Triggering mechanics for transverse vibrations of a circular cylinder in a shear flow: Wall-proximity effects

Abstract

Vortex-Induced Vibrations (VIVs) of a near-wall cylinder involve complex interactions between the structure and the boundary layers in the shear flow. Based on dimensional analyses, the wall-proximity effects on the triggering of transverse VIVs of a circular cylinder are physically investigated with flume observations. An accessorial low structural-damping VIV simulation device was developed and an upward-illumination Particle Image Velocimetry (PIV) system was employed in the physical modeling. Although the regular Kármán-like vortex-shedding could be suppressed for small gap-to-diameter ratios (e.g., e/D < 0.40) as reported in previous studies, the present power spectral analyses on the velocity fluctuations in the lee-wake indicate that the characteristic frequency can still be well identified, which could further trigger the vibrations of a low mass-damping cylinder. PIV measurements show that, as the cylinder approaches the bottom, the asymmetry of swirling-strengths in the lee-wake gets more remarkable, and the values of Strouhal number increase correspondingly. Four typical stages can be identified in the process of the vibration being triggered under the wall-free condition, including (a) the fully-stationary, (b) the intermittent-vibrating, (c) the sharp-jump, and (d) the upper-branch vibration stages. But, for e/D≤ 0.20, the intermittent-vibrating stage disappeared. During the sharp-jump stage, the peak jump-amplitude decreases dramatically as the cylinder approaches the bottom, which has negative correlation with the vibration frequency. The effects of the swirling-strength attenuation and the downward collisions on the vibration responses are discussed. The relationship between critical reduced velocity V rcr and e/D is finally established by fitting the experimental results. It is indicated that the wall-proximity effects could be significant and should be taken into account while evaluating the critical velocity for triggering transverse vibrations of the near-wall cylinder in the engineering practices.