A study on the effects of axial stiffness and pre-tension on VIV dynamics of a flexible cylinder in uniform cross-flow

摘要

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. The parameters 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's mass/mass of replaced water), and high aspect ratio (length/diameter = 162). The tests were conducted in the subcritical Reynolds number regime (Re = 2000-20,000). The effects of pre-tension and axial stiffness were studied for five different cases in which these parameters were varied. Our results showed that high pre-tension, which reduces vibration amplitude, can significantly raise the lift coefficient. Specifically, a fourfold increase in pre tension from 73.5 N through 294 N was found to correspond to an increase of approximately 57% in lift coefficient and 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 bandwidth increased with increase in pre-tension. The fluctuating lift has a secondary peak at the point of maximum fluctuating drag. In addition, high applied pre-tensions delay the excitation of higher modal frequencies compared to lower pre-tensions within the same range of flow velocities. Spectral analysis of all the data showed a broadband frequency response in the lower branch. 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 broadeningof the region in which this ratio is over 2.0. The rate of mean and fluctuating tensions increased with increasing axial stiffness and decreased with increasing pre-tension. For constant pre-tension, the increase of axial stiffness resulted in higher frequency increase rates, while the frequency increase rate slowed with the increase of pre-tension. Hence, the parameter TL~2/EI may not be a reliable criterion for assessment of the frequency increase rate regardless of pre-tension. Regarding the inference of the VIV response from tension measurements, we found that the fluctuating tension component showed a similar trend with corresponding CF vibration amplitude and increased for higher vibrational modes. In addition, we were able to simply extract excited IL and CF frequencies from tension measurements.