The Flow around a Cantilevered Circular Cylinder with Finite Span-length under Flow-Induced In-line Oscillation (Vortex Frequency Distribution along Span Direction)

Abstract

The flow-induced in-line oscillation of a centilevered circular cylinder was experimentally studied by free oscillation tests in a water tunnel. Two hot-film probes were set in the wake of a test cylinder to measure the fluctuating velocity in the near-wake. These probes were moved along the span direction at some reduced velocities Vγ. Two types of test cylinders, AR 10 with the aspect ratio of 10 and AR 21 with the aspect ratio of 21, were investigated. AR 21 with an end plate had two excitation regions against Vγ but AR 10 had one excitation region. The vortex structure of the AR 21 cylinder with the end plate was the symmetrical vortex at the first excitation region and was the alternate vortex from the end of the first excitation region to the second excitation region. So, the valley between these excitation regions is caused by the characteristics of the alternate vortex. Because the vortex structure of the AR 10 cylinder was mainly symmetrical vortex at any reduced velocity, the excitatio region of AR 10 is caused by the instability related to the symmetrical vortex. The vibration characteristic that was caused by aspect ratio was explained by the flow around the tip of cylinder, because AR 10 with end plate had two excitation regions. The aspect ratio of the cylinder influeces the vortex structures and the characteristics of vibration.