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

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 Ur0=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 Ur0 range of 1.45<Ur0<7.25, where lock-in occurs at Ur0=5.31, while w yM ′ is dominant in the post-lock-in Ur0 range of 8.21<Ur0<12.08. 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 Ur0 range of 1.45<Ur0<8.21 range and w yM ′ in the Ur0 range of 9.18<Ur0<12.08. 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.