Vortex-induced vibration of a single degree-of-freedom flexibly-mounted horizontal cylinder near the free surface

Abstract

This paper reports findings from an experimental study on vortex-induced vibration of a horizontal, flexibly-mounted cylinder positioned near the free surface of fluid flow. The cylinder is allowed to oscillate in the vertical direction only. Oscillation amplitude and frequency data are presented as a function of reduced velocity and dimensionless depth from the free surface. Data are presented for cases of decreasing depth from where the resting cylinder is fully-submerged until its centerline is even with the free surface of the water. For a fully-submerged cylinder, VIV behavior is consistent with published findings of similar systems, producing a well-documented lock-in region with initial, upper, and lower branches. Broadly, as the cylinder is raised from the fully-submerged case, the amplitudes diminish and the lock-in region decreases in range and shifts toward higher reduced velocities. When the cylinder is very near the surface, a second region of oscillations occurs at higher reduced velocities. This region emerges in cases where the cylinder’s top edge is half a diameter below the free surface. For a short range of depths, the lock-in region and the non-zero amplitude region at higher reduced velocities coexist. But after a critical depth, the lock-in region disappears and oscillations are observed only at the higher reduced velocity region. Multi-frequency oscillations and regions of hysteresis are observed for some cases of low depth and high reduced velocity.