EXPERIMENTAL STUDY ON VIV DYNAMICS OF A FLEXIBLE CYLINDER IN UNIFORM CROSS-FLOW

In this paper we present the experimental results of a study on the effects of pre-tension and axial stiffness on vortex-induced vibration (VIV) of a horizontally mounted flexible cylinder. Theparameters examined included vibration amplitude and its suppression, mean and fluctuating tensions, frequency increase rate, drag and lift coefficients, and in-line (IL) and cross-flow (CF) harmonics. The test model was selected with a low bending-stiffness, low mass ratio (cylinder’smass/mass of replaced water), and high aspect ratio (length/diameter=162). The tests were conducted in the subcritical Reynolds number regime (Re=2000-20000). The effects of pretension and axial stiffness were studied for five different cases in which these parameters werevaried. Our results showed that high pre-tension, which reduces vibration amplitude, can significantly raise the lift coefficient. Specifically, a four-fold increase in pre-tension from 73.5 N through 294 N was found to correspond to an increase of approximately 57% in lift coefficientand a decrease of approximately 30% in vibration amplitude. We also observed that the lock-in bandwidth of amplitude response narrowed with increase in pre-tension, whereas, it broadened with axial stiffness. In contrast to the vibration amplitude bandwidth, the lift bandwidthincreased with increase in pre-tension. The ratio of dominant IL-to-CF frequency was almost 2.0 except for the IL lock-in and upper branch regions. In the IL lock-in region the ratio was found to be 1, and in the upper branch the ratio reached 4.0 for the highest pre-tension (294 N), accompanied by broadening of the region in which this ratio is over 2.0.

کلیدواژه ها:

Vortex-induced vibration (VIV) ، Flexible cylinder ، Hydrodynamic forces ، Vibration amplitude ، Pre-Tension ، axial Stiffness