Vortex-Induced Vibrations And Jump Phenomenon: Experiments With a Clamped Flexible Cylinder In Water

Vortex Induced Vibrations experiments have been carried out with a clamped, lightly damped and long, flexible, circular cylinder suspended from a towing tank carriage. The lock-in phenomenon has been observed through bending strain measurements taken along the tube span, within a range of speeds — Reynolds number varying from 6¥103 to 4¥104 — fully exciting the first flexural eigenmode. Experimental results clearly show 2 resonance branches, in accordance with similar experiments performed in water with rigid cylinders mounted in linearly elastic supports (Khalak and Williamson, 1996; Parra, 1996), or in air (Feng, 1968). But now, an upper overlapping branch of oscillation appears at higher reduced velocities than in the case of rigid models. For the first natural mode of flexural vibration the nondimensional mass-damping parameter in water, (m*+Ca)za in Khalak’s and Williamson’s 1996 work, takes the value 0.016 if only the structural damping in water is considered. This value has the same low order of magnitude observed by those authors. As could be expected, a jump phenomenon, from the lower to the upper branch, has been experimentally observed. This jump takes place at a reduced velocity value close to Vr @ 8.30, giving further evidences of a nonlinear scenario, as discussed, e.g., by Bearman (1984), Brika and Laneville (1993), Khalak and Williamson (1996) and first addressed by Feng (1968). *ISOPE Member. Received January 4, 1999; revised manuscript received by the editors August 17, 1999. The original version (prior to the final revised manuscript) was presented at the Eighth International Offshore and Polar Engineering Conference (ISOPE-99), Montréal, Canada, May 2429, 1997.