last Modified:	2010-04-19
Downloads:	2 View Download Statistics