Free-stream turbulence effects on vortex-induced vibration and flow-induced force of an elastic cylinder

摘要

The effect of free-stream turbulence on vortex-induced vibration of an elastic cylinder in a cross-flow and the associated fluid forces was investigated experimentally. The range of Reynolds number investigated is 5000-41 000. A turbulence generation grid was used to generate turbulence intensity around 10% in the upstream flow. Cylinder displacements in the transverse direction at cylinder mid-span were measured in the reduced velocity range U r0=1.45-12.08. The cylinder vibration has two dominant components, one at the frequency of vortex shedding and another at the natural frequency of the fluid-cylinder system. These are represented by their root-mean-square (r.m.s) values, w yV ′ and w yM ′, respectively. For a nonturbulent uniform flow, w yV ′ is dominant in the U r0 range of 1.45U r07.25, where lock-in occurs at U r0=5.31, while w yM ′ is dominant in the post-lock-in U r0 range of 8.21U r012.08. Free-stream turbulence increases the vortex shedding frequency in the post-lock-in region but does not change the system natural frequency. It also substantially increases w yV ′ in the U r0 range of 1.45U r08.21 range and w yM ′ in the U r0 range of 9.18U r012.08. These features are further studied using flow-induced forces deduced from vibration data. The overall effect of free-stream turbulence can thus be interpreted from the ratio of energy increment; free-stream turbulence feeds energy to the cylinder in general, and this energy transmission reaches a maximum at the lock-in point, and drops rapidly in the off-lock-in region. Therefore, the lock-in region is of main concern when free-stream turbulence is present.